Excited-state magnetic moments of the magnesium isotopes

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Transient field(TF) is an effective magnetic field acting on ions passing through ferromagnetic foils. Because of large TF strengths, a large number of nuclear magnetic moments have been measured for short-lifetime excited states on stable nuclei. Such measurements have been carried out using stable-ion beams at velocities v << Z^・ vo, since the velocity dependence of the TF strength B_<TF> is well known in this region, where Z and vo denote the atomic number and the Bohr velocity. Taking into account the fact that radioactive-isotope beams produced in the projectile-fragmentation reaction have high velocities v 【greater than or equal】 Z^・ vo, it is important to determine the velocity dependence of B_<TF> at a velocity of approximately Z^・ vo or higher. From measurements using E=135 MeV ^<22>Ne and ^<24>Mg beams obtained from the K=70 RIKEN AVF cyclotron and the targets consisting of two layers, Au and Gd, we have determined preliminary values of B_<TF> strengths. Their values were smaller than the expected strengths from those determined in the lower velocity region. Theoretically, a sharp decrease in TF strength was predicted at velocities v【greater than or equal】Z^・ vo. The obtained strengths, however, are much larger than this prediction. It rather supports a recent model, with which a rather constant or even increasing trend of B_<TF> strength is predicted at velocities v【greater than or equal】Z^・ vo. Moreover, this observation is in agreement with other measurements obtained at similar velocities. This finding allows us to measure the excited-state magnetic moments of unstable nuclei produced in the projectile-fragmentation reactions.