Q Value of 115In→115Sn(3/2+): The Lowest Known Energy β Decay

Using precision, cryogenic, Penning-trap mass spectrometry we have measured the atomic mass difference M[In-115]-M[Sn-115] to be 497.489(10) keV/c(2). Combined with the energy of the 3/2(+) first excited state of Sn-115 [J. Blachot, Nuclear Data Sheets 104, 967 (2005)], the Q value of the In-115(9/2(+))-> Sn-115(3/2(+)) beta decay is determined to be 155(24) eV. This confirms the inference of Cattadori [Nucl. Phys. A 748, 333 (2005)] that this small branching-ratio beta decay has the lowest-known Q value. We also report improved values of the individual atomic masses: M[In-115]=114.903 878 774(16) u and M[Sn-115]=114.903 344 697(17) u.