First observation of α decay of 190Pt to the first excited level (E_exc = 137.2 keV) of 186Os

The α decays of naturally occurring platinum isotopes, which are accompanied by the emission of γ quanta, have been searched for deep underground (3600 m water equivalent) at the Gran Sasso National Laboratories of the INFN (Italy). A sample of Pt with a mass of 42.5 g and a natural isotopic composition has been measured with a low background HP Ge detector (468 cm3) during 1815 h. The α decay of 190Pt to the first excited level of 186Os (Jπ=2+, Eexc=137.2 keV) has been observed for the first time, with the half-life determined as T1/2=2.6-0.3+0.4(stat.)±0.6(syst.)×1014 yr. The T1/2 limits for the α decays of other Pt isotopes have been determined at the level of T1/2≃1016–1020 yr. These limits have been set for the first time or they are better than those known from earlier experiments.