We investigate the residual energy distribution in the electron capture (EC) process of ${}^{159}\mathrm{Dy}$, emphasizing the role of subthreshold atomic states, which have typically been omitted in conventional spectral analyses. By incorporating these energetically forbidden hole states into the spectral function $P(E)$, we demonstrate a significant enhancement—over an order of magnitude—in the EC rate near the zero-momentum neutrino emission region. This enhancement increases further with larger $Q$ values, contrary to standard expectations. Our analysis shows that $P(E)$ can be experimentally determined, resolving ambiguities near the end point and enabling new approaches to “ultralow $Q$ value” EC reactions for neutrino mass studies.

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• Electron & muon capture