S T U D I E S W I T H S C I N T I L L A T I O N C O I N C I D E N C E S P E C T R O M E T E R : Cs T M

BY GIRISH CHANDRA (Tata Instttate of Fundamental Research, Bombay-l)

Recewed August 23, 1956

(Commumcatcd by Dr. B. Peters, F,A.SC )

ABSTRACT

The set-up of 'Slow-Fast' conicldence scintillation spectrometer is described. Gamma-gamma coincidences in Cs TM have been studied. The 604-kev. gamma transition is found to be in cascade with 460 + 20. 555 + 15, 794 i 15 and 1349 • 30 kev. gamma transitions. The 794-key. gamma transition is found to be in cascade with 604 i 10 and 555 ~ 15 kev. gamma transitions. The results are consistent vr the decay scheme of Cs TM proposed by Forster et al. ix

INTRODUCTION

COINCIDENCE studies with scintillation counters have been found to be a very useful method of determimng the sequence of energy levels in a nucleus formed in a complex beta-decay of a radio-active nucleus. For this purpose a 'slow-fast ' coincidence spectrometer as described below has been set up. This instrument was used to investigate the gamma transitions in Ba TM, resulting from the complex beta-decay of Cs1% This isotope has been studied by several workers 1-11 and in particular, the gamma-ray energies were determined from the photo-electron spectrum taken in the Siegbahn- Slatis beta-ray spectrometer 7 at this Institute. In the decay of Cs TM, eleven gamma transitions of the following energies were reported--467, 553, 571, 607, 794, 1027, 1164, 1368 and 1401 key. Of these the two gamma-rays at 604 kev. and 794 kev. are the most intense ones. The aim of this work was to determine which of the other gamma transitions are in cascade with these two well-known gamma-rays in Ba TM.

In a previous report on beta-gamma and gamma-gamma coincidence study of this isotope, G. Bertollini et a l? o, employed discriminators, one in each channel, for energy dxscrlmination, the resolving time of the coinci- dence unit being 0.5/z sec. In this work, gamma-gamma coincidences have been studied, employing a shorter resolving time, 1.5• 10 -s sec. of the coincidence unit and single channel pulse analysers, one in each channel, for energy selection. This helps in deciding more definitely the energies of gamma-rays that are in cascade and throws some light on the question

194

Studies with Scintillation Coincidence Spectrometer: Cs 1~ 195

of the sequence of levels in Ba TM. In particular, as a result of this investi- gation, it can be argued that the first excited state in Ba TM is 604 kev. rather than 794 kev.

EXPERIMENTAL ARRANGEMENT

The block diagram of the conventional ' s low-fast ' coincidence scintillation spectrometer is shown in Fig. 1. For gamma-gamma coinci- dence measurements two NaI (TI) crystals each 1" in diameter and 1" thick, viewed by two EMI6260 photomultipliers have been used. Both the crystals were shielded with �89 thick lead collimators. They were kept 120 ~ apart so that they do not see each other thus minimizing the false coinci- dences arising from scattering of gamma rays from one crystal to the other.

-i N H "''~ CATHODE !.1 klE#d~ CHANNEL FOLIkOWE'R AMPLIFIER PULSE I ~-

ANAL'VSER

' LIMITER

COIMGIOEHCE I r I'~,- IOSSec J [g " 41,r i COINCIOENCEI '~ J I 1 II

To'rAL C,O I NC

~ I M I T E P

ANALVSER

O EI..AY I O ~ S

" r : 4 p , s ] I I I G HAFIC s

COINr

FIG. 1. Block Diagram of Slow-Fast Coincidence Scmtdlatmn Spectrometer.

Pulses from the anode of the photomultiplier are limited to an ampli- tude of �89 volt by a limiter circuit. The limiter output pulses are carried to the fast coincidence unit by a coaxial cable RG8U which is terminated at both ends by its characteristic impedance to eliminate any reflections. These pulses are shaped at the input to the coincidence circuit by 400 cm. long RG8U coaxial cable shorted at the other end. The fast coincidence circuit is a bridge type circuit using crystal diodes. By inserting known delays in

196 GnusH CBA~ZDRA

either channel the prompt coincidence due to Co ~~ gammas were counted and from the curve thus obtained, the resolving time of the coincidence unit was determined to be 1.5 • 10 -a sec.

3o

6O4- KEV

IOP_.O I I G O 1 3 4 0

1 T "r I0

6

i ' 6

- 5

Z

60

50

4 0

754. I<EV

I I I I I I I I I I I I I ~ O ~oo p_.oo ~ o o 34-0 5 8 o 4~.o 4GO

~ I A S

FZG. 2. Gamma-ray Spectrum by a SCmtlllatlon Spectrometer

Pulses from the third dynode, after amplification, are fed to a single Channel analyser for energy selection. The fast coincidence output and those

Studies with Scintillation Coincidence Spectrometer: Cs TM 197

from the two single channel analysers are fed to a slow triple coincidence unit(resolving time 4/zs.) thus enabling coincidences between any two selected energies of gamma-rays to be counted. This indirect method of introducing energy selection to coincidence circuits is necessary, because the fast coinci- dence unit must be operated by the pulse derived from the first few photo- electrons produced in the photomultiplier, whereas the analyser is required to be triggered by a pulse proportional to the total number of photo-electrons released by the light scintillation from the phosphor. The analyser output pulse will therefore be considerably delayed with respect to the pulse operat- ing the fast coincidence unit and, moreover, this delay will vary with pulse amplitude. The fast coincidence circuit therefore cannot follow the analyser. The chance coincidences are also counted simultaneously by feeding the same three pulses to another triple coincidence umt (resolving time 4 t~s.) with one channel delayed by 10/zs.

l 50

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8 0 i- 0

IU

< Q: 60

Z I- Z

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. . . . .COINCID- ENCE

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,~_.,ANNING CH~ BIAS FIo. 3. Spectrum in Coincidence with 794 kev.

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p- Z

IOO 0 U

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Z

U

J O

1 9 8 G I R I S H C H A N D R A

R E S U L T S

Gamma-ray spectra in coincidence with 604 kev., 794 key. and 1349 key. respectively were taken. A very strong source was used. The following results are obtained as shown in Figs. 3, 4 and 5 : - -

0 Z I - Z

0 u

Id ..1

Z

SO

4 0

3o

2O

I0

6o4~. 41'V v

, , SlI~GLE 5PECTI~UM

COINCIDENCE SPECTRUM /

0 4 o 0

7a,(- KI{V

4 0 - 604 KEV FIXED WIDTH CH 1 H

d ~ o 2

Iz I,I n

2 0 0 Z D 0 U hi U Z

I 0 0 bl Q u Z

u

o L J ~ ~ o Jo ,5 20 g5 3 0

S C A N N I N O C H ~ B I ~ S

FIG. 4. Spec t rum in Coincidence with 604 key.

1. One channel was fixed to allow full photo-peak of 794 kev. as shown in the inset of Fig. 3. Both crystals were shielded with lead and were kept 120 ~ apart and were arranged to be 2" from the source. The gamma transi- tions in coincidence with the 794 key. gamma-ray are found to be of energies 604 4- 10 and 555 4- 15 kev. There is also strong evidence for the existence of a gamma-ray of energy 460 + 20 key., which is not in coincidence with

794 kev. (Fig. 3). 2. One channel was fixed to allow a small fraction of the 604 key.

photo-peak as shown in the inset of Fig. 4. In this case the geometry was the same as above. The gamma-rays in coincidence with 604 key. gamma-ray are found to be of energies 460 4-4- 20 key., 555 4- 15 key. and 794 • 15 kev,

3. Finally, one channel was fixed to allow the full photo-peak of 1,349 key. gamma-ray. Since this is weak, the crystal corresponding to this

Studies with Scintillation Coincidence Spectrometer: Cs TM 199

I0 gO4- KEV

~IMGLE ~PtrCTRU~f

" ' - COIMC IDEI'JC lr A 5 P I [ C ' T R I J ? 1

: H ~ I- -I

o ~zv \

~5 4 0 4 5

714. KEY

1

o

;,,b\ /%

400 500 600

KEV

187~.

1842

~643

i

o so

t o o ~

Z 3

8 i

5O U Z hJ o U z

S C A N N I N G C H . t B I A S

FIG. 5. Spectrum in Coincidence with 1350 kev.

134. 134. Cs (e-~v) 5a

140~, I,~67

11153

1168

40

.-3O

t,I

(Z

t~ 2 i-

0 U IO_ ;11 ,.I O Z

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0

F[o. 6. Decay-Scheme of Cs TM Propose, d by Forster and Wiggins.

200 GIRISH CHANDRA

channel was unshielded and kept at a distance of 1.5 cm. from the source. A lead plate, 1 cm. thick, was interposed between this crystal and the source in order to reduce the 604 key. and 794 key. radiations. The 1349 key. gamma-ray is found to be in coincidence with 604 key. gamma-ray but not with 794 kev. gamma-ray as seen from Fig. 5.

DISCUSSION

All the results obtained in this work, namely, the triple cascade 604 key.- 794 kev.-555 kev. and the double cascades 460 kev.-604 kev. and 1349 kev.- 604 kev., are consistent with the decay-scheme of Cs TM shown in Fig. 6 which is the decay-scheme proposed by Forster and Wiggins. 11

The main disagreement between the different decay-schemes proposed for Cs TM seems to be about the energy of the first excited state in Ba TM.

Since the weak gamma 1349 kev. (1369 kev. of Forster 11) is in cascade with the intense 604 kev. gamma-ray, 604 kev. as the energy of the first excited state seems to be more justified than 794 kev. If 794 kev. is assumed to be the energy of the first excited state, then the 1349 kev. gamma transition has to be to the ground state and the 604 kev. gamma-ray has to be emitted by the highest level 1972 kev. so as to be in cascade with the 1349 kev. transition. In that case, the 604 key. gamma transition would be a weak one since the 1972 kev. level is formed by a rather weak beta-ray group (80 kev., 28~on). In fact 604 key. gamma-ray has been found to be the most intense one. Hence it is preferable to assume 604 key. as the energy of the first excited state.

Author's thanks are due to Dr. B. V. Thosar for his interest and helpful suggestions and to Dr. S. Jha for discussion.

REFERENCES

1 Elhot, G. and Bell, R.E. Phys. Rev, 1947, 72, 979 2 Slegbahn, K and Ibid, 1948, 73, 410.

Deutseh, M. 3. Peacock, C L. and Ibtd, 1951, 83, 484.

Brand, J. L. 4. Waggoner, M. A , et al IbM, 1950, 80, 420 5. Schmldt, F. H and Ibid., 1952, 86, 632.

Keister, G. L. 6. Cork, J. M., et al. Ibld, 1953, 90, 444. 7. Joshi, M. C. and Thosar, Ibid, 1954, 96, 1022.

B.V. 8. Lu, D. and Wiedenbock, Ibid., 1954, 94, 501.

M.L . 9. Keister, G. L., et al Ibid, 1955, 97, 451.

10. Bertohm, G e t al I1 Nuovo Cimento, 1955, 2, 273. 11. Forster, H H. and Ibtd, 1955, 2, 854

Wiggins, J. S.