The Cryogenic Underground Observatory for Rare Events (CUORE) is a particle physics facility located at the Laboratori Nazionali del Gran Sasso in central Italy. CUORE was designed primarily as a search for neutrinoless double beta decay (0νββ) in 130Te. It uses tellurium dioxide (TeO2) crystals as both the source of the decay and as bolometers to detect the resulting electrons. These decays have never been observed; an observation would conclusively show that neutrinos are Majorana fermions; that is, they are their own antiparticles. This is relevant to many topics in particle physics, including lepton number conservation, nuclear structure, and neutrino masses and properties. CUORE searches for the characteristic signal of 0νββ, a small peak in the observed energy spectrum around the known decay energy: Q = 2527.518±0.013 keV. It also can search for axion signals.
The CUORE collaboration involves physicists from several countries, primarily from the United States and Italy. CUORE is funded by the Istituto Nazionale di Fisica Nucleare (INFN) of Italy, the United States Department of Energy (DOE), and National Science Foundation (NSF) of the United States.
In September 2014, as part of the testing of the CUORE dilution refrigerator, scientists in the CUORE collaboration cooled a copper vessel with a volume of one cubic meter to 6 mK (0.006 K, −273.144 °C) for 15 days, setting a record for the lowest temperature in the universe over such a large contiguous volume.
The CUORE Detectors use beta decaying TeO2 crystals as low heat capacity bolometers, contained in copper towers, chilled in cryostats (dilution refrigerators) to approximately 10 mK. The detectors are isolated from environmental thermal, electromagnetic, and other particle backgrounds by ultrapure low-radioactivity shielding. Temperature spikes from electrons emitted in Te beta decays are collected for spectrum analysis. 232Th, a gamma emitter with several decay lines up to 2615 keV, is used to calibrate the detectors.
Considerable effort was aimed to minimize radioactive contamination in the construction, which creates background noise. The crystals were grown by Shanghai Institute of Ceramics for radiopurity. Neutron-doped Ge thermistors, oxygen-free copper, N2 flushing, and clean rooms, were used in construction and assembly to minimize particles. Impurities(214Bi, 40K, 208Tl, 60Co, and 228Ac) in the experiment and environment emit α and γ rays into the detectors. Roman lead and borated polyethylene are used for shielding for gamma rays and neutrons. Coincidence algorithms have been used to reject these signals from nearby crystals and other methods are being developed.
CUORICINO, the prototype was built to determine if backgrounds could be reduced enough. It had 62 mostly unenriched TeO2 crystals, 11.34 kg 130Te, with multiple Roman lead shields, mechanical vibration protection, chilled to 8 mK in an old dilution fridge and a Faraday cage.
CUORE-0 confirmed the design and process improvements before production of the CUORE array. It has 52 improved TeO2 crystals in a copper tower with better surface purity, a 20 cm lead shield, and 10 cm neutron shield, in a large dilution refrigerator at 13–15 mK, including several tons of low-radioactivity lead recovered from an ancient shipwreck It began operation Mar 2013 and will continue running until CUORE is completed.
CUORE will include 19 similar towers built in Hall A, containing 988 5×5×5 cm3 crystals of 203 kg TeO2, in one cryostat at ≈10 mK, with improved coincidence analysis, more effective shielding from copper and cryostat backgrounds, and reduced surface contamination, to reach a target background of 0.01 counts/(keV·kg·y). It will have a 73-ton octagonal shield for environmental γ rays and neutrons. Cosmic muons will be excluded by tracking cuts and won't affect sensitivity. Data taking is expected to begin in 2015.
CUORICINO, the first detector, ran April 2003 to June 2008. Final results using 19.75 kg·y of 130Te, set 90% limits T 0ν
½ > 2.8×1024 yr, and mν ≤ 710 eV, with a background count of 0.18±0.01/(keV·kg·yr). Axion mass limits were also set.
CUORE-0 initial performance was published in Aug 2014 using data taken March to September 2013, with 7.1 kg·y exposure, showing backgrounds slashed by 6 vs CUORICINO, and energy resolution of 5.7 keV. CUORE-0 reduced background to 0.019 ± 0.002 counts/(keV·kg·y), and is expected to surpass the CUORICINO bound after one year of data. An improved limit was presented in April 2015, combining 9.8 kg·yr of CUORE-0 exposure to reach the bound T 0ν
½ > 4.0×1024 yr.
CUORE will have 988 crystals in 19 similar towers and a background goal of 0.01·counts/(keV·kg·y). After 5 years, they expect to have a 90% CL half-life sensitivity of: 9.5×1025 yr, and mass sensitivity of 0.13 meV, which overlaps the inverted neutrino hierarchy.
Research and Development
ABSuRD is "A Background Surface Rejection Detector" research and development project for the CUORE detector. The project aims to develop a scintillating bolometer with the ability to veto ionizing background radiation.
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