Novel techniques for liquid xenon time projection chambers: A Ar-37 calibration source for dark matter searches and characterization of silicon photomultipliers
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Abstract
Astronomical observations of the dynamics of galaxies and galaxy clusters suggest they have to contain substantially more mass than is directly observed, the so-called Dark Matter (DM). This finding is supported by measurements of the cosmic microwave background (CMB), which requires five times more inert mass than regular, 'baryonic' matter participating in acoustic oscillations of the primordial plasma. Furthermore, the observed structures in the universe constrain the DM to be non-relativistic at the epoch of matter-radiation equality. The combined observations exclude every known particle in the standard model of particle physics, leaving the explanation of the phenomenon to new physics.
The XENON detectors utilize dual-phase time projection chambers (TPCs), dedicated to the search for DM, focused in particular on one suitable candidate, the weakly interacting massive particles (WIMPs). The last two detectors of the series contain 2 t and 5.9 t as an active liquid xenon target for XENON1T and XENONnT, respectively. Although no DM has been detected yet, new exclusion limits were set.
To produce reliable results, the detector characteristics need to be well known. This is achieved with calibrations, where the detector is exposed to radiation with well-known properties. These can be external sources, but also radioactive isotopes mixed directly into the xenon can be used to verify detection efficiency and uniformity throughout the detector volume.
This work focuses on the introduction of a new internal low-energy source, the radioactive isotope Ar-37. Two transitions at energies of 2.82 keV and 0.27 keV, respectively, are used for calibration. The source can be produced on-site at the University of Mainz at the TRIGA research reactor located on campus.
On the hardware side, a dosing system was developed to fine-dose the amount of activity injected into the TPC. Also, the procedure to remove the source from the xenon was successfully performed, by the cryogenic distillation column of the XENON1T/nT recirculation system.
A complete calibration run was performed at the end of data taking of XENON1T and after the first science run of XENONnT. The high statistics and uniformity of Ar-37 data stressed the impact of field non-uniformity on charge carrier and photon production in xenon and therefore on the applied event corrections. Based on this calibration, we found an event-reconstruction anomaly in the top part of the TPC for low energies and developed a correction.
With the calibration data, the photon and electron yields of xenon for the 2.82 keV decay were measured at 32.27 ± 0.52 ph/keV and 41.02 ± 1.06 e-/keV, respectively. For the lower value of 0.27 keV an electron yield of 68.0+6.3-3.7 e-/keV was found.
Planned improvements of liquid xenon TPCs include a possible replacement of current photo multiplier tubess (PMTs) with modern, compact silicon photomultipliers
(SiPMs). To evaluate the feasibility and performance of SiPMs in xenon detectors, a test setup was developed to characterize the sensors under detector conditions. This setup and first steps in data analysis are presented in this work, with a possible future upgrade of the local 'MainzTPC' in mind.