Digital pulse shape discrimination applied to capture-gated neutron detectors P.J. Sellin, S. Jastaniah, W. Catford

Department of Physics University of Surrey Guildford, UK [email protected]

www.ph.surrey.ac.uk/cnrp Paul Sellin, Radiation Imaging Group

Contents

An overview of the neutron detector work carried out recently at the University of Surrey:  Background to the project:    Pulse shape discrimination (PSD) in liquid scintillators Digital PSD algorithms 10 B -loaded scintillator for capture-gated fast neutron detection  Results from the digital system:    Digital PSD from integrated and current pulses PSD Figure of Merit (FOM) capture-gated neutron detection   Conclusions References: this presentation covers data recently published: 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

”, submitted to NIM A.

Paul Sellin, Radiation Imaging Group

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 700 600 500 400 300 200 Neutron Fluence to Ambient Dose Equivalent conversion factor Conversion factor from B. Siebert, Rad. Prot. Dosim.

58/3 (1995) 177-183, derived from ICRU -60  Full-energy fast neutron spectrometry has particular advantages for dosimetry detectors: 100 0 0.001

0.01

0.1

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

1 10

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 Paul Sellin, Radiation Imaging Group

Integrated vs current pulses

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

Large time constant RC>>

t

:

integrated pulse - event energy extracted from pulse amplitude t

extracted from pulse risetime

:

v

(

t

) 

C Q

1 

e

 t

t for t



RC

Short time constant RC<<

t

:

current pulse - event energy extracted from pulse integral t

extracted from pulse decay time: v

(

t

) 

Q e

 t

t for t



RC

Paul Sellin, Radiation Imaging Group

Pulse risetime algorithms Integrated pulses

- using a PMT preamplifier Improved signal-noise ratio Risetime limited by preamp (~10ns) 

10-90% risetime algorithm

Current pulses

- anode connected directly to 50 W Simple circuitry, fastest response PSD algorithm affected by signal noise 

‘time over threshold’ algorithm

0.0

(a) -0.2

-0.4

-0.6

-0.8

0.0

-0.1

-0.2

(b) pulse risetime time over threshold

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

-0.3

-0.4

0 50 Time (ns) Integrated pulse 10% level 90% level threshold Current pulse 200

10

B loaded liquid scintillator

We have investigated liquid scintillator enriched with 10 B - BC523A Often used for thermal neutron detection, 10B-loaded scintillator can also be used for ‘capture-gated’ neutron spectroscopy: 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 Paul Sellin, Radiation Imaging Group

Waveform Digitiser

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 Paul Sellin, Radiation Imaging Group

Detector Cells

Various liquid scintillator cells were made (100 ml and 700 ml), containing BC501A and BC523A When filling the cells, the scintillator was bubbled with N 2 gas to purge the oxygen.

A two-detector cell was made, with a BGO embedded in the BC523A to detect coincident 478 keV gamma rays from 10 B reaction Paul Sellin, Radiation Imaging Group

Energy Calibration

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

High Gain:

 photopeak for X/ g -rays < 60 keV: Ba, Tb K X-rays 241Am g -ray

Low Gain:

 Compton edge for high energy g -rays: 57 Co 137 Cs 60 Co 44 keV Tb X-ray 8-bit digital DAQ 44 keV Tb X-ray 12-bit analogue DAQ Paul Sellin, Radiation Imaging Group

PSD at low gain

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

120

(a) BC501A

100 80

Neutron events

60 40 20 0 0

>100 counts 5 -- 100 0.2 -- 5 0.01 -- 0.2

120 100

(b) BC523A

80

Neutron events

60 40 50 20

Gamma ray events

100 150

Pulse height (a.u.)

200 250

>100 counts 5 -- 100 0.2 -- 5 0.01 -- 0.2

50

Gamma ray events

100 150

Pulse height (a.u.)

200

Paul Sellin, Radiation Imaging Group

250

No PSD in plastic BC454

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

120

BC408

100 80 60 40 20 0 0

all events > 100 counts 5 -- 100.0

0.2 -- 5 0.01 -- 0.2

Gamma ray events

50 100 150

Pulse height (a.u.)

200 250

Paul Sellin, Radiation Imaging Group

PSD at high gain

120 100

At high gain, the 10 B capture peak is visible due to simultaneous detection of 7 Li and a no significant PSD is observed (a) BC501A Neutron events > 100 Counts 5 -- 100 0.2 -- 5 0.01 -- 0.2

120 100

(b) BC523A > 100 counts 5 -- 100.0

0.2 -- 5 0.01 -- 0.2

80 80

Thermal neutron events

60 60 40 40

Gamma ray events Gamma ray events

20 20 0 0 50 100 150 200 250

Pulse height (a.u.) Pulse height (a.u.) Lack of PSD is due to quenching of slow component from heavy ions - alpha particle PSD has been seen in ‘special’ 10 B-loaded scintillator

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

50 100 150 200

Paul Sellin, Radiation Imaging Group

250

PSD Figure of Merit

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

FOM



F n S n

g 

F

g

S n

g

F n,

g = separation of two peaks = n, g peak centroid position Vertical ‘slices’ from the 2D spectra give risetime histograms: Method is similar to conventional analogue PSD techniques FOM is extracted digitally in software FOM>1 required for ‘good’ PSD 4000 3500 3000 2500 2000 1500 1000 500 0 0 25 g

(a)

low energy FOM = 1.4

50 n 1000 800 600 400 200 FOM = 1.5

75 100 0 0 Rise time (ns) 25 50 75 100 Paul Sellin, Radiation Imaging Group

PSD from current pulses

120 100

Current pulses use ‘time over threshold’ in place of risetime - the 2D plot has a different shape FOM is slightly worse than for integrated pulses (1.1 - 1.2) with poorer valley separation 8000 BC501A > 100 counts 5 -- 100 0.2 -- 5 0.01 -- 0.2

7000 6000 Neutron events 5000

80

4000

60

3000

40

2000

20

Gamma ray events 1000

0 0 50 100 150

Pulse height (a.u.)

200 250

0 0 (a) 2000 1750 1500 1250 1000 750 500 250 25 50 75 100 0 0 Risetime (ns) Paul Sellin, Radiation Imaging Group 25 (b) 50 75 100

Capture-gated neutron detection

Capture-gated neutron detection gives very clean fast neutron signature Trigger event rate is low, requiring full moderation of neutron within the scintillator  volume dependant PSD can be used to further reject false TAC start pulses Paul Sellin, Radiation Imaging Group

Fast neutron capture lifetime

Fast neutron capture lifetime t has an exponential distribution:

p

(

t

)  t  1 exp( 

t

t ) where t depends only on 10B concentration, since s  1/v: t  

N

10

B

s   1 Scintillator BC523A BC523 BC454 10 B (%) ~ 5 ~ 1 ~ 1 t (  s) 0.49

2.25

2.13

6 tac4 vs ln(c-b) 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 5

Mean capture time:

t

= 470+/- 80 ns

4 720

Fit equation: Y= -2.15X10

-3 X + 6.8

740 760 780 800 Paul Sellin, Radiation Imaging Group Time difference (ns)

Conclusions

 High speed waveform digitisers are opening up new techniques for neutron and gamma ray detection in fast organic scintillators   The performance of 1 ns sampling time, 8-bit resolution, digitisers has been successfully demonstrated Good n/ g PSD has been demonstrated using computationally simple digital pulse risetime algorithms  The application of digital techniques to capture-gated fast neutron detection is a powerful technique for fast neutron monitors

Issues for the future:

 Multi-channel 1 GS/s digitisers are still expensive  Digitisers are not yet available in a ‘laptop’ format  True neutron spectroscopy from capture-gated 10 B-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. 7 Li+ a from 10 B).

Paul Sellin, Radiation Imaging Group