# Phonon-mediated crystal detectors with metallic film coating capable of
rejecting $\alpha$ and $\beta$ events induced by surface radioactivity

I.C. Bandac  Laboratorio Subterráneo de Canfranc, 22880 Canfranc-Estación,
Spain  A.S. Barabash  National Research Centre Kurchatov Institute, Institute
of Theoretical and Experimental Physics, 117218 Moscow, Russia  L. Bergé  Ch.
Bourgeois  Université Paris-Saclay, CNRS/IN2P3, IJCLab, 91405 Orsay, France
J.M. Calvo-Mozota  Laboratorio Subterráneo de Canfranc, 22880 Canfranc-
Estación, Spain  P. Carniti  INFN, Sezione di Milano Bicocca, I-20126 Milano,
Italy  M. Chapellier  Université Paris-Saclay, CNRS/IN2P3, IJCLab, 91405
Orsay, France  M. de Combarieu  IRFU, CEA, Université Paris-Saclay, F-91191
Gif-sur-Yvette, France  I. Dafinei  INFN, Sezione di Roma, I-00185, Rome,
Italy  F.A. Danevich  Institute for Nuclear Research of NASU, 03028 Kyiv,
Ukraine  L. Dumoulin  Université Paris-Saclay, CNRS/IN2P3, IJCLab, 91405
Orsay, France  F. Ferri  IRFU, CEA, Université Paris-Saclay, F-91191 Gif-sur-
Yvette, France  A. Giuliani<EMAIL_ADDRESS>Université
Paris-Saclay, CNRS/IN2P3, IJCLab, 91405 Orsay, France  C. Gotti  INFN, Sezione
di Milano Bicocca, I-20126 Milano, Italy  Ph. Gras  IRFU, CEA, Université
Paris-Saclay, F-91191 Gif-sur-Yvette, France  E. Guerard  Université Paris-
Saclay, CNRS/IN2P3, IJCLab, 91405 Orsay, France  A. Ianni  INFN, Laboratori
Nazionali del Gran Sasso, I-67100 Assergi (AQ), Italy  H. Khalife  Université
Paris-Saclay, CNRS/IN2P3, IJCLab, 91405 Orsay, France  S.I. Konovalov
National Research Centre Kurchatov Institute, Institute of Theoretical and
Experimental Physics, 117218 Moscow, Russia  P. Loaiza  M. Madhukuttan  P. de
Marcillac  R. Mariam  S. Marnieros  C.A. Marrache-Kikuchi  Université Paris-
Saclay, CNRS/IN2P3, IJCLab, 91405 Orsay, France  M. Martinez  Fundación ARAID
& Centro de Astropartículas y Física de Altas Energías, Universidad de
Zaragoza, Zaragoza 50009, Spain  C. Nones  IRFU, CEA, Université Paris-Saclay,
F-91191 Gif-sur-Yvette, France  E. Olivieri  Université Paris-Saclay,
CNRS/IN2P3, IJCLab, 91405 Orsay, France  G. Pessina  INFN, Sezione di Milano
Bicocca, I-20126 Milano, Italy  D.V. Poda  Th. Redon  J.-A. Scarpaci
Université Paris-Saclay, CNRS/IN2P3, IJCLab, 91405 Orsay, France  V.I. Tretyak
Institute for Nuclear Research of NASU, 03028 Kyiv, Ukraine  V.I. Umatov
National Research Centre Kurchatov Institute, Institute of Theoretical and
Experimental Physics, 117218 Moscow, Russia  M.M. Zarytskyy  Institute for
Nuclear Research of NASU, 03028 Kyiv, Ukraine  A.S. Zolotarova  Université
Paris-Saclay, CNRS/IN2P3, IJCLab, 91405 Orsay, France

###### Abstract

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 $20\times 20\times 10$ mm3
Li2MoO4 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 $20\times 20$ mm2 face.
Discrimination capabilities were tested with 238U 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 reduction of the background level can be achieved in
experiments searching for neutrinoless double-beta decay.

###### pacs:

07.20.Mc, 63.20.-e, 68.65.Ac, 14.60.Pq

Phonon-mediated particle detectors Enss and McCammon (2008) (often defined
“bolometers”) have nowadays important applications in neutrino physics,
Giuliani (2012); Nucciotti (2014) dark-matter searches Pirro and Mauskopf
(2017) and rare nuclear decay investigations. Belli  _et al._ (2019) They also
provide outstanding $\alpha$, $\beta$, $\gamma$, X-ray and neutron
spectroscopy. Enss and McCammon (2008); Pirro and Mauskopf (2017); Belli  _et
al._ (2019); Bekker  _et al._ (2016)

Neutrinoless double-beta ($0\nu 2\beta$) decay Dolinski, Poon, and Rodejohann
(2019) is a hypothetical rare nuclear transition of an even-even nucleus to an
isobar with two more protons, with the emission of just two electrons. Its
observation would provide a unique insight into neutrino physics. Vergados,
Ejiri, and Šimkovic (2016) Bolometers based on Li2MoO4 crystals are promising
detectors for a next-generation $0\nu 2\beta$ decay experiment. Bekker  _et
al._ (2016); Armengaud  _et al._ (2017a, 2020a) They embed the favorable
candidate 100Mo, maximising the detection efficiency. The $0\nu 2\beta$ decay
signature is a peak in the sum energy spectrum of the two emitted electrons,
expected at 3.034 MeV for 100Mo. In a bolometer, the energy deposited by a
particle in the crystal is converted into phonons, which are then detected by
a suitable sensor.

The highest challenge in $0\nu 2\beta$ decay search is the control of the
radioactive background, due to the long expected lifetime of the process
($>10^{25}-10^{26}$ y). Gando  _et al._ (2016); Agostini  _et al._ (2020);
Adams  _et al._ (2020) The experiments are located underground under heavy
shielding. To reduce the current background level of bolometric experiments,
it is mandatory to reject $\alpha$ or $\beta$ events — defined “surface
$\alpha$’s or $\beta$’s” in the following for brevity — induced by radioactive
impurities located either close to the surface of the crystal itself or to
that of the surrounding structure.Artusa  _et al._ (2014); Alduino  _et al._
(2017) Surface $\alpha$’s can be rejected in scintillating materials — such as
Li2MoO4 — by detecting simultaneously scintillation and phonons for the same
event Pirro  _et al._ (2006); Artusa  _et al._ (2014); Poda and Giuliani
(2017) and exploiting the generally lower light yield of $\alpha$’s with
respect to $\beta$’s, Tretyak (2010) but the rejection of surface $\beta$’s
requires dedicated techniques capable of tagging surface events in bolometers.
Foggetta  _et al._ (2005); Marnieros  _et al._ (2008); Nones (2010); Nones
_et al._ (2012); Agnese  _et al._ (2013)

In this letter, we report an effective method to identify both surface
$\alpha$’s and $\beta$’s in Li2MoO4 bolometers. The discrimination is achieved
by coating a Li2MoO4 crystal side with a metallic film acting as a pulse-shape
modifier for events that release energy close to the coated face. When an
ionizing event occurs in a dielectric crystal kept at mK temperature, the
deposited energy is readily converted to athermal phonons with typical
energies of the order of tens of meV, to be compared with the few $\mu$eV
thermal-bath energy. The energy down-conversion of these athermal phonons
occurs mainly by anharmonic decay and is progressively slowing down, as the
phonon lifetime scales as the fifth power of the energy. Orbach and Vredevoe
(1964); Bron, Levinson, and O’Connor (1982) If a sensor sensitive mainly to
thermal phonons is used (as in this work), the rise time of the signal is in
the $\sim 10$ ms range, which corresponds to the typical thermalization time
of the deposited energy. However, thermalization can speed up via a metallic
film covering a crystal side. If the particle is absorbed close to the film, a
significant fraction of its energy is trapped in the metal in the form of hot
electrons, excited by the absorption of the particle-generated athermal
phonons. The energy is quickly thermalised in the electron system, so that
phonons of much lower energies are re-injected in the crystal from the film.
Signals from events occurring close to the film will present therefore a
shorter rise time and a modified time evolution. We show here that surface
events can be tagged according to this approach.

All the detectors in this work share a common basic structure, which is
similar to that used in the $0\nu 2\beta$ experiments CUORE, Adams  _et al._
(2020) LUMINEU, Armengaud  _et al._ (2017a) CUPID-Mo, Armengaud  _et al._
(2020a, b) CUPID-0 Azzolini  _et al._ (2019) and in the dark-matter experiment
EDELWEISS Armengaud  _et al._ (2017b) as far as the phonon readout is
concerned. The surface sensitivity was studied above ground with prototypes of
reduced size with respect to the final $0\nu 2\beta$ bolometers. The energy
absorber of the bolometers described here is a single Li2MoO4 crystal
Grigorieva  _et al._ (2017) with a size of $20\times 20\times 10$ mm3 and a
mass of $\sim 12$ g. All the tests involve just a single $20\times 20$ mm2
coated side.

Figure 1:  Scheme (left) and photograph (right) of the detector assembly with
a bare Li2MoO4 crystal. A Ge thermistor and a Si:P heater (used to stabilize
the bolometric response) are glued on the upper face of the crystal, which is
held by polytetrafluoroethylene (PTFE) elements, not shown in the scheme. A
uranium source — visible in transparency in the photograph — is placed below
the crystal. A bolometric light detector — removed to take the photograph —
faces the upper side of the crystal. The reflecting foil forms an optical
cavity that aids light collection.

The phonon sensor is a neutron transmutation doped Ge thermistor Haller  _et
al._ (1984) (NTD) with a size of $3\times 3\times 1$ mm3. Its resistivity
increases exponentially as the temperature decreases. Efros and Shklovskii
(1975) The NTD is glued on the crystal by means a two-component epoxy. The
glue provides a slow transmission interface, making the NTD sensitive mainly
to thermal phonons.

We used uranium radioactive sources to test the detector surface sensitivity.
They were obtained by drying up a drop of uranium acid solution on a copper
foil. These sources provide two main $\alpha$ lines at $\sim 4.2$ and $\sim
4.7$ MeV from 238U and 234U respectively, affected by a significant straggling
due to partial $\alpha$ absorption in the source residues and/or in the copper
substrate. 238U disintegration is followed by two consecutive $\beta$
emissions, from 234Th (with a half-life of 24.1 d and an end-point of 0.27
MeV) and from 234mPa (with a half-life of 1.2 min and an end-point of 2.27
MeV). The 238U $\alpha$ rate and the 234mPa $\beta$ rate are extremely close.

Figure 2:  Particle identification obtained by a Li2MoO4 detector with a
10-nm-thick Pd coating (see inset on the left) exposed to a uranium source.
Left: The pulse-shape parameter $m/S_{m}$ is plotted as a function of the heat
energy (estimated by $S_{m}$) deposited in the Li2MoO4 crystal, with a
$\gamma$-based calibration. Surface events appear as a population with lower
$m/S_{m}$ values. They are selected by visual inspection and highlighted in
red. The neutron-capture line from the reaction 6Li(n,t)$\alpha$ lays in the
interior-event band. The $\alpha$ particles are mis-calibrated by about $20$ %
due to both pulse-shape effects and intrinsic different responses for
$\alpha$’s and $\beta$/$\gamma$’s. Right: The Li2MoO4 scintillation light
yield (LY) is plotted for the same pulses on the same energy scale. The LY is
expressed by the ratio of the energy deposited in the light detector by
scintillation photons (in keV) to that deposited in the Li2MoO4 crystal as
heat (in MeV), for the same event. The same surface events highlighted in the
left panel are shown in red. Surface $\beta$’s lay in the high-LY band, while
$\alpha$’s and the neutron capture events are well separated in the low-LY
band.

The detector assembly is shown in Fig. 1. A first test was conducted with a
Li2MoO4 crystal without coating to establish the bare detector performance.
All subsequent tests have adopted the configuration shown in Fig. 1, where the
metal-coated side, which is optically polished before the film deposition,
faces the radioactive source. A bolometric light detector based on a Ge wafer
Armengaud  _et al._ (2017a, 2020a) is used to separate $\alpha$ from
$\beta$/$\gamma$/muon events by detecting the scintillation light. Pirro  _et
al._ (2006); Artusa  _et al._ (2014); Poda and Giuliani (2017)

The detectors were cooled down in a dilution refrigerator located in IJCLab,
Orsay, France. Mancuso  _et al._ (2014) All the data discussed here have been
collected with the detector thermalized to a copper plate at $\sim 22$ mK
(Fig. 1). A current of the order of $\sim 5$ nA is injected in the NTD, rising
the detector temperature to about $\sim 25$ mK, at which the NTD resistance is
about 0.5 M$\Omega$. The voltage signal amplitude across the NTD is $\sim 60$
$\mu$V/MeV for the bare crystal, corresponding to an NTD temperature change of
$\sim 0.5$ mK/MeV. The pulse rise time (from 10% to 90% of maximum amplitude)
is typically in the 3–10 ms range and the pulse decay time (from 90% to 30% of
maximum amplitude) is tens of ms. The signals are read out by a DC-coupled
low-noise voltage-sensitive amplifier. Arnaboldi  _et al._ (2002) In all the
tests, the Li2MoO4 detector is energy-calibrated using $\gamma$ peaks of the
environmental background, and the light detector using the cosmic muons
crossing the Ge wafer Novati  _et al._ (2019). In the Li2MoO4 heat channel, we
obtained routinely good energy resolutions of 5–10 keV FWHM for environmental
$\gamma$ peaks in the 0.2–1 MeV region.

The first test to attain surface sensitivity was performed with a
10-$\mu$m-thick Al-film coating. The details of the achieved results are
reported elsewhere. Bandac  _et al._ (2020); Khalife  _et al._ (2020) We
remind here that an excellent separation of surface $\alpha$ particles was
demonstrated thanks to pulse-shape discrimination.

The best separation between surface $\alpha$’s and any type of interior events
was obtained via a specially developed pulse-shape parameter — extensively
used here — that we will designate as $m/S_{m}$. Bandac  _et al._ (2020) To
construct it, the signals are passed through a digital optimal filter, Gatti
and Manfredi (1986) whose transfer function is built using the noise power
spectrum and the pulse shape of an interior event. This filter provides the
best estimator of the signal amplitude $S_{m}$ (i.e. energy). An individual
pulse $S(t)$ is plotted point by point against an average pulse $A(t)$ —
formed from a large sample of interior events and normalized to 1 — obtaining
approximately a straight line. The related slope parameter $m$ is an estimator
of the pulse amplitude as well. The ratio $m/S_{m}$ turns out to be very
sensitive to the pulse shape. Interior events have $m/S_{m}\sim 1$, as
expected. On the contrary, $m/S_{m}$ deviates from 1 for surface $\alpha$
events.

For the Al-coated Li2MoO4 crystal, the separation between the interior and the
surface $\alpha$ events from a uranium source is better than $10\sigma$ in
terms of $m/S_{m}$ distributions. Bandac  _et al._ (2020) Unfortunately, only
a slight hint of separation of the surface $\beta$ events emitted by the same
source was observed, Bandac  _et al._ (2020); Khalife (2021) ruling out Al
coating as a viable method for a complete surface-event tagging.

Aluminum was chosen as it is superconductive at the bolometer operation
temperature, with a critical temperature $T_{C}(\mathrm{Al})\sim 1.2$ K.
Cochran and Mapother (1958) This leads to a negligible contribution to the
heat capacity of the full bolometer, as the electron specific heat of
superconductors vanishes exponentially with the temperature. We remark that
the heat capacity of a bolometer must be as low as possible to achieve high
signal amplitudes. In fact, no deterioration of the detector sensitivity was
observed with respect to the bare Li2MoO4 crystal. However, the behaviour of
superconductors can spoil surface particle tagging. The prompt absorption of
athermal phonons by the film breaks Cooper pairs and forms quasi-particles.
Theoretically, quasi-particle lifetime diverges as the temperature of the
superconductor decreases, Kaplan  _et al._ (1976); Barends  _et al._ (2008)
although it is often experimentally found to saturate at low temperatures. de
Visser  _et al._ (2014); Fyhrie  _et al._ (2020) In aluminum, at very low
temperatures such as ours ($T/T_{C}<0.02$), we expect the quasi-particle
lifetime to be as large as several ms, Schnagl  _et al._ (2000); Baselmans and
Yates (2009); Fyhrie  _et al._ (2020); de Visser  _et al._ (2014) similar to
the thermalization time of interior events. This mechanism competes with the
faster thermalization that should be provided by the film.

Driven by these considerations, we tested a Li2MoO4 bolometer with a normal-
metal coating. At low temperatures, the electron specific heat of normal
metals is proportional to the temperature and tends to dominate over the
crystal heat capacity, which scales as $T^{3}$ according to the Debye law. The
thickness of normal-metal films must be substantially smaller than the
aluminum ones. We chose palladium as a coating material as it can provide
continuous thin films down to 2 nm thickness and no challenging radioactive
isotopes are present in its natural composition. A thickness of 10 nm was
chosen as a good compromise between heat capacity reduction and phonon
absorption probability. The particle-identification results are encouraging,
as shown in Fig. 2: both surface $\alpha$’s and $\beta$’s are well separated
from the interior events.

Unfortunately, the heat capacity of the Pd film Mizutani, Massalski, and Bevk
(1976) competes with that of the Li2MoO4 crystal Musikhin  _et al._ (2015)
affecting seriously the sensitivity of the detector, which was only $\sim 23$
$\mu$V/MeV, about one third of that achieved with the bare crystal. Therefore,
this option is not viable for a full coating of the crystal.

To overcome the heat-capacity problem, we developed a detector coated with an
Al-Pd bi-layer (100 nm and 10 nm thick respectively, with Al on the top),
which is superconducting by proximity effect below $T_{C}$(Al-Pd) $=0.65$ K.
The superconductive gap induced in Pd by the Al film reduces substantially the
Pd specific heat with respect to the normal state. This gap is however low
enough to ensure the fast thermalization of the energy deposited by surface
events. In fact, the surface-event discrimination capability was fully
maintained (see Fig. 3, left). The detector sensitivity was measured to be 43
$\mu$V/MeV, almost doubled with respect to the pure Pd film.

Figure 3:  Particle identification obtained by a Li2MoO4 detector with an Al-
Pd coating exposed to a uranium source. The $\alpha$ events are removed by a
light-yield cut. Left: in a plot of the pulse-shape parameter $m/S_{m}$ versus
energy, the surface events (in red) lay below a black curve defining a
3$\sigma$ acceptance region for the interior events (in blue). The analysis is
carried out in the [0-3000] keV energy interval, which is divided in several
sub-intervals. For each of them, a double Gaussian fit of the $m/S_{m}$
distribution is performed to separate the two populations. An example is
provided in the inset. The black point is located 3$\sigma$ at the left of the
mean of the Gaussian of the interior events. The black curve fits the black
points by a power-law function. Right: Energy spectra (with and without
source) of the surface events selected according to the procedure illustrated
on the left. The live times of the two measurements are normalized. The fit of
the source data accounts for the two simulated $\beta$ contributions of the
uranium source and that of the background.  Figure 4:  Energy spectrum
collected by a Li2MoO4 detector with an Al-Pd coating exposed to a uranium
source after selection of the $\alpha$ events by a light-yield cut with $\sim
100$% efficiency. The spectrum is calibrated using the $\alpha$-line
positions. The measurement is the same that provided the source data shown in
Fig. 3. The straggling can be reproduced by assuming five source components in
copper. Each component is a 6 mm diameter disk with a given thickness. The
active nuclei are assumed to be uniformly distributed in each disk. The exact
source structure is unknown, but our goal here is to set up a phenomenological
model capable of explaining the observed straggling. 238U and 234U are not in
secular equilibrium, as already observed in these types of liquid sources.
Figure 5:  Particle identification obtained by a Li2MoO4 detector with an Al-
Pd grid coating exposed to a uranium source. The event selection is performed
as in Fig. 3, left. In the top inset, the grid-coated crystal is shown. In the
bottom inset, pulses from a surface (red) and an interior (blue) event are
shown, corresponding to a deposited energy of about 1 MeV.

We performed two runs with the bi-layer detector. In the first, a uranium
source was present, while the second was a background measurement in the same
configuration. The trigger rate was $\sim 0.2$ Hz with the source. The
contribution of the source is at the level of $\sim 0.03$ Hz. First, we
developed a method to separate the surface $\beta$ component. The events below
the black curve in the left panel of Fig. 3 — collected in a source run — are
selected as surface events, while those above represent more than 99% of the
interior event population. The same analysis was performed for the background
run. By means of Geant4-based Agostinelli  _et al._ (2003) Monte-Carlo
simulations (using the G4RadioactiveDecay and Decay0 Ponkratenko, Tretyak, and
Zdesenko (2000) event generators), we were then able to confirm that the
surface $\beta$ events isolated at low energies come actually from the
radioactive source. We built a model to predict the $\beta$ spectrum shape
considering the observed $\alpha$ straggling (Fig. 4) and the $\beta$
interactions in the detector. We fitted then the experimental $\beta$ spectrum
using the predicted shape and taking the background into account (right panel
in Fig. 3). The total number of 234mPa decay events returned by the fit — the
only free parameter — is 3526(81). To build the source model, we set a
uranium-source depth profile capable of reproducing the observed $\alpha$
spectrum and the related straggling, as shown in Fig. 4. From the model and
the experimental number of $\alpha$ counts it was possible to predict
independently the expected total number of 234mPa events, which resulted to be
3455(273), in excellent agreement with that deduced from the selection of the
source $\beta$ events. The efficiency in selecting surface $\beta$ events can
be estimated as 102(8)%.

The $\beta$-particle range in Li2MoO4 is of the order of 2 mm at 1 MeV and 4
mm at 2 MeV. Therefore, we can separate events that deposit a significant
amount of energy up to $\sim 4$ mm from the film, well beyond its thickness.
We performed then a last test by replacing the continuous Al-Pd film with an
Al-Pd grid. The width of the grid lines was 70 $\mu$m and the spacing between
each line was 700 $\mu$m (see inset in Fig. 5). The purpose of using a grid is
manifold: (1) further reduction of the heat capacity of the coating; (2)
possibility to extract scintillation light through the coating; (3)
availability of geometrical parameters to possibly tune the discrimination
depth. The grid was tested with another uranium source, prepared with the same
method as the first one, but about twice less intense. The detector with grid
coating can separate surface $\beta$ events (see Fig. 5). The $\beta$
selection efficiency resulted to be 93(10)%, in good agreement with the
continuous-film results. In addition, we measured a discrimination power of
about 4.5$\sigma$ for surface $\alpha$ events using the $m/S_{m}$ parameter.
In terms of detector performance, we observed an almost full recovering of the
detector sensitivity, that was $\sim 51$ $\mu$V/MeV for $\beta$/$\gamma$
events. Therefore, the grid method is currently our protocol for surface event
discrimination.

In conclusion, we have shown that both $\alpha$ and $\beta$ particles absorbed
close to a metal-coated surface of a Li2MoO4 bolometer can be rejected with
high efficiency by pulse-shape discrimination. The prospects of this approach
for $0\nu 2\beta$ searches are promising. In fact, the current background
model of the future $0\nu 2\beta$ experiment CUPID The CUPID Group of Interest
(2019) predicts a background level of 0.1 counts/(tonne y keV). Next-to-next
generation experiments aim to a reduction of an additional factor 10. Since
surface $\beta$ events contribute significantly to the current background
level, a necessary condition to achieve the desired reduction is to reject
them with an efficiency up to 90%. This is achievable with the technique here
described.

This work is supported by the European Commission (Project CROSS, Grant
ERC-2016-ADG, ID 742345). The ITEP group was supported by the Russian
Scientific Foundation (grant No. 18-12-00003). F.A.D., V.I.T. and M.M.T. were
supported in part by the National Research Foundation of Ukraine (grant No.
2020.02/0011). The PhD fellowship of H.K. has been partially funded by the
P2IO LabEx (ANR-10-LABX-0038) managed by the Agence Nationale de la Recherche
(France). The dilution refrigerator used for the tests and installed at IJCLab
(Orsay, France) was donated by the Dipartimento di Scienza e Alta Tecnologia
of the Insubria University (Como, Italy).

The data that support the findings of this study are available from the
corresponding author upon reasonable request.

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