CUORE

Coordinates: 42°27′N 13°34′E / 42.450°N 13.567°E / 42.450; 13.567 Coordinates: 42°27′N 13°34′E / 42.450°N 13.567°E / 42.450; 13.567 The Cryogenic Underground Observatory for Rare Events (CUORE, pronounced ) is a particle physics facility located underground at the Laboratori Nazionali del Gran Sasso in Assergi, Italy. CUORE was designed primarily as a search for neutrinoless double beta decay in 130Te, a process that has never been observed. It uses tellurium dioxide (TeO2) crystals as both the source of the decay and as bolometers to detect the resulting electrons. CUORE searches for the characteristic signal of neutrinoless double beta decay, a small peak in the observed energy spectrum around the known decay energy; for 130Te, this is Q = 2527.518 ± 0.013 keV. CUORE can also search for signals from dark matter candidates, such as axions and WIMPs. An observation of neutrinoless double beta decay 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. 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 of Italy, the United States Department of Energy, and the National Science Foundation 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 are TeO2 crystals used as low heat capacity bolometers, arranged into towers and cooled in a large cryostat to approximately 10 mK with a dilution refrigerator. The detectors are isolated from environmental thermal, electromagnetic, and other particle backgrounds by ultrapure low-radioactivity shielding. Temperature spikes from electrons emitted in Te double beta decays are collected for spectrum analysis. The detectors are calibrated using 232Th, the first element in a long decay chain that includes several prominent gamma rays up to 2615 keV. For the construction of CUORE, the collaboration followed several procedures to minimize radioactive contamination that can cause the detectors to register background events at energies close to the energy released in neutrinoless double beta decay. The crystals were grown by the Shanghai Institute of Ceramics at the Chinese Academy of Sciences with strict radiopurity requirements. The crystals are held in place by PTFE support in towers constructed from oxygen-free high thermal conductivity copper and were assembled under nitrogen inside gloveboxes in cleanrooms. Copper, lead, ancient low-radioactivity Roman lead, and borated polyethylene are used to shield the detectors. Coincidence algorithms are also used to reject events that caused multiple channels to trigger, such as would be caused by an incoming cosmic ray muon or a gamma ray that Compton scatters in multiple crystals. Cuoricino was the first large-scale bolometer tower used for a rare event search and was operated from 2003 to 2008. It had 62 TeO2 crystals (11 kg of 130Te), with some crystals enriched in 130Te and others with natural isotopic abundance, and some slightly larger and some smaller crystals. The tower was similar in construction to the CUORE tower, and was shielded with copper, lead, and Roman lead. Cuoricino was operated near 8 mK in a relatively small dilution refrigerator. Using the results of Cuoricino, the final details of the CUORE detector towers were finalized, and an assembly sequence was set up for the construction of these 19 towers. CUORE-0 was the first detector tower produced on this assembly line. It had 52 improved TeO2 crystals in a copper tower with better surface purity and significantly reduced radon and other contamination. It was operated in the Cuoricino cryostat from 2013 to 2015 as a first test of the new CUORE assembly procedures as the assembly of the CUORE towers was completed.