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

In the spectrum collected with the Pt sample (see figure), the peak at energy (137.10.1) keV is observed while it is absent in the background spectrum. The fit of the Pt spectrum in the energy region (120 170) keV with a Gaussian peak and linear background gives area of (132 17) counts, inconsistent with zero at about 8σ. The 137.1 keV peak can be explained with the α decay of 190Pt to the first excited level of 186Os. If populated, this level deexcites with the emission of a quantum with energy E=(137.157 0.008) keV which is in nice agreement with that of the observed peak. We did not find alternative processes which could mimic this effect. Thus, the measured T1/2 of the α decay of the 190Pt to the first excited level (Eexc = 137.2 keV) of 186Os decay is: 2.6+0.4-0.3(stat.) 0.6(syst.)× 1014 yr. This α decay has been observed for the first time.

Searches for α decays of other Pt isotopes - related with the emission of γ quanta - have also been searched for. Limits of order of 1017-1019 yr have been obtained.