Exploring coherent neutrino-nucleus scattering with NUCLEUS

Neutrinos are electrically neutral particles which are hardly detected owing to their weak interaction with matter. Typically, detectors with a huge volume are required. A new technique, established only quite recently, the coherent neutrino-nucleus scattering increases the detection probability for neutrinos considerably. For that purpose, suitable crystals are grown and cooled to low temperatures close to absolute zero, we speak of cryogenic detectors. In case a neutrino scatters at an atomic nucleus, vibrations are induced in the crystal lattice which is equivalent to a rise in temperature. Dedicated sensors monitor these temperature changes. Instead of detectors weighing tons, now crystals with a total weight of a few grams are sufficient. By this method, the NUCLEUS experiment aims at the detection and characterization of neutrinos which are emitted after nuclear fission by the reactor fuel of a nuclear power plant. In this context it is important to understand how the detector crystals will respond to the energy change caused by scattering of the neutrinos. For the expected energy range of low energies this detector response is currently not known sufficiently well and would be estimated from higher energy behavior, only. To obtain a reliable calibration of the crystals in the relevant energy range, the CRAB project has been initiated. Here, we rely on another neutral elementary particle, the neutron. After absorption of a neutron by an atomic nucleus, this nucleus is in an excited state and will release the excess energy by emission of gamma radiation with precisely defined energy. This causes a recoil of the nucleus for which the response of the detector can be determined experimentally. In that way the energy scale of the crystals becomes calibrated for neutrino scattering. In the frame of the project a neutron beam out of the TRIGA reactor of TU Wien at the Atominstitut will be suitably tailored and directed onto a cryostat where the detector crystal at low temperatures is located. To detect the interaction sequence of neutron absorption, gamma emission and crystal excitation we will use the identical sensors as for detecting coherent neutrino-nucleus scattering. This fact ensures a high quality of the calibration procedure. Neutrino research with cryogenic detectors aims at a precise test of the successful standard model of particle physics. Processes which deviate from this model are looked for in a multitude of research areas. In our case this would lead to evidence of a neutrino interaction with matter that does not fulfil the predictions of the standard model. Also, in the search for Dark Matter, cryogenic detectors are employed. Therefore, the energy calibration provided by CRAB will find direct application in this field of research as well. Additional applications may be expected both in nuclear physics and in reactor physics.