Study of Bohr-Weisskopf effect using low-temperature nuclear orientation

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The hyperfine anomaly arises from the hyperfine interaction of the finite nuclear volume of nuclear magnetization and the hyperfine field due to Fermi-contact. The difference between the point nuclear magnetic structure and the finite magnetic structure is referred as Bohr-Weisskopf effect [2], which depends on the nuclear structure. If the spin and orbital contributions to the magnetic moment have opposite sign, the large hyperfine anomaly is expected. The study of Bohr-Weisskopf effect by this project was carried on for two years from 2001.Nuclear magnetic resonance on oriented nuclei (NMR-ON) experiments on Sc isotopes (A=44, 44m, 46, 47, 48) have been performed at about 10mK. The samples were prepared by recoil implantation into Fe foils using α- beams at CYRIC cyclotron, Tohoku University. The activated part of the foil was cooled down to about 10mK by a ^3He/^4He dilution refrigerator. All NMR-ON resonances for Sc isotopes (A=44, 44m, 46, 47, 48) were observed. The values of magnetic moments except ^<48>Sc are already known by the atomic beam method. From these results, the values of the hyperfine field of ^<47>ScFe and the magnetic moment of ^<48>Sc were determined as -13, 17(5) T and 3.79(1)μ_<N_3> respectively. Comparing with ^<44m>Sc the effects of Bohr-Weisskopf were determined as; ^<44>ScΔ^<44m>Sc = -1.2(12)%, ^<46>ScΔ^<44m>Sc = 1.3(7)%, ^<47>ScΔ^<44m>Sc = -1.3(7)%. We also made the sample of ^<91g.m>YFe using the mass separator at Kyoto Univ.. The NMR-ON resonances for both isotopes were observed. From the values of the resonance frequencies versus the external magnetic fields, the effects of Bohr-Weisskopf was determined as ^<91>YΔ^<91m>Y = -1(3)%. The ^<91>Y case is that the spin and orbital contributions to the magnetic moment have opposite sign. The theoretical estimation from Fujita and Arima yields that the effect is -4-5%. Detailed analysis are now in progress.