Precession measurements following Coulomb excitation with oxygen ions

1.E.3:

Nuclear Physics AI09 (1968) 201--204; (~) North-Holland Publishing Co., Amsterdam

2.H

Not to be reproduced by photoprint or microfilm without written permission from the publisher

PRECESSION

MEASUREMENTS

FOLLOWING

COULOMB EXCITATION

WITH OXYGEN IONS

(V). Hf ions recoiling into liquid gallium I. BEN-ZVI, P. G I L A D , G. G O L D R I N G , P. H I L L M A N A. S C H W A R Z S C H I L D + and Z. V A G E R

The Weizmann Institute of Science, Rehocoth, Israel

Received 1 November 1967

Abstract: Precession measurements were carried out for the 2+ states in 17~Hf, '~SHf and l~°Hf. The nuclei were excited by oxygen ions of 34 MeV and embedded by recoil into liquid gallium which was found to provide an unperturbed environment. E

NUCLEAR REACTIONS 176,178,18°Hf(~60, 160'F), E = 34 MeV; measured a(E~,,O, H). lr6,178,XS°Hflevel deduced g. Enriched target.

1. Introduction Previous precession m e a s u r e m e n t s in external m a g n e t i c fields r e p o r t e d in this series 1- 3) were c a r r i e d o u t with ions e m b e d d e d by recoil into copper. This p r o v i d e d an e n v i r o n m e n t suitable for precession m e a s u r e m e n t s for ions o f Os, N d a n d the isot o p e ' 8 6 W in the excited nuclear states t h a t were studied. In some other cases, the 2 + states o f Y b isotopes a n d 15ZSm, the a n g u l a r d i s t r i b u t i o n s were f o u n d to be strongly p e r t u r b e d , while in m e a s u r e m e n t s 2) involving H f isotopes, the p e r t u r b a t i o n was small e n o u g h for m e a n i n g f u l m e a s u r e m e n t s to be carried o u t but was nevertheless large e n o u g h to constitute the largest source o f u n c e r t a i n t y in the d e t e r m i n a t i o n of the g-factors o f the 2 + states, in the w o r k presented here, H f nuclei were precessed after recoil into liquid gallium. This p r o v e d to be an ideal p e r t u r b a t i o n - f r e e environm e n t and allowed precession m e a s u r e m e n t s o f m u c h higher accuracy to be carried out.

2. Experimental procedure The targets were m o u n t e d in the m a n n e r shown in fig. 1. A s it is essential to a v o i d even s h o r t recoil p a t h s in v a c u u m 4), and as the gallium was f o u n d n o t to wet the target material, it was necessary to press the gallium firmly against the target leaving no air pockets or v a c u u m bubbles. The design o f fig. 1 p r o v e d to be satisfactory for this purpose. The g a l l i u m was c o n t r o l l e d by the tightening screw shown, the whole o p e r a t i o n being carried out in vacuo. The m e a s u r e m e n t s were carried out in the usual way, exciting a n d aligning the nuclei by C o u l o m b excitation with b a c k s c a t t e r e d oxygens ions o f 34 MeV. * Present address: Brookhaven National Laboratory U p t o n , New York 11973, USA. 201

202

L BEN-ZVIet al.

Angular distribution measurements were carried out for the 2 + ~ 0 + transition in 176Hf, 178Hf and ~8°Hf in the range 45 ° < 0 < 90 ° and were analysed in terms of the attenuation coefficients G2 and G [cf. ref. ~)] G = 5 B z G2 "t- 9B4 G,,

5B2 + 9B4 where B k are the geometrical attenuation coefficients representing the finite apertures of the counters. The measurements were corrected as in ref. 2) for the admixture of

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Fig. 1. Schematic view of target assembly.

radiation following the excitation of the 4 + levels. A small correction was also applied to account for the radiation scattered from the gallium backing. The relative intensity of the scattered radiation was determined by placing a radioactive source (~7°Tm Er = 84 keV) at the position of the target and observing the increase in counting rate (l.7 to 3.9 %) when the gallium holder was brought close to it. The values of G determined in this way are very close to unity. As all three isotopes have similar lifetimes and are also similar in their magnetic and electric quadrupole moments, the attenuation coefficients are expected to be very nearly the same for all of them. We can therefore combine the three measurements and obtain a common value Gm a m = 0.982__+0.023.

Precession measurements were carried out with these targets in the usual way with

COULOMB EXCITATION (V)

203

the counters at + 112.5 °, determining the double ratio for counts in the two counters a n d with the field up T or down J, F/i(~)

• HI(~)

--

n,l(T) " nn(+)

ep -

(1

+el 2

\1~--~/ '

zX obs

e~ accounts for the beam turning qS, which in these measurements a m o u n t e d to = 0.17 o ___0.04o . This corresponds to eA 0.010__0.003. The coefficient X is introduced to account for possible perturbations 5) and was calculated f r o m the mean attenuation coefficient Gm as Arm = 0.985-t-0.015. The pertinent information on the precession measurements is summarized in table 1. TABLE 1

Summary of precession measurements Nucleus

Energy (keV)

v (nsec)

o~T New measurement (rad)

cot Ref. 2) (rad)

g New measurement

g Ref. 2).

l~eHf

88.3

2.01 i 0 . 0 6 a) 0.0439±0.0030

0.266±0.021

lrSHf

93.1

2.12 ±0.04 b) 0.0412~0.0018 0.0461±0.0018 0.237±0.014

0.300Jz0.035

18°Hf

93.3

2.164±0.029 e) 0.0467±0.0020 0.0492±0.0025 0.263~0.015

0.313±0.035

a) Ref. 0). b) Ref. 7). e) Ref. 8). 3. Results The g-factors o f the 2 + states in the three hafnium isotopes are given in table 1 together with the previous measurements reported in ref. 2). The comparison of the two measurements suggest that the value of 2" for recoil into copper is close to unity, and this implies a fast perturbation relative to the mean life of the excited states. A n u m b e r o f calculations o f these g-factors has been carried out recently, based on specific models of the collective nuclear rotation. The relevant values are presented in fig. 2 and c o m p a r e d with the measured values. The values of b o t h K u m a r and Baranger 9) and case A o f Nilsson and Prior 10) are in excellent agreement with the measurements. The other values derived in ref. 11) with various choices of the relevant parameters fit the data less well. The most conspicuous difference between

204

I. BEN-ZVIet al.

refs. 10,11) is in the assumed value of gs in the nucleus; gs = gsfree in ref. 10) and g~ = 0.6 g~ tree in ref. 11). However, as other parameters also have different values in the two calculations it is difficult to reach definite and specific conclusions f r o m the comparison. 0.35

I

I Ref II} case I j - ~ ' ~ . 11..I-

./"j

"-~

/./"

~.,~

__~

0.30

I

./. j Ref

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__

Ref IOI cose B

Ref IO) cose A

0.25

Ref 91

0.20

I

I

I

!76

178

180

Hf isolopes Fig. 2. Summary of g-factor measurements and theoretical values.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11)

G. Goldring et al., Nuclear Physics 80 (1966) 33 P. Gilad et al., Nuclear Physics A91 (1967) 85 I. Ben-Zvi et al., Nuclear Physics A96 (1967) 138 D. Ashery et al., Nuclear Physics A101 (1967) 51 G. Goldring and Z. Vager, Phys. Rev. 127 (1962) 929 D. B. Fossan and H. Herskind, Nuclear Physics 40 (1963) 24 N.D.S., weighted average 7;J delay measurements N.D.S., weighted average Baranger and Kumar, Nuclear Physics, to be published S. G. Nilsson and O. Prior, Mat. Fys. Medd. Dan. Vid. Selsk. 32, No. 16 (1961) O. Prior, private communication