Measuring the gyromagnetic factor of the 2+ state in 126Sn via the transient field technique
The gyromagnetic ratio (g factor) of the first 2+ state in 126Sn was measured to be g(2+)=-0.25(21) using Coulomb excitation of the beam and the transient field technique. Despite the large statistical uncertainty, the sign has been determined to be negative and continues the trend, see figure 1, observed in the stable isotopes that were previously measured. The negative sign indicates the neutron occupation of the h11/2 and d3/2 orbitals as the N=82 shell closure is approached. Large-scale shell model calculations (LSSM), quasi-particle random phase approximation (QRPA) and quasiparticle phonon model (QPM) calculations were done in order to compare with experiment. Both approaches can explain the data with the QRPA results favoring smaller g factor values for 126Sn. We note that in another HRIBF radioactive beam Coulomb excitation measurement, the recoil-in-vacuum technique was used to determine the magnitude (but not the sign) of the g factor of the state to be 0.12(3).
The transient field g factor measurements requires a thick target composed of several layers of material clamped across the pole pieces of an electromagnet that is cooled to liquid nitrogen temperatures. For the present case, the 126Sn ions were Coulomb excited by 1.0 mg/cm2 carbon that was backed by 5.01 mg/cm2 Gd, 1.1 mg/cm2 Ta, and 5.04 mg/cm2 of Cu. The scattered Sn ions are stopped in the target while the carbon atoms exit the target and strike one of three solar cells located at 0 degrees with respect to the beam and above and below the beam axis. This arrangement selects the appropriate 126Sn ions that emit gamma-rays in the reaction plane. The magnet saturates the Gd layer such that the fast moving ions experience a hyperfine magnetic field of 2.4 kT resulting in a spin precession about the external field axis. This field is flipped periodically. The resulting changes in gamma-ray yield allow one to determine the g factor. In addition, the lifetime of the 2+ state was measured via the Doppler shift attenuation method.
The Holifield Radioactive Ion Beam Facility produced the 126Sn beam through proton-induced fission in a uranium carbide target located in an ion source. The atoms diffused out of the target and formed tin sulfide that was then ionized and mass analyzed by a low-resolution magnet. The molecules were passed through a charge exchange cell where the molecules were broken resulting in negatively ionized 126Sn that was accelerated to 200 keV. This beam passed through the high-resolution isobar separator and was injected into the tandem where it was accelerated to 378 MeV or about 20% below the Coulomb barrier. This molecular transport technique suppresses 126Sb (19.15 m, 12.35 d) and 126Te isobaric contaminants by 4 or 5 orders of magnitude. This experiment could not have been done without using this beam purification technique, even though 126Sn would have been over 90% of the total beam. The buildup of radioactive 126Sb in the thick target would have caused very high counting rates in the Ge detectors.
G. J. Kumbartzki et al., “Transient field g factor and mean-life measurements with a rare isotope beam of 126Sn,” Phys. Rev. C 86, 034319 (2012).