Session C20: Neutrinos I – Neutrinoless Double-Beta Decay: Background Challenges

Live

The new era of precision neutrino physics has given us access to several previously-unmeasured neutrino properties and allowed dramatically improved limits on others, including properties predicted by physics Beyond the Standard Model. Limits on as-yet-unmeasured Standard Model properties of neutrinos, like their masses, mass hierarchy, and CP-violating oscillation phase, have seen significant improvements in the last few years; next-generation experiments are expected to have the sensitivity needed to make conclusive measurements of some of these properties. Current and planned measurements of coherent elastic neutrino-nuclear scattering will allow us to measure or set limits on the neutrino magnetic moment and many varieties of non-standard interactions, and higher-sensitivity neutrinoless double-beta decay searches are probing the potential Majorana nature of the neutrino. Persistent tensions between short baseline measurements and the three-flavor oscillation model point to the exciting possibility of additional generations of neutrinos -- or simply unresolved systematics in our measurements. Future precision measurements of neutrino and nuclear physics could hold the answer to this and other important questions.   

Live

The nEXO Collaboration is developing a ton-scale neutrinoless double beta decay experiment employing an enriched Xe-136 target. The enriched liquid xenon is operated as a time projection chamber (TPC) providing event timing and position reconstruction. The goal is to search for excess events at the 2458 keV endpoint of the Xe-136 double beta-decay energy spectrum. An event excess at this energy would imply the existence of a decay branch that does not emit the otherwise required two antineutrinos that should accompany the beta particles. Current measurements set a limit on the neutrinoless double beta decay of Xe-136 at a half-life of greater than $10^{26}$ years. To investigate the possibility of neutrinoless double beta decay with a longer half-life of order $10^{28}$ years, a large 5-ton volume of LXe will be operated in a time projection chamber format. This presentation will describe the single-phase, LXe time projection chamber design concept, emphasizing the choices made to minimize sources of radioactive backgrounds while enhancing the ability to discriminate single-site, signal-like events from multiple-site, background-like events. Some of the technical challenges of designing such a large time project chamber will also be presented.   

Live

The nEXO experiment will use 5 tons of liquid Xe enriched to 90\% in the double beta emitting isotope 136 in a Time Projection Chamber (TPC) to search for neutrinoless double-beta ($0\nu\beta\beta$) decay. To achieve the chemical purity necessary in the TPC, the xenon will be continuously passed through a high temperature getter in the gas phase at a rate of 350 SLPM. We describe a simulation of the nEXO Xe plant created using the AspenTech software suite for steady state and dynamic processes, including changes in the flow rate and other transient events. The model was validated by simulating the Xe plant for the predecessor to nEXO, EXO-200, where the model is compared with experimental data. The Xe plant simulations for nEXO allow modeling of alternatives for specific components such that the stability and response to off-normal upsets can be optimized. As an example of this, two different evaporation/liquefaction schemes will be presented, with and without a counter flow heat exchanger, demonstrating the capability of using this model to design the full Xe plant for nEXO.   

Live

nEXO is a planned \textasciitilde 5000 kg neutrinoless double beta decay experiment. It will utilize a single-phase liquid xenon time projection chamber (TPC) with isotopically enriched $^{\mathrm{136}}$Xe. In order to achieve the half-life sensitivity target of 10$^{\mathrm{28}}$ years, extremely low backgrounds are required. The ($\alpha $, n) reaction on light-Z detector materials is identified as a source of internal background in nEXO. Material surface exposure to ambient air during the fabrication and transportation phases can accumulate $^{\mathrm{210}}$Pb and $^{\mathrm{210}}$Po, which in turn will lead to neutron-induced background. This results in stringent requirements on the allowable air exposure. This talk will present an ongoing study of the attachment of radon daughters to Teflon and copper surfaces. Attachment rates of $^{\mathrm{210}}$Pb obtained by low-background alpha-spectrometry of $^{\mathrm{218}}$Po and $^{\mathrm{214}}$Po will be presented.   

Live

The next-generation Enriched Xenon Observatory (nEXO) is a planned experiment utilizing 5 tonnes of isotopically-enriched liquid xenon in a time projection chamber (TPC) to search for neutrinoless double beta decay of $^{136}$Xe. The large, monolithic design of the nEXO TPC provides excellent shielding from the dominant background source - $\gamma$ rays that originate from external materials. With an exceptionally clean central region of the TPC, we need to consider and quantify backgrounds that have previously been considered to be small relative to backgrounds from $\gamma$ rays. I will present recent studies of two of these unconventional background sources - various low-energy neutron sources and $^{42}$Ar in enriched liquid xenon.   

On Demand

The nEXO experiment is a next-generation neutrinoless double-beta decay search with the isotope $^{\mathrm{136}}$Xe and a half-life sensitivity goal of 10$^{\mathrm{28}}$ years. The nEXO experiment plans to take full advantage of the self-shielding effects of the liquid xenon and exploit as large a fiducial mass as possible; therefore minimizing external contributions to the background radiation entering the nEXO time projection chamber (TPC) is required. In order to accomplish this task, an outer detector, in which the nEXO TPC and cryostat are fully submerged, consisting of a tank filled with ultra-pure deionized water and instrumented with 8-inch PMTs will provide the outer shielding and act as a muon veto for nEXO. The initial design along with the passive and active shielding capability for external backgrounds will be presented.   

On Demand

The search for neutrinoless double beta ($0\nu\beta\beta$) decay is the most sensitive technique to establish the Majorana nature of neutrinos. An extremely low background radiation environment in the $0\nu\beta\beta$ decay energy range is required in order to detect this hypothetical decay mode. The Large Enriched Germanium Experiment for Neutrinoless $\beta\beta$ Decay (LEGEND) collaboration is considering various shielding and active veto designs to sufficiently reduce external gamma and neutron backgrounds in order to achieve a discovery potential for a $10^{28}$ year $0\nu\beta\beta$ half life. This work presents these options with accompanying simulations using both MCNP6 and GEANT4 packages. Comparisons with literature values are also presented to validate the simulations. Impact on the $0\nu\beta\beta$ discovery potential is extrapolated.