Session MB: Mini-Symposium: Neutrinos and Nuclei X: Double Beta Decay Detectors

Assembly and test of a prototype nEXO charge-readout module with built-in, cryogenic ASIC readout

The nEXO experiment aims to discover neutrinoless double beta decay of xenon 136, with a lifetime sensitivity goal of greater than 10^28 years. Compared to using long cables to transmit signals outside of the detector, mounting amplification and digitization circuitry directly on detector submodules reduces noise and improves measurement fidelity. A cryogenic application specific integrated circuit (ASIC) called CRYO ASIC has been designed by SLAC and fabricated for direct attachment to the nEXO charge readout modules. In this talk, the electrical characteristics of the nEXO charge readout will be discussed along with ASIC performance considerations. A prototype nEXO charge-readout module with attached ASIC has been assembled and operated in a liquid xenon time projection chamber; this module’s performance using a full chain of ASIC-controlling circuit boards will be presented.     

Calibration of nEXO light response.

nEXO is a proposed 5 tonne liquid xenon experiment which seeks to detect \gls{onbb} in Xe-136 using \gls{tpc} technology. The experiment will use the combina tion of scintillation and ionization signals to reconstruct events with an energy resolution of $<$1\% $\sigma/E$ at the \gls{onbb} Q-value. The scintillati on light will be collected by silicon photomultipliers (SiPMs) around the sides of the detector, and their collection efficiency will vary as a function of ev ent position. Using nEXO Monte Carlo simulation data, we have demonstrated the use of machine learning techniques to effectively map and correct the light re sponse of the detector while minimizing calibration time. In this talk, we present a strategy for calibrating the light response in the nEXO detector using alpha decays from ${}^{222}$Rn calibration source dissolved into the liquid xenon.   

The Research, Development, and Production of Neutron Transmutation Doped Germanium Thermistors for the CUPID Experiment.

CUPID—the CUORE Upgrade with Particle Identification—is an upcoming experiment at Laboratori Nazionali del Gran Sasso, searching for the neutrinoless double-beta (0νββ) decay of ¹⁰⁰Mo with a large array of scintillating Li₂MoO₄ bolometers enriched in the isotope of interest. Operating at ~10 mK temperatures, these detectors measure absorbed particles' energies with high resolution via their thermal signatures. Secondary bolometers detecting scintillation light from the Li₂MoO₄ crystals provide active background discrimination through particle identification. Each of the primary and secondary bolometers (~4000 readout channels) is instrumented with a neutron transmutation doped (NTD) Ge thermistor—a temperature sensor, whose electrical resistance depends on temperature as R(T) = R₀e ^{√(T₀ /T)} at sub-Kelvin temperatures, with target R₀ ≈ 1–2 Ω and T₀ ≈ 4K giving a sensitivity of ~0.3–1 MΩ/μK. These parameters require high (~10¹⁷ cm^–3 net p-type), yet very uniform, doping of the high purity Ge. This is achieved by the unique neutron transmutation doping process, with wafers irradiated in a nuclear reactor at nominal doses of ~10¹⁸ n/cm³. In this talk, we present the research and development of novel NTD geometries and readout schemes, as well as the CUPID NTD mass production program.   

Low-temperature light detectors for the CUPID experiment

CUPID is a proposed next-generation experiment that will search for neutrinoless double-β (0νββ) decay in ¹⁰⁰Mo using ~1600 Li₂¹⁰⁰MoO₄ scintillating crystals operated as low-temperature calorimeters close to ∼10mK. It will leverage the crystal’s energy loss mechanism to tag particle type by simultaneously measuring both the thermal and scintillation signals. We will use an auxiliary low-temperature calorimeter to detect light with high photon collection efficiency. The light detectors must have a very low energy threshold ??(100eV) and good timing resolution < 1 ms to tag α background and 2νββ pile-up events in the region of interest. The light detectors are crucial to reach the CUPID background goal of <10^-4 counts/(keV.kg.yr) for its baseline design. In this talk, I will briefly discuss the baseline design of the CUPID light detectors and the R&D status of a future upgrade with transition-edge sensor (TES) based light detectors which may further help suppress the pile-up background.   

Acceptance Characterization and Performance of 76Ge Detectors for LEGEND-200

The Large Enriched Germanium Experiment for Neutrinoless double-beta Decay (LEGEND) is pursuing a phased approach to develop a ton-scale experiment searching for neutrinoless double-beta decay (0nbb) in ⁷⁶Ge. The experiment builds on the successes of two previous 0nbb experiments, the MAJORANA DEMONSTRATOR and the GERDA experiment. The first phase, LEGEND-200, will deploy ~200-kg of ⁷⁶Ge detectors enriched to >86%. 60-kg will be reused from GERDA and MAJORANA detectors. 140-kg of new detectors will be fabricated in the novel Inverted Coaxial Point Contact (ICPC) geometry developed to increase the mass of the detectors without sacrificing their excellent pulse shape discrimination (PSD) capabilities or energy resolution. LEGEND plans to begin commissioning and data-taking in late 2021. Before the detectors can be deployed, they need to be fully characterized. These tests include using a  ²²⁸Th source to measure each detector’s efficiency, energy resolution, and timing response as well as additional radial and longitudinal scans with ²⁴¹Am and ¹³³Ba sources to determine the dead-layer. A description of the characterization program and the performance of the ICPC detectors will be presented.   

Internal Scanner for Rapid Characterization of 76Ge detectors used in LEGEND-200

The LEGEND collaboration is developing an experimental search for neutrinoless double beta decay (0νββ) in the ⁷⁶Ge isotope with a discovery potential of a half-life beyond 10²⁸ years. The first phase, LEGEND-200, is an experimental search using 200 kg of ⁷⁶Ge-enriched germanium, with data taking beginning in 2021. The search for 0νββ  requires a precise understanding of the behaviour of germanium detectors, necessitating extensive detector characterization. As characterization for LEGEND-200 is underway, there is an effort to develop a scanner for the characterization of the α, β, and γ response on and near the passivated surface of inverted coaxial point-contact detectors (ICPC) to be deployed in the experiment. Scanning a selected sample of points in a low background environment allows for rapid characterization. An ²⁴¹Am source provides 5.45 MeV α's for measuring the α response on the passivated surface while a ¹³⁷Cs source provides 625 keV internal conversion electrons for studying the β response. The development and initial results of the internal scanner will be presented.   

Development of advanced germanium detectors using the crystal grown at the University of South Dakota

Germanium (Ge) detectors are used extensively in searching for rare-event physics such as dark matter and neutrinoless double beta decay. At the University of South Dakota (USD), we purify Ge ingots through the process of zone-refining and then grow large-size single crystals using the Czochralski technique. Detector-grade crystals are then fabricated into different detector-contact geometries. We have successfully made about 30 detectors (planar, guard-ring, point contact). This paper will summarize the properties of these detectors. The information obtained from the detector characterization provides valuable feedback to the crystal growth and detector fabrication. The fabrication of detectors with different geometries addresses the requirements of rare-event physics experiments.   

Characterization of Ge detectors using pulse shapes generated by alphas

A method to accurately determine the charge carrier mobility in a HPGe detector is demonstrated experimentally, where a combination of a small thin-contact planar HPGe detector and an alpha source is utilized. The thin-contact eliminates the influence of surface dead layer. The planar configuration insures a simple electric field distribution. The small size guarantees a nearly constant impurity concentration. And alpha particles deposit energy right on the surface of the detector, which reduces the uncertainty of the starting point of the drift of charge carriers to the minimum. Electric pulses are simulated with input physics parameters and then fit to measured pulses to precisely determine the parameter values. The mobility of a home-made HPGe detector from a home-grown crystal at USD has been determined as 3.4x10⁴ cm²/Vs, and other important detector characteristics such as net impurity concentration have been measured using the same method and compared to other measurement techniques. This method can be used to determine some critical properties of crystals used to make HPGe detectors for scientific research, such as neutrinoless double beta decay search, etc., as long as a thin slice of wafer from the same crystal is made into a thin-contact planar detector.