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

In the context of the LENS R&D solar neutrino project, the gamma 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 gamma 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 gamma quantum following the beta decay of In-115 to the first excited state of Sn-115 ((9/2)(+) --> (3/2)(+)). This decay channel of In-115, which is reported here for the first time, has an extremely low Q(beta)-value, Q(beta) = (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) x 10(-6). This could be the beta decay with the lowest known Q(beta)-value. The limit on charge nonconserving beta decay of In-115 is set at 90% C.L. as tau(CNC) > 4.1 X 10(20) yr. (C) 2004 Elsevier B.V. All rights reserved.