  "Ultra-low" Q value $\beta$ decays are referred to as such due to their low
decay energies of less than $\sim$1 keV. Such a low energy decay is possible
when the parent nucleus decays into an excited state in the daughter, with an
energy close to that of the Q value. These decays are of interest as potential
new candidates for neutrino mass determination experiments and as a testing
ground for studies of atomic interference effects in the nuclear decay process.
In this paper, we provide an updated evaluation of atomic mass data and nuclear
energy level data to identify potential ultra-low Q value $\beta$ decay
candidates. For many of these candidates, more precise and accurate atomic mass
data is needed to determine if the Q value of the potential ultra-low decay
branch is energetically allowed and in fact ultra-low. The precise atomic mass
measurements can be achieved via Penning trap mass spectrometry.
