Session GD: Mini-Symposium: The Physics of Double Beta Decay - Detector Development I

Observation of De-Excitation Gamma-Rays from Excited States of 136Cs.

Excited states in the isotope 136Cs are of interest both as intermediate states in the double beta decay of 136Xe and as final-state products in neutrino charged-current interactions in next-generation liquid-xenon-based particle physics experiments. In the latter case, the presence of long-lived isomeric states may enable background-free detection of solar neutrinos with sensitivity down to ~700 keV, which would enable new measurements of CNO neutrinos in upcoming experiments such as nEXO or DARWIN. Measurements of the 136Cs excited states are sparse, though recent QRPA and shell model theoretical calculations have been made. In this talk, we will describe an experimental program to characterize the nuclear levels of 136Cs and search for long-lived isomeric states.  We will show new results from experiments performed at the Triangle Universities Nuclear Laboratory using (p,n) reactions on a 136Xe target, including the first observation of 31 de-excitation gamma-rays in 136Cs.  Ongoing gamma-gamma coincidence measurements to elucidate the decay schemes of particular excited states will also be discussed.

    

The Design of the Photon Readout Electronics for nEXO

nEXO is next-generation neutrino-less double beta decay search (0νββ) experiment based on a 5 ton liquid-Xe TPC enriched with Xe-136, and is designed to achieve a sensitivity to 0νββ of T_½ ~ 10²⁸ yr.  The total energy resolution goal of nEXO is σ_E/E = 1% at the Q_ββ, and consists of contributions from the charge and photon signals.  The nEXO photon readout electronics system (PRE) has several requirements in order to achieve sensitivity goal: severe constraints on radiopurity and outgassing, excellent single photon resolution (despite SiPM capacitances as high as 8 nF/cm²), and enough segmentation to identify Bi-214 decays.  These challenges have led to a design based on cryogenic ASICs inside the TPC to maximize signal readout performance and vastly improve radiopurity.  In this talk we will describe the nEXO PRE design in detail, and highlight the design decisions that were made to produce a solution to the numerous challenges in the photon readout for nEXO.   

VUV SiPMs for nEXO

The nEXO detector features a 5 tonne time projection chamber filled with liquid xenon enriched to 90% in the A=136 isotope.  nEXO aims to detect neutrinoless double beta (0??ββ) decay) of ¹³⁶Xe with a 90% CL sensitivity = 1.35 x 10²⁸ years using vacuum ultraviolet (VUV) silicon photomultipliers (SiPMs) as scintillation sensors. This talk will focus on testing of the SiPM devices that are under consideration for use in nEXO. We present the characterization methods and results, highlighting the photon detection efficiency (PDE), single photoelectron (SPE) gain, temperature-dependent breakdown voltage, dark count rate, and correlated avalanche contribution.   

Light simulation for nEXO

nEXO is a tonne scale liquid xenon time projection chamber (TPC) that aims to detect neutrinoless double beta decay in ¹³⁶Xe with a projected 90 % CL sensitivity of 1.35 x 10²⁸ yr. nEXO will be able to measure energy deposits from both ionization electrons and scintillation photons from events that occur inside the detector. Silicon photomultiplers will surround the sides of the TPC to detect scintillation light, providing a more highly pixellated light collection system than previous liquid Xe TPCs. With this design, the detector will have high light collection efficiency and energy resolution of σ/Q_ββ < 1% with low amounts of plastic as well as the ability to image the location of interactions based on their light signals. This talk presents the simulation of the light response in the nEXO detector and its impact on the experiment's sensitivity along with possible future background reduction through the use of light based position reconstruction in the experiment.   

A First Principles Solution to Developing an Adaptable W-Value Parameter in NEST

The Noble Element Simulation Technique (NEST) is a C++ package with optional GEANT4 integration and a Python equivalent (nestpy) with a strong track record of accurately simulating the scintillation, ionization, and electroluminescence processes in xenon and argon - elements key to understanding rare event searches such as those for neutrinoless double beta decay and dark matter. Using a combination of empirical and first principle methods, NEST models the intrinsic physics of noble detectors while maintaining a format that is accessible and customizable for users. On behalf of the NEST collaboration, I will present key outputs including energy resolution and light and charge yields of various interactions with noble elements. I will also discuss my personal contributions to the code addressing the discrepancy between NEST and recent publications on the W-value. This work utilizes a modified Thomas Imel Box Model as a first principles solution to making NEST readily adaptable to future experiments.   

Development of Germanium (Ge) Ring Contact Detectors for Ge-based Neutrinoless Double-Beta Decay Experiment

The next generation neutrinoless double beta (0νββ) decay experiments aim to achieve the decay half-life of ~10²⁸ years. Germanium (Ge)-based experiment has great chance to achieve the discovery potential for this rare decay process due to its excellent energy resolution to reject 2 decay events.  LEGEND-1000 prefers large-size detectors (>3 kg per detector), which will further reduce backgrounds, complexity, and cost. This talk will explore large-size detectors through conducting research and development (R&D) of Ge ring-contact (GeRC) detectors using high-purity Ge crystals grown at USD. The detector is designed between ORNL, UNC, and USD. We fabricate GeRC detectors at TAMU and test them at UNC. The GeRC detector technology can potentially increase the mass per Ge detector for LEGEND-1000. Due to the physical properties of Ge crystal and the geometric structure of the detector, the fabrication process is complex. At TAMU, we have overcome many difficulties and gained a lot of experience in making GeRC detectors. This talk will report some initial results about the GeRC detector fabricated at TAMU using the USD-grown crystals.

    

Characterization of 76Ge detectors for LEGEND-200

The Large Enriched Germanium Experiment for Neutrinoless double-beta Decay (LEGEND) is an experimental search for neutrinoless double beta decay (0νββ) in ⁷⁶Ge 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 commissioning data-taking having started in 2022. LEGEND-200 will reuse germanium detectors from GERDA and the MAJORANA DEMONSTRATOR with 2/3 of the active mass as state-of-the-art inverted coaxial point contact detectors. Extensive characterization of these detectors is necessary to quantify the detector response and ensure each detector reaches the performance standards necessary for LEGEND. Characterization tests include the use of a ²²⁸Th source to measure the detector’s energy resolution and pulse-shape discrimination capabilities. An overview of the LEGEND characterization program and its results will be presented.