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Eur. Phys. J. C (2020) 80 :44 Page 3 of 15 44
The present work describes the CUPID-Mo experimental
setup, currently operating in the EDELWEISS-III [30,31]
cryostat at LSM. The detector was constructed in the clean
rooms of the Laboratoire de l’Accélélarateur Linéaire (LAL)
and the Centre de Sciences Nucléaires et de Sciences de la
Matiére (CSNSM, Orsay, France) in the fall of 2017 and then
moved to LSM and installed in the cryostat in January 2018.
The detector was successfully operated through the summer
of 2018 (Commissioning I). The fall of 2018 was devoted to
cryostat maintenance, after a severe cryogenic failure, and
detector upgrades. After optimization of the cryogenic sys-
tem and detectors over the winter of 2019 (Commissioning
II), the experiment has been collecting data in a stable con-
figuration since the end of March 2019 (Physics run). In this
paper we present the CUPID-Mo detector concept and con-
struction (Sect. 2), the operation and initial performance of
the first Physics run dataset (Sect. 3), and the prospects of
the experiment in 0νββ decay search (Sect. 4).
2 Experimental setup
CUPID-Mo consists of an array of 20 scintillating bolometer
modules arranged in five towers, each with four modules, as
showninFig.1. Each module contains one Li
2
100
MoO
4
crys-
tal and one germanium wafer assembled inside a single-piece
copper housing, instrumented with Neutron Transmutation
Doped (NTD) Ge thermistors. All the materials used for the
towers’ construction were carefully selected, and addition-
ally cleaned as needed to minimize radioactive contamina-
tion. The detector construction, transportation, and assembly
into the underground cryogenic facility were performed in a
clean environment. The key ingredients of the detector, its
assembly, and the cryogenic apparatus are detailed below.
Fig. 1 Rendering of a CUPID-Mo single detector module (left)
designed to hold a Li
2
100
MoO
4
scintillating element. A module is
comprised of a crystal of size 44 × 45 mm and a Ge wafer of
44 × 0.175 mm. The full 20-detector bolometric array is arranged
in five suspended towers containing four detector modules each (right)
2.1 Li
2
100
MoO
4
crystals
CUPID-Mo operates the four existing Li
2
100
MoO
4
crystals
previously used in LUMINEU [28,29]. An additional sixteen
new Li
2
100
MoO
4
crystals were fabricated with the identical
procedure as that employed by LUMINEU [28,32,33]. All
crystals have a cylindrical shape with ∼ 44 mm diameter and
∼ 45 mm height, and a mass of ∼ 0.2 kg. The crystals were
produced at the Nikolaev Institute of Inorganic Chemistry
(NIIC, Novosibirsk, Russia) as follows:
• purification of the ∼ 97% enriched molybdenum [32],
previously used in the NEMO-3 experiment [34];
• selection of lithium carbonate with low U/Th and
40
K
content [28] and purified
100
Mo oxide [33];
• crystal growth via a double crystallization process using
the low-thermal-gradient Czochralski technique [28,33];
• slicing of the scintillation elements, and treatment of their
surfaces with radio-pure SiO powder.
The total mass ofthe20Li
2
100
MoO
4
crystals used in CUPID-
Mo is 4.158 kg, corresponding to 2.264 kg of
100
Mo.
2.2 Ge slabs
The high-purity Ge wafers (Umicore Electro-Optical Mate-
rial, Geel, Belgium), used as absorbers for the scintillation
light, have a diameter of 44.5 mm and a 175 µm thickness.
A ∼ 70 nm SiO coating was evaporated on both sides of the
Ge wafer to make them opaque, thus increasing the light col-
lection by ∼ 35% [35]. A small part of the wafer surface was
left uncoated to ease the gluing of a temperature sensor.
2.3 Sensors
CUPID-Mo employs NTD Ge thermistors [36] as thermal
sensors. These thermistors were provided by the Lawrence
Berkeley National Laboratory (LBNL, Berkeley, USA) and
come from a single production batch. The NTDs used for
the Li
2
MoO
4
(LMO) bolometers are 3.0 × 3.0 × 1.0mm
3
in dimension, and have a temperature-dependent resistance
given by R = R
0
· e
(T
0
/T )
0.5
where the average values for the
parameters are T
0
= 3.8 K and R
0
= 1.5 . Given the lower
heat capacity of the Ge absorbers for the bolometric light
detector (LD), we opted to better match and reduce the heat
capacity of their sensors by dicing the NTDs into multiple
pieces.
In Commissioning I, we produced three sensors with
3.0 × 0.8 × 0.4mm
3
dimensions from the slicing of a single
NTD in two directions. The LDs with these sensors showed
an unexpectedlyhighnoise with a strong 1/ f [Hz] component
reaching frequencies up to hundreds of Hz. For Commission-
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