Neutron-gamma discrimination based on leading edge shape measurement

Nuclear Instruments and Methods in Physics Research A270 (1988) 487-491 North-Holland, Amsterdam

NEUTRON-GAMMA

DISCRIMINATION

Suvendu BOSE, Mihir Baran CHATTERJEE,

BASED

487

ON LEADING

Bedanta Kumar SINHA

EDGE

SHAPE

MEASUREMENT

and Rangalal BHATTACHARYYA

Saha Institute of Nuclear Physics, Calcutta - 700 064, India

Received 1 December 1987

A neutron-gamma discriminator using a NE213 liquid scintillator and an amplitude independent pulse risetime measurement technique is presented. A separation index S has been proposed in place of the figure of merit M to evaluate the performance. Performance of the n - y discriminator has been studied with a 30 mCi Am-Be source in the light of S and other possible performance indices. A good performance in an in-beam experiment has been obtained for a dynamic range of 20 keV to 10 MeV electron equivalent energy.

1. Introduction Pulse shape discriminators (PSD) applied to nuclear spectroscopy have b e e n the subject of intense study for a long time. The m o s t fruitful application of this techn i q u e has b e e n in the case of separation of n e u t r o n s a n d y-rays a n d a rather large n u m b e r of studies has b e e n devoted towards this topic [1-14]. T h e technique has received a new impetus due to recent interest in (HI, x n) reactions a n d use of n e u t r o n multiplicity detectors. Here one aims to separate the y-rays in the different c h a n n e l s c o r r e s p o n d i n g to the different multiplicity of the neutrons. It is evident that unless one uses some k i n d of n - 7 discrimination n o fruitful i n f o r m a t i o n can be d r a w n from such experiments. It is therefore felt necessary to have a discriminator system with as few c o m p o n e n t modules as possible so that inclusion of m a n y of t h e m in a large experimental a r r a n g e m e n t becomes easy with respect to cost a n d overall size. It is k n o w n that the time course of the light intensity as well as the integrated light o u t p u t from some scintillators when excited b y y-rays are different from those w h e n excited by neutrons. Such a difference results in unequal risetimes in the two cases at the o u t p u t of the p h o t o m u l t i p l i e r coupled to the scintillator. N e u t r o n s which are detected t h r o u g h recoil p r o t o n s are f o u n d to yield a longer risetime t h a n that due to y-rays. This variation in risetime is often exploited to discriminate between n e u t r o n s a n d -(-rays in a PSD. Several techniques have b e e n developed a n d are used for this purpose. To describe the p e r f o r m a n c e of a P S D for n - y discrimination, Reid a n d H u m m e l [2] define M , the figure of merit. Sperr et al. [7], however, argue t h a t the value of M c a n n o t b e considered as a p r o p e r index for evaluation of the p e r f o r m a n c e of a P S D a n d they used the y-peak to valley ratio to specify the p e r f o r m a n c e 0 1 6 8 - 9 0 0 2 / 8 8 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

while A h m e d [8] prefers using the n e u t r o n peak to valley ratio. It is felt that the p e r f o r m a n c e of the n - y discriminator can be b e t t e r described in terms of a separation index S. Fig. 1 shows a n o t i o n a l P S D response curve. Referring to this figure S is defined as S=

P~-P~ VHWHM + n HWHM '

where Py a n d Pn are the c h a n n e l n u m b e r s corresponding to the peak p o i n t s due to y a n d n e u t r o n respectively, a n d Y H W H M a n d n H W H M are the half-widths

~E
HM 7/HW

o

E

PC

V

G

~ v (z:,< I--LU laj DZ

P~

H

CHANNEL NUMBER

Fig. 1. A notional n - 7 discriminator response curve showing

the gamma peak, valley region and the neutron peak. The gamma and the neutron peaks when extrapolated meet the abscissa at E and H respectively.

488

S. Bose et al. / Neutron - g a m m a discrimination

at half-maximum of the right hand side (RHS) and the left side (LHS) of the ,/ and neutron peaks respectively. Figs. 3 and 4 show that the y peak is fairly symmetric whereas the neutron peak is not. In view of this large asymmetry of the neutron peak the value of M cannot correctly represent the performance of the PSD. The discrimination of neutrons from y-rays can be better characterised in terms of the half-width at half-maximum (HWHM) of the RHS of the 3` peak and H W H M of the LHS of the neutron peak instead of taking the F W H M of both peaks. This is because, if there is any mixing between neutrons and ,/-rays, it is most likely to be reflected in the shape of these two halves only (fig. 1). It is seen from the reports on the earlier works that measurements on the leading edge shape (LES) of the integrated photomnltiplier output pulses have not been done to achieve n - y discrimination. Kinbara and Kumahara [4] have separated neutrons from 3,-rays by measuring the risetime of the pulses after amplification in a slow linear amplifier. Since the present-day photomultipliers can give large output pulses at an appreciably low noise level, it is instructive to subject these faster pulses direct from the anode or any suitable dynode of the photomultiplier to measurement in a LES discriminator. This procedure is expected to help eliminate the slow linear amplifier and consequently open up the possibility of increasing the pulse rate handling capability. The present work is devoted to the study of the performance of such a LES discriminator system to be employed for n - 7 discrimination.

2. Principle of operation The principle of risetime measurement discussed and used on HPGe detector pulses by Bose et al. [15] has

been adapted to serve as a LES discriminator. In this application, the clipped input pulse (fig. 2) is suitably delayed and compared with a part X% of the unclipped pulse in contrast to the arrangement of ref. [15], where the comparison was done with respect to a fixed dc voltage level. Thus for any pulse height, the shape information contained in the time between X% and (100 - X)% of the leading edge of the pulse is converted into a voltage pulse height. This way of comparison makes the arrangement substantially independent of input pulse height and hence the energy of the incident radiation. The value of X can be easily set by adjusting the potentiometer P. For X = 10%, the arrangement works as a risetime to pulse height converter. The arrangement is shown as a schematic in fig. 2. The processed pulses at various stages are also shown at the right hand side of this figure. The clipped pulses of width 150 ns are delayed by 100 ns and compared with a part (set by the potentiometer P) of the input unclipped pulse by a high speed comparator LM 360. The output pulses from the comparator then have a width which is related to the shape of the leading edge of the input pulse. It can be seen that for a pulse having zero risetime the comparator yields a pulse of width 150 ns, the minimum possible for a clippling time of 150 ns [15]. The constant current generator (T2, R2) charges the capacitor C 3 only for the time for which the output of LM 360 is zero and for the rest of the time the current from the generator is steered away and the capacitor discharges via R 3. The voltage across C 3 is proportional to the width of the comparator output pulse. The constant current generator, the capacitor C 3 and the steering diodes (Oa, 02) thus form an inexpensive and simple pulse width to amplitude converter (PWAC). The buffer B4 isolates capacitor C 3 from the external circuit. The choice of 150 ns for the clipping

A

®

@

INPUT

-lzv -

-

time

Fig. 2. Schematic circuit diagram of the leading edge shape (LES) discriminator. P - potentiometer; B1, B2, B 3 , B 4 - unity gain buffers: D1, D 2 - diodes 1N4148; T1 - transistor 2N 918, T2 FET 2N 4416. R> R2, R 3 and C 3 are explained in the text.

489

S. Bose et al. / Neutron - g a m m a discrimination

time is not critical. It is found that the circuit gives comparable performance for a clipping time of 80 ns and above. The rate handling capacity is, however, much improved in the case of a smaller clipping width. A positive feedback is incorporated in the comparator LM 360 and as a result the output (point 5) is at logic H I G H even in the absence of any input pulse and the point A is - 4 mV more positive than B. Again if we account for the effect of feedback resistances only, the point B becomes - 4 mV more positive than point A, when the output (point 5) is at logic zero. For large input pulses the effect of this hysteresis in setting the value of X at A is negligible. For very small input pulses (say a 10 mV clipped pulse at B), the normal setting of X by P is affected by this hysteresis. For smaller input pulses, therefore, X tends to increase with respect to the set value resulting in a reduction in the comparator output pulse width and hence a smaller output pulse height [15]. Thus pulses of low amplitudes will be falsely recognised as pulses having risetimes shorter than the actual. In the present application, exact measurement of risetime ( X = 10%) is not a hard and fast requirement. Any suitable value of X, even if somewhat different from 10% and adjusted to obtain the largest possible value for S, is acceptable. For too small a value of X, P- - Pv as also H W H M and HWTM increase while for too large an X, Pn -- Pv decreases. The most favourable value of X is set by trial. The discriminator works well with input pulses of height above 25 mV which implies that the useful dynamic range is from 25 mV to 5 V. The upper limit is set by the comparator characteristics. If now 5 V is made to correspond to 10 MeV y-rays, the useful dynamic range becomes 50 keV to 10 MeV. Large PMT output pulses which saturate the buffer will be recognised as pulses having risetimes shorter than the actual. It is important to avoid such pulses. A dynamic range expander, an example of which appears in the work of Drosg [11], will help avoid these pulses and at the same time increase the dynamic range substantially.

3, Performance studies

A liquid scintillator NE213 (50 mm × 50 mm) coupled to an XP2020 photomultiplier is used in the present studies. Two pulses are delivered from the photomultiplier. The dynode pulse is fed into the n - y discriminator while the anode pulse goes into the side channel to facilitate selection of the threshold energy E 0•

Fig. 3 shows the performance of the n - y discriminator for different threshold energies (E0) from 50 to 200 keV in equivalent electron pulse height using a 30 mCi

GAMMA )EAK

NEUTRON PEAK

_J

k

."''~...,

Eo = 200 keV

"'....~.............

__..../

;%,

U3 I,,Z

..e "'....,. Eo =150 keY ~=.,.__J

°"--°.~ ..........

0 (J

=~,°°

y-

"-.......... Eo=100keV

.2*'°,.

.."

.......,J L,......... ".. t.7 178 CHANNEL

Eo=50 keV

NUMBER

Fig. 3. LES discriminator response curves using a NE213 scintillator and a 30 mCi Am-Be source, for four different threshold settings.

A m - B e source. It is seen that the y-rays and neutrons are almost completely separated for threshold settings above 150 keV, while good enough discrimination is achieved even for a setting as low as 50 keV (table 1). The peaks in fig. 4 show the performance of the discriminator for only y-rays from 1 3 7 C s and 2°7Bi as examples. The clear and sharp peaks with an insignificant tail on their right hand edge suggest that almost no y pulses are likely to be recorded under the neutron peak of the LES spectra of fig. 3. It is also observed that the y peak positions in fig. 4 do not change with variation of y-ray energies. The tail at the left hand edge of these peaks occurs for lower E 0 settings only. The y peak becomes symmetrical above E 0 = 100 keV. Similar results have been obtained with other V sources like 133Ba and 6°C0. The LES spectrum with a 241Am y source at E 0 = 50 keV shows a shift of the peak position towards lower channel number by a few channels only. This shift is expected in view of the reasons already stated in connection with smaller pulses being falsely recorded as having somewhat shorter risetime than actual. The nonzero valley region in the LES spectra for E 0 = 5 0 keV and E 0 = 1 0 0 keV are due to low level neutron pulses which have been recognised by the discriminator as having risetimes comparatively shorter

490

S. Bose et al. / Neutron-gamma discrimination

Table 1 Performance indices at different threshold settings. Source: 30 mCi Am-Be, scintillator: NE213, PMT: XP2020 Threshold equivalent electron energy E o [keV]

A n / A v a)

50 100 150 200

0.93 1.12 1.22 1.28

P n - Pv M = v FWHM + n FWHM (refs. [2,6])

S

1.38 1.45 1.49 1.57

3.4 3.7 4.1 4.5

P n - P~, v HWHM + n HWHM

Vpeak valley (ref. [7])

npeak R = valle~"-y (ref. [8])

NA +VA D = - VA

22 47 59 76

6 10 14 21

11 19 22 21

") A v = area under 3' peak bounded by E and F of fig. 1. A n = area under neutron peak bounded by G and H of fig. 1.

t h a n the majority of its class, yet s o m e w h a t longer t h a n that for 2/ pulses. H e n c e they occur in between the two peaks. T h e shaded area F G s h o w n in fig. 1 defines the valley area VA b o u n d e d by the curve between the points F a n d G. The n e u t r o n peak area N A is b o u n d e d b y the curve between the points G a n d H. Here F a n d G correspond to channels having ordinates equal to n xM where n TM is one t e n t h of the n e u t r o n peak count. However, a discrimination factor D can be defined as D = ( N A + V A ) / V A to determine the p e r f o r m a n c e of a P S D in a very special way. Evaluation of D as a function of E 0 (table 1) done from the curves of fig. 3 shows that D varies very little over 100 keV. Thus from Eo=5OkeV Eo=150keV

a knowledge of D as a f u n c t i o n of E 0, one c a n select the threshold bias to b e used in an experiment. In the present case, E 0 = 100 keV is a good choice o n this account. F o r a value of E0 above 100 keV the valley height is reduced to a very small level. However, prop o r t i o n a t e r e d u c t i o n in the n u m b e r of n e u t r o n pulses u n d e r the region G H also ensues. A second c o m p a r a t o r c a n be integrated into the circuit to obviate the need of a separate SCA operating in the integral mode. Such a c o m p a r a t o r c o n n e c t e d to p o i n t (1) of fig. 2 c a n select pulses above a preselected threshold E 0 a n d allow the o u t p u t of L M 360 for P W A C actuation. The rate h a n d l i n g capacity of the circuit (fig. 2) can b e increased b y using a clipped reference pulse of width a b o u t 300 ns in the c o m p a r a t o r in place of the unclipped pulse at the p o i n t (1). If this clipped pulse, a t t e n u a t e d b y the p o t e n t i o m e t e r P, is fed to the comp a r a t o r the pileup in the reference of the c o m p a r a t o r can be reduced. T h e time c o n s t a n t R3C 3 m a y then be decreased to e n h a n c e the rate h a n d l i n g capacity. F o r a very high c o u n t i n g rate the capacitor C 3 has to be discharged b y a properly timed semiconductor switch in place of R 3. The n-2/ discriminator discussed above has been used for studying the 2/-rays detected in a 76 m m × 76 m m NaI(T1) detector in coincidence with the n e u t r o n s selected from the LES s p e c t r u m ( L L D - 200 keV, U L D integral) in gate. T h e spectrum in fig. 5 shows the 4,44 M e V 2/ peak originating from the deexcitation of 12C* p r o d u c e d t h r o u g h the 9 B e + ct ~ 1 3 C * ---, 12 C * + n reaction in a n A m - B e source. T h e n e u t r o n spectrum (not shown) in coincidence with the "/-rays selected from the LES s p e c t r u m in gate indicate that almost no n e u t r o n pulses are present u n d e r the 2/ peak. A typical LES s p e c t r u m in a n i n - b e a m experiment with 50 M e V alphas o n t a n t a l u m is shown in fig. 6. N o side c h a n n e l was used here. However, the hysteresis i n t r o d u c e d in the c o m p a r a t o r L M 360 in fig. 2 sets a m i n i m u m E 0 of a b o u t 20 keV. The low energy h u m p on the left side of the peak due to "/-rays arises from such a -

CHANNEL

NUMBER

Fig. 4. LES discriminator response curves using a NE213

scintillator and

137Cs, 2°7Bi gamma

sources for two different threshold settings.

491

S. Bose et al. / Neutron - g a m m a discrimination

.7 800 4

SOURCE:30mCi Am-Be

%,-. 600

* . ~ ~"

V e•

(sq I.-

•

-vt

z 4o0 O ¢J

GAMMA DETECTOR : 200

76ram @x 76ram NaI(TI) 50mm~x50mm NE213

NEUTRON DETECTOR:

I

I

I

I

I

!

I

t

G A M M A ENERGY IN M e V

Fig. 5. Gamma energy spectrum measured with an Am-Be source gated by the neutrons selected from the spectrum of fig. 3, showing the photopeak of the 4.44 MeV 7-ray along with the two escape peaks.

GAMMA PEAK 6.0

IN BEAM MEASUREMENT 50MeV ALPHA ON TANTALUM PMT: X P 2 0 2 0 SCINT: NE 213(50mm~xS0mm)

. %

+ XP2020 P M T for this work. They also wish to express their gratitude to the Director of the Variable Energy Cyclotron Centre, Calcutta for allowing them to d o the in-beam experiment, and to Dr. G. Muthukrishnan for lending the A m - B e source.

References X v

z

•

3.0

•

O

•

~

o

: • 50

100

NEUTRON PEAK 150

200

CHANNEL NUMBER Fig. 6. An in-beam measurement using a LES discriminator. No side channel was used to set E 0.

low effective threshold but even then it can be seen that the n - 7 discrimination is quite good.

Acknowledgements The authors are grateful to Dr. A r u n K u m a r Chatterjee of Bose Institute, Calcutta for lending his NE213

[1] L. Varga, Nucl. Instr. and Meth. 14 (1961) 24. [2] W.B. Reid and R.H. Hummel, Can. Nucl. Tech. 5 (1966) 36. [3] B. Sabbah and A. Suhami, Nucl. Instr. and Meth. 58 (1986) 103. [4] S. Kinbara and T. Kumahara, Nucl. Instr. and Meth. 70 (1969) 173. [5] G.W. McBeth, J.E. Lutkin and R.A. Winyard, Nucl. Instr. and Meth. 93 (1971) 99. [6] R.A. Winyard, J.E. Lutkin and G.W. McBeth, Nucl. Instr. and Meth. 95 (1971) 141. [7] P. Sperr, H. Spieler, M.R. Maier and D. Evers, Nucl. Instr. and Meth. 116 (1974) 55. [8] M. Ahmed, Nucl. Instr. and Meth. 143 (1977) 255. [9] F.D. Brooks, Nucl. Instr. and Meth. 162 (1979) 477 and references therein. [10] Z.W. Bell, Nucl. Instr. and Meth. 188 (1981) 105. [11] M. Drosg, Nucl. Instr. and Meth. 196 (1982) 449. [12] T. Kumahara and H. Tominaga, IEEE Trans. Nucl. Sci. NS-31 (1984) 451. [13] R.B. Piercey, J.E. McKisson, M.A. Herath Banda and M.R. Shavers, IEEE Trans. Nucl. Sci. NS-34 (1987) 82. [14] K. Neelkantan and V,R. Seshadri, NucL Instr. and Meth. A256 (1987) 112. [15] S. Bose, M.B. Chatterjee, B.K. Sinha and R. Bhattacharya, Nucl. Instr. and Meth. A254 (1987) 79.