Study of Collision and Γ -Cascade Times Following Neutron-Capture Processes in Cryogenic Detectors

The emission of gamma-rays after neutron capture in a cryogenic detector can generate monoenergetic nuclear recoils in the sub-keV regime, which is of direct interest for the calibration of dark matter and coherent elastic neutrino-nucleus scattering experiments. Here we show that accurate predictions of the spectra of total energy deposition induced by neutron captures require taking into account the interplay between the development in time of the deexcitation gamma-ray cascade of the target nucleus and that of the associated atomic collisions in matter. We present detailed simulations coupling the FIFRELIN code for the description of the gamma-ray cascades and the IRADINA code for the modeling of the fast atomic movements in matter. Spectra of total energy deposition are predicted, and made available to the community, for concrete cases of Al2O3, Si, Ge, and CaWO4 crystals exposed to a low intensity beam of thermal neutrons. We find that timing effects cause new calibration peaks to emerge in the recoil spectra and also impact the shape of the continuous recoil distribution. We discuss how they could give access to a rich physics program, spanning the accurate study of the response of cryogenic detectors in the sub-keV range, tests of solid state physics simulations, and nuclear models.