Digital%20techniques%20for%20neutron%20detection%20and%20pulse%20shape%20discrimination%20in%20liquid%20scintillators

1
Digital techniques for neutron detection and
pulse shape discrimination in liquid scintillators

• P.J. Sellin, S. Jastaniah, G. Jaffar
• Department of Physics
• University of Surrey
• Guildford, UK
• p.sellin_at_surrey.ac.uk
• www.ph.surrey.ac.uk/cnrp

2
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

3
Introduction

• 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

• Emphasis on fast computationally-simple digital
  algorithms suitable for field instruments
• Efficient n/g 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

See also A. Rasolonjatovo et al, NIM A492 2002
423-433
4
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 t
5
Integrated vs current pulses

• Extraction of scintillation decay lifetime t
  depends on the RC time constant of the external
  circuit
• Large time constant RCgtgtt
• integrated pulse - event energy extracted from
  pulse amplitude
• t extracted from pulse risetime
• Short time constant RCltltt
• current pulse - event energy extracted from pulse
  integral
• t extracted from pulse decay time

6
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 50W
• 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

7
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 ratio

The Q-Ratio signal amplitude A is PSD
parameter is
8
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)
9
XIA performance
Simple rise time inspection gives reasonable a, g
separation More sophisticated algorithms allow
good discrimination of p, a, g
10
Other PSD techniques

• Other 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.

where s(t) is the measured pulse signal, r(t,t)
is the detector response function, and f(t) is
the scintillator light pulse
PMT response function
s(t) expt data and fit
This technique is computationally intensive and
not suitable for portable instruments
11
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

12
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 than
• from gammas
• Technique need as fast digitiser
• with nanosecond timing.

Graph shows calculated average time between
hydrogen recoils vs neutron energy
13
Requirements for the direct technique

• Reeders 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

14
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

15
The Surrey waveform digitiser system

• 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

• 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

16
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
17
10B capture peak

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

The 10B capture peak is observed at 60 keV
electron-equivalent energy.
18
Energy Calibration
44 keV Tb X-ray 8-bit digital DAQ

• Liquid scintillator operated at 2 gain settings,
  with separate energy calibrations
• High Gain
• photopeak for X/g-rays lt 60 keV
• Ba, Tb K X-rays
• 241Am g-ray
• Low Gain
• Compton edge for high energy g-rays
• 57Co
• 137Cs
• 60Co

44 keV Tb X-ray 12-bit analogue DAQ
19
Digital DAQ calibration
low energy photopeak calibration
typical photopeak spectra - 8 bit digital system
high energy Compton edge calibration
20
PSD at low gain

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

21
No PSD in plastic BC454

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

all events
22
PSD at high gain

• At high gain, the 10B capture peak is visible due
  to simultaneous detection of 7Li and a ? 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
23
PSD Figure of Merit

• Quality of PSD is described using a Figure of
  Merit (FOM)
• Vertical slices from the 2D spectra give
  risetime histograms

Sng separation of two peaks Fn,g n,g peak
centroid position
high energy FOM 1.5
low energy FOM 1.4
Method is similar to conventional analogue PSD
techniques FOM is extracted digitally in
software FOMgt1 required for good PSD
g
n
24
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

25
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

26
FOM plots from Q-Ratio algorithm

FOM values are 1.1 for both energy ranges - the
Q-ratio algorithm gives better overall PSD
performance for current pulses
27
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

where ERMAX is the maximum recoil energy of
nucleus with atomic mass A For protons, A1 and
ERMAXEN
28
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

Neutron capture n 10B ? 7Li a 478 keV g
(Q 2.31 MeV, 92) n 10B ? 7Li a (Q
2.79 MeV, 6)

• Characteristic double-pulse sequence of
  moderation capture provides clean fast neutron
  signature.
• Capture pulse has fixed amplitude (10Bn Q value)
• Amplitude of moderation pulse gives incident
  neutron kinetic energy
• ?? true full energy neutron spectrometer

29
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 ms The BGO
provides an additional signature for the
coincident 478 keV gamma ray from deexcitation of
7Li -gt 7Li
30
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
31
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 measurements MC 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.
32
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 ms 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
33
Neutron capture lifetimes

• After moderation in the scintillator, the
  neutron capture lifetime is dependent only on the
  10B concentration (s ? 1/v)
• and the thermal neutron probability distribution
  is given by
• The calculated capture lifetimes for the various
  commercially-available boron loaded scintillators
  are

34
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
35
Capture-gated neutron detection
Neutron capture n 10B ? 7Li a Q 2.31 MeV
(92) Q 2.79 MeV (6)

• 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 lifetime
36
Capture-gated TAC spectrum
37
Fast neutron capture lifetime

• Neutron capture lifetime t has an exponential
  distribution
• where t depends only on 10B
• concentration, since s ?1/v

Scintillator 10B () ? (?s) BC523A
5 0.49 BC523 1 2.25 BC454 1 2.13
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
38
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/g 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, not
• from BC454
• no alpha/lithium - gamma PSD
• observed in either material
• Boron loaded pastics quench the
• long-lived triplet state that is normally
• filled mainly by heavy charged particles

39
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 samples
• is still difficult

40
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/g 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. 7Lia from 10B).

41
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/g pulse shape
discrimination and capture-gated neutron
spectroscopy using liquid scintillators, in
press NIM A.