Zusammenfassung
Experimental evidence from astronomy and cosmology has shown that dark matter is the dominant component of our Universe. Despite this, its nature remains a mystery. Over the past decades, several detector technologies have been developed to directly detect dark matter particles. Among these, the XENONnT experiment aims to detect WIMPs using a dual-phase xenon TPC. Experiments searching for rare events, such as interaction of dark matter particles with ordinary matter, require extremely high sensitivity due to the expected low event rate. Achieving such sensitivities demands careful study of background contributions and the implementation of mitigation techniques. In XENONnT, as in other experiments using noble gases as targets, one of the most significant background sources arises from the radioactive isotope 222Rn, a decay product of the primordial 238U series. This isotope is present in all detector materials and has a relatively long half-life (3.8 days) and a high mobility in the material, allowing it to emanate into the detector’s active region from material surfaces. Among its decay products, 214Pb undergoes beta decay with an endpoint energy of about 1 MeV, representing the dominant source of electronic recoil (ER) events for recoil energies ≤ 30 keV. To meet the stringent background level for 222Rn of 1 μBq/kg set by the XENONnT experiment, considerable experimental effort is dedicated to understanding and suppressing this background. What is more, for the next generation experiment DARWIN/XLZD a suppression of one order of magnitude more is foreseen, which requires the development of new mitigation techniques.
To accurately characterize 222Rn emanation and implement mitigation strategies, reliable 222Rn emanation sources are needed. This work focuses on the production and characterization of such sources using various methods of radon emanation measurements. Firstly, the methodology for producing 222Rn sources is described. These sources are obtained by implanting 226Ra atoms into samples of different materials, such as stainless steel, titanium, copper, lead, and polytetrafluoroethylene (PTFE). The implantation process is carried out at the Ion Separator On-Line Device (ISOLDE) facility at CERN, producing samples with activities ranging from approximately a few mBq to a few Bq. Additionally, a simulation of the 226Ra atom implantation process in different sample materials is performed with the use of the TRIDYN software, enabling a detailed study of the implantation profile and its dependence on parameters such as fluence and material density, along with an estimation of the radon emanation fraction. Once produced, the sources are characterized and studied at the Max Planck Institut für Kerniphysik (MPIK) using local facilities. The implanted activity is verified through alpha and gamma spectrometries, while the radon emanation rate of the samples is analyzed using an electrostatic radon monitor and miniaturized proportional counters. The emanation fraction of 222Rn is then estimated for all samples with different materials and activities. The production and characterization of reliable 222Rn emanating sources enables detector calibration studies as well as studies of radon mitigation techniques, such as the coating technique. Furthermore, this work serves as a starting point for deeper studies of radon emanation from different materials, which could eventually provide guidance for high-purity material selection in the next generation experiments.
