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 : Se 75 a n d Sb 125

BY GIRISH CHANDRA (Tata Institute o f Fundamental Research, Bombay-l)

Received August 23, 1957 (Communicated by Professor B. Peters, r.A.Sc.)

ABSTRACT

Cascade relationships between gamma-transitions in the decay of Se ~5 and Sb 1~6 have been investigated with the help of a scintillation coin- r spectrometer. A decay scheme for Se v5 has been proposed.

SELE~UM-75

THE decay scheme of Se ~5 has been studied by a number of workers. It is well established that it decays by pure electron-capture process to As 7a, which emits a large number of gamma-radiations. Jensen et al. 1 in their study of internal conversion and photo-electron spectra of this isotope find the follow- ing gamma-radiations : 66, 76, 98, 124, 138, 203, 268, 281,307 and 405 Kev. Later Lu et aL 2 who studied this isotope by the summing technique using a scintillation spectrometer, concluded that--

(a) 405 Kev is the highest excited level.

(b) 138 Kev and 269 Kev radiations are in cascade.

(c) 124 Kev transition is in cascade with some gamma-transition whose energy is not known.

(d) 98 Kev transition is perhaps not in coincidence with any transition.

It was considered desirable to study this isotope in detail with the help of a 'Slow-Fast ' scintillation coincidence spectrometer. This instrument has been described in an earlier paper)

RESULTS AND DISCUSSIONS

Gamma-spectra in coincidence with 138 Kev and 269 Kev transitions respectively were recorded and the following results were obtained :--

1. 138 Kev and 269 Kev transitions are found to be in coincidence in agreement with Jensen et al. 1 and Lu et aL ~ (Figs. 2 and 3).

2. 138 Kev transition is found to be in coincidence with 200 -4- 5 Kev transition (Fig. 2 b) confirming the assumption of Jensen et a D and LU et aL 2

360

Studies with Scintillation Coincidence Spectrometer: Se ~5 and Sb ~" 361

3. A definite indication of a weak gamma-transition of about 95 4- 3 Kev, in cascade with 138 Key (Fig..2 a), has been found. Lu et al. ~ have

150

kl

~ 1 0 0

Z I- 2

O 5 0

O ~ O

7 7 KE~

1 / Ioo

FIo. 1.

158 Ks

1

P 200

\

2 6 9 KEV

1

5 o o 4 0 0 5 0 0 6 0 o

BIAS Gamma Spectrum of Sr rS.

I00

Ill

r162 50

2 t- Z 0 u 0 I I

50 sSo a

~O01"- L i ~oo_.5 KEV l

I 0 0 I I

600[ r" 26,9 K~v

400

20O

0 I ~ o o b 5OO ~50 550 C 4 5 0

B I A S

FIG. 2. Gamma Spectra in Coincidence with 138 KoV Gamma Radiations of 8r 75.

found by summing technique that the 98 Kev transition is not in coincidence with any transition. Thus, there are possibly two transitions of nearly the same energy of 98 Kev.

4. Contrary to the assumption of Jensen et al. 1 and Lu et al. ~, 66 d- 3 Key transition has been found to be in coincidence with 269 Kev transi-

362 GmisH CHANDRA

tion and not with 138 Kev (Fig. 3). 77 Kev transition is not found to be in coincidence with either of 138 Kev or 269 Kev transitions.

5. Jensen et al. 1 and Lu e t a l . ~ infer 124 Kev and 280 Kev transitions to be in cascade. It has been shown by Lu e t al . ~ that 124 Kev transition is in coincidence with some gamma-transition whose energy is not known. To check the assumption that 124 Kev and 280 Kev are in cascade the follow- ing experiment was performed:

1500

-J I0OO ! v

500 ~ /

J!ZZ__ 0 0 lOO 200 ~Soo

BIAS FIG. 3. Gamma Spectrum in Coincidence with 270 KeV of Se ~5.

la

IZ

Z I- Z :) 0 0

18o

8O

4 0

C O

136 KEY

4.05 K E V

1

e~o KEV

sO0 ~oo 500 400 500 600 BIAS

FIG. 4. Gamma Spectrum of Se ~5 with source at the Centre of the Crystal.

Studies with Scintillation Coincidence Spectrometer: Se 75 and Sb 1~5 363

Gamma-spectrum of Se 75 with the source at the centre of a 2~• 2" NaI (T1) crystal was taken (Fig. 4). Since 138 Kev and 269 Kev peaks are summed up to 405 Kev, the peak at 280 Kev is easily brought out as a result of considerable reduction in the intensity of the 269 Kev peak which ordi- narily would mask the 280 Kev line. Gamma-spectrum in coincidence with this 280 Kev is taken in the region of 124 Kev (Fig. 5). In this case the crystal of the other channel was kept opposite to the hole of the well-type crystal. In Fig. 5, the singles and coincidence spectra are shown normalised

s38 KEV

L ~ ~ SINGLE5 SPECTI~UM ,~I~ECTRUM 11.4 C.OING

QO KE Wl*lrH ~80 REV.

-~-x DIFF BETWEF.,)J SINGLE8 AND COINC. 6PECTIRA.

I 0

" b.

0 / ~ ! I lOO P.O0 3 0 0

FIG. 5. Spectrum in Coincidence with 280 KcV of Sc vb

at the peak. The difference between the two spectra gives a peak at about 90 Kev, showing that the coincidence spectrum does not contain 90 Kev line as expected but contains 138 Kev as well as 124 Key lines. This indicates the possibility that the 124 Key and 280 Kev transitions are in cascade.

On the basis of these results a modified decay scheme of Se 75 is proposed (Fig. 6). This explains all the above mentioned results. Since the 77 Kev transition is not found to be in coincidence with either of 138 Key and 269 Key transitions, it is assumed that the 77 Kev level is mainly formed by the electron-capture process, 66 Key transition being much weaker than the 77 Key transition.

ANTIMONY-125

This isotope has been studied by a number of workers with the help of beta-ray spectrometers. It is found to have a complex beta spectrum 4 of energy groul~s at 621 Kev and 288 Key. There were indications of lower energy groups but ~heir energies could not be measured because of a large number of internal conversion lines. On the basis of beta-gamma coinci- dence experiments with a magnetic lens spectrometer Moreau 6 reported the following beta groups with maximum energy at 612, 444, 300 and 125 Key.

A4

364 GmlSH CHhNDRh

Spectrometer studies 4 have shown the existence of the following gamma- transitions: 110, 125, 174, 431,466, 609 and 646 Kev. It has been shown ~-s that there exists an excited state of 145 Kev in Te 1~5 which is a metastable state of a half-life of 58 days.

Se'zs

KEY ,405

_~41

280

~ 5 3

158

9 0

75

~05 I~4 263

ZSO

158

I

507

I

I Zo3~

I

I

i / I I

g.5 I I I

I f I

G4 . i I i

7~ g8

FIG. 6. Proposed Decay Scheme.

To establish the cascade relationships between different gamma-transi- tions in the decay of Sb x25 coincidence studies were undertaken.

Studies with Scintillation Coincidence Spectrometer: Se 7s and Sb z~5 365

RESULTS

Gamma-spectra in coincidence (Fig. 7) with 175 Kev and 427 Kev transi- tions respectively were taken with the following results:

IO

[ 17IS K [V .4.e'r 46as SbI~5 I KIEV K E Y

8

SINGLE SPECTRUM

IKEV t10 + IO 2t0• 3~O+_.tO 4 2 5 . +~0 595 KEV \ /

i m �9

W I T H 1 7 5 K E V = , , - f ",,~ 00 " iiii'

I I I I 0 I00 200 ~00 4,oo 500 i500 700

B IAS

FzG. 7. Gamma Coincidence Spectra of Sb z=5.

I. Gamma-transitions in coincidence with 175 Key are found to be: I i0 -I- 10, 210 ~ 15, 320 i 10 and 425 ~ 10Key.

2. Gamma-transitions in coincidence with 427 Key are found to be: 105 ~ 10, 175 -I- 5 and 210 ~ 15 Key.

These results are in good agreement with those of a recent coincidence study by Lazar. 8

366 GIRISrI CI-IA~I~RA

Author's thanks are due to Dr. B. V. Thosar for his interest and helpful suggestions, to Dr. S. Jha for discussion and to Mr. K. S. Bhatki for prepa- ration and chemical purification of the sources.

REFERENCES

1. Jensen, N., et aL P.R., 1953, 90, 557.

2. Lu, D. C., et al. Ibid., 1955, 97, 139.

3. Girish Chandra Proc. Ind. Acad. ScL, 1956, 44A, 195.

4. Kern, D., et aL P.R., 1949, 76, 94.

5. Moreau, P.J. Arkiv For Fysik, 1954, 7, 391.

6. Hill et al. P.R., 1949, 75, 324.

7. Siegbahn, K. and Forsling, W. Arkiv For Fysik, "1950, 1, 505.

8. Bowe, J. C. and Axel, P. .. P.R., 1952, 85, 858.

9. Lazar, N. H, .. Ibid., 1956, 102, 1058.