Session D15: Mini-Symposium: Lepton Number Violation II

Active Noise Cancellation in CUORE Data

The Cryogenic Observatory for Rare Events (CUORE) experiment searches for the neutrinoless double beta decay using tellurium dioxide crystals as a source and detection method. Due to the highly sensitive nature of the detectors, they will experience acoustic noise that will worsen the resolution of the signal. Auxiliary devices such as accelerometers and microphones are currently being used alongside the detectors to measure acoustic noise and actively subtract it out of the data. This is done by analyzing the frequency-dependent coherence between the auxiliary devices and detectors and using this to predict the detector noise. Recent results have shown that an approximation of the stable noise in CUORE can be used to denoise calibration data that have few noise events. Similar methods, combined with machine learning techniques, can be used to increase the resolution of the signals in the detectors.

I will present a brief overview on the denoising algorithm used for CUORE and describe the role of the auxiliary devices for active noise cancellation and how a generic filter has been used to denoise data without the direct use of auxiliary device data.   

Radon progeny attachment to surfaces as a background source in nEXO experiment

nEXO is a next-generation, tonne-scale neutrinoless double beta decay experiment. The experiment utilizes a time projection chamber and 5 tonnes of xenon enriched in the mass-136 isotope. The projected 90% CL half-life sensitivity is 1.35·10²⁸ yr after 10 yr of exposure. Stringent radioactive background control and careful material selection are necessary to achieve such a sensitivity.

Radon daughter plate-out is a background source for nEXO. The exposure of detector materials to air will lead to the accumulation of ²¹⁰Po, whose α-decay can produce neutrons from (α, n) reactions. Some of these neutrons will capture on ¹³⁶Xe to form ¹³⁷Xe, whose β-decay has a Q-value of >4 MeV, above the double beta end point. The nEXO group at the University of Alabama conducted a comprehensive study of radon progeny attachment to surfaces of 9 materials relevant for nEXO. More than 1200 measurements were performed in several environmental conditions with careful monitoring and control of the exposure parameters. The impact of environmental parameters on attachment rates is studied. Attachment modelling, using the Jacobi model, is compared to data. The first measurement of the physical collection length for radon progeny on copper, corrected for chain equilibrium breakage, will be reported.   

Radioassay data management for nEXO

Neutrinoless double beta decay (0νββ) is a hypothetical nuclear process that violates lepton number conservation. Its detection would prove that neutrinos are Majorana particles. The next-generation Enriched Xenon Observatory (nEXO), a successor to the EXO-200 experiment, aims to observe 0νββ using a liquid xenon (LXe) time projection chamber (TPC) and about 5 tonnes of xenon enriched to 90% in ¹³⁶Xe. To reach a half-life sensitivity of 1.35 × 10²⁸ years at 90% C.L. with 10 years of data taking, controlling background is a major challenge. The majority of the background comes from the detector components. To search for the appropriate radiopure materials that also satisfy mechanical, cryogenic, and other constraints, hundreds of materials have been screened for radioactivity. Such a radioassay campaign will generate a large amount of data which will be used in the design of the nEXO detector. There is, thus, a need for a tool to store the data, and to facilitate the use of the data in the design process. The nEXO Materials Database is aimed to fulfil this need. In this presentation, I will discuss the requirements for the Database, its implementation, and its performance.   

Breakdown Voltage of VUV SiPMs as a Function of Temperature

The tonne-scale nEXO experiment will search for 0νββ decay using a liquid xenon time projection chamber enriched to 90% ¹³⁶Xe. The chamber will be instrumented with vacuum ultraviolet (VUV) silicon photomultipliers (SiPMs) to sense the scintillation light. We present our study of the breakdown voltage of an FBK VUVHD3 SiPM as a linear function of temperature, in the context of the nEXO collaboration. We measure the breakdown voltage of the SiPM in different environmental conditions, including vacuum, gaseous xenon, gaseous nitrogen, and liquid xenon. We describe our experimental methodology, analysis of the data, calculation of systematic errors, and the interpretation of the results. In the course of this analysis we also report on the absolute gain of the SiPM and correlated avalanche probability.   

Developments in the MAJORANA DEMONSTRATOR Background Model

The MAJORANA DEMONSTRATOR neutrinoless double-beta decay experiment operated 44 kg of Ge detectors, of which 30 kg were enriched in the candidate isotope ⁷⁶Ge. The DEMONSTRATOR achieved one of the lowest background rates in the region of the 2039 keV Q-value, 15.7 cnts/(FWHM t y). This background rate, however, is significantly higher than the rate of 2.9 cnts/(FWHM t y) projected by material assays and simulations. An excess of ²³²Th decay chain events unevenly distributed between the two modules of the DEMONSTRATOR has been observed. Background model fits aim to understand this deviation from assay-based projections, potentially determine the source location(s) of observed backgrounds, and allow a precision measurement of the two-neutrino double-beta decay half-life. Fits indicate the origin of the ²³²Th excess is not from near-detector components, which informed design decisions for the next-generation LEGEND experiment. Recent findings from background model fits and radioassays have further narrowed the suspected locations for the ²³²Th excess. A study of systematic uncertainties is also underway with the overall goal of completing a background model.  

    

Final Results of the MAJORANA DEMONSTRATOR

The MAJORANA DEMONSTRATOR is a low background experiment searching for neutrinoless double-beta decay (0νββ) in ⁷⁶Ge and other rare events originating from physics beyond the standard model. The DEMONSTRATOR consisted of an array of high purity germanium detectors using three different p-type point-contact geometries (PPC, ICPC and BEGe), housed at the Sanford Underground Research Facility in Lead, SD. Thanks to its ultra-low background materials, world-leading energy resolution, and pulse-shape analysis background rejection techniques, the experiment achieved one of the lowest background indices to date for a 0νββ search. The experiment finished taking data with enriched detectors in March 2021 and, with 64.5 kg-yr of enriched exposure, produced a final half-life limit of T_1/2>8.3×10²⁵ yr. This talk will present the final half-life limit, other recent results, and ongoing analysis efforts for the MAJORANA DEMONSTRATOR.   

Light yield of cryogenic CsI down to 5.9 keV

We measured the light yield of an undoped cryogenic CsI scintillator at four energies including the first measurement at 5.9 keV that is close to the detector threshold. A light yield of 33.4 ± 1.7 photoelectrons per keV electron-equivalent (PE/keV_ee) was achieved with an undoped CsI crystal coupled with a photomultiplier tube (PMT) at 77 Kelvin using the 5.9 keV X-ray from Fe-55 source and a light yield of 40.0 ± 2.0 PE/keV_ee was observed from the 59.5 keV γ-ray from an Am-241 source. Analysis procedures and reconstruction methods will be presented.   

Fabrication and Characterization of High-Purity Germanium Detectors with Thin (Amorphous Germanium) Contacts

Large high-purity germanium (HPGe) detectors are widely used in neutrinoless double-beta decay and dark matter experiments. Currently, large (diameter > 11 cm) HPGe crystals can be grown successfully at the University of South Dakota (USD) and small planar detectors with thin contacts can be fabricated and characterized. Compared with lithium contacts, thin contacts (Amorphous Germanium) have a smaller dead layer. This increases the active volume of the detector. Thanks to NSF, USD has purchased a more advanced sputtering system this year, which can make large-size Ge detectors with thin-contacts. We will make detectors and conduct a R&D to deposit a layer of PEN material on the amorphous germanium contacts through this new sputtering system. This talk will report the latest progress in USD detector fabrication.

    

Searching for Beyond-Standard-Model Physics with LEGEND-1000

LEGEND-1000 is a next-generation experiment to search for neutrinoless double-beta decay of the Ge-76 isotope. This ton-scale experiment uses enriched high-purity Ge detectors surrounded by a large active liquid Ar shield, deployed deep underground. Because of the low noise and low energy thresholds of these detectors, along with the low background design of LEGEND-1000, this experiment provides an excellent opportunity for searches for new physics beyond neutrinoless double-beta decay. These include searches for dark matter candidates, exotic nuclear decays, tests of fundamental symmetries, emissions of additional particles during two-neutrino double-beta decays, and more. This talk will briefly describe the LEGEND-1000 program, continuing with a focus on the strategies and expected sensitivities of the experiment for these searches for physics beyond the standard model.