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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
www.ph.surrey.ac.uk/cnrp
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
) (
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
)(
Paul Sellin, Radiation Imaging Group
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)
Paul Sellin, Radiation Imaging Group
XIA performance
Simple rise time inspection gives reasonable , separation
More sophisticated algorithms allow good discrimination of p, ,
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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.
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
Digital DAQ calibration
low energy photopeak calibration
high energy Compton edge calibration
typical photopeak spectra- 8 bit digital system
Paul Sellin, Radiation Imaging Group
PSD at low gain
Risetime versus pulse height plot at low gain setting showing n/ PSD from (a) BC501A, and (b) from BC523A.
Paul Sellin, Radiation Imaging Group
No PSD in plastic BC454
We also tested PSD in plastic scintillator BC454 - no discrimination was seen for neutron scatter events
all events
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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%)
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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:
Paul Sellin, Radiation Imaging Group
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.
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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
Paul Sellin, Radiation Imaging Group
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).
Paul Sellin, Radiation Imaging Group
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.