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Register a new userCryogenic light detectors with enhanced performance for rare events physics
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We have developed and tested a new way of coupling bolometric light detectors to scintillating crystal bolometers based upon simply resting the light detector on the crystal surface, held in position only by gravity. This straightforward mounting results in three important improvements: (1) it decreases the amount of non-active materials needed to assemble the detector, (2) it substantially increases the light collection efficiency by minimizing the light losses induced by the mounting structure, and (3) it enhances the thermal signal induced in the light detector thanks to the extremely weak thermal link to the thermal bath. We tested this new technique with a 16 cm$^2$ Ge light detector with thermistor readout sitting on the surface of a large TeO$_2$ bolometer. The light collection efficiency was increased by greater than 50% compared to previously tested alternative mountings. We obtained a baseline energy resolution on the light detector of 20~eV RMS that, together with increased light collection, enabled us to obtain the best $alpha$ vs $beta/gamma$ discrimination ever obtained with massive TeO$_2$ crystals. At the same time we achieved rise and decay times of 0.8 and 1.6 ms, respectively. This superb performance meets all of the requirements for the CUPID (CUORE Upgrade with Particle IDentification) experiment, which is a 1-ton scintillating bolometer follow up to CUORE.
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Simulations of photon propagation in scintillation detectors were performed with the aim to find the optimal scintillator geometry, surface treatment, and shape of external reflector in order to achieve maximum light collection efficiency for detector configurations that avoid direct optical coupling, a situation that is commonly found in cryogenic scintillating bolometers in experimental searches for double beta decay and dark matter. To evaluate the light collection efficiency of various geometrical configurations we used the ZEMAX ray-tracing software. It was found that scintillators in the shape of a triangular prism with an external mirror shaped as truncated cone gives the highest light collection efficiency. The results of the simulations were confirmed by carrying out measurements of the light collection efficiencies of CaWO4 crystal scintillators. A comparison of simulated and measured values of light output shows good agreement
A new generation of cryogenic light detectors exploiting Neganov-Luke effect to enhance the thermal signal has been used to detect the Cherenkov light emitted by the electrons interacting in TeO$_{2}$ crystals. With this mechanism a high significance event-by-event discrimination between alpha and beta/gamma interactions at the $^{130}$Te neutrino-less double beta decay Q-value - (2527.515 $pm$ 0.013) keV - has been demonstrated. This measurement opens the possibility of drastically reducing the background in cryogenic experiments based on TeO$_{2}$.
Progress in nuclear physics is driven by the experimental observation that requires state of the art detectors to measure various kinematic properties, such as energy, momentum, position etc. of the particles produced in a nuclear reaction. Advances in detector technology has enabled nuclear physicists to measure these quantities with better precision, and the reduced cost of the detection system has helped to have larger detection systems (array of detectors) to measure the rare processes with greater sensitivity. Several detection systems have been designed, developed and built in India over last few decades and are being used by the physicists. In this article, I will focus on such developments of detection systems at Variable Energy Cyclotron Centre (VECC), Kolkata.
For the first time, planar high-purity germanium detectors with thin amorphous germanium contacts were successfully operated directly in liquid nitrogen and liquid argon in a cryostat at the Max-Planck-Institut fuer Physics in Munich. The detectors were fabricated at the Lawrence Berkeley National Laboratory and the University of South Dakota, using crystals grown at the University of South Dakota. They survived long-distance transportation and multiple thermal cycles in both cryogenic liquids and showed reasonable leakage currents and spectroscopic performance. Also discussed are the pros and cons of using thin amorphous semiconductor materials as an alternative contact technology in large-scale germanium experiments searching for physics beyond the Standard Model.
The continuous emanation of radon due to trace amounts of uranium and thorium in detector materials introduces radon to the active detection volume of low-background rare event search detectors. $^{222}$Rn produces a particularly problematic background in the physics region of interest by the ``naked beta decay of its $^{214}$Pb daughter nucleus. While charcoal-based adsorption traps are expected to be effective for radon reduction in auxiliary circulation loops that service the warm components of current {ton-scale} detectors at slow flow rates $(0.5-2;SLPM)$, radon reduction in the entire circulation loop at high flow rates $mathcal{O}({100s;SLPM})$ is necessary to reach high sensitivity in future generation experiments. In this article we explore radon dynamics with a charcoal-based radon reduction system in the main circulation loop of time projection chamber detectors. We find that even for perfect radon traps, circulation speeds of $2,000;SLPM$ are needed to reduce radon concentration in a 10,ton detector by 90%. This is faster by a factor of four than the highest circulation speeds currently achieved in dark matter detectors. We further find that the effectiveness of vacuum swing adsorption systems, which have been employed very successfully at reducing atmospheric radon levels in clean-rooms, is limited by the intrinsic radon activity of the charcoal adsorbent in ultra-low radon environments. Adsorbents with significantly lower intrinsic radon activity than in currently available activated charcoals would be necessary to build effective vacuum swing adsorption systems operated at room temperature for rare event search experiments. If such VSA systems are cooled to about $190,K$, this requirement relaxes drastically.
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