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The experiment

CUORE is the largest volume of cryogenic tellurium bolometers ever built. It looks for neutrinoless double beta decay, which is the only way to investigate whether neutrinos are their own anti-particles. The tellurium crystals form both the source material and the detector: a radioactive decay of 130-Te is detected when a crystal heats up minutely. All 741 kg of the crystals, plus the associated support structures, are cooled to about 10mK in a cryostat that uses a dual-phase helium dilution refrigerator. The location in the underground Gran Sasso National Laboratory cuts out any background from cosmic rays. To further shield the detector from any outside interference, CUORE uses a layer of ancient Roman lead that was retrieved from a sunken ship by physicists and archeologists. Prototypes for CUORE are already producing world-class results about nuclear interactions, and CUORE will be one of the definitive neutrino experiments of the decade.


The crystals

The heart of the CUORE detector is made of crystal bolometers of Tellurim oxide. Each time a nucleus decays, it releases minuscule amount of heat (up to 2.5 MeV for 130-Te double beta decay) which makes the crystal heat up. This small change in temperature is measured by a NTD thermometer glued to each crystal. The CUORE bolometers are arranged into 19 towers, supported by copper and PTFE which have been impeccably cleaned.
Tellurium is a good choice for making this detector for a few reasons. In its 130 isotope, it exhibits double beta decay, which is the primary requirement. Out of the elements that have double beta decays, Tellurium decays at a relatively high energy, which makes it easier to eliminate systematics errors and background signals. Tellurium oxide has low intrinsic background and is mechanically stable, so it can be operated for years.
CUORE measures the energy spectrum of individual, very rare radioactive decays, and the measurement needs to be precise enough that we can avoid false signals. This is why CUORE is located under the Gran Sasso mountain: it cuts out the background from cosmic rays. Other background signals can creep in from radioactive contamination that, under normal circumstances, would be so small and harmless that they would go unnoticed. All the parts of the CUORE detector were made from specially chosen radio-pure materials, then cleaned with chemical and electropolish procedures, then stored in a pure nitrogen environment to prevent natural Radon gas from getting too close to the immaculate detector.
The crystals were assembled using a robotic arm that ensured the same spacing between each crystal and its thermometers. This helped make the responses from all the crystals more similar to each other, thus improving the entire detector response.


The cryostat

The cryostat in which the CUORE detector operates is the largest of its kind. It cools all of the tellurium crystals to less than 10mK, making the detector the coldest cubic meter in the universe . This cooling is accomplished in several layers, each one colder than the ones outside. There is a new layer roughly for each half-order of magnitude in absolute temperature, from 300K down to a few mK.
At the very center of the cryostat, performing the last stage of cooling, is a dual-phase helium dilution unit. Mixtures of helium isotopes exist in two phases, one more dense than the other. The mixture can be cooled by pumping 3-He out from the dilute phase, allowing more 3-He to pass from the dense phase to the dilute phase. A similar process helps cool more familiar liquids: we can blow the steam off a cup of tea, and by allowing more tea to evaporate into steam, we induce the whole cup to cool faster.
Outside of the cryostat, several layers of shielding prevent outside activity from interfering with the measurement. Nested copper vessels preserve a vacuum and temperature gradient. Outside that is a layer of lead to stop large nuclear interactions, then borate polyethylene to stop small atoms and neutrons. The Gran Sasso mountain itself forms the final layer of shielding against cosmic rays.


The Roman lead

There is naturally occurring radioactivity which mixes with any material as it is shaped. Specifically, lead contains the isotope 210-Pb as a result of natural contamination from the 238-U chain. This radioactive isotope is chemically indistinguishable from other stable isotopes and gets mixed in when lead is metallurgically extracted from ore. The halflife of 210-Pb is 20 years, so as the lead ages, the 210-Pb decays away, leaving the lead cleaner the older it gets. Because of this, the oldest possible pieces of lead are the best for low-background experiments.
CUORE has been fortunate to use lead that was shaped during the height of the ancient Roman empire. In each kilogram of naturally occurring modern lead, the activity rate is between a few to hundreds of decays per second. This activity has completely decayed away in the Roman lead in CUORE. We don't know how they did it, but when Roman metallurgists purified this lead they got rid of the natural contaminants so well that it compares to the best prepared modern lead samples. Using this lead gives CUORE a unique opportunity to create an extremely radio-pure environment.
The lead in CUORE is from a ship that sunk between 50 and 80 B.C. off the coast of Sardinia. There were about a thousand ingots of 33kg each extracted in a campaign of diving expeditions funded by INFN in collaboration with the archaeological heritage ministery. Out of these, 270 are currently being used for physics experiments. Each of the lead ingots has a unique stamp that records some of its manufacturing history: the name of the Roman who cast it. These inscriptions are priceless archeological sources, and are being studied at the National Archaeological Museum in Cagliari, southern Sardinia.
This lead shielding is cooled to 4K-- that's a huge mass to keep so cold. The process of getting it down to that temperature requires a special system with extra cooling power, beyond simple heat conduction. This "fast cooling system" forces helium gas through the cryostat as it cools, convectively cooling the added mass of the lead shields.
CUORE is the culmination of decades of development of bolometer research for neutrino mass studies. The prototype experiment Cuoricino validated the basic techniques for CUORE, and more recently the CUORE-0 experiment used a single CUORE-style tower to demonstrate improvements in many aspects of CUORE purification, assembly, and construction. These efforts have made CUORE a worldwide leader in the search for neutrinoless double beta decay.
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C.Alduino1, K.Alfonso2, D.R.Artusa1,3, F.T.Avignone III1, O.Azzolini4, M.Balata3, T.I.Banks5,6,3, G.Bari7, J.W.Beeman8, F.Bellini9,10, A.Bersani11, D.Biare6, M.Biassoni 12,13, C.Brofferio 12,13, A.Buccheri 10, C.Bucci 3, C.Bulfon10, A.Camacho4, A.Caminata11, L.Canonica3, X.G.Cao14, S.Capelli12,13, M.Capodiferro10, L.Cappelli 3,15, L.Carbone13, L.Cardani9,10, M.Cariello11, N.Casali3,16, L.Cassina12,13, R.Cereseto11, G.Ceruti13, A.Chiarini7, D.Chiesa12,13, N.Chott1, M.Clemenza12,13, S.Copello17, C.Cosmelli9,10, O.Cremonesi13,♛, C.Crescentini7, R.J.Creswick 1, J.S.Cushman18, I.Dafinei10, A.Dally19, C.J.Davis18, F.Del Corso7, S.Dell’Oro3,20, M.M.Deninno7, S.Di Domizio17,11, M.L.di Vacri3,16, L.DiPaolo6, A.Drobizhev5,6, L.Ejzak19, G.Erme3,15, D.Q.Fang14, M.Faverzani12,13, J.Feintzeig6, G.Fernandes17,11, E.Ferri12,13, F.Ferroni9,10, S.Finelli7, E.Fiorini13,12, M.A.Franceschi21, S.J.Freedman6,5,±, B.K.Fujikawa6, R.Gaigher13, A.Giachero12,13, L.Gironi12,13, A.Giuliani22, L.Gladstone23, P.Gorla3, C.Gotti12,13, M.Guerzoni7, M.Guetti3, T.D.Gutierrez24, E.E.Haller8,25, K.Han18,6, E.Hansen23,2, K.M.Heeger18, R.Hennings-Yeomans5,6, K.P.Hickerson2, H.Z.Huang2, M.Iannone10, L.Ioannucci3, R.Kadel26, G.Keppel4, Yu.G.Kolomensky5,26, A.Leder23, C.Ligi21, K.E.Lim18, X.Liu2, Y.G.Ma14, M.Maino12,13, L.Marini17,11, M.Martinez27, R.H.Maruyama18, R.Mazza13, Y.Mei6, R.Michinelli7, N.Moggi28,7, S.Morganti10, P.J.Mosteiro10 T.Napolitano21, M.Nastasi12,13, S.Nisi3, C.Nones29, E.B.Norman30,31, A.Nucciotti12,13, T.O’Donnell5,6, F.Orio10, D.Orlandi3, J.L.Ouellet5,6, C.E.Pagliarone3,15, M.Pallavicini17,11, V.Palmieri4, G.Pancaldi7, L.Pattavina3, M.Pavan12,13, R.Pedrota32, A.Pelosi10, M.Perego13, G.Pessina13, V.Pettinacci10, G.Piperno9,10, C.Pira4, S.Pirro3, S.Pozzi12,13, E.Previtali13, C.Rosenfeld1, C.Rusconi13, E.Sala12,13, S.Sangiorgio30, D.Santone3,16, N.D.Scielzo30, V.Singh5, M.Sisti12,13, A.R.Smith6, F.Stivanello4, L.Taffarello32, L.Tatananni,3 M.Tenconi22, F.Terranova12,13, M.Tessaro32, C.Tomei10, S.Trentalange2, G.Ventura33,34, M.Vignati10, S.L.Wagaarachchi5,6, J.Wallig35, B.S.Wang30,31, H.W.Wang14, L.Wielgus19, J.Wilson1, L.A.Winslow23, T.Wise18,19, A.Woodcraft36, L.Zanotti12,13, C.Zarra3, G.Q.Zhang14, B.X.Zhu2, S.Zimmermann35, S.Zucchelli37,7.

1 Department of Physics and Astronomy, University of South Carolina, Columbia, SC 29208 - USA 
2 Department of Physics and Astronomy, University of California, Los Angeles, CA 90095 - USA 
3 INFN - Laboratori Nazionali del Gran Sasso, Assergi (L’Aquila) I-67010 - Italy 
4 INFN - Laboratori Nazionali di Legnaro, Legnaro (Padova) I-35020 - Italy 
5 Department of Physics, University of California, Berkeley, CA 94720 - USA 
6 Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 - USA 
7 INFN - Sezione di Bologna, Bologna I-40127 - Italy 
8 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 - USA 
9 Dipartimento di Fisica, Sapienza Università di Roma, Roma I-00185 - Italy 
10 INFN - Sezione di Roma, Roma I-00185 - Italy 
11 INFN - Sezione di Genova, Genova I-16146 - Italy 
12 Dipartimento di Fisica, Università di Milano-Bicocca, Milano I-20126 - Italy 
13 INFN - Sezione di Milano Bicocca, Milano I-20126 - Italy 
14 Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800 - China 
15 Dipartimento di Ingegneria Civile e Meccanica, Università degli Studi di Cassino e del Lazio Meridionale, Cassino I-03043 - Italy 
16 Dipartimento di Scienze Fisiche e Chimiche, Università dell’Aquila, L’Aquila I-67100 - Italy 
17 Dipartimento di Fisica, Università di Genova, Genova I-16146 - Italy 
18 Department of Physics, Yale University, New Haven, CT 06520 - USA 
19 Department of Physics, University of Wisconsin, Madison, WI 53706 - USA 
20 INFN - Gran Sasso Science Institute, L’Aquila I-67100 - Italy 
21 INFN - Laboratori Nazionali di Frascati, Frascati (Roma) I-00044 - Italy 
22 Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse, 91405 Orsay Campus - France 
23 Massachusetts Institute of Technology, Cambridge, MA 02139 - USA 
24 Physics Department, California Polytechnic State University, San Luis Obispo, CA 93407 - USA 
25 Department of Materials Science and Engineering, University of California, Berkeley, CA 94720 - USA 
26 Physics Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 - USA 
27 Laboratorio de Fisica Nuclear y Astroparticulas, Universidad de Zaragoza, Zaragoza 50009 - Spain 
28 Dipartimento di Scienze per la Qualità della Vita, Alma Mater Studiorum - Università di Bologna, Bologna I-47921 - Italy 
29 Service de Physique des Particules, CEA / Saclay, 91191 Gif-sur-Yvette - France 
30 Lawrence Livermore National Laboratory, Livermore, CA 94550 - USA 
31 Department of Nuclear Engineering, University of California, Berkeley, CA 94720 - USA 
32 INFN - Sezione di Padova, Padova I-35131 - Italy 
33 Dipartimento di Fisica, Università di Firenze, Firenze I-50125 - Italy 
34 INFN - Sezione di Firenze, Firenze I-50125 - Italy 
35 Engineering Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 - USA 
36 SUPA, Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ - UK 
37 Dipartimento di Fisica e Astronomia, Alma Mater Studiorum - Università di Bologna, Bologna I-40127 - Italy 

 ±  Deceased 
♛ Spokesperson 
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