Magnetic moments of ¹⁷N and ¹⁷B

The magnetic moments of ¹⁷N and ¹⁷B were measured by using spin-polarized radioactive nuclear beams which were obtained from the projectile fragmentation reaction. The observed magnetic moment of ¹⁷N, |μ(¹⁷N)|=(0.352±0.002)μ_N, where μ_N is the nuclear magneton, falls outside the Schmidt lines. By virtue of a simplifying feature of nuclear structure inherent in a p_1/2 valence nucleus, the deviation from the Schmidt value is attributed on firm ground to admixing of the configurations in which two neutrons in the sd shell are coupled to J^π=2⁺. This interpretation is confirmed in standard shell-model calculations. The calculations reproduce fairly well the experimentally inferred amount of 2⁺ admixture, as well as the experimental magnetic moment itself. The magnetic moment for ¹⁷B was determined as |μ(¹⁷B)|=(2.545±0.020)μ_N. The result is substantially smaller than the πp_1/2 single-particle value, and the shell-model calculations indicate that the quenching of μ largely stems from J^π=2⁺ configurations of the sd neutrons. The observed amount of quenching, however, is larger than the shell-model predictions, suggesting an enhanced contribution of the 2⁺ neutron configurations. This result is explained if the pairing energy for neutrons in the sd shell of a neutron-rich nucleus is assumed to diminish by about 30%. We also find that the use of the reduced pairing energy improves agreements in the magnetic moment and low-lying energy levels of ¹⁷N as well.