M E A S U R E M E N T O F T R U E A N G U L A R C O R R E L A T I O N I N F e ~7 I N A S I N G L E C R Y S T A L O F M E T A L L I C

C O B A L T

BY GIRISH CHANDRA

(Tata In.r of Fundamental Re~earch, Bomba),-5, India)

Reeeived August 24, 1966

(Communieated by Dr. B. V. Thosar, F.A.Sc.)

ABSTRAL"r

Unperturbed angular correlation in Fe 57 has been observed in a single erystal of metallie cobalt. The value of the angular correlation coet¡ Aa = - 0"042-4-0.016 is obtained whieh gives the mixing ratio �91 .-= -- 0.17 4- 0.03 for the 122 keV gamma-transition.

INTRODUCTION

IN the measurement of angular correlaª betwccn nuclear radiations of a cascade whose intermediate state has largc half-lifc--of the ordcr of l0 -g sec. or more, it is gcnerally a dit¡ problem to find a suitable medium for cmbedding the source in order to determine unperturbexi angular corrcla- tion. The presence of intcrnal magnetic ficld and electric field gradicnt can strongly attenuate the anisotropy. Unperturbed angular correlation can be observed in an axially symmetdc single crystal. 1 The population of the substates quanfised along the symmetry axis remain constant and hence the correlation is undisturbed if the symmetry axis coincides with the direction of emission of the first gamma-radiation of the cascade. Single crystal of metaUic cobalt is uniaxial and has the hexagonal close-packed structure.~,3 Ir has one symmetry axis along the hexagonal axis which is also the direction of easy magnetisation at room temperature, and when unmagnetised all the domains lie in one of the two orientations paraUel to this axis. Above 270 ~ C., the hexagonal axis becomes the most difficult direction of magneti- sation, 2 all directions at right angles to the axis being equally easy. Above 430 ~ C. the crystal is whoUy face-centred cubic.4 Thus a single crystal of cobalt is a suitable medium for observing unperturbed angular correlations. The hyperfine structure of Fe 5r in metallic cobalt has been studied in a Mossbauer expedment.S, 6 Ir gives the effective internal maguetic field acting at F ~ 7 nuclei in cobalt metal as H,rt = 3" 1 • 105 Oe, Very small quadruple

25

26 GIRISH CtD, NDRA

interaction was observed which will not appreciably perturb the angular correlation.

The angular correlation between 122 keV gamma-rays from Fe s7 was measured in several types of sources by Lindquist and Heer.7 The angular correlation is quite likely to be attenuated as the intermediate level at 14 keV is known to have large half-life. Some reeent measurementsa, 9 gave the valuc for half-life Tal2 = 0.98 • --7 sec. The spins of 136 keV and 14 keV levels ate 5/2 and 3/2 respectively and that of ground state is 1/2. The mag- netic moment of 14 keV level is -- 0. 1552 nm lo and it also possesses fairly large electric quadruple moment? o Thus both electric and magnetie inter- actions can cause perturbation of angular correlation. The 14 ieV is estab- lished to be a pure MI transition from the measurement of internal conver- sion coefficient? The admixture of E2 in the 122 keV transition has been the subject of several investigations. 7, 1~-~3 Measurements have been made by studying anisotropy of 122 keV y-rays from oriented Co ~7 nuclei. 1~ In such measurements there are two types of " ' " uncertalntles which can cause error in the determination of E2 admixture. One is the reorientation effect ~4 in the 136 keV state whose half-life is 8.8 • 10 .9 sec. 15 and the second is the presence of the 136 keV gamma line which cannot be resolved from the 122 keV line by scintillation detectors. The E2 admixture has also been determined f rom a M Sssbauer coincidence experiment? 3 This method has the advantage that the result is not affected due to long half-life of the intermediate state because the populations of the sub-levels of the intermediate state are independent of the precession of the nuclear spin axis around the field direetion.

EXPERIMENTAL PROCEDURE AND R E S ~ T S

We have measured the unperturbed angular correlation between the 122 keV and 14 keV gamma-rays from Fe 57 in a single crystal of metallie eobalt. The crystal was taken in the form of a dise of 3 mm. diameter and 0"2 mm. thiekness with the hexagonal axis in the plane of the dise.* Co 67 was electrodeposited on the erystal and thermal diffusion was allowed to take place for about 20 days at 400 ~ C. in high vacuum. Higher temperature was not used as the crystal beeomes cubic above 420 ~ C. It was eooled down to room temperature in about two days. The axis of easy magnetisa- tion was loeated with the h.elp of a sensitive astatie magnetometer.

For the measurement of angular correlation the crystal was mounted on a perspex rod, the plane of the dise being horizontal with its axis point,

) The cobalt crvstal was obtained from Metah Research Ltd., England T

Angular Correlation in Fe 57 in a Single Crystal of Metallic Cobalt 27

ing towards the fixed detector set for 122 keV gamma-radiations. This detector consisted of 1"• cylindrical NaI (TI) crystal coupled to RCA 6810 A ph0tomultiplier. The movable detector, for 14 keV radiation, con- sisted of a 1.5" diameter and 1 mm. thick NaI (TI) crystal coupled to a RCA 6810A photomultiplier. The distances between the source and crysta ls were 10cm. Usual slow-fast coincidence set-up was used, single-channel analysers were used in each channel for selecting the photopeaks,for the two gamma-radiations. A bddge-type time-to-amplitude convertcr TM was used to record the time spectrum of the coincidences between 122 keV and 14 keV gamma-rays on a ND 512-channel analyser. It is shown in Fig. 1. More than six half-lives could be followed after correcting for chance coinci- dences. The calibration was obtained from chance coincidence rate in each channel. The half-life for 14 keV level thus obtained 0"1/2 : 98 q-3 • 10 -9 sec.) is in good agreement with some recent measurement.S, 9 This spectrum was used to determine the bias setting of the discriminator following the time- to-amplitude converter in order to ensure negligiblc loss in true coincidence counting rate for angular correlation measurements. The bias used is indicated in Fig. 1.

The centering of the source was better than 1% as determined from the singles counting rate in the movable detector. The measurements were

2 0 0 0

I 0 0 0 N~

8 0 0 co s7 6 0 0

m 4 0 0 I.... Z ~ 2 0 0 ~ O

"J ~OOns o o . ,,, I 00

o

~ a0 - , 6() Q

4 0 oO " Z

oo o o 2 0

o o

1o 8 6

4 3

30 40 50 60 r o 8 0 90 I 0 0

CHANNEL NUMBER

FIQ. I. Time spectrum of coincidenccs betwcen 122 keV and 14 keV garnma-rays from Fe 57. Promvt c u r v e is o b t a i n e d w i t h H a u source,

28 GmISH C~~'DI~,

made at two angles 90 ~ and 180 ~ as no AL terna in the angular correlation was expected. The coincidence counts were corrected for chance coincidences. After correcting for solid angle attenuation the angular correlation function is found to be,

W (0) = 1 -- (0.042 4- 0.016) P= (cos 8).

This value of A2 is larger than that obtained by Lindquist and Heer 5 (see Table I) showing that in their measurement angular correlation was

TABLE I

Authors A= �91 Method

L!ndquist and Heer 7 --0-024-b0.003 .. Angular correlation Present work --0.042-b0.016 --0-17 +0 .03 do. Bishop et al. u .. -bG. 19 ~0 .02 Nuclear orientation Strohm and Sapp 12 .. +0" 148~:0.01 do. Waard and Woude ~3 .. --0.15 -b0.035 Mtissbauer effect

attenuated. Figure 2 shows the theoretical curve for A2 vs. 8 (the square- root of the ratio of intensities of E2 and MI) and determination of the value

0"4

0"3

0'2

OI o,{

o At,

EXP -0'1

-0"2

id =

Fxo. 2. g mma-ray.

ANG. CORR. BETWEEN 122 keV AND 14keV GAMMA RAYS / FROM Fe 57

Ÿ

- ' o ~ ' d 2 4 6 810 2 4 6 81 t 4. 6 800 2 4 6 81

�91 Analysis of angul0r r162 measurement to determine the valur of 8 for 122 kr

Angular Correlation in Fe 67 in a Single Crystal of Metallic Cobalt 29

of �91 from our experimental value of A2. This gives �91 : --0.17-t-0.03 for the 122 keV gamma-radiation. This is in agreement within the limits of error, with the values of �91 obtained by others x1-13 given in Table I. This measurement demonstrates that it should be possible to measure unperturbed angular correlation in case of other nuclei, without the use of any external magnetic field, by thermally diffusing them into the lattice of cobalt crystal.

ACKNOWLEDGEMENTS

The author is thankful to Shri V. R. Pandharipande and Shri T. S. Radha- krishnan for their help in the measurement, to Drs. C. Radhakrishnamurty and P. W. Sahasrabudhe for permitting us to use their astatic magnetometer and to Shri B. M. Joshi for electrodeposition of source.

1. Steffen, R. M. and Frauenfelder, H.

2. Bozorth, R. M.

3 , �9 �9

4. Sucksmith, W. ..

5. Wertheim, G .K. . .

6. Perlow, G. J., Johnson, C. E. and Marshall, W.

Lindquist, T. and Heer, E.

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14. Steenberg, R .R . . .

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16. Girish Chandra

In Perturbed Angular Correlation, North Holland Publishing Co., Amsterdam, 1964, p. 379.

Phil. Mag., 1955, 46, 951.

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Ibid., 1963, 129, 1342.

Ibid., 1954, 95, 982.

In Perturbed Angular Correlation, North Holland Publishing Co., Amsterdam, 1964, p. 441.

Nuovo Cimento, 1964, 31, 297.