Light Readout for a 1 ton Liquid Argon Dark Matter Detector

Evidence for dark matter (DM) has been reported using astronomical observations in systems such as the Bullet cluster. Weakly interactive massive particles (WIMPs), in particular the lightest neutralino, are the most popular DM candidates within the Minimal Supersymmetric Standard Model (MSSM). Many groups in the world are focussing their attention on the direct detection of DM in the laboratory. The detectors should have large target masses and excellent noise rejection capabilities because of the small cross section between DM and ordinary matter (σWIMP−nucleon < 4 · 10−8 pb). Noble liquids are today considered to be one of the best options for large-size DM experiments, as they have a relatively low ionization energy, good scintillation properties and long electron lifetime. Moreover noble liquid detectors are easily scalable to large masses. This thesis deals with the development of a large (1 ton) LAr WIMP detector (ArDM) which could measure simultaneously light and charge from the scintillation and the ion- ization of liquid argon. Argon vacuum ultra violet (VUV) scintillation light has to be converted with wavelength shifter (WLS) materials such as TetraPhenyl Butadiene (TPB) in order to be detected by photomultipliers with bialkali photocathodes. This thesis de- scribes the research and developments performed to detect the 128 nm VUV light with high efficiency and low threshold. The scintillation light from α particles in gaseous (GAr) and liquid argon (LAr) was used for the development of an optimal WLS coated reflector. Different coating techniques were investigated and optimized. The best results were achieved using the evaporation technique, a teflon membrane (Tetratex) as reflector material, and a WLS surface density of 1 mg/cm2 (TPB). The LAr scintillation light was studied for different projectiles. The relative intensities of the scintillation components were measured for α−particles, electrons and muons. Neutron induced argon nuclear recoils were also measured. An evaporator chamber was constructed for the evaporation of the WLS on large surfaces. This research and development was preparatory for the construction of the ArDM light readout which is composed mainly of two parts: the side reflector and the light detector. The side reflector was segmented in panels which were coated using the large evaporator. The light detector is composed of an array of 14 × 8” cryogenic photomultipliers (PMTs). Different cryogenic PMTs were tested in the laboratory. A light readout system with the side reflector and 8 PMT modules with different coatings on the photocathodes, was assembled at CERN at the end of 2007. The data acquisition (DAQ) system was developed and tested. The performances of the light readout were measured in GAr and LAr using a movable internal Am α−source and external radioactive sources in different configurations. A light yield of 0.50 ± 0.05 p.e./keVee (photoelectrons per keV electron-equivalent) was obtained for 7 out 14 PMTs. The Hama- matsu R5912-02 mod PMT was chosen for the construction of the 1 ton LAr detector which was assembled and tested in October 2009.