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

超精细异常产生于有限核体积的核磁化和费米接触引起的超精细场的超精细相互作用。点核磁结构和有限磁结构之间的差异被称为玻尔-韦斯科普夫效应[2]，它取决于核结构。如果自旋和轨道对磁矩的贡献具有相反的符号，则预计会出现大的超精细异常。从2001年开始，本项目对玻尔-外斯科普夫效应进行了为期两年的研究。在10 mK左右对Sc同位素（A=44，44 m，46，47，48）进行了定向核核磁共振实验。样品是在日本东北大学CYRIC回旋加速器上用α束反冲注入Fe箔制备的。用^3He/^4He稀释制冷机将箔的活化部分冷却到约10 mK。观察到Sc同位素（A=44，44 m，46，47，48）的所有NMR-ON共振。除了^ Sc之外的磁矩值<48>已经通过原子束方法已知。由这些结果确定了^ ScFe的超精细场<47>和^ Sc的磁矩<48>分别为-13，17（5）T和3.79（1）μ L<N_3>。与^ Sc相比<44m>，Bohr-Weisskopf效应分别为：^<44>ScΔ^<44m>Sc = -1.2（12）%，^<46>ScΔ^<44m>Sc = 1.3（7）%，^<47>ScΔ^<44m>Sc = -1.3（7）%。我们还<91g.m>使用京都大学的质量分离器制备了^ YFe样品。观察到两种同位素的NMR-ON共振。根据共振频率与外磁场的关系，确定玻尔-魏斯科普夫效应为^<91>YΔ^<91m>Y = -1（3）%。^<91>Y的情况是自旋和轨道对磁矩的贡献具有相反的符号。Fujita和Arima的理论估计得出的效果为-4-5%。详细分析正在进行中。

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.