Session JD: Mini-Symposium on Experimental Challenges for Double Beta Decay Experiments III

Development of a new $^{48}$Ca enrichment method and the CANDLES experiment

CANDLES is a project to study double beta decay of $^{48}$Ca. CANDLES could become the most competitive experiment if we could have an efficient method to enrich $^{48}$Ca. We developed a new method for enrichment of large amount of calcium isotopes. The method is called Multi-Channel Counter-Current Electrophoresis (MCCCE) which can be found elsewhre.\footnote{T. Kishimoto et al., PTEP (2015) 033D03; doi:10.1093/ptep/ptv020} Essential point is the increase of the power density in the migration path. In MCCCE, ions migrate in multi-channels on a boron nitride (BN) plate by which substantial increase of the power density was achieved. We made a tiny prototype instrument with a 10 mm thick BN plate and obtained 3 for an enrichment factor for the ratio of abundance of $^{48}$Ca to $^{43}$Ca over that of natural abundance. It corresponds to 6 for the enrichment factor of $^{48}$Ca to $^{40}$Ca. Recently we obtained 10 for the enrichment factor by using 20 mm BN plate. This remarkably large enrichment factor demonstrates that the MCCCE is a realistic and promising method for the enrichment of large amount of ions. This method can be applied to many other elements and compounds. I will describe MCCCE and its effect on the study of double beta decay and other fields.   

A search for double beta decays of $^{136}$Xe to the excited state of $^{136}$Ba with EXO-200

EXO-200 is one of the most sensitive searches for neutrinoless double beta decay of $^{136}$Xe in the world. The experiment uses 110 kg of active enriched liquid xenon in an ultralow background time projection chamber installed at the Waste Isolation Pilot Plant, a salt mine with a 1600 m water equivalent overburden. This detector has demonstrated excellent energy resolution and background rejection capabilities. While the experiment is designed to search for the double beta decays of $^{136}$Xe to the ground state of $^{136}$Ba, transitions to the excited states of $^{136}$Ba are also plausible. The $\beta\beta$2$\nu$ decay to the first 0$^+$ excited state of the daughter nuclei has been observed for $^{100}$Mo and $^{150}$Nd; this particular transition for $^{136}$Xe has a theoretical lifetime on the order of 10$^{25}$ year, which is right around the sensitivity of EXO-200. We present the results from the search of double beta decays to the excited state using two years of EXO-200 data.   

The $^{76}$Ge(n,p)$^{76}$Ga reaction and its relevance to searches for the neutrino-less double-beta decay of $^{76}$Ge

The $^{76}$Ge(n,p)$^{76}$Ga reaction and the subsequent $\beta $ decay of $^{76}$Ga to $^{76}$Ge has been used to excite the 3951.9 keV state of $^{76}$Ge, which decays by emission of a 2040.7 keV $\gamma $ ray. Using HPGe detectors, the associated pulse-height signal may be undistinguishable from the potential signal produced in neutrino-less double-beta decay of $^{76}$Ge with its Q-value of 2039.0 keV. In the neutron energy range between 10 and 20 MeV the production cross section of the 2040.7 keV $\gamma $ ray is approximately 0.1 mb. In the same experiment $\gamma $ rays of energy 2037.9 keV resulting from the $^{76}$Ge(n,$\gamma $)$^{77}$Ge reaction were clearly observed. Adding the $^{76}$Ge(n,n${'}$$\gamma $)$^{76}$Ge reaction, which also produces the 2040.7 keV $\gamma $ ray with a cross section value of the order of 0.1 mb clearly shows that great care has to be taken to eliminate neutron-induced backgrounds in searches for neutrino-less double-beta decay of $^{76}$Ge.   

Neutron induced radio-isotopes and background for Ge double beta decay experiments

Environmental neutrons, mostly produced by muons in the cosmic rays, might contribute backgrounds to the search for neutrinoless double beta decays. These neutrons can interact with materials and generate radio-isotopes, which can decay and produce radioactive backgrounds. Some of these neutron-induced isotopes have a signature of a time-delayed coincidence, allowing us to study these infrequent events. For example, such isotopes can decay by beta decay to metastable states and then decay by gamma decay to the ground state. Considering the time-delayed coincidence of these two processes, we can determine candidates for these neutron-induced isotopes in the data and estimate the flux of neutrons in the deep underground environment. In this report, we will list possible neutron-induced isotopes and the methodology to detect them, especially those that can affect the search for neutrinoless double beta decays in $^{76}$Ge. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, the Particle Astrophysics Program of the National Science Foundation, and the Sanford Underground Research Facility. We acknowledge the support of the U.S. Department of Energy through the LANL/LDRD Program.   

Simulation studies of muon-produced background events deep underground and consequences for double beta decay experiments

Cosmic radiation creates a significant background for low count rate experiments. The {\sc Majorana demonstrator} experiment is located at the Sanford Underground Research Facility at a depth of 4850ft below the surface but it can still be penetrated by cosmic muons with initial energies above the TeV range. The interaction of muons with the rock, the shielding material in the lab and the detector itself can produce showers of secondary particles, like fast neutrons, which are able to travel through shielding material and can produce high-energy $\gamma$-rays via capture or inelastic scattering. The energy deposition of these $\gamma$ rays in the detector can overlap with energy region of interest for the neutrino-less double beta decay. Recent studies for cosmic muons penetrating the {\sc Majorana demonstrator} are made with the {\sc Geant4} code. The results of these simulations will be presented in this talk and an overview of the interaction of the shower particles with the detector, shielding and veto system will be given. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, the Particle Astrophysics Program of the National Science Foundation, and the Sanford Underground Research Facility.   

Online Data Monitoring for the CUORE Neutrinoless Double-beta Decay Experiment

The Cryogenic Underground Observatory for Rare Events (CUORE) is an upcoming bolometric experiment that will search for neutrinoless double-beta decay at Gran Sasso, Italy. Crystals of tellurium dioxide are instrumented with neutron transmutation doped (NTD) thermistors to observe the heat pulse caused by a double beta decay event. Currently under construction, CUORE will contain 988 independent bolometers. The CUORE-0 detector, consisting of the first 52 bolometers, took data from 2013-2015. After briefly reviewing results from a neutrinoless double-beta decay search with CUORE-0, I will outline recent work to improve data analysis and online data quality monitoring for the upcoming CUORE detector.   

Performance of the Majorana Demonstrator Muon Veto System

The \textsc{Majorana Demonstrator} is a neutrinoless double beta decay experiment operating at the 4850-ft. level of the Sanford Underground Research Facility in Lead, SD. The low-background goals of this Ge-based experiment require a muon veto system. The operation of the partial veto panel array (2/3 coverage) provides the first opportunity to study muon events during the commissioning of the Ge detectors. The Prototype Ge detector module operated in the \textsc{Demonstrator} shield for a total exposure of over 600 kg*day with the partial veto system. The operation of Module 1, consisting of 22.5 kg of Ge mass, in the shield with full veto panel coverage will provide a complete array to study muon-induced events in the experiment. The veto panels are synchronized with Ge detectors using a common 100MHz clock, presenting a unique opportunity to 1) study the flux and angular distribution of muons incident on the \textsc{Demonstrator} using the experiment's modular veto panel design, and 2) examine the effect of muon-related events on the Ge detectors. In this talk the performance of the muon veto system, including an analysis of the coincidence patterns of the incident muons and the corresponding spectra produced in the Ge detectors, is presented.