Identification of a 3.2?s isomer in76Rb

Abstract

Fusion evaporation reactions were investigated to search for short lived isomeric states of nuclei near the proton drip line. Gamma spectra were measured, both singles and in delayed coincidence with evaporation residues implanted into a silicon detector after a velocity separation. A short lived activity was measured in the40Ca+40Ca →80Zr* reaction at excitation energies between 55 and 79 MeV. A half-life of 3.20 (10) μs was determined from delayed coincidences between evaporation residues and gamma rays. AtE *=55 MeV the cross section is 9 mb. The activity was assigned to an isomeric state in76Rb by investigation of excitation functions. The isomer decays by emission of four gamma rays with energies of 70.55(5), 101.30(4), 145.11 (5), and 246.32(10) keV. A first level scheme is proposed assigning to the isomeric state an energy of 316.94(7) keV above the ground state. The isomer decaying into the low spin 1(−) ground state band is explained from systematics as a band head of a high spin (4+) (πg9/2,νg9/2) structure. A high hindrance factor of 3×106 forE1 radiation compared to a single particle transition is due possibly to a change of the core particle structure in the transition.