Paul Sellin, Radiation Imaging Group Digital techniques for neutron detection and pulse shape...

Paul Sellin, Radiation Imaging Group

Digital techniques for neutron detection and pulse shape discrimination in liquid

scintillators

P.J. Sellin, S. Jastaniah, G. Jaffar

Department of PhysicsUniversity of Surrey

Guildford, UK

[email protected]

www.ph.surrey.ac.uk/cnrp


Contents Motivation for this work Pulse shape discrimination (PSD) in organic scintillators:

traditional PSD in liquid scintillators direct detection of neutron scatter events digital PSD algorithms

Results from the Surrey digital setup: Digital PSD from integrated and current pulses PSD Figure of Merit (FOM)

10B-loaded scintillator for fast neutron detection: review of capture-gated neutron detection in BC454 the use of BC523/BC523A boron-loaded liquid scintillators current status and limitations of a portable capture-gated neutron detector

New material developments Conclusions


Introduction

Emphasis on fast computationally-simple digital algorithms suitable for field instruments Efficient n/ discrimination is essential - the extraction of a weak fast neutron flux against a strong gamma ray background

Full-energy fast neutron spectrometry has particular advantages for dosimetry detectors:

Motivation for this work:

Development of digital neutron monitors for neutron field measurements, homeland security, and neutron dosimetry

Portable instruments can take advantage of compact digital pulse processing technology

See also: A. Rasolonjatovo et al, NIM A492 2002 423-433


Pulse shape discrimination

Pulse shape discrimination (PSD) in organic scintillators has been known for many years - particularly liquid scintillators (NE213 / BC501A)

PSD is due to long-lived decay of scintillator light caused by high de/dx particles - neutron scatter interactions events causing

proton recoils:

mean decay time


Integrated vs current pulses

Extraction of scintillation decay lifetime depends on the RC time constant of the external circuit:

Large time constant RC>>:

integrated pulse - event energy extracted from pulse amplitude

extracted from pulse risetime:

Short time constant RC<<:

current pulse - event energy extracted from pulse integral

extracted from pulse decay time:

RC for eC

Qt v

t

1 ) (

RC for e Q t vt

) (


Pulse risetime algorithms (1)

Integrated pulses - using a PMT preamplifier

Improved signal-noise ratio

Risetime limited by preamp (~10ns)

1. 10-90% risetime algorithm

Current pulses - anode connected directly to 50

Simple circuitry, fastest response

Two PSD algorithms have been investigated:

2. ‘time over threshold’ algorithm

Other techniques use a full least-squares fit to the pulse shape, eg. N.V. Kornilov et al, NIM A497 (2003) 467-478.

S. Marrone et al, NIM A490 (2002) 299-307


Pulse risetime algorithms (2)

3. ‘Q-Ratio’ algorithm

A digital implementation of the common charge integration PSD algorithm - the current pulse is integrated within a ‘short’ and a ‘long’ time window

eg. D. Wolski et al, NIM A360 (1995) 584-593

Advantage of this technique compared to ‘Time over Threshold’ is that all the data in the pulse is sampled

better S/R ratioThe Q-Ratio ‘signal amplitude’ A is:

PSD parameter is:

2

0

)(tt

tt

dttiA

A

dtti

PSD

tt

tt

1

0

)(


Digital PSD on inorganic scintillators

Digital implementations of PSD algorithms have been already applied to commercial systems, suitable for slower inorganic scintillators

Eg. The XIA digital data acquisition system, sampling at 40 MHz, time interval 25 ns.

See: W. Skulski and M. Momayezi, NIM A458 (2001) 759-771

photon interaction in silicon photodiode

scintillation interactions in CsI(Tl)


XIA performance

Simple rise time inspection gives reasonable , separation

More sophisticated algorithms allow good discrimination of p, ,


Other PSD techniquesOther techniques use a full least-squares fit to the pulse shape:

eg. by de-convolution of the scintillator light pulse from the detector response function:

N.V. Kornilov et al, NIM A497 (2003) 467-478.

''' )(),()( dttfttrts

where s(t) is the measured pulse signal, r(t,t’) is the detector response function, and f(t’) is the scintillator light pulse

s(t) expt data and fit

PMT response function

This technique is computationally intensive and not suitable for portable instruments


Least square fitting of scintillator pulses

Fast digital sampling of liquid scintillators has been combined with full linear-regression curve fitting:

S. Marrone et al, NIM A490 (2002) 299-307

• Convolution of the detector response function with a single exponential decay term does not fit the observed pulse shapes• a two-component exponential function is required:

• a complex iterative fitting procedure is required to optimise all 6 free parameters very computationally intensive

)(

)()()()(

)()(

00

00

tttt

tttt

L

S

eeB

eeAts


Direct discrimination of fast neutrons

In principal, direct discrimination of fast neutrons can be attempted by observing the time delays between fast neutron scatters.

This has been reported by Reeder et al, NIM A422 (1999) 84-88. 1 MeV neutron travels at 5% of c, with a 90% chance of

interaction in 10cm of plastic scintillator Time delay between 1st and 2nd neutron scatter is ~3 ns 1 MeV gamma has mean free path of ~13 cm, with a flight time

of 0.45 ns The fast neutron pulse in plastic

scintillator should be broader thanfrom gammas

Technique need as fast digitiserwith nanosecond timing.

Graph shows calculated average time between hydrogen recoils vs neutron

energy


Requirements for the direct technique

Reeder’s method used a digital oscilloscope to capture pulse shapes - direct record of fast neutron scatters prior to significant moderation.

Better efficiency that ‘capture gated’ methods since only 2-3 scatters are required - the neutron can then escape from the scintillator.

Requires timing resolution ~1 ns or better Single neutron scatter events cannot be distinguished from

gammas 252Cf time-of-flight system used to provide tagged 1 MeV

neutrons


Results of direct discrimination

Results: average width of 100 gamma pulses: 3.3 ns average width of 100 neutron pulses: 3.5 ns

Why are the gamma pulses so broad (not expected by MCNP studies)?• Fast light pulses directly into PMT gives width ~1.4ns• single photon fluorescence confirmed plastic decay time• scintillator shows asymmetric pulse shape which washes out the expected time differences


The Surrey waveform digitiser system

Single channel specification:

8 bit resolution

1 GS/s, 500 MHz

2 Mpoints waveform memory

80 MB/s sustained data transfer rate to PC

(12 bit cards, up to 400 MS/s also available)

Custom LabView software for real-time pulse analysis and histogramming

High speed waveform digitisers now provide 1ns sampling times (1 GS/s), 8 bit resolution, high speed data transfer to PC:

We use the Cougar system from Acqiris - www.acqiris.com

4 channel compactPCI crate-based system, expandable up to 80 channels


Detector Cells

PSD measurements were initially made with small-volume (100 ml) commercial cells, containing BC501A (no boron) and BC523A (5% 10B enriched)

A similar size cell of BC454 plastic was also studied (5% natural boron, ~1% 10B)

A larger 700 ml cell was the constructed to investigate capture-gated neutron detection. This cell included an embedded 30mm diameter BGO scintillator

When filling the cells, the scintillator was bubbled with N2 gas to purge the oxygen.

A fume cupboard is required, and careful adhesion (Torrseal) of the glass window to the metal canister is necessary to prevent evaporation/leakage


10B capture peak

Typical pulse height spectrum from a BC523A cell, acquired with the digital data acquisition system:

Channel number

0 20 40 60 80 100 120 140 160 180 200

Cou

nts

1

10

100

1000

10000

Boron-10 thermal neutron capture peak at 60 keVee

Proton recoils due to fast neutronscattering, and -rays events

The 10B capture peak is observed at 60 keV electron-equivalent energy.


Energy Calibration

Liquid scintillator operated at 2 gain settings, with separate energy calibrations:

High Gain: photopeak for X/-rays

< 60 keV:Ba, Tb K X-rays

241Am -ray

Low Gain: Compton edge for high

energy -rays:57Co137Cs60Co

44 keV Tb X-ray8-bit digital DAQ

44 keV Tb X-ray12-bit analogue DAQ


Digital DAQ calibration

low energy photopeak calibration

high energy Compton edge calibration

typical photopeak spectra- 8 bit digital system


PSD at low gain

Risetime versus pulse height plot at low gain setting showing n/ PSD from (a) BC501A, and (b) from BC523A.


No PSD in plastic BC454

We also tested PSD in plastic scintillator BC454 - no discrimination was seen for neutron scatter events

all events


PSD at high gain

At high gain, the 10B capture peak is visible due to simultaneous detection of 7Li and no significant PSD is observed

Lack of PSD is due to quenching of slow component from heavy ions - limited PSD has been seen in ‘special’ 10B-loaded scintillator

S. Normand et al, NIM A484 2002 342-350


PSD Figure of Merit

Quality of PSD is described using a Figure of Merit (FOM):

Vertical ‘slices’ from the 2D spectra give risetime histograms:

0 25 50 75 100

Cou

nts

0

500

1000

1500

2000

2500

3000

3500

4000

Rise time (ns)

0 25 50 75 1000

200

400

600

800

1000(a) (b)

FF

SFOM

n

n

low energyFOM = 1.4

high energyFOM = 1.5Method is similar to

conventional analogue PSD techniques

FOM is extracted digitally in software

FOM>1 required for ‘good’ PSD

n

Sn = separation of two peaksFn, = n, peak centroid position


PSD from current pulses (1)

‘Time over Threshold’ current pulse algorithm - the 2D plot has a different shape

FOM is slightly worse than for integrated pulses with poorer valley separation, particularly at low signal amplitude

0 50 100 150 200 2500

20

40

60

80

100

120 BC501A

Neutron events

Gamma ray events

> 100 counts

Pulse height (a.u.)

Ris

e tim

e (n

s)

5 -- 100 0.2 -- 5 0.01 -- 0.2


PSD from current pulses (2)

‘Q-Ratio’ current pulse algorithm - the 2D plot has well separated locii across the full energy range

PSD performance at low signal amplitude is considerably better than ‘time over threshold’ algorithm


FOM plots from Q-Ratio algorithm

(a) Low energy gate

QT/QP

0 50 100 150 200 250 300

Co

un

ts

0

200

400

600

800

1000

1200

1400

1600

(b) High energy gate

0 50 100 150 200 250 300

Co

unt

s

0

100

200

300

400

500

FOM values are 1.1 for both energy ranges - the Q-ratio algorithm gives better overall PSD performance for current pulses


10B loaded liquid scintillator

We have investigated liquid scintillator enriched with 10B - BC523A

Often used for thermal neutron detection, 10B-loaded scintillator can also be used for ‘capture-gated’ neutron spectroscopy:

Fast neutron spectroscopy routinely measures the energy of proton recoil events:

NRMAX EA

AE

2)1(

4

where ERMAX is the maximum recoil energy of nucleus with atomic mass A

For protons, A=1 and ERMAX=EN


Capture gated timing signals

The method of ‘capture-gated’ neutron spectroscopy uses the technique of ‘moderate + capture’. If moderation occurs within the active detector, the full energy of the neutron EN can be uniquely measured

Characteristic double-pulse sequence of moderation + capture provides clean fast neutron signature.

Capture pulse has fixed amplitude (10B+n Q value)

Amplitude of moderation pulse gives incident neutron kinetic energy

true ‘full energy’ neutron spectrometer

Neutron capture: n + 10B 7Li* + + 478 keVQ = 2.31 MeV, 92%) n + 10B 7Li + (Q = 2.79 MeV, 6%)


First capture-gated experiments

Capture-gated neutron measurements were first reported in 1986 - 1991, initially with BC454 - plastic loaded with 5% natural boron

WC Feldman et al (NIM A306 (1991) 350-365 and NIM A422 (1999) 562-566) developed a BC454 + BGO detector for the NASA Lunar Prospector

The neutron capture lifetime was measured as 2.2 s

The BGO provides an additional signature for the coincident 478 keV gamma ray from deexcitation of 7Li* -> 7Li


Large-volume experiments

Large-volume capture-gated experiments, again with BC454, were carried out by Miller.

An array of 10 BC-454 detectors, each optically coupled to BGO and a photomultiplier.

The 10B capture peak (Q ~ 2.3 MeV) was observed at an electron equivalent energy of 93 keV:


Multi-detector system

The array of 10 detectors was arranged in a ring, to accommodate a central sample chamber.

Designed at Los Alamos for neutron assay measurementsMC Miller et al, Appl Rad Isotopes 47 (1997) 1549-1555 and NIM A422 (1999) 89-94

In both the Los Alamos and NASA systems, no PSD was available from the plastic scintillator, and only analogue readout electronics was used.


First measurements with liquid BC523

Boron-loaded liquid scintillator was developed to combine fast neutron detection properties with PSD for gamma rejection.

T Aoyama et al, NIM A333 (1993) 492-501 measure a neutron capture lifetime of 2.2 s in BC523 - 5% natural Boron

The capture-gated spectroscopic performance of BC523 to monoenergetic neutrons was measured:

non-linear light yield vs recoil energy produces poor resolution spectra a major limitation to the spectroscopic performance of this technique


Neutron capture lifetimes

After moderation in the scintillator, the neutron capture lifetime is dependent only on the 10B concentration ( 1/v):

and the thermal neutron probability distribution is given by:

The calculated capture lifetimes for the various commercially-available boron loaded scintillators are:

)exp()( 1

ttp

110

BN

Scintillator 10B (%) (s)

BC523A ~ 5 0.49

BC523 ~ 1 2.25

BC454 ~ 1 2.13


The Surrey BC523A detector head

The 700ml volume BC523A cell was fabricated from aluminium, with an embedded BGO detector to measure coincident 478 keV gamma rays from 10B reaction


Capture-gated neutron detection

Capture-gated neutron detection gives very clean fast neutron signature

Trigger event rate is low: requires full moderation of neutron within the scintillator volume dependant

Full energy spectrometer - fast neutron energy obtained from amplitude of recoil pulse

PSD can be used to further reject false TAC start pulses

Neutron capture:n + 10B 7Li +

Q = 2.31 MeV (92%)Q= 2.79 MeV (6%)

neutron capture lifetime


Capture-gated TAC spectrum

f3_tac4

Time difference (ns)

600 650 700 750 800 850

Cou

nts

0

100

200

300

400

500

Chance coincidence events

True coincidence eventsAfter pulse

events


Fast neutron capture lifetime

Neutron capture lifetime has an exponential distribution:

where depends only on 10B concentration, since 1/v:

Scintillator 10B (%) (s)BC523A ~ 5 0.49BC523 ~ 1 2.25BC454 ~ 1 2.13

)exp()( 1

ttp

1

10

BN

Short neutron capture times allow high event rates for the capture-gated detection mode

Event rate with our 10GBq AmBe neutron source: ~20Hz for 700ml BC523A cell


New materials

New loaded scintillator materials offer much potential for future development of neutron detection methods. Some promising candidates include:

1. Boron loaded plastics showing n/ PSD

Norman et al (NIM A484 (2002) 432-350) have shown limited fast neutron - gamma PSD from boron-loaded plastic, not previously observed in BC454:

limited PSD was seen from scintillator grown at CEA, notfrom BC454 no alpha/lithium - gamma PSDobserved in either material

Boron loaded pastics quench thelong-lived triplet state that is normallyfilled mainly by heavy charged particles


New materials (2)

2. Lithium gadolinium borate

J Bart Czirr et al (NIM A476 (2002) 309-312) have produced a new loaded plastic scintillator, lithium gadolinium borate, which contains a mixture of high cross-section materials:

This material is still under test - obtaining large-volume samplesis still difficult


Conclusions

Digital PSD techniques in organic scintillators are being developed that rival traditional analogue methods - the performance of high speed waveform digitisers is key to these developments

Good n/ PSD performance of 1 ns sampling time, 8-bit resolution, digitisers has been successfully demonstrated, using computationally-simple algorithms suitable for field-portable instruments

The application of digital techniques to capture-gated fast neutron detection is under development, and offers a useful technique for fast neutron monitors

Issues for the future: Fast waveform digitisers are still expensive and non-portable True neutron spectroscopy from capture-gated 10B-loaded scintillator

is currently limited by the non-linear light output of these materials New loaded scintillators need to be developed offering good PSD of

the neutron capture reaction (eg. 7Li+ from 10B).


References:

SD Jastaniah and PJ Sellin, “Digital pulse-shape algorithms for scintillation-based neutron detectors”, IEEE Trans Nucl Sci 49/4 (2002) 1824-1828.

SD Jastaniah and PJ Sellin, “Digital techniques for n/ pulse shape discrimination and capture-gated neutron spectroscopy using liquid scintillators”, in press NIM A.

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