Theg-factor of the 10? isomeric level in208At

Abstract

The g-factor of the 10-level at 1,090 keV in 2~ has been determined by the Time Differential Perturbed Angular Distribution technique. The result g(10 )= +0.269(3) is in very good agreement with the shell model prediction for a pure (7~h9/2 vil~z)loconfiguration. PACS: 21.10.KyThe 10 state is known [1, 2] to be isomeric in light and even bismuth isotopes. In 2~ which has one proton and one neutron hole compared to the doubly magic nucleus 2~ the nuclear structure is dominated by the coupling of the odd proton and the neutron hole in different orbitals. The configuration of the 10-state in 2~is an almost pure (7~h9/2 vi13~2)10-configuration [3] and this state remains as a seniority two state also in the lighter bismuth isotopes. Isomeric 10-states have also been observed in 21~176176 [zV6] and the same two quasiparticle configuration as in bismuth is suggested for these states. Using the effective g-factors + 0.91347 (5) for the zc9/2 orbital [7] and -0.1496 (18) for the vil-3'/2 orbital [8] an estimate of the g-factor for a pure configuration becomes + 0.2660 (11). The g-factor of the 2.5 ms 10-isomeric state in 2~ has been measured [3] with a high precision and found to be +0.2672(14) in very good agreement with the expected value. It is of interest to study the influence of the removal of neutrons and/or the addition of protons on the g-factor of the (7~h9/2 -1 v~3/2)1o-configuration. From the measured g-factors of the 10-states in 2~[9, 10], 2~[11] and 2~ [12], which are nuclei with 1, 2 and 3 neutron pairs removed from the core, the conclusion is that the g-factors decrease slightly when neutrons are removed. The influence of adding a proton pair to the bismuth isotopes can be studied by measuring g-factors of the 10-states in the even astatine isotopes. In this letter we report conclusive results about the g-factor of the 10-state in 2~ This nucleus corresponds to 2~ with a proton pair added. The 10-state in 2~ is reported [5] to have a half-life of 45 ns and an excitation energy of 1,090 keV (cf. Fig. 1). Thus a g-factor measurement using the standard TDPAD-method [13] was found feasible. 2~ was produced in the reaction 2~ 5n) with alpha particles from the Stockholm 225-cm cyclotron. The deexciting gamma rays were detected with two Gedetectors positioned at + -135 degrees with respect to the beam direction. Data were recorded between the beam pulses in a two parameter energy versus time mode. A beam pulse separation time of 1.2 gs was achieved by suppressing twelve out of thirteen of the natural beam pulses with the external beam pulsing system [14]. In order to avoid electric quadrupole interactions the target was kept molten during the experiments. The results obtained with a gate on