Material Effects on Electron-Capture Decay in Cryogenic Sensors

Abstract

Several current searches for physics beyond the standard model are based on measuring the electron-capture (EC) decay of radionuclides implanted into cryogenic high-resolution sensors. The sensitivity of these experiments has already reached the level where systematic effects related to atomic state energy changes from the host material are a limiting factor. One example is a neutrino mass study based on the nuclear EC decay of ${}^{7}\mathrm{Be}$ to ${}^{7}\mathrm{Li}$ inside cryogenic $\mathrm{Ta}$-based sensors. To understand the material effects at the required level, we use density-functional theory to model the electronic structure of lithium atoms in different atomic environments of the polycrystalline $\mathrm{Ta}$ absorber film. The calculations reveal that the $\mathrm{Li}$ 1s binding energies can vary by more than 2 eV due to insertion at different lattice sites, at grain boundaries, in disordered $\mathrm{Ta}$, and in the vicinity of various impurities. However, the total range of $\mathrm{Li}$ 1s shifts does not exceed 4 eV, even for extreme amorphous disorder. Furthermore, when investigating the effects on the $\mathrm{Li}$ 2s levels, we find broadening of more than 5 eV due to hybridization with the $\mathrm{Ta}$ band structure. Material effects are shown to contribute significantly to peak broadening in $\mathrm{Ta}$-based sensors that are used to search for physics beyond the standard model in the EC decay of ${}^{7}\mathrm{Be}$, but they do not explain the full extent of observed broadening. Understanding these in-medium effects will be required for current- and future-generation experiments that observe low-energy radiation from the EC decay of implanted isotopes to evaluate potential limitations on the measurement sensitivity.