Semina.Serienee.. · The theory of the neutrons scattering by atomic nuclei : ... Fundamentals...

Pierre-François.Lory - Felix.Kandzia - Simon.Wulle.

17.03.2015

.Seminar.Series.“All.you.need.is.Neutrons”

Neutron.Detectors.and.Data.Acquisition

OUTLINE

• 1/ Introduction – Outline – Neutrons detectors concepts – Applications

• 2/ Technologies of detectors

– Gas proportional counters and ionization chambers : Technology concept, advantages/inconvenience and ILL instruments

– Scintillation detectors : Technology concept

– Neutron image plate detectors: Technology concept and example VIVALDI

– CCD Camera Detector : Technology concept and example Orient-express

• 3/ Data acquisition 2

3

Neutron Applications

1. To give a measure of the power in nuclear

reactors (Hot neutrons)

2. To allows the characterization of the structure

and/or dynamic of materials

3. Radioprotections : the relative biological action

4. Detection of fissile materials : 233U and 239Pu

5. The observatories of neutrinos

6. Detection cosmic radiation

Neutron Detectors

• What does it mean to “detect” a neutron?

– Need to produce some sort of measurable

quantitative (countable) electrical signal

– Can’t directly “detect” neutrons

• Need to use nuclear reactions to “convert”

neutrons into charged particles

4

Cold neutrons Thermal neutrons Hot neutrons

20 K 300 K 1500K

1.724 meV 25.86 meV 130 meV

Elastic scattering Absorption of neutrons

5

Scattering cross-section

The theory of the neutrons scattering by atomic nuclei :

- We considered the neutrons as limited (10-17 cm)

-The potential of interaction is spherical

-The typical value of the effective scattering section is

of 10-24 cm2 = 1 barn

- Coherent scattering cross-section : σcoh = 4πb2

σtot = total number of

neutrons scattered per

second / Φ

Ref : Squires, G. L. (2012). Introduction to the theory of thermal neutron scattering. Cambridge university press.

Cross-section

6

Ref : Database NEA N ENDF/B-VII.1 using janis software

Fundamentals Handbook, Nuclear Physics and Reactor Theory, DOE-HDBK-1019/1-93

Cross-section

Reaction ratio

Neutrons

Neutrons interactions

• Lower energy neutrons (thermal or near thermal) are likely to undergo absorption reactions with atoms in their environment

• Fast neutrons are most likely to undergo scatter interactions with atoms in their environment

- Elastic Scatter dominate for lower energy of the fast neutrons

- Inelastic Scatter < 1 Mev

7

Slow-Neutron Capture reaction

1/ Neutrons is absorbed by particles (large σ)

2/ Production of heavy and light particles (3H and 1H)

Particles share the reaction energy Q inversely

according to their masses 8

n + 3He → 3H +1H M(3H) = 3.0160492 u M(1H) = 1,0072765 u

Fast neutrons

• Probability to detect a neutrons depend on σ and the neutrons energy; If En increase σ is low

Solution 1: Need to reduce the neutron velocity (energy), Use a material to moderates, to slows down the fast neutrons => to diffuse the neutrons (polyethylene layer)

Solution 2 : The Elastic scattering processing => advantages the kinetic energy is conserved

9

10

Fast neutrons - Elastic scattering

• Elastic scattering and neutrons with nuclei of the

gas, i-e production of recoil protons

• Recoils nucleus energy in terms of its own angular

recoil and atomic mass A is given by :

Er /En= [4A/(A+1)2] cos2(θ)

• A = mass of target nucleus

• En = energy incident neutrons

• Er = Recoil nucleus kinetic

energy

• θ = scattering angle of the

recoil nucleus

11

Target nucleus Er/En|max = 4A/(1+A)2

1H 1

2H 0.0889

3He 0.750

4He 0.640

12C 0.284

16O 0.221

• The maximum fractional

energy transfer

increases as the mass

of target nuclei decrease

• Nuclei with lower mass

are more effective on a

“per collision” basis for

slowing down neutrons

Er /En= [4A/(A+1)2] cos2(θ)

Fast neutrons - Elastic scattering

12

Neutron Detectors Materials

Atomic number name xs(barns) 10 Technology Remarks

64 Gd157 259000 Scintillation detectors γ-ray

64 Gd155 61100 Scintillation detectors γ-ray

64 Gd 49700 Scintillation detectors Oxidized by the water, toxicity,

Monazite, γ-ray

62 Sm149 42080 - No abundant

48 Cd113 20600 - Toxicity, expensive

63 Eu151 9100 - No abundant

62 Sm 5920 control rod of nuclear

reactors Abundant (40th)

2 He3 5330 Gas proportional Efficiency (<90%)

63 Eu 4530 - No abundant

5 B10 3840 Scintillation detectors Stable, available

80 Hg196 3080 - Toxicity

76 Os184 3000 - Abundant, stable

66 Dy164 2840 - Expensive

48 Cd 2520 sample environment Product of zinc, toxicity

70 Yb168 2230 - Reaction with air

80 Hg199 2150 - Toxicity

71 Lu176 2070 - No pure

94 Pu239 1020 - Dangerous

66 Dy 994 - Expensive

77 Ir191 954 - Radioactive material

3 Li6 940 Scintillation detectors Expensive, No abundant

5 B 767 Scintillation detectors Toxicity

64 Gd152 735 - Toxicity and γ-ray

92 U235 681 Nuclear reaction Dangerous

Ref : nMoldyn database, NIST and web-element

Nuclear Reactions

13

o n + 3He → 3H +1H + 0.764 MeV o n + 6Li → 4He + 3H + 4.79 MeV o n + 10B → 7Li* + 4He → 7Li + 4He +2.31 MeV+ γ (0.48 MeV) (93%) → 7Li + 4He +2.79 MeV (7%) o n + 155Gd → Gd* → γ -ray spectrum + conversion electron spectrum (~70 keV) o n + 157Gd → Gd* → γ -ray spectrum + conversion electron spectrum (~70 keV)

o n + 235U → xn + fission fragments + ~160 MeV (<x> ~ 2.5) o n + 239Pu → xn + fission fragments + ~160 MeV (<x> ~ 2.5) o 197Au(4.906 eV), 115In( 1.46 eV), 181Ta(4.28 eV), 238U(6.67, 10.25 eV); Remarks: energy-selective detectors, narrow resonances, prompt capture gamma rays

Neutron Detectors

1. Gas Ionisaiton Detectors

2. Scintillators

3. Semiconductors

The General Detection Process

The principle of ’detection’

I radiation → interaction → response → analysis

The General Detection Process

The principle of ’detection’

I radiation → interaction→ response︸︷︷︸detector

→ analysis︸︷︷︸readout

Why is spectroscopy so difficult for neutrons?

I wide energy range (meV - MeV)

I interaction (no e.m.)

I strongly energy dependent/low cross sections

Response function

dN

dH=

∫R(H,E )S(E ) dE (1)

H...pulse height,R...response function, −→S ...spectrum of the radiation

dN/dE

d E

Response function

dN

dH=

∫R(H,E )S(E ) dE (1)


dN/dE

d E

in practice: energy bins

Ni =∑j

RijSj i = 1, ..,M; j = 1, .., L (2)

Response function

dN

dH=

∫R(H,E )S(E ) dE (1)


dN/dE

d E

in practice: energy bins

Ni =∑j

RijSj i = 1, ..,M; j = 1, .., L (2)

I Rij diagonal for response functions with small distributions →proportionality

I wide/ irregular response functions → deconvolution necessaryI M ≥ L: M linear equations, straightforward solutionI M < L: no unique solution → ’best estimate’

Discovery of the neutron

J. Chadwick (1932):

I α + 9Be → 12C + n

I detection of recoil nuclei inI cloud chamberI gas ionisation chamber

I electric neutral particles withmass ≈ mp

Science & Technology Facilities Council,http://www.stfc.ac.uk/2685.aspx

,

Discovery of the neutron

J. Chadwick (1932):

I α + 9Be → 12C + nI detection of recoil nuclei in

I cloud chamberI gas ionisation chamber

I electric neutral particles withmass ≈ mp

Science & Technology Facilities Council,http://www.stfc.ac.uk/2685.aspx

J.Chadwick, The Existence of a Neutron,in: Proc. Roy. Soc., A, 136, p. 692-708, 1932

Proportional counters / ionisation chambers

I ionising particles produce e−/ionpairs

I drift to electrodes, due to ~E -field

⇒ charge induced

I |~E | = Vr ln(ro/ri )

⇒ main amplification near anode

I charge multiplication depends onapplied voltage

I normally used in proportional mode

Glenn Knoll, ”Radiation Detection and Measurement”,Wiley & Sons

Counting gas

I counting gasI good charge multiplicationI short drift times

I quenching gasI keep avalanche localI ’reset’ counter after signal

⇒ often gas mixture used

I chemically stable/ durable against radiation

I e.g. 3He + CF4

Counting gas

I counting gasI good charge multiplicationI short drift times

I quenching gasI keep avalanche localI ’reset’ counter after signal

⇒ often gas mixture used

I chemically stable/ durable against radiation

I e.g. 3He + CF4

I Neutron sensitive material:I counting gas (3He, BF3 (toxic))I coating of the inner wall (BN, uran oxide, ...)I plates within the gas volumeI recoil nuclei (fast neutrons only)

He-3 counter

I 3He + n −→ 3H + p + 764 keV

↙ ↘191 keV 573 keV

I slow neutrons:I deposited energy = Q-valueI range p (@ 1 atm) ∼ cm⇒ ’wall effect’

I fast neutrons:I deposited energy = Q-value + recoilI epithermal peak caused by

moderated neutrons

He-3 counter

I 3He + n −→ 3H + p + 764 keV

↙ ↘191 keV 573 keV

I slow neutrons:I deposited energy = Q-valueI range p (@ 1 atm) ∼ cm⇒ ’wall effect’

I fast neutrons:I deposited energy = Q-value + recoilI epithermal peak caused by

moderated neutrons

I background discrimination: Glenn Knoll, ”Radiation Detection and Measurement”

I gammas are indirectly ionising (e−)I range fast electrons ∼ 10 cm⇒ deposit only small fraction of their energy⇒ discrimination via pulse height

Diffractometer D2B

I 128 3Hepositionsensitivedetectors(PSD)

I spacing 1.25◦

I scans insteps of0.05◦

Position Sensitive Detectors

I objective: get position information

I charge divisionI x = L · Qa

Qa+Qb

I Drift ChamberI several anode wiresI cathode wires/stripes perpendicular

I Microstrip DetectorI alternating strip electrodesI conducting material sputtered on

substrateI electrodes etched into surfaceI modular structure


I objective: get position informationI charge division

I x = L · Qa

Qa+Qb




---+ +

+L

a

b



I x = L · Qa

Qa+Qb




---+ +

+L

a

b

anode wires

cathode

cathode



I x = L · Qa

Qa+Qb




---+ +

+L

a

b

anode wires

cathode

cathode

cathodes anodesconductingglass substrate

D19, D20

I D19I drift chamberI 2 dim. (120◦x30◦)I resolution: 0.19◦ horiz.

0.12◦ vert.

I D20I microstrip detectorI 1 dim. (154◦), 4 m x 15 cmI 48 plates a 32 cells

(a 2.6 mm = 0.1◦)

Time of Flight Measurements

I preconditionI well defined flight pathI well known starting moment

}⇒ (artificially) pulsed sources

I Example: IN5

I TOF after scattering depends onenergy transfer to sample

I (P) 2, counter rotating,create pulse

I (CO) 1, reduces bandwith

I (FO) 1, can be used tosuppress pulses (avoidpile-up)

I (M) 2, counter rotating,chop a narrow bandwidth

Fast Neutron Spectroscopy - Bonner Spheres

I (slow) neutron counter surrounded with moderating material

I moderator thickness defines sensitivity to different neutron energies

Fast Neutron Spectroscopy - Bonner Spheres

I (slow) neutron counter surrounded with moderating material

I moderator thickness defines sensitivity to different neutron energies

Ni =∑

j RijSj

Scintillation process

I radiation causes lightemission

I nb photons ∼ incidentenergy

Scintillation process

I radiation causes lightemission

I nb photons ∼ incidentenergy

I anorganic:

valence band

conduction bandexciton 'band'

impurities

I production of excitons

I organic scintillation process:

S

S*

I excitation of (aromatic)molecules

Scintillator materials

I anorganic crystals (LiI(Eu))

I organic crystalsI organic liquids, plastics

I scintillating material dispersed in solvent/plasticI large, flexible in shapeI mostly wavelength-shifter required

I gasI also in liquid form (higher density)I UV light ⇒ wavelength-shifter

I glasI e.g. boron silicate

I neutron sensitivity:I doped with neutron sensitive materialI recoil nuclei (fast neutrons only)

Scintillators - recoil nuclei from fast neutron scattering

I applicable for most detectors

I best performance for H

⇒ use of hydrogen rich materials

(e.g. organic liquids)

I widely used: NE213

reminder:

Target Emax,n = 4A(1+A)2

11H 121H 8/9=0.8932He 3/4=0.7542He 16/25=0.64126 C 48/169=0.28168 O 64/289=0.22

Pulse shape discrimination

I generally possible for organic scintillatorsI based on different types of excited states:

I singlet (spin 0): 1/√

2 (| ↑↓〉 − | ↓↑〉) , τf ∼ ns

I triplet (spin 1):

| ↑↑〉1/√

2 (| ↑↓〉+ | ↓↑〉)| ↓↓〉

, τs ∼100 ns

I population of the states depends on dEdx of particles

Applications

OrientExpressI scintillator:

I 6LiF with ZnS(Ag)I thickness: 0.45 mmI active area: 252 x 198 mm2

I CCD readout (res150 x 150µm)

I Laue diffraction pattern in a few sec

CCDImage Intensifier Lens

ScintillatorElectronics

Sealed Tube

Image Plate Detectors

I use of x-ray image plates (BaFBr, doped with Eu2+)

I Gd2O3 added for neutron conversion

I 1. exposure → 2. readout → 3. reset → exposure ...

(photostimulated luminescence)

Eu2+

valence band

conduction band

traps1

2

I Very Intense, Vertical-Axis Laue-DIffractometerI pixel size 100x100µm2 to 400x400µm2

I offline readout, ∼ 3.5 min

Semiconductors

I p-n junction

I applied voltage creates depleted area

A C

E

n+ p+n

I high density and atomic number

⇒ very sensitive to background

Semiconductors - neutron conversion

I boron layer (BN): n+10B → 7Li + α(∼1.5 MeV)

→ α-range ∼ 5µm

I partially/completely energy loss within conversion layer

I but: thin conversion layer ⇒ low efficiency

n

I fast neutrons: detect (n,p), (n,α) reactions

2

Data Acquisition – Main Goal

• Convert the events arriving at the

Detector into numerical or graphical

representation

• Direct analysis or storage

3

• Requirements in Neutron Detection

• Accurate Timing

• Minimal Dead-Time

• Handle High Data Throughput

• Handle High Event Time

Data Acquisition – Main Goal

4[1] Courtesy of the Instrument Control Service Group

[1]

Position and/or Trajectory

Particle

Detector

Timing Charge = Energy

Data Acquisition – Detector Signal

5

Digital

Analog

TTL

Address

Pulse-Shape

Current/Voltage

Phase

0x7f66

0 V

5 V

[1] Courtesy of the Instrument Control Service Group

[1]

Data Acquisition – Digital / Analog Approach

6

+-

Charge Sensitive

Preamplifier Shaping

Amplifier

Peak Sensing

ADC

Discriminator

Logic

Unit

TDC

Scaler

Threshold

Shaping Time,

Gain

Fast

Slow

Fast

Amplifier

-> Energy

-> Pos.

-> Time

-> Count


[1]

Data Acquisition – The Analog Chain

7

Data Acquisition – The Analog Chain

8

A/D DDP Interface

Energy

Time

Count

Position

Digitizer

Samples

IN

• Very High data throughput

• Requirement to reduce data flow to

relevant quantities

• -> Acquisition Modes


[1]

Data Acquisition – The Digital Chain

9

• A Trigger is generated when the signal amplitude crosses a certain

threshold values

• Noise can generate bad triggers

• Events can be missed because of signal pile-up

Data Acquisition – Timing / Trigger

10

• Using digital CR-Filter to transform into a bipolar signal

• Zero Crossing at the position of signal peak is detectable with high

precision


11

• Using digital Filters for noise suppression and base-line straightening

• RC-Filter for high frequencies

• CR-Filter for low frequencies


12

• Using a trapezoidal Filter transforms exponential decay

Exponential Decay

Time Constant (M)Rise Time (b)

INPUT

TRAPEZOID

Trapezoid Rise/Fall

Time (k)

Trapezoid Flat Top (m)

Flat Top DelayNumber of samples

for the peak average

Moving Average Window Size

for the calculation of the trap. baseline

peaking area


[1]

Data Acquisition – Energy / Pulse Height Analysis

13

• Using a trapezoidal Filter transforms exponential decay


[1]

ENERGY LIST

SAVE TO MEMORY (n pulses)

TIME

THRESHOLD

Data Acquisition – Energy / Pulse Height Analysis

14

• Advantages

• Simplicity: One module can provide energy, timing and pulse shape analysis

• Low Cost per Channel

• Easier to maintain

• In general lower dead time of acquisition system

• Easier to synchronize over several data channels

• Disadvantages

• Limited by sampling rate of the acquisition card , loss of resolution with fast

signals

• Requires extended knowledge of digital circuits

Data Acquisition – Digital vs. Analog comparison

15

• PowerPC based Cards using different plug-in modules for detector

acquisition

Data Acquisition – Acquisition Cards

• Main functions

• Collecting raw data from detector

• Reducing data according to acquisition

mode

• Output rearranged Data for live display

and storage

16

• Acquisition Mode depends on the requirements of the experiment

• Simple Count

• Simple Image of the detector with or without masking

• Timing Information not considered

• Time-of-flight

• Timing Information is used

• Events are arranged as a function of their travel time from source to detector

• Timescale: 100 ns to 100 ms

• Kinetic Mode

• Same principle as TOF

• Timescale: 100 ms to 100 s

• Investigating dynamic changes in the sample

Data Acquisition – Acquisition Modes

17

• Acquisition Mode depends on the requirements of the experiment

• Time-of-flight / Kinetic

• Advantages of both modes combined

• For each kinetic time slice the events are arranged according to their TOF

• Doppler Mode

• Special mode for Instruments with a Doppler drive

• DDP Mode

• Complete digital collection of events at the detector

• Energy and Time of each event is saved in a list

• Very high data consumtion

Data Acquisition – Acquisition Modes

Pierre-François.Lory - Felix.Kandzia - Simon.Wulle.

17.03.2015

Thank.you.for.your.attention0

Semina.Serienee.. · The theory of the neutrons scattering by atomic nuclei : ... Fundamentals...

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