Magnetic moment of a suggested three-quasiparticle state in 125Te

Nuclear Physics A154 (1970) 369-384; Not to be reproduced

by photoprint

MAGNETIC

@ North-Holland

or microlilm

MOMENT

D. W. CRUSE, of Physics,

K. JOHANSSON University

without written permission

Co., Amsterdam from the publisher

OF A SUGGESTED

THREE-QUASIPARTICLE

Institute

Publishing

STATE IN ‘25Te and E. KARLSSON

of Uppsala,

Uppsala,

Sweden

Received 8 May 1970 Abstract: The magnetic moments of the 321 keV $- and 464 keV 8” levels of lz5Te have been determined by the PAC method to bep = - (0.918 f0.032) n.m., and p = - (0.58 +0.27) n.m., respectively. The result for the 321 keV level agrees with the prediction of Kisslinger’s threequasiparticle model. The values of the angular correlation coefficients of the 321- 177 keV and 204-177 keV cascades are, after correction for background coincidences and solid angle, A2 = -0.168f0.012, Aq = -0.003+0.015 and A2 = -0.403+0.014, A4 = +0.003+0.020 respectively. The mixing ratio for the 204 keV (p- -$-) transition is 6(E2/Ml) = - 1.88f0.22 giving an E2 content of Q(E2) = 0.78j=O.O4, while for the 321 keV (4+-g-) transition 0 5 Q(M2) 5 0.010 and -0.22 s 6(M2/El) 5 0. The magnetic hyperfine field for tellurium in gadolinium has been measured with the result Bhf = -38 + 10 kG. The electric quadrupole interaction frequency of the 321 keV level for Te in Gd is wt = (27OA70) . lo6 rad/s. E

RADIOACTIVITY lz5Sb; measured yy(8) and yy(0, H) in liquid, Cu, Fe, Ni and Gd lattices; deduced hypetine fields. ‘*‘Te levels, deduced p, y-mixing.

1. Introduction

The odd-parity excited states of lz5Te are thought to be based upon the h+ neutron orbital. The e- state at 321 keV in particular, has been interpreted by Kisslinger ‘) as a three-quasiparticle state based on the y isomer. An earlier determination of the magnetic moment of this state by Singh et al. “) disagrees with the theoretical prediction for such a three-quasiparticle excitation, but measurements of the E2 transition rate for the $-+y-ray 3,4) correspond closely to Kisslinger’s prediction. A recent measurement of the nuclear g-factor of the 2- state by Knapek et al. 24) also supports this interpretation. Since there are several experimental pitfalls involved in the perturbed angular correlation measurement it is worth-while to remeasure the g-factor of the 321 keV state, in order to help establish if threequasiparticle excitations exist at these low energies. Experimental considerations included the question of the environment for the Te atoms, the selection of a suitable y-ray cascade and the corrections from spurious coincidences. 2. Decay scheme The most recent investigation of the decay of “‘Sb has been carried out by Inamura 5), whose level scheme is shown in fig. 1. The half-lives marked a) in fig. 1 369

keV ,671.6

a)

(40223) ps n\

j/642.3

w2+ 7/2+

'636.2

7/2+

. 525.2

7/2-

463.5 "443.6

. 321.1

d) “\

5/2+ (5/2-)

9/2-

- 144.7 1112-

58 d

"'(1.45i0.03)ns ,?

I

t

_

] ’ !

35.5

-

0

3/2+ 112+

'%73

Fig. 1. Decay scheme of 125Sb [ref. 5)]. Lifetime data: a) Marelius et al. 4), b) and c) weighted mean values from table 1, d) Friedlander ef al. *).

b)

600-l--

635.5

606 2 671.3 i

J

A

Fig. 2. Gamma-ray spectrum of lzsSb measured with (1 cm* x 10 mm) Ge(Li) detector. The energy resolution was 2.0 keV at 428 keV. a) W-400 keV and b) 400-700 keV.

MAGNETIC

TABLE

371

MOMENT

1

Half-lives of the 321 keV and 464 keV states Level (keV) 321.1

463.5

Half-life (ns)

Ref.

0.87 kO.08 0.695*0.015 0.68 f0.03 0.68 hO.03

Inamura s) Kownacki et al. 3, Hosangdi et al. “) Marelius et al. 4,

0.695&0.012

weighted mean value

0.013 kO.002 0.014f0.006

Metzger et al. ‘) Marelius et al. 4,

0.013+0.002

weighted mean value

Singles 'Sp4Mll

d) 04

177

k_

L Fig. 3. Coincidence spectra. In a) and b) the corresponding singles spectra are also shown. Gate settings: a) G3 = 396-490 keV, b) G5 = 200-240 keV, c) G1 = 160-220 keV, d) G6 = 95-135 keV.

D. W. CRIJSE et

372

d.

are from the measurement by Marelius et al. “) while those for the 321.1 keV and 463.5 keV states are the weighted mean values taken from table 1. In fig. 2 a Ge(Li) singles y-ray spectrum is shown. The energies of the photopeaks have been taken from Inamura ‘) by comparing the corresponding spectra. Inamura reported a 122.1 keV transition which was not seen by Stone et al. ‘) and does not appear in our Ge(Li) spectrum. Since the gate settings are sensitive in the g-factor measurements, some yy coincidence spectra were taken with 3.8 cm diam x 5.1 cm long scintillation detectors. These spectra, shown in fig. 3, are not corrected for accidental coincidences. The angle between the detectors was 90” in order to avoid backscattered coincidences from the 600 keV (group), 428 keV and 463 keV y-rays. Fig. 3a shows that the 428 keV and the 463 keV transitions are in coincidence with those at 172 keV and 209 keV. From fig. 3b it is seen that the 177 keV transition is in coincidence with the 204 keV transition. Fig. 3c shows that the 204 keV, 320 keV, 428 keV and 463 keV transitions are in coincidence with that at 177 keV. Although the 320 keV y-ray is not seen in the singles spectrum it is very clear in the coincidence spectrum. 3. Source considerations With level life-times of the order of 1 ns the chemical form of the source must be chosen carefully in order to avoid all other perturbations than that due to the magnetic field. Liquid sources in the form of chloride complexes in strong HCl were prepared after chemical separation of the active lzsSb from the original activity lo), which contained large amounts of inactive Sn (bought from Amersham). In addition a metallic source, made by electroplating almost carrier-free lzsSb onto a copper foil followed by melting and annealing at 900” C for 24 hours, was also prepared. Copper, which forms a matrix of cubic symmetry for impurities has been found to provide a suitable environment, free from static and time-dependent electric quadrupole perturbations “). TABLE

The Az coefficient of the 204-177

2 keV cascade for different sources

Source cu liquid [SbClJliquid [SbClJ-

-42

(high acidity)

The backscatter contribution

-0.101 &to.006 -0.117*0.004 -0.110*0.005

is not subtracted.

Preliminary angular correlation measurements were performed to compare two liquid sources of different acidity with the metallic source. The comparison - which was made using the anisotropy of the 204-177 keV cascade uncorrected for background - gave the results shown in table 2. The errors quoted are statistical. A further

MAGNETIC

MOMENT

313

test of source effects was made in the main series of experiments by comparison of effective or-values, which in this case could be done more accurately than the above comparisons of AZ, because of serious back-scattering effects, to be described later. From table 2 and the comparison in table 5, which shows almost identical or-results for copper and liquid sources, it is concluded that quadrupole effects are in all cases negligible. The use of the internal magnetic fields existing in ferromagnetic alloys sometimes increases the sensitivity of the method. For this reason dilute solid solutions of I2 5Sb in nickel and iron were also prepared, in a way similar to that described for antimony in copper. Both nickel and iron fulfil the conditions for vanishing electric quadrupole effects as both have cubic symmetry. In addition to giving increased perturbation effects as compared to that obtainable in available external fields (about 50 kG), data for the nuclear precession in the three different fields provide a check on the consistency of the evaluation of wz from the measured shift in the angular correlation. 4. Angular correlation coefficients It is important that the angular correlation coefficients of the gamma cascades selected for measurement are known as accurately as possible if mz is large in the gfactor determination. In “‘Te the situation is particularly difficult in that all the cascades of interest have at least one y-ray lying in the energy region 150-220 keV which is also the energy region of back-scattered radiation. Thus one of the gates in each of the angular correlation measurements included back-scattered gamma radiation resulting in spurious coincidence peaks in the coincidence spectrum. For ’ 25Te, back-scattering of the very strong 428.1 keV and 462.9 keV lines produces coincidence peaks at 250 keV, and the 600 keV line complex gives rise to coincidence peaks at 420 keV. Without adequate precautions these spurious coincidence peaks will seriously affect the measurements. Large effects were found in the angular region 135”-180”-225” and in fact even with precautions it was found impossible to rely upon data taken in the 180” position. The comparison of liquid and copper environments for Te in table 2, using the highly anisotropic 204.3-176.7 keV cascade demonstrates the effect of the backscattered radiation. The angular correlations were measured at 90”, 180” and 270”. The measured coefficients are very much smaller than the corrected values in table 3. However, this does not invalidate their use for comparison amongst themselves since they were measured under identical conditions with identical gate settings. The angular correlations were measured on a conventional two-channel apparatus, the more difficult cascades being measured twice. In both cases a coincidence spectrum was recorded for 24 hat each of the angles 90”, 135” and 157.5”. One detector was used as a gate to select only those y-rays in the interval 160-220 keV. The resulting coincidence spectrum from the second detector was recorded on a LABEN multi-channel

374

D.

w.

CRUSE

et

al.

analyser using 512 channels. With the aid of standard line shapes these spectra were stripped to obtain the angular correlation coefficients corrected for all background TABLE

3

Angular correlation coefficients corrected for interfering cascades, coincident back-scattering solid angle Cascade (keV)

AZ

‘-L

321keV level 321-177 method 1 method 2 weighted value Inamura S, Knapek et al. 24)

-0.165~0.016 -0.171f0.018 -0.168f0.012 -0.169*0.027 -0.165~0.004

-0.002~0.018 -0.005&0.027 -0.003*0.015 0.018*0.034 -0.007*0.005

321 keV level 204-l 77 method 1 method 2 weighted value Inamura 5, Bajaj et al. 16)

-0.42110.019 -0.383f0.020 -0.403 40.014 -0.364&0.048 -0.38 f0.17

+0.016&0.028 -0.011 kO.030 +0.003 kO.020 0.100~0.075 0.32 zbO.13

464 keV level 209-428+464 method 1

-0.137f0.012

0.009+0.020

Gate settings (see fig. 3)

Gz-GI

Gl-GI

Gzi-G3

The error includes both statistical and background uncertainties.

1.0 !Y s!

0.5

6 9 0.2 ii

2

0.1

5 .

0.05

Y e

0.02

0.01

Fig. 4. Resolved coincidence spectra at 90”, 135” and 157.5”. Gate G1 in fig. 3.

and

MAGNETIC

MOMENT

315

contributions. The shape of the back-scatter spectra from the 400 keV and 600 keV groups were obtained from back-scatter coincidence spectra taken using 19sAu and ’ 3 ‘Cs. Some coincidence spectra are shown in fig. 4. In the first case, called method 1 in table 3, the standard detectors (3.8 cm diam x 2.54 cm long and 3.8 cm diamx 5.1 cm long NaI crystals the smaller crystal being used for the lower energy gate) with normal lead shielding cones were used and 2 mm lead flanked by 1 mm copper shielding was positioned such that there was no line of sight between the two detectors at the angles used. This eliminated most of the backscattered radiation. In the second case, called method 2 in table 3, two of the detectors from the gfactor apparatus were put onto the two-channel apparatus together with pole tips to reproduce the same scattering conditions as in the magnet. The detectors were both NaI scintillators (3.8 cm diam x 5.1 cm long) but had poorer energy resolution because a light guide coupled each crystal and photomultiplier. The dummy pole tips effectively blocked most of the backscattered radiation so that it was not necessary to use additional back-scattering shielding. The 321.1 keV level. Two cascades were measured for this level. They were the 320.9-176.7 keV and 204.3-176.7 keV cascades and were selected from considerations of intensity, degree of anisotropy and freedom from nearby interfering cascades. Each cascade was measured by both of the methods described above. The gate settings corresponded to the energy intervals: 160-220 keV and 300-340 keV for the 320.9-176.7 keV cascade, while both gates were set at 160-220 keV for the 204.3-176.7 keV cascade. For this latter cascade the combined 204.3-176.7 keV and 176.7-204.3 keV angular correlation was measured. The gate settings are illustrated in fig. 3. The 463.5 keV level. The most suitable cascades for this level are the 209.9-428.1 keV and 208.9-462.9 keV cascades. Since both the 428.1 keV and 462.9 keV y-rays originate from the same level and are similar in energy it is difficult to separate them with NaI detectors. However, this is not necessary for the g-factor measurement and the combined 208.9-428.1, 462.9 keV angular correlation was measured. The measurement using back-scatter shielding was considered to be reliable since there are no higher energy coincidence lines. The gate settings corresponded to the energy intervals 20@-220 keV and 400-490 keV. The results are given in table 3 where they are compared with previous measurements.

5. g-factor measurement The g-factors of the 321 keV and 464 keV levels were determined by measuring the precession angle of the angular correlation pattern when a magnetic field was applied to the Te nuclei. These measurements were carried out with a four-detector system having 8 coincidence combinations I’) and installed in a 6 T electromagnet ’ “). The total number of coincidences for + and - field directions were, after sub-

376

D. W. CRUSE r?td.

traction of accidentals and back-scatter contributions,

combined in the expressions

To obtain or for the desired cascade the weighted mean of P, and P, was compared with

where Wj are the angular correlation functions and a, the relative contributions from the different cascades i in the total number of recorded coincidences I*). The a, coefficients, calculated from coincidence spectra and decay scheme according to the method in ref. I*) are given in table 4 for both cascades used. TABLE

Percentage

contributions

4

from different cascades to the total coincidence factor measurements

counting rate in the g-

Contribution Energy 172-428 172-464 177-204 177-321 209-428,464 428,464-209

321-177 keV 0.121 0.190 -0.369 -0.155 -0.127 -0.127

0.016 0.0 +0.002 -0.002 0.007 0.007

5.4 1.8 0.0 87.5 5.5 0.0

in percent 204-177 keV 6.2 1.9 60.6 8.7 6.3 16.3

Since it is important that the gate setting for the higher energy y-ray does not include the lower energy y-ray the discriminator settings for the 204-176 keV cascade were chosen carefully from a resolved coincidence spectrum, shown in fig. 5. If these precautions are not taken coincidences of the type yl-yz will be mixed with y2-y1 coincidences which show an opposing rotation of the angular correlation pattern in a magnetic field. The results of the g-factor measurements are shown in table 5. As our preliminary results for the g-factor of the 321 keV state did not at all agree with the value given by Singh et al. “) great care was taken in the PAC experiments by using different types of sources and different cascades. The result by Knapek et al. 24) became available after the completion of all these measurements.

MAGNETIC

MOMENT

311

In the error of the m/B,,, ratios are included the contributions from P,, AZ, A, and Beff . In the Cu and liquid sources the magnetic field is equal to the externally applied field while the magnetic hyperfine fields for Te in nickel and iron are well known from Mijssbauer measurements 15). The latter were performed at very low temperatures, but the correction for room temperature can in this case be reliably estimated since no effects of local magnetic moments are to be expected. The relation between the ef-

Fig. 5. Resolved singles spectrum showing the gate settings in the g-factor measurement.

fective and hyperfme fields is Beft = Bhf + B, + ($c - D)M, where B. is the externally applied polarizing field, A4 the magnetization and D the demagnetization factor. Unfortunately, the P, value of the 321-177 keV cascade for the Ni and Fe sources lies near the maximum of the P = f(m) curve, where this function is very sensitive to the errors in A2 and Ad. This is reflected in the large Am in the Ni and Fe results. From the equation g = (h/,~)(or)/B r, where r is the meanlife, we deduce for the 321 keV level g = -0.204rfiO.007 or p = - (0.981 kO.032) n.m. and for the 464 keV level g = -0.23+0.11 or p = -(0.58+0.27) n.m. The results for both levels do not agree with those of Singh et al. “) who found p = - (2.61 kO.27) n.m. for the 321 keV state and /J = -(1.98+0.60) n.m. for the 464 keV state. However, our result for the 321 keV state is in excellent agreement with the value of g = -0.202+0.016 reported by Knapek et al. 24).

Q-

4’

321 “),

464,

cu Ni Fe

[SbC14][SbC16]cu Ni

Fe

204-177

209(428 +464)

Source

321-177

(keV)

Cascade

1.003(13)

679

1.040(14)

1.211(28) 1.227(29) 1.225(15) 1.975(42)

1.022(10)

weighted mean

1.216(19) 1.188(19) 1.210(10) 2.017(30)

0.014(6)

0.0542(48) 0.0478(48) 0.0528(25) 0.234(17)

1.221(26) 1.157(26) 1.197(14) 2.058(41)

1.088(7) 1.327(12) 1.479(16)

51.5 51.5 51.5 236

1.095(10) 1.327(17) 1.479(22)

0.0477(33) 0.179(;$) 0.83 6;)

on

1.081(10) 1.327(17) 1.473(22)

P”

51.5 236 679

PZ (rad)

PI

B en (kc)

“) Used AZ and A4 from weighted result in table 3.

Spin

Level (keV)

TABLE 5 Results of g-factor measurements for the 321 keV and 464 keV states

0.021(9)

0.976(30)

1.052(93) 0.928(93) 1.025(49) 0.991(72)

0.926(62) 0.76 (;;) 1.24 (;z)

WB,ff (rad/MG)

0.23(11)

-0.204(7)

B Q 5

-0.206

%

:: -0.213

P

P -0.219 -0.192

-0.192 -0.16 -0.25

9

MAGNETIC

379

MOMENT

6. Discussion 6.1. NUCLEAR

PROPERTIES

The excited states of “‘Te can be compared to the predictions by Kisslinger and Sorensen “) w h o used a pairing plus quadrupole interaction. The states are built up by quasiparticles and phonons interacting through a quadrupole force. The $Jstate at 321 keV, which is of interest here, is not expected in this model at such a low energy as 177 keV above the ystate. Therefore Kisslinger suggested ‘) that this state and a few other similar cases in odd-mass isotopes, e.g. in ‘lV, “‘Ag and 7/2+-\

\

\

\

_

s/2*

\ ---_

3/2+ -\ \

\

/

/

’,y------

/-

7/2+ /

;;;z: 7712

+

512 +

(512-j

,/ /

5/2+

912 -

11/2- -\

\

\

\

\

\

.I

1112-

-----

3/2+ l/2+ Kisslinger Sorensen

Fig. 6. Comparison

-

of experimental

312 + 112 + Experiment

and theoretical energy levels of lz5Te.

logAg, are formed by coupling three quasiparticles in the shell-model state j to a state (j”)j_ 1. In Kisslinger’s model it is explained why this level is depressed due to the quadrupole component of the effective nuclear interaction much more than other states formed in the same way. An “intruder state” is thus brought in at a relatively low energy (see fig. 6). A check whether this interpretation is correct is provided by data on electromagnetic moments for such states and the transition rates, especially for the 3qplqp transition to the state j. The g-factor for a three-quasiparticle state of Kisslinger’s type is - as pointed out by Stone et al. ‘) - exactly that of a single nucleon in the j-state. For the $- state in 12’Te, which is based on the Js- shell-model state at 145 keV, we can compare with

380

D.

W.

CRUSE

f?t cd.

the Schmidt value for h, : g = -0.35. The measured factors gj for h, states in neighbouring nuclei are, however, gj,exp = -0.198 for ‘lJmCd and gj,enp = -0.190 for 115mCd, and we take instead the mean value of these two as the most probable value for the J$-state, i.e. g1 = - 0.194. Our experimental value for the q-state, g = -0.204+0.007 is thus in excellent agreement with the prediction made for a (y-)i three-quasiparticle state. Additional evidence for the three-quasiparticle nature is given by the measured E2 transition probability to the %i-- state. The theoretical E2 enhancement has been compared to the experimental one by Kownacki et al. “) and later by Marelius et al. “). Both measurements strongly support Kisslinger’s description of the 321 keV level. The earlier discrepancy, resulting from the very high g-value reported by Singh et al. ‘) has thus been removed. The g-factor for the 3- state in ‘IV measured by Kesztheleyi et al. ‘*) was found to be g(3) = 1.54kO.13. Since it was not mentioned by the authors we should like to point out that this value is within the limits of error equal to the g-factor for the ground state g(s) = 1.47 and therefore supports a three-quasiparticle description for the 3 state. The 464 keV level is probably quite well described in the one-quasiparticle + phonon model and the Kisslinger-Sorensen wave function is found to be “) Is+> = 0.4625&O, 0>+0.65351+, 1, 2) +0.22991+, 1, 2)+0.30711& 1, 2) +0.117615, 1,2), from which a prediction for the magnetic moment of p = 1.2 n.m., g = -0.52 is derived. Our experimental value, which is of very limited accuracy, is g = - 0.23 + 0.11 and can be compared to the value published by Singh et al.: -0.79kO.24. Considering the large errors involved none of these results can be said to disagree with the theory. However, our value comes closer to the KS prediction than to the single-particle value, g = -0.77. 6.2. MULTIPOLE

MIXING

ANALYSIS

The angular correlation coefficients for the 204 and 321 keV transitions can be used to derive more accurate values for the mixing ratios. In fig. 7 the method by Arm and Wiedenbeck I’) is used for analysing possible spin sequences for a cascade. Possible spins for the 525 keV state are 3-, $- and q-. The spin 4 cannot reproduce AZ = -0.40 for the 204-177 keV cascade when d2 = - 0.624kO.034 for the 177 keV transition “9 ‘). U n for t unately, the angular correlation results cannot discriminate between the spins 3- and Tl1 -. However, if the K-conversion coefficients of the 111, 117, 204 and 380 keV transitions are taken into account the most likely

MAGNETIC

381

MOMENT

spin assignment is s-. This spin was excluded by Stone et al. ‘) and by Andrews et al. ‘“) on the basis of nuclear orientation experiments. However, the results of these orientation experiments are rather’ sensitive to a small M3 admixture in the 379.5 keV y-ray. A value of 6(M3/E2) = -0.10 is sufficient to make the 3 spin assignment compatible with their measurements. The improved b-value of the 204 keV ($--+-) transition, which is of theoretical interest here since the 3- state might be interpreted as the (~-)~state in Kisslinger’s

At=-Ql68*Q012

A2=-a403ta014 -04

Fig. 7. Mixing ratio analysis of the 204-177 keV and the 321-177 keV transitions.

model, (see fig. 6) is s(E2/Ml) = -1.88kO.22 giving an E2 content of Q(E2) = 0.78kO.04. This high E2 content supports the interpretation of the 3- state suggested above. For the 321 keV (s’+-) transition the analysis (see fig. 7) gives 0 5 Q(M2) 6 0.010 and -0.22 s 6(M2/El) 5 0.

7. Hypertine fields for Te in ferromagnetic 7.1. FIELD

IN NICKEL

AND IRON

metals

HOSTS

The ratios of the Larmor precession frequencies w for the external field (Cu source) and the internal fields (Ni and Fe sources) give directly the ratios of the fields at the nucleus in the three cases. These ratios can be compared to the ratios between the

D. W. CRUSEet al.

382

fields obtamed

from Miissbauer

measurements

’ ‘):

B(Fe):

B(Ni): Bext = 3.2: 1: 0.26,

o(Fe):

w(Ni): w,,~ = 4: 1: 0.29.

The agreement is satisfactory and shows that the use of internal fields in PAC is justified. The most sensitive region of the P versus wr curve, used for the determination of unknown fields, corresponds to or-values in the range O-O.4 rad, which for this particular nuclear state means that internal fields in the range O-40 T can be determined. 7.2. HYPERFINE

FIELD FOR TELLURIUM

IMPURITIES

IN GADOLINIUM

Since the PAC method can be used to determine ratios of hyperfine fields in different materials with fairly high accuracy an attempt was made to introduce the r2 5Sb activity into gadolinium metal, for which the hyperfine field at antimony impurities is not known. The gadolinium sources were prepared in two different ways: (i) melting and annealing at 1000” C for 50 hours, (ii) ion-implantation in a mass-separator with 50 kV acceleration voltage. TABLE 6 Hyperrine field for Te in Gd Source preparation

Activity Temp (rel. to Cu (“IQ source)

I. melting+ annealing

II. a) melting+ annealing II. b) remelting without annealing

0.60

300 77

0.07 0.07

1.123(21) 0.97(3)

51 -12(13)

51 32

0 -44(13)

77

0.95(6)

-21(24)

32

-53(25)

77 300

1.01(6)

+4(25)

32

-28(25)

III. melting + annealing

0.02

77

IV. ion implantation No annealing

0.02

77 300

0.70(7)

0.55(10)

1.10(16)

42(65)

32

+11(65)

1.12(10)

50(40)

30

+20(40)

The solubility of antimony in gadolinium is presumably very small and it is difficult to establish by chemical methods to what extent the antimony forms a solid solution or exists as an antimony-rich phase. Therefore three different sources were prepared by method (i). The antimony concentration of the first one was estimated to be appreciably less than 1 %. The second and third sources consisted of approx-

MAGNETIC

MOMENT

383

imately & and 2c, respectively, of the antimony content of the first one. Since the nuclei in an Sb-rich phase are not expected to experience any hyperfine field a comparison of the mean nuclear precessions for the three sources would indicate whether or not they represented dilute solid solutions. The fraction of the nuclei that are subject to the hyperfine field would increase when lowering the concentration, resulting in a higher wr value for the mean precession. The PAC results for different types of Sb-Gd sources are presented in table 6. All measurements were performed using the 204-177 keV cascade. The first source was measured both at room temperature (which is above the Curie temperature To = 289°K) and at 77” K. The value Gz = 0.70+0.07, obtained at room temperature from an angular correlation measurement on the source IIb in table 6, corresponds to an electric quadrupole interaction frequency c$ = (270+ 70) - lo6 rad/s. The P-value in the 51 kG magnetic field is decreased by this combined interaction to 1.12(2), as compared to 1.21(l) expected for 51 kG with a pure magnetic interaction. According to the theory for a combined interaction the precession angle d0 vs o,r curve is linear for small angles. This is also the case in the presence of an o$ of the above magnitude. With sufficient accuracy we can use the calibration B,, = k(P,- 1) with k = 415 kG. The effective field Betf is the sum of the external fields. Since field Bext, the hyperfine field Bhf and the Lorentz and demagnetization the latter two approximately cancel each other we have

In order to find out whether heat treatment caused the segregation of antimony into Sb-rich phases, source II was measured twice; first after conventional heat treatment, then after remelting and quenching to liquid nitrogen temperature. For the weak samples prepared through melting as well as for the ion implanted sample, the coincidence counting rate was very low and only order of magnitude estimates for the hyperfine field could be obtained. The low Gz value measured for the ion implanted source may possibly be due to radiation damage. The weighted mean of the results given in table 6 is Bhf = -38* 10 kG. Negative fields in gadolinium have also been found for Sn [ref. “‘)I, Cd [ref. ““I) and Pd [ref. ““)I impurities, with values of Bhf = - 302+ 4, - 312+ 10 and - 62+ 9 kG respectively, at 77” K. These results together with the present measurement support the theoretical calculations of Campbell 26,27) based on the Daniel-Friedel conduction electron polarization model. The authors are indebted to Professor K. Siegbahn for his interest in this work. The assistance of Fil. mag. Tor Noreland and Fil. mag. Arne Arnesen, who prepared the mass-separated sources, is gratefully acknowledged. The work was financially supported by the Swedish Atomic Research Council. D.W.C. wishes to thank The Royal Society (England) for a Post-Doctoral European Program Fellowship which made his work at Uppsala possible.

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