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

Recently the first-excited state in Sb-135 has been observed at the excitation energy of only 282 keV and, due to its properties, interpreted as representing mainly a configuration of a d(5/2) proton coupled to the (134)Su core. It was suggested that its low-excitation is due to a relative shift of the proton d(5/2) and d(7/2) 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 beta gamma gamma(t) method at the OSIRIS fission product mass separator at Studsvik. The preliminarily measured half-life, T-1/2 = 6.0(7) us, yields an exceptionally low B(M1; 5/2(1)(+) -> 7/2(1)(+)) value of <= 2.9 x 10(-4) mu(N)(2). The result is discussed in the framework of shell model calculations.