A new way to measure neutrino masses

Internal bremsstrahlung electron capture (IBEC) is the decay of an atom into the one preceeding it in the periodic table, a neutrino, and a photon with energy in the X-ray domain. The endpoint shape of the photon spectrum is sensitive to the neutrino mass, m_v. IBEC experiments could significantly constrain m_v if the rates were sufficient. This is presumably not the case for typical K-capturing nuclides. But, in e-capturing isotopes in which K-(or L-) capture is forbidden, the fractional counting rate at the endpoint is enhanced by several orders of magnitude. The IBEC theory for these isotopes is developed, with emphasis on ¹⁹³Pt and ¹⁶³Ho. The theoretical and practical merits of β-decay and IBEC experiments - as tests for massive neutrinos - are compared.