CUORE

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The Cryogenic Underground Observatory for Rare Events (CUORE) is a particle physics facility located at the Laboratori Nazionali del Gran Sasso in Italy. It uses a tellurium oxide crystal bolometer-thermistor setup to search for neutrinoless double beta decay (0νββ) in 130Te, a process not yet ever detected. Observing such decays would mean that neutrinos are their own antiparticles and Majorana fermions. This is relevant to many topics in particle physics: lepton number conservation, nuclear structure, as well as neutrino masses and properties. CUORE searches for a rare spike in the decay spectrum around the known decay energy: Q = 2527.518±0.013 keV, that would be evidence of 0νββ events. It also can search for axion signals.

The CUORE collaboration involves physicists from several countries including the US and Italy,[1] and has published papers about the performance of various parts of the detector and simulation.[2] 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, scientists in the CUORE collaboration at the Laboratori Nazionali del Gran Sasso in Italy 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[3]

Detectors[edit]

The CUORE Detectors use beta decaying TeO2 crystals as low heat capacity bolometers, contained in copper towers, chilled in cryostats to ~10mK, isolated from environmental thermal, electromagnetic, and particle influences by ultrapure low-radioactivity shielding. Temperature spikes from Te β decays are collected for spectrum analysis. 232Th, a gamma emitter with 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 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.[4]

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[5] 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 not affect sensitivity. Data taking is expected to begin in 2015.

Results[edit]

CUORICINO the first detector, ran April 2003 to June 2008. Final results using 19.75 kg·y of 130Te, set a 90% limits: T 0νββ
½
 
≥ 2.8×1024 yr, mν ≤ 710 eV, with a background count of 0.18±0.01/(keV·kg·yr).[6] Axion mass limits were also set.[7]

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.[8] 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, with results expected in 2015.

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.[9]

References[edit]

  1. ^ CUORE Collaboration. "Cuore - Institutions". Retrieved 2013-11-08. 
  2. ^ CUORE Collaboration. "Public List of Published Papers". Retrieved 2013-11-08. 
  3. ^ "CUORE: The Coldest Heart in the Known Universe". INFN Press Release. Retrieved 21 October 2014. 
  4. ^ Arnaboldi, C.; Brofferio, C.; Cremonesi, O .; Gironi, L.; Pavan, M.; Pessina, G.; Pirro, S.; Previtali, E. (6 March 2011). "A novel technique of particle identification with bolometric detectors". Astroparticle Physics 34 (11): 797–804. doi:10.1016/j.astropartphys.2011.02.006. Retrieved 21 October 2014. 
  5. ^ Nosengo, Nicola (15 April 2010). "Roman ingots to shield particle detector". Nature. doi:10.1038/news.2010.186.  edit
  6. ^ Andreotti, E. et al. (2011). "130Te neutrinoless double-beta decay with CUORICINO". Astroparticle Physics 34 (11): 822. doi:10.1016/j.astropartphys.2011.02.002.  edit
  7. ^ The CUORE collaboration (May 2013). "Search for 14.4 keV solar axions from M1 transition of 57Fe with CUORE crystals". Journal of Cosmology and Astroparticle Physics 2013 (5): 007–1–007–16. arXiv:1209.2800. doi:10.1088/1475-7516/2013/05/007. 
  8. ^ CUORE Collaboration (2014). "Initial performance of the CUORE-0 experiment". arXiv:1402.0922 [physics.ins-det].
  9. ^ CUORE Collaboration (25 Feb 2014). "Searching for neutrinoless double-beta decay of 130Te with CUORE". arXiv:1402.6072 [physics.ins-det].

External links[edit]