Hyperfine Interactions 78 (1993) 147-151

147

Magnetic dipole moment of 127Sbby NMR/ON M. Booth a, M. Lindroos a,b, I. Oliveira a, p. Richards a, J. Rikovska a, N.J. Stone a, B. Fogelberg c and M. Veskovic d,e a Clarendon Laboratory, Parks Road, Oxford, OJ(13PU, UK b Physics Department, Chalmers University of Technology, Gothenburg, Sweden c Studsvik Science Research Laboratory, Nyk@ing, Sweden d CERN, Geneva, Switzerland oInstitute of Physics, University of Novi Sad, Novi Sad, Yugoslavia

The technique of NMR on oriented nuclei has been applied to 1278bto measure the magnetic dipole moment of the 127Sbground state. Resonant destruction of gamma-ray anisotropy from 127Sbg (I~ = 7/2+) has been observed at 139.6(2) MHz for Bapp= 0.30(1) T and at 138.7(1) MHz for Bapp= 0.25(1) T. The deducedmagneticmomentis I~1 = 2.697(6)#N.

1. I n t r o d u c t i o n The accurate measurement of the magnetic dipole m o m e n t s of nuclear states and their variation with changes in proton and neutron number provides valuable nuclear structure information. Measurements of this type can be made on nuclei far from stability using N M R technique on oriented nuclei ( N M R / O N ) where the orientation is produced at low temperature after mass separation and implantation into a polarised host, normally iron. In this method, a destruction of nuclear orientation (a destruction of gamma-ray anisotropy) is expected, due to a partial equalisation of the nuclear magnetic substate populations by resonant R F absorption. The resonant frequency corresponding to the gB-product is determined unambiguously with a precision which is typically ,,~ 10 -3 [1]. Near double-closed shells rather than pure shell model configurations are expected to dominate the structure of the ground state and low-lying excitations in odd-A nuclei. Antimony, with Z = 51, one proton above the strong magic number Z = 50, should exhibit only small deformation effects and thus measured nuclear m o m e n t s should fall within the scope of a nearly spherical shell-model description. High accuracy (1 in 103) measurements of the magnetic dipole m o m e n t s of 127-133Sb using low-temperature nuclear orientation of isotope-separator implanted short-lived radioisotopes and N M R / O N can be used to examine how the collective c o m p o n e n t in the 7/2 + Sb ground state magnetic dipole m o m e n t var© J.C. Baltzer AG, SciencePublishers

148

M. Booth et al. / Magnetic dipole moment o f 12zSb

ies as a function of neutron number. Coupling schemes characterising the odd nucleons and the ground state deformation can be extracted from the nuclear moments. With (double magic + 1) 1338b as the reference, the main goal of these experiments is to examine whether the collective component in 7/2 + Sb ground state magnetic dipole moment varies as expected according to particle-corecoupling calculations carried out for Sb (Z = 51) isotopes [2]. Comparison of the 1-proton-particle excitations in Sb to 1-proton-hole states in In nuclei will shed light on differences between particle and hole excitations as understood within the present model. Comparison of results on Sb isotopes with those in T1 will yield information on the effect of the differing underlying shell structure upon the mean field at the beginning and the end of the 10-82 proton shell. The present experiment is thus the starting point in a systematic study of magnetic dipole moments of heavy odd-A Sb isotopes. Previous to this study Krane and Steyert carried out the nuclear orientation study of gamma-rays emitted by 127Sb polarised in iron [3]. From the 1-90 ° anisotropics of the angular distributions, the ground state magnetic dipole moment was deduced to be: [/z(127Sb)l = 2.59(12)/zN. This Value is in agreement with systematics of other g7/2 levels in the odd-mass Sb isotopes, but poor accuracy does not allow any precise comparison with particle-core-coupling calculations.

2. Experimental details and results The radioactive source of 127Sb was prepared by mass separation of A = 127 nuclei from the fission products of 235y at the OSIRIS facility mass separator online to the nuclear reactor at Studsvik [4]. The sample employed in the experiment was a disk of diameter 6 mm and at the start of the experiment the activity was of order of 3 ~tCi. The sample was soft-soldered to the copper cold finger of a 3He/4He dilution refrigerator. The temperature of the sample was measured using a 54MnNi sample whose well known moment and decay scheme allow an accurate temperature measurement. The gamma-rays following the 13- decay of the 1278b into 127Te were detected using two Ge detectors, one placed along(axial) and the other perpendicular (equatorial) to the applied magnetic field used to polarise the iron foil and so define the axis of nuclear orientation. Two gamma transitions Ev = 686 keV and Ev = 473 keV were used to look for the N M R resonant destruction ofanisotropy. A search for the 127SbgFe resonance was made with two different samples in the range 132.5-142.5 MHz. The first experiment was made at Bapp = 0.25(1) T over 132.5-142.5 MHz in steps of 1 MHz with +1 MHz modulation at 500 Hz. The second experiment was made at Bapp = 0.30(1) T over 136-142.5 MHz in steps of 0.5 MHz with 4-1 MHz modulation at 500 Hz. At each frequency the count rate in

149

M. Booth et aLI Magnetic dipole moment of 127Sb

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Fig. 1. Partial decayof 1278b to the levelsin 127Te. the axial detector was measured with the frequency modulation on (NFM) and with a carrier wave only present (Ncw). The percentage destruction of anisotropy may be defined as (NFM

-- Ncw

D (%)= (Nw-Ncw)

)

x 100,

(1)

where Nw is the axial count rate when the iron foil is warm (T ~ 1 K) giving no nuclear orientation. All count rates were corrected for source decay. In the resonance case eq. (2) is fulfilled, hvo = gtZN[Bhf + Bapp(1 + K)].

(2)

The resonance centre frequency can be expressed in terms of the magnetic moment #, the hyperfine field Bre and the external applied field Bapp by vo = ( I d l Z h ) [ B h f + B0(1 + K)],

(3)

M. Booth et al. / Magnetic dipole moment of 127Sb

150

where K is the Knight shift parameter which is estimated to be IK(SbFe)I ~ 2 x 10 -3 using the Korringa constant for 125SbFe [5] and the Korringa relationship [6]. Adopting K = 0 in eq. (3) introduces a negligible error into our result for l#(127Sb)l . The most accurate determination of the hyperfine field at antimony isotopes in iron is the spin-echo experiment ofKoi et al. [7]. they gave Bhf(123SbFe) = +23.387(10) T. A fit to the variation of D with frequency (evaluated for each transition separately and summed) for Bapp = 0.25(1)T gave the resonance frequency centre at u0(Bapp = 0.25(1) T) = 138.7(1) Mhz with a full width at half maximum (FWHM) of 1.7 M H z (see fig. 2) and for Bapp = 0.30(1)T, u0(Bapp = 0.3(1) T) = 139.6(2) M H z with F W H M = 1.3 MHz. These results yield for the ground state magnetic moment the value

I#1 =

2.697(6)/~N.

The sign of the magnetic moment cannot be determined using N M R / O N but it

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M. Booth et al. / Magnetic dipole moment of 12~Sb

151

Table 1 Measured magnetic moments of 7 / 2~-odd-A Sb isotopes [8]. Isotope

State

# (/zN)

121Sbm 123Sb

(37 Ke V-3.5 ns) ground state ground state ground state

+2.518(7) +2.5498 (2) +2.5859(13) a +2.697(3)

125Sb 127Sb

Recalculated from v(B = 0) = 131.71(3) MHz and Bhf = 23.387(10) T [7,9].

is assumed to be positive on the basis of the systematics of other g7/2 levels in the odd-mass Sb isotopes (see table 1). Particle-core-vibration coupling calculations carried out for the Sb isotopes [2] predict that the variation in moment should reflect the variation in first 2+ state of the Sn core nucleus. As this energy rises only very slowly in 124,126,X28Sn the observed continued slow change in odd-A Sb magnetic moment is as expected, greater changes being predicted closer to the 132 shell closure.

References [1] I. Berkes, in: Low-Temperature Nuclear Orientation, eds. N.J. Stone and H. Postma (North-Holland, Amsterdam, 1986). [2] K. Heyde, private communication (1989). [3] K.S. Krane and W.a. Steyert, Phys. Rev. C 6 (1972) 2268. [4] G. Rudstam, Nuel. Instr. Meth. 139 (1976) 239. [5] E. Klein, in: Low-Temperature Nuclear Orientation, eds. N.J. Stone and H. Postma (North-Holland, Amsterdam, 1986). [6] J.Korringa, Physiea 16(1950)601. [7] Y. Koi, M. Kawakamni, T. Hihara andA. Tsujimura, J. Phys. Soe. Japan 33 (1972) 267. [8] C. Bengston et al., Phys. Seripta 30 (1984) 164 R. Neugart et al., Phys. Rev. Lett. 55 (1985) 1559; E. Otten, in: Treatise on Heavy Ion Science, Vol. 8 ed. D.A. Bromley (Plenum Press, New York, 1989) eh. 7. [9] J.A. Barclay et al., Proc. Conf. HFS and Nuclear Radiations, Asilomar 1967, eds. Matthias and Shirley (North-Holland, Amsterdam, 1968) p. 902.