First observation of α decay of 190Pt to the first excited level (Eexc = 137.2 keV) of 186Os
All the six naturally occurring isotopes of platinum are potentially unstable in relation to
α decay but only for one of them, the 190Pt (with the biggest energy
release of Q. = 3251(6) keV), this process was experimentally observed to-date
with half life estimations at level of 1011 yr.
In all the previous works the 190Pt α decay was observed only to the ground state (g.s.) of 186Os. However, the first excited level of the daughter nuclide 186Os has a quite low energy: Eexc = 137.2 keV, and the energy available to the α particle in the decay to this level is not much lower than that in the g.s. to g.s. transition. Our theoretical estimates of the corresponding half-life give values in the range of 1013 - 1014 yr.
By using the HP Ge detector of LNGS Ge facility, we have measured Pt sample with total mass of 42.5 g (see figure). In order to observe the rare α decay of 190Pt to the first excite level of 186Os, we have studied the presence of the 137.2 keV quantum emitted in the deexcitation of the 186Os nucleus. The Pt sample has a natural isotopic composition with very low percentage of 190Pt (0.014%).
The data with the Pt sample were collected with HP Ge detector (GeCris, 468 cm3) over 1815.4 h. The background spectrum of the detector was measured during 1045.6 h. The comparison of the Pt and of the background spectra shows that the platinum sample practically is not contaminated by "usual" radioactive contaminants (U/Th series, 40K, 60Co) but the presence of the radioactive 192Ir has been observed (activity: (49 ± 3) mBq/kg).