Observation of β decay of 115In to the first excited level of 115Sn

In the context of the LENS R and D solar neutrino project, the γ spectrum of a sample of metallic indium was measured using a single experimental setup of 4 HP-Ge detectors located underground at the Gran Sasso National Laboratories (LNGS), Italy. A γ line at the energy (497.48+/-0.21) keV was found that is not present in the background spectrum and that can be identified as a γ quantum following the β decay of 115In to the first excited state of 115Sn ((9/2)+->(3/2)+). This decay channel of 115In, which is reported here for the first time, has an extremely low Qβ-value, Qβ=(2+/-4) keV, and has a much lower probability than the well-known ground state-ground state transition, being the branching ratio b=(1.18+/-0.31)x10-6. This could be the β decay with the lowest known Qβ-value. The limit on charge nonconserving β decay of 115In is set at 90% C.L. as τCNC>4.1x1020 yr