Testing Shell Model on Exotic Nuclei at 135Sb

Recently the first excited state in ¹³⁵Sb has been observed at the excitation energy of only 282 keV and interpreted as mainly d_5/2 proton coupled to the ¹³⁴Sn core. It was suggested that its low-excitation energy is related to a relative shift of the proton d_5/2 and g_7/2 orbits induced by the neutron excess. With the aim to provide more spectroscopic information on this anomalously low-lying 5/2⁺ state, we have measured its lifetime by the Advanced Time-Delayed βγγ(t) method at the OSIRIS fission product mass separator at Studsvik. The M1 and E2 transition rates from the 282 keV state are strongly hindered, similarly to what occurs in ²¹¹Bi for the transition de-populating the first excited state at 405 keV. However, more data are needed above ¹³²Sn especially on the transition matrix elements. Thus our investigation was extended to include lifetime measurement of the 5/2⁺ 243 keV state in ¹³⁷I, which has an extra pair of protons above ¹³⁵Sb. Results of shell model calculations are presented.