|
0nDBD 130Te
half-life
>
2.4
1024 y (90%C.L.)statistics (8.38 kg 130Te * y) updated at May 2006 |
CUORICINO is an
array of 62 TeO2
bolometers designed to search for the
neutrinoless double beta decay of 130Te. With a total mass
of 40.7 kg
of TeO2, corresponding to about 11 kg of 130Te, this is one
of the more sensitive experiments presently running. In few years CUORICINO will be
able to
reach a sensitivity on the Majorana mass of neutrino of the order of
0.1 - 0.5 eV, morover it is the pilot experiment for the next
generation
Double Beta Decay experiment CUORE.
Designed on the basis of the experience gained by the Milano-Como-Gran Sasso groups with the MiDBD experiment (a 20 bolometers array), the CUORICINO detector is an array of 44+18 TeO2 crystals used as bolometers and arranged in a 13 plane tower structure.
Eleven
of these planes are 4-crystal modules identical to those forseen for
CUORE. Two additional planes are made of 9-crystal modules, in these
two planes 18 of the 20 crystals used for the MiDBD experiment have
been housed. Therefore CUORICINO contains 44 TeO2
crystals of size 5x5x5 cm3 and mass 790 g and 18 TeO2 crystals of size
3x3x6 cm3 and mass 330 g. All the crystals are made of natural
tellurium but 4 of the small size ones. These are made with enriched
materials: two of them are enriched to 75% in 130Te and two are
enriched to 82.3% in 128Te.
Designed on the basis of the experience gained by the Milano-Como-Gran Sasso groups with the MiDBD experiment (a 20 bolometers array), the CUORICINO detector is an array of 44+18 TeO2 crystals used as bolometers and arranged in a 13 plane tower structure.
Eleven
of these planes are 4-crystal modules identical to those forseen for
CUORE. Two additional planes are made of 9-crystal modules, in these
two planes 18 of the 20 crystals used for the MiDBD experiment have
been housed. Therefore CUORICINO contains 44 TeO2
crystals of size 5x5x5 cm3 and mass 790 g and 18 TeO2 crystals of size
3x3x6 cm3 and mass 330 g. All the crystals are made of natural
tellurium but 4 of the small size ones. These are made with enriched
materials: two of them are enriched to 75% in 130Te and two are
enriched to 82.3% in 128Te.
To fulfill the
background
requirements typical of rare events physics,
particular care was dedicated to the selection and treatment of the
materials used for the construction of the CUORICINO array: the
crystals where grown with low contamination materials
in China and shipped to Italy where they have been optically polished
with specially selected low
contamination powders. The mechanical structure of the array was made
exclusively in OFHC copper and PTFE, both these materials have an
extremely low radioactive content. All the copper and PTFE parts of the
mounting structure were separetely treated with acids to remove any possible
surface contamination. Finally the array was assembled in an
underground clean room in a Nitrogen atmosphere to avoid Radon
contamination.
The pictures show the CUORICINO array once completely
mounted, and the two different modules used: the one with the large
size crystals (4-crystal module) and that
with the small size ones (9-crystal module).
Once closed inside its copper box the array was mounted in the dilution refrigerator installed in Hall A of the underground Laboratori Nazionali del Gran Sasso (L'Aquila, Italy), a location that guarantees a high degree of suppression of cosmic ray flux thanks to the ~ 3500 m.w.e. depth.
The array, closed in a copper structure and hang in vacuum inside the Inner Vaccum Chamber of the refrigerator (the detectors are at a temperature of about 10 mK) is surrounded by a ~ 1 cm thick roman lead cylindrical shield closed with a bottom and a top lead discs of thicknesses of 7.5 cm and 10 cm respectively. The refrigerator itself is shielded with a 20 cm thickness of low activity lead and a 10 cm thickness of borated PET. Nitrogen is fluxed between the external lead shield and the cryostat to avoid any Rn contribution to the detectors background.
CUORICINO was cooled at the beginning of year 2003 (Run 1), during the cooling procedure some of the signal wires disconnected so that only 32 of the large size crystals and 16 of the small ones could be read. That problem was solved in 2004 by warming and opening the cryostat to recover the electrical connections. Only for two 790 g crystals this was not possible. CUORICINO Run2 started in September 2004 with 40 big size and 18 small size crystals used to search for 0nDBD (indeed two big size crystals have an excess noise and are not used).
The live time for the 0nDBD measurement in Run2 two is of ~ 64%, where the dead time is partially due to the periodic calibration of the detectors with an external Th source (every 4-5 weeks) and partially to the maintenance of the cryogenic apparatus.
The performances of the detectors are quite good: the average FWHM resolution measured during the calibration of the detectors with a 232Th source is of ~ 8 keV for the big size crystals and of ~ 10 keV for the small size crystals. Both these values are measured on the 208Tl gamma line at 2615 keV, just above the position (2528.8 keV) where the 0nDBD peak should appear. Similar resolutions are measured on the sum background spectra of big and small crystals.
To search for 0nDBD the detectors are operated in anticoincidence, rejecting a fraction of background events. The efficiency for a complete containment of the 0nDBD event being ~ 85%.
The sum background spectra (in the so called "gamma region") of big and small size crystals are shown below. The peaks clearly appearent in the spectra are due to gamma emission of radioactive contaminants present in the materials: the 238U and 232Th chain gammas as well as the 40K gamma are clearly evident.
In the DBD region two peaks appear, one below the 0nDBD Q-value and one above. They are due respectively to the 60Co sum line (2505.5 keV) and the 208Tl line (2615 keV). The first is ascribed to a 60Co contamination of the copper structures due to cosmogenic activation while the second is ascribed to 232Th contamination outside the internal roman lead shield. While the first line do not give any contribution to the 0nDBD bkg the second prove that a contribution from 232Th through multicompton of the 2615 keV line is present. This bkg cannot however explain the whole bkg counting rate in the 0nDBD region that - according to the present interpretation - receive a non negligible contribution from alpha (from U and Th chains) surface contaminations of the array (the crystals themselves and the inert materials directely facing the crystals). A detailed study of the bkg sources is on the way.
The statistics collected up to now is summarized in the following table:
The pictures show the CUORICINO array once completely
mounted, and the two different modules used: the one with the large
size crystals (4-crystal module) and that
with the small size ones (9-crystal module).
Once closed inside its copper box the array was mounted in the dilution refrigerator installed in Hall A of the underground Laboratori Nazionali del Gran Sasso (L'Aquila, Italy), a location that guarantees a high degree of suppression of cosmic ray flux thanks to the ~ 3500 m.w.e. depth.
The array, closed in a copper structure and hang in vacuum inside the Inner Vaccum Chamber of the refrigerator (the detectors are at a temperature of about 10 mK) is surrounded by a ~ 1 cm thick roman lead cylindrical shield closed with a bottom and a top lead discs of thicknesses of 7.5 cm and 10 cm respectively. The refrigerator itself is shielded with a 20 cm thickness of low activity lead and a 10 cm thickness of borated PET. Nitrogen is fluxed between the external lead shield and the cryostat to avoid any Rn contribution to the detectors background.
CUORICINO was cooled at the beginning of year 2003 (Run 1), during the cooling procedure some of the signal wires disconnected so that only 32 of the large size crystals and 16 of the small ones could be read. That problem was solved in 2004 by warming and opening the cryostat to recover the electrical connections. Only for two 790 g crystals this was not possible. CUORICINO Run2 started in September 2004 with 40 big size and 18 small size crystals used to search for 0nDBD (indeed two big size crystals have an excess noise and are not used).
The live time for the 0nDBD measurement in Run2 two is of ~ 64%, where the dead time is partially due to the periodic calibration of the detectors with an external Th source (every 4-5 weeks) and partially to the maintenance of the cryogenic apparatus.
The performances of the detectors are quite good: the average FWHM resolution measured during the calibration of the detectors with a 232Th source is of ~ 8 keV for the big size crystals and of ~ 10 keV for the small size crystals. Both these values are measured on the 208Tl gamma line at 2615 keV, just above the position (2528.8 keV) where the 0nDBD peak should appear. Similar resolutions are measured on the sum background spectra of big and small crystals.
To search for 0nDBD the detectors are operated in anticoincidence, rejecting a fraction of background events. The efficiency for a complete containment of the 0nDBD event being ~ 85%.
The sum background spectra (in the so called "gamma region") of big and small size crystals are shown below. The peaks clearly appearent in the spectra are due to gamma emission of radioactive contaminants present in the materials: the 238U and 232Th chain gammas as well as the 40K gamma are clearly evident.
 |
 |
In the DBD region two peaks appear, one below the 0nDBD Q-value and one above. They are due respectively to the 60Co sum line (2505.5 keV) and the 208Tl line (2615 keV). The first is ascribed to a 60Co contamination of the copper structures due to cosmogenic activation while the second is ascribed to 232Th contamination outside the internal roman lead shield. While the first line do not give any contribution to the 0nDBD bkg the second prove that a contribution from 232Th through multicompton of the 2615 keV line is present. This bkg cannot however explain the whole bkg counting rate in the 0nDBD region that - according to the present interpretation - receive a non negligible contribution from alpha (from U and Th chains) surface contaminations of the array (the crystals themselves and the inert materials directely facing the crystals). A detailed study of the bkg sources is on the way.
The statistics collected up to now is summarized in the following table:
|
Detector |
Statistics |
FWHM
@ 2615 keV |
0nDBD
background counting rate |
| kg (130Te) · y | keV |
c/keV/kg(TeO2)/y | |
| 5x5x5 cm3 natural crystals |
4.3 | 7.5 |
0.19±0.02 |
| 3x3x6 cm3 natural crystals |
0.5 | 12 | 0.19±0.05 |
| 3x3x6 cm3 130Te enriched crystals | 0.2 |
18 |
0.55±0.22 |
- No peak appears
at the 0nDBD Q-value and with a Maximum Likelihood procedure an upper
limit of 2 1024 y (90%C.L.) is set for the 0nDBD of 130Te. The limit
is evaluated using anticoincidence sum spectra and considering
separately the sum spectra of big, small and enriched crystals and
distinguishing the two runs (this to take in account the different i.a.
and/or efficiency and/or background). In order to account for the
spread in energy resolutions of the detectors, the response function
used in the fit is the sum of N gaussian (N being the number of
detectors summed in the considered spectrum) centered at the same
energy but with different sigma (si). For each detector the
average si, measured at 2615 keV in all the 232Th
calibration performed during the two runs, is used.
The 2505 keV 60Co peak is included in the window used for the analysis, while the background undelying the peaks is fitted with a flat/linear function. A very shallow dependence on the energy window used for the analysis is observed, resulting in variation of the oder of 5% of the limit when using a flat or a linear background and when including or not the 2615 keV peak.
This lower bound on the 130Te 0nDBD half-life, once converted in terms of the Majorana mass of neutrino, results in an upper bound between 0.2 and 1.1 eV depending on the value of the nuclear matrix element used in the calculation.