Proton-deuteron discrimination with a single semiconductor detector

NUCLEAR

INSTRUMENTS

A N D M E T H O D S 57 (1967) 131-136; © N O R T H - H O L L A N D

PUBLISHING

CO.

PROTON-DEUTERON DISCRIMINATION WITH A SINGLE SEMICONDUCTOR DETECTOR A. ALBERIGI Q U A R A N T A

lstituto di Fisica dell" Universit& di Modena, Italy M. MARTINI and G. OTTAVIANI

INFN Centro Elettronica, lstituto di Fisica dell'Universit~ di Bologna, Italy and G. Z A N A R I N I

lstituto di Matematica dell'Universit~ di Bologna, Italy Received 28 July 1967 A novel method is presented which permits to discriminate protons from deuterons with a single surface barrier detector and

a very simple electronic circuitry. The energy measurement is not influenced by the particle discrimination system.

1. Introduction Charge collection time in semiconductor detectors is known to depend on the range R and on the impact angle 3 of the ionizing heavy particles which are detected i - 4). Therefore it is possible to obtain useful information on R or ~9 by measuring the rise time of the voltage pulse supplied by the detector, or the amplitude of the current pulse. The first experimenters working on this subject 5-7) tried to discriminate a type of particle, which was stopped within the depleted region of the counter, against a more penetrating radiation, which reached the undepleted bulk material, giving an undesirable background. Obviously, in this case, the electronic technique is rather simple because the charge produced outside the depletion region is collected in a much slower time, depending upon the diffusion constant of the material. On the other hand, only the energies of the shorter range particles can be measured, because the diffusion times may easily be several tens of microseconds long, and this causes the loss of some charge for recombination and demands impractically large differentiating time constants in the charge sensitive amplifier. In more refined experiments two or more kinds of particles are identified which are all stopped within the depleted region of the detector. The first positive results of this type were obtained by Ammerlaan et al.l), employing a 2.2 mm thick silicon lithium drifted detector. Identification of particles stopped within the depletion layer of a surface barrier or a diffused detector requires much more sophisticated electronic techniques, because the charge collection times are much faster. An alpha - proton identification experiment in the

3 to 6 MeV energy range was performed by Siffert and Coche 8) utilizing the same principle employed in Ammerlaan's experiment. In both the previously quoted experiments, the identification signal is obtained by delay line clipping the voltage pulse from the semiconductor detector. A simpler system consists in directly analyzing the current pulse which may be collected at one of the detector's leads, while the other lead is employed for the charge measurementg). This system allows the use of very simple electronic circuitry (basically an integral discriminator). In the present work a theoretical analysis is performed of the possibility of distinguishing protons from deuterons by means of this technique; moreover, some experimental results are reported which are in reasonably good agreement with the theory. 2. Theoretical previsions In some published papers z'4) the induced charge was studied due to the motion of the charge carriers created by ionization in the space charge layer of a solid state detector. If the mobilities of both carriers are assumed to be constant (this assumption will be discussed later) and if the following expression for the energy loss is assumed to hold 1) Q(t)

( d E / d x ) - ' = - a l l~(R -- x) ~b- ,)th,

where x = dE/dx = R = a = = = b = 131

space coordinate (pm); specific energy loss ( M e V / p m ) ; range (pm); 12 for protons, 7.23 for deuterons, 1.09 for alpha particles; 1.7 (for energies less than 50 MeV);

132

A. ALBERIGI QUARANTA

we have*

Q(t) = (qE/Eo) [En(t) + H , ( t ) ] , where q = electronic charge; E = energy o f the incident p a r t i c l e ; E o = energy necessary for creating a p a i r (-~ 3.65 eV); E,(t) = n o r m a l i z e d c o m p o n e n t o f the voltage signal c o n t r i b u t e d by electrons; H.(t) = n o r m a l i z e d c o m p o n e n t o f the voltage signal c o n t r i b u t e d by holes;

En(t)-I-Hn(t)--,

l,

for

t~oo.

et al.

mobilities a s s u m p t i o n , by which this simple expression for Io was o b t a i n e d , does not let us consider the obtained results generally valid ; on the other hand, as it is shown in 2), if the resistivity o f the m a t e r i a l a n d the bias voltage are such t h a t the m a x i m u m value o f the electric field is not s u p e r i o r to 3-4 kV/cm, the f o r m e r a s s u m p tion m a y be considered to be correct. Fig. 1 shows t h a t it is possible to discriminate p r o t o n s from deuterons in a given energy range by t a k i n g a d v a n t a g e o f the d e p e n d e n c e o f I o on the energy and the range o f the incident particles. In fact, let us assume c h o o s i n g the detector's width in such a way t h a t it is sufficient j u s t to stop the highest energy p r o t o n s ; then

E.(t) and H,(t) are given by the following expressions: E.(t) = [ l - R b / { w ( b + l ) } ] [ 1 - e x p { - t / ( p s ) } ] ,

~o(x,o5)

{bt(b + l)} {R (b+l)/b_

-[R-w+wexp{-t/(3pe)}]

,6 (b+l>/b.

,s

d, W . 3 0 0 p

%

•exp{t/(3pe)}}, for t < T0, H.(t) = "~

,~

where TO is defined by R = w[l - e x p {--TO/(3pO}]; S2

#f

Rb/{w(b+l)}, for t > T O ; where p = resistivity o f the material, = dielectric c o n s t a n t , w = space charge region width, The current pulse ( o b t a i n e d by differentiating the charge pulse) reaches its peak value at the initial instant. This value is given by the following r e l a t i o n :

d,W=200

p

1o

9

p,W-300

ta

s \

7

" p .Wi 200 p

d, W - 100 ,u

6

l(O) = (dQ/dt),=o = = 411

- R b / { w ( b + I)}] {q/(pe)}(E/Eo).

It should be noticed t h a t the m a x i m u m a m p l i t u d e o f the c u r r e n t pulse increases when the energy increases, while it decreases when the R/w ratio or the resistivity o f the material increase. If we consider, for the sake o f simplicity, a n o r m a lized a m p l i t u d e for the current pulse Io(t ), we have, for different values o f w, the curves Io(0) vs the energy o f the incident particles ( p r o t o n s or d e u t e r o n s ) shown in fig. 1; here, by definition, lo = l/{q/(pe)}. It is worthwhile p o i n t i n g o u t t h a t the c o n s t a n t • Assuming that we neglect the time necessary to create electronhole pairs and plasma time. The first assumption is justified in H)), the second one later.

s

p W= 100 p

3

p,W I 50

2

f

0

1

2

3

4

5

6

7

8

E. M e V

Fig. 1. Normalized current pulse I0 as a function of the energy E of the incident protons (p) or deuterons (d) for various detector widths w. The effect of the equivalent circuit is not taken into account.

PROTON-DEUTERON DISCRIMINATION

E,HeV 8

7 6-

5

4 3 2 f

0

q 100

] 200

i 300

W, p

Fig. 2. Proton-deuteron discriminability energy range as a function of the detector width w (#m).

the energy range in which it is possible to discriminate between protons and deuterons has an upper limit given by the energy corresponding to R = w for protons and a lower limit given by the energy at which the amplitude of the deuterons current pulse equals the maximum amplitude reached by the protons current pulse. Fig. 2 shows the computed discriminability energy

range as a function of the detector width ; observe that it does not depend on resistivity. On the other hand, the shape of the current signals does depend on resistivity: in fact2), they are faster (i.e. shorter) when the resistivity is lower. This fact can introduce a dependence of the practical discriminability range on resistivity, owing to the integration effect associated with the detector equivalent circuit. If the detector is assumed to behave as a pure capacitance (of some tens of pF) and if the input impedance of the current amplifier is 50 ohm, a time constant T of the order of some ns is obtained. If the resistivity is very low (of the order of some hundred ohm-cm), so that the time constant ~ is comparable with the charge collection time the result of the integration is a signal whose maximum amplitude depends practically only on the charge, i.e. on the energy. This fact may be clearly seen by considering the limiting case when the charge collection time is much smaller than the time constant of the equivalent circuit. In fact, in this case the current pulse supplied by the detector may be considered as a 6-function, and the current pulse at the amplifier input is given by

l(t)

START OUTPUT I MASTER INPUTS C A L E R I I

ORTECMOO20t

START PULSE

SURFACEBARRIER OETECTOR N.C.PHA.

PARTICLE IDENTIFYING ARM ~ FASTCURRENT AMPLIFIER

L-i~ ? 50~ CABLE

SLAVE]

YPUTSCALERI START STOP INPUT INPUTI

] Fig. 3. The experimental apparatus.

(QI ) e-",

Owing to this unavoidable integration effect, we must then expect the practical discriminability range to shrink when the resistivity is lowered. Before beginning the discussion of the experimental

ENERGY MEASURING ARM

I

=

w h e r e Q -- q E / E o.

II

45cm 50J~CABLEIORTECI.K)Ofor

133

1__

134

A. ALBERIGI QUARANTA et al.

results, it is necessary to emphasize that the current pulse is influenced not only by the equivalent circuit, as above stated, but also by the presence, at the other lead of the detector, of the charge amplifier (fig. 3) which tends to deform the current pulse itself, lowering its amplitudeS1). The input impedance of the charge amplifier has a rather unusual and poorly known time dependence, and therefore an exact calculation about its influence on the current pulse is very difficult and hardly worthwhile, because it would hold only for one type of charge amplifier. However, we found out experimentally that the adverse effect of the charge amplifier on the current pulse may be reduced by connecting the amplifier to the detector by means of a short coaxial cable instead of directly. The presence of the cable obviously increases the noise at the output of the charge amplifier; however, in many practical cases, the noise is dominated by the detector reverse current rather than by the capacitance at the amplifier input.*

3. Experimental set-up and results 3. I. THE DETECTOR The detector employed for the experimental check is an Ortec type 210 Itm thick detector manufactured from 5500 ohm. cm n-silicon. In order to obtain the optimum sensitivity of discrimination, the detector is biased at a voltage equal to the depletion voltage (35 V). With this detector the plasma time is surely negligible for protons and deuterons in the energy range employed in our experiment ~2). 3.2. ELECTRONIC CIRCUITRY AND NOISE PERFORMANCE

Fig. 3 shows the experimental apparatus that we employed at the 5.5 MeV Van de Graaff accelerator of the University of Padua. It should be stressed that the apparatus of fig. 3 employs only quite conventional commercially available circuitry, while similar experiments ~,8) required more complicated solutions. The proton or deuteron beam was elastically scattered by a thin gold foil and sent through a collimator to the detector biased at the depletion voltage. The current pulse, amplified 64 times by a 50 ohm input impedance and 3 ns rise time transistor amplifier, was analyzed by a fast integral discriminator, whose output was fed to the coincidence input of the multichannel PHA. The counting was normalized by means ofthe two scalers shown in fig. 3. As previously stated, the connection detector-charge amplifier was made by means of a short cable. In our case 50 cm was found to be the * The noise coming from the fast arm is discussed later.

Fig. 4a

Fig. 4b

Fig. 4c Fig. 4. Current pulse obtained (a) with the charge amplifier substituted with a short circuit; (b) with the charge amplifier connected directly to the detector; (c) with the 50 cm cable between the amplifier and the detector. All these pulses were obtained with 5.5 MeV c~-particles. The horizontal sensitivity is 10 ns/div.

optimum length for a 50 ohm cable with a 52 pF measured capacitance. Fig. 4 shows the current pulse obtained with the charge amplifier substituted with a short circuit (fig. 4a), with the charge amplifier connected directly to the detector (fig. 4b) and with the 50 cm cable between the amplifier and the detector (fig. 4c). All these pulses were obtained with 5.5 MeV c~-particles and 35 V bias voltage. In measuring the noise performance of our apparatus we carefully checked not only the increase due to the above mentioned cable, but also the effect on the charge measuring arm of the shot noise at the input of the fast current amplifier (fig. 3) whose first stage was a common base transistor. This noise is coupled to the energy measuring arm through the detector capacitance. The situation is summarized in table 1. It is easily seen that

PROTON-DEUTERON

DISCRIMINATION

135 TABLE

Noise

SETTING f SETTING *

---

-

I

fwhm at the

charge amplifier

Conditions

output

(keV)

10

charge amplifier

only

(no cable - no detector) 12

charge amplifier+cable

21

(no detector) charge amplifier

+ cable + detector

(fast arm short circuited) 23 Fig. 5. Counting rons (upper

efficiency

vs protons

(lower

curves) and deute-

curves) energy range for two threshold fast discriminator.

main

detector It

contribution reverse

should

be

experimental charge

A

3.3. EXPERIMENTAL

nevertheless particles i3 keV).

the

noise

comes

from

the

current.

own

quite

that

the

also

if not

noise

could

acceptable

experiments

(the

noise

level

optimized easily

in

this

fwhm

of

of

our

(e.g.,

the

be lowered), kind

the

of

5 shows

threshold seen

noticed

set up,

amplifier

to

disposition

settings of the

Fig. the

actual

the results

settings

that

RESULTS

a good

obtained

for

two

of the fast discriminator. proton-deuteron

different It is easily

discrimination

is

is

heavy

spectrum

is

_

30

20

t

Fig. 7. Theoretical lines

are

achieved

deuterons

is never

less than

spectra

0

4

3.95 FNCRGY,

Fig. 6. The 4 MeV

spectra

range from

1 (the

Fig. 6 shows

P

with

for the employed

by taking into account equivalent circuit.

in the energy

setting ratio

previsions

calculated

the

threshold

4

detector. the

Dotted

effect

4.0 to 5.25 MeV

to protons

counting

of the

with

efficiency

16: 1). MeV setting

deuterons

and

protons

1.

4.05 3.4. COMPARISON

MoV

for deuterons

and protons

discriminator adjusted on threshold setting

I.

with

the

The

WITH THEORETICAL

discriminability

narrower

range

than the theoretical

PREVISIONS

experimentally

found

one (fig. 2) because

is

of the

136

A. ALBER1GI QUARANTA et al.

a b o v e q u o t e d effects (equivalent circuit a n d charge amplifier) and because o f the u n a v o i d a b l e presence o f noise. A r o u g h calculation to o b t a i n a m o r e realistic theoretical prevision, a c c o u n t i n g for the equivalent circuit a n d the charge amplifier, m a y be p e r f o r m e d in the following way. The c u r r e n t pulse is c o n s i d e r e d to be d a m p e d by the integral time c o n s t a n t due to the k n o w n c a p a c i t a n c e o f the d e t e c t o r (25 p F at 35 V) a n d a 100 o h m resistance. T h e latter is due to the series o f the i n p u t resistance o f the fast c u r r e n t amplifier plus the resistance " s e e n " by the pulse in the a r m where the charge amplifier is connected. Owing to the 50 o h m cable, the i m p e d a n c e in this a r m is, at least initially, 50 ohm. Fig. 7 shows the theoretical prevision o b t a i n e d in this way, which is very near to the e x p e r i m e n t a l results. O u r t h a n k s are due to dr. R. L. C h a s e and dr. R. Radeka, of Brookhaven National Laboratories (U.S.A.) for some helpful discussions. W e t h a n k prof. A. Rostagni, prof. C. Villi for having k i n d l y p u t at o u r d i s p o s a l the P a d u a University

a c c e l e r a t o r and all the accelerator labs p e o p l e for their helpful assistance. W e also gratefully a c k n o w l e d g e the c o n t r i b u t i o n s o f Mr. A. Borghi a n d Mr. P. C a n t o n i in p e r f o r m i n g the measurements.

References

1)~C. A. J. Ammerlaan, R. F. Rumphorst and L. A. Ch. Koerts, [~Nucl. Instr. and Meth. 22 (1963) 189. '-')~A. Alberigi Quaranta, G. Casadei, M. Martini, G. Ottaviani and G. Zanarini, Nucl. Instr. and Meth. 35 (1965) 93. 3) A. Alberigi Quaranta, M. Martini, G. Ottaviani and G. Zanarini, IEEE Trans. Nucl. Sci. NS-13, no. 1 (1966) 752. a) N. J. Hansen, Progress in Nuclear Energy, Ser. IX, 4, no. 1 (Pergamon Press, Oxford. 1964). 5) G. Amsel, P. Baruch and O. Smulkowski, Nucl. Instr. and Meth. 8 (1960) 92. 6) H. O. Funsten, IRE Trans. Nucl. Sci. NS-9, no. 3 (1962) 190. 7) j. A. Scheer, Nucl. Instr. and Meth. 22 (1963) 45. s) p. Siffert and A. Coche, IEEE Trans. Nucl. Sci. NS-13, no. 1 (1966) 757. ,3) A. Alberigi Quaranta, M. Martini, G. Ottaviani and G. Zanarini, Nucl. Instr. and Meth. 50 (1967) 169. lo) p. A. Tove and K. Falk, Nucl. Instr. and Meth. 12 (1961) 278. it) A. Alberigi Quaranta, M. Martini and G. Ottaviani, Nucl. Instr. and Meth. 47 (1967) 10. 12) p. A. Tove and W. Seibt, Nucl. Instr. and Meth. 51 (1967) 261.