A magnetic moment measurement of the first-excited 2+ state of22Ne

Abstract

The magnetic moment of the first-excited 2 + state of 22Ne has been measured with the time-differential recoil-into-vacuum technique with the use of a plunger. The state was populated with the reaction 4He(19F, p)22Ne at E(19F) =41.4 MeV. Two helium targets prepared with a thickness of approximately 4/ag. cm -2 by implantation into nickel at 30 keV were used. Protons were detected at forward angles between 0 ~ and 10 ~ in a silicon surface-barrier detector shielded by a 90/~m gold absorber. Gamma rays were detected in coincidence with protons in four 12.7 cm X 12.7 cm NaI(T1) detectors at angles +45 ~ +90 ~ -90 ~ and -135 ~ and in an 125 cm 3 Ge(Li) detector at an angle of 0 ~ with respect to the beam direction. The outgoing 22Ne ions (o/c = 0.048) were stopped in a 30/~m thick stretched gold foil. The unperturbed "y-ray angular correlation on recoil into silver showed an 85% m = 0 substate population. An increasing linear background under the Pl peak in the particle spectrum due to the reaction 12C(19F, pn)29Si was caused by carbon deposited on the stopper and the target foil during the experiment. The target foil was continuously monitored for flatness and stability by a laser interferometer, which was also used to determine the distance of closest approach (6/~m) as described in an earlier paper [1]. In a preliminary analysis including contributions from the various atomic states of the free ion, the g-factor of the first-excited 2 + state was determined to be [g[ = 0.32 -+ 0.03. The lifetime of this state deduced from the Ge(Li) detector data was in good agreement with the compiled value [2]. Recent shell-model calculations [3] yield values of the g-factor between 0.30 and 0.35. The same reaction on a He-implanted Fe foil was used to measure the transient field precession at a high recoil velocity. This measurement yielded an integral precession angle 4)/g = 17 -+ 4 mrad, which is 15 -+ 4 mrad higher than the LindhardWinther (LW) prediction [4]. An earlier experiment [5] at low recoil velocity (u/c = 0.017) yielded dp/g = 2.9 -+ 0.7 mrad in fair agreement with the LW theory. The deviation from the LW value at high recoil velocity can be understood as capture of polarized electrons from Fe into ls vacancies of the moving 22Ne ion, see ref. [6].