Alignment of 56Co nuclei

Gallaher, L. J.

Physica XXI 1 17-123

W h i t t l e , Ch.

Beun, J. A. Diddens, A. N. Gorter, C. J. Steenland, M. J.

ALIGNMENT P A R T i. E X P E R I l V I E N T A L

O F ~6Co N U C L E I

RESULTS;

DETERMINATION

OF MAGNETIC

MOMENT by L. J. G A L L A H E R *), CH. W H I T T L E *), J. A. BEUN, A. N. DIDDENS, C. J. G O R T E R and M. J. S T E E N L A N D NatuurkuHdig Laboratorium, Rijksuniversiteit, (;roninge,, Nederiattd CommuuieatioH No. 298c from the Kamerlillgh Onm.s l.aboratorium, I.eiden, Noderl;md Synopsis

A d i l u t e c o p p e r a n m a o n i u m T u t t o n salt, c o n t a i n i n g a s m a l l a m o u n t of 56Co, was cooled b y a d i a b a t i c d e m a g n e t i z a t i o n to a b o u t 0 . 0 2 ° K a n d a n a n i s o t r o p y in t h e a n g u l a r d i s t r i b u t i o n of t h e g a m m a i n t e n s i t y , r e s u l t i n g f r o m t h e n u c l e a r a l i g n m e n t , w a s f o u n d in e a c h of t h e six g a m m a rays. A n e s t i m a t e of t h e m a g n e t i c m o m e n t of 56Co y i e l d e d 2.8 5 : 0 . 9 n u c l e a r m a g n e t o n s .

1. Introduction. The cobalt isotopes 57Co, 58Co and 6°Co have been studied

b y the method of nuclear alignment or nuclear polarization by a number of authors 1) 2) 3). In this paper similar experiments on 25~Co29will be described. ~Co decays with K-capture, positron- and gamma-emission. S a k a i, Dick, Anderson and K u r b a t o v 4 ) have identified two positron groups, with endpoints 1.50 and 0.44 MeV, and six gamma rays of energies 0.845, 1.24, 1.75, 2.30, 2.60 and 3.25 MeV and have given a decay scheme. In our experiments a Tutton salt single crystal containing a small amount of 56Co was cooled by adiabatic demagnetization to about 0.02°K. At this temperature a considerable nuclear alignment was found to exist. An anisotropy in the angular distribution of all gamma rays was measured though the effect in the 2.30 MeV radiation is somewhat questionable because of the low intensity of this line. In the following paper **) the results of these measurements will be used to make conclusions about the multipole order of the gamma rays and about the spins and parities of SrCo and the various levels in its daughternucleus SrFe. A spin assignment made, we were able to estimate the magnetic moment of 5°Co by comparing the measured and the theoretical anisotropies. 2. Cryogenics and source. A single crystal of (Co,Cu,Zn) (NH4)2(SO4) 2. 6H20 *) Fulbright Fellowship 1953-1954. **) P a r t l I : O . J. P o p p e m a ,

J.G. Siekman,

R. v a u W a g e n i n g e n

Commun. Suppl. No. 10%; Physica 21 ((1955) (forthcoming). --

117

--

and H.A. T o l h o e k ,

1 18

L. J. G A L L A H E R , CH. W H I T T L E , J. A. B E U N , A. N. D I D D E N S , C. J. G O R T E R

was grown from a solution with a relative Co++, Cu ++, Zn ++ ratio of 1 : 20 : 79. The 56Co content was about 20/~C; the crystal weight 1.4 grams. For crystallographic data we refer to the description of experiments on oriented 6°Co nuclei ~), where similar crystals were used. The mounting of the crystal was also the same as in the previous experiments, except that quartz instead of glass supports were used to avoid the influence of the paramagnetism of the glass on the susceptibility measurements. The crystal was demagnetized from an initial field of 22000 oersted at a temperature of about 1°K. In this way a magnetic temperature T* somewhat below 0.02 ° could be reached. The rise in temperature of the crystal was mainly due to absorption of 56Co/5+-radiation. Usually about 1 minute after the moment of demagnetization a T* of 0.02 ° was measured, while after about 12 minutes T* was 0.06 °. The isotope 56Co was kindly prepared for us in the cyclotron of the I.K.O. at Amsterdam and contained about 25% STCo and a negligible amount of 58Co.

3. Detection apparatus. The gamma ray detection apparatus consisted of two scintillation spectrometers *) (Channel I and Channel II). Each channel was made up of two NaT(T1)- crystals, mounted on two R.C.A. 6199 photomultiplier tubes, both feeding into the same amplifier, differential-discriminator and scaler. The four counters were mounted on a ring, their axes making angles of 90 °, the source being in the centre of the ring and photomultiplier tubes opposite each other feeding the same channel. The counters could then be rotated in the K1-K2-plane of the Tutton salt crystal. The distance from source to counter was 6 cm. The gamma spectrum of SSCo measured with these spectrometers was found to be identical with that given by S a k a i eta/. 4). Within 45 seconds after demagnetization the counters were positioned around the cryostat, one channel along the K1 axis, the other along the K2 axis. After each counting interval of 18 seconds the counters were rotated through 90 ° . Thus each channel recorded alternatively the intensity in the direction of maximum and of minimum anisotropy. About 25 such counting periods of 18 seconds were available before the temperature had risen to 0.06°K. For the results quoted in the present paper we needed about 20 demagnetizations. When most of the anisotropy had disappeared the intensity of the isotropic radiation (J,) was measured (source at I°K) for normalization of the anisotropic radiation intensity Ja. The use of two scintillation counters, placed opposite to each other not only has the advantage of double intensity, but also of a smaller influence of a possible eccentricity of the source relative to the scintillation crystals. *) This equipment was obtained in part under a grant from the Research Corporation, New Yot k, N.¥.

AND

M.

j.

STEENLAND,

ALIGNIvIENT

OF 56Co

NUCLEI

1 19

4. Measurements. In the K1 and K2 direction J J J, was measured for the gamma rays of 0.845; 1.24; 1.75" 2.30; 2.60 and 3.25 MeV. Fig. 1 represents the graphs of J,/.[, against 1/T* for all these lines. TABLE nleasured

Y

0.845 1.24

1.75 2.30 2.60 3.25

1

error

T* = 0.040

T* = 0.020

column

+ 0.034

+ 0.093

+ 0.03 -- 0.02

F 0.07 -- 0.05

--

0.02

--

0.06

i

--

0.0

--

0.02

0.02 0.02

0.04

+

0.12

5:_ 0 . 0 2

+

corrected

Probable

(Ja/Jr)K2-

(3IeV)

I Probable

(Ja/Jr)K2---

in

I

e r r o r in COlUInII 6

T* = 0.040

T* = 0.020

4- 0 . 0 0 6

+ 0.04

± 0.01 ± 0.01

F 0.04 - 0.03 -- 0.02?

+ 0.10 0.09 - 0.07 -- 0.05 ? --0.15

q- 0.01 Jz 0 . 0 2

+ 0.12

± 0.02

3

0.04 + 0.04 --

i

0.02 large

n

0.06

In table I columns 2 and 3 give the measured effects (J,/J,) - 1 in the K2 direction ( 0 = 9 0 ° in the formulae derived by T o l h o e k and C o x S ) ; ]a/J, corresponds to ½W(O)) for the magnetic temperatures T* = 0.020 and T * = 0.040 respectively', the effects being linear in 1/T* between these temperatures. In the K1 direction (0 ---- 34 °) the effect has about the same magnitude but opposite sign. Columns 5 and 6 give the data, corrected for the background of the Compton scattering in the scintillation crystal from the higher energy gamma rays (obtained from the gamma scintillation spectrum and by making use of the results given by M a e d e r et al. 6)). Because of its weak intensity and the large correction to be applied, the results for the 2.30 MeV line are thought to be doubtful.

5. Discussion. The accuracy of the magnetic temperature measurements was better than 10°/o. The errors in column 4 of table I are of a statistical nature. Those in column 7 are the estimated errors in the corrected value of (J,/J,) - l, due mainly to the uncertainty in the correction for the background of the Compton scattering in the scintillation crystal. The corrections for scattering in the source and cryostat and for the solid angles were thought to be negligible. T o 1 h o e k and C o x give the angular distribution W(O) of the gamma rays of oriented nuclei in the case of axial symmetry as a function of 0 (angle between direction of observation and axis of nuclear orientation), of the multipole order L of the gamma ray, of the spins I~ of the initial level and 1 / of the final level belonging to this gamma transition and of fl = (#H)/(IokT), ;i, and I o being the magnefit moment and spin, respectively, of the original oriented nucleus, H the magnetic field at the nucleus, k Boltzmann's constant and T the absolute temperature 5). Table II gives for the K2 direction the expected deviation from isotropy ½W(O) -- 1, calculated from these formulae, assuming the groundstate spin

120

L. J. GALLAHER, CH. WHITTLE, j. A. BEUN, A. N. DIDDENS, C. J. GORTER

1.15

{ 0.845 MeV

1.10 1.05 1.00

'

I

J

20

I

30

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l l . ~ ~ . ~

T

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IT

310

i

41

i

SO

i

610

o.gs'

Fig. 1. N o r m a l i z e d i n t e n s i t y Ja/Jt of g a m m a r a d i a t i o n a l o n g K1 a x i s (o) a n d K 2 axis (x) of t h e six g a m m a r a y s in 56Co. E a c h g r a p h r e p r e s e n t s t h e a v e r a g e d r e s u l t s of s e v e r a l series. N o t e t h e d i f f e r e n t v e r t i c a l scales.

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122

L. J. GALLAHER,

CH.

WHITTLE,

J. A. BEUN,

TABLE

A. N. DIDDENS,

l

I0 = 4; fl = 0 . 5 3

10 = 5; fl = 0 . 4 5

1) 1)

-- 0.083 -- 0.042

-- 0.083 -- 0.048

12, --- 2) (2, + 2)

t- 0 . 1 1 4 + 0.060 + 0.116

+ 0.113 + 0.068 + 0.125

(1, -¢1..4-

(l,

o)

GORTER

II .11'V (~z/2) - -

type

C. J.

I 0 of the oriented 5SCo nucleus to be 4 or 5, respectively, and for a value of r, which m a y a p p r o x i m a t e l y correspond to T* = 0.020, viz. fl = 0.53 and 0.45. The various g a m m a transitions are denoted by the symbol (L, AI) with AI ---- I t -- I; and L the multipole order of the g a m m a transition. Assuming t h a t all the transitions in the decay of 56Co have m i n i m u m multipole order, in which case the parameters [k 5) do not change sign (except in the case of a (1,0) transition with I i ---- 1) after a g a m m a transition, it can be readily seen from the sign of the effect, t h a t the 1.75 and 2.60 MeV g a m m a rays are dipole and the 0.845, 1.24 and 3.25 MeV are quadrupole or dipole with A I -- 0. In the following paper an analysis of the possible multipoh, orders of tile g a m m a transitions and of the spin and p a r i t y assignments to the levels investigated will be made. The results are shown in fig. 1 and table VI of t h a t paper.

6. Magnetic moment o! S6Co. The spins being determined according to scheme A, B or C (see following paper), we are able to calculate ½W(O) for t h e 0.845 MeV g a m m a ray as a function of fl -= (l~H)/(fokT). These results can be compared with the curve of the 0.845 MeV line in fig. 1 after applying the Compton correction to it. This gives us a relation between fl and T*. (The 0.845 MeV line has been chosen for this comparison, because it has been measured most accurately and is almost certain to be a (2, --2) transition). For T* sufficiently high, fiT* will approach (~H)/(Iok).H/k for crystals without copper is known to be 0.0132°K per nuclear m a g n e t o n v) s). But by comparing the limiting values of fiT* for crystals without copper and with 20% copper, from the experiments mentioned in ref. 2) on S°Co in both types of crystals, one rather finds for crystals containing 20°/,, of copper H/k = 0.0 ! 5 0.0015°K per nuclear magneton. Accepting this latter value for H/k and I o = 4 one obtains t, == 2.8 i 0.9 nuclear m a g n e t o n s . This calculation has been made in the same way as in former work ~). The sign of ~, cannot be found in this way. The error in the value of t~ arises from 1 : U n c e r t a i n t y in H/k ; 2: E s t i m a t e d error in the corrected value of (J,/J,) - 1 ; 3 : I n a c c u r a c y in the determination of T* ; 4: U n c e r t a i n t y in the determination of the direction of the K2 axis ; 5: U n c e r t a i n t y in the t y p e of interaction in the r+ transitions and K-captures with 3 I = 0. Not taken into account are differences between T and T*, t h a t

AND M.

j.

STEENLAND,

A L I G N M E N T OF 56Co N U C L E I

123

still might exist at T m 0.06°K, possible spin precessions, and the occurrence of mixtures of gamma transitions of different multipole order. We are indebted to Prof. H. B r i n k m a n and Dr. O. J. P o p p e m a of the University of Groningen and to Prof. A. H. W. A t e n of the"Instituut voor kernphysisch onderzoek" at Amsterdam for their stimulating interest and active support, to Messrs L. B. B e e n t j e s , M. S. R. C h a r i and J. v a n W e e s e 1 for help in the experiments. One of us (A.N.D.) was enabled to partake in these investigations by a grant of the "Stichting voor Fundamenteel Onderzoek der Materie (F.O.M.)" which is financially supported by the "Nederlandse Organisatie voor Zuiver Wetenschappelijk Onderzoek (Z.W.O.). Received 25-11-54.

RFFERENCES I) B l i H - S t o y l e , R. J., ( ; r a c e , M. A. and H a l b a n , H., Progress in nuclear physics (Editor O. R. Friseh), 3 (1953) 63. 2) P o p p e m a , 0. J., S t e e n l a n d , M. J., B e u , , J. A. and G o r t e r , C. j., Commun. Kamerlingh Onnes Lab., Leiden No. 298b; Physica 21 (19,55) (forthcoming). 3) G r a c e, M. A., Colloque international no. 54 sur ,,Le r61e du cortege ~leetronique clans h's ph6,om~nes radioactifs" du Centre National de la Recherche Scientifique, Paris 19,54. 4) S a k a i , M., D i c k , J. L., A n d e r s o n , W. S. and K u r b a t o v , J. D., Phys. Rev. S)5 (1954) I01. 5) T o l h o e k, H. A. and C o x , J. A. M., Physica 19 {1953) 101, 673. 6) M a e d e r , D., M t i l l e r , R. and W i n t e r s t e i g e r , V., Helv. phys. Acta 27 {1954) 3. 7) B l e a n e y, B. and I n g r a m , D. J. E., Proc. roy. Soc. London A 20B (1951) 143. 8) P r o e t o r , ~V. (;. and Y u, F.C., Phys. Rev. 77 (1950} 716.