A charged particle identification system using a fast multiplier

NUCLEAR INSTRUMENTS

A N D M E T H O D S 44 (I966) 9 3 - 9 7 ;

© NORTH-HOLLAND

PUBLISHING

CO.

A CHARGED PARTICLE IDENTIFICATION SYSTFA-I USING A FAST M U L T I P L I E R S. K. MARK* and R. B. MOORE

Foster Radiation Laboratory, McGill University, Montreal, Canad_a Received 2 April 1966 A charged particle identification system based on the principle (E+ Eo+kdE/dx)dF./dx is described. It is found that charged particles with energy in the range from 20 to 100 MeV can be separated by using plastic scintillator Naton 136 for the measure-

ment of E and dE/dx of the particles. A relatively fast transistor multiplication circuit is presented together with its performance. The performance of the system in separating deuterons from protons is given.

1. Introduction To study nuclear reactions involving the emission of charged particles such as protons, deuterons and tritons, a system which is capable of indentifying them is required. Systems based on the simultaneous determination of dE/dx, the specific energy loss of the particle in passing through matter, and E, the total energy of the particle, have been widely used 1-7). They all depend on the well-known relation 8) that over a limited energy range the product of E and dE/dx is approximately proportional to MZ 2. By using plastic scintillators for the measurement of E and dE/dx, Griffiths et al. 7) have found that the function (E+Eo)dE/dx, where E 0 is a constant, could be used to identify particles over an energy range

of 8 to 50 MeV. However, for higher energy ranges the function (E + Eo + k dE/dx) dE/dx, first used by Stokes et a l ) ) where k is constant, provides better particle identification. The system to be described here is based on the particle separation function (E + Eo + k dE/dx)dE/dx. By making use of plastic scintillators for the measurement of E and dE/dx, this system is capable of separating charged particles over an energy range of 20 MeV to 100 MeV. It has a dead time of 1 #sec, making it faster than most of the previously described systems and the electronic circuits involved are simple and easy to construct. 2. Particle separation function The plastic scintillator Naton 136 (CHo.997, density = 1.047 g/cm 3) was used to measure dE/dx and E of

* N.R.C. (Canada) post-doctoral fellow at the Department of Physics, University of Manitoba, Winnipeg.

25 (n

)-

|

/

20

TmTON

DEUTERON Z

+

t$

PHOTON I0

.3 + J J


O

I0

i

t

,

ZO

30

40

i

DO ENERGY

)

i

i

t

60

70

80

90

IN

i

IO0

IlO

MeV

Fig. 1. The particle separation function L(L + Lo + kAL) vs particle energy for dE/dx scintillator of thickness 0.209 g/cm2. L0 is equivalent to the total amount of light output of a 23 MeV proton and k -- 1.55.

93

94

s . K . MARK AND R. B. MOORE

the particles. The dE/dx scintillator was 2.0 mm thick and the E scintillator had a thickness slightly greater than the full range of a 100 MeV proton. The dE/dx scintillator acted as an absorber of the thickness required to linearize the response of the E scintillator to the particle energy of protons and deuterons and at the same time gave a sufficiently small Landau spread for the highest energy protons. Following the method of Gooding and Pugh9), the light response of these scintillators was computed for the three singly charged particles over the energy range of interest. Using this light response the particle separation function (L + L o + kAL)AL was calculated for the three different types ofparticlesas a function of particle energy. L and AL are the light outputs of the E and the dE/dx scintillators respectively, and the parameters L o and k were varied so as to give optimum particle separation. The values obtained for these parameters were k = 1.55 and L o equal to the total light output for a 23 MeV proton. Fig. 1 shows the plot of the resulting separation function, showing a good separation of singly charged particles over the energy range 20 MeV to 100 MeV. The doubly charged particles have very much larger values of the separation function and offer no interference.

3. Description of system A block diagram of the system is shown in fig. 2.

AMPL.

1

COINC.

AMPL.

I tt t

t

50 ns

[CONSTAN I

LINEAR TWIN-GATE

IVoGENEI:~T.J

VaF (VE+ V o " ~

I

I

MULTIPLIER]

FAST

I ADDER I ....

~'(VE*V~r~

t V~VE 4"Vo*,"kVa¢) SINGLE CHANNEL1 ANALYSER | Fig. 2. The block diagram of the charged particle identification system.

From each detector, a voltage pulse is obtained from a photomultiplier dynode and a current pulse is obtained from the photomultiplier anode. The current pulses from the two counters trigger a fast coincidence circuit which in turn operates two other circuits; a constant amplitude (V o) pulse generator and a 50 ns linear twin-gate. The two voltage pulses VE and VnE, corresponding to L and AL respectively, are used to measure the energy deposited by the particle in the two counters. Ve and VnE and Vo are summed and multiplied by Vne to give a pulse corresponding to (L + L o + kdL) AL. A single channel analyzer is used to select any group of pulses corresponding to a particular type of particle. The amplitude of Vo and the relative gain k are adjusted for best particle separation. The fast coincidence circuit was used to ensure that the same particle was detected in each counter of the d E / d x - E telescope. It is built with tunnel diodes and is an improved version of the one described by Gorodetzky et al.l°). This circuit has a resolving time of 20 ns and can handle a pulse repetition rate of about 10 Mc et each of its input channels. The 50 ns linear twin-gate is used to prevent noncoincident events from entering the multiplication circuit. It is actually two single fast gates operated by a single gating signal. This circuit operates on a principle similar to the one given by Liu and Loeti]er11). The linearity is better than 1% for input pulses from 0.2 V to 5 V, feedthrough of the input pulse is not observable and the gating signal is made to produce less than a 10 mV pulse at the output by using an adjustable cancelling pedestal. The circuit, from input to output, is capable of following pulses with rise and fall times down to 10 ns. The natural dead time of the gate is 0.1 #s. However, to prevent pile-up in the multiplication circuit, which can take up to 1 lts to finish a multiplication, a means of extending the dead time of the gate up to 1/~s has been provided. The Vo generator is a 10 Mc monostable which produces 50 ns square pulses with adjustable amplitude. The adding circuit is a fast operational adder.

4. Detail of multiplication circuit In the past decade many forms of electronic multiplication circuits have been developed in conjunction with nuclear reaction studies. Most of them operate on one or another of the following principles: (a) generating a logarithmic function by means of biasing an array of diodes 6) or utilizing the logarithmic characteristics of certain transistors or diodesa2'aa); (b) the use of special "squaring" tubes 3-~) to obtain an identity

95

A C H A R G E D P A R T I C L E I D E N T I F I C A T I O N SYSTEM

-I?V ,I

1,5 nt

v, Hk~T,

L.I

F4rg

llql~J'[

~, I

--

-~

.I

~OU.'~UT

Te

o

o

+1;"

vz

INPUT

o~j

"I',o

'

tl

T~a

Fig. 3. The multiplication circuit. TI, T2 and T 9 are 2N2717; T3, T10 are 2N964; T4, T5 and Tn are 2NS01A; TT, Ta and TI2 are 2N2257;

TH is 2N2259; D is ID 10-050; Zt and Z2 are 2 mA constant current devices (circuitdyne CP3-2.05). 4AB = (A + B) 2 -- (A - B) 2 ; (c) using the amplitude

to time conversion method 1' 2, ~) to handle one of the input pulses. The multiplier to be described is based on the last principle. It is simple, easy to construct and capable of operating at relatively high speed. The detailed circuit is given in fig. 3. This circuit is designed to accept two simultaneous negative pulses, each having amplitude varying from 0.2 to 5 V, with rise time about 12 ns. These correspond to the outputs of the 50 ns linear twin-gate and the adder. Let the pulse corresponding to dE]dx be called VI and the pulse from the adder be called I"2. When these two pulses arrive at the inputs of the circuit, V1 is stretched and V2 is used to generate a rectangular pulse, with duration proportional to V2. This time analogous pulse of V2 is then used to gate the stretched version of V1. The resultant pulse with amplitude proportional to V1 and duration proportional to V2 is then integrated to produce a pulse with amplitude proportional to the product of VI and V2. The stretching of V1 is effected by T x, T 2 and C1. During the rise time of V1, C1 is charged negatively to the amplitude of V1 through T 1. However, when the input pulse starts to fall, TI is turned off because of the charges stored in C1. At the same time the time analogue pulse of I"2 arrives at the base of T2 and turns off T2. This effectively isolates C~, which then retains the peak values of the input VI.

The time analogue pulse of I"2 is formed by Tg, Tlo, C2 and Z 2. Using the same method as in the stretcher, C2 is charged up to the peak value o f V 2 and discharges linearly through the constant current device Z2. The output of the limiter Tlo is a rectangular pulse with duration proportional to V2. The pulse across C1 with amplitude proportional to V: and duration proportional to V2 is fed into an integrator T 6, Z t and C 3 through the isolating stages T4 and Ts. Z1 is a constant current device. T a and T 7 are used to return C1 and C 3 respectively back to their normal conditions when the time analogue pulse of V2 ends. This is achieved by differentiating the trailing edge of the time analogue pulse at the input of T 11, which produces two discharging signals, through T12 , for T 3 and T7 respectively. The linearly rising pulse with maximum height proportional to the product of V1 and V2 is obtained at the emitter o f T a. Normally, the input Vx is delayed for about 15 ns to allow the generation of the time analogue pulse of V2. The dead time of the circuit is almost just that required for the amplitude to time conversion of the I"2 pulse, this conversion rate being 4 V/#s. In the absence of VI, V2 gives nothing at the output of the circuit. In the absence of/"2, V1 produces a small signal at the output, but the coincidence circuit and the twin-gate rule out the possibility of single pulses entering the multiplication circuit.

96

s.K.

MARK

AND

6. Performance of system To check the performance of the particle identification system, targets of beryllium and carbon were bombarded by a 100 MeV external proton beam of the McGill Synchrocyclotron. The counter telescope with brass collimators was placed at an angle about 35 ° with respect to the incident proton beam direction. The dE/dx scintillator (2 m m x 1 cm diameter Naton 136) was viewed edgewise by a R C A 6342A photomultiplier. A cone-shaped aluminized mylar reflector with entrance

iO

>t

3.0

Oo~

\

\

\

_\

N

\

~0^

\

x\

\x

\

if) J 0 > Z

%

\x

g\>,

R . B. M O O R E

% 6000

1.0

PROTONS

\: \ \

n Z

9 3000

Be

139 mg/cm2

z

8

0.3

~. 9 0 0 0 o

600

DEUTERONS

=

i

0.1

i

i

t

,

i

O. 3

i

300

i

I.O

3.0

INPUT 2 IN V O L T S Fig.

4. Performance

of

the

multiplication

circuit.

I00

5. Performance of the multiplication circuit Fig. 4 illustrates the performance of the multiplication circuit. The two simultaneous input pulses were obtained from a fast pulser and shaped so as to be equivalent to those available at the outputs of the twingate and the adder. The amplitude of each of the inputs was independently controlled by means of helipots and the output product pulse was measured by a discriminator, the bias level of which was a linear function of a helipot setting. The range of the variation for each input was from about 0.2 to 5 V. Each straight line of the graph represents a constant product of the two inputs. By holding either one of the inputs constant, the response to the other input showed an increase in slope of about 4% over a range of 0.2 to 5 V. An increase of a factor of 15 in both inputs gave a factor for the output 7% greater than 225. The greater part of this error is due to the non-linear response and finite reverse emitter-base junction impedance of Tz and T 9. However, this error is tolerable in charged particle identification since the Landau spread and the light collection efficiency of the dE/dx counter usually give a greater spread than this to the VaE input pulses.

40

60 CHANNEL

i

6000

PROTO~

I00

BO NUMBER

!

120

i

12

C

89 mg/cmz

3000

0¢.1 u. I 0 0 0 0 n,.

DEUTERONS

6o0

300

I00

I

40

60 CHANNEL

80

I O0

120

NUMBER

Fig. 5. Performance of the system: the separation between protons and deu~rons.

A CHARGED

PARTICLE

and exit windows 0.8 mg/cm 2 thick was used. The E scintillator was a 8.9 cm long and 3.8 cm dia. plastic N a t o n 136 directly coupled to a R C A 6342A photomultiplier. The discrimination levels of the inputs of the fast coincidence circuit were adjusted so that only particles within the energy range of 20 MeV to 100 MeV were capable of triggering the coincidence circuit. The multiplication circuit output was first amplified and then fed into a multichannel analyzer. After empirical adjustment of Vo and k, the multiplier output pulses fell into two discrete groups as shown in fig. 5. A measurement of Vo and k resulting in the best proton-deuteron separation, corresponded very closely to the values of Lo and k used to calculate the separation function shown in fig. 1. This system has been used in this laboratory to study (p,p') and (p,d) reactions in light nuclei with 100 MeV protons. Energy resolutions of 1.5 MeV for protons and 1.8 MeV for deuterons have been obtained in the E counter. No effort has been made to study the actual performance of the system to separate the tritons or multiply charged particles because of their scarcity relative to scattered protons. The authors wish to thank Dr. T. M. Kavanagh, now

IDENTIFICATION

SYSTEM

97

of Nuclear Chicago Corporation, for his guidance and help to one of us (S.K.M.) in the development of this system. This work was supported by a grant from the Atomic Energy Control Board (Canada).

References l) B. Wolfe, A. Silverman and J. W. DeWire, Rev. Sci. Instr. 26 (1955) 504. 2) F. A. Aschenbrenner, Phys. Rcv. 98 (1955) 657. 3) R. G. Stokes, J. A. Northrop and K. Boyer, Rev. Sci. Instr.

29 (1958) 61. 4) w. L. Briscoe, Rev. Sci. Instr. 29 (1958) 401. s) R. G. Stokes, Rev. Sci. Instr. 31 (1960) 768. 4) L. Wahlin, Nucl. Instr. and Meth. 14 (1961) 281. 7) R. J. Grifliths, K. M. Knight, C. J. Candy and J. Cole, Nucl. Instr. and Meth. 15 (1962) 309. 8) M. S. Livingston and H. A. Bethe, Rev. Mod. Phys. 9 (1937) 263. 9) T. J. Gooding and H. G. Pugh, Nucl. Instr. and Meth. 7 (1960) 189. 10) S. Gorodetzky, A. Muser, J. Zen and R. Armbruster, Nucl. Instr. and Meth. 14 (1961) 205. 11) F. F. Liu and F. J. Loefl]er, Nucl. Instr. and Meth. 12 (1961) 124. 12) G. Gianneli and L. Stanchi, Nucl. Instr. and Meth. 8 (1960)79. 13) S. Deb and J. K. Sen, Rev. Sci. Instr. 32 (1961) 189.