  We investigate weak nuclear decays with extremely small kinetic energy
release (Q value) and thus extremely good sensitivity to the absolute neutrino
mass scale. In particular, we consider decays into excited daughter states, and
we show that partial ionization of the parent atom can help to tune Q values to
<< 1 keV. We discuss several candidate isotopes undergoing beta+, beta-, bound
state beta, or electron capture decay, and come to the conclusion that a
neutrino mass measurement using low-Q decays might only be feasible if no
ionization is required, and if future improvements in isotope production
technology, nuclear mass spectroscopy, and atomic structure calculations are
possible. Experiments using ions, however, are extremely challenging due to the
large number of ions that must be stored. New precision data on nuclear
excitation levels could help to identify further isotopes with low-Q decay
modes and possibly less challenging requirements.
