Development of 100 Mo -containing scintillating bolometers for a high-sensitivity neutrinoless double-beta decay search

Abstract

This paper reports on the development of a technology involving ${}^{100}\text{Mo}$ -enriched scintillating bolometers, compatible with the goals of CUPID, a proposed next-generation bolometric experiment to search for neutrinoless double-beta decay. Large mass ( $\sim 1\text{kg}$ ), high optical quality, radiopure ${}^{100}\text{Mo}$ -containing zinc and lithium molybdate crystals have been produced and used to develop high performance single detector modules based on 0.2-0.4 kg scintillating bolometers. In particular, the energy resolution of the lithium molybdate detectors near the Q-value of the double-beta transition of ${}^{100}\text{Mo}$ (3034 keV) is 4-6 keV FWHM. The rejection of the $\alpha$ -induced dominant background above 2.6 MeV is better than $8\sigma$ . Less than $10\mu \text{Bq/kg}$ activity of ${}^{232}\text{Th}{(}^{228}\text{Th})$ and ${}^{226}\text{Ra}$ in the crystals is ensured by boule recrystallization. The potential of ${}^{100}\text{Mo}$ -enriched scintillating bolometers to perform high sensitivity double-beta decay searches has been demonstrated with only $10\text{kg}\times \text{d}$ exposure: the two neutrino double-beta decay half-life of ${}^{100}\text{Mo}$ has been measured with the up-to-date highest accuracy as $T_{1/2}$ = [6.90 ± 0.15(stat.) ± 0.37(syst.)] $\times {10}^{18}\text{years}$ . Both crystallization and detector technologies favor lithium molybdate, which has been selected for the ongoing construction of the CUPID-0/Mo demonstrator, containing several kg of ${}^{100}\text{Mo}$ .

Fig. 1

Photographs of the first large-mass ${}^{100}\text{Mo}$-enriched scintillators: the $\sim 1.4\text{kg}{\text{Zn}}^{100}{\text{MoO}}_4$ crystal boule with the cut $\sim 0.38\text{kg}$ scintillation element enrZMO-t (top panels), and the $\sim 0.5\text{kg}$ boule of ${\text{Li}}_2^{100}{\text{MoO}}_4$ crystal with the produced $\sim 0.2\text{kg}$ sample enrLMO-t (bottom panels). Both scintillation elements were cut from the top part of the boules. Color and transparency of the enrZMO-t crystal are different from the ones of the boule due to artificial light source and grinded side surface. The photo on the top left panel is reprinted from [37]

Fig. 2

Photographs of a three-spring suspended tower (first column) and two LUMINEU scintillating bolometers (second column): the $334\text{g}{\text{ZnMoO}}_4$ (top photo; ZMO-b, $\oslash 50\times 40\text{mm}$) and the $186\text{g}{\text{Li}}_2^{100}{\text{MoO}}_4$ (bottom photo; enrLMO-t, $\oslash 44\times 40\text{mm}$) bolometers together with two identical $\oslash 44\text{-mm}$ Ge light detectors (third column). A third detector of the tower (bottom in the left photo, not shown in details) is a 0.2 kg Ge bolometer

Fig. 3

The energy spectra of the ${}^{232}\text{Th}\mathit{\gamma }$ source measured by the $\sim 0.4\text{kg}{\text{Zn}}^{100}{\text{MoO}}_4$ (enrZMO-t; dashed histogram) and $0.2\text{kg}{\text{Li}}_2^{100}{\text{MoO}}_4$ (enrLMO-b; solid histogram) bolometers over 64 h and 168 h, respectively, at LNGS (left figure). The energy bin is 5 keV. The 2615 keV peak of the ${}^{208}\text{Tl}\mathit{\gamma }$ quanta accumulated by the detectors is shown in the inset. The energy dependence of the energy resolution of the ZMO-t and enrLMO-b detectors (right figure). The fits to the data by a function FWHM $(\text{keV})=\sqrt{(}{p}_1+p_2\times E_{\mathit{\gamma }}(keV))$ ($p_1$ and $p_2$ are free parameters) are shown by the dashed lines. The parameters of fits are $7.9(5){\text{keV}}^2$ and 0.0282(6) keV for the ZMO-t and $1.6(2){\text{keV}}^2$ and 0.0091(3) keV for the enrLMO-b. The dotted line indicates the $Q_{\mathit{\beta }\mathit{\beta }}$ value of ${}^{100}\text{Mo}$ (3034 keV)

Fig. 4

Scatter plots of light-versus-heat signals of the background data collected with scintillating bolometers based on $334\text{g}{\text{ZnMoO}}_4$ (ZMO-b, top figures), $382\text{g}{\text{Zn}}^{100}{\text{MoO}}_4$ (enrZMO-b, bottom left), and 186 g ${\text{Li}}_2^{100}{\text{MoO}}_4$ (enrLMO-t, bottom right) crystals over 2767, 1300, 593, and 1303 h, respectively. The ${\text{Zn}}^{100}{\text{MoO}}_4$ detector was operated in the CUPID R&D cryostat, while the other data were accumulated in the EDELWEISS-III set-up (the light signals of the latter are in analog-to-digit units, ADU). The heat channels were calibrated with $\mathit{\gamma }$ quanta. The $\mathit{\gamma }$($\mathit{\beta }$) and $\mathit{\alpha }$ events populations are distinguished in color by using the cuts on the heat energy and the light yield parameters (see the text). The particle identification capability of the ${\text{ZnMoO}}_4$ detector affected by vibration noise (top left) was substantially improved in the suspended tower (top right). The features of the $\mathit{\alpha }$ particle populations are discussed in the text

Fig. 5

The light yield distributions of $\mathit{\alpha }$ particles from a smeared $\mathit{\alpha }$ source and $\mathit{\gamma }$($\mathit{\beta }$) events collected by a $379\text{g}{\text{Zn}}^{100}{\text{MoO}}_4$ scintillating bolometer (enrZMO-t) over 593 h of background and 78 h of ${}^{232}\text{Th}$ calibration measurements in the CUPID R&D set-up at LNGS (Italy). The distributions are fitted by Gaussian functions shown by solid lines. The corresponding discrimination power is $D{P}_{\mathit{\alpha }/\mathit{\gamma }(\mathit{\beta })}=7.8$. The intervals containing 99.9% of both event types and $\pm 7$ sigma interval of the $\mathit{\alpha }$ band are also given

Fig. 6

The light-versus-heat data accumulated with the 186 g ${\text{Li}}_2^{100}{\text{MoO}}_4$ scintillating bolometer (enrLMO-t) in the EDELWEISS-III set-up (21–20 mK data) under neutron irradiation of an AmBe neutron source ($\approx 21\text{n/s}$) over 33.5 h. Three populations ascribed to $\mathit{\gamma }$($\mathit{\beta }$)’s, $\mathit{\alpha }{+}^3\text{H}$ events and nuclear recoils due to neutron scattering are well separated. (Inset) The ${}^6\text{Li}$ thermal neutron capture peak, calibrated to the nominal energy of the reaction, together with a Gaussian fit. The energy resolution is $\text{FWHM}=5.9\text{keV}$

Fig. 7

Light-versus-heat scatter-plot obtained in a 20.5 h AmBe ($\sim 100\text{n/s}$) calibration measurement with a 151 g ${\text{Li}}_2{\text{MoO}}_4$ scintillating bolometer (LMO-1; left figure). Rise and decay times as functions of the energy (right figures). The populations of $\mathit{\gamma }(\mathit{\beta })$ and ${}^6\text{Li}(\text{n},\text{t})\mathit{\alpha }$ events used for the evaluation of the discrimination power are marked by black cycles and red triangles, respectively. The calculated discrimination power is 19, 5.4, and 8.1 by means of the light yield, the rise time, and the decay time parameters, respectively

Fig. 8

The background energy spectra measured with the LMO-1 (over 237.5 h), LMO-2 (135 h), and LMO-3 (135 h) scintillating bolometers in the CUPID R&D set-up. The energy bin is 10 keV. The $\mathit{\alpha }$ events in red are selected by the LY parameter (the events of the ${}^{238}\text{U}$ smeared $\mathit{\alpha }$ source for the LMO-2 detector are not shown below 3.25 MeV). An internal potassium contamination of the LMO-1 crystal generates the continuum up to $\approx 1.3\text{MeV}$ and the $\mathit{\gamma }$ de-excitation peak at 1464 keV. The ${}^{208}\text{Tl}$ line visible in the LMO-1 data can be ascribed to the thorium contamination of the set-up. The $\mathit{\beta }$ spectrum of ${}^{234m}\text{Pa}$ in the data of the LMO-2 detector is due to the presence of the smeared ${}^{238}\text{U}\mathit{\alpha }$ source. The $\mathit{\alpha }$ peaks of ${}^{210}\text{Po}$ (common for all the crystals) and ${}^{226}\text{Ra}$ with daughters (in LMO-3) are caused by the contamination of the ${\text{Li}}_2{\text{MoO}}_4$ crystals

Fig. 9

The energy spectra of $\mathit{\alpha }$ events detected by the $0.4\text{kg}{\text{Zn}}^{100}{\text{MoO}}_4$ (left) and the $0.2\text{kg}{\text{Li}}_2^{100}{\text{MoO}}_4$ (right) scintillating bolometers. The energy bin is 20 keV and 10 keV, respectively. The data of enrZMO-t and enrZMO-b (both over 593 h), and enrLMO-b (487 h) detectors were collected in the low-background measurements in the CUPID R&D cryostat at LNGS. The enrLMO-t (1303 h) bolometer has been operated in the EDELWEISS-III set-up at LSM

Fig. 10

The normalized energy spectra of $\mathit{\gamma }(\mathit{\beta }$) events accumulated in low-background measurements with the $334\text{g}{\text{ZnMoO}}_4$ scintillating bolometer in the EDELWEISS-III set-up. A Monte-Carlo-simulated energy spectrum of the $2\mathit{\nu }2\mathit{\beta }$ decay of ${}^{100}\text{Mo}$ with half-life $T_{1/2}=6.90\times {10}^{18}\text{years}$ (measured in the present work, see Sect. 5.4.3) is shown (upper panel). The energy bin is 10 keV. The same data in the 2.5–3.5 MeV energy interval (lower panel)

Fig. 11

The energy spectra of $\mathit{\gamma }(\mathit{\beta }$) events measured by the $\sim 0.4\text{kg}{\text{Zn}}^{100}{\text{MoO}}_4$ (left) and $\sim 0.2\text{kg}{\text{Li}}_2^{100}{\text{MoO}}_4$ (right) scintillating bolometers. The energy bin is 10 keV. The data of both ${\text{Zn}}^{100}{\text{MoO}}_4$ detectors and one ${\text{Li}}_2^{100}{\text{MoO}}_4$ (enrLMO-b) detector were accumulated in the CUPID R&D cryostat (593 and 319 h of data taking, respectively), while the enrLMO-t bolometer was measured in the EDELWEISS-III set-up (over 1303 h). The origin of the most intensive $\mathit{\gamma }$ peaks is marked

Fig. 12

The $\mathit{\gamma }$($\mathit{\beta }$) background spectrum accumulated over 1303 h with the 186 g ${\text{Li}}_2^{100}{\text{MoO}}_4$-based detector (enrLMO-t) in the EDELWEISS-III set-up together with the fit by a simplified background model built from the $2\mathit{\nu }2\mathit{\beta }$ distribution of ${}^{100}\text{Mo}$ ($T_{1/2}=6.9\times {10}^{18}\text{years}$), internal ${}^{40}\text{K}$ ($2.4\text{mBq/kg}$), and external $\mathit{\gamma }$ quanta represented by exponential background (ext $\mathit{\gamma }$), external ${}^{40}\text{K}$ and ${}^{232}\text{Th}$. The $2\mathit{\nu }2\mathit{\beta }$ signal-to-background ratio above 1.5 MeV is 8:1