CUORE - Experiment
Where should the field of double beta decay (DBD) go from the present situation? Neutrino oscillation experiments have given clear evidence that neutrinos oscillate. A number of theoretical interpretations of these experiments data imply that the effective Majorana mass of the electron neutrino (as measured in neutrinoless DBD) could be in the range 0.01 eV to the present bounds. Considering this range, could a next generation 0nDBD experiment detect it? If so, what technique would be the best for a possible discovery experiment?
With a total mass of 741 kg of TeO2 crystals operated as bolometers at very low temperatures CUORE represents one of the best approaches presently available. It can be launched without isotopic enrichment nor extensive R&D , and it can achieve next generation sensitivity. CUORE originates as a natural extension of the succesfull MiDBD and Cuoricino 130Te experiments where for the first time a large array of bolometers was used to search for 0nDBD . The successful operation of these detectors has demonstrated that the bolometric technique, although novel is competitive and alternative to the traditional calorimetric Ge technique.
CUORE is 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 (34.3 % abundance), cold dark matter, solar axions, and rare nuclear decays. The expected performance and sensitivity, based on Monte Carlo simulations and extrapolations of presently available results indicate that CUORE will be able to test the 0.02-0.05 eV region for <mn>.
The principle of operation of CUORE bolometers is quite simple. 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 kΩ, 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. A stack of 13, 4-detector modules, supported by a copper structure forms a tower.
The whole detector is designed as a 19 tower configuration hosted inside a ultra clean copper fcage. The construction of all the 19 towers of CUORE has been completed in the summer 2014. 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 a temperature of about 10 mK. This requires an extremely powerful dilution refrigerator (DR) and a very effective cryostat. The DR has been built by Leiden Cryogenics and has demonstrated an excellent performance. The dilution refrigerator has been constructed from materials specially selected for low levels of radioactivity. The cryogenic system has been designed and built by the CUORE collaboration by minimizing he parasitic power the detector and 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). One important design feature is the clear access to the mixing chamber to allow three rods, suspended from an external structure, to support the detector array while minimizing vibrations from direct connection to the mixing chamber. Furthermore, the system is cryogen free, the cooling power being provided by a system of 5 pulse tubes. Two cold shields realized with ancient roman lead are installed close to the detector and maintained at 4K. They accomplish shielding against the dewar, and reduce the total amount of lead required. A heavy shield against environmental radioactivity surrounds the CUORE cryostat. This consists of 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. All the materials used to built the detectors, their mounting structure, the cryostat and the shieldings themselves were selected to ensure that only low radioactive contamination materials be used. A particular care was devoted to the detectors which were grown from ultrapure TeO2 powders also minimzing their exposure to cosmic rays. Great care has been devoted also to their surface treatment. Copper and Teflon used to construct the CUORE array has been selected for their low contamination. Once machined all the copper pieces have undergone a complex and specifically designed surface cleaning procedure to guarantee the required low level of surface radioactive contamination for those parts that directely face the detectors. All the 19 twers have been assembled underground in a specially designed assembly line consisting of a series of glove boxes that have avoided any contact with Rn contaminated air. The 19 towers will be assembled in a low Rn clean room to avoid Rn daughters contamination duringthe fall 2015.
A dedicated front-end electronics furnishing a bias current to the NTD thermistors and receiving and processing the resulting signal-bearing voltage outputs has been designed and produced. The modules will be installed next to the cryogenic system. The connection with the thermistors will be via shielded twisted pairs.The design of the electronics addresses the 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 is 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 2012 using the same materials and procedures developed for CUORE. It was attached to the mixing chamber of the same dilution refrigerator (DR) used to host the Cuoricino experiment and operated to test the effectiveness of the new design and assembly procedure of the CUORE detector. This detector has been called CUORE-0 and started operation in Spring 2013 as an independent experiment. The CUORE detector can then be considered as the final development of a phased program fo bolometric detectors of increasing mass and improved performance. This, plus the fact that CUORE requires no isotopic enrichment, (the isotopic abundance of the DBD emitter 130Te is 33.8 %) makes CUORE a truly next generation 0nDBD experiments. The technology, though novel, is developed and to a large degree proven. An extensive R&D was carried out, starting since 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 of 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).
last updated on: 09.04.2015, 00:28 by Ollie