Fast-timing measurements performed at ALTO on 137,139Cs

Fast-timing measurements were performed on 137,139Cs obtained from the ?? decay of 137'139Xe. The neutron-rich Xe nuclei were produced by the 238IJ photofission induced by the 50 MeV electron beam delivered by the ALTO facility. The half-lives of the first excited state in 137Cs and of six excited states in 139Cs were measured for the first time. These T1/2 results give new information on the structure of the 137,13955Cs82,84 nuclei located near the neutron closed shell N = 82. In particular, it is shown that, in both nuclei, the first excited 5/2+ level corresponds mainly to the one-proton state issued from the ?2d5/2 orbital. From the comparison with other N = 84 isotones, the most probable spin values for the states located at 289.8, 393.5, and 393.8 keV in 139Cs appear to be 5/2+, 3/2+, and 1/2+.