Transistorized nuclear counting instruments for a low-level counting laboratory

NUCLEAR

I N S T R U M E N T S AND METHODS 5 °

(i967)325-332;

©

NORTH-HOLLAND

PUBLISHING

CO.

T R A N S I S T O R I Z E D NUCLEAR COUNTING INSTRUMENTS FOR A LOW-LEVEL COUNTING LABORATORY S. RAGUPATHY

Air Monitoring Section, Atomic Energy Establishment Trombay, Bombay, India Received 10 January 1967 Simple transistorized circuits adoptable for low-level counting work have been described in this paper. These include: I. a dekatron scaler with five decades; 2. a low background beta counting system with coincidence and anticoincidence circuits; 3. a single channel pulse height analyzer for gamma spectro-

metry. The performance of these circuits in low-level counting work has also been reported. The special features of the circuits described here are their simplicity, good long-term stability and reliability, With fastness adequate for most type of low-level counting work.

1. Introduction

applications are given. Fig. 1 shows the three instruments described in this paper.

The transistorized nuclear counting instruments described here have been developed for use at the low-level counting facility at the Air Monitoring Section, Atomic Energy Establishment Trombay. A review of the published work 1.3) in this field of instrumentation shows that the trend is towards the development of fast circuits for general purpose instruments. The use of such fast circuits for routine low level counting work may not be always essential. In designing these instruments more importance was given to the circuit simplicity, stability and reliability than to the circuit fastness. In this paper, a dekatron scaler, a low background beta counting system with anticoincidence shielding and a single channel pulse height analyser are described. A brief description of the circuits designed and their performance and

2. Dekatron scaler

2.1. DESCRIPTIONOF THE CIRCUIT The dekatron scaler (fig. 2) consists of a two stage cascaded emitter follower (T 1,2) at the input, a simple two stage cascaded amplifier (Ta-Ts) with individual emitter feed-back, a voltage discriminator (T6-Ta) and five similar dekatron scaling stages (T9-T19). Almost all nuclear radiation detectors are high impedance current generators and to couple these efficiently to any low impedance transistor stage an emitter follower is imperative. The cascaded emitter follower (T 1,2) in this configuration serves this purpose. The output of the cascaded emitter follower is fed

Fig. 1. Single channel pulse height analyser; low background beta counting system; Dekatron scaler (left to right),

325

326 < > o

x k

÷

÷

<

o

I°

1 I

~J

I I

w o .¢¢r¢~

ol

",~0 01

_

__..]

I I

I I I

o~

al

•"v'- 0 0 1

,...- j

1 ~-D.--

_ _

.._[

;o

_+ t--

o m

~J-

,_++ x

'

ii v

++

'- : ,

~i

-i

TRANSISTORIZED NUCLEAR COUNTING INSTRUMENTS through a six position step attenuator to a simple two stage cascaded amplifier (T3-Ts). Individual emitter feed-back technique 4) is employed to stabilize the gain and improve the frequency response. With a feed-back fraction of 5 the gain of the amplifier per loop is 7 and a rise time of 0.5/ts has been achieved. The amplifier accepts negative pulses and delivers negative pulses of maximum 6 V without overloading. The voltage discrimination is done in the reverse biased transistor stage (T6). As this transistor is normally cut off, the base leakage current is not amplified in the collector circuit but it does flow through the base circuit resistors affecting the stability of the discriminator. The current normally flowing through the threshold potentiometer is large compared to the base leakage current and so the variation of base leakage current due to temperature changes does not disturb the threshold level of the discriminator too much. The normally reverse biased and hence cut off transistor (T6) is switched on when the pulse applied reaches the triggering level which is set by the threshold helipot (P~). The positive going trigger pulse thus produced at the collector of T 6 and T 7 triggers the collector coupled univibrator (T7,8). The combined gain and its stability determines the discriminator switching speed, sensitivity and offset error. If triggered the collector coupled univibrator produces 60/zs negative pulse which goes to trigger the inverter (T9). This (T9) produces 60/~s positive pulses which are applied to the first dekatron scaling stage. Each dekatron scaling stage consists of a cascaded two transistor driving circuit and one GC I0 B5) double pulse dekatron tube. On applying the 60/~s positive pulse through a properly set differentiating circuit to the base of the first transistor (T10) it conducts producing a negative pulse at its collector. This negative pulse is applied to the 1st guides of the dekatron tube and so the glow is transferred from the on zero cathode to the nearest I st guide. The same negative pulse is differentiated and the resultant positive going pulse is applied to the base of the second transistor (T~I) which on conduction produces a negative pulse at its collector. This negative pulse is applied to the 2 nd guides of the dekatron tube and so the glow is transferred from the on l st guide to the nearest 2 nd guide. When this negative pulse ceases the glow is transferred from the on 2 "a guide to the 1 st cathode, which is more negative at that moment and thus the glow transfer process completes. At the end of ten similar transfers the glow returns to the zero cathode, also known as the output cathode, producing a positive pulse which is applied to the

327

next dekatron scaling stage for further scaling. The differentiating circuit at the base of the first transistor determines the width of the first guide drive pulse and the differentiating circuit at the base of the second transistor determines the width of the second guide drive pulse. The first differentiating circuit also determines the required time delay between these two guide drive pulses which is important though not critical, to achieve reliable glow transfer process. 2.2. PERFORMANCEAND APPLICATIONS The dekatron scaler accepts negative pulses and can be used for gross alpha, beta and gamma counting with suitable detector probes. The scaler functions are reliably upto 2 kc/s. 3. Low background beta counting system

3.l. DESCRIPTIONOF THE CIRCUIT The low background beta counting system (fig. 3) consists of two cascaded emitter followers (T 1,2 and T9,1o ) at each input, two simple cascaded amplifiers with individual negative feed-back (T3,4, 5 and Tl1,12,13), two voltage discriminators (T6.7, 8 and T ~4, ~5,~ 6), one coincidence and anticoincidence circuit (T 17-T20) and a two channel dekatron scaling circuit. The emitter followers, amplifiers, voltage discriminators and dekatron scaling circuits are similar to those used in the dekatron scaler described above. The channel " A " has been designed to accept negative pulses directly from a GM counter and the channel " B " has been modified to accept pulses directly from the GM counter or from the scintillation counter. With proper counters at the two inputs and the gain and discriminating levels set properly, the " A " channel discriminator (T6-Ts) delivers a 100 ¢ts positive pulse and the " B " channel discriminator (T14-T16) delivers 200/~s positive and negative pulses to the coincidence and anticoincidence circuit (T17 and T20). The coincidence and anticoincidence circuit consists of one simple series coincidence circuit (T18 and T2o) and one series anticoincidence circuit (TI7 and T19). The 100/Ls positive pulse is differentiated and the delayed negative going trailing edge of the pulse is applied to the bases of the normally cut off transistors (T]7A8). The 200/zs positive gating pulse is applied to the base of the normally forward biased transistor (T19) and the 200/~s negative gating pulse is applied to the base of the normally cut off transistor (T20). Whenever two pulses arrive at the two inputs simultaneously, that is, in coincidence, the 200/zs negative gating pulse from channel " B " opens the normally

o

I- - ~

z

Z

o

0.

- -


c

_ >

:

0

+

~z'

r-

8 0~

o o

o

.c 0

o

1

o

~

.

m

~-~--~

~ o o~ ~o °~

J

~ ,@-

,A/vW~ i1.

p-

g z II1

329

5~ 0

~r Z

\ ¢ +

-t1'

< m

~iL.

o

o

u.

i. e.o e-

:

:Pr~ o

o m

o

~ +

e--

Y

,,n:

m

~2

~n m u

,-

3

Y

oz . ( ~

~

~rOOS "V'OS~ -trOOZ

"u'OOt

-U'O£

330

s. RA.GUPATH Y

closed gate (T20) and after 100/zs the negative trigger pulse from channel " A " triggers the normally cut off transistor (T~s) producing a positive trigger pulse at its collector, which goes to trigger the coincidence channel scaling circuits. If and when a single pulse arrives at the " A " input, that is, in anticoincidence, the negative trigger pulse from channel " A " triggers the normally cut off transistor (TI7) producing a positive trigger pulse at its collector, which goes to trigger the anticoincidence channel scaling circuits. When two pulses arrive at both the inputs or a single pulse arrives at the " B " input alone, the 200/ts positive gating pulse inhibits the anticoincidence circuit for 200/ts and during this period pulses arriving at " A " input are blocked. For the proper functioning of the system with maximum counting efficiency and minimum background, proper gain, discriminator level and " A " and " B " channel pulse width should be set properly. 3.2. PERFORMANCE AND APPLICATIONS The performance of the circuits was studied with two Van Duuren type GM counters, Mullard Type MX 157, ref.6), operating in anticoincidence, at the two inputs. With a 6" mild steel shielding for the counter, the system gives a background of I cpm and a counting efficiency of 2 0 ~ for 45Ca, 17~ for 9°St, 17% for 11lAg and 29% for 9Oy. The performance of these circuits was also studied with one GM counter, G.E.C. Type G.M. 4/LB, at the " A " input and one plastic scintillation counter ( 4 1 " x 6 " o . d . and lgZ"x4 " i.d,) v) at the " B " input. The channel " B " discriminator was set approximately at 500 keV and the " A " and " B " channel pulse widths were set at 100/~s and 200/zs respectively. With this setting, the system background was observed to be 0.5 cpm in a lead shield of 3" wall thickness and a counting efficiency of 8% for aSCa, 12% for 9°Sr, 14% for ~l~Ag and 20% for 90y were achieved.

4. Single channel pulse height analyser 4.1. DESCRIPTION OF THE CIRCU1T The single channel pulse height analyser has been designed for the routine low-level measurements of radioisotopes by gamma spectrometry. A single photopeak can be covered by one broad window. A lower level discriminator and an upper level discliminator determines this broad window which can be placed anywhere within the linear range of the spectrometer. The single channel pulse height analyser (fig. 4) consists of a linear amplifier (T1-T6) at the input,

a lower level voltage discriminator (Tio-T12)an upper level voltage discriminator (Tv-T9) , an anticoincidence circuit (T13,14) and a five stage dekatron scaling circuit. The two voltage discriminators, the anticoincidence circuits and the dekatron scaling circuits are similar to those used in the previous instruments described above. The linear amplifier (T1-T6) is designed to accept a negative pulse from scintillation counter and the signal is applied to the amplifier through a six position step attenuator. The two loops of the linear amplifier are almost similar in configuration but for the difference in dc levels to handle signals of different amplitudes. Collector to base feed-back technique s ) is employed to stabilise the gain and to improve the frequency response. The emitter followers help to isolate the circuits and avoid undesirable circuit loading. With a feed-back factor of approximately 20 for each loop the overall gain is about 400 and a rise of 0.5/ts is obtained. It delivers negative pulse of maximum 6 V without over loading. This negative pulse is applied to the lower level and upper level voltage discriminators (T7-T 12) which are connected in parallel, and if triggered they deliver 5 ltS and l0 ¢ts positive pulses respectively to the anticoincidence circuit. The anticoincidence circuit (T13,14) is one of series type and the top transistor (T~s) is normally reverse biased and the bottom one (T14) is normally forward biased. The 5/zs positive pulse from the lower level discriminator is differentiated and the delayed negative going trailing edge is applied to the base of the normally reverse biased transistor (T~3) and the 10/is positive gating pulse from the upper level discriminator is applied to the base of the normally forward biased transistor (T14). If the pulse applied is able to trigger the lower level discriminator only and not the upper one, the anticoincidence circuit is triggered and a positive trigger pulse produced at the collector of transistor (TI 3) goes to trigger the dekatron scaling circuit. If the pulse applied triggers both the discriminators, the 10/ts positive gating pulse from the upper level discriminator blocks the anticoincidence circuit and so the negative going trailing edge of the pulse from the lower level discriminator reaching the circuit 5/ts after the block pulse will not be able to trigger the anticoincidence circuit and thus the pulse is rejected. The 5/ts delay compensates for the unavoidable time jitter due to the finite rise time of the pulse to be analysed. 4.2. PERFORMANCEAND APPLICATIONS The performance study made on the single channel

TRANSISTORIZED

NUCLEAR

COUNTING

~g

25 K

~oc

0,01.~ :>

H.T. OUTPUT

OOK ~ , L

~2uF3Kv

L85A2v ;:(.I 7 TUBIE5")

0'0"1u)~'

,=-

331

INSTRUMENTS

4.7M

J

4.7M 100/MA

I•

H.T.

KSK E250C200

?

1OH

5K

-400V

5K

C .105V

l o: KSK E2SO C~'O0

SR400

B2mO

F12V

O0 IJ.F ~'12 V3W Z EN IER L.T" ~0

SRIO0

---- 8 2 ..,t~

~

O

-12V

12V3WZ EN ER

SRIO0

O $R100

4-6V

00.~1F - ~ 6 V 3W Z ~lN R

SR400

120-00

.~RIO0

i-sv

6V3W ZE NNNER

I Fig. 5. Low and high voltage power supply.

pulse height analyser, using a mercury relay precision pulse generator indicated 0.1 ~ integral linearity and 0.5~o threshold stability (for a period of 24h). The circuit functions are reliably upto 50 kc/s. The performance of the single channel pulse height analyser was also studied with a cylindrical (2½" x2½") NaI(T1) crystal (Harshaw lntegral line, Type 10S10) as the detector. A lead shield of 6" wall thickness was

used to reduce the background. The feasibility of using this spectrometer for low level measurement of gamma emitting radioisotopes was studied by assaying ~37Cs and 4°K present in milk. The single broad channel was set to cover the full photopeak of 137Cs and the surplus channel was used for 4°K measurement. The radioactive assay of a typical milk sample containing 21.7 pCi of 137Cs and 9.2 nCi of natural 4°K

332

s. RAGUPATHY

was carried out in a counting time of one hour, with fairly good accuracy. A background of 11 cpm and 71 cpm was observed in these respective channels. The possibility of adopting this single channel pulse height analyser circuit along with the coincidence circuit for complex gamma spectrometry work after chemical group separation is under study.

instruments performance study was made is air conditioned and is provided with regulated power. The author wishes to thank Dr. K.G. Vohra from whom guidance for this development work and suggestions for writing this paper were received. References

5. Concluding remarks on the performance of the instruments

The three instruments described above are being used for routine low level counting work and their performance was found to be satisfactory. Fig. 5 gives the schematic diagram of the zener regulated low voltage power supply and VR regulated high voltage power supply used to operate the instruments described above. The low level counting laboratory where the

1) j.R. Gilland, Rev. Sci. Instr. 30 (1959) 479. 2) T.L. Emmer, IRE Trans. Nucl. Sci. NS-9, no. 3 (1962) 305. 3) M.G. Strauss, Argonne National Laboratory Report, ANL6123 (1960). 4) F.C. Franklin, Transistor circuit analysis and design (Van Nostrand, 1960)p. 196. 5) R.C. Bacon and J.R. Pollard, Electr. Eng. 26 (1950) 173. 6) K. Van Duuren, A.J.M. Jaspers and J. Hermsen, Nucleonics 17 (1950) 86. 7) U.C. Mishra and R. Kerala Varma, Nucl. Instr. and Meth. 24 (1963) 473. 8) F.C. Franklin, ref. 4) p. 195.