Simulating and Testing the TRIUMF Bragg Ionisation Chamber · UMF [6]. The facility is divided into...

Simulating and Testing the TRIUMF Bragg

Ionisation Chamber

Chantal Nobs

February 2013

A dissertation submitted to the Physics Department at the University of Surrey in partial

fulfilment of the degree of Master in Physics.

DEPARTMENT OF PHYSICS

University of Surrey

Guildford

Surrey

GU2 7XH

Abstract

At TRIUMF the process of charge breeding radioactive ion beams, for the purposes of

acceleration, introduces high levels of stable contamination. The resulting cocktail beam makes

tuning the relatively weak radioactive beam difficult. A Bragg chamber is a type of gas ionisation

chamber, which produces signals with information about the atomic number and mass of the

isotopes contained in the beam; making it a useful tool for beam tuning and diagnostics. In

addition to tuning the beam it is also necessary in experiments to know the constituents of a

beam and this is best done through particle identification.

In the framework of this dissertation a Bragg chamber (TBragg) was implemented at TRI-

UMF’s Isotope Separator and Accelerator (ISAC) facility. Simulations were conducted to op-

timise the pressure and voltage for each experiment, and also to provide a comparison with

data. Two in-beam experiments have been conducted this year, the first of which was with

an 18O beam. The main aim of this experiment was to provide experimental evidence to show

that non-flammable P-8 gas could be used to replace flammable P-10 gas, without having to

compromise on resolution. The second experiment was conducted with a 94Sr beam, where the

TBragg was one out of a series of detectors utilised to test beam quality. The data collected

with the 94Sr beam has shown that the TBragg is capable of identifying beam contamination

with an energy resolution of 0.9–1.7 %, and a Z resolution of 1/67–1/75.

Acknowledgements

A special thanks is extended to Reiner Kruecken for not only providing me with this unique

opportunity to work at TRIUMF, which has been one of the most rewarding and exciting years

of my life, but for also being consistently supportive. I would like to thank Greg Hackman for

his valued assistance and advice throughout my project. Also to Peter Bender for his assistance

in data analysis and preparation for experiments conducted with the TBragg, and for always

being available and willing to provide me with the support I needed.

I am grateful to have had the pleasure of meeting Robert Openshaw, in addition to designing

and constructing the TBragg gas handling system he was always happy to spend his time sharing

his limitless knowledge with me. My thanks also to Roman Gernhaeuser for his assistance in

helping us problem solve in the initial stages of testing the TBragg. Of course, my gratitude

to all those who helped take shifts during beam time Greg, Peter, Carl, David M., David C.,

Corina, Adam and Lee, and also Gordon Ball for his valued assistance interpreting data. Of

course, I would like to thank everyone at TRIUMF who have made me feel very welcomed and

helped me to settle in Adam, Mustafa, Carl, Peter, Steffan and David to name but a few.

Finally, without the support from Paddy Regan I would not have spent my year at TRIUMF

and I would not have gained the valuable knowledge this year has given me. A huge thanks to

Paddy for all the hard work he puts into securing research positions for students every year.

This project was partly funded by NSERC.

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Contents

1 Introduction 1

2 Theory 6

2.1 Theory of Energy loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Theory of Electron Drift and Diffusion . . . . . . . . . . . . . . . . . . . . . . . . 10

3 Background and Design 13

3.1 The TRIUMF Bragg Ionisation Chamber (TBragg) . . . . . . . . . . . . . . . . . 13

3.1.1 Frisch Grid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.2 Gas Handling System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.2.1 EPICS - Experimental Physics Control System . . . . . . . . . . . . . . . 20

3.2.2 Working With Flammable Gas . . . . . . . . . . . . . . . . . . . . . . . . 21

3.3 Testing Window Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.4 Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4 Simulations 25

4.1 SRIM 2011 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.1.1 SRIM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.1.2 TRIM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.2 Drift Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.2.1 Simulating the anode signal . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.3 Pulse Shape Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

5 Preliminary Tests 40

5.1 Experimental Set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

5.2 Data Collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

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5.3 Using the non-flammable gas alternative, CF4 . . . . . . . . . . . . . . . . . . . . 46

6 Low Mass Beam Tests 49

6.1 ISAC I Set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

6.2 ISAC I Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

6.2.1 Gold target . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

6.2.2 Lithium-fluoride target . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

6.2.3 Carbon target . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

6.3 ISAC I Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

7 High Mass Beam Tests 61

7.1 ISAC-II Set Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

7.1.1 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

7.2 ISAC II Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

8 Conclusions 71

8.1 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

8.2 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

Appendix A Instructions for the TBragg GHS 74

A.1 Operating with a non-flammable gas . . . . . . . . . . . . . . . . . . . . . . . . . 74

A.2 Operating with flammable gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

A.3 Setting Pressures and Flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

Appendix B EPICS interlocks 81

Appendix C CUPC Abstract 82

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List of Figures

1.1 A schematic of the ISAC (Isotope Separator and Accelerator) facility at TRI-

UMF [6]. The facility is divided into two sections, in ISAC-I beams are used

directly from the targets for mass measurements, laser spectroscopy, decay spec-

troscopy and precision decay studies; additionally accelerated beams are used to

study nuclear reactions. Whereas ISAC-II uses high energy radioactive isotope

beams to study direct reactions of exotic nuclei. . . . . . . . . . . . . . . . . . . . 3

1.2 The post-accelerator used to accelerate radioactive beams above the Coulomb

barrier has an A/Q acceptance of < 7, therefore prior to acceleration the beam is

charge bred. This graph shows the typical mass spectrum of ions extracted from

the charge state booster, which increases the charge state of ions, thus lowering

the A/Q value [7]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1 These graphs were produced using SRIM to show how the Bragg curve varies

when increasing the energy and the atomic number of incident ions. (a) 12C ions

at 2, 3 and 4 MeV/u stopping in P-10 at 1.15 bar. (b) 14N, 12C and 11B at

36 MeV stopping in P-10 at 1.05 bar. . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 A schematic of the geometry for a heavy ion in close proximity to an atomic

electron [14]. Where e is the charge of an electron, v is the velocity of the heavy

ion, M is the mass of the heavy ion, ze is the charge of the heavy ion, b is the

distance of the heavy ion to the electron. . . . . . . . . . . . . . . . . . . . . . . . 7

iv

3.1 This figure shows a modified schematic of the Bragg ionisation chamber from the

Diploma thesis by W. Weinzierl [18]. It highlights the four key components of

this detector; gas fills the inside of the chamber to stop ions that pass through the

window (1), as this happens electrons are ionised in the gas. The electrode rings

(2) accelerate these electrons to the anode (4), which is shielded by the Frisch

grid (3). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.2 (a) The 1.5 µm aluminised Mylar entrance window which acts as the first barrier

for ions. Typically 1–2 MeV for A¿30 is lost by ions passing through the window.

(b) The supporting grid is required in order for the window to contain high

pressures (>2 bars) in the TBragg. . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.3 This graph shows how the percentage change in energy varies for ions of different

atomic number passing through the Mylar entrance window, of thickness 1.5 µm,

calculated using SRIM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.4 The TBragg contains a set of 20 electrode rings, which create the constant electric

field required to drift the ionised electrons towards the anode. The rings are

tapered in order to minimise edge effects, which arise due to the finite size of

the rings. The electrode rings are connected via a resistor chain to create a field

typically of 11 V/cm between the window and the Frisch grid and 500 V/cm

between the Frisch grid and the anode. . . . . . . . . . . . . . . . . . . . . . . . . 15

3.5 This figure shows the effect of electrons moving relative to a conductor, under the

effect of an electric field. (a) A static electric field means the charges inside the

conductor are stationary. (b) As the electron approaches the conductor the elec-

tric field lines get closer together, internal charges move to maintain equilibrium.

If connected to a charge sensitive preamplifier this movement of charges would

cause a signal to be generated. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.6 This figure shows the effect of introducing a Frisch grid between charged particles

created in the gas and the anode. (a) The electric field lines for all charged

particles will terminate at the Frisch grid, shielding the anode. (b) When the

electrons pass through the grid the anode will collect the charge, while remaining

shielded from other charged particles . . . . . . . . . . . . . . . . . . . . . . . . . 18

v

3.7 This figure shows the gas handling system constructed by R. Openshaw [21] and

M. Goyette [22] in order to provide gas to the TBragg, and to maintain a constant

pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.8 This readout box shows the flow rate (left), and the pressure (right) in the TBragg.

The proportional-integral-differential controller (right) is used to set the desired

pressure (green) and show the current pressure (red). A pressure gauge is con-

nected to the TBragg that reads into the proportional-integral-differential con-

troller, which then adjusts the flow rate to maintain a constant pressure. . . . . . 20

3.9 The schematic of the TBragg gas handling system, provided by R. Openshaw,

which is used to provide gas to the TBragg, and maintain a constant pressure for

prolonged periods of time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.10 The experimental set up used to locate the pressure limits of the Mylar windows,

which was required before further experiments could be conducted. . . . . . . . . 22

3.11 This figure shows a circuit diagram for the TBragg preamplifier box. It includes

the resistive chain that connects the electrodes inside the chamber. . . . . . . . . 23

3.12 This figure shows the effect the preamplifier box has on the anode signal. The

RC circuit will produce a signal with a fast rise time and a slow decay time. The

preamplifier chips will then amplify this new signal. . . . . . . . . . . . . . . . . 24

3.13 This figure shows a signal that has been digitised through use of an analog to

digital converter (ADC). The TBragg ADC was set to sample at a frequency of

50 MHz (every 20 ns). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4.1 This figure shows input screen for SRIM, a program which calculates the range

of ions in matter for a variety of different stopping materials [15]. . . . . . . . . . 25

4.2 This graph shows the pressure required to stop ions at 9 cm into the TBragg, for

all four gases available. CF4 and isobutane are better suited for stopping lighter

ion, as opposed to P-10 and P-8. The red lines mark the upper and lower limits

allowed by the gas handling system. In this plot the data points for P-10 and P-8

overlap completely, and are thus indistinguishable. . . . . . . . . . . . . . . . . . 26

4.3 These graphs show the Bragg curve (energy loss as a function of distance) for

24Mg at 4 MeV/u stopping in P-10 at (a) 3 cm (b) 6 cm and (c) 9 cm, which was

achieved by lowering the gas pressure. These plot were created using SRIM. . . . 28

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4.4 These graphs show how for higher masses the Bragg peak becomes less defined

(a) 32S at 3 MeV/u in P-10 (b) 40K at 3 MeV/u in P-10 (c) 64Zn at 3 MeV/u in

P-10. All of these plots were created using SRIM. . . . . . . . . . . . . . . . . . . 28

4.5 (a) Shows the TRIM input screen, one of the programs available in SRIM 2011

that calculates the energy loss of ions in matter. (b) Shows some of the graphs

that can be produced using TRIM. . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.6 These figures show the relationship between the drift velocity of electrons and

E/P for (a) isobutane, (b) CF4 and (c) P-10. This data was obtained experimen-

tally [26] (isobutane: Figure 4.5, CF4: Figure 4.6 and P-10: Figure 4.7). . . . . . 30

4.7 A diagram of ions moving through a 10 cm drift volume and ionising electrons in

the gas. The majority of electrons are produced at the end of the ions path, and

will reach the anode first when drifted under the influence of an electric field. . . 32

4.8 This graph provides a visual representation of the “forecast” function, which is a

linear interpolation formula used to calculate the energy of simulated ions through

10 cm of gas, using data obtained through TRIM. Where x1, y1 and x2, y2 are

the closest sets of simulated data points to New(x) and New(y). . . . . . . . . . . 33

4.9 These plots show how the simulated anode signals vary for ions with (a) different

energies and different atomic numbers (b) the same energies but different atomic

numbers and (c) the same atomic numbers but different energies, stopping in

isobutane at 127 mbar with a drift velocity of 4.1 cm/µs. . . . . . . . . . . . . . 35

4.10 This diagram highlights the process for applying a trapezoidal filter with one

window (filter function), and shows the signal before and after application of the

filter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.11 (a) This diagram shows the trapezoidal filter used to analyse TBragg data, which

utilises two windows, the width of each is defined by tP and the separation between

the leading edge of the two windows is defined by tG. (b) Shows the application of

the trapezoidal filter to a typical anode signal. “A” shows the short filter, which

must have a gap time much smaller than the rise time in order to sample the

slope. Whereas “B” shows the long filter, which requires a gap time longer than

the rise time of the signal, in order to sample the signal height. . . . . . . . . . . 37

4.12 This figure shows the simulated particle identification (PID) plot obtained using

simulated data from SRIM, and applying pulse shape analysis methods. . . . . . 39

vii

5.1 The set up used for bench testing the TBragg. Isobutane was used to stop alpha

particles, emitted from a triple alpha source, in order to test the operation of the

TBragg. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

5.2 The signals were first observed using an oscilloscope, in addition to the alpha

signals shown in (a), a number of noise signals, shown in (b), were also observed

with a much faster rise time. These signals were contributed to impurities in the

detector, that were removed through conditioning. . . . . . . . . . . . . . . . . . 42

5.3 This plot shows the data collected from stopping alpha particles emitted from

a triple alpha source in isobutane, this data was collected using an analog data

acquisition system over a 12 hour period. A resolution of 2% was achieved. . . . 42

5.4 This figure shows the data collected from stopping alpha particles emitted from

a triple alpha source in isobutane using the digital data acquisition system, data

was collected over a 12 hour period which provided an energy resolution of 1.5%. 43

5.5 This PID plot was obtained from stopping alpha particles emitted from the source

at three different energies in 212 torr of isobutane. . . . . . . . . . . . . . . . . . 44

5.6 This data was simulated using SRIM, and shows how the range of different ions

and the short shaping parameters affect the evaluation of the atomic number

and therefore the position of data points on the PID plot. The “high” short

parameters had a peaking time of 2 µs and a gap time of 6 µs. The “low” short

parameters had a peaking time of 0.5 µs and a gap time of 1.8 µs. The long

shaping parameters were kept constant with a peaking time of 1 µs and a gap

time of 9.2 µs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5.7 This PID plot was constructed from three different runs, stopping alpha particles

in isobutane. Group 1 shows the initial run with the distance between the window

and the source at vacuum. Group 2 shows the effect of introducing 80 torr between

the source and the window. Finally, group 3 shows the effect of introducing

150 torr between the source and the window. . . . . . . . . . . . . . . . . . . . . 46

5.8 This figure shows the oscilloscope signals obtained using CF4 gas to stop alpha

particles from the triple alpha source. . . . . . . . . . . . . . . . . . . . . . . . . 46

viii

5.9 This figure shows the data collected, with an analog data acquisition system

over a 12 hour period, from stopping alpha particles emitted from a triple alpha

source in CF4. Due to high amplitude background noise the alpha signals were

indistinguishable. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.10 Figures (a) and (b) show the projected x axis from the PID plot constructed with

data from stopping alpha particles in isobutane, there is clear separation in the

energy of these particles. Whereas, figures (c) and (d) show the projected x axis

from the PID plot constructed with data from stopping alpha particles in CF4.

Although higher levels of noise were present in the CF4 tests the three alpha

energies remain distinguishable. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

5.11 This figure shows the PID plot constructed with data obtained from stopping

alpha particles in CF4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

6.1 The target wheel used to mount the three targets (gold, LiF and carbon), and

made the process of changing between each target quicker and easier during the

experiment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

6.2 For set up for the first in-beam experiment, conducted with an 18O beam. The

TBragg was mounted to a scattering chamber at 24◦ with respect to the beam

line. A target wheel, with three targets attached, was positioned in the centre of

the scattering chamber, in the beam axis. . . . . . . . . . . . . . . . . . . . . . . 50

6.3 This diagram illustrates the beam of ions colliding with a target; the products

are emitted at a variety of angles, those emitted at 24◦ are accepted by the TBragg. 51

6.4 This diagram shows the effect of the Coulomb repulsion on the path of an incident

particle interacting with a stationary particle. Where b is the impact parameter

and d is the distance of closest approach [36]. . . . . . . . . . . . . . . . . . . . . 51

6.5 These figures show the PID plots produced for 18O scattered from a gold target

and stopping in (a) P-10 at 300 torr and 300 V (b) P-8 at 300 torr and 700 V. . 52

6.6 These plots were produced from the scattering of the 18O beam on a gold target.

(a) and (c) show the projected x axis of the PID plot, showing the uncalibrated

energy peaks for P-10 and P-8 respectively. Whereas (b) and (d) show the pro-

jected y axis of the PID plot, showing the uncalibrated atomic number peaks for

P-10 and P-8 respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

ix

6.7 Alpha particles are produced, with a range of energies, in fusion evaporation.

Higher energy alpha particles will travel further in the gas eventually punching

through the back of the detector, resulting in this “arrow head” arrangement. . . 54

6.8 These figures show the PID plots obtained when stopping 18O ions scattered from

the LiF target in (a) P-10 at 500 torr and 300 V (b) P-8 at 700 torr and 300 V . 56

6.9 These plots were produced from the scattering of the 18O beam on the LiF target.

(a) and (c) show the projected x axis of the PID plot, showing the uncalibrated

energy peaks for P-10 and P-8 respectively. Whereas (b) and (d) show the pro-

jected y axis of the PID plot, showing the uncalibrated atomic number peaks for

P-10 and P-8 respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

6.10 These PID plots show the data collected from the scattering of the 18O on a 12C

target. (a) and (c) were produced using simulated data, whereas (b) and (d) show

the experimental data. 18O was detected in both its ground state and an excited

state (Q=1.98 MeV) due to elastic and inelastic scattering, respectively. . . . . . 58

6.11 These plots were produced from the scattering of the 18O beam on a carbon

target. (a) and (c) show the projected x axis of the PID plot, showing the uncali-

brated energy peaks for P-10 and P-8 respectively. Whereas (b) and (d) show the

projected y axis of the PID plot, showing the uncalibrated atomic number peaks

for P-10 and P-8 respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

7.1 This image shows the beam line between the DTL and the SC-Linac, referred to

as the S-bend. The position of both of the carbon stripping foils used to purify the

beam are labelled as “Degrader #1” and “Degrader #2”. The PRAGUE magnet

was used in conjunction with the TBragg to monitor beam contaminants. [39] . . 62

7.2 Two carbon foils were used to strip contaminants from the beam according to

A/Q value. The first filter was set to A/Q=6.260 and the second at A/Q=4.268.

Isotopes that lie in the region where these two A/Q selections cross over are the

most expected beam constituents [42]. . . . . . . . . . . . . . . . . . . . . . . . . 63

7.3 This figure shows the experimental set up for the high mass beam experiment

conducted in the ISAC-II facility. The TBragg was mounted to a vacuum chamber

at the end of a beam line and a Faraday cup was used to measure the beam

intensity and ensure the count rate did not exceed the limit of the DAQ system. 63

x

7.5 This image illustrates an effect known as pile-up, where a second signal is collected

before the first signal has finished decaying. The energy information of the second

signal, which is given by the height of the signal, is distorted as it rides on the

tail of the first signal. [14] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

7.4 12C was used as the test beam for this high mass experiment, this figure shows

the PID plot obtained. Both 12C and 16O are observed with charge state 3+ and

4+ respectively. Pile-up effects are also shown in this plot as the 12C ions arrive

close together. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

7.6 This PID plot shows the data collected with the 94Sr beam. 69Ga, 119Sn and

94Mo were used as reference points to calibrate this PID plot and identify the

other isotopes contained in the cocktail beam. 94Sr was not observed, possibly as

the beam lines were not fully optimised. . . . . . . . . . . . . . . . . . . . . . . . 66

7.7 This graph shows the comparison between PID points obtained for the simulated

and calibrated experimental data collected with the 94Sr beam. Good comparison

is shown suggesting the constituents of the cocktail beam were correctly identified.

The slight variation in position is partly due to an energy spread in the beam. . 67

7.8 This figure shows the measured energy spread of a few beams at a variety of

energies from the DTL, and that ∆E/E has a typical range between ∼ -1–1% [44]. 67

7.9 This graph was produced with simulated data and shows the required energy

spread in the beam for the simulations to match the calibrated experimental

data. It shows that ∆E/E would have to be between -1.4–1.9% for the data to

match. The actual energy spread is from about -1–1% which suggests that an

energy spread is only part of the reason for the variation between simulated and

experimental data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

7.10 (a) This figure shows the projected x axis from the 94Sr beam, showing the separa-

tion in energy of the different isotopes observed. Whereas (b) shows the projected

y axis for the 94Sr beam and shows the separation in atomic number of the ob-

served isotopes, in particular there is some separation between 94Zr and 94Mo

which have ∆Z=2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

B.1 TBragg EPICS interlock logic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

xi

List of Tables

2.1 The variables used to derive the Bethe Bloch equation, which provides an approx-

imate method of explaining the energy loss of ions in matter. . . . . . . . . . . . 10

2.2 The variables used to derive a simple equation for the drift velocity of electrons

in a gas under the effect of an electric field. It is only valid for low electric fields

and pure gases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.1 Variables used in this section to describe how a signal is created from the move-

ment of electrons, and the function of a Frisch grid. . . . . . . . . . . . . . . . . . 18

3.2 Useful values for the TBragg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.1 An example xarray and yarray . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.2 The simulated Z and energy resolution for a variety of ions, whose energy loss

was simulated using SRIM and pulse shape analysis methods were applied using

Microsoft Excel to produce comparable data to experiments. . . . . . . . . . . . 39

5.1 This table lists the alpha particle energies for the triple alpha source used to

bench test the TBragg. Values were obtained from nuclear data sheets [28–30]. . 40

6.1 This table provides the energy and the range of the gold at oxygen ions, stopping

in P-10 at 300 torr and P-8 at 300 torr. The energy at this angle was calculated

using Catkin and the ranges were calculated using SRIM. . . . . . . . . . . . . . 52

6.2 This table provides values for the resolution achieved for P-10 and P-8 both at

300 torr, using a gold target to scatter the 18O beam. . . . . . . . . . . . . . . . 53

6.3 This table lists the expected energy, as calculated using Catkin, of each ions

emitted at 24◦ in the scattering of the 18O beam on a LiF target, and the range

of these ions in both P-10 and P-8, calculated using SRIM. . . . . . . . . . . . . 54

xii

6.4 This figure shows the resolution obtained from the scattering of the 18O beam on

the LiF target and stopping the ions in P-10 at 700 torr and P-8 at 300 torr. . . 55

6.5 This table lists the expected energy and range of ions emitted at 24◦ from the

scattering of the 18O beam on a 12C target, and stopping ions in P-10 and P-8

both at 500 torr. At these pressures all ions have the same range in both gases,

as calculated using SRIM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

6.6 This table lists the resolution achieved for both simulated and experimental data

for the scattering of 18O ions on a 12C target and stopping in P-10 at 300 torr

and 300 V and P-8 at 500 torr and 1000 V. . . . . . . . . . . . . . . . . . . . . . 59

7.1 This table presents the energy and Z resolution obtained for both experimental

and simulated data for the stopping of a 94Sr beam in P-8. . . . . . . . . . . . . 70

xiii

Chapter 1

Introduction

TRIUMF is one of the world’s leading subatomic physics laboratories, and is located in

Vancouver, Canada. At TRIUMF negatively charged hydrogen ions are injected into the centre

of the cyclotron, and accelerated negatively charged hydrogen ions injected into its centre, to

75% of the speed of light through use of intense magnetic and electric fields. The electrons are

then stripped from the H− ions, leaving a proton beam, which is directed out of the cyclotron

through four main beam lines. One of these beam lines transports 475–500 MeV protons to the

Isotope Separator and Accelerator (ISAC) facility, which uses the Isotope Separation OnLine

(ISOL) technique to produce radioactive ion beams (RIBs).

The ISOL system consists of directing the proton beam onto one of two production targets.

The reaction products diffuse to the surface, and the target is heated (∼2000◦C) to allow these

products to effuse through a heated tube into an ion source [1]. The ions are then extracted,

with a 1+ charge state, at an energy of 20–60 keV [2]. This process produces a cocktail beam,

however, through use of laser ionisation this process is improved, producing cleaner beams.

A mass separator is then used to strip away some of the contamination, through the use of

bending magnets. ISAC-I uses beams directly from the targets for mass measurements, laser

spectroscopy, decay spectroscopy and precision decay studies.

Alternatively, the RIBs can be injected into the post accelerator for use in high energy

experiments. In ISAC-I accelerated beams (up to 1.8 MeV/u) are used to study nuclear reactions

relevant for quiescent and explosive nucleosynthesis in various astrophysical scenarios. Whereas,

ISAC-II focuses on high energy radioactive isotope beams (typically 7 MeV/u, for A < 30 nuclei)

in order to study direct reactions of exotic nuclei, focusing on nucleon transfer reactions and

inelastic scattering. The post accelerator consists of a Radio Frequency Quadrupole (RFQ),

1

Introduction

a Drift Tube Linac (DTL), and a Superconducting Linear accelerator (SC-Linac). The RFQ

uses alternating electric quadrupole fields to provide radial confinement of charged particles

travelling along the axis of the RFQ. At TRIUMF the RFQ consists of four rods, segmented

into six sections. This allows both axial confinement, from DC fields applied to each segment,

and operation in either continuous or bunched beam mode [3]. The RIB is pre-bunched at the

entrance of the RFQ, by means of a three harmonics electric buncher. This improves the beam

emittance while limiting the energy spread of each bunch [4]. The RFQ has an A/Q acceptance

< 30, and accelerates the beams from to 2–150 keV [1]. A chopper is placed after the RFQ,

which filters the beam according to A/Q value and has a theoretical resolution of 1/1000.

The DTL is composed of five interdigital H-type (IH) structures, each operating at 105 MHz,

providing acceleration up to 1.8 MeV/u. Between each of the IH tanks are quadrupole triplets

and three split ring bunchers, which provide periodic transverse and longitudinal focusing [5] [4].

Following the DTL a stripping foil is used to further tune the beam, and has a theoretical

resolution in A/Q of 1/800. The SC-linac is composed of five cryomodules, each containing four

superconducting cavities, for acceleration above the Coulomb barrier (5–11 MeV/u)); and one

superconducting solenoid, for transverse focusing [4]. Both the DTL and SC-Linac have an A/Q

acceptance of < 7; for ions with mass < 30 stripping foils can be used to increase the charge

state, but for ions with higher mass this is achieved through charge state boosting [2].

The two main types of ion sources typically used for charge breeding are an Electron Beam Ion

Source (EBIS) and Electron Cyclotron Resonance (ECR). EBIS can produce highly charged ions

with an efficiency of up to 10% for a single charge state [2]. However, it requires pulsed beams,

which limits the efficiency and the intensity of the beam. An ECR source on the other hand,

accepts a continuous beam, but this set up introduces high levels of stable beam contamination.

TRIUMF has adopted the use of an ECR source, due to its potential high efficiency in producing

intermediate A/Q values (A/Q = 6).

An ECR ion source contains a high density plasma (typically 1013/cc [7]), which is mag-

netically confined. Voltage is applied to the ion source, which slows the RIB as it approaches.

Microwaves at a frequency 14.5 GHz [8] are used to heat the plasma. The actual temperature of

the plasma is not directly known, although, it is generally agreed that the energy of the electrons

can be divided into two groups: low energy electrons of a few keV, and high energy electrons of

approximately 100 keV [8].

As the ions enter the ECR source they will be “cold” compared to the plasma, and as a result

2

Introduction

Figure 1.1: A schematic of the ISAC (Isotope Separator and Accelerator) facilityat TRIUMF [6]. The facility is divided into two sections, in ISAC-I beams are useddirectly from the targets for mass measurements, laser spectroscopy, decay spectroscopyand precision decay studies; additionally accelerated beams are used to study nuclearreactions. Whereas ISAC-II uses high energy radioactive isotope beams to study directreactions of exotic nuclei.

the plasma will work to heat them up using the 14.5 GHz microwaves. The beam is ionised to

high charge states in the plasma, through collisions with the high energy electrons [2]. The ions

leave the charge state booster (CSB) with A/Q values up to ∼ 20, and enter a second mass

separator with a mass resolution of 1/100 [8]. The separator is designed to only allow ions with

A/Q < 7 to pass through, and continue on to the post accelerator. Selecting the correct A/Q

value at this point removes the requirement of additional stripper foils later, which would reduce

the beam intensity.

Typically, the beam current produced by the ECR source is 100 µA (6.24 × 1014 parti-

cles per second) [8]. The intensity of the radioactive beam extracted from the CSB is dependent

on the initial intensity entering the plasma, typically this will be 1×107 particles per second [8].

However, when extracted from the ion source this will have decreased to about 1 × 105 parti-

cles per second, for an individual charge state [8], which is a very small fraction of the total

beam. Once this beam has passed through the separator the total beam current will decrease

3

Introduction

to 10–100 pA [8]. Figure 1.2 shows a typical mass spectrum of stable ions extracted from the

CSB. All of the peaks are seen with an intensity greater than 5 pA, whereas, the radioactive

beam will have an intensity of only 0.016 pA.

Figure 1.2: The post-accelerator usedto accelerate radioactive beams above theCoulomb barrier has an A/Q acceptanceof < 7, therefore prior to acceleration thebeam is charge bred. This graph showsthe typical mass spectrum of ions extractedfrom the charge state booster, which in-creases the charge state of ions, thus low-ering the A/Q value [7].

The three main contributors to this stable

contamination are residual gases, the support

gas and sputtered ions [9]. As the ion source is

not operated at perfect vacuum, some air and

also a little water can reside in the chamber [8];

this allows isotopes, mainly 16O and 14N, to

become ionised in the plasma. 4He is inserted

into the chamber to act as a support gas for the

plasma; resulting in the ionisation of some of

the helium atoms. Finally, the plasma is mag-

netically confined inside the chamber, however,

it is possible for some ions to escape and col-

lide with the walls of the chamber; knocking

out additional ions, which can then be ionised

in the plasma. When using a beam that is so heavily dominated by stable contamination, it

can be difficult to detect reactions originating from the isotopes of interest. For all experiments

it is essential to measure the intensities of radioactive and stable components, and this is best

achieved through particle identification. Knowledge of the beam components is also required for

measurements such as the excitation cross sections of rare accelerated ions.

A Bragg ionisation chamber is a gas filled detector. Typically, gas ionisation chambers drift

the ionised electrons in a path that is perpendicular to the beam axis, and the charge collected

corresponds to the total energy of the ion. Conversely, in a Bragg ionisation chamber the ionised

electrons are drifted in a path that is parallel to the beam axis, resulting in an anode signal

containing information about both the atomic number and the total energy of the ions. The

TRIUMF Bragg ionisation chamber (TBragg) is a twin of a Bragg detector designed and built

in Germany. The original Munich Bragg chamber is currently used at REX-ISOLDE in CERN

as a standard beam diagnostic device [10]. Prior to experiments, the MINIBALL gamma array

is typically used to identify radioactive contamination, and the stable contamination is observed

using the Munich Bragg detector [11]. The same approach could be adopted at TRIUMF,

4

Introduction

using the TIGRESS gamma array in the future; however, for the experiments discussed in

this dissertation, the TBragg was used to measure all beam components independently. Due

to the fact the TBragg was a new detector at TRIUMF, the aim of this project was centred

around its preparation for a radioactive beam experiment, and to ensure the best possible results

were obtained. To this end, a number of preliminary tests were conducted, to understand the

operation of the TBragg, and test the electronics and the gas handling system. Furthermore, to

enable the understanding of these tests and to ensure the correct operating parameters were used,

simulated data was generated. Understanding how to operate the TBragg and its apparatus was

essential to ensure the safety of the equipment and the users, and to ensure the optimisation

of the data collected. Ultimately, the TBragg will be used to tune the mass separators, and

other components along the beam lines, to provide cleaner beams. Following this project it

will be necessary for the TBragg to be operated remotely (as much as possible) and for the

data collection and analysis to be almost instantaneous; therefore, the aims of this project were

fulfilled with these additional requirements in mind.

Following this introduction, this Chapter 2 covers the theory of relevant physical processes

and the equations required to understand how the aims will be achieved. Chapter 3, provides

an insight into the design of the Bragg detector, and the electronics that were used for data

collection. Chapter 4 details the development of the simulations, which utilise a number of

programs to ensure the correct set up of the TBragg. Chapter 5 explains the preliminary exper-

iments conducted, which were designed to provide information about how the TBragg operates

and to test the electronics. Chapter 6 will discuss the first in-beam experiment conducted with

the TBragg, using an 18O beam to test the change in resolution when replacing a flammable

gas with a non-flammable gas. Finally, Chapter 7 describes the radioactive beam experiment

conducted with a 94Sr beam; which measured the TBragg’s ability to identify the beam quality,

and quantified the degree to which the aims of this project were successfully completed.

5

Chapter 2

Theory

2.1 Theory of Energy loss

The energy loss of ions travelling through a material can be separated into two components:

1. Inelastic collisions with the atomic electrons of the gas atoms (electronic collisions).

2. Elastic scattering from nuclei (nuclear collisions).

Electronic stopping is the dominant mechanism for the energy loss of high energy ions;

whereas, nuclear stopping is important for very low energies. Through electronic collisions

charged particles interact primarily via Coulomb forces between the positive charge of the par-

ticle, and the negative charge of the orbital electrons. Depending on the proximity of these

encounters, this impulse may be sufficient to either raise the electron to a higher lying shell

within the absorber atom (excitation), or to remove it completely (ionisation). Ions can be

formed either by direct interaction with the incident particle, or through a secondary process

in which some of the particle’s energy is transferred to an energetic electron, or “delta ray”. If

the electron produced has acquired sufficient energy it will go on to cause further ionisation of

the material. The energy transfer to the electron means the velocity of the charged particle is

decreased [12]. Individually, only a small fraction of the particle’s total energy is transferred; but

over the full path many collisions occur, resulting in a cumulative effect stopping the particle.

William Henry Bragg was the first to discover a defined peak on the Bragg curve immediately

before the ions come to rest; where the Bragg curve is a plot of energy loss against the depth

through matter. As a heavy particle slows down in matter, the reaction cross-section increases.

Therefore, the rate of energy loss will increase as the kinetic energy decreases, and more energy

6

Theory 2.1. Theory of Energy loss

per unit length will be deposited towards the end of its path rather than at the beginning [13].

This can be seen in Figures 2.1a and 2.1b, which show the Bragg curves for three ions of the

same atomic number but different energies, and three ions of the same energies but different

atomic numbers, respectively. These ions were stopped in P-10 and SRIM was used to simulate

the energy loss, a program which is introduced in Section 4.1. At the end of the path more

collisions occur, consequently, the energy loss reaches its peak followed by a rapid drop to zero

as the particle stops.

(a) (b)

Figure 2.1: These graphs were produced using SRIM to show how the Bragg curvevaries when increasing the energy and the atomic number of incident ions. (a) 12C ionsat 2, 3 and 4 MeV/u stopping in P-10 at 1.15 bar. (b) 14N, 12C and 11B at 36 MeVstopping in P-10 at 1.05 bar.

Figure 2.2: A schematic of the geome-try for a heavy ion in close proximity toan atomic electron [14]. Where e is thecharge of an electron, v is the velocityof the heavy ion, M is the mass of theheavy ion, ze is the charge of the heavyion, b is the distance of the heavy ion tothe electron.

The primary dependencies of the stopping

power on the ion velocity and the elemental charge

can be understood using a simple model, which is

introduced hereafter. Figure 2.2 shows a heavy

charged particle, of mass M and charge ze, mov-

ing through some material with velocity v. An

electron is in close proximity to this ion (a dis-

tance b), and is assumed to be free and at rest.

It is also assumed that the electron moves very

7


slightly during interaction with the charged particle, so the electric field acting on the electron

may be taken at its initial position. Furthermore, it is assumed the charged particle is undeviated

from its path following the collision, due to its larger mass.

The momentum impulse an atomic electron receives from colliding with a heavy particle is

given by Equation 2.1 [14].

I = e

∫E⊥

dx

v(2.1)

This equation highlights the fact that only the perpendicular component of the electric field,

(E⊥), with respect to the particle’s trajectory is required, due to symmetry. Gauss’ Law can

then be used to calculate this integral over an infinitely long cylinder centred on the path of the

heavy particle, as shown in Figure 2.2, to find [14]:

I =2ze2

bv(2.2)

The energy gained by the electron is therefore [14]:

∆E (b) =I2

2me(2.3)

Where me is the mass of an electron. By substitution of 2.2 into 2.3, the energy lost to all

electrons between b and b+ db in a thickness dx is [14]:

−dEdx

=4πz2e4

mev2Neln

bmax

bmin(2.4)

Where Ne is the density of electrons. The limits of integration cannot be between zero and

infinity, as for large b the interaction would not take place over a short period of time; whereas,

b = 0 gives infinite energy transfer. Therefore, bmax and bmin are used. Classically, the maximum

energy transferable is in a head on collision where the electron obtains energy of [14]:

1

2me (2v)2 (2.5)

However, when taking relativity into account this becomes [14]:

2γ2mev2 (2.6)

8


where γ = 1/√

1− β2 and β = v/c. Using equation 2.3 it can be shown that [14]:

bmin =ze2

γmev2(2.7)

When calculating bmax it is important to remember that the electrons are not free, but are

instead bound to atoms with some orbital frequency ν. In order for the electron to absorb energy

the interaction must take place in a time that is short compared to the orbital period of the

electron, τ = 1/ν, otherwise no energy is transferred. This makes the typical interaction time

t ≈ b/v. Relativistically, this becomes t = t/γ = b/γv ≤ τ = 1/ν [14]. Where ν is the mean

frequency averaged over all bound states. Therefore, bmax is [14]:

bmax =γv

ν(2.8)

Substituting this into equation 2.4 gives [14]:

−dEdx

=4πz2e4

mev2Neln

γ2mv3

ze2ν(2.9)

Taking this simplified approach a relationship between energy loss and z2/v2 can already be

seen. However, for ions lighter than alpha particles, such as protons, this relationship breaks

down due to quantum effects.

Bethe and Bloch approached this problem using Quantum Mechanics, and the fundamental

equations for the stopping of fast particles in a quantised material (derived from the Born

approximation) [15]. The Bethe Bloch formula (Equation 2.10) defines the energy transfer

parametrised in terms of momentum transfer rather than the impact parameter. It contains two

corrections, the density effect correction δ, and the shell correction, C [14].

−dEdx

= 2πNare2mec

2ρZ

A

z2

β2

[ln

(2meγ

2v2Wmax

I2

)− 2β2 − δ − 2C

Z

](2.10)

There still remains a clear dependence on z2 and v2, which means all the information required

to determine the isotopes stopped in the medium are provided by the energy loss curve. However,

with the discovery of nuclear fission, and the energetic heavy particles produced from nuclear

disintegration, the problem became describing the interaction of partially stripped ions [15]. This

is known as the “effective charge problem”, which further complicates the Bethe Bloch formula.

9

Theory 2.2. Theory of Electron Drift and Diffusion

The full theory of the slowing of charged particles in matter is implemented in simulation codes

like SRIM which are used later, Section 4.1.

Table 2.1: The variables used to derive the Bethe Bloch equation, which provides anapproximate method of explaining the energy loss of ions in matter.

I: The mean excitation potential

e: The charge of an electron (1.6 x 10−19 C)

E⊥: The perpendicular component of the electric field

z: The charge of the incident particle, in units of e

b: The distance of the incident particle from the atomic electron of a nearby atom

v: The velocity of the incident particle

me: The mass of an electron (9.11 x 10−31 kg)

Ne: The density of electrons

γ: 1/√

1− β2

re: The classical electron radius (2.82 x 10−15 m)

ρ: The density of the absorbing material

β: v/c of the incident particle

δ: The density correction

C: The shell correction

Wmax: The maximum energy transfer in a single collision

NA: Avogadro’s constant (6.022 x 1023 mol−1)

Z: The atomic number of the absorbing material

A: The atomic weight of the absorbing material

2.2 Theory of Electron Drift and Diffusion

Knowledge of the factors that affect the drift velocity of electrons in a gas is necessary to be

able to the performance of gas ionisation chambers. The kinetic energy of the ionised electrons

will vary, depending on the energy transferred in a collision between an ion and gas atom.

Nonetheless, they will quickly lose additional energy, through collisions with the gas atoms, in

order to maintain the average thermal energy distribution of the gas, εT [16]:

εT =3

2kT (2.11)

Where T is the temperature of the gas and k is the Boltzmann constant. Electrons, of mass me,

will diffuse in the gas with an average velocity, 〈v〉 [17]:

〈v〉 =

√3kT

me(2.12)

10


The distribution of the electron energies follows Maxwell’s distribution [17]:

dN

dε= F (ε) = C

√εexp

(−εkT

)(2.13)

Where C is a constant and ε is the energy of the electrons.

In the absence of external forces, after a time t the electrons will diffuse by a distance x

following a Gaussian distribution [17]:

dN

N=

1√4πDt

exp

(− x2

4Dt

)dx (2.14)

Where D is the diffusion constant, and N is the number of gas atoms. However, when an

electric field is applied it will accelerate the electrons, each with a charge q, in the direction of

the electric field E [17]:

~a =q ~E

me(2.15)

As the electrons are accelerated towards the anode they will continue to collide with the gas

atoms, causing them to change direction. However, the electric field will cause the electrons

to then continue drifting towards the anode. On a microscopic level the movement will appear

random, but on a macroscopic level there will be definite drift in one direction. The average

distance between the collisions of the electrons and gas atoms is referred to as the mean free

path, λ [17]. The mean free path depends on the cross section for interaction, σ and the density

of matter, ρ [17]:

λ =1

Nσ (ε)=

A

NAρσ (ε)(2.16)

Where A is the relative atomic mass of the matter, NA is Avogadro’s number and N is the

number of particles per volume.

For an ideal gas, of pressure p and volume V [17]:

pV = NRT (2.17)

11


Where R is the gas constant. Thus, with constant volume, one finds the proportionality:

1

N∝ T

p(2.18)

Using the proportionality between the density of gas atoms and the free path length, in a

gas of approximately constant temperature T , the drift velocity of electrons becomes: [17]:

vd ∝E

p(2.19)

This clearly shows that the drift velocity is directly affected by both the electric field and gas

pressure; these values must be chosen suitably to achieve high drift velocity, which will enable

fast charge collection. It has been found experimentally that this proportionality between drift

velocity and E/p is typically only valid for low electric fields. In addition, this classical approach

becomes unsatisfactory when describing gas mixtures. A more complicated gas transport theory

is required to reproduce the effects measured experimentally. These relationships have been

measured for various gas mixtures and this empirical data has been used in TBragg simulations

(discussed in Section 4.2).

Table 2.2: The variables used to derive a simple equation for the drift velocity ofelectrons in a gas under the effect of an electric field. It is only valid for low electricfields and pure gases.

ε: Energy of electrons

k: Boltzmann constant (1.38 x 10−23 m2 kg s−2 K−1)

T : Gas temperature

me: Mass of electron (9.11 x 10−31 kg)dNN : The fraction of charges found in the element dx

D: Diffusion Constant

E: Electric field

q: Charge of an electron (1.6 x 10−19 C)

N : Number of gas atoms

A: Relative atomic mass number

NA: Avogadro’s constant (6.022 x 1023 mol−1)

ρ: Gas density

p: Gas pressure

V : Volume of gas

R: Gas constant (8.314 J K−1 mol−1)

12

Chapter 3

Background and Design

3.1 The TRIUMF Bragg Ionisation Chamber (TBragg)

The Bragg ionisation chamber employed in the present work was developed at TU Munich,

according to the framework of the Diploma thesis of W. Weinzierl [18].

Figure 3.1: This figure shows a modified schematic of the Bragg ionisation chamberfrom the Diploma thesis by W. Weinzierl [18]. It highlights the four key componentsof this detector; gas fills the inside of the chamber to stop ions that pass through thewindow (1), as this happens electrons are ionised in the gas. The electrode rings (2)accelerate these electrons to the anode (4), which is shielded by the Frisch grid (3).

There are four main components which constitute the TBragg, illustrated in Figure 3.1:

1. An aluminised Mylar entrance window, which is grounded

2. A series of 20 electrode rings, connected via a resistor chain

3. A Frisch grid, connected to a high voltage supply

4. An anode, connected to a separate high voltage supply from the Frisch grid

13

Background and Design 3.1. The TRIUMF Bragg Ionisation Chamber (TBragg)

(a) (b)

Figure 3.2: (a) The 1.5 µm aluminised Mylar entrance window which acts as the firstbarrier for ions. Typically 1–2 MeV for A¿30 is lost by ions passing through the window.(b) The supporting grid is required in order for the window to contain high pressures(>2 bars) in the TBragg.

The Mylar entrance window (shown in Figure 3.2a) has a thickness of 1.5 µm, and is the

first barrier for ions to pass through. The window is capable of holding > 2 bars of differential

pressure in the chamber, made possible through the use of the supporting grid (Figure 3.2b).

These pressure limits were tested at TRIUMF, which is discussed further in Section 3.3. The grid

is made of 1 mm thick copper with a number of 0.8 mm holes, and provides ∼ 80% transmission.

As the ions pass through the window they lose energy from collisions with the atoms in the

Mylar foil. The amount of energy lost by each ion will vary due to statistical fluctuations in the

number of collisions each ion has in the foil, known as energy loss straggling. The amount of

energy lost depends on the atomic number and the initial energy of the ions, according to the

Bethe Bloch formula. Figure 3.3, shows how the percentage change in the energy of the ions,

after passing through the window, varies with atomic number and energy.

When the ions collide with the atoms in the foil they can produce delta electrons, through

ionisation. These electrons can remain in the foil increasing the surface charge, which could

discharge and damage an element of the detector. To prevent this, the window is aluminised;

this allows it to be grounded by connecting it to an electron source, which will draw out these

ionised electrons and replace them in the foil. Additionally, some of the ionised electrons will

be knocked out of the foil, most of which will recombine with particles in the gas, but a few will

drift to the anode and contribute to the anode signal. Therefore, it is necessary to be aware of

the energy lost by the ions in the Mylar window to be able to account for a slightly lower total

energy. Reducing the thickness of the window would reduce this energy loss and energy loss

straggling, but it would also reduce the pressure limits.

14


Figure 3.3: This graph shows how the percentage change in energy varies for ionsof different atomic number passing through the Mylar entrance window, of thickness1.5 µm, calculated using SRIM.

Figure 3.4: The TBraggcontains a set of 20 electroderings, which create the con-stant electric field requiredto drift the ionised electronstowards the anode. Therings are tapered in order tominimise edge effects, whicharise due to the finite sizeof the rings. The electroderings are connected via a re-sistor chain to create a fieldtypically of 11 V/cm be-tween the window and theFrisch grid and 500 V/cmbetween the Frisch grid andthe anode.

The majority of the energy lost by the incident ions occurs

through electronic collisions with the gas, particles producing

electron ion pairs; signals are then produced from the collec-

tion of these ionised electrons at the anode. In the absence

of an electric field the electrons would collide with the gas

particles to assume the thermal energy of the gas, and pos-

sibly recombine. However, through application of an electric

field these ions are drifted towards the anode. There are 20

electrode rings in the TBragg, spaced 5 mm from each other,

which provide a constant electric field. As shown in Figure 3.4,

the electrode rings have a large opening in the centre (diame-

ter 28 mm), for the ions to pass through. The electrode rings

are tapered (2 mm at the outer edge, 1 mm at the centre and

0.5 mm at the inner opening) in order to minimise edge ef-

fects, which arise from the finite size of the electrodes. This

ensures a homogeneous electric field along the detector axis

for the electrons to drift at constant velocity. The electrodes

create a typical electric field of 11 V/cm between the entrance

window and the Frisch grid, and 500 V/cm between the Frisch

15


grid and the anode [18].

The Frisch grid is comprised 50 µm gold plated tungsten wires, in parallel, equally spaced

at 0.5 mm, and connected to the high voltage. The grid is located 1 mm in front of the

anode and acts to shield it from charged particles in the first part of the chamber (discussed in

Section 2.2). Both the Frisch grid and anode are connected to a separate high voltage to avoid

current noise [18].

The signal provided by the anode is all that is required for particle identification. A data

acquisition (DAQ) system was used to record these signals, this process is described in more

detail in Section 4.3.

3.1.1 Frisch Grid

In different types of atoms the electrons have different degrees of freedom, materials with

high electron mobility are known as conductors; inside, the outer electrons are bound so loosely

they are able to move throughout the material under the influence of electric forces. When a

conductor is placed inside a constant electric field E these free electrons, with charge q, will

reposition themselves in a way that will make the net internal electric field zero [19]. According

to Gauss’ Law, the total electric field through a closed surface S is proportional to the total

charge contained in S [19]:

∫SE · dS =

1

ε0

∑i

qi (3.1)

Where ε0 is the permittivity of free space. Therefore, when the electric flux is zero the

internal charge will also be zero. This requires that the charge of the conductor has to be on the

surface, and it is distributed with a surface charge density σ [19]. The magnitude of the electric

field on the surface of the conductor is given by [19]:

E =σ

ε0(3.2)

When high energy ions move through a gas they can produce electron ion pairs. The electrons

and positive ions will move in opposite directions under the influence of an electric field, and the

collection of these electrons will produce a signal. Figures 3.5a and 3.5b show how the electric

field lines change as the particles move towards and away from an anode, which is made from

conducting material. At the surface of the anode the electric field lines are distorted to become

16


perpendicular to the conductor’s surface, a phenomenon known as electrostatic polarisation [19].

When these electric field lines vary the surface charge density will redistribute, in order to

maintain equilibrium.

(a) (b)

Figure 3.5: This figure shows the effect of electrons moving relative to a conductor,under the effect of an electric field. (a) A static electric field means the charges inside theconductor are stationary. (b) As the electron approaches the conductor the electric fieldlines get closer together, internal charges move to maintain equilibrium. If connectedto a charge sensitive preamplifier this movement of charges would cause a signal to begenerated.

The electric field between each particle and the anode will cause the internal charges of the

anode to rearrange themselves. Positive charges are pulled to the surface to balance the negative

charge of the electron; and negative charges are drawn to the surface to balance the positively

charged ion. As the electron approaches the anode the electric field lines will get closer together;

according to Equation 3.2 this increase in electric field will lead to an increase in the surface

charge density (as the electrons inside the anode get closer together). If the anode is connected

to a charge sensitive preamplifier this increase in surface charge will be detected as a charge

signal, the height of which will be representative of, but not equal to, the energy lost by the

ion. It will not be equal, because the presence of the positive ion causes the signal height to

be lowered. As the positive ion moves away from the anode the height of the signal will rise,

as the surface charge density of the anode becomes less influenced by the positive ion. In order

to calculate the actual energy lost by the ion, from the electron, a correction would have to be

made to account for the effect of the positive ion. In addition, the further the ionised electron

has to travel to reach the anode, the longer the rise time of the signal will be, which can be

difficult to detect.

O.R. Frisch was the first to suggest the use of a grid in front of the anode, to screen it

from these effects and removing the need for corrections [20]. A Frisch grid is also made from

conducting material, so the electric field lines from both of the particles will now terminate

17


(a) (b)

Figure 3.6: This figure shows the effect of introducing a Frisch grid between chargedparticles created in the gas and the anode. (a) The electric field lines for all chargedparticles will terminate at the Frisch grid, shielding the anode. (b) When the electronspass through the grid the anode will collect the charge, while remaining shielded fromother charged particles

at the Frisch grid (shown in Figure 3.6a). As the electrons and positive ions move towards

and away from the Frisch grid, respectively, the anode will be unaffected. When the electron

passes through the Frisch grid the electric field lines will then extend to the anode (shown in

Figure 3.6b), but the field from the positive ion will still terminate at the Frisch grid. This

means that as the electron approaches the anode, the signal collected will be generated purely

from the electron; therefore the height is equal to the energy lost by the incident ion. It is

important that the distance between the Frisch grid and the anode is short, as this will produce

a signal with a faster rise time making it easier to detect.

Table 3.1: Variables used in this section to describe how a signal is created from themovement of electrons, and the function of a Frisch grid.

E: Electric field∫S E · dS: The electric field through a closed surface S

ε0: Permittivity of free space (electric constant)

q: Charge

σ: Surface charge density

Table 3.2: Useful values for the TBragg

Thickness of the Mylar window: 1.5 µm

Diameter of the window: 10 mm

Forward burst pressure: > 2.1 bar

Reverse burst pressure: 0.33 bar

Maximum field cage voltage: 5 kV

Electrode-electrode distances: Min: 3 mm Max: 5 mm

Length of field cage: 10 cm ± 0.5 cm

Distance between Frisch grid and anode: 1 mm

18

Background and Design 3.2. Gas Handling System

3.2 Gas Handling System

Figure 3.7: This fig-ure shows the gas han-dling system constructedby R. Openshaw [21] andM. Goyette [22] in or-der to provide gas to theTBragg, and to maintaina constant pressure.

The gas handling system for the TBragg (Figure 3.7) was de-

signed and commissioned by R. Openshaw [21], and assembled by

M. Goyette [22]. A schematic of the gas handling system has been

included at the end of this section, Figure 3.9. Detailed instruc-

tions for operating the gas handling system have been included

in Appendix A.

The TBragg identifies particles through the electrons ionised

in the elastic collisions with the gas atoms. However, there are

a number of other processes that can occur in a gas that will

lead to energy loss, but will not contribute to the final signals. It

is therefore important to use a gas that is highly susceptible to

producing electron ion pairs. This is indicated by the W value,

a measurement of the mean energy required to form an electron

ion pair. Four gases were chosen:

1. CF4 (non-flammable, non-toxic).

2. Isobutane (flammable).

3. P-10 (Ar/CH4 90:10, flammable, non-toxic).

4. P-8 (Ar/CH4 92:08, non-flammable, non-toxic).

A gas handling system was required to control the flow of these four gases, through the use of

two mass flow controllers (MFCs). MFCs work by measuring the heat transferred by the flowing

gas; thus their flow sensitivity and full scale range is a direct function of the heat capacity of the

gas [21]. Two MFCs were required because argon based gas mixtures (P-10 and P-8) have much

lower heat capacity compared to isobutane and CF4. A constant pressure is maintained during

experiments by constantly flowing the gas through the detector. Fluctuations in the pressure

will mainly arise from variations in the room temperature; an increase of 3◦C can cause a 1%

density change of the gas, equivalent to +9 torr, when operating at 900 torr [21]. This will affect

the stopping power of the ions and the drift velocity of the ionised electrons, both of which will

affect the anode signals.

19


Pressure control is provided by an absolute pressure transducer, mass flow controller and

proportional-integral-differential controller. The pressure inside the TBragg, as measured by

the transducer, is the input to the proportional-integral-differential controller. This device then

outputs a signal to adjust the flow rate through the MFCs, such that the PID set point is

maintained. Figure 3.8 shows the readout box for the flow rate (cc/min) and pressure (torr).

A maximum pressure of 999 torr (1.33 bar) is set by the gas handling system; although the

window is capable of holding higher pressures, the proportional-integral-differential controller

cannot control pressures higher than this limit.

Figure 3.8: This readout box shows the flow rate (left), and the pressure (right)in the TBragg. The proportional-integral-differential controller (right) is used to setthe desired pressure (green) and show the current pressure (red). A pressure gauge isconnected to the TBragg that reads into the proportional-integral-differential controller,which then adjusts the flow rate to maintain a constant pressure.

3.2.1 EPICS - Experimental Physics Control System

The experimental physics and industrial control system (EPICS) allows the active recording

and archiving of the data collected from the vacuum gauges; and provides active personnel and

equipment safety protection through the use of interlocks [23]. The TBragg interlock logic is

included in Appendix B.1.

Interlocks will close, or prevent the opening of valves if specific conditions have not been

adhered to. It was important to implement interlocks for the TBragg for a variety of reasons.

Firstly, the window will be damaged under reverse pressure in excess of 0.33 bar. Therefore, by

preventing the user from pumping the detector to vacuum, unless it is connected to a vacuum

chamber, the window is protected. In addition, interlocks were set to close the gas input and

exhaust valves if the pressure started to drop inside the TBragg, indicative of a gas leak; this was

particularly important when working with flammable and toxic gases. Furthermore, monitoring

these values allows the protection of the preamplifier chips. If the pressure decreases rapidly,

while the high voltage remains turned on, the leakage current will increase, which will damage

the preamplifier chips. In such a situation, an EPICS interlock will switch off the high voltage

instantaneously in this situation.

20


3.2.2 Working With Flammable Gas

Isobutane and P-10 are the only two flammable gases that are currently used in the TBragg;

and when in use additional precautions must be taken (provided in Appendix A.2), primarily

the use of a safe intermediate gas. lower explosive limit (LEL) and upper explosive limit (UEL)

define the percentage of oxygen required to mix a flammable gas in order for it to ignite. For

isobutane, LEL = 1.8 % and UEL = 8.4 %. Whereas for P-10, LEL = 49 % and UEL = 51 % [21].

Knowledge of these limits is required when selecting the upper and lower set points for the MFCs,

and is monitored by the EPICS system. If the flow rate does not lie within these set points the

high voltage will be turned off to inhibit any possibility of igniting the gas.

Figure 3.9: The schematic of the TBragg gas handling system, provided by R. Open-shaw, which is used to provide gas to the TBragg, and maintain a constant pressure forprolonged periods of time.

21

Background and Design 3.3. Testing Window Strength

3.3 Testing Window Strength

Figure 3.10: The experimental set up used to locate the pressure limits of the Mylarwindows, which was required before further experiments could be conducted.

For safety reasons, before experiments could be conducted the pressure limits on the window

had to be tested, Figure 3.10 shows the setup used. To prevent internal damage to the TBragg,

from the window bursting, a cylinder of similar volume was used instead.

Initially, the window was attached with the supporting grid on the outside, as it would be in

normal operation. The pressure was gradually increased in steps of 0.3 bar and held for 5 minutes

at each step, to ensure the prolonged strength of the window at each pressure. 2.06 bar was

reached, without the window breaking, and the chamber was held at this pressure for 12 hours.

As the gas handling system cannot control pressures above 1.33 bar it was not necessary to

increase the pressure any further.

The window was then reversed, so the grid was on the inside. Once again, pressure was

increased in steps; but this time each step was 0.02 bar, as the window was expected to be

much weaker in this direction. The window could only support 0.33 bar, therefore, the TBragg

must be connected to a vacuum chamber during experiments to prevent reverse pressure from

damaging the window.

22

Background and Design 3.4. Electronics

3.4 Electronics

In the TBragg the ionised electrons are drifted at a constant velocity and collected at the

anode. The signal produced contains all the information needed for particle identification, thus

making the electronics simple to use.

As shown in Figure 3.11, the anode signal is first directed to a charge sensitive preamplifier.

The preamplifier passes the signal through an RC circuit which gives the signal a fast rise time

and slow decay time, shown in Figure 3.12. Either of the two preamplifier chips can be used to

amplify the signal, they differ mainly by a factor of ∼10 in their gain. The CR-110 (rev. 2) chip

has a gain of 1.4 volts/pC [24], whereas, the CR-111 chip has 0.13 volts/pC [25]. It is necessary

to have two chips available when using the TBragg, as lower energy beams will produce smaller

signals, and will therefore require larger amplification, and vice versa. The preamplifier provides

two identical output signals, both of which are sent to the ADC (Analog to Digital Converter)

for further pulse shape analysis.

Figure 3.11: This figure shows a circuit diagram for the TBragg preamplifier box. Itincludes the resistive chain that connects the electrodes inside the chamber.

The decay time of the signal (τ) is dependent on the electronics, and so the only useful parts

of this signal are the rise time and the height, which are both dependent on the ion. Thus,

anode signals are typically presented as just the first part of this signal, as shown in Figure 3.13.

The experimental setup was designed such that the distance, and therefore length of cable,

23

Background and Design 3.4. Electronics

Figure 3.12: This figure shows the effect the preamplifier box has on the anode signal.The RC circuit will produce a signal with a fast rise time and a slow decay time. Thepreamplifier chips will then amplify this new signal.

between the anode and the preamplifier box is minimised, as this reduces any noise in the signal

before amplification. Once the signal has been amplified it is sent to the SIS3302 sampling

ADC. The distance it will have to travel is much longer than before and this is where most of

the noise comes from. However, the “real” signal has been amplified and is much larger than

the background noise.

The SIS3302 ADC has eight channels available and a maximum sampling rate of 100 MHz;

but only two of these eight channels were used to record the anode signals. Furthermore, the card

was limited to 512 samples per signal; as the signals produced from the TBragg typically have

a long rise time (3 µs) the sampling frequency was limited to 50 MHz (20 ns). After digitising

the anode signals (shown in Figure 3.13), two filters were applied, one to each channel. The

filters are used to extract the two key pieces of information required for particle identification,

total energy and atomic number (described in detail in Section 4.3). Both the raw waveforms

and the signals produced after application of the filters can be saved for later analysis.

Figure 3.13: This figure shows a signal that has been digitised through use of ananalog to digital converter (ADC). The TBragg ADC was set to sample at a frequencyof 50 MHz (every 20 ns).

24

Chapter 4

Simulations

Before experiments could be conducted it was vital to ensure the operating parameters were

chosen in such a way that would optimise the efficiency of the data collected. In addition, the

simulations provided a means for comparison against experimental data, enabling the identifi-

cation of isotopes. This chapter will describe how and why values such as gas type, pressure

and voltage were chosen, when setting up the TBragg.

4.1 SRIM 2011

Figure 4.1: This figure shows input screenfor SRIM, a program which calculates therange of ions in matter for a variety of dif-ferent stopping materials [15].

SRIM 2011 [15] is a collection of pro-

grams that calculate the range and stopping

of ions in matter, through the use of algo-

rithms developed from equations introduced

in Section 2.1. When simulating the TBragg

only two programs from this collection were

required, SRIM and TRIM.

4.1.1 SRIM

SRIM calculates the range of ions in mat-

ter, Figure 4.1 shows the input screen. The

program requires information about both the

ion and the target; only one target layer can

be included in each SRIM simulation.

25

Simulations 4.1. SRIM 2011

Initially, SRIM was used to calculate the amount of energy ions would lose going through

the Mylar window (seen in Figure 3.3 in Section 3.1). This was to identify the remaining energy

of the ions when they entered the gas, which ensured the correct pressure was set. Due to the

upper and lower pressure limits, set by the gas handling system, the choice of gas was important.

Figure 4.2 shows how the pressure must be varied with atomic number to stop ions in 9 cm each

gas. The red lines indicate the maximum and minimum pressure limits. It can be seen that for

ions with atomic number less than ∼10 it was necessary to use either isobutane or CF4; and for

ions with atomic number greater than ∼10 it was necessary to use P-10 or P-8.

Figure 4.2: This graph shows the pressure required to stop ions at 9 cm into theTBragg, for all four gases available. CF4 and isobutane are better suited for stoppinglighter ion, as opposed to P-10 and P-8. The red lines mark the upper and lower limitsallowed by the gas handling system. In this plot the data points for P-10 and P-8overlap completely, and are thus indistinguishable.

In SRIM, the target material can be selected either by element or through use of the com-

pound dictionary, which lists common gas and solid targets. This dictionary includes Mylar,

isobutane, CF4 and P-10, but not P-8 specifically. In order to provide accurate simulations for

P-8, the stoichiometry of the similar gas P-10 was adjusted. This was also done with P-10, as

the default values were not correct. Stoichiometry refers to the percentage quantities of the

various types of atoms in a gas mixture.

26


The molar quantities of P-10 are:

Ar = 90

CH4 = 10 [C = 10,H = 4O]

Whereas P-8 consists of:

Ar = 92

CH4 = 8 [C = 8,H = 32]

Using these values, the stoichiometric percentages of each atom are:

Ar = 914 = 64.2%

C = 114 = 7.1%

H = 414 = 28.6%

For P-8, the stoichiometric values are:

Ar = 92132 = 69.7%

C = 8132 = 6.1%

H = 32132 = 24.2%

These values were then entered into the “stoich” column in SRIM. Once the gas type was

chosen, the pressure was established using SRIM to ensure the ions stop at the desired distance.

For the TBragg, the drift volume is ∼ 10 cm long, and the ions were typically stopped between

8–9 cm. Stopping the ions at this depth made the signals easier to analyse. If the ions are

stopped at shorter distances in the gas the Bragg curve becomes condensed, which will produce

an anode signal with a faster rise time and make pulse shape analysis more difficult. Figure 4.3

shows how the Bragg curve changes as 24Mg, at 4 MeV/u, is stopped at 3 cm, 6 cm and 9 cm

into the detector. Pulse shape analysis is further explained in Section 4.3, and problems related

to the range of ions are discussed in Chapter 5.

27


(a) (b) (c)

Figure 4.3: These graphs show the Bragg curve (energy loss as a function of distance)for 24Mg at 4 MeV/u stopping in P-10 at (a) 3 cm (b) 6 cm and (c) 9 cm, which wasachieved by lowering the gas pressure. These plot were created using SRIM.

By integrating the area under the Bragg curve the total energy is obtained, and the Bragg

peak holds information about the atomic number. The shorter the range, and the more con-

densed the Bragg curve, the more difficult it becomes to distinguish between the two. It was

also observed that for higher masses the Bragg peak becomes less defined. Figure 4.4 shows the

Bragg curves for 32S, 40K and 64Zn at 3 MeV/u, stopping in P-10.

(a) (b) (c)

Figure 4.4: These graphs show how for higher masses the Bragg peak becomes lessdefined (a) 32S at 3 MeV/u in P-10 (b) 40K at 3 MeV/u in P-10 (c) 64Zn at 3 MeV/uin P-10. All of these plots were created using SRIM.

28

Simulations 4.2. Drift Velocity

4.1.2 TRIM

Once the operating parameters had been determined TRIM was used to simulate the energy

loss of the ions. Figure 4.5a shows the input screen for TRIM; as with SRIM it requires infor-

mation about the ion and the target. However, TRIM allows the inclusion of multiple targets in

the simulation; which made it possible to include the entrance window as the first target layer,

and the gas as the second layer. This enabled energy loss straggling, through the Mylar, to be

accounted for in the simulations.

Figure 4.5b shows some of the graphs that can be produced with TRIM. In addition, various

data files were available, which detailed information about the stopping of ions in the gas. The

collision file was the most useful for the TBragg simulations, as it provided information about

every collision each ion had with the gas atoms, and the depth at which each collision occurred.

This collision file was then imported into Microsoft Excel for further analysis.

(a) (b)

Figure 4.5: (a) Shows the TRIM input screen, one of the programs available in SRIM2011 that calculates the energy loss of ions in matter. (b) Shows some of the graphsthat can be produced using TRIM.

4.2 Drift Velocity

In order to enable the fast collection of signals a high electron drift velocity was required.

The relationship between drift velocity, electric field and pressure was introduced in Section 2.2.

However, the specifics of this relationship vary for different gas types. Figure 4.6 shows the drift

velocity graphs for isobutane, CF4 and P-10; these graphs were constructed from experimentally

collected data [26].

29


(a)

(b)

(c)

Figure 4.6: These figures show the relationship between the drift velocity of electronsand E/P for (a) isobutane, (b) CF4 and (c) P-10. This data was obtained experimen-tally [26] (isobutane: Figure 4.5, CF4: Figure 4.6 and P-10: Figure 4.7).

30


The following values list the typical drift velocities used for each gas (P-8 was interpolated

using the drift velocity information for P-5, P-10 and P-20):

1. P-10: 5.3 cm/µs at 160 Vcm bar

2. P-8 : 4.8 cm/µs at 140 Vcm bar

3. CF4 : 7.8 cm/µs at 0.5 Vcm torr (∼ 375 V

cm bar)

4. Isobutane: 1.3 cm/µs at 384.6 Vcm bar

From the drift velocity values and the pressure the operating voltage required to maximise

drift velocity was calculated.

The drift velocity was also useful for simulations. The TRIM collision file provided data

about the ion energy at various distances through the gas; this data could be used directly from

TRIM form to produce the Bragg curve, however, it is not easy to extract information from this

graph. A more useful graph is the anode signal, a plot of cumulative energy loss as a function

of time taken for electrons to reach the anode.

4.2.1 Simulating the anode signal

In order to calculate the anode signal from simulated data, the ion energy was first calculated

at defined distances through the gas to enable the calculation of the cumulative energy loss. Once

this was done the trapezoidal shaping filters were applied to produce a simulated PID plot.

As mentioned in Section 3.4, the ADC samples the pulse height every 20 ns. Consequently,

the simulations calculated the energy of the ions every 20 ns, using the TRIM data which was

inserted into Microsoft Excel. In order to interpolate the ion energy, the drift velocity was first

used to determine the distance travelled by electrons in 20 ns. However, this calculated the

distance with respect to the anode, whereas TRIM measures the distance with respect to the

window. Therefore, this value was then deducted from 10 cm, which is the length of the drift

volume. These distance steps, as measured from the window, will be referred to as New(x).

It is important to remember that the TRIM file details the energy of the ion as it moves

away from the window (illustrated in Figure 4.7). As a result, the energy will be decreasing as

the distance from the window increases, while the rate of energy loss will be increasing. But,

the anode will see this in reverse; as all of the ionised charges are drifted with a constant drift

velocity, the charges ionised at the end of the ion’s path will be collected first. The ion will

31


therefore be seen by the anode to have its energy increasing with distance, resulting in a reverse

Bragg curve. Thus the anode signal increases rapidly to begin with, as a high density of electrons

are collected, and eventually reaches a plateau when the total charge is collected.

Figure 4.7: A diagram of ions moving through a 10 cm drift volume and ionisingelectrons in the gas. The majority of electrons are produced at the end of the ions path,and will reach the anode first when drifted under the influence of an electric field.

Using the TRIM data the energy was interpolated at each value of New(x), using the following

formula [27].

ENew(x) = [Forecast(New(x),Offset(yvalue),Offset(xvalue))]− EMylar (4.1)

Where,

Offset(yvalue) = Offset(yarray,Match(New(x), xarray, 1)− 1, 0, 2, 1) (4.2)

Offset(xvalue) = Offset(xarray,Match(New(x), xarray, 1)− 1, 0, 2, 1) (4.3)

In this notation, xarray is the distance array, from the TRIM data; and yarray is the energy

array, from the TRIM file. EMylar is the energy lost in the window, specific to each ion, and was

calculated using the following formula:

EMylar = Einitial − [Forecast(15000,Offset(yMylar),Offset(xMylar))] (4.4)

Where,

Offset(yMylar) = Offset(yarray,Match(15000, xarray, 1) + 1, 0, 2) (4.5)

Offset(xMylar) = Offset(xarray,Match(15000, xarray, 1) + 1, 0, 2) (4.6)

32


In Equations 4.4,4.5 and 4.6, 15000 refers to the width of the Mylar window, in angstrom,

and Einitial is the incident energy of that ion.

Equations 4.1 and 4.4 can be broken down into three main functions:

1. Forecast: Forecast(New(x), yarray, xarray)

2. Offset: Offset(ref,row offset,column offset,row count,column count)

3. Match: Match(New(x), lookup array, match type)

Figure 4.8: This graph provides a visual rep-resentation of the “forecast” function, which is alinear interpolation formula used to calculate theenergy of simulated ions through 10 cm of gas,using data obtained through TRIM. Where x1,y1 and x2, y2 are the closest sets of simulateddata points to New(x) and New(y).

In order to calculate the new energy,

New(y), the forecast function used the

TRIM data to locate the values of (x1,y1)

and (x2,y2) in the xarray and the yArray

(illustrated in Figure 4.8). Where x1

is the largest distance that is less than

New(x) in the xarray; x2 is the smallest

distance that is larger than New(x) in the

xarray; and y1 and y2 are the correspond-

ing values in the yarray, to x1 and x2.

To find values for (x1,y1) and (x2,y2)

two offset functions were required. Equa-

tion 4.2 calculated y1 and y2, thus requiring the yarray as the “reference”. Whereas, Equation 4.3

calculated x1 and x2, using xarray as the “reference”.

Taking Equation 4.2, in order to calculate the “row offset”, and locate y1, a match function

was used. This function matches New(x) with the largest value that is less than New(x) in

the xarray, and will return its position. Subtracting 1 from this position gives the “row offset”,

which is the number of rows from row 1 to y1. For example, in Table 4.1, if New(x) was equal

to 6 the match function would return the position as 3. Subtracting 1 from this means that in

the yarray, the function will start at row 1, move down 2 cells down, and record y1 as 10.

The “reference” ensures the offset function reads from the correct column, therefore the

“column offset” was set to zero.

In order to calculate y2 the “row count” was equal to 2, making the function record y2 as

the value listed after y1 in the yarray. This works due to the fact y1 was recorded as the y value

33


Table 4.1: An example xarray and yarray

xarray yarray

1 15

4 13

5 10

9 7

11 5

corresponding to the highest x value that is lower than New(x). This means the number directly

following y1, in the yarray, is the y value corresponding to the lowest x value that is higher than

New(x); referring back to Table 4.1, y2 will be 7. It was therefore important that the xarray was

sorted in ascending order. However, if the xarray was in descending order the match type would

be -1 to initially return the position of the smallest x value that is larger than New(x). Finally,

the “column count” was set to 1, as only values from one column were recorded in each offset

function.

The same process was repeated to find x1 and x2, however the “reference” was changed to

the xarray. Once both (x1,y1) and (x2,y2) were calculated the forecast function assumed a linear

relationship between these two points and using New(x) it calculated the value for New(y). In

this way the energy was calculated at defined distances through the gas; from these energy values

the energy loss of the ion was calculated, from one time step to the next. Following this, it was

possible to calculate the cumulative energy loss. Plotting the cumulative energy loss against

the time taken for the charges to reach the anode produced the simulated anode signal (where

t = 0 is at a distance 10 cm from the window). The height of this signal corresponding to the

total energy of the ion, which was previously represented by the area underneath the Bragg

curve. Additionally, the rise of the signal represents the atomic number of the ion, which was

previously given by the Bragg peak. Through pulse shape analysis methods it was possible to

extract the information required for particle identification (covered in Section 4.3).

Figure 4.9 shows how the anode signal varies for different isotopes, different energies and

different atomic numbers; all of these ions were stopped in isobutane at 127 mbar and a drift

velocity of 4.1 cm/µs was applied. Figure 4.9a shows the anode signal for two different ions,

with different energies. As expected, the gradient of the slope is slightly different for both Si

and Ar, as they have different atomic numbers (14 and 18 respectively); and the height of each

signal is different as they both have different energies (56 MeV and 40 MeV respectively). The

34


two anode signals start at different times because the range of both ions is different; the ions

that stop shorter in the gas will have further to travel before being collected at the anode, and

therefore the signal starts later. Figure 4.9b shows the anode signal for two different ions, Si

and Ar, with the same energy (40 MeV), which means the height of both signals is the same but

the gradient of the slopes are different. Finally, Figure 4.9c shows the anode signals for two Ar

ions with different incident energies (40 MeV and 80 MeV), therefore the height of the signal

differs but the gradient of the slope is the same. However, there is separation between the two

ions as the ion with lower energy stops at a shorter distance.

(a) (b)

(c)

Figure 4.9: These plots show how the simulated anode signals vary for ions with(a) different energies and different atomic numbers (b) the same energies but differentatomic numbers and (c) the same atomic numbers but different energies, stopping inisobutane at 127 mbar with a drift velocity of 4.1 cm/µs.

35

Simulations 4.3. Pulse Shape Analysis

4.3 Pulse Shape Analysis

Figure 4.10: This diagram highlights the process forapplying a trapezoidal filter with one window (filterfunction), and shows the signal before and after appli-cation of the filter.

Using the TRIM data file a simu-

lated anode signal was produced, as

discussed in Subsection 4.2.1. Pulse

shape analysis was then used to ex-

tract information about total energy

and atomic number. Figure 4.10

shows the mechanism for applying

a simple trapezoidal filter, with one

window, also referred to as the filter

function. The signal values that lie

within this window are averaged and

the new value is recorded. These

filter values produce a new signal,

which in this case has simply cal-

culated the energy at various points

along the anode signal.

In order to identify the change in cumulative energy loss, a filter with two windows was

required. This filter works in the same way, signal values inside the windows are averaged;

however, now these values are subtracted from each other to obtain the filter value. As shown

in Figure 4.11a, the peaking time, tP determines the width of the windows; whereas, the gap

time, tG, refers to the distance between the leading edge of the two windows. By making tG

larger than the rise time of the signal, the height was obtained, which corresponds to the total

energy of the ion and is referred to as the long filter. Subsequently, making tG smaller than the

rise time the difference between two points close together on the slope was calculated, this gave

the change in energy loss, and is referred to as the short filter. Typically, tP was 140 ns and

tG was 500 ns for the short filter; whereas tP was 240 ns and tG was 4500 ns for the long filter.

These long and short filters are illustrated in Figure 4.11b.

In Excel, the long and short filters were applied to the simulated cumulative energy loss,

using Equations 4.7 and 4.8. The two windows were calculated separately and then subtracted

from each other. The same formulas were used for both the long and short filters but different

36


(a)(b)

Figure 4.11: (a) This diagram shows the trapezoidal filter used to analyse TBraggdata, which utilises two windows, the width of each is defined by tP and the separationbetween the leading edge of the two windows is defined by tG. (b) Shows the applicationof the trapezoidal filter to a typical anode signal. “A” shows the short filter, which musthave a gap time much smaller than the rise time in order to sample the slope. Whereas“B” shows the long filter, which requires a gap time longer than the rise time of thesignal, in order to sample the signal height.

tP and tG values were specified. The following formula was used to calculate window 1:

window 1 = Average[Num 1 : Offset(Num 1,tPts, 0, 1, 1)] (4.7)

Where ts is the time step (20 ns for the TBragg), and Num 1 is the starting point which

defines the leading edge of window 1. The end point was calculated using an offset function,

where the “reference” was Num 1, and the “row offset” was tP /time steps.

In the TBragg simulations, tP for the short window was typically 0.14 µs, and the time steps

were 0.02 µs (20 ns); which gave a “row offset” of 7 cells. The“column offset” was zero, as the

correct column is being read due to the “reference”. Finally, “row count” and “column count”

were both equal to 1, to return a value that is one cell high and one cell wide.

The same variables were used to calculate window two:

window 2 = Average[(Offset(Num 1,tGts, 0, 1, 1)) : Offset(Num 1, (

tGts

+tPts

), 0, 1, 1)] (4.8)

The maximum value given by the long filter is directly equal to the total energy of the ion.

However, the maximum given by the short filter is only related to the atomic number; it is also

affected by other qualities, such as the stopping distance. Therefore, an empirical normalisation

37


was required to calculate the atomic number from this maximum value. This was typically done

by taking maximum value for a number of different isotopes, and as the atomic numbers of

these isotopes are known, they were plotted and the equation relating the two axes was used to

calculate the atomic number for other isotopes.

A PID plot was obtained by plotting the total energy against the atomic number for these

simulated ions (shown in Figure 4.12). This plot was used for comparison with PID plots

constructed from experimental data, to ensure isotopes were correctly identified.

Typically 20 ions were simulated for each isotope, and the average and the standard deviation

of this data was then calculated. Using Equation 4.9 [12] the Z and energy resolution was

calculated. The energy resolution is typically multiplied by 100% and presented as a percentage;

whereas, the Z resolution is usually presented as a fraction.

Resolution R =FWHM

H0(4.9)

Where the Full Width at Half Maximum (FWHM) is equal to 2.35σ, in this notation σ is

the standard deviation of a data set, and H0 is the peak centroid value.

Table 4.2 shows that the simulations produce data with a high resolution. The Z resolution

for simulated data was typically equal to 1/135, which means that simulated data can resolve

isotopes up to an atomic number of 135. In contrast, the simulated energy resolution was

typically 0.15%. A Z resolution or 1/45 and an energy resolution of 0.6% was achieved by

the original Munich Bragg detector [10], and it was expected that the TBragg would provide

approximately the same resolution. The main reason the simulations produce a better resolution

than was realistic in the experiment is the fact that the simulations are a simplified version of

the actual operation of the TBragg. No consideration for the electronic processes were included

in the simulations, and the beam was assumed to enter the TBragg at exactly 90◦ to the window

and that all particles contained in the beam had exactly the same energy.

38


Figure 4.12: This figure shows the simulated particle identification (PID) plot obtainedusing simulated data from SRIM, and applying pulse shape analysis methods.

Table 4.2: The simulated Z and energy resolution for a variety of ions, whose energyloss was simulated using SRIM and pulse shape analysis methods were applied usingMicrosoft Excel to produce comparable data to experiments.

Isotope Z resolution E resolution (%)

86Kr 0.008(2) ≈ 1/123 0.035(3)76Se 0.009(2) ≈ 1/116 0.018(3)71Ga 0.004(2) ≈ 1/250 0.019(2)66Zn 0.006(1) ≈ 1/153 0.024(2)12C 0.011(1) ≈ 1/90 0.579(50)16O 0.013(1) ≈ 1/78 0.178(20)

39

Chapter 5

Preliminary Tests

Before the TBragg could be used for in-beam experiments it was bench tested using a triple

alpha source. This was to test the operation of the gas handling system, and to ensure it was

able to keep the TBragg at a constant pressure for an extended period of time. It was also

necessary to test the electronics, and ensure the trigger thresholds and the filter parameters (tG

and tP ) had been set correctly. Table 5.1 shows the average energy of each alpha particle leaving

the source, and the amount of energy they should lose, on average, passing through the Mylar

window, which was calculated using SRIM.

Table 5.1: This table lists the alpha particle energies for the triple alpha source usedto bench test the TBragg. Values were obtained from nuclear data sheets [28–30].

Parent Nucleus Energy (keV) Energy lost in window (keV)

244Cm 5804.77(5) 160241Am 5485.56(12) 166239Pu 5156.59(14) 174

5.1 Experimental Set-up

The triple alpha source was mounted inside the vacuum chamber, as indicated in Figure 5.1.

The gas handling system, without the use of EPICS, was used to fill the TBragg with isobutane

at a pressure of 212 torr in order to stop the alpha particles emitted from 244Cm, 241Am and

239Pu at 5.4 cm, 4.9 cm and 4.5 cm, respectively. 1500 V was applied to the chamber, in order

to achieve a drift velocity of 9 cm/µs.

Initially, an oscilloscope was used to observe the signals from the preamplifier (shown in

40

Preliminary Tests 5.1. Experimental Set-up

Figure 5.1: The set up used for bench testing the TBragg. Isobutane was used to stopalpha particles, emitted from a triple alpha source, in order to test the operation of theTBragg.

Figure 5.2a). The W value is a measure of the mean energy required to form an electron ion

pair. It is dependent on the properties of the irradiated material, and only weakly on the

energy and nature of the incident particles, if they are much faster than the valence electrons

in the molecules of the material [31]. For isobutane the W value is 23 eV/electron ion pair [32].

For these tests the preamplifier chip with the higher gain (1.4 pC/V) was used. With use of

Equation 5.1, the expected signal height on the oscilloscope was calculated to be ∼ 25 mV,

which corresponds with the signals (shown in Figure 5.2a).

Signal height =E q

gain W(5.1)

Where E is the energy of the alpha particle in eV, q is the charge of an electron in pC, the

gain of the preamplifier is in pC/V and the W value is in eV/electron ion pair.

Initially, many signals with a comparatively fast rise time and large amplitude were observed

(shown in Figure 5.2b). This was attributed to impurities in the chamber, as these signals

increased in frequency with voltage, and decreased in frequency over time. The detector was left

to condition for a week in order to remove as many of these impurities as possible; the voltage

was increased gradually during this time, up to 3000 V.

41

Preliminary Tests 5.2. Data Collection

(a) (b)

Figure 5.2: The signals were first observed using an oscilloscope, in addition to thealpha signals shown in (a), a number of noise signals, shown in (b), were also observedwith a much faster rise time. These signals were contributed to impurities in the detec-tor, that were removed through conditioning.

5.2 Data Collection

The PID plots were analysed using ROOT [33], a program developed to handle large amounts

of data efficiently; which provides a number of packages which enable data to be presented in a

variety of ways.

Following the conditioning process the preamplifier outputs were connected to the analog

system for data collection. The data presented in Figure 5.3 was collected over 12 hours.

Figure 5.3: This plot shows the data collected from stopping alpha particles emittedfrom a triple alpha source in isobutane, this data was collected using an analog dataacquisition system over a 12 hour period. A resolution of 2% was achieved.

42


Figure 5.3 shows a double peak at each alpha energy; this was due to the oscillating baseline,

from electronic and sonic noise, at a frequency of 250 Hz and a magnitude of 10 mV. As a result

an energy resolution of 2% was achieved with the analog system. The preamplifier outputs were

then connected to the DAQ. The short filter used a peaking time of 1000 ns and a gap time

of 200 ns; whereas, the long filter used a peaking time of 4000 ns and gap time of 2000 ns.

From Figure 5.4 it is clear that the DAQ was more efficient at removing the baseline oscillation,

and thus improved the energy resolution to ∼1.5%. The PID plot, in Figure 5.5, shows the

separation achieved between the three alpha energies.

Figure 5.4: This figure shows the data collected from stopping alpha particles emittedfrom a triple alpha source in isobutane using the digital data acquisition system, datawas collected over a 12 hour period which provided an energy resolution of 1.5%.

Three main features can be seen in this PID plot; firstly, the straight, diagonal line is from

the background noise. The noise signals had very fast rise times compared to alpha signals, and

therefore values calculated by the short filter were higher than for the alpha signals. Although,

due to the smaller integration area, the short filter still evaluated a smaller number than the

long filter.

The second aspect of Figure 5.5 is the separation of the three alpha points in the y axis,

which should not be present as they all have the same atomic number. However, due to the low

energies the alpha particles had a range between 5.4–4.5 cm. With a drift velocity of 9 cm/µs

43


Figure 5.5: This PID plot was obtained from stopping alpha particles emitted fromthe source at three different energies in 212 torr of isobutane.

the rise time was ∼ 0.5 µs, but the peaking time of the short filter was set twice as long as this.

This short rise time meant that the short trapezoidal filter was not able to correctly calculate

the atomic number. There are two methods to correct this, either increasing the range of the

ions or altering the short shaping parameters. The range can be extended either by increasing

the incident energy or by decreasing the gas pressure.

The energy loss of 4He, 7Li and 12C ions in 500 mbar of isobutane was simulated using SRIM.

The ions were given a range of energies, enabling them to travel different ranges through the

15 cm simulated drift length. A constant drift velocity of 1.3 cm/µs was applied to the simulated

data, and two sets of short shaping parameters were used to produce the simulated PID plot

shown in Figure 5.6. The “high” short parameters had a peaking time of 2 µs and a gap time

of 6 µs. The “low” short parameters had a peaking time of 0.5 µs and a gap time of 1.8 µs.

The long shaping parameters were kept constant with a peaking time of 1 µs and a gap time of

9.2 µs. No normalisation was applied to this data to correct the y axis.

It can be seen that regardless of the shaping parameters, the heavier ions reach the plateau

at a higher energy. However, when the appropriate shaping parameters are used the height of

44


Figure 5.6: This data was simulated using SRIM, and shows how the range of differentions and the short shaping parameters affect the evaluation of the atomic number andtherefore the position of data points on the PID plot. The “high” short parametershad a peaking time of 2 µs and a gap time of 6 µs. The “low” short parameters hada peaking time of 0.5 µs and a gap time of 1.8 µs. The long shaping parameters werekept constant with a peaking time of 1 µs and a gap time of 9.2 µs.

the plateau is lowered, as the atomic number is calculated more effectively. This effect is present

for all isotopes, however the higher the atomic number the more incident energy is required

for particles to travel the range necessary to reach a plateau. Therefore, the plateau will start

further along the x axis, compared to lighter particles travelling through the same gas at the

same pressure.

The third and final feature of the PID plot is the tail, which extends from the three alpha

points to the origin, with a gradient that is less steep than the noise. This tail is due to straggling

effects, and is particularly noticeable here as the alpha particles have such low energies. The

distance between the source and the window was ∼10 cm, and although a good vacuum was

maintained the source was not collimated. Therefore, it was possible for alpha particles to lose

energy through collisions with other alpha particles. A lower energy means a smaller range, and

therefore points will fall further down the low energy tails, shown in Figure 5.6. In order to

test this tail was energy dependent, gas was introduced between the source and the window to

degrade the energy of the alpha particles before entering the TBragg. It is clear from Figure 5.7

that this relationship was reproduced, and was due to the energy of the alpha particles. Group

3 only shows points attributed to only two of the alpha energies, because the alpha particles

with the lowest initial energy, emitted from 239 Pu, has been degraded so much that the data

lies in the noise.

45

Preliminary Tests 5.3. Using the non-flammable gas alternative, CF4

Figure 5.7: This PID plot was constructed from three different runs, stopping alphaparticles in isobutane. Group 1 shows the initial run with the distance between thewindow and the source at vacuum. Group 2 shows the effect of introducing 80 torrbetween the source and the window. Finally, group 3 shows the effect of introducing150 torr between the source and the window.

5.3 Using the non-flammable gas alternative, CF4

Figure 5.8: This figure shows the oscillo-scope signals obtained using CF4 gas to stopalpha particles from the triple alpha source.

Following on from these tests, CF4 was

used as a replacement gas for the isobutane to

stop alpha particles. The main purpose was

to test whether a non-flammable gas would

degrade the resolution. The pressure was

set to 200 torr and 1500 V was applied to

the TBragg, which enabled a drift velocity of

∼ 9 cm/µs. This pressure stopped alpha parti-

cles emitted from 244Cm, 241Am and 239Pu at

6.2 cm, 5.7 cm and 5.2 cm, respectively. The

W value of CF4 is 54 eV/electron ion pair [34],

46


Figure 5.9: This figure shows the data collected, with an analog data acquisitionsystem over a 12 hour period, from stopping alpha particles emitted from a triple al-pha source in CF4. Due to high amplitude background noise the alpha signals wereindistinguishable.

resulting in an expected signal height of ∼ 12.2 mV, which corresponds to Figure 5.8. The alpha

signals were also expected to have a rise time of ∼ 0.6 µs. As with the isobutane tests, data

was first collected over a 12 hour period with an analog system. In addition to the pulse heights

being lowered, by the change of gas, the noise levels were also higher, at ∼ 15 mV. As a result

the analog system was unable to differentiate between alpha signals and noise signals, which is

reflected in Figure 5.9.

However, with the use of the DAQ it was possible to differentiate the three alpha energies.

The same filter parameters were used as with the isobutane tests. Figure 5.10 shows the effect

of higher background noise levels. In addition to higher noise levels, the count rate was also

significantly lower than with isobutane, at approximately 1–2 counts every 5 minutes. This is

due to the higher W value, which results partly from the high probability of producing photons

in CF4 [34].

As a result of higher background noise, and a much lower count rate, the energy resolution

was between 2.62–3.07%. Figure 5.11 shows the PID plot obtained with this data. Although

47


the resolution had been slightly degraded, it was decided that CF4 will be used to replace

isobutane in further experiments, as CF4 provides sufficient resolution. There are also a number

of health and safety benefits to using CF4 over isobutane, as it is non-flammable and non-toxic;

particularly when using radioactive beams.

(a) (b)

(c) (d)

Figure 5.10: Figures (a) and (b) show the projected x axis from the PID plot con-structed with data from stopping alpha particles in isobutane, there is clear separationin the energy of these particles. Whereas, figures (c) and (d) show the projected xaxis from the PID plot constructed with data from stopping alpha particles in CF4.Although higher levels of noise were present in the CF4 tests the three alpha energiesremain distinguishable.

Figure 5.11: This figure shows the PID plot constructed with data obtained fromstopping alpha particles in CF4.

48

Chapter 6

Low Mass Beam Tests

Preliminary testing, described in the previous section, confirmed that the TBragg apparatus

was functioning correctly for in-beam experiments. The first of which was conducted with a

stable 18O beam, at 1.5 MeV/u. The aim of this experiment was to determine whether non-

flammable P-8 gas could be used to replace flammable P-10 gas, without having to compromise

on resolution. This is particularly important for experiments with radioactive beams, as, for

health and safety reasons, the nuclear exhaust can not be used if the gas is flammable.

6.1 ISAC I Set-up

Figure 6.1: The target wheel usedto mount the three targets (gold,LiF and carbon), and made the pro-cess of changing between each tar-get quicker and easier during the ex-periment.

During this experiment some of the EPICS con-

trols were available, primarily the voltage trip off. This

control ensured the voltage would be automatically

switched off if the pressure inside the TBragg fluctu-

ated rapidly, as this would suggest a gas leak or loss of

gas control, and is necessary to prevent sparks igniting

the flammable gas.

As can be seen in Figure 6.2, the TBragg was

mounted to a scattering chamber at 24◦ with respect to

the beam. Inside the scattering chamber three targets

were used; carbon, lithium fluoride and gold. These

targets were mounted on the target wheel shown in

Figure 6.1.

49

Low Mass Beam Tests 6.1. ISAC I Set-up

Figure 6.2: For set up for the first in-beam experiment, conducted with an 18O beam.The TBragg was mounted to a scattering chamber at 24◦ with respect to the beamline. A target wheel, with three targets attached, was positioned in the centre of thescattering chamber, in the beam axis.

Catkin is an Excel spreadsheet designed to calculate the kinematics for two-body collisions;

as well as calculating various other nuclear physics quantities, such as separation energy, Q-value

and multiple scattering [35]. It was used to calculate the expected energies of the 18O, following

the collision with the target, and the energy of target projectiles at 24◦. This information was

required for simulations and also to calculate, using SRIM, suitable pressure limits to ensure the

ions would stop in both gases.

Initially the TBragg was filled with P-10 and the pressure was set to 300 torr, 500 torr and

700 torr. At each of these pressures the voltage was set to 300 V, 500 V, 700 V and 1000 V,

and data was collected for 5 minutes at each pressure and voltage. The gas was then replaced

with P-8 and this process was repeated. The DAQ was used to collect data, the short filter was

set to a peaking time of 140 ns and a gap time of 240 ns; the long filter was set to a peaking

time of 500 ns and a gap time of 4500 ns. The lower gain preamplifier chip was used, which has

a gain of 0.13 V/pC.

50

Low Mass Beam Tests 6.1. ISAC I Set-up

Figure 6.3: This diagram illustratesthe beam of ions colliding with a target;the products are emitted at a variety ofangles, those emitted at 24◦ are acceptedby the TBragg.

As the 18O ions move through the target they

will interact with many particles in the target.

The exact number of collisions will vary for dif-

ferent ions, which will contribute to the energy

loss straggling. Products will be emitted at a va-

riety of angles due to Coulomb scattering (also

known as Rutherford scattering), which is defined

as the deviation of a particle’s path as a result of

Coulomb forces.

The amount of deviation depends on both the energy of the 18O and the proximity of the

beam ions and target particles. The impact parameter b is a measure of the proximity of the

two particles, before the incident particle has been deviated. When b = 0 the incident particle

will collide head-on with the target, provided enough energy is given to counter the Coulomb

repulsion. There is a limit to the value of b before the incident particle is deviated by an angle

greater than 90◦, and does not pass through the target foil. For interactions where b is below

this threshold, and the target has comparable mass to the 18O, the Coulomb repulsion will result

in the target and incident particles moving in the opposite directions.

Figure 6.4: This diagram shows the effect of the Coulomb repulsion on the path of anincident particle interacting with a stationary particle. Where b is the impact parameterand d is the distance of closest approach [36].

51

Low Mass Beam Tests 6.2. ISAC I Data Analysis

6.2 ISAC I Data Analysis

As with preliminary experiments, ROOT was used to view and analyse the sorted data [33].

6.2.1 Gold target

When using the gold target, (listed in Table 6.1), only 18O was expected to be present in

the PID plot. This is because the gold particles are significantly heavier than the oxygen and

therefore require more energy to be moved. Figures 6.5 show the data collected using P-10 and

P-8, respectively, to stop the ions in the TBragg. In both these graphs there is a prominent tail

extending from the 18O data due to energy loss straggling.

Table 6.1: This table provides the energy and the range of the gold at oxygen ions, stoppingin P-10 at 300 torr and P-8 at 300 torr. The energy at this angle was calculated using Catkinand the ranges were calculated using SRIM.

Isotope Energy at 24◦ (MeV) Range in P-10 (cm) Range in P-8 (cm)

18O 26.6 6.7 6.7197Au 7.0 0.5 0.4

(a) Gold P-10 300 V 300 torr (b) Gold P-8 700 V 300 torr

Figure 6.5: These figures show the PID plots produced for 18O scattered from a goldtarget and stopping in (a) P-10 at 300 torr and 300 V (b) P-8 at 300 torr and 700 V.

Figure 6.6 show how the projected short and long axes of the PID plots, shown in Figure 6.5,

vary for P-10 and P-8. It would appear that P-8 shows higher noise levels than P-10; however,

the resolution for both P-10 and P-8 is shown in Table 6.2. Both gases provide similar resolution,

which suggests that P-8 is a suitable replacement for P-10, although slightly different voltages

are required to achieve this similar resolution.

52


(a) (b)

(c) (d)

Figure 6.6: These plots were produced from the scattering of the 18O beam on a goldtarget. (a) and (c) show the projected x axis of the PID plot, showing the uncalibratedenergy peaks for P-10 and P-8 respectively. Whereas (b) and (d) show the projected yaxis of the PID plot, showing the uncalibrated atomic number peaks for P-10 and P-8respectively.

Table 6.2: This table provides values for the resolution achieved for P-10 and P-8 bothat 300 torr, using a gold target to scatter the 18O beam.

Gas Isotope z resolution Energy resolution (%)

P-10 18O 0.02777(5) ≈ 136 6.7662(1)

P-8 18O 0.02857(6) ≈ 135 6.6754(1)

53


6.2.2 Lithium-fluoride target

Table 6.3: This table lists the expected energy, as calculated using Catkin, of eachions emitted at 24◦ in the scattering of the 18O beam on a LiF target, and the range ofthese ions in both P-10 and P-8, calculated using SRIM.

Isotope Energy (MeV) Range in P-10 (cm) Range in P-8 (cm)

19F 22.5 2.8 2.018O † 22.8 3.4 2.47Li - - -12C 21.2 4.8 3.418O †† 19.8 2.8 2.0

† scattered from 19F†† scattered from 12C

Figure 6.7: Alpha particles are produced,with a range of energies, in fusion evaporation.Higher energy alpha particles will travel furtherin the gas eventually punching through the backof the detector, resulting in this “arrow head”arrangement.

With the LiF targets, the aim was to see

if 18O and 19F would be distinguishable us-

ing the TBragg. Although the energies in Ta-

ble 6.3 would suggest fluorine would be de-

tectable this was not the case, possibly due

to a low percentage of 19F in the target. The

PID plots in Figure 6.8 show three key fea-

tures. The first is the 18O, which is accom-

panied by an energy loss straggling tail. The

second feature is a collection of points with a

lower atomic number than the 18O, but slightly higher energy. A lower atomic number means

these points cannot be 19F; nor can they be 7Li as, according to Catkin, the kinematics do not

allow 7Li to be seen at this angle. However, typically targets such as LiF are mounted onto a

12C backing, which is most likely the identity of these particles. The third interesting feature

of this graph is the collection of points closer to the origin. In some plots of the other runs

they are arranged in an arrow head formation shown in Figure 6.7, but here there is just the

initial diagonal line of increasing energy and atomic number. These points are attributed to

alpha particles, emitted from fusion evaporation in the target. In addition to elastic scattering,

the 18O could collide head on with a target particle, if this happens at the right energy the two

particles will merge. This will produce a hot, heavy nucleus which will emit protons, neutrons

and alpha particles at a range of energies [37]. It is unlikely that protons and neutrons will

54


be detectable with the TBragg, but the alpha particles were. The higher the alpha energy,

the further along the x axis the data points will be; when the energy is high enough the alpha

particles will “punch-through” the back of the TBragg, without stopping. Thus not all of the

energy was lost in the gas and so the data points then start to move back down the x axis, even

though energy is actually still increasing. In addition, the position of the alpha particles on the

y axis is determined by their range, as was discussed in Chapter 5. The higher the energy the

longer the range of the alpha particles, and so the higher their position on the y axis, as the

short filter is better calculating the atomic number. When ions begin to punch through the back

of the detector the detected range will stop increasing and so the points will maintain a certain

y value.

The resolution obtained from this experiment was much lower than expected, there are a

number of reasons as to why this might be. It could partly be due to the fact the targets had

been used many times previously, and as no 19F was detected they could be at the end of their

lifetime. They were also very thin targets and had to be stacked together, each run was only

5 minutes long, however, if some of the targets were breaking during this time 18O ions passing

through the target stack during later runs will not have lost as much energy as if all the targets

had been intact. The shaping times may also not have been optimised for the long and short

filters. However, the drift velocity is approximately 2.5 cm/µs in P-10, and 1.8 cm/µs in P-8;

this results in a rise time for 12C of 1.92 µs in P-10 and 1.89 µs in P-8, and for 18O a rise time

of 1.12 µs in P-10 and 1.33 µs in P-8. The values for the short and long filters are suitable

considering these numbers, and therefore it is more likely a problem with the target than the

DAQ. Table 6.4 lists the resolution obtained for both gases, and the projection of the x and y

axes of the PID plots are shown in Figure 6.9.

Table 6.4: This figure shows the resolution obtained from the scattering of the 18Obeam on the LiF target and stopping the ions in P-10 at 700 torr and P-8 at 300 torr.

Gas Isotope Z resolution Energy resolution(%)

P-10 18O 0.1739(10) ≈ 16 123.16(1)

P-10 12C 0.0477(8) ≈ 121 -

P-8 18O 0.2000(10) ≈ 15 130.59(1)

P-8 12C 0.0667(10) ≈ 115 130.59(4)

55


(a) (b)

Figure 6.8: These figures show the PID plots obtained when stopping 18O ions scat-tered from the LiF target in (a) P-10 at 500 torr and 300 V (b) P-8 at 700 torr and300 V

(a) (b)

(c) (d)

Figure 6.9: These plots were produced from the scattering of the 18O beam on the LiFtarget. (a) and (c) show the projected x axis of the PID plot, showing the uncalibratedenergy peaks for P-10 and P-8 respectively. Whereas (b) and (d) show the projected yaxis of the PID plot, showing the uncalibrated atomic number peaks for P-10 and P-8respectively.

56


6.2.3 Carbon target

The carbon target provided the most interesting set of data, and the most unexpected.

Figure 6.10 shows two of the PID plots obtained; the 18O and 12C points are well defined in

both plots, however, there appears to be two sets of data points for each isotope. This interaction

can either be elastic or inelastic. For elastic scattering the final products are identical to the

initial products (illustrated by Equation 6.1.

12C(

18O, 18O)12

C (6.1)

This applies to the sets of data points with the higher energy in the PID plot. Whereas, for

inelastic scattering one possibility is for the 18O to become excited during the interaction. This

reaction has a Q value of 1.98 MeV, where the Q value is the amount of energy released by a

reaction, given in equation 6.2 [36]. A positive Q value means this reaction is exothermic, and

binding energy is released as kinetic energy to the final products [36].

Q = Tfinal − Tinitial (6.2)

Due to Equation 6.3, this results in the final products having lower energy than those result-

ing from elastic scattering. Therefore, inelastic scattering is indicated by the set of data points

with the lower energy, and is given in Equation 6.4.

Einitial = T18O = Efinal = T18O + T12C + E∗(18O) (6.3)

12C(

18O, 18O∗)12

C (6.4)

The elastic scattering and inelastic scattering products were simulated passing through the

TBragg, using both P-10 and P-8. The simulated PID plots are shown in Figures 6.10a and

6.10c, which show good correlation between the PID plots in Figure 6.10b and 6.10d obtained

with experimental data. Figure 6.11 shows the comparison between the projected long and short

axes, for P-10 and P-8, with experimental data. Table 6.6 lists the resolution achieved for P-10

and P-8, and how this compares to the simulated data. A better resolution, for both energy and

Z, was achieved with the 12C targets, compared to the LiF targets. However, when compared

to simulated data there is still clearly room for improvement. The fact that excited 18O was

distinguishable from ground state 18O, and the 12C was clearly identified as having two energies,

57


means the method of particle identification adopted by the TBragg is very efficient. Without

plotting the energy, as well as the atomic number, in the PID plots the different energies of each

isotope would not have been observed, this was an unexpected ability.

(a) (b)

(c) (d)

Figure 6.10: These PID plots show the data collected from the scattering of the 18Oon a 12C target. (a) and (c) were produced using simulated data, whereas (b) and (d)show the experimental data. 18O was detected in both its ground state and an excitedstate (Q=1.98 MeV) due to elastic and inelastic scattering, respectively.

Table 6.5: This table lists the expected energy and range of ions emitted at 24◦ fromthe scattering of the 18O beam on a 12C target, and stopping ions in P-10 and P-8 bothat 500 torr. At these pressures all ions have the same range in both gases, as calculatedusing SRIM.

Isotope Q value Energy (MeV) Range (cm) Energy lost in window (MeV)

12C 0 21.2 4.6 0.812C 1.98 18.7 4.0 0.818O 0 19.8 2.8 1.418O 1.98 17.6 2.4 1.5

58


(a) (b)

(c) (d)

Figure 6.11: These plots were produced from the scattering of the 18O beam on acarbon target. (a) and (c) show the projected x axis of the PID plot, showing theuncalibrated energy peaks for P-10 and P-8 respectively. Whereas (b) and (d) show theprojected y axis of the PID plot, showing the uncalibrated atomic number peaks forP-10 and P-8 respectively.

Table 6.6: This table lists the resolution achieved for both simulated and experimentaldata for the scattering of 18O ions on a 12C target and stopping in P-10 at 300 torr and300 V and P-8 at 500 torr and 1000 V.

Gas Isotope Z resolution E resolution (%)

Exp

erim

enta

l

P-1012C 0.0365(10) ≈ 1

27 17.8407(60)

18O 0.0294(3) ≈ 134 11.0922(10)

P-812C 0.0755(20) ≈ 1

13 22.2528(60)

18O 0.0579(4) ≈ 117 10.6053(8)

Sim

ula

ted P-10

12C 0.026(5) ≈ 139 1.49(30)

18O 0.011(2) ≈ 191 0.27(5)

P-812C 0.011(2) ≈ 1

87 0.55(10)

18O 0.015(3) ≈ 178 0.48(9)

59

Low Mass Beam Tests 6.3. ISAC I Conclusion

6.3 ISAC I Conclusion

The best resolution from this experiment was achieved using the gold target, with an energy

resolution of 6.77% for P-10 and 6.66% for P-8 and a Z resolution of 1/36 for P-10 and 1/37 for

P-8. When comparing the resolution of this detector to other Bragg detectors, the Z resolution

obtained was comparable to other Bragg detectors, but the energy resolution could be improved.

As mentioned previously, the original Munich Bragg detector has an energy resolution of 0.6%

and a Z resolution of 1/45. Additionally, the Bragg detector used by J.M. Asselineau [38] has

an energy resolution of 0.8% and a Z resolution of 1/37. There were many aspects of the set

up of this experiment that could be improved, for example the alignment and position of the

targets was difficult to perfect. Nevertheless, the aim of this experiment was to test whether

P-8 is a suitable replacement for P-10. Considering the gold target, the resolution was slightly

better when using P-8, whereas, for the other two targets the resolution was slightly degraded

when using P-8. Nonetheless, in all PID plots any isotopes observed on the PID plots for P-10

gas were also not only visible when using P-8, but also distinguishable. By also taking into

consideration the health and safety benefits of using P-8 it was decided that P-8 would be used

to replace P-10 for future experiments.

The simulations provide a guideline for the best possible resolution. A few simplifications

are made when simulating these experiments, such as angle of entrance and interactions with

other ions in the beam but they still provide a good first approximation for what should be seen

during the experiment.

60

Chapter 7

High Mass Beam Tests

Previous experiments confirmed that the TBragg and its apparatus were suitable to take

radioactive beam. The High Mass Task Force (HMTF) is a collaboration between the TRIUMF

Accelerator division and the Science division, with the common aims of reaching higher beam

energies (above the Coulomb barrier) and delivering purer, high mass RIBs (beyond A=30) [39].

The CSB is the key to producing these high mass radioactive beams, however, as discussed in

the introduction it also produces high levels of stable contamination. Therefore, the TBragg is

essential for providing the information required for tuning, in order to purify these high mass

beams. This section will describe the 94Sr beam experiment, which was the first of several

successful radioactive beam experiments conducted with the TBragg this year.

The experiment consisted of irradiating a uranium-oxide target with 10 µA of 500 MeV

protons. The singly-charged products were extracted from the ISAC target and passed through

the CSB to produce the radioactive cocktail beam, which was accelerated to 4.21 MeV/u in the

ISAC-II accelerator. In order to monitor the purity and intensity of the RIB a series of three

detectors were utilised, in addition to the TBragg:

1. A silicon detector (PSID5), positioned just after the ISAC-I DTL accelerator stage, which

identifies the beam components from their total energy. Thus, isotopes of the same mass

but different atomic number cannot be distinguished [39].

2. A 90◦ bending magnet, known as the PRAGUE magnet, which disperses the energy of the

beam on a transverse profile monitor (PRAGUE harp), before going through the S-bend.

It is therefore able to identify the mass of isotopes but not the atomic number [40].

3. A ∆E-E silicon detector telescope, which consists of a thin transmission detector in front

61

High Mass Beam Tests

of a thick stopping detector. Where the first measures the energy loss and the second

measures the full energy, in order to identify their charge and mass. As with all silicon

detectors, the incident current needs to be restricted, so as to not damage the detectors [39].

Figure 7.1: This image shows the beam line between the DTL and the SC-Linac,referred to as the S-bend. The position of both of the carbon stripping foils used topurify the beam are labelled as “Degrader #1” and “Degrader #2”. The PRAGUEmagnet was used in conjunction with the TBragg to monitor beam contaminants. [39]

As shown in Figure 7.1 two 44 µg/cm2 carbon stripping foils [41] were placed between the

DTL and the SC-Linac, labelled as “Degrader #1” and “Degrader #2”. These foils create a

velocity difference depending on the atomic number of the particle, by means of energy loss.

The velocity difference allows magnetic selection around the DTL to SC-Linac (DSB) beam

lines, also referred to as the S-Bend [39]. The RIB was first tuned to the PRAGUE magnet at

a charge state of 15+, and then stripped to a higher charge state of 23+ before reaching the

TBragg. The resulting beam had a range of A/Q values between 5.932–6.934 from the CSB to

the DSB, and 3.580–4.528 from the DSB to the TBragg, for charge states populated by more

than 5% [41]. As the beam constituents are only filtered out by A/Q value it was possible for

many contaminants to remain in the beam. Thus, when possible, charge states of the desired

beam are chosen such that the majority of these contaminants are extracted. Figure 7.2 shows

the likely components that were expected to reside in the 94Sr beam, according to their A/Q

value, which was calculated using the CSB assistant [42].

62

High Mass Beam Tests 7.1. ISAC-II Set Up

Figure 7.2: Two carbon foils were used to strip contaminants from the beam accordingto A/Q value. The first filter was set to A/Q=6.260 and the second at A/Q=4.268.Isotopes that lie in the region where these two A/Q selections cross over are the mostexpected beam constituents [42].

7.1 ISAC-II Set Up

Figure 7.3: This figure shows the experimental set up for the high mass beam experi-ment conducted in the ISAC-II facility. The TBragg was mounted to a vacuum chamberat the end of a beam line and a Faraday cup was used to measure the beam intensityand ensure the count rate did not exceed the limit of the DAQ system.

63

High Mass Beam Tests 7.1. ISAC-II Set Up

The TBragg was mounted to the end of a beam line in the ISAC-II hall, as shown in

Figure 7.3. Two Faraday cups were also connected to the vacuum chamber, however only

one was functional at the time of this experiment. A Faraday cup is a common beam diagnostic

device which intercepts the beam and, through use of a current meter, measures the beam

intensity [43]. The TBragg is also capable of measuring the beam intensity, from the number of

counts per second recorded by the DAQ. However, at the time of this experiment the beam rate

into the TBragg was limited to 1000 particles per second by the DAQ system. Therefore, the

Faraday cup was used to ensure this limit was not exceeded, so the DAQ did not crash. Due to

the fact it completely stops the beam, the Faraday cup would have been particularly useful if

there was a problem with either the TBragg or the beam line, and the beam had to be stopped

in an emergency [43].

7.1.1 Calibration

Prior to producing the high mass beam it was necessary to use a calibration beam to tune

the post accelerator sections, and ensure the TBragg had been correctly set up. For this purpose

a 12C beam, with the same A/Q value as the RIB, was produced from the offline ion source

(OLIS), and delivered to the TBragg at 4.21 MeV/u. In order to stop ions at ∼ 9 cm, the

TBragg was filled with P-8 at 900 torr, and 1560 V was applied. The short shaping parameters

used a peaking time of 140 ns and a gap time of 240 ns; the long shaping parameters used a

peaking time of 500 ns and a gap time of 4500 ns. Full EPICS controls were available for this

experiment; the EPICS logic can be found in Appendix B.

Figure 7.5: This image illustrates an effectknown as pile-up, where a second signal is col-lected before the first signal has finished de-caying. The energy information of the secondsignal, which is given by the height of the sig-nal, is distorted as it rides on the tail of thefirst signal. [14]

From Figure 7.4 it is clear that two iso-

topes were contained in the calibration beam.

The dominant isotope being 12C, with a charge

state of 3+, which was accompanied by an en-

ergy loss straggling tail. The second point on

this plot was identified as 16O, with a charge

state of 4+; it was contained in the beam due

to oxidisation of the OLIS target. In addition,

pile-up effects were also observed; when signals

are collected and passed through the preamplifier, as discussed in Section 3.4, the signal has a

decay time τ . The amplitude of the signals are proportional to the total energy of the ion; if

64

High Mass Beam Tests 7.2. ISAC II Data Analysis

Figure 7.4: 12C was used as the test beam for this high mass experiment, this figureshows the PID plot obtained. Both 12C and 16O are observed with charge state 3+ and4+ respectively. Pile-up effects are also shown in this plot as the 12C ions arrive closetogether.

a second ion arrives within a time τ it will ride on the tail of the first signal [14], and a signal

such as the one shown in Figure 7.5 is collected. Therefore, its amplitude will be increased and

the energy information of the signal will be distorted. This effect is known as pile-up; it can be

reduced by either lowering the counting rate to 1/τ counts per second, or by shortening the tail

by reshaping the signal [14]. The latter is typically the preferred option, but it requires the use

of an amplifier following the preamplifier. However, it was not necessary during this experiment

as the pile-up effects did not affect data analysis. In Figure 7.4 the points aligned horizontally

are attributed to two 12C ions entering the detector within a time τ of each other. As τ tends to

zero the signals will have twice the height, and therefore twice the energy is calculated, which

is the maximum energy for two ions, and the PID points are aligned vertically at this energy.

7.2 ISAC II Data Analysis

Once the calibrations were complete the experiment with the radioactive beam could begin.

The TBragg was set to the same pressure and voltage as used in the 12C test run, and the short

and long shaping parameters were also kept constant. Figure 7.6 shows the PID plot obtained

and Figures 7.10a and 7.10b show the x and y projections of this PID plot. Once again, ROOT

was used for analysing the PID plots, and the projected x and y axes, to identify the beam

constituents [33].

65


Figure 7.6: This PID plot shows the data collected with the 94Sr beam. 69Ga, 119Snand 94Mo were used as reference points to calibrate this PID plot and identify the otherisotopes contained in the cocktail beam. 94Sr was not observed, possibly as the beamlines were not fully optimised.

In order to produce the labels shown in Figure 7.6 some known isotopes were required as

references. The PRAGUE magnet detected isotopes of A=69 and A=119; according to the CSB

assistant, figure 7.2, it was likely that these were 69Ga and 119Sn. The strongest PID point

observed was most likely to have A=94 and, as stable isotopes are produced from the CSB with

considerably higher intensity, this point was labelled as 94Mo. These three isotopes were used

as calibration points to obtain the following equations:

short : y = 0.4858x+ 3.488 (7.1)

long : y = 0.1003x− 46.573 (7.2)

Using Equations 7.1 and 7.2 initial estimations of identities of the other isotopes were made.

In order to test the accuracy of these labels simulations were conducted. Figure 7.7 shows a PID

plot comparing this simulated data to the calibrated experimental data; this shows is very good

agreement between the two sets of data points, which suggests the labels given in the calibrated

PID plot are correct. It is important to note that all of these isotopes have been identified as

stable isotopes; it was expected that the main peak would be 94Sr, but this was not the case. The

most probable reason is that the beam lines were not optimised in a way that enabled 94Sr to

66


reach the TBragg. This highlights the importance for a device such as the TBragg at TRIUMF,

as with accurate and detailed information of beam constituents the accelerator division will be

able to tune future beams more successfully.

Figure 7.7: This graph shows the comparison between PID points obtained for thesimulated and calibrated experimental data collected with the 94Sr beam. Good compar-ison is shown suggesting the constituents of the cocktail beam were correctly identified.The slight variation in position is partly due to an energy spread in the beam.

Figure 7.8: This figure shows the measuredenergy spread of a few beams at a variety ofenergies from the DTL, and that ∆E/E hasa typical range between ∼ -1–1% [44].

Additionally, there is a slight discrepancy

between the simulated and experimental data,

which could be the result of a number of

factors. However, after further investigation

the main reason was an energy spread in the

beam, which was not incorporated in the sim-

ulations. Figure 7.9 shows what the energy

of each isotope would have been if this en-

ergy spread was present, assuming it is the

only factor contributing to this discrepancy.

An energy spread is known to be present in

many cocktail beams produced at TRIUMF,

and it comes from the process of accelerating

the beam. Using this data the beam delivery group can make adjustments to the beam line

67


set up in order to minimise this spread. Additionally, this information is useful for experiments

utilising the cocktail beams.

Figure 7.9: This graph was produced with simulated data and shows the requiredenergy spread in the beam for the simulations to match the calibrated experimentaldata. It shows that ∆E/E would have to be between -1.4–1.9% for the data to match.The actual energy spread is from about -1–1% which suggests that an energy spread isonly part of the reason for the variation between simulated and experimental data.

Figure 7.8 shows that the previously measured energy spread after the DTL, for a range of

sample energies between 341–1217 keV/u, was between roughly -1–1%. However, the energy

spread predicted by the simulations in Figure 7.9 is between roughly -1.4–1.9%. This suggests

that the energy spread of the beam is not the only factor contributing to the discrepancy between

simulated and experimental data, but it was the dominant factor.

Figure 7.10a shows the projection of the x axis of the PID plot, which shows the uncalibrated

energy peaks of the isotopes detected. The strongest peak was attributed to the A=94 isotopes,

94Mo and 94Zr; the surrounding peaks are clearly distinguishable from this dominant peak, but

their intensity is at least a factor of 10 lower. This shows that the beam was delivered with

good selection of the desired mass. Figure 7.10b shows the projection of the y axis of the PID

plot, which shows the separation in atomic number. It was decided previously that the strongest

peak has A=94; in this plot of atomic number a double peak is observed, which was attributed

to 94Mo and 94Zr, with 94Mo having the higher intensity. Being able to separate these two

peaks, which have ∆Z=2, shows how powerful this detector is as a diagnostic tool. Following

68


(a)

(b)

Figure 7.10: (a) This figure shows the projected x axis from the 94Sr beam, showingthe separation in energy of the different isotopes observed. Whereas (b) shows theprojected y axis for the 94Sr beam and shows the separation in atomic number of theobserved isotopes, in particular there is some separation between 94Zr and 94Mo whichhave ∆Z=2.

the A=94 peak there is a low intensity peak, which was contributed to 107Ag; according to the

PID plot, and the projected x axis, two peaks should follow, 113In and 119Sn, but only one

peak is observed. This suggests the TBragg was unable to provide separation at this level, with

the short parameters used in this experiment. Table 7.1 details the resolution achieved for both

experimental and simulated data for this experiment. It is clear that this was the most successful

experiment conducted thus far with the TBragg, exceeding the resolution achieved with other

Bragg detectors mentioned throughout this thesis.

The Z resolution for this experiment was between 1/67–1/75, whereas, the average energy

resolution was between 0.9–1.7%. When comparing these values to other Bragg detectors, the

Z resolution of this detector is very impressive, and exceeded expectations; the original Munich

69


Bragg detector has an energy resolution of 0.6% and a Z resolution of 1/45 [10]. Whereas, the

Bragg detector used by J.M. Asselineau [38] has an energy resolution of 0.8% and a Z resolution

of 1/37. The TBragg was clearly used much more efficiently in this experiment, compared to

the 18O experiment; both of these experiments provided a lot of information about the TBragg,

and this knowledge was used in future experiments to ensure the capabilities of the TBragg were

constantly improved.

Table 7.1: This table presents the energy and Z resolution obtained for both experi-mental and simulated data for the stopping of a 94Sr beam in P-8.

Isotope Z † E †† z † † † E † † †Resolution

Z E (%)

Exp

erim

enta

l

69Ga 31 281.4 31.200(4) 286.49(4) 0.01450(70) ≈ 169 1.3900(80)

94Zr 40 381.9 40.500(3) 384.10(3) 0.01490(30) ≈ 167 1.6200(30)

94Mo 42 382.8 41.800(1) 380.95(1) 0.01430(4) ≈ 170 1.6200(5)

107Ag 47 434.7 46.500(10) 431.35(9) 0.01370(80) ≈ 173 1.3000(90)

113In 49 459.2 49.200(8) 454.96(10) 0.01490(50) ≈ 167 1.8400(60)

119Sn 50 484.1 49.500(1) 477.16(1) 0.01490(9) ≈ 167 1.5400(9)

132Xe 54 537.2 54.600(10) 547.04(10) 0.01350(80) ≈ 174 1.1900(90)

Sim

ula

ted

69Ga 31 281.4 30.50(3) 272.43(3) 0.012(7) ≈ 185 0.10(8)

94Zr 40 382.8 39.07(5) 370.49(10) 0.013(5) ≈ 178 0.33(6)

94Mo 42 381.9 41.85(5) 367.46(3) 0.013(5) ≈ 175 0.08(6)

107Ag 47 434.7 47.19(4) 418.75(5) 0.009(5) ≈ 1111 0.11(5)

113In 49 459.2 48.93(5) 442.15(5) 0.010(5) ≈ 1101 0.13(5)

119Sn 50 484.1 49.76(3) 467.15(6) 0.007(4) ≈ 1136 0.14(5)

132Xe 54 537.2 54.78(7) 518.19(5) 0.013(4) ≈ 180 0.10(4)

† Atomic number†† Total energy after passing through the Mylar window† † † Calculated from calibrated experimental and simulated data respectively

70

Chapter 8

Conclusions

At TRIUMF a post accelerator, consisting of an RFQ, DTL and SC-Linac, is used to ac-

celerate high mass radioactive beams above the Coulomb barrier; and has an A/Q acceptance

of < 7, for ions with mass < 30. Therefore, charge breeding is required to reduce the charge

state of the beam constituents; this is achieved using an ECR ion source. However, it introduces

high levels of stable contamination into the beam. A combination of stripping foils and bending

magnets are used to purify the beams, by the A/Q value of the constituents. The main purpose

of the TBragg is particle identification of these cocktails beams. Not only is it essential for many

experiments to know the exact composition of the beam, but this data will also enable more

efficient tuning of the beam lines.

The TBragg was a new detector at TRIUMF, arriving at the beginning of my placement, and

it was tasked to me to set up and test the TBragg ready for the radioactive beam experiment. It

required a gas handling system that could handle four different gas types, and it had to control

the pressure inside the chamber for prolonged periods of time. With regards to the DAQ, pulse

shape analysis methods had to be understood and optimised to ensure the detected isotopes

were correctly identified.

Alongside preparing the TBragg for experiments, it was necessary to obtain simulated data.

This part of the project was entirely my own, and through combining the capabilities of SRIM,

TRIM, Catkin and Excel I developed simulated PID plots for each experiment, and ensured

the ideal gas and operating values were used. The simulations also provided a rough guide to

choosing the shaping parameters for pulse shape analysis. Even though the simulations adopt

a simplified view of the data collection process, the results produced from the simulations have

shown strong comparison to experimental data. From the simulations an expected Z resolution

71

Conclusions

of the TBragg was 1/135, and the expected energy resolution was 0.15%. Comparing this to

the original Munich Bragg detector, currently used at REX-ISOLDE in CERN, which has a Z

resolution of 1/45 and an energy resolution of 0.6%. It was clear the simulations were overly

optimistic, due to assumptions in the simulations of the data collection process and of the beam.

Following initial testing of the TBragg with alpha sources, the first in-beam experiment

was conducted; scattering an 18O beam, accelerated to 1.5 MeV/u, on a gold, LiF and carbon

target. The aim of this experiment was to show that using a non-flammable gas (P-8) would

not degrade the resolution achieved when using a flammable gas (P-10), which was successfully

proven. The best energy resolution achieved during this test was 6.77% for P-10 and 6.66% for

P-8, and a Z resolution of 1/36 for P-10 and 1/37 for P-8. When compared to the resolution

of the simulations and the other Bragg detectors shows there is room for improvement in the

resolution obtained in this experiment. However, these results do support the fact that the use

of non-flammable gases does not reduce the resolution of data collected.

The second experiment was conducted with the radioactive beam, where the TBragg was

one of four diagnostic devices utilised to monitor the purity of the 94Sr beam (accelerated to

4.21 MeV/u). The TBragg identified seven stable isotopes in the beam, with an average Z

resolution of 1/70 and energy resolution of 1.5%. The absence of any radioactive components

was an indicator that the beam lines had not been optimised for the delivery of 94Sr to the

ISAC-II facility, and highlighted the necessity for the TBragg at TRIUMF, as it is the only

radiation hard detector capable of such high resolving power.

In conclusion, the aim of this project was to test and optimise the TBragg, for a variety of

gases and isotopes, in preparation for the radioactive beam experiment; this aim was fulfilled

and provided results that in many ways exceeded expectations. An additional consideration

throughout this project has been the necessity for the TBragg to ultimately be used remotely.

In order to satisfy this requirement a number of interlocks were integrated into the EPICS

system. Furthermore, it was proven that non-flammable gases are just as efficient as flammable

gases; thus removing the necessity for flammable gases, which require constant supervision when

in use. Following this 94Sr beam experiment I was involved in a number of other experiments,

with the TBragg. The TBragg has become a particularly useful tool at TRIUMF not only due to

its high resolving power, but also the fact it is simple to operate and the necessary information

is easily viewable online during the experiment, and offline for further analysis.

72

Conclusions 8.1. Future Work

8.1 Future Work

My aims for this project were completed, with the TBragg detector available to use, relatively

remotely, in the future by the accelerator group. In spite of this I feel that there is still more

of the TBragg that should be explored in order to possibly achieve higher resolution. My main

role within the TBragg group was to provide the simulations; even though they were successful

in providing a good comparison to experimental data, the method adopted was not the most

efficient, with hindsight. It has inspired me to develop my skills in computer programming, to

make future work not only more efficient but also user friendly; given more time at TRIUMF the

next stage would have been to build a simulation code. With regard to improving the resolution

of the detector there are two aspects that I believe would achieve this. First, the entrance

window is mounted to a supporting grid, which reduces the intensity of the beam and could

cause further energy loss of the ions, and possibly some scattering. For this reason I feel that by

removing this supporting grid and experimenting with the thickness of the Mylar window may

help to improve the energy resolution. Furthermore, the simulations provided some guidance

for setting the short and long parameters for the trapezoidal filters, additionally, the expected

rise time of the signals is typically calculated and the filter parameters are selected accordingly.

However, I believe it may be beneficial to conduct an experiment with a few isotopes of varying

energies and atomic numbers, and possibly also a couple of different gases and pressure settings,

to ensure the filter parameters are fully optimised.

8.2 Concluding Remarks

This project has really enabled me to further my knowledge in the field of Nuclear physics,

although the TBragg is primarily a beam diagnostic tool it has provided me with opportunities

to get involved with other experiments throughout the year. Joining this project from the very

beginning has been an enriching experience, as I have been able to take control of all aspects

of the project; including organising weekly meetings, communicating with people outside of the

TBragg group like R. Openshaw who developed the gas handling system, and setting up the

TBragg for upcoming experiments. Through being so involved I was able to operate the TBragg

and the gas handling system independently and I feel that my contribution to this project has

made a positive impact to the beam production process at TRIUMF.

73

Appendix A

Instructions for the TBragg GHS

At least one day prior to the start of the experiment, contact R. Openshaw or M. Goyette

to make arrangements to set up gas supply system, leak check and purge gas supply lines.

A.1 Operating with a non-flammable gas

Assumed starting condition: Diagnostic box filled with air and TBragg filled with air or

dry N2 at atmospheric pressure or greater. Gas supply lines purged and currently filled with

operating gas.

I. Pump diagnostic box to vacuum. (Check for gross leaks in diagnostic box, etc.)

II. Pump TBragg to vacuum

1. Ensure all plumbing between TBragg, GHS, gas supply panel, and nuclear exhaust is

connected correctly.

2. Close supply valve VA1 (EPICS).

3. Turn on both MFCs (channels 1 and 2 on MFC PS/Display box). (This ensures that any

contaminant gas in the tubing between VA1 and the valves of the MFCs is pumped out).

4. Close manual valve VB4.

5. Turn on pump SP1 (EPICS).

6. Once PA2 < 200 torr, open VA3 (EPICS).

7. Slightly open manual valve VB4 until PA1 pressure starts to rapidly decrease (typically

opening VB4 to where the micro-switch clicks). After PA1 pressure has significantly de-

74

Instructions for the TBragg GHS A.1. Operating with a non-flammable gas

creased, and PA2 pressure is comfortably below 100 torr, slowly open manual valve VB4

to the completely open position. If VB4 is opened too quickly, the gas pressure surge at

the inlet to the pump causes PA2 > 200 torr, which trips off VA3. If this occurs, just close

VB4, reset VA3, open VA3, and try again.

8. Allow PA1 pressure to decrease to < 0.3 torr.

9. Close VA3 (EPICS), and ensure there are no large air leaks into TBragg (i.e. PA1 pressure

remains reasonably stable with time)

10. Open VA3 (EPICS).

III. Fill TBragg with non-flammable gas (Assumes gas supply lines from gas shack have been

purged and are currently flowing operating gas, and TBragg has been pumped out, leak tested,

and is currently at vacuum)

1. Ensure supply valve VB1 on gas supply panel is open.

2. Ensure VA2 (EPICS) is closed.

3. Ensure VA3 (EPICS) is open.

4. Close manual valve VB4, and (gently) close manual needle valve VN2.

5. Set desired TBragg operating pressure on PID.

6. Select appropriate MFC (’1’ for CF4, ’2’ for P-8) with manual valve VB2.

7. Turn on appropriate MFC channel on MFC PS/display box (’1’ for CF4, ’2’ for P-8)

8. Open VA1 (EPICS) PA1 pressure slowly increases.

9. When pressure approaches set point, slowly open VN2. Adjust VN2 such that flow sta-

bilises at 70 cc/min with PA1 pressure stable at set point value.

10. Once flow and pressure have stabilised, reset ’INTHV’ interlock (EPICS), turn on HV PS

and adjust to desired voltage.

IV. End of run procedure It is necessary that the TBragg pressure be no more than∼200 mbar

below the diagnostic box pressure at all times. Thus it is essential to bring the TBragg to

atmospheric pressure before venting the diagnostic box. (Assumes gas is currently flowing at

last used pressure in TBragg)

75

Instructions for the TBragg GHS A.2. Operating with flammable gas

1. Ensure HV supply is turned off.

2. If PA1 pressure currently >800 torr, skip to step 4.

3. Set PID set-point to 800 torr pressure starts increasing, when it reaches 800 torr go to

step 4

4. Close VA3 (EPICS).

5. Turn off pump SP1 (EPICS) and turn off MFC (MFC PS/display box).

6. Close VA1 (EPICS) and manual valve VB1 (gas supply panel

7. Contact R. Openshaw to shut down gas supply in the gas shack.

8. Vent diagnostic box to air (if desired).

A.2 Operating with flammable gas

Assumed starting condition: The diagnostic box and TBragg are both filled with air, at

atmospheric pressure. The gas supply lines have been purged with N2, and are currently filled

with N2 or operating gas.

I. Pump diagnostic box to vacuum. (Check for gross leaks in diagnostic box, etc.)

II. Pump TBragg to vacuum

1. Ensure all plumbing between TBragg and GHS is connected correctly

2. Ensure exhaust valve (VB6) on gas supply panel is open

3. Ensure MFCs are turned off (Channels 1 and 2)

4. Ensure VB4 is open

5. Turn on pump (SP1), and allow PA1 pressure to decrease to <0.3 torr

6. Turn off pump, and ensure no large air leaks into TBragg (i.e. PA1 pressure remains

reasonably stable with time)

7. Turn pump back on

76


III. Purge TBragg and GHS with nitrogen (N2) This step must be done before introducing

a flammable gas (P-10, isobutane) into the GHS, if the GHS and/or TBragg are currently, or

just recently, filled with air. There are two ways the N2 purge can be done. If the supply lines

from the gas shack are still flowing N2, use method III(a). If the supply lines from the gas shack

are flowing flammable gas, use method III(b).

III(a). Purge from supply lines running N2

1. Ensure all plumbing between the GHS and the gas supply panel are connected correctly.

2. Ensure supply valve (VB1) on gas supply panel is open

3. Ensure VA2 is close (ball valve on air line)

4. Open VA1 (EPICS)

5. Close VB4 and (gently) close VN2

6. Set desired TBragg operating pressure to 760 torr on PID

7. Select appropriate MFC (’1’ for isobutane, ’2’ for P-10) with VB2

8. Turn on appropriate MFC channel (’1’ for isobutane, ’2’ for P-10) PA1 pressure slowly

increases

9. When pressure approaches set point, slowly open VN2. Adjust N2 such that flow stabilises

at 70 cc/min. Allow N2 to flow for 34 minutes

10. Turn off selected MFC channel.

11. Open VB4 and allow pump to reduce PA1 pressure to <0.3 torr

12. Proceed to section IV

III(b). Purge from nitrogen (N2) cylinder through VA2

1. Ensure VA1 is closed (EPICS), and VA2 is closed (ball valve on air supply line to VA2).

2. Ensure plumbing between GHS and N2 cylinder is connected correctly.

3. Open N2 cylinder and regulator valves. Ensure regulator is set to 10 PSIG.

4. Open VA2 (ball valve on air supply)

77



6. Set desired TBragg operating pressure to 760 torr on PID



increases


at 70 cc/min. Allow N2 to flow for 34 minutes

10. Turn off selected MFC channel.

11. Open VB4 and allow pump to reduce PA1 pressure to <0.3 torr

12. Close VA2

IV. Fill TBragg with operating gas (Assumes gas supply lines from gas shack have been

purged and are currently flowing the desired gas, if necessary, assumes TBragg has been purged

with N2)

1. Ensure all plumbing between GHS and gas supply panel is connected correctly

2. Ensure supply valve (VB1) on gas supply panel is open

3. Ensure VA2 is closed (ball valve on air supply line)

4. Open VA1 (EPICS)


6. Set desired TBragg operating pressure on PID



increases


at 70 cc/min.

78

Instructions for the TBragg GHS A.3. Setting Pressures and Flows

10. Once flow and pressure have stabilised, turn on HV and adjust to desired voltage. (Note,

you may have to reset the INTHV interlock on the EPICS screen.)

V. End of run procedure if using flammable gas in TBragg

1. Turn off MFC channel (whichever one selected by VB2) input flow stops

2. Open VB4 and pump out TBragg to <0.3 torr


4. Turn off pump

5. Set PID to set point to 800 torr

6. Ensure tubing connecting GHS to N2 cylinder is OK, cylinder pressure regulator set to

10 PSIG, cylinder and regulator valves open.

7. Close VA1 (EPICS screen) and open VA2 (ball valve on air line)

8. Turn on selected MFC channel, pressure begins to increase

9. When pressure reaches 800 torr, turn off MFC, close VA2 and cylinder valves

10. Close VB1 (gas panel supply). Before closing VB6 (gas panel exhaust), ensure that the

pump is off by unplugging it. After closing VB6 *immediately* disconnect the pump

exhaust tube from the pump. Running the pump exhaust into a closed pipe will destroy

the scroll tips on the pump.

11. Contact R. Openshaw to purge the gas lines from the gas shack with N2.

12. If the TBragg is to be left in this state for a long (days) time, remove the gas supply tube

at the TBragg to leave the detector vented to air.

13. Vent diagnostic box to air

A.3 Setting Pressures and Flows

Once the TBragg has been purged, and operating gas flow established, the only activity most

users will need to do, is to change the operating pressure as required to obtain good data from

the TBragg. The following is a brief description of the procedure for setting TBragg pressure

and gas flow rate:

79

Instructions for the TBragg GHS A.3. Setting Pressures and Flows

1. Turn off HV power supply.

2. Press the Index button on the PID controller.

3. Press and hold the up or down arrow button on PID controller until desired pressure is

displayed. (Note: the functional set-point wont change until Enter is pressed in step 4

below).

4. Press the Enter button on PID controller.

5. Press the Index button to return to the normal PID display.

6. As the TBragg pressure (displayed on the PID controller and also on PA1 (EPICS))

converges on the set-point pressure, adjust manual valve VN2 to restore flow to ∼70

cc/min with pressure stabilised at the new set-point pressure.

7. Reset INTHV interlock (EPICS).

8. Turn on HV power supply and adjust to desired operating voltage for the newly set TBragg

pressure.

80

Appendix B

EPICS interlocks

Figure B.1: TBragg EPICS interlock logic

81

Appendix C

CUPC Abstract

The experiments discussed in the framework of this thesis were first presented during the

Exotic Beam Summer School (EBSS), held this year at Argonne National Laboratory (ANL), as

a poster presentation. Following this, the data was presented at the Canadian Undergraduate

Physics Conference (CUPC), held this year at the University of British Colombia (UBC), as an

oral presentation. The abstract for this conference has been provided below.

ABSTRACT

The TRIUMF Bragg ionisation chamber (TBragg) is used for beam tuning and diagnostics.

Incident beams are stopped within the TBragg, and ionized electrons are drifted along the beam

axis and collected at the anode. A particle identification plot can be produced from applying two

trapezoidal filters to the signals. One filter samples the rise time, which contains information

about the proton number; and the other samples the height of the signals, which corresponds

to the total energy.

Prior to experiments, this process of identifying isotopes from their energy loss is simulated

to provide information on ideal experimental parameters such at the gas type, pressure and

voltage. Two in beam experiments have been conducted this year. The first with an 18O

beam scattering on a 12C target, the main aim of this experiment was to provide experimental

evidence to show non-flammable P-8 gas can be used to replace flammable P-10 gas, without

having to compromise on resolution. The second experiment was conducted with 94Sr beam,

the TBragg was one detector in a series of detectors designed to test beam quality. The design

and performance of the TBragg detector will be discussed in this talk.

82

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