Accurate Calibration of Nuclear Recoils at the 100 eV Scale Using Neutron Capture

Abstract

Searches for light dark matter (DM) and studies of coherent elastic neutrino-nucleus scattering (CEvNS) imply the detection of nuclear recoils in the 100 eV range. However, an absolute energy calibration in this regime is still missing. The CRAB project proposes a method based on nuclear recoils induced by the emission of an MeV gamma following thermal neutron capture. A detailed feasibility study has shown that this method yields distinct nuclear recoil calibration peaks at 112 eV and 160 eV above background for tungsten. In the first phase, the CRAB project foresees to perform a nuclear recoil calibration of cryogenic CaWO4 detectors read-out by TES, similar to the detectors used in CRESST and NUCLEUS. The low-power TRIGA reactor in Vienna provides a clean beam of thermal neutrons well suited for such a measurement. Newly developed and compact sub-keV calibration sources based on x-ray fluorescence (XRF) provide an absolute energy calibration during operation at the research reactor as well as in the DM/CEvNS experiments. In the second phase, additional tagging of the photons produced in the de-excitation process will allow extending the calibration method to even lower energies and to a wider range of detector materials, such as Ge. Combined with the XRF source, CRAB may allow measuring energy quenching in the sub-keV regime.