| General Public | Photos | Collaboration | Publications
Introduction

A number of recent theoretical interpretations of neutrino oscillation experiments imply an effective Majorana neutrino mass of the order of 0.01 eV. The good results obtained so far with the bolometric technique (MIBETA) prove that this technique, although novel is competitive and alternative to the traditional calorimetric Ge technique.
The CUORE project, proposed as a natural extension of the succesfull MIBETA, will be a tightly packed array of 1000 TeO2 bolometers, each being a cube 5 cm3 on a side with a mass of 760 g. The array consists of 25 vertical towers, arranged in a square of 5 towers by 5 towers, each containing 10 layers of 4 crystals.
The technical feasibility of CUORE will be tested in a smaller scale experiment called CUORICINO (which in Italian means small CUORE).

Experimental setup

CUORICINO consists in an array of 62 TeO2 bolometers assembled in a tower structure, with a total mass of TeO2 of ~ 40 kg, the array will be mounted inside the same dilution refrigerator used in MiDBD experiment.
The design of the detector is very similar to that of the single CUORE tower. Optimized for ultralow-background searches CUORICINO is not only a test bed for CUORE but also a self consistent experiment that will improve, within few years, the present sensitivity of < mn > obtained with isotopically enriched Ge detectors.

CUORICINO is presently mounted and will start collecting data at the beginning of year 2003. The predicted background is of about 0.1 c/keV/kg/y (one half of that measured in the MiDBD experiment) implying a sensitivity on < mn > of 0.1 - 0.7 eV (depending on which evaluation of the nuclear matrix element is used to extract < mn > from the 130Te half-life).

A CUORICINO detector consists of a TeO2 single crystal that acts both as a detector and source. 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 measurable temperature rise.
This temperature change is recorded using a Neutron Transmutation Doped (NTD) germanium thermistors glued on the crystal surface.

These devices were developed and are produced at the Lawrence Berkeley National Laboratory (LBNL) and UC Berkeley Department of Material Science. They were specifically prepared for the CUORICINO array in order to present similar thermal performance. Finally a resistor of 100-200 kW, realized with a heavily doped meander implanted on a 1 mm3 silicon chip, is attached to each absorber and acts as a heater to calibrate and stabilize the gain of the bolometer over long running periods.

CUORICINO is an array of 62 TeO2 such bolometers arranged in a tower structure similar to the single tower of CUORE. The tower consists in 13 floors where two differens size crystals are arranged in a copper structure. Eleven floors contain a CUORE-like module i.e. 4 cubic (5x5x5 cm3) crystals of natural TeO2 with a mass of about 790 g each. Two floors contain a 9 crystal module where 18 of the 3x3x6 cm3 crystals used for the MiDBD experiment are mounted. These smaller TeO2 detectors have a mass of about 330 g, 14 of them are made with natural tellurium, while 4 are made with enriched tellurium: two in 130Te (i.a. 75%) and two in 128Te (i.a. 82.3% ).

The modules have a copper structure consisting into two frames joined with four columns, the crystals are held in this structure by means of PTFE tip. The modules are assembled in the tower by means of two copper bars, the entire structure is then closed inside a copper box and is surrounded by a 1 cm thick Roman lead shield with a contamination < 4 mBq/kg in 210Pb.

The tower is connected via an OFHC copper cold finger to the mixing chamber (coldest point) of the same dilution refrigerator used for the MiDBD experiment (underground Hall A on the LNGS). The refrigerator was specially constructed with previously tested low radioactivity materials, it is shielded with two layers of lead of 10 cm minimum thickness each. The outer one is made of common low radioactivity lead, the inner of special lead with a contamination of 16 ± 4 Bq/kg in 210Pb. The lead shield is surrounded by 10 cm of borated-polyethylene to reduce the neutron flux and by a Plexiglas anti-radon box fluxed with clean N2 from a liquid nitrogen evaporator. Finally the entire set up is contained inside a Faraday cage to eliminate electromagnetic interference.

 

 


 


Scheme of the experimental setup of the CUORICINO experiment @ LNGS (Hall A)

The front-end electronics of each detector is located at room temperature and consists of a differential voltage sensitive preamplifier followed by a second stage and an antialiasing filter. A pair of room temperature load resistors (30 GW metal films) serves to bias each bolometer in a symmetric way.

The array will work at about 10 mK, with a temperature spread of ~1 mK. The detectors will be calibrated by means of regular exposures to a combined radioactive source of 238U and 232Th placed just outside the dilution refrigerator. According to the results obtained in the previous tests of the 4 crystal modules the FWHM energy resolution of the 5x5x5 cm3 detectors will be similar to that obtained with the 3x3x6 cm3 detectors in the MiDBD experiment i.e of about 5 keVat the DBD transition energy (~ 2528 keV). While an energy resolution of about 1 keV should be obtained near threshold (~ 10 keV).


 

 Calibration spectrum obtained exposing 5x5x5 cm3 CUORE-like detectors to a combined radioactive source of 238U and 232Th placed just outside the (Hall C, LNGS) dilution refrigerator.

The predicted background is of about 0.1 c/keV/kg/y (one half of that measured in the MiDBD experiment) implying a sensitivity on < mn > of 0.1-0.7 eV (depending on which evaluation of the nuclear matrix element is used to extract < mn > from the 130Te half-life).

Webmaster