CUORE is a proposed tightly packed array of 988 TeO2
bolometers, each being a cube 5 cm3 on a side with a
mass of 750 g. The array consists of 19 vertical towers, arranged in
a cylindrical structure as shown in the figure. Each tower will consist 13 layers of 4
crystals. The design of the detector is optimized for
ultralow-background searches: for neutrinoless double beta decay of
130Te (33.8 % abundance), cold dark matter, solar axions,
and rare nuclear decays. 
A
preliminary experiment involving 20
crystals 3x3x6 cm3 of 340 g (MiDBD) has been completed,
and a single CUORE tower is being constructed as a smaller
scale experiment called CUORICINO. The expected performance and
sensitivity, based on Monte Carlo simulations and extrapolations of
present results indicate that CUORE will be able to test the
0.02-0.05 eV region for <mn>.
The CUORE detector will consist of an array of 988 TeO2
bolometers arranged in a cylindrical configuration of 19 towers with
53 crystals each. The principle of operation of these bolometers is
now well understood. Tellurium Oxide is a dielectric and diamagnetic
material. According to 
the Debye Law, the heat capacity of a single crystal at low temperature is proportional to the ratio T/TD3 where TD is the Debye Temperature of TeO2. Thus, providing that the temperature is extremely low, a small energy release in the crystal results in to a measurabletemperature rise. This temperature change can be recorded with thermal sensors and in particular using Neutron Transmutation Doped (NTD) germanium thermistors. These devices were developed and produced at the Lawrence Berkeley National Laboratory (LBNL) and UC Berkeley Department of Material Science, they have been made unique in their uniformity of response and sensitivity by neutron exposure control with neutron absorbing foils accompanying the germanium in the reactor. The TeO2 crystals are produced by the Shanghai Quinhua Material Company (SQM) in Shanghai, China and they will be the source of 750 g TeO2 crystals for CUORE.
A single CUORE detector consists of a 5x5x5 cm3 single crystal of TeO2 that acts both as a detector and source. The temperature sensors are Neutron Transmutation Doped (NTD) Ge thermistors, specifically prepared in order to present similar thermal performance. Proper resistors of 100-200 kW, realized with a heavily doped meander implanted on a 1 mm3 silicon chip, are attached to each absorber in order to calibrate and stabilize the gain of the bolometer over long running periods. Detectors reproducibility was tested on the MI-DBD array, which is a significant number of detectors (20) operating simultaneously. CUORE crystals are grouped in elementary modules of four elements held between two copper frames joined by copper columns. TEFLON pieces are inserted between the copper and TeO2, as a heat impedance and to clamp the crystals. There is a 6 mm gap between crystals with no material between them. The four detectors are mechanically coupled; some of the TEFLON blocks and springs act simultaneously on two crystals. Tests on this and a similar structure but with 3x3x6 cm3 crystals clearly demonstrate that the CUORE technology is viable. A stack of 10, 4-detector modules, will be connected by two vertical copper bars to form a tower.
The whole detector will be
supported in a copper frame in a 25 tower configuration. The towers
will be mechanically connected to a OFHC copper top plate elastically
suspended from the dilution refrigerator mixing chamber (coldest
point). The frame, and dilution refrigerator mixing chamber to which
it is thermally connected, forms the heat sink, while the teflon
stand-offs provide the thermal impedance which delays the re-cooling
of the bolometers.
The bolometers operate at ~7-10 mK. This will require an extremely powerful dilution refrigerator (DR). Estimates were made of the parasitic power the detectorand DR would receive from: heat transfer of the residual helium gas in the inner vacuum chamber (IVC), power radiated from the 50 mK shield facing the detector, and from vibrational energy (microphonic noise). A system similar to that of the Nautilus Collaboration that cools a 2-ton gravitational antenna will be required. One important design feature is the 50 mm diameter clear access to the mixing chamberto allow a rod, suspended from an external structure, to suspend the detector array to minimize vibrations from direct connection to the mixing chamber. The dewar housing the DR will have a jacket of liquid nitrogen (LN), to avoid the need of superinsulation, which is not free of radioactivity. Liquifiers will provide constant liquid levels for the two baths over long periods of operation. A heavy shield against environmental radioactivity will characterize the CUORE setup. Part of the bulk lead shielding will be placed inside of the cryostat, and part outside. This will accomplish shielding against the dewar, and will reduce the total amount of lead required. A 4p layer of ultra-low background lead will constitute 3 cm thick walls of the cubic structure of the array. This layer will be Roman lead. The dilution refrigerator will be constructed from materials specially selected for low levels of radioactivity. Nevertheless, these levels might be higher than can be tolerated by the sensitivity requirements. The top of the detector array will be protected by two layers 1x1 m2, with a 10 cm diameter central bore to accommodate the copper cold finger that supports the detector and the narrow neck of two radiation shields of the refrigerator that are at temperatures of 50 mK and 600 mK. The layer close to the detector will be 10 cm thick, made from high quality lead with an activity of 16 Bq/Kg of 210Pb. The upper layer, also 10 cm thick, will bemade of modern lead with an activity of 210Pb of 150 Bq/Kg. Another layer of lead 17 cm thick, and 40 cm by 40 cm will be placed directely on the top face of the detector. It will be constructed from low activity lead of 16 Bq/Kg of 210Pb. This configuration is designed so that the minimum path to the detector from the IVC and the dilution unit is 20 cm of lead. Finally, outside the dewar, there will be two 10 cm thicknesses of lead, 16 Bq/Kg of 210Pb for the inner layer, and 150 Bq/Kg for the outer layer. The lead shield will be surrounded with a 10 cm thick box of borated polyethylene that will also function as an hermetically sealed enclosure to exclude radon. It will be flushed constantly with dry nitrogen. The entire dewar, detector, and shield will be enclosed in a Faraday cage to exclude electromagnetic disturbances that also constitute a source of background, albeit at low energies, important to dark matter and solar axion searches.A heavy shielding structure of CUORE All the materials used to built the detectors, their mounting structure, the cryostat and the shieldings themselves will be selected to ensure that only low radioactive contamination materials will be empolyed. A particular care will be obviously devoted to the detectors which will be grown from ultrapure TeO2 powders also minimzing their exposure to cosmic rays. Great care will be devoted also to their surface treatment. Copper and Teflon used to construct the CUORE array will be selected for their low contamination. Once machined all the copper and Teflon pieces will undergo a surface cleaning procedure that will guarantee the required low level of surface radioactive contamination for those parts that directely face the detectors. The array will be assembled underground, in a low Rn clean room to avoid Rn daughters contamination.
A front-end electronics furnishing a bias current to the NTD thermistors and receiving and processing the resulting signal-bearing voltage outputs will be installed next to the cryogenic system. Two solutions, mainly differing just for the addition of a cold buffer stage inserted between the detector and the room temperature preamplifier are under investigation. The connection with the thermistors will be via shielded twisted pairs.The design of the electronics will addressthe following major issues: 1) minimization of the biasing circuitry and preamplifier noise; 2) capability to manage the spread of bolometer/thermistor responses; 3) in situ measurement of the bolometer characteristics; 4) high level of remote programmability in order to avoid manual in-situ parameter adjustments which could interfere with bolometer measurements.
CUORE will be located in the underground halls of Laboratori Nazionali del Gran Sasso (L'Aquila - Italy) at a depth of 3400 m.w.e.
A single tower of CUORE was built in 2002. It was attached to the mixing chamber of the same dilution refrigerator (DR) used in the Milano 20 crystal array experiment (MiDBD) and run not only as a test bed for CUORE but will also be an experiment called CUORICINO (which in Italian means small CUORE) designed to improve on the present sensitivity of < mn > obtained with isotopically enriched Ge detectors. CUORICINO will prove the feasibility of the extension of the MiDBD technology to large arrays. The smaller array of 20, 3x3x3 cm3 crystals was operated successfully for 31,000 hours kg. The data yield a bound T1/2(130Te) > 2 x 1023 y, corresponding to <mn> < 1-2 eV, depending on the nuclear matrix element used. The CUORE detector then will be the final version of a three step development comprising: MiDBD, CUORICINO, and finally CUORE. This, plus the fact that CUORE requires no isotopic enrichment, (the isotopic abundance of the DBD emitter 130Te is 33.8 %) puts CUORE ahead of other options of truly next generation 0nDBD experiments. The technology, though novel, is developed and to a large degree proven. An extensive R&D was started in 2000 (Hall C cryogenic installation) to test and improve the performance of the proposed CUORE detectors. Various single 4-detector modules were tested with excellent results in terms of stability and energy resolution. Dedicated background measurements to in vestigate bulk and surface radioactive contaminations of the mounting materials were also carried out.
The goal of CUORE is to achieve a background rate in the range 0.001 to 0.01 counts/(keV\;kg\;y) at the 0nDBD transition energy of 130Te (2528 keV). A low counting rate near threshold (that will be of the order of ~5 keV) is also foreseen and will allow to have results in the Dark Matter and Axions research fields. Radioactive contaminations of individual construction materials, as well as the laboratory environment, were measured and the impact on detector performance determined by Monte Carlo computations (Geant-4). The following background sources were considered: 1) bulk and surface contamination (238U, 232Th, 40K and 210Pb )of the construction materials ; 2) bulk contamination of construction materials due to cosmogenic activation; 3) neutron and muon flux in the Gran Sasso Laboratory; 4) gamma ray flux from natural radioactivity in the Gran Sasso Laboratory; 5) background from the 2nDBD. Main bulk contributions tcome from the heavvy structures near the detectors and from the detectors themselves. The assumed contamination levels of radioactivity as well as the 60Co cosmogenic contamination of copper were deduced from the 90% C.L. upper limits obtained for the contaminations of the constructing materials of the MiDBD experiment and from low activity Ge spectrometry measurements. In both cases no evidence of a bulk contamination is obtained with the achievable sensitivity and only upper limits could be produced. To obtain a real evaluation of bulk contribution to CUORE background higher sensitivity measurement of bulk contamination of the construction materials are required. Cosmogenic activation and muons, neutrons and gamma rays from the Laboratory environment would produce reduced contribution to CUORE background thanks to the underground storage of construction materials and the optimization of the lead and neutron shields. The unavoidable background produced by the 2nDBD is lower than 10-4 counts/(keV kg d).
Although background levels of the order of 0.001 counts/(keV kg y) at the 0nDBD decay transition energy seem to be viable by an accurate selection of construction material and by the optimization of the surface treatment a more conservative level of 0.01 counts/(keV kg y) can be also considered for CUORE sensitivity . The situation is summarized in the table, for one year of measurement and an assumed 5 keV energy resolution.
|
Background (counts/keV kg y) |
mn>
(10-3 eV) |
T1/2 (1025 y) |
| 10-2 | 30-60 | 9.6 |
| 10-3 | 20-30 | 3 |
- C. Arnaboldi et al. - CUORE: A Cryogenic Underground Observatory for Rare Events. Subm. to NIM A (2002)
- O. Cremonesi: Neutrinoless double beta decay : present and future, Nucl.Phys. B (in the press), arXiv:hep-ex/0210007
- A. Alessandrello et al: A Cryogenic underground Observatory fo Rare Events: CUORE, an update, arXiv:hep-ex/0201038