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Material Screening with Mass Spectrometry
Review

Enriched Crystal Scintillators for 2β Experiments

Institute for Nuclear Research of NASU, 03028 Kyiv, Ukraine
Received: 31 December 2020 / Revised: 27 February 2021 / Accepted: 1 March 2021 / Published: 9 March 2021
(This article belongs to the Special Issue Radiation Spectroscopy with Solid Scintillators for Rare Events)

Abstract

The investigation of 2β decay is an important issue in modern physics, allowing the test of the Standard Model of elementary particles and the study of the nature and properties of neutrinos. The crystal scintillators, especially made of isotopically-enriched materials, are powerful detectors for 2β decay experiments thanks to the high radiopurity level and the possibility to realize the calorimetric “source = detector” approach with a high detection efficiency. For the moment, the 2ν2β processes have been observed at the level of 1019–1024 years with enriched crystals; the sensitivity to the 0ν mode have reached the level of 1024–1026 years in some decay channels for different nuclides allowing one to calculate the upper limits on the effective mass of the Majorana neutrino at the level of 0.1–0.6 eV. The paper is intended to be a review on the latest results to investigate 2β processes with crystal scintillators enriched in 48Ca, 106Cd, and 116Cd.
Keywords: enriched crystals; scintillators; double beta decay enriched crystals; scintillators; double beta decay

1. Introduction

2β decay is the rarest process observed in nature in which nuclei with atomic mass A spontaneously change their charge Z by two units: (A,Z) → (A,Z ± 2) involving the simultaneous emission of two electrons or positrons. The 2ν mode (the transition accompanied by two (anti)neutrinos (2ν2β)) is allowed by the Standard Model (SM), and can be expressed for nucleus as X Z A X Z + 2 A + 2 e + 2 ν ¯ ( X Z A X Z 2 A + 2 e + + 2 ν ). The probability of the decay is inversely proportional to the half-life T 1 / 2 2 ν 1 = G 2 ν M 2 ν 2 , where G 2 ν   is a phase space factor and M 2 ν   is the nuclear matrix element (NME) [1]. While the G 2 ν   can be precisely calculated, the calculations of NMEs are complex and depend on the selected nuclear model [2,3,4]. The half-life (or the lower limit on the half-life) relative to different channels of 2β decay can be estimated using the following formula:
T 1 / 2 ( or   lim T 1 / 2 ) = N × ln 2 × η det × η sel × t / S ( or   lim S ) ,
where N is the number of nuclei in the sample, η det is the detection efficiency of the decay (the ratio of the number of events in the simulated distribution to the number of generated events), η sel is the efficiency of events selection, t is the time of measurement, and S is the number of events of the effect searched for, which can be excluded at a given confidence level. The precise measurements of T 1 / 2 for 2ν2β decay for different nuclei could help to specify the methods and parameters of theoretical calculations of NMEs.
After more than 75 years of investigations, the 2ν2β decay to the ground state has been observed for 11 nuclides (48Ca, 76Ge, 82Se, 96Zr, 100Mo, 116Cd, 128Te, 130Te, 150Nd, 136Xe, and 238U) with half-lives in the range of 1018–1024 years [5] in direct, geochemical, and radiochemical experiments. The average values of the half-lives relative to 2ν2β decay are shown in Figure 1 (black squares). The 2ν2β decay to the first 0+ excited level of the daughter nucleus in 100Mo and 150Nd has also been detected with half-lives of T 1 / 2 ~ 1020 years [5,6]. The processes of 2β decay with nuclear charge decreasing have been observed only for 124Xe [7] and 78Kr [8] in direct experiments, but also for 130Ba in geochemical experiments [9,10].
The neutrinoless 2β decay (0ν2β), a transformation which changes the charge of a nucleus by two units without (anti)neutrino emission ( X Z A X Z + 2 A + 2 e ), is forbidden in the SM due to the lepton number violation. The process has not been observed yet, only the most stringent half-life limits have been set at the level of 1025–1026 years (red circles in Figure 1). Thanks to the observation of neutrino oscillations, the neutrino is supposed to be a particle with non-zero mass [11,12]. While the oscillation experiments are sensitive to the squared neutrino mass eigenstates difference, 0ν2β decay is a possible way to test the lepton number conservation in weak interactions, clarify the nature of neutrino (Dirac or Majorana particle), and its observation would provide the information about the neutrino mass scale [13,14,15,16,17,18,19,20,21,22,23,24]. The investigation of neutrinoless electron capture with positron emission (0νεβ+) and neutrinoless double positron emission (0ν2β+) is one of possible ways to refine the mechanism of the decay due to the light ν mass or due to the right-handed currents admixture [25]. The development of experimental methods for the research of different 2β isotopes is highly required because of big uncertainties of the theoretical estimations for 0ν2β decay probability [26].
The highest experimental values on the lower limits of the half-lives related to 0ν2β decay for 48Ca [27], 76Ge [28], 78Kr [8], 82Se [29], 96Zr [30], 100Mo [31], 116Cd [32], 128Te [33,34], 130Te [35], 136Xe [36], and 150Nd [37] are shown in Figure 1 (red circles). The sensitivity of the experiments with enriched crystals reached the level of 1024–1026 years for some 2β channels for different nuclides that are close to the theoretical estimations allowing to calculate the upper limits on the effective mass, ‹ m υ ›, of the Majorana neutrino at the level of ‹ m υ › ~ 0.1–0.6 eV; see [38] for a review and the results obtained in the experimental works [28,35,36].
Crystal scintillators are widely utilized in particle physics, in particular, for search for 2β decay, dark matter (DM) particles, and the study of rare nuclear decays (alpha, beta, cluster decays, etc.); see [39] and references therein. The sensitivity of the 2β decay experiment depends on the following values [40,41]:
lim T 1 / 2 ~ ε δ m t F W H M B g ,
where ε is the detection efficiency, δ is the abundance of the investigated isotope in the sample, m and FWHM (full width at half maximum) are the total mass and the energy resolution of the detector, t is the time of measurement, and Bg is the background rate in the region of interest for the searched effect. The lim T 1 / 2 value directly is proportional to the detection efficiency and the enrichment of the isotope, while the other parameters are under the square root. So the development of isotopically enriched materials that contain the maximum value of the investigated isotopes for 2β decay experiments is strongly required. Based on Equation (2), the crystal scintillators are considered as powerful detectors to search for 2β processes due to the following general advantages:
-
realization of the “source = detector” approach by detecting events whose origins are within the crystal; in this case the detection efficiency for searched effects could reach 80−90%;
-
possibility of isotope enrichment for many of the crystals [42]; some enrichment techniques (like the gas centrifuge method) are not so expensive, while others (for example, the multi-channel counter current electrophoresis (MCCCE) or laser separation) are much complicated and expensive;
-
an ability to increase the sensitivity of the experiments by the use of big-size crystals [43,44] or large array of crystals;
-
low level of the internal radioactive contamination (typically the crystals for low background measurements have contamination by 226Ra and 228Th on the level of ~(0.001−0.1) mBq/kg;
-
possibility of the development of radiopure scintillators;
-
the good pulse-shape discrimination factor;
-
very good operating stability (for years).
Furthermore, since the investigation of such rare processes requires special conditions such as a low level of external radioactive background, the experiments are carried out mostly at the low-background underground laboratories.
In order to investigate the 0ν2β decay, isotopes with the high decay energy are favored due to the increasing probability of the decay (as E5), but also the sharp decreasing of the natural radioactivity after 2615 keV (energy of γ quanta of 208Tl from 232Th chain) and lower cosmogenic activations at a high-energy region; this advantage simplifies the problem of induced background. Another challenge in the research is a possible contribution from the 2ν2β mode to the 0ν2β decay region. A good energy resolution of the crystal scintillators is preferable to overcome this problem. It could be provided by using enriched crystals as cryogenic detectors for which effective particle discrimination by the simultaneous detection of phonon and scintillation signals is inherent. The most perspective low-temperature scintillating detectors for the 2β decay searches are the scintillators enriched by 100Mo: Calcium molybdates ( Ca 100 MoO 4 ) [45,46], lithium molybdates ( Li 100 MoO 4 ) [47,48], and zinc molybdates ( Zn 100 MoO 4 ) [49,50]. 100Mo is one the most promising 2β isotope thanks to a high natural isotopic abundance δ = 9.8% [51] (it can also be enriched using relatively inexpensive ultra-speed centrifuge technology) and its large energy release Q 2 β = 3034.40(17) keV [52] above the edge of natural radioactivity. In particular, cryogenic scintillation bolometers with molybdate crystals enriched in 100Mo with a relatively high energy resolution (a few keV at Q 2 β ) are accepted for large-scale projects of the new generation to search for 0ν2β decay aiming at a half-life sensitivity ≥1027 years (with sensitivity to neutrino mass at the level of the inverse scheme of neutrino mass states 0.02–0.05 eV), like AMORE (Advanced Mo-based Rare process Experiment) [53] and CUPID (CUORE Upgrade with Particle Identification) [54]. The best up-to-date sensitivity to 0ν2β decay in 100Mo was reached: T 1 / 2 0 ν 2 β > 1.5 × 10 24 years at 90% confidence level (C.L.) which corresponds to the effective Majorana mass of neutrino ‹ m 2 β › < (0.31–0.54) eV [31]). Some other enriched crystals using bolometers (130 TeO 2 [55], Zn92Se [29], 116 CdWO 4 [56]) are also a perspective for high-sensitivity 0νββ decay searches.
The paper focuses on some isotopes for 2β processes investigation that could be used as crystal scintillators at room temperature and fall under most demands listed above (48Ca, 106Cd, and 116Cd). The review on the latest results for these isotopes is given. Since the mass of these detectors does not exceed a few kilograms, they could be used mostly for the 2ν2β decay search. The precise measurements of the 2ν mode is important for NME calculations because there are still large differences in the literature between the NME values.

2. Crystal Growth and Radiopurity of Crystal Scintillators

2β experiments, at different stages over the world, running or being under development, require crystal scintillators with as low as possible radioactive contamination ~(0.1–1) μBq/kg by U/Th daughters (the secular equilibrium of the U/Th chains could be broken and should be considered separately), 40K, surface contamination and cosmogenic activation by 110mAg, 56Co, 88Y, and 88Zr, radioactive isotopes of rare-earth elements could also be found in the crystals [57,58,59], as well as anthropogenic 90Sr-90Y and 137Cs caused by the Chernobyl (1986), and the Fukushima Daiichi (2011) nuclear disasters [60]. As additional contamination in low-background experiments, one should consider also an initial activity of the different compounds of the crystals (for example, 2β activity of 48Ca, 100Mo, and 116Cd can contribute in the regions of the interest for the rare processes of these isotopes). The main objectives of different projects are investigation and R&D (research and development) of deeply radiopure crystal scintillators for low-background measurements to investigate 2β decay and search for DM. R&D of very low radioactive scintillators calls for a deep collaboration of chemists and crystal growers with low radioactivity experts. Crystal producers should pay more attention on radioactive contamination and behavior of tiny bulk and surface pollutions because even a small fraction of some radioactive impurities can play a crucial role in the low-counting measurements. It is important to realize a fast screening of crystal samples to investigate the radioactivity of all the crystal growth equipment, equilibrium issues, surface contamination, cosmogenic activation, and radon deposit. A very low radioactive contamination, required by the next generation of 2β decay experiments, can be measured by using a low background “source = detector” approach. The method can provide the sensitivity at the level of μBq/kg for 40K, 137Cs, 228Th, and U/Th daughters [61]. The efforts in the research of new materials and purification techniques as well as the improvement of the crystal growth procedures allows a wide choice among inorganic scintillators for a variety of uses and reaching progress in 2β investigations.
The radiopurity level of isotopically enriched crystals is typically lower than it is required for the low-background experiments, whose requirements on radiopurity level are ~(0.1–1) μBq/kg (for example, the total internal α activity in the cadmium tungstate crystals enriched by 106/116Cd is ~(2–3) mBq/kg, while for natural crystal it is ~(0.2–0.3) mBq/kg). The choice of the best purification and production cycle of enriched scintillators includes:
(1)
minimization of the initial materials and possible radioactive components of the growing set-ups;
(2)
a test of the radiopurity level of the detectors;
(3)
a study of the effect of double crystallization, development of protocol for surface treatment;
(4)
an application of methods to purify enriched isotopes by using combination of chemical (recrystallization) and physical (vacuum distillation, filtration, zone melting) approaches;
(5)
minimization of cosmogenic/neutrons activation (the production line and transportation of the crystals to the experiment site should be provided with the safest way, by ground-based) and radioactive background deposit from radon.
For the moment, one of the best techniques of enriched crystal production is the low-thermal gradient Czochralski crystal growth method (LTG CZ) [62] which provides large volume radiopure crystal scintillators with an excellent optical quality, relatively high yield of crystalline boules (for example, ~87% of the initial charge for CdWO4, and an acceptable level of irrecoverable losses of expensive initial enriched materials (~2% for CdWO4).
The improvement of a radiopurity level of enriched crystal scintillators could also be achieved by recrystallization [63,64]. For example, comparing the data of experiments for 116CdWO4 crystals, before and after recrystallization, one can conclude that segregation coefficient for K, Th, and especially of Ra is very low during crystal growth. It gives a strong signature that the radioactive contamination of the crystals by 40K, 228Th, and 226Ra can be substantially reduced by recrystallization. The total α activity due to the U/Th chains after recrystallization has been improved by a factor ~3; the contamination by thorium (228Th) has been reduced by one order of magnitude to the level of 0.01 mBq/kg (see Table 1). The same behavior was also discovered for molybdates (including crystals enriched in 100Mo) [50] and zinc tungsten crystals [64]. The development of other crystals by LTG CZ (for example, calcium and lead molybdates (CdWO4, PbMoO4) is in progress.

3. Calcium Fluoride Crystal Scintillators Enriched in 48Ca

The 48Ca is the lightest nucleus among potential 2β decay isotopes with the highest energy release ( Q 2 β = 4268.08(8) keV [52]; a simplified 48Ca decay scheme is shown in Figure 2), promising theoretical calculations, and, despite the low natural isotopic abundance (δ = 0.187(21)%), a possibility for enrichment up to almost 100%.
The first experiment with europium-activated calcium fluoride crystal scintillator, CaF 2 Eu , enriched in 48Ca up to 96.59% has been performed in 1966 at the Brookhaven National Laboratory with the aim to search 2β processes in 48Ca. The detector of 2 cm × 1.8 cm with a total mass of 48Ca about 10.6 g worked in anticoincidence mode with a plastic scintillator shield. Only the limit on 0ν2β decay was set as T 1 / 2 0 ν 2 β > 2 × 10 20 years [65].
There were few attempts with CaF 2 crystals with a natural abundance of 48Ca to investigate 2β processes in 48Ca. The limits on 0ν2β decay were established as T 1 / 2 0 ν 2 β > 1.4 × 10 22 years and T 1 / 2 0 ν 2 β > 5.8 × 10 22 years at 90% C.L. with the help of CaF 2 and CaF 2 Eu scintillators in [66,67], respectively.
In addition, the CANDLES experiment (CAlcium fluoride for the study of Neutrinos and Dark matters by Low Energy Spectrometer) is in progress at the Kamioka underground observatory (2700 m of water equivalent), Japan. The use of enriched 48 CaF 2 Eu crystals up to 2% (and then to 50%) by 48Ca is expected; the collaboration has been developing the 48Ca enrichment with MCCCE or laser separation and the possibility of large-scale enrichment is still under investigation. Meanwhile, for the first stage, 96 CaF 2 cylindrical crystals (of the diameter and length, Ø5 cm × 5 cm, with natural abundance by 48Ca) with a total mass of 305 kg are using as scintillating bolometers. A liquid scintillator provides an active 4π shield. The energy resolutions for the detectors at the Q-value of 48Ca is ~2.6% (a better energy resolution is required to reduce an irremovable background 2νββ background), and of 4.0–4.5% at 1460.8 keV and 2.9–3.3% at 2614.5 keV. The new limit on 0ν2β decay was set as T 1 / 2 0 ν 2 β > 6.2 × 10 22 year at 90% C.L. [27,68].

4. Cadmium Tungstate Crystal Scintillators Enriched in 106Cd

106Cd is one of the most appealing candidates for 2β+ decay search with natural abundance δ = 1.245(22)% with a possibility of enrichment up to 100% by using relatively inexpensive gas centrifugation techniques. Energy release is high enough, Q 2 β = 2775.39(10) keV [52], for different processes like double positron emission decay (2β+), electron capture with positron emission (εβ+), and double electron capture (2ε). A simplified 106Cd decay scheme is shown in Figure 3. The isotope is also interesting by its possible near resonant neutrinoless 2ε captures (0ν2ε) from different shells (K, L1, L3) to the excited level of 106Pd (2718 keV for 0ν2K, 2741 keV for 0νKL1 and 2748 keV for 0νKL3). For some modes of the 2β processes, theoretical T1/2 are promising 1020–1022 years (ground state (g.s.) to g.s.) and reachable by modern low-counting techniques [69].
106CdWO4 crystal enriched in 106Cd up to 66.4% have been developed for the high-sensitivity experiments to search for 2β decay of 106Cd [71]. Cadmium samples (including enriched ones) were deeply purified by a vacuum distillation technique with filtering on getter filters and cadmium tungstate compounds were synthesized from solutions [72,73]. The isotopic composition was precisely measured by thermal ionization mass-spectrometry (TIMS). The crystal boule of 106CdWO4 with a mass of 231 g was grown by the low-thermal-gradient Czochralski technique [62,74]. Then it was cut and the surface has been polished to improve the light collection (see Figure 4). The final total mass became 216 g.
Few stages of experiments with the 106CdWO4 crystal enriched in 106Cd up to 66% were carried out at the Gran Sasso underground laboratory of the INFN (LNGS, Assergi, Italy) at a depth of 3600 m of water equivalent. The scintillator shows excellent optical and luminescence properties, good α/γ discrimination capability, and low levels of intrinsic contamination by the primordial 40K and radionuclides from U/Th chains, anthropogenic 90Sr and 137Cs, cosmogenic 110mAg (in particular, <1.4 mBq/kg of 40K, 0.005 mBq/kg of 228Th, 0.012 mBq/kg of 226Ra, and with the total α activity ~2.1(2) mBq/kg). The low energy resolution of the enriched detector (FWHM ~ 10% at 662 keV), which was measured with 22Na, 60Co, 133Ba, 137Cs, and 228Th γ sources, could be explained by its elongated shape, which leads to a low and non-uniform collection of scintillation light [69,71,75].
At the first stage, the 106CdWO4 scintillator (216 g) enriched in 106Cd up to 66% was installed in the low-background DAMA R&D set-up. After 6590 h of data taking, new half-life limits on the 2β decay processes were set at the level of 1019–1021 years [69].
For the second stage, a lead tungstate (PbWO4) light-guide was developed from archeological lead to suppress the background owing to the radioactive contamination from the photomultiplier tube (PMT) [76,77]. The same crystal, as in the first stage, was installed in an ultra-low-background high purity germanium (HPGe) γ spectrometer GeMulti of the STELLA (SubTErranean Low Level Assay) facility [78] in a well in the center of one cryostat with four HPGe detectors (~225 cm3 each). The scintillator was viewed through the PbWO4 crystal light-guide (Ø40 mm × 83 mm) by 3-inch low-radioactive PMT Hamamatsu R6233MOD (see Figure 5). The detection of γ quanta, inherent for the most 106Cd decay channels, was expected, including the annihilation γ quanta emitted in decay modes with positron emissions. Despite the protection of the crystal from the radioactive contamination of PMT, the use of archeological PbWO4 crystal led to a decreasing of the energy resolution due to its non-perfect transparency [79]. The improved limits have been set after 13,085 h of data taking with respect to different 2β decay channels at the level of T 1 / 2 ~ (1020–1021) years [75]. In particular, the half-life limit on the two-neutrino electron capture with positron emission has reached the region of theoretical predictions: T 1 / 2 2 ν ε β + > 1.1 × 10 21 years at 90% C.L (see Table 2).
At the third stage, the 106CdWO4 detector was processing in the low-background DAMA/Crys set-up in coincidences/anti-coincidences with two large volumes of 106CdWO4 crystal scintillators in close geometry with the aim to increase the detection efficiency of γ quanta. The 106CdWO4 crystal scintillator was viewed through the PbWO4 crystal light-guide (Ø40 mm × 83 mm) by 3-inch low-radioactive PMT (Hamamatsu R6233MOD). Two cylindrical detectors from natural cadmium CdWO4 (Ø70 mm × 38 mm) were viewed by two 3-inch low-background PMTs (EPI EMI9265B53/FL) through optical light-guides composed of two parts: Ultrapure quartz (Ø66 mm × 100 mm) and polystyrene (Ø66 mm × 100 mm) (see Figure 6 and Figure 7). The passive shield consists of copper (11 cm), lead (10 cm), cadmium (2 mm), and polyethylene (10 cm) with additional flushing by high-purity nitrogen gas to suppress the radon-caused background. The amplitude, arrival time, and pulse shape of each event were recorded.
After 26033 h of data taking there are no features that can be attributed to the processes of 2β decay of the 106Cd in the experimental data. The lower limits on the half-life of 106Cd relative to different channels of 2β decay were estimated by using Equation (1). The responses of the detector system to different channels of 2β decay of 106Cd, as well as the contribution of radioactive contamination of the 106CdWO4 crystal scintillator, including cosmogenic 110mAg and 2ν2β decay of 116Cd with the half-life T 1 / 2 2 ν 2 β = 2.63 × 10 19 years [32], external γ quanta from the setup materials (232Th, 238U, and 40K in the cryostat of the HPGe detector, PMT, light guide, 26Al in the aluminum well of the cryostat), distribution of α particles (which passed the pulse-shape discrimination cut to select β and γ events) were simulated by using the EGSnrc package [81] with the initial kinematics given by the event generator DECAY0 [82].
Table 2. Half-life limits on some 2β processes in 106Cd. The theoretical estimations are also given.
Table 2. Half-life limits on some 2β processes in 106Cd. The theoretical estimations are also given.
Decay Channel,
Level of 106Pd (keV)
lim T 1 / 2 ( years )   at   90 %   C . L .
Best LimitsTheory
2ν2ε, 01+ 1134≥9.9 × 1020 [80]1.1 × 1024 [83]
0ν2ε, g.s.≥1.1 × 1021 [69]
2νεβ+, g.s≥2.1 × 1021 [80]8.3 × 1020 [84]
2.7 × 1022 [83]
7.7 × 1022 [85]
2νεβ+, 01+ 1134≥1.1 × 1021 [69]1.1 × 1027 [83]
0νεβ+, g.s.≥1.4 × 1022 [80]3.4 × 1026 [25]
2ν2β+, g.s.≥2.3 × 1021 [69]2.4 × 1027 [83]
3.1 × 1027 [85]
0ν2β+, g.s.≥5.9 × 1021 [80]4.8 × 1027 [25]
(1.9–3.2) × 1027 [86]
Res. 0ν2ε, 2718≥2.9 × 1021 [80]
Res. 0ν2ε, 2741≥9.5 × 1020 [69]
Res. 0ν2ε, 2748≥1.4 × 1021 [69]
The best half-life limits (as well as the theoretical estimations) on some 2β processes in 106Cd are presented in Table 2 [80]. In particular, new limits on 2ε, εβ+, and 2β+ processes were set on the level of T 1 / 2 > (1020–1022) years. The half-life limit on the 2νεβ+ decay to the ground state, T 1 / 2 2 ν ε β + > 2.1 × 10 21 years, reached the region of theoretical predictions; for 0ν2ε resonant captures the limits were set as T 1 / 2 0 ν 2 ε > ( 0.35 2.9 ) × 10 21 years.
After almost 1 year of data taking there are no features that can be attributed to the processes of 2β decay of the 106Cd in the experimental data. The sensitivity of the experiment for the different channels of 2β decay of 106Cd is expected to be 1021–1022 years during five years of measurements that is near the theoretical predictions (see Table 2). The measurements and data analysis are in progress.

5. Cadmium Tungstate Crystal Scintillators Enriched in 116Cd

116Cd is a promising isotope to search for 0ν2β decay thanks to its high energy release ( Q 2 β = 2813.49(13) keV) [52], isotopic abundance (δ = 7.49(18)%), possible enrichment in large amount (despite the relatively high price), and promising theoretical calculations for 2β processes. A simplified decay scheme of 116Cd is shown in Figure 8.
CdWO4 crystal scintillators have excellent scintillation properties, low level of internal radioactive contamination, good particle discrimination ability, and could be used in the “source = detector” approach. In particular, the light output of the detectors is ~20% in comparison with NaI(Tl), the wavelength of emission maximum is 480 nm with principal decay time 13 μs.
For the first measurement, the 116CdWO4 crystal scintillators, enriched in 116Cd to 83%, have been developed for the 2β decay investigation [88]. Four 116CdWO4 crystals with a total mass of 330 g were installed inside a set-up at the Solotvina Underground Laboratory (Ukraine). They were viewed by a low-background 5-inch PMT through a light-guide (Ø10 cm × 55 cm) which was composed of 25 cm of high purity quartz and 30 cm of plastic scintillator. The active shield consisted of 15 CdWO4 crystal scintillators with a total mass of 20.6 kg. The set-up is described in details in [89]. The pulse-shape discrimination technique was applied to obtain the energy spectra of β(γ) events with the aim to investigate different modes of 2β decay in 116Cd. The results of the experiment can be found in [90]. In particular, the half-life value on the 2ν2β decay of 116Cd was set as T 1 / 2 2 ν 2 β = 2.9 0.3 + 0.4 × 10 19 years, and the new half-life limit on the 0ν2β decay of 116Cd has been established as T 1 / 2 0 ν 2 β > 1.7 × 10 23 years at 68% C.L. Other 2β processes in 106Cd, 108Cd, 114Cd, 180W, and 186W nuclei were restricted at level 1017–1021 years [89,90].
For the second experiment, called Aurora, to investigate 2β processes in 116Cd, new 116CdWO4 crystals enriched in 116Cd up to 82% have been developed [91]. A large-mass boule (~1.9 kg) of the enriched 116CdWO4 crystal (see Figure 9) was cut on three cylindrical parts; two of them (580 g and 582 g) were used for the experiment. The scintillating detectors shown good optical and scintillation properties obtained thanks to the deep purification of 116Cd and W, and the advantage of the low-thermal-gradient Czochralski technique to grow the crystal.
The experiment with two cadmium tungsten crystal scintillators has been run in the low background DAMA/R&D set-up at the LNGS with several upgrades to improve the detector background counting rate and energy resolution [92,93,94]. The radioactive contamination by radionuclides from U/Th chains and K of the scintillating elements increases longitudinal during the crystal growth that shows a segregation of radio-impurities: The conic part of the boule, the beginning of the crystal growth, appeared the cleanest one. On Figure 10 the total energy spectrum accumulated with two 116CdWO4 detectors over 26,831 h and selected by the pulse-shape and the front-edge analysis is shown: Raw data, spectra of γ(β), α, and 212Bi−212Po events (denoted “Bi-Po”). The spectra α(1) and α(2) demonstrate the distributions of alpha events accumulated by the detectors No. 1 from the upper part of the boule and No. 2, from the bottom part.
The analysis of the recrystallized bottom part of the boule [64] shows that the application of an additional crystallization after the remelting of the grown crystal boule (double crystallization procedure) helps to reduce the radioactive contamination of the crystal. This feature is important for the production of radiopure crystal scintillators, especially from expensive enriched materials.
The results of the Aurora experiment to investigate 2β processes in 116Cd with 1.162 kg of 116CdWO4 scintillators, enriched in up to 82% in 116Cd, are presented in [32]. In particular, the most accurate value of the half-life of 116Cd relative to the 2ν2β decay to the ground state of 116Sn was set: T 1 / 2 2 ν 2 β = 2.63 0.12 + 0.11 × 10 19 years, as well as a new half-life limit on the 0ν2β decay of 116Cd to the ground state of 116Sn: T 1 / 2 0 ν 2 β > 2.2 × 10 23 year at 90% C.L. that corresponds to the limit on the effective Majorana neutrino mass ‹ m υ › ≤ (1.2−1.5) eV (depending on the nuclear matrix elements used in the estimations of the neutrino mass limit). Other 2β processes, including 0ν2β decay with different majorons (χ0) emission were restricted at level 1020–1022 years. The theoretical predictions for the transitions in 116Cd are on the level of T 1 / 2 ~ 1021−1024 years (see [95,96]). The best half-life limits on 2β processes in 116Cd are presented in Table 3 [32].
116 CdWO 4 crystals can be used also as bolometers: High energy resolution and particle discrimination capability were tested at milli-Kelvin temperature with a 34.5 g enriched 116CdWO4 sample with the sensitivity on the level of ~0.1 mBq/kg for 232Th, 235U, and 238U (and their daughters) [99].
In addition, the same crystals that were used in the AURORA experiment, operated recently as scintillating bolometers in new measurements in two underground laboratories: One at the Laboratoire Souterrain de Modane, France (LSM) and another one at Laboratorio Subterraneo de Canfranc, Spain (LSC). Promising results were shown in regards to using them as bolometers with high energy resolution (11–16 keV at 2615 keV) and extremely high PSD for α and γ(β) particles (up to ~20σ) (see [56]).

6. Conclusions

Crystal scintillators, in particular, with a high concentration of the isotopes of interest, are powerful detectors for 2β experiments thanks to the low radiopurity level and the possibility to implement calorimetric “source = detector” approach with a high detection efficiency. The deep purification of the initial materials, application of the low-thermal-gradient Czochralski technique, which provides a large volume of radiopure crystal scintillators with high optical quality and low losses, as well as the development of isotopically enriched crystals, are strongly required for the 2β decay investigations. The best half-life limit on the 2νεβ+ decay in 106Cd to the ground state with the help of 106CdWO4 crystal (216 g) enriched in 106Cd up to 66.4% was set as T 1 / 2 2 ν ε β + > 2.1 × 10 21 years, which reached the region of theoretical predictions. In the AURORA experiment, enriched in 82% in 116Cd 116CdWO4 scintillators, the most accurate value on the half-life of 116Cd relative to the 2ν2β decay to the ground state of 116Sn ( T 1 / 2 2 ν 2 β = 2.63 0.12 + 0.11 × 10 19 years) and a new half-life limit on the 0ν2β decay of 116Cd to the ground state of 116Sn ( T 1 / 2 0 ν 2 β > 2.2 × 10 23 years at 90% C.L.) were set. The search for the 0ν mode of the 2β decay could be provided by using enriched crystal scintillators as bolometers that met the requirements of sensitive experimental equipment capable of registering extremely rare nuclear decays with half-lives of 1025–1028 years (the possibility of using scintillation crystals enriched in 116Cd as bolometers is under development).

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The average values of the half-lives (in years) relative to 2ν2β decay (black squares) [5] and the highest experimental values on the lower limits of the half-lives related to 0ν2β decay for different isotopes (red circles, see text). Errors are given at 68% C.L., while the lower limits are given at 90% C.L. 2ν2β decay for the 128Te isotope is observed only in the geochemical experiment.
Figure 1. The average values of the half-lives (in years) relative to 2ν2β decay (black squares) [5] and the highest experimental values on the lower limits of the half-lives related to 0ν2β decay for different isotopes (red circles, see text). Errors are given at 68% C.L., while the lower limits are given at 90% C.L. 2ν2β decay for the 128Te isotope is observed only in the geochemical experiment.
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Figure 2. A simplified 48Ca decay scheme. Energies of the excited levels and of the emitted γ quanta are in keV. The relative intensities of γ quanta are given in parentheses [52].
Figure 2. A simplified 48Ca decay scheme. Energies of the excited levels and of the emitted γ quanta are in keV. The relative intensities of γ quanta are given in parentheses [52].
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Figure 3. A simplified 106Cd decay scheme [70]. Energies of the excited levels and of the emitted γ quanta are in keV. The relative intensities of γ quanta are given in parentheses. For the levels of 106Pd with energies 2718 keV, 2741 keV, and 2748 keV the resonant 0ν2EC decays are possible.
Figure 3. A simplified 106Cd decay scheme [70]. Energies of the excited levels and of the emitted γ quanta are in keV. The relative intensities of γ quanta are given in parentheses. For the levels of 106Pd with energies 2718 keV, 2741 keV, and 2748 keV the resonant 0ν2EC decays are possible.
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Figure 4. The crystal boule of 106 CdWO4 (231 g) was grown by the low-thermal-gradient Czochralski technique (left). 106CdWO4 crystal scintillator (216 g) with diffused surface for better light collection (right). The figure was taken from [71].
Figure 4. The crystal boule of 106 CdWO4 (231 g) was grown by the low-thermal-gradient Czochralski technique (left). 106CdWO4 crystal scintillator (216 g) with diffused surface for better light collection (right). The figure was taken from [71].
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Figure 5. A scheme of the experimental GeMulti set-up with 106CdWO4.
Figure 5. A scheme of the experimental GeMulti set-up with 106CdWO4.
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Figure 6. A scheme of the experimental set-up with 106CdWO4. 106CdWO4 crystal scintillator (1) is viewed through the PbWO4 light-guide (2) with the help of Photomultiplier Tube (PMT) (3). Two CdWO4 crystal scintillators (4) are viewed by two PMT (7) through optical light-guides composed of two parts: Ultrapure quartz (5) and polystyrene (6). The passive shield consists of copper (8—some parts are shown), lead, polyethylene, and cadmium. The figure was taken from [80].
Figure 6. A scheme of the experimental set-up with 106CdWO4. 106CdWO4 crystal scintillator (1) is viewed through the PbWO4 light-guide (2) with the help of Photomultiplier Tube (PMT) (3). Two CdWO4 crystal scintillators (4) are viewed by two PMT (7) through optical light-guides composed of two parts: Ultrapure quartz (5) and polystyrene (6). The passive shield consists of copper (8—some parts are shown), lead, polyethylene, and cadmium. The figure was taken from [80].
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Figure 7. 106CdWO4 crystal scintillator (1), teflon stand for the 106CdWO4 crystal (2), CdWO4 crystal scintillators (3), quartz light-guide (4), and “internal” copper shield (5).
Figure 7. 106CdWO4 crystal scintillator (1), teflon stand for the 106CdWO4 crystal (2), CdWO4 crystal scintillators (3), quartz light-guide (4), and “internal” copper shield (5).
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Figure 8. Simplified decay scheme of 116Cd [87]. Energies of the excited levels and of the emitted γ quanta are in keV. The relative intensities of γ quanta are given in parentheses.
Figure 8. Simplified decay scheme of 116Cd [87]. Energies of the excited levels and of the emitted γ quanta are in keV. The relative intensities of γ quanta are given in parentheses.
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Figure 9. A large-mass crystal boule (~1.9 kg) of the enriched 116CdWO4 crystal was used in the experiment Aurora [91].
Figure 9. A large-mass crystal boule (~1.9 kg) of the enriched 116CdWO4 crystal was used in the experiment Aurora [91].
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Figure 10. The sum energy spectrum accumulated with two 116CdWO4 detectors over 26,831 h (Raw data) and spectra of γ(β), α and 212Bi-212Po events (denoted “Bi-Po”) selected by the pulse-shape analysis. The spectra α(1) and α(2) denote the distributions of alpha events accumulated by the detectors No. 1 from the upper part of the boule and No. 2, from the bottom part.
Figure 10. The sum energy spectrum accumulated with two 116CdWO4 detectors over 26,831 h (Raw data) and spectra of γ(β), α and 212Bi-212Po events (denoted “Bi-Po”) selected by the pulse-shape analysis. The spectra α(1) and α(2) denote the distributions of alpha events accumulated by the detectors No. 1 from the upper part of the boule and No. 2, from the bottom part.
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Table 1. Radioactive contamination of the 116 CdWO4 crystal before and after recrystallization [64].
Table 1. Radioactive contamination of the 116 CdWO4 crystal before and after recrystallization [64].
ChainNuclideActivity, mBq/kg
Before RecrystallizationAfter Recrystallization
232Th232Th0.13 (7)0.03 (2)
228Th0.10 (1)0.010 (3)
238U238U1.8 (2)0.8 (2)
226Ra≤0.1≤0.015
234U + 230Th0.6 (2)0.4 (1)
210Po1.6 (2)0.4 (1)
Total α 4.44 (4)1.62 (4)
Table 3. The best half-life limits on 2β processes in 116Cd [32]. (Here, n defines the spectral index and LV denotes Lorentz-violating decay.)
Table 3. The best half-life limits on 2β processes in 116Cd [32]. (Here, n defines the spectral index and LV denotes Lorentz-violating decay.)
Decay Channel, Level of 116Sn (keV) lim T 1 / 2 ( years )   at   90 %   C . L .
2ν2β, g.s. 2.63 0.12 + 0.11 × 10 19
2ν2β, 2+(1294)≥2.3 × 1021 [97]
2ν2β, 0+(1757)≥5.9 × 1021 [97]
2ν2β, 0+(2027)≥2.0 × 1021 [97]
2ν2β, 2+(2112)≥2.5 × 1021
2ν2β, 2+(2225)≥7.5 × 1021
0ν2β, g.s.≥2.2 × 1023
0ν2β, 2+(1294)≥7.1 × 1022
0ν2β, 0+(1757)≥4.5 × 1022
0ν2β, 0+(2027)≥3.1 × 1022
0ν2β, 2+(2112)≥3.7 × 1022
0ν2β, 2+(2225)≥3.4 × 1022
0νχ0 n = 1, g.s.≥8.5 × 1021 [98]
0νχ0 n = 2, g.s.≥4.1 × 1021
0νχ0 n = 3, g.s.≥2.6 × 1021
0νχ0χ0 n = 3, g.s.≥2.6 × 1021
2νχ0LV n = 1, g.s.≥1.2 × 1021
0νχ0χ0 n = 7, g.s.≥8.9 × 1020
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