The magnetic moment of the 344 keV level of 152Gd

Nuclear Physics AI30 (1969) 541--544; ( ~ North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

T H E MAGNETIC M O M E N T OF T H E 344 keV LEVEL OF tS2Gd H. Z M O R A , M. BLAU and S. O F E R

Department of Physics, The Hebrew University, Jerusalem, Israel t Received 24 February 1969 Abstract: The magnetic m o m e n t of the 344 keV level of lS2Gd was determined from measurements

of the rotations of the angular correlations of ~,-y cascades involving this level in the magnetic hyperfine field of Gd metal. The value found for g r is (2.434-0.22)" 10 -11 sec. Taking z = (7.6 + 1.3) • 10-11 sec, a value of 0.32 +0.06 was derived for the gyromagnetic ratio of this level.

E I RADIOACTIVITY lSZEu; measured ~3'(0, H). lS2Gd level deduced g, p. Natural target. ]

I

I 1. Introduction

In recent years, the range of excited nuclear states whose magnetic moments can be measured by the perturbed angular correlation technique has been greatly extended due to the accumulation of data on magnetic hyperfine fields tt. In this work we describe a measurement of the magnetic moment of the 344 keV level of 1s 2Gd using the known value of the internal magnetic field acting on the gadolinium nucleus in Gd metal.

2. Experimental details and results A source of 1SZEu (12 y) was prepared by the irradiation of europium metal with thermal neutrons. The source was introduced homogeneously into metallic gadolinium by melting the metals in an argon atmosphere. Special care was taken to prevent oxidation of the europium metal. The atomic concentration of europium in the sample was ~ 0.05 %. The samples were cut in the form of small needles and were held perpendicular to the correlation plane. A conventional setup for measuring integral angular correlations was used. The detectors were a 5.1 c m x 5 . 1 cm NaI(TI) crystal for the low-energy ~,-rays and a 7.6 cm x 7.6 cm crystal for the high-energy v-rays. The time resolution of the coincidence circuit was 35 ns, and the data (single and coincidence counts) were accumulated in a split memory 400-channel pulse-height analyser. A partial decay scheme of 12 y tSZEu is given in fig. 1, which is taken from ref. 2) and the V-spectrum is shown in fig. 2. f Work supported in part by the Israel Academy of Sciences and Humanities. tt See ref. 1) for a compilation of earlier work on hyperfine fields. 541

H. ZMORAet al.

542

In the absence of perturbations, the angular correlation function can be written as

kl~ax

w(o) =

(])

E b2kcos(2k0).

,+

k=l

s3Eu Is2

3-

2-

,?it!, {

2+

/ / ~ 26% / / / ~ 17./. , E C / ~ - 23./. //,/ 1530~ ~///

17% 15,0%

1235 ~//"

;3:;: I ,J9

3I I 11o~7

1642 1298

1

112~

8~I 756 /'i2

689

615 {

12 ~

344

344

2+ 0+

62Smls2

~Gd ~s2

1o

Fig. 1. A partial decay scheme of lS2Eu.

10¢ 779b

~2~g ~.o~

COUNTS

103--

\ 1020

I

20

I

40

[

60

I

80 CHANNEL

[

100

120

140

Fig. 2. The gamma spectrum of tSZEu (12 y). a) single spectrum; b) coincidences with the 344 keV line.

543

344 keV LEVEL OF 152Gd

In many cases, all terms higher than b 2 a r e equal to zero, and the correlation function obtains the form W(O) = l + b 2 cos(E0). (2) When a magnetic field H acts on the sample normal to the correlation plane, one obtains (assuming that bEg ~- 0 for k > 2)

w(180 °, H ) - W(90 °,

l

S = 2 W( 180°, H ) + W(90 °, H ) = 262 1 +(Ecoz) 5'

(3)

where co is the Larmor frequency and z the mean lifetime of the intermediate level. When the angle between the detectors is fixed at 135 ° and the sense of the transverse magnetic field is changed intermittently, one obtains (with bEk = 0, for k __> 2) R = 2 W(135°' H T ) - W(135°' H~) = 2b2 2coz W(135 °, H i ' ) + W(135 °, H~.) 1 + (Eeoz)z"

(4)

TABL~ 1 S u m m a r y of experimental results Cascade 779-344 1298-344

b2

S

--0.060 4=_0.002 -;-0.153 ± 0 . 0 0 5

-- 0.117 ± 0 . 0 0 2 + 0.310 -_t:0.013

R 0.0096 ± 0.00087 0.024 ± 0.005

2co-r 0.074-t-0.0067 0.077 + 0 . 0 1 5 0

In order to determine the magnetic moment of the 344 keV level, the rotations of the 779 keV-344 keV and 1298 keV-344 keV ~-cascades were measured (the window settings are indicated in fig. 2). For both cascades, the anisotropy at room temperature (above the Curie point of gadolinium) was measured in agreement with previous results for liquid sources 3) (for these two cascades, terms higher than b2 are small and may be neglected). Measurements of S and R [eqs. (3) and (4) above] at 85°K were carried out in an external magnetic field of 4500 Oe. In the measurements, care was taken to reduce the influence of the magnetic field on the photomultipliers. The singles counting rates for the two directions of the field differed by less than 0.1 ~ . The results are summarized in table 1. The average value found for 2o9T is 0.075+0.0065. The magnetic field acting on the gadolinium at 85 ° K was calculated using the relation H(85OK ) = __MH(OOK)_Hd, _H,x, '

(5)

Ms where M and Ms are the magnetizations of gadolinium at 85°K and 0 ° K, respectively, Hd,m the demagnization field 4~/3 and M and Hext the external applied fields. Taking ( - 348 + 13) kOe for g (0) [ref. 1)], a value of ( - 320 + 15) kOe is derived for H(85 ° K),

544

H. ZMORA el aL

and the value found for #z (where a is the gyromagnetic ratio of the 344 keV level) is (2.43+0.22)x l0 - t l sec. The mean lifetime of the 344 keV level was measured by Burde et al. 4) to be z = (7.6+ 1.3)x 10-11 sec. It is quite safe to assume that in this range of lifetimes there are no "after effects" present in the metal ~), and that the field acting on the 152Gd during its stay in the 344 keV level is equal to the regular value of Herf in Gd metal. In order to assure that the europium was properly introduced into the gadolinium lattice, the magnetic hyperfine field acting on the t 52Sm nucleus in our x52Eu in Gd radioactive sample was measured at 130°K (this source decays by K-capture to 52Sm, see fig. 1). This temperature is above the magnetic ordering temperature of europium metal and below that of gadolinium metal. By measuring the anisotropy of the 1408 keV-122 keV cascade of t52Sm with and without an external field, an absolute value of m 300 kOe was found for the value of Heft in agreement with the value of ( - 2 9 5 + 3 0 ) kOe derived by Murnick et aL 6) for 15°Sm in Gd at II0°K. The value found for the #-factor of the 344 keV level of ~52Gd (using the value of (7.6+ 1.3) x l0 -11 sec for r) is 0.32+0.06. 3. Discussion

The ~52Gd nucleus has 88 neutrons and 64 protons, and lies therefore in the transition region between spherical and deformed nuclei. According to the hydrodynamic description of the nucleus, the collective gyromagnetic ratio #R is equal to Z/A (0.42 for ~52Gd), but as is well known, the experimental values are found to be consistently smaller than this value. There are two approaches to the calculation of this reduction. One is macroscopic 7), which states that the deformation of the protons is different from that of the neutrons. Calculations following this approach carried out by Greiner 7) lead to a value of 0.36 for #a of the 344 keV level. The second approach is microscopic and is based on detailed calculations of the nuclear properties using the quasi-particle model. Calculations of Baranger and Kumar a) using the pairing-plus-quadrupole model lead to a value of #~ = 0.299. Both of these calculated values are consistent with the measured value of 0.32+0.06. References 1) D. A. Shirley, in Hyperfine structure and nuclear radiations, ed. by E. Matthias and D. A. Shirley (North-Holland Publ. Co., Amsterdam, 1968) p. 979 2) Nuclear Data Sheets 3) Z. Grabowski, Ark. Fys. 20 (1961) 177 4) J. Burde, M. Rakavy and S. Ofer, Phys. Rev. 124 (1961) 1911 5) U. Biiverstam, A. Johansson and T. R. Gerholm, Ark. Fys. 35 (1968) 451 6) D. E. Murnick, L. Grodzins, J. D. Bronson, B. Herskind and R. R. Borchers, Phys. Rev. 163 (1967) 254 7) R. Greinel, Nucl. Phys. 80 (1965) 417 8) K. Kumar and M. Baranger, Nucl. Phys. A l l 0 (1968) 529