The CUPID-Mo experiment at the Laboratoire Souterrain de Modane (France) is a demonstrator for CUPID, the next-generation ton-scale bolometric 0νββ experiment. It consists of a 4.2 kg array of 20 ...enriched Li_{2}^{100}MoO_{4} scintillating bolometers to search for the lepton-number-violating process of 0νββ decay in ^{100}Mo. With more than one year of operation (^{100}Mo exposure of 1.17 kg×yr for physics data), no event in the region of interest and, hence, no evidence for 0νββ is observed. We report a new limit on the half-life of 0νββ decay in ^{100}Mo of T_{1/2}>1.5×10^{24} yr at 90% C.I. The limit corresponds to an effective Majorana neutrino mass ⟨m_{ββ}⟩<(0.31-0.54) eV, dependent on the nuclear matrix element in the light Majorana neutrino exchange interpretation.
Random coincidences of events could be one of the main sources of background
in the search for neutrino-less double-beta decay of $^{100}$Mo with
macro-bolometers, due to their modest time ...resolution. Scintillating bolometers
as those based on Li$_2$MoO$_4$ crystals and employed in the CROSS and CUPID
experiments can eventually exploit the coincident fast signal detected in a
light detector to reduce this background. However, the scintillation provides a
modest signal-to-noise ratio, making difficult a pile-up pulse-shape
recognition and rejection at timescales shorter than a few ms.
Neganov-Trofimov-Luke assisted light detectors (NTL-LDs) offer the possibility
to effectively increase the signal-to-noise ratio, preserving a fast
time-response, and enhance the capability of pile-up rejection via pulse shape
analysis. In this article we present: a) an experimental work performed with a
Li$_2$MoO$_4$ scintillating bolometer, studied in the framework of the CROSS
experiment, and utilizing a NTL-LD; b) a simulation method to reproduce,
synthetically, randomly coincident two-neutrino double-beta decay events; c) a
new analysis method based on a pulse-shape discrimination algorithm capable of
providing high pile-up rejection efficiencies. We finally show how the NTL-LDs
offer a balanced solution between performance and complexity to reach
background index $\sim$$10^{-4}$ counts/keV/kg/year with 280~g Li$_2$MoO$_4$
($^{100}$Mo enriched) bolometers at 3034 keV, the Q-value of the double-beta
decay, and target the goal of a next generation experiment like CUPID.
In preparation to the CROSS $2\beta$ decay experiment, we installed a new
detector suspension with magnetic dumping inside a pulse-tube cryostat of a
dedicated low-background facility at the LSC ...(Spain). The suspension was tested
with two scintillating bolometers based on large-volume 116CdWO4 (CWO-enr) and
Li2MoO4 (LMO) crystals. The former, a reference device, was used for testing
new noise conditions and for comparing bolometric performance of an advanced
Li2MoO4 crystal developed in the framework of the CLYMENE project, in view of
next-generation double-beta decay experiments like CUPID. We cooled down
detectors to 15 mK and achieved high performance for all tested devices. In
particular both CWO-enr and LMO bolometers demonstrated the energy resolution
of 6 keV FWHM for the 2.6 MeV gamma quanta, among the best for thermal
detectors based on such compounds. The baseline noise resolution (FWHM) of the
CWO-enr detector was improved by 2 keV, compared to the best previous
measurement of this detector in the CROSS facility, while the noise of the
Ge-based optical bolometer was improved by a factor 2, to 100 eV FWHM. Despite
of the evident progress in the improving of noise conditions of the set-up, we
see high-frequency harmonics of a pulse-tube induced noise, suggesting a noise
pick-up by cabling. Another Ge light detector was assisted with the signal
amplification exploiting the Neganov-Trofimov-Luke effect, which allowed to
reach 20 eV FWHM noise resolution by applying 60 V electrode bias.
Highly-efficient particle identification was achieved with both detectors,
despite a low scintillation efficiency of the LMO material. The radiopurity
level of the LMO crystal is rather high; only traces of 210Po and 226Ra were
detected (0.1 mBq/kg each), while the 228Th activity is expected to be at least
an order of magnitude lower, as well as a 40K activity is found to be < 6
mBq/kg.
The current experiments searching for neutrinoless double-$\beta$
($0\nu\beta\beta$) decay also collect large statistics of Standard Model
allowed two-neutrino double-$\beta$ ($2\nu\beta\beta$) decay ...events. These can
be used to search for Beyond Standard Model (BSM) physics via $2\nu\beta\beta$
decay spectral distortions. $^{100}$Mo has a natural advantage due to its
relatively short half-life, allowing higher $2\nu\beta\beta$ decay statistics
at equal exposures compared to the other isotopes. We demonstrate the potential
of the dual read-out bolometric technique exploiting a $^{100}$Mo exposure of
1.47 kg $\times$ y, acquired in the CUPID-Mo experiment at the Modane
underground laboratory (France). We set limits on $0\nu\beta\beta$ decays with
the emission of one or more Majorons, on $2\nu\beta\beta$ decay with Lorentz
violation, and $2\nu\beta\beta$ decay with a sterile neutrino emission. In this
analysis, we investigate the systematic uncertainty induced by modeling the
$2\nu\beta\beta$ decay spectral shape parameterized through an improved model,
an effect never considered before. This work motivates searches for BSM
processes in the upcoming CUPID experiment, which will collect the largest
amount of $2\nu\beta\beta$ decay events among the next-generation experiments.
We report on the development of scintillating bolometers based on lithium
molybdate crystals containing molybdenum depleted in the double-$\beta$ active
isotope $^{100}$Mo ...(Li$_2$$^{100\textrm{depl}}$MoO$_4$). We used two
Li$_2$$^{100\textrm{depl}}$MoO$_4$ cubic samples, 45 mm side and 0.28 kg each,
produced following purification and crystallization protocols developed for
double-$\beta$ search experiments with $^{100}$Mo-enriched Li$_2$MoO$_4$
crystals. Bolometric Ge detectors were utilized to register scintillation
photons emitted by the Li$_2$$^{100\textrm{depl}}$MoO$_4$ crystal
scintillators. The measurements were performed in the CROSS cryogenic set-up at
the Canfranc underground laboratory (Spain). We observed that the
Li$_2$$^{100\textrm{depl}}$MoO$_4$ scintillating bolometers are characterized
by excellent spectrometric performance ($\sim$3--6 keV FWHM at 0.24--2.6 MeV
$\gamma$'s), moderate scintillation signal ($\sim$0.3--0.6 keV/MeV depending on
light collection conditions) and high radiopurity ($^{228}$Th and $^{226}$Ra
activities are below a few $\mu$Bq/kg), comparable to the best reported results
of low-temperature detectors based on Li$_2$MoO$_4$ with natural or
$^{100}$Mo-enriched molybdenum content. Prospects of
Li$_2$$^{100\textrm{depl}}$MoO$_4$ bolometers for use in rare-event search
experiments are briefly discussed.
Neutrinoless double beta decay ($0\nu\beta\beta$) is a yet unobserved nuclear
process which would demonstrate Lepton Number violation, a clear evidence of
beyond Standard Model physics. The process ...two neutrino double beta decay
($2\nu\beta\beta)$ is allowed by the Standard Model and has been measured in
numerous experiments. In this letter, we report a measurement of
$2\nu\beta\beta$ decay half-life of $^{100}$Mo to the ground state of
$^{100}$Ru of
$(7.07~\pm~0.02~\text{(stat.)}~\pm~0.11~\text{(syst.)})~\times~10^{18}$~yr by
the CUPID-Mo experiment. With a relative precision of $\pm~1.6$ \% this is the
most precise measurement to date of a $2\nu\beta\beta$ decay rate in
$^{100}$Mo. In addition, we constrain higher-order corrections to the spectral
shape which provides complementary nuclear structure information. We report a
novel measurement of the shape factor $\xi_{3,1}=0.45~\pm 0.03~\text{(stat.)} \
\pm 0.05 \ \text{(syst.)}$, which is compared to theoretical predictions for
different nuclear models. We also extract the first value for the effective
axial vector coupling constant obtained from a spectral shape study of
$2\nu\beta\beta$ decay.
CUPID-Mo, located in the Laboratoire Souterrain de Modane (France), was a
demonstrator for the next generation $0\nu\beta\beta$ decay experiment, CUPID.
It consisted of an array of 20 enriched ...Li$_{2}$$ ^{100}$MoO$_4$ bolometers and
20 Ge light detectors and has demonstrated that the technology of scintillating
bolometers with particle identification capabilities is mature. Furthermore,
CUPID-Mo can inform and validate the background prediction for CUPID. In this
paper, we present a detailed model of the CUPID-Mo backgrounds. This model is
able to describe well the features of the experimental data and enables studies
of the $2\nu\beta\beta$ decay and other processes with high precision. We also
measure the radio-purity of the Li$_{2}$$^{100}$MoO$_4$ crystals which are
found to be sufficient for the CUPID goals. Finally, we also obtain a
background index in the region of interest of
3.7$^{+0.9}_{-0.8}$(stat)$^{+1.5}_{-0.7}$(syst)$\times10^{-3}$counts/$\Delta$E$_{FWHM}$/mol$_{iso}$/yr,
the lowest in a bolometric $0\nu\beta\beta$ decay experiment.
The CUPID-Mo experiment, located at Laboratoire Souterrain de Modane (France), was a demonstrator experiment for CUPID. It consisted of an array of 20 Li\(_2^{100}\)MoO\(_4\) (LMO) calorimeters each ...equipped with a Ge light detector (LD) for particle identification. In this work, we present the result of a search for two-neutrino and neutrinoless double beta decays of \(^{100}\)Mo to the first 0\(^+\) and \(2^+\) excited states of \(^{100}\)Ru using the full CUPID-Mo exposure (2.71 kg\(\times\)yr of LMO). We measure the half-life of \(2\nu\beta\beta\) decay to the \(0^{+}_1\) state as \(T_{1/2}^{2\nu \rightarrow 0_1^+}=7.5\pm 0.8 \ \text{(stat.)} \ ^{+ 0.4}_{-0.3} \ \text{(syst.)} )\times 10^{20} \ \mathrm{yr}\). The bolometric technique enables measurement of the electron energies as well as the gamma rays from nuclear de-excitation and this allows us to set new limits on the two-neutrino decay to the \(2_1^+\) state of \(T^{2\nu \rightarrow 2_1^+}_{1/2}>4.4\times 10^{21} \ \mathrm{yr} \ \text{(90 % c.i.)}\) and on the neutrinoless modes of \(T_{1/2}^{0\nu\rightarrow 2_1^+}>2.1\times10^{23} \ \mathrm{yr}\ \text{(90 % c.i.)}\), \(T_{1/2}^{0\nu\rightarrow 0_1^+}>1.2\times10^{23} \ \mathrm{yr}\ \text{(90 % c.i.)}\). Information on the electrons spectral shape is obtained which allows us to make the first comparison of the single state (SSD) and higher state (HSD) \(2\nu\beta\beta\) decay models for the \(0_1^+\) excited state of \(^{100}\)Ru.
Appl. Phys. Lett. 118 (2021) 184105 Phonon-mediated particle detectors based on single crystals and operated at
millikelvin temperatures are used in rare-event experiments for neutrino
physics and ...dark-matter searches. In general, these devices are not sensitive
to the particle impact point, especially if the detection is mediated by
thermal phonons. In this Letter, we demonstrate that excellent discrimination
between interior and surface $\beta$ and $\alpha$ events can be achieved by
coating a crystal face with a thin metallic film, either continuous or in the
form of a grid. The coating affects the phonon energy down-conversion cascade
that follows the particle interaction, leading to a modified signal shape for
close-to-film events. An efficient identification of surface events was
demonstrated with detectors based on a rectangular $20 \times 20 \times 10$
mm$^3$ Li$_2$MoO$_4$ crystal coated with a Pd normal-metal film (10~nm thick)
and with Al-Pd superconductive bi-layers (100~nm-10~nm thick) on a $20 \times
20$ mm$^2$ face. Discrimination capabilities were tested with $^{238}$U sources
emitting both $\alpha$ and $\beta$ particles. Surface events are identified for
energy depositions down to millimeter-scale depths from the coated surface.
With this technology, a substantial improvement of the background figure can be
achieved in experiments searching for neutrinoless double-beta decay.
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