On the structure of the anomalously low-lying 5/2+ state of 135Sb

Recently the first-excited state in 135Sb has been observed at the excitation energy of only 282keV and, due to its properties, interpreted as representing mainly a configuration of a d5/2 proton coupled to the 134Sn core. It was suggested that its low-excitation is due to a relative shift of the proton d5/2 and g7/2 orbits due to 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 preliminarily measured half-life, T1/2 = 6.0(7)ns, yields an exceptionally low B(M1;5/21+ → 7/21+) value of ≤2.9×10-4μ2N. The result is discussed in the framework of shell model calculations.