Experimental setup
The Mi-DBD array (tower) consists in a stack of five floors, each consisting
of 4 detectors. Each detector consists of a massive absorber and a proper
temperature sensor in thermal contact with it. The array operates inside
a dilution refrigerator in the Gran Sasso Underground Laboratory. The twenty
absorbers are crystals of TeO2 of 3x3x6 cm3 volume
with a total active mass of about 6.8 kg, the largest in any cryogenic experiment.
Sixteen of these crystals are made of natural telluride. Two contain tellurium
isotopically enriched at 82.3 % in 128Te and other two at 75.0
% in 130Te.
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Simplified scheme of the two detector
setups of the MiDBD experiment.
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The temperature sensors are Neutron Transmutation
Doped (NTD) Ge thermistors. They were specifically prepared in order to present
similar thermal performance. 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. The tower
is connected via an OFHC copper cold finger to the mixing chamber (coldest
point) of a dilution refrigerator specially constructed with previously tested
low radioactivity materials. The entire setup 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 refrigerator is surrounded by a Plexiglas anti-radon box fluxed with
clean N2 from a liquid nitrogen evaporator, and by a Faraday cage
to eliminate electromagnetic interference.
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Scheme of the experimental setup of
the MiDBD experiment @ LNGS (Hall A)
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Experiment runs were carried out in two separate runs. A series of improvements
were implemented for the latter one: 1) surface treatment of all crystal
and mounting structure copper surfaces in order to reduce the radioactive
contaminations introduced in the preparation processes; 2) more compact assembling
of the crystals; 3) elastic suspension of the tower by means of a steel spring,
in order to reduce vibrations.
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. In both runs the array was cooled down to temperatures
around 8 mK with a temperature spread of ~1 mK The detectors were calibrated
by means of regular exposures to a combined radioactive source of 238U
and 232Th placed just outside the dilution refrigerator. The detectors
FWHM energy resolutions at the 2615 keV line of 208Tl range from
5 to 15 keV.
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Sum calibration spectrum obtained
exposing the array detectors to a combined radioactive source
of 238U and 232Th placed just outside the
dilution refrigerator.
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Results
The two set-up have been operated for effective running times of
~31,508 and 5,690 hours x kg, respectively. In all background spectra the main
lines due to the natural activity of the
232Th and
238U
chains, of
40K and the lines at 1173 and at 1332 keV due to cosmogenic
60Co
are present. In addition there are lines at 89 and 248 keV and an excess activity
in the line at 144 keV which can be attributed to
123mTe,
125mTe
and
127mTe produced by activation of tellurium from cosmic rays.
No peak appears in the region of neutrinoless DBD of
130Te, where
the rates are of
0.59 ± 0.06 and
0.33 ± 0.11
counts keV-1 kg-1 year-1 for the former
and latter run, respectively. No peak appear at the energies corresponding
to neutrinoless DBD of
130Te to excited levels of
130Xe,
and at the energy of 867 keV corresponding to neutrinoless DBD of
128Te.
The region above 2000 keV of the total collected spectrum,
corresponding to ~3.56 kg x year of TeO2 and to
~0.98 kg x year of 130Te is shown in Figure. The
clear peaks corresponding to the lines at 2104 keV (single
escape of the 2615 keV 208Tl line ), at 2204 keV
(214Bi) and at 2615 keV ( 208Tl ),
confirms the reproducibility of the array during both runs.
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High energy region
of the background spctrum of the MiDBD experiment
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The 90 % c.l. limits for the various decay processes are
reported in the Table. For what concerns 0nDBD
they correspond to lower limits on <mn> which
are the most stringent after those obtained with Germanium
diodes.
-
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Isotope
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Transition
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T1/2 (y)
|
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130Te
|
0nDBD(0+)
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< 2.1 x 1023
|
|
130Te
|
0nDBD(2+)
|
< 1.4 x 1023
|
|
130Te
|
2nDBD(0+)
|
< 3.8 x 1020
|
|
130Te
|
1cDBD(0+)
|
< 2.2 x 1021
|
|
130Te
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2cDBD(0+)
|
< 0.9 x 1021
|
|
128Te
|
0nDBD(0+)
|
<1.1 x 1023
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