CUORE and CUORE-0 status: toward a next-generation neutrinoless double
beta decay experiment
M. Pedretti25, F. Alessandria,1 R. Ardito,2 D. R. Artusa,3, 4 F. T. Avignone III,3 O. Azzolini,5 M. Balata,4
T. I. Banks,4, 6, 7 G. Bari,8 J. Beeman,9 F. Bellini,10, 11 A. Bersani,12 M. Biassoni,13, 14 T. Bloxham,7 C. Brofferio,13, 14
C. Bucci,4 X. Z. Cai,15 L. Canonica,4 S. Capelli,13, 14 L. Carbone,14 L. Cardani,10, 11 M. Carrettoni,13, 14 N. Casali,4
N. Chott,3 M. Clemenza,13, 14 C. Cosmelli,10, 11 O. Cremonesi,14 R. J. Creswick,3 I. Dafinei,11 A. Dally,16
V. Datskov,14 A. De Biasi,5 M. P. Decowski,6, 7, * M. M. Deninno,8 S. Di Domizio,12, 17 M. L. di Vacri,4 L. Ejzak,16
R. Faccini,10, 11 D. Q. Fang,15 H. A. Farach,3 E. Ferri,13, 14 F. Ferroni,10, 11 E. Fiorini,13, 14 M. A. Franceschi,18
S. J. Freedman,6, 7 B. K. Fujikawa,7 A. Giachero,14 L. Gironi,13, 14 A. Giuliani,19 J. Goett,4 P. Gorla,20 C. Gotti,13, 14
E. Guardincerri,4, 7, † T. D. Gutierrez,21 E. E. Haller,9, 22 K. Han,7 K. M. Heeger,16 H. Z. Huang,23 R. Kadel,24
K. Kazkaz,25 G. Keppel,5 L. Kogler,6, 7, ‡ Yu. G. Kolomensky,6, 24 D. Lenz,16 Y. L. Li,15 C. Ligi,18 X. Liu,23
Y. G. Ma,15 C. Maiano,13, 14 M. Maino,13, 14 M. Martinez,26 R. H. Maruyama,16 N. Moggi,8 S. Morganti,11
T. Napolitano,18 S. Newman,3, 4 S. Nisi,4 C. Nones,27 E. B. Norman,25, 28 A. Nucciotti,13, 14 F. Orio,11 D. Orlandi,4
J. L. Ouellet,6, 7 M. Pallavicini,12, 17 V. Palmieri,5 L. Pattavina,14 M. Pavan,13, 14 G. Pessina,14 S. Pirro,14
E. Previtali,14 V. Rampazzo,5 F. Rimondi,8, 29, § C. Rosenfeld ,3 C. Rusconi,14 S. Sangiorgio,25 N. D. Scielzo,25
M. Sisti,13, 14 A. R. Smith,30 F. Stivanello,5 L. Taffarello,31 M. Tenconi,19 W. D. Tian,15 C. Tomei,11
S. Trentalange,23 G. Ventura,32, 33 M. Vignati,11 B. S. Wang,25, 28 H. W. Wang,15 C. A. Whitten Jr.,23, § T. Wise,16
A. Woodcraft,34 L. Zanotti,13, 14 C. Zarra,4 B. X. Zhu,23 S. Zucchelli8, 29
a
Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA
1
INFN - Sezione di Milano, Milano I-20133 - Italy
2
Dipartimento di Ingegneria Strutturale, Politecnico di Milano, Milano I-20133 - Italy
3
Department of Physics and Astronomy, University of South Carolina, Columbia, SC 29208 - USA
4
INFN - Laboratori Nazionali del Gran Sasso, Assergi (L'Aquila) I-67010 - Italy
5
INFN - Laboratori Nazionali di Legnaro, Legnaro (Padova) I-35020 - Italy
6
Department of Physics, University of California, Berkeley, CA 94720 - USA
7
Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 - USA
8
INFN - Sezione di Bologna, Bologna I-40127 - Italy
9
Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 - USA
10
Dipartimento di Fisica, Sapienza Università di Roma, Roma I-00185 - Italy
11
INFN - Sezione di Roma, Roma I-00185 - Italy
12
INFN - Sezione di Genova, Genova I-16146 - Italy
13
Dipartimento di Fisica, Università di Milano-Bicocca, Milano I-20126 - Italy
14
INFN - Sezione di Milano Bicocca, Milano I-20126 - Italy
15
Shanghai Institute of Applied Physics (Chinese Academy of Sciences), Shanghai 201800 - China
16
Department of Physics, University of Wisconsin, Madison, WI 53706 - USA
17
Dipartimento di Fisica, Università di Genova, Genova I-16146 - Italy
18
INFN - Laboratori Nazionali di Frascati, Frascati (Roma) I-00044 - Italy
19
Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse, 91405 Orsay Campus - France
20
INFN - Sezione di Roma Tor Vergata, Roma I-00133 - Italy
21
Physics Department, California Polytechnic State University, San Luis Obispo, CA 93407 - USA
22
Department of Materials Science and Engineering, University of California, Berkeley, CA 94720 - USA
23
Department of Physics and Astronomy, University of California, Los Angeles, CA 90095 - USA
24
Physics Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 - USA
25
Lawrence Livermore National Laboratory, Livermore, CA 94550 - USA
26
Laboratorio de Fisica Nuclear y Astroparticulas, Universidad de Zaragoza, Zaragoza 50009 - Spain
27
Service de Physique des Particules, CEA / Saclay, 91191 Gif-sur-Yvette - France
28
Department of Nuclear Engineering, University of California, Berkeley, CA 94720 - USA
29
Dipartimento di Fisica, Università di Bologna, Bologna I-40127 - Italy
30
EH&S Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 - USA
31
INFN - Sezione di Padova, Padova I-35131 - Italy
32
Dipartimento di Fisica, Università di Firenze, Firenze I-50125 - Italy
33
INFN - Sezione di Firenze, Firenze I-50125 - Italy
SUPA, Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ - UK
*
Presently at: Nikhef, 1098 XG Amsterdam - The Netherlands
†
Presently at: Los Alamos National Laboratory, Los Alamos, NM 87545 - USA
‡
Presently at: Sandia National Laboratories, Livermore, CA 94551 - USA
§
Deceased
34
Abstract
The Cryogenic Underground Observatory for Rare Events (CUORE) experiment will search for the neutrinoless double beta
decay of 130Te and other rare events. The first step towards the experiment is CUORE-0, a single CUORE-like tower that will
operate in the former CUORICINO cryostat. CUORE-0 will validate the assembly procedure and serve as a sensitive
experiment in its own right. The status of CUORE and CUORE-0 and their sensitivities are reported.
Keywords: Neutrinoless double beta decay, neutrino mass, Majorana particle, bolometer.
1. Introduction: CUORE and CUORICINO
Neutrinoless double beta decay (0νDBD) is a rare
nuclear transition that is not allowed within the
Standard Model framework. Its discovery would
indicate the Majorana nature of neutrinos, support the
“seesaw mechanism” scenario for the explanation of
the matter-antimatter asymmetry in the universe, and
provide information on the hierarchy of the neutrino
masses. The 0νDBD signature is a peak at the decay
transition energy (Q value) in the spectrum of the
total energy of the emitted electrons. [1]
CUORE is a bolometric experiment searching for
the neutrinoless double beta decay of 130Te [2]. The
0νDBD Q value for this nucleus is 2527 keV [3], [4],
[5]. In the bolometric technique, the energy deposited
by a particle is measured as a temperature increase in
the crystal absorber, where particles interact. In
CUORE, each bolometer will use a cubic 5×5×5-cm3
TeO2 crystal with a mass of about 750 g as the
absorber. In this way, the detector serves also as the
source, achieving high efficiency (~ 87%) and good
energy resolution. The temperature variations of the
crystal are detected as resistance changes of the
thermistor. The thermistor is a neutron transmutation
doped (NTD) Ge semiconductor, which exhibits an
exponential increase of the resistance at low
temperatures. The crystals are held and thermally
coupled to a heat sink – a copper structure cooled
down to about 10 mK – by means of eight
polytetrafluoroethylene (PTFE) supports. Typical
signals in the thermistor correspond to about 100
μV/MeV of deposited energy. An example of TeO2
bolometric detectors is shown in Fig. 1.
The CUORE detector will be composed of 988
natural TeO2 bolometers for a total mass of 741 kg.
As the natural abundance of 130Te is ~ 34%, the total
amount of 0νDBD active mass is about 206 kg. The
detectors will be arranged in 19 towers of 13 layers
each. The towers will be placed in a roughly
cylindrical compact configuration in a new low
radioactivity custom dilution refrigerator, to be
commissioned and installed in the Gran Sasso
Underground Laboratory, in Italy.
Fig. 1 A photo of four single-module bolometers used in the
CUORICINO experiment.
A demonstrator experiment, CUORICINO, was
operated in the same laboratory in 2003 - 2008. The
CUORICINO detector was composed of 62 TeO2
bolometers, for a total mass of 40.7 kg. The acquired
statistics was 19.75 kg (130Te) × y, and no peak
indicative of 0νDBD signal was found. The final
background spectrum obtained in CUORICINO in
the region of interest is shown in Fig. 2. The
background level in this region was 0.169 ± 0.006
counts/keV/kg/y and the corresponding lower limit
on the 0νDBD half-life of 130Te is 2.8 × 1024 y (90%
C.L.) [6]. This limit translates to an upper limit on the
neutrino effective Majorana mass ranging from 300
to 710 meV, depending on the nuclear matrix
elements considered in the computation [7], [8], [9].
Fig. 2 The CUORICINO spectrum in the 0νDBD energy region.
The best fit, and the peaks at the 130Te Q value that can be
accommodated by the data (at 68% and 90% confidence levels) are
shown.
2. The CUORE experimental challenges
The aim of future experiments, such as CUORE, is to
probe the inverted hierarchy region of neutrino
masses. To achieve the necessary sensitivity, the key
experimental parameters are i) a large sample of the
nuclei to be studied, ii) good detector energy
resolution, iii) very low radioactive backgrounds and
iv) long live time.
The CUORE Collaboration has already proved
that the bolometric technique is able to provide very
good energy resolutions. In CUORICINO, the
average resolution at the energy of the 208Tl line
(2615 keV), near the region of interest, was 6.3 ± 2.5
keV (full-width at half maximum). The performance
of the bolometers produced for the CUORE R&D has
been improved thanks to a new detector layout that
uses new copper frames and new PTFE crystal
supports, and the target energy resolution is 5 keV
[11].
A challenge related to the detector performance is
the uniformity of their behaviour. In CUORICINO, a
large spread in the pulse shape among the detectors
was observed. This led to complications in the data
analysis that could be even more challenging to deal
with for an experiment like CUORE that has about
1000 detectors. To improve the detector uniformity,
particular care has been devoted to the realization of
a new detector assembly system, as described in 3.1.
2.1. Background
The real challenge for CUORE, common to all
experiments searching for rare events, is the
radioactive background. The CUORICINO results are
very useful for understanding the most dangerous
sources of background for the CUORE experiment.
Using Monte Carlo simulations and experimental
signatures from outside the 0νDBD region, the count
rate in the 0νDBD could be attributed primarily to
three sources:
 30 ± 10%: multi-Compton events caused by the
208
Tl 2614.5-keV gamma radiation from 232Th
contamination of the cryostat or its shields
 10 ± 5%: degraded alpha particles (that lost part
of their energy in a passive part of the detector)
from surface contamination of the TeO2 crystals
with 232Th, 238U, or 210Pb
 50 ± 20%: degraded alpha particles from the
surfaces of inert materials surrounding the
crystals, most likely the copper frames
The degraded alpha-particle events can produce a
flat background that extends from the full alpha
energy peak down to lower energies. CUORE will
benefit from a completely new cryogenic set-up that
has been designed to reduce to negligible levels the
background induced by environmental and material
radioactivity (and in particular, the 2615-keV multiCompton events). The main construction materials
(copper and lead) were selected in order to be able to
reach this goal and the radioactivity of any object
installed in the cryostat has to satisfy specific
radiopurity requirements. The radioactivity of the
detector (and of any other material included near the
detector) will likely determine the final background
of the experiment.
The production of the CUORE crystals began in
March 2008 at the Shanghai Institute of Ceramics,
Chinese Academy of Sciences (SICCAS) and is
nearly complete [10]. A dedicated protocol was
defined for the quality control of the crystal
production process. Upon arrival at LNGS, a few
crystals are randomly selected from each batch and
tested as bolometers [11]. Thus far, these
measurements have given no indication of bulk or
surface contamination from the uranium and thorium
decay chains, or from 210Pb (out of equilibrium). An
extrapolation of these results yield a conservative
upper limit of the CUORE background in the 0νDBD
energy region of 1.1 × 10-4 counts/keV/kg/y for the
bulk contamination and 5.5 × 10-3 counts/keV/kg/y
for surface contamination.
For the copper structure, the copper, produced by
the Norddeutsche Affinerie [12], was selected for the
experiment because of its high RRR parameter
(certified to be higher than 400) — a constraint from
our cryogenic application — and because samples
were found to be extremely radio-pure. HPGe and
neutron-activition analysis (NAA) measurements
limit bulk contamination to less than 5 pg/g of
238
U 0.5 pg/g of 232Th [13]. A dedicated test, the
Three Tower Test (TTT), was undertaken to compare
three different methods used to clean the copper of
the detector tower structure. The copper of the first
tower was wrapped with polyethylene film, the
copper of the second one was treated with an etching
procedure, and the copper of the third one was treated
with a TECM procedure (Tumbler, Electro-polishing,
Chemical-etching, and Magnetron plasma cleaning).
The test demonstrated that surface contamination
levels lower than 7×10−8 Bq/cm2 for 232Th and 238U
and below 9 × 10−7 Bq/cm2for 210Pb can be achieved.
Extrapolating these values to CUORE gives a 90%
C.L. upper limit of 0.02 counts/keV/kg/y in the
0νDBD energy region if the TECM cleaning
procedure is used or 0.03 counts/keV/kg/y if the
copper parts are wrapped in a polyethylene film. The
reason why these are upper limits is that for the
extrapolation it is assumed that the entire contribution
of the 3 - 4 MeV background observed in the TTT is
due to copper surface contamination.
The best measurement of backgrounds from the
PTFE supports also comes from the TTT test. If the
entire 3 - 4 MeV counting rate in the TTT test is
produced by PTFE contamination, then the upper
limit for the CUORE background is equal to 6 × 10 -2
counts/keV/kg/y. Note that the upper limits reported
above for the copper and PTFE contribution are
mutually exclusive. A higher sensitivity measurement
for PTFE is planned in the near future.
The contribution of the bulk contamination of Ge
thermistors is negligible because signals induced by
radioactive impurities in the sensor produce deformed
signals that can be efficiently rejected by pulse shape
analysis.
The background contribution in the region of
interest of other small detector parts is already
constrained
by
Si
surface-barrier
detector
measurements to be negligible.
3. CUORE-0
CUORE-0 consists of a single CUORE-like tower
constructed with the assembly line and procedures
that are intended for CUORE. The CUORE-0
detector will operate in the CUORICINO dilution
refrigerator. The main goal of this project is to
perform a full test of the hardware and procedures
developed in recent years for the CUORE detector. It
will also allow a test of the detector uniformity, a
verification that the radioactive backgrounds are
under control, and an opportunity to optimize the
analysis tools prepared for CUORE. Moreover, it will
be a stand-alone 0νDBD experiment able to improve
the limits achieved by CUORICINO.
The two main steps necessary to realize the
CUORE-0 detector are the sensor-to-crystal
connection and the assembly of the tower. These
operations have been performed inside the CUORE
clean room in the CUORE Hut.
3.1. The new sensor-to-crystal connection system
The connection of the sensors for CUORE
bolometers is achieved with glue spots of about 1mm diameter and 50-m height. The glue used is
Araldite Rapid glue, a bicomponent epoxy with a
short pot-life (3 minutes), whose radioactive
contaminations have been shown to be very low. This
connection is crucial as it drives the detector
performances. Data from CUORICINO and previous
tests indicated that this connection was also source of
the irreproducibility that we observed in our detector
behavior. A new gluing system for the sensor-tocrystal connection has been developed for the
CUORE and CUORE-0 detectors. The three main
goals of the new system are to:
 to obtain a reliable and reproducible sensor-toabsorber thermal and mechanical connection;
 avoid the problem of crystal surface
recontamination. For this reason, all the
operations are performed inside a nitrogen-fluxed
glove box and all the material used have been
carefully selected for radiopurity
 be a fast procedure allowing a high rate of
instrumented crystals
The new semiautomated system (see Fig. 3) was
commissioned during summer 2011 and in September
2011, and the CUORE-0 crystals were successfully
instrumented with their sensors. The gluing process
took about 10 days. Details on each glued sensor
(including photographs) were recorded in a dedicated
database. This information enables the correlation of
possible experimental effects with variations in
gluing parameters and the extraction of possible
information to improve the CUORE detector
construction.
electrical connection of the sensors to the tower
wires, which consist of PEN strips with a copper
deposition forming the wires. This connection is
obtained by bonding four 25-m wires from the
sensor gold pads to the copper pads on the wire strips.
The assembly of the CUORE-0 tower (Fig. 4) was
successfully completed in April 2012.
Fig. 3 Picture of the semiautomated system used to make the
sensor-to-crystal connections during commissioning in the CUORE
clean room. The main components of the system are the
anthropomorphic robot that manipulates the crystals, the cartesian
system that moves the glue dispenser, and the two positioners that
place the sensors at the proper distance from the crystal.
Fig. 4 On the left, a picture of the CUORE-0 detector without
any shielding is shown. The picture on the right shows the
CUORE-0 detector installed under the former CUORICINO
dilution refrigerator.
3.3. CUORE-0 status and sensitivity
3.2. The new tower assembly system
Due to the extraordinarily stringent radiopurity
requirements of the experiment, the CUORE towers
must be assembled following strict protocols and
under extremely clean conditions. This not only
requires that all the assembly be performed inside
glove-boxes flushed with nitrogen gas (to avoid
possible radon contamination), but also that strict
controls are implemented on all materials that come
into contact with the tower components during
assembly. Contact with tools is also kept to a
minimum. The assembly procedures have to be
simple, fast, and reproducible even when carried out
by different people. The assembly line consists of
five separate glove boxes for specialized operations
on the detector components and a main table
containing an elevator platform to facilitate tower
assembly. The assembly also requires making the
Once the CUORE-0 tower was completed, the
detector was moved from the CUORE clean room to
the CUORICINO clean room to install it in the
dilution refrigerator (Fig. 4). In August 2012, the
detector cool down began and the first results from
the CUORE-0 detector should be available shortly.
The CUORE-0 background will be dominated by
the contribution coming from the cryogenic set-up.
This will limit the lowest achievable background to
0.05 counts/keV/kg/y. If the limits for the detector
materials discussed earlier are used, the CUORE-0
detector configuration is taken into account, and the
fact that the anti-coincidence analysis is less efficient
for a single tower, a background level of
approximately 0.11 counts/keV/kg/y can be expected.
A plot of the expected 1σ background-fluctuation
sensitivity of CUORE-0 as a function of live time for
the two possible background levels configuration is
shown in Fig. 5, [14].
The commissioning of the CUORE cryostat
started during summer 2012. The CUORE crystal
production will finish in early 2013. The CUORE
detector assembly is foreseen to end in the first half
of 2014 and the start of data taking is foreseen for the
end of 2014. A plot of the CUORE experimental
sensitivity as a function of the live time and exposure
is shown in Fig. 6. Assuming a background of 10-2
counts/keV/kg/y and a five-year live time, the
CUORE half-life sensitivities at 1σ would be 1.6 ×
1026 years. This would mean a sensitivity to the
effective Majorana neutrino mass ranging between 41
and 95 meV, depending on the nuclear matrix
elements used. In Fig. 7, the expected sensitivity of
CUORE is compared with the preferred values of the
neutrino mass parameters obtained from neutrino
oscillation experiments. The sensitivity of CUORE
will allow the investigation of the upper portion of
the inverted hierarchy region for the neutrino mass
spectrum.
Exposure [kg y]
n
T01/2
[y] 1s Sensitivity
0
5
10
15
20
25
30
35
CUORE-0 - bkg: 0.11 cts/(keV kg y)
1
1.5
2
2.5
3
10-1
Cuoricino exclusion 90% C.L.
76
Ge claim
CUORE 1s sensitivity
D m223<0
10-2
D m223>0
10-3
10-4 -4
10
10-3
10-2
10-1
1
mlightest [eV]
Fig. 7 Plot of the effective Majorana neutrino mass as function
of the lightest neutrino. The CUORICINO result and the expected
CUORE sensitivity are overlaid on the plot that shows the bands
preferred by neutrino oscillation data. Both normal (red) and
inverted (green) hierarchies are shown. See [14] for details on the
oscillation parameter inputs.
This work was supported by the Istituto Nazionale di
Fisica Nucleare (INFN), the Commission of the
European Community under Contract No. HPRNCT- 2002-00322, by the US Department of Energy
under Contract No. DE-AC03-76-SF00098, and DOE
W-7405-Eng-48, and by the National Science
Foundation Grant Nos. PHY-0139294 and PHY0500337.
CUORE-0 - bkg: 0.05 cts/(keV kg y)
0.5
1
40
1025
0
mbb [eV]
4. CUORE status and sensitivity
3.5
4
Live time [y]
References
Fig. 5 CUORE-0 sensitivity as function of live time for two
possible background scenarios.
Exposure [kg y]
n
T01/2
[y] 1s Sensitivity
0
10
200 400 600 800 1000 1200 1400 1600 1800 2000
27
1026
CUORE w/future R&D - bkg: 0.001 cts/(keV kg y)
CUORE - bkg: 0.01 cts/(keV kg y)
0
1
2
3
4
5
6
7
8
9
10
Live time [y]
Fig. 6 CUORE sensitivity as function of the live time for two
possible background scenarios.
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