Session J15: Mini-Symposium: Neutrino Detector Development II

Radioactive Source Calibration for nEXO

The nEXO experiment aims to observe neutrinoless double beta decay of ¹³⁶Xe with an expected sensitivity that exceeds 10²⁸ years. If neutrinoless double beta decay occurs, it would suggest that the neutrino is a Majorana particle and that lepton number is not conserved. To observe neutrinoless double beta decay, the nEXO experiment will use a time projection chamber filled with 5 tonnes of isotopically enriched liquid xenon. The signature of neutrinoless double beta decay is a peak at the endpoint of the two-neutrino double beta decay spectrum, so the energy response of the detector must be well understood. To achieve its sensitivity goal, the experiment will need to be carefully calibrated by multiple complementary methods, including externally deployed gamma-ray sources (point-like sources) and sources dissolved in the liquid xenon (volumetric sources). This presentation will describe the radioactive source calibration methods planned for nEXO.   

Radon Mitigation by Distillation in nEXO

nEXO is an experiment aiming to observe neutrinoless double beta (0vBB) decay with 5 tonnes of enriched liquid xenon contained within a time projection chamber (TPC). If observed, 0vBB decay would imply the existence of Majorana neutrinos that are their own antiparticle. The nEXO detector collects ionization electrons and scintillation photons produced by particle interactions in xenon to reconstruct deposited energy, location, and topology. Half of the background in nEXO is expected to come from Radon-222 which emanates from parts in the system and uniformly mixes with the xenon. Radon-222 decay daughters can plate out onto surfaces in the detector and produce radioactivity with a similar energy signature to 0vBB decay. The Radon-222 background requirements for nEXO are stringent and all materials in contact with the xenon will be screened for radon emanation. In addition, a prototype cryogenic distillation column is being developed to define design parameters of such a radon-suppression device that could be added to the current nEXO design. We present the design and construction of this distillation column.   

Characterization of CRYO ASIC for the nEXO Experiment

nEXO is a next-generation liquid xenon experiment to search for the neutrino-less double beta decay of $^{136}$Xe, with a lifetime sensitivity goal of greater than 10$^{28}$ years. The experiment will use an array of charge tiles composed of crossed metal strips to record ionization electrons. An in-xenon cryogenic application-specific integrated circuit (ASIC) named CRYO ASIC has been designed by SLAC for amplification, digitization and multiplexing of the charge signals. The CRYO ASIC, mounted on an auxiliary board, is characterized in liquid xenon to mimic the nEXO experiment. This work presents the results of CRYO ASIC tests in a liquid xenon environment.   

Characterizing CRYO ASIC and Charge Readout in nEXO

Neutrinoless double beta decay (0vbb) is a lepton-number-violating nuclear transition forbidden in the Standard Model. The observation of 0vbb would fundamentally imply that neutrinos are their own anti-particles, indicating their Majorana nature and might explain the asymmetry of matter and anti-matter in the universe. nEXO is a proposed next-generation 0vbb experiment with 5000 kg of isotopically enriched liquid xenon (LXe) in a time projection chamber (TPC). Ionization charge produced by ionizing particles drifts along the electric field towards the anode where it is read out by arrays of crossed strips, deposited on 10 cm*10 cmdielectric tiles. The signal pre-processing, digitization and read-out steps are carried out at cryogenic temperatures by a follow-up novel System-on-Chip CRYO ASIC deployed on the back side of the tiles. nEXO requires a high precision of reconstruction of the electron pair’s energy to effectively discriminate signals from backgrounds. In this study, we focus on tests at SLAC and Stanford to characterize the CRYO ASIC, along and combined with charge readout tiles, and understand noise and signal reconstruction properties necessary to elucidate the charge reconstruction and physical potentials of nEXO.   

Photo-induced Charge Calibration R&D for nEXO

The nEXO experiment is designed to search for the elusive neutrinoless double beta decay of ¹³⁶Xe with a half-life sensitivity goal of >10²⁸ years using a 5-tonne liquid xenon (LXe) 1.3 m diameter cylindrical TPC. The calibration of the  ionization and scintillation light response of the detector involves the regular deployment of external radioactive sources, challenging because of the excellent self-shielding properties of LXe, and the occasional injection of ²²⁰Rn and ¹²⁷Xe. Several other risk-mitigating R&D efforts are ongoing to incorporate additional calibration techniques to regularly monitor the drift electrons and the light response of silicon photomultipliers in liquid xenon. This presentation presents the status, including preliminary results, of an effort at the University of Massachusetts Amherst to  develop gold photocathodes to generate photoelectrons in LXe using a small, dual grid ionization chamber. The use of multiple such photocathodes could allow to monitor the ionization electron lifetime almost continuously in nEXO.   

Monte Carlo Modeling of eV-Scale Backgrounds in Superconducting Tunnel Junctions from Gamma-Ray Interactions

The BeEST experiment searches for physics beyond the standard model (BSM) in the neutrino sector by utilizing the electron capture (EC) decay of 7Be. The 7Be is embedded in superconducting tunnel junction (STJ) sensors such that the low-energy (eV-scale) radiation is detected with high efficiency. In 10% of the decays, a 478 keV gamma-ray is emitted in the dexcitation of the 7Li daughter nucleus. While the gamma ray does not interact in the STJ (~0.5um), it scatters in the silicon substrate that the STJ is deposited on. This then results in a low-energy background seen on the energy spectra read from the quantum sensor. In this talk I will discuss our modeling of this background using a python based Monte Carlo of light-matter interactions and subsequent phonon propagation.   

CryoCsI crystal shape optimization based on optical simulations

COHERENT collaboration is the first that observed Coherent elastic neutrino-nucleus scatter (CEvNS) events in 2017. A 14.6 kg doped CsI at room temperature was placed 20 meters away from the 1.4 MW Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL). The SNS pulsed proton beam provides a uniquely high background rejection and high-intensity neutrinos. CryoCsI is a cryogenic undoped CsI scintillating detector, which has a much lower energy threshold potentially down to 0.5 keVnr. Working with other COHERENT detectors, CryoCsI can produce world-leading sensitivities on broad physics topics in and beyond the Standard Model. In this talk, I'll present optical simulation studies on optimizing the shape of a ~10 kg CryoCsI crystal.   

Characterization of Ge detectors' passivation surface for LEGEND using Kr

The LEGEND experiment is searching for neutrinoless double beta decay in ⁷⁶Ge using arrays of high purity enriched Ge crystals. Each element of these arrays is a Ge diode weighing ~1 kg, with a nominal 1 μm thick passivation surface made up of non-crystalline Ge situated around the contact point of the detectors. However, the low energy spectrum from Majorana Demonstrator has revealed that the effective thickness of these layers is larger than expected, which may cause bleeding of high energy α particles into the Q_ββ region of interest of the experiment. A test-stand is set up at University of Washington, in joint collaboration with Indiana University, to investigate the effect of the effective thickness of the passivation surface using Kr as a low-energy mono-energetic electron source. Recent results from this have been able to reproduce the similar energy degradation in that passivation surface like Majorana. In this talk, I will present those results and also discuss a surface charge model which is showing promises at modifying the effective dead layer to explain this data.   

Crosstalk Characterization for the LEGEND-200 Experiment

The discovery of the lepton-number-violating neutrinoless double-beta decay (0νββ) process would determine if the neutrino mass is Majorana or Dirac in nature, give insight to the origin of neutrino mass, and provide an explanation for possible leptogenesis processes in the early universe.  Searches for 0νββ using ⁷⁶Ge-enriched High-Purity Germanium (HPGe) detectors have proven to be very successful, with the previous-generation experiments the MAJORANA DEMONSTRATOR (MJD) and GERDA demonstrating the best energy resolution and lowest backgrounds in the field, respectively. This program is rapidly advancing, with the present-generation LEGEND-200 experiment now successfully taking data.

 

Ultra-precise experiments such as these require the ability to reconstruct the energy deposition and topology of the particle interactions in the ⁷⁶Ge detectors to an extreme degree in order to recover a potential 0νββ decay, and any effects which could affect the energy collected by a detector must be understood and corrected. An example of one such effect is the presence of crosstalk in complex detector electronic circuit systems, whereby a transient energy signal present in one subsystem (e.g. a pulse generated by a particle interactionin an individual germanium detector channel) can bleed over to another subsystem, resulting in an incorrect energy being recorded for the event. The LEGEND-200 experiment is one such complex electronics system, consisting of an array of over 100 individual ⁷⁶Ge semiconductor detectors, a liquid argon (Lar) instrumentation system with dozens of silicon photomultiplier (SiPM) detectors, and a muon veto system with dozens of photomultiplier (PMT) detectors. This talk will present the techniques and algorithms being designed and implemented in the LEGEND-200 analysis framework to quantify and correct for these effects in the LEGEND-200 experiment.