Development of Mo-containing scintillating bolometers for a high-sensitivity neutrinoless double-beta decay search
This paper reports on the development of a technology involving 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 (1 kg), high optical quality, radiopure 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 -value of the double-beta transition of Mo (3034 keV) is 4–6 keV FWHM. The rejection of the -induced dominant background above 2.6 MeV is better than 8. Less than 10 Bq/kg activity of Th (Th) and Ra in the crystals is ensured by boule recrystallization. The potential of Mo-enriched scintillating bolometers to perform high sensitivity double-beta decay searches has been demonstrated with only 10 kgd exposure: the two neutrino double-beta decay half-life of Mo has been measured with the up-to-date highest accuracy as = [6.90 0.15(stat.) 0.37(syst.)] 10 yr. 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 Mo.
Keywords:Double-beta decay Cryogenic detectors Scintillating bolometers Scintillators Enriched crystals Mo Zinc molybdate Lithium molybdate Particle identification Low background Radiopurity
Neutrinoless double-beta () decay, a yet-to-be-observed nuclear transition, consists in the transformation of an even-even nucleus into a lighter isobar containing two more protons with emission of two electrons and no other particles, resulting in a violation of the total lepton number by two units: (e.g. see Ref. Vergados:2017 ()). This hypothetical transition is energetically allowed for 35 nuclei Tretyak:2002 (). The detection of decay would have profound implications for our understanding of nature, proving that neutrinos are their own antiparticles (Majorana fermions), fixing the absolute neutrino mass scale and offering also a clue for the creation of matter abundance in the primordial universe (see recent reviews Vergados:2017 (); DellOro:2016 () and references therein). It is to remark that this process is much more than a neutrino-physics experiment, because decay is a powerful, inclusive test of lepton number violation. Non-conservation of the total lepton number is as important as baryon number violation and is naturally incorporated by many theories beyond the Standard Model (SM). The current most stringent lower limits on the decay half-lives are in the range of 10–10 yr Vergados:2017 (); Gando:2017 (). The SM allowed process two neutrino double-beta () decay is the rarest observed nuclear transition and it has been measured in 11 nuclides with the half-lives in the range of 10–10 yr Barabash:2015 ().
There are a number of proposed next-generation decay experiments, based on upgrades of the most promising current technologies (e.g. see Refs. Vergados:2017 (); Cremonesi:2014 (); Beeman:2012 (); Beeman:2012a (); Artusa:2014a ()). The goal of these future searches is to improve by up to two orders of magnitude the present best limits on the half-life with a sensitivity to the effective Majorana neutrino mass (a measure of the absolute neutrino mass scale) at the level of 10–20 meV, covering the so-called inverted hierarchy region of the neutrino mass pattern. The bolometric approach is amongst the most powerful methods to investigate decay. Particularly, one of the most stringent constrains on the effective Majorana neutrino mass Vergados:2017 () have been set by the results of Cuoricino and CUORE-0, precursors of the Cryogenic Underground Observatory for Rare Events (CUORE) Arnaboldi:2004a (), which studies the candidate isotope Te with the help of TeO bolometers. CUORE, a ton-scale decay experiment, is now taking data in the Gran Sasso National Laboratories (Italy) and will be in operation for several years. A large group of interest is proposing a next-generation bolometric experiment, CUORE Upgrade with Particle ID (CUPID) CUPID (); CUPID_RD (), to follow CUORE after the completion of its physics program. The nuclei Te, Mo, Se and Cd are the candidates considered for CUPID. A selection of the CUPID technologies, isotopes and materials is foreseen in 2018/2019.
Scintillating bolometers, the devices used in the present work, are favorable nuclear detectors for the conduction of sensitive decay searches, as they offer high detection efficiency, excellent energy resolution (at the level of 0.1%), efficient /() particle separation and potentially low intrinsic background Artusa:2014a (); Alessandrello:1992 (); Bobin:1997 (); Alessandrello:1998 (); Meunier:1999 (); Pirro:2006 (); Giuliani:2012 (). The Mo isotope is one of the most promising candidates, since its signal is expected at = 3034 keV Rahaman:2008 () (-value of the transition), while the environmental background mainly ends at 2615 keV. The candidate is embedded in zinc and lithium molybdate crystals (ZnMoO and LiMoO), working both as low-temperature bolometers and scintillators. An auxiliary bolometer, consisting of a thin Ge wafer, faces each Mo-containing crystal in order to detect the scintillation light. The energy region above 2.6 MeV is dominated by events produced by radioactive contamination of surfaces, especially particles (e.g. as shown by the Cuoricino Andreotti:2011 () and CUORE-0 Alfonso:2015 ()). Scintillation light yield from alpha interactions is usually quenched when compared to the () interactions of the same energy Tretyak:2010 (). Combined with the fact that the thermal response for and () interactions are nearly equivalent, this allows for dual channel scintillating bolometer readouts to perform an effective event-by-event active background rejection Beeman:2012 (); Beeman:2012a (); Artusa:2014a (); Pirro:2006 ().
The development of a reproducible crystallization and detector technologies is needed for the scintillating bolometer technique to be applicable to a large-scale experiment, like CUPID. The specific requirements to be fulfilled by a crystallization technology of Mo-containing scintillators are Berge:2014 (): large enough crystal boule size; limited losses of the high-cost enriched isotope in the purification-crystallization chain; good optical properties; high scintillation yield; exceptionally low radioactive contamination. The size of a boule should be enough to produce at least one 70–100 cm scintillation element. The volume of the Mo-containing crystal is bounded to the aformentioned value in order to avoid a significant impact on background from random coincidences of the decay events of Mo Chernyak:2012 (); Chernyak:2014 (); Chernyak:2017 (). Irrecoverable losses of the enriched material are acceptable at the level of a few % taking into account that the price of the enriched isotope Mo is 80 $/g Giuliani:2012a (). High transmittance (no less than 30 cm absorption length at the emission maximum) is welcome to reduce the amount of the trapped light and therefore to improve the scintillation light yield Chernyak:2013 (). ZnMoO and LiMoO crystals have a reasonable scintillation yield, at the level of 1 keV/MeV, which do not require ultra-low-noise bolometric light detectors. (Baseline noise at the level of a few hundreds of eV are sufficient to provide efficient light-assisted particle identification) According to Monte Carlo simulations of experiments based on Mo-containing scintillating bolometers Beeman:2012 (); Beeman:2012a (); Artusa:2014a (); Danevich:2015 (); Luqman:2017 (), a crystal bulk contamination of the order of 0.01 mBq/kg of Th would result to a minor contribution to the background in the region of interest (ROI; e.g. FWHM wide centered at ), at the level of 10 counts/yr/kg/keV Artusa:2014a (). As far as Ra is concerned, a specific activity of even an order of magnitude higher would provide the significantly lower contribution to the background (e.g. see Danevich:2015 (); Luqman:2017 ()). The total activity of other radionuclides from the U/Th chains should not be higher than few mBq/kg to avoid pile-up effects. The main demands concerning the detector performance at the ROI are Beeman:2012a (); Artusa:2014a (); Danevich:2015 (): better than 10 keV FWHM energy resolution (5 keV FWHM is the CUPID goal CUPID ()); at least 99.9% rejection of -induced events (with ()s acceptance larger than 90%) to suppress this background component to less than 10 counts/yr/kg/keV.
Preliminary results have been achieved in the past with bolometers containing molybdenum with natural isotopic composition in the Gran Sasso underground laboratory in Italy Beeman:2012a (); Gironi:2010 (); Beeman:2012b (); Barinova:2010 (); Cardani:2013 (), in the Modane underground laboratory in France Danevich:2015 (); Armengaud:2015 (); Poda:2016 (); Poda:2015 () and in an aboveground cryogenic laboratory located at CSNSM (Orsay, France) Berge:2014 (); Chernyak:2013 (); Beeman:2012c (); Mancuso:2014a (); Chernyak:2015 (); Bekker:2016 (). In the latter set-up, the first small Mo-enriched ZnMoO two-detector array has been tested recently Barabash:2014 (). Most of these R&D activities were conducted in the framework of the scintillating-bolometer research programs of LUCIFER lucifer () — focused on ZnSe for the decay candidate Se but involving also Mo-containing scintillators — and of LUMINEU lumineu (), dedicated to the investigation of Mo.
The present work represents a crucial step forward in the development of radiopure scintillating bolometers based on ZnMoO and LiMoO crystals grown from Mo-enriched molybdenum. A protocol for crystal growth was developed, and several prototypes were tested showing excellent energy resolution, efficient background rejection power and remarkable radiopurity. The results described here prove in particular that the LiMoO technology is mature enough to carry out a pilot experiment on a several-kilogram scale. This technology demonstrator will provide essential information for the choice of the CUPID technique by clarifying the merits and the drawbacks of the Mo option.
2 R&D on natural and Mo-enriched zinc and lithium molybdates
Important milestones were achieved by LUMINEU in the R&D on zinc molybdate scintillators: the development of a molybdenum purification procedure Berge:2014 (); the growth of large (1 kg) ZnMoO Armengaud:2015 () and small (0.17 kg) ZnMoO Barabash:2014 () crystals with the help of the low-temperature-gradient Czochralski (LTG Cz) method Pavlyuk:1992 (); Borovlev:2001 (); further optimization of the ZnMoO growth process Chernyak:2015 (). The R&D goal has been accomplished by the successful development of a large-mass ZnMoO crystal boule (1.4 kg in weight, Mo enrichment is 99%) shown in Fig. 1 (top left). Even though there is still room to improve the ZnMoO crystal quality — the boule exhibits a faceted structure and contains inclusions, mainly in the bottom part — the developed ZnMoO crystallization technology ensures the growth of reasonably good quality scintillators with a mass of about 1 kg — which represents more than 80% yield from the initial charge of the powder in the crucible — and below 4% irrecoverable losses of the enriched material.
In the present work, we report about the study of four massive (0.3–0.4 kg) ZnMoO crystals operated as scintillating bolometers at (10–20) mK. Two scintillation elements have been cut from a boule containing molybdenum of natural isotopic composition Armengaud:2015 (), while the other two are obtained from the ZnMoO boule (Fig. 1, top left). The information about the applied molybdenum purification, the size and the mass of the produced samples are listed in Table 1. The size of crystals is chosen to minimize the material losses and to produce similar-size samples from each boule. According to Danevich:2014 (), a hexagonal shape of ZnMoO elements (e.g. see Fig. 1, top right) should provide a higher light output than the cylindrical one.
|sublimation||crystallization||ID||in boule||(h mm)||(g)|
Because of some experienced difficulties with the ZnMoO crystallization process111We observed the formation of a second phase due to an unstable melt in the ZnO-MoO system Chernyak:2015 ()., which prevented us from obtaining top quality large-mass crystals, the LUMINEU collaboration initiated an R&D on the production of large-mass radiopure lithium molybdate scintillators Bekker:2016 (); Velazquez:2017 (). Thanks to the low and congruent melting point of LiMoO, the growth process is expected to be comparatively easier than that of ZnMoO. However, the chemical affinity of lithium and potassium results in a considerably high contamination of K (0.1 Bq/kg) in LiMoO crystal scintillators, as it was observed in early studies of this material Barinova:2009 (). Despite the low of K, random coincidences of K and events of Mo can produce background in the ROI Chernyak:2014 (). In particular, a contamination level around 0.06 Bq/kg of K in a LiMoO detector with dimension mm provides the same background counting rate in the ROI as the random coincidences of the events. So, in addition to the LUMINEU specifications on U/Th contamination, the acceptable K activity in LiMoO crystals is of the order of a few mBq/kg. Therefore, the R&D on LiMoO scintillators included the radioactive screening and selection of commercial lithium carbonate samples, the optimization of the LTG Cz crystal growth and the investigation of the segregation of radioactive elements in the crystallization process.
Three samples of high purity lithium carbonate were measured by HPGe spectrometry at the STELLA facility of the Gran Sasso underground laboratory (Italy): 1) 99.99% purity grade powder produced by Novosibirsk Rare Metal Plant (NRMP, Novosibirsk, Russia) NRMP (); 2) 99.998% lithium carbonate by Puratronic (Alfa Aesar GmbH & Co KG, France) Alfa-Aesar (); 3) 99.99% raw material by Sigma-Aldrich (USA) Sigma-Aldrich (). The results are given in Table 2. The lithium carbonate produced by NRMP, the material of highest radiopurity, was selected for LiMoO crystals production. Due to the high K contamination, the Sigma-Aldrich material was rejected for further investigation.
|Chain||Nuclide||Activity in LiCO powder (mBq/kg)|
Even first attempts of the LiMoO growth by the LTG Cz technique were successful providing high quality crystal boules with masses of 0.1–0.4 kg Bekker:2016 (). The growing conditions have been optimized to extend the LiMoO crystal size up to 100 mm in length and 55 mm in diameter (0.5–0.6 kg mass) Grigorieva:2017 () allowing us to produce two large scintillating elements of about 0.2 kg each from one boule. For the present study, we developed three large LiMoO scintillators by using highly purified molybdenum oxide and high purity grade lithium carbonate. Two of them have been grown from the NRMP LiCO compound by applying a single (LMO-1 sample in Table 1) and a double (LMO-2) crystallization, while the last one (LMO-3) was grown by the single crystallization from the Alfa Aesar Li-containing powder.
|Standard||Heat||Support||Reflector||NTD sensor type|
|LUMINEU||ZMO-t||Holder||L- and S-||RMF||HR||HR|
|LUMINEU||ZMO-b||Holder||L- and S-||ESR||LR||–|
Once the LTG Cz growth of LiMoO crystals containing molybdenum of natural isotopic composition was established, we started to process molybdenum enriched in Mo. Fig. 1 (bottom left) shows a first large-mass (0.5 kg) Mo-enriched crystal boule grown at the beginning of 2016. The crystal was produced by a triple crystallization due to an accident that happened during the second crystal growth process. The second massive LiMoO crystal boule (0.6 kg; see Fig. 4 in Grigorieva:2017 ()) was grown by double crystallization at the end of May 2016. Both enriched crystals demonstrated high optical quality and have the size required for the production of two similar transparent LiMoO scintillation elements with masses of 0.2 kg (see Table 1 and Fig. 1, bottom right). Two cylindrical samples produced from the first LiMoO crystal boule were used for the bolometric tests described in the present work222All these LiMoO elements have been recently operated as a four-bolometer array in the EDELWEISS set-up at Modane Underground Laboratory (France) Poda:2017 ()..
3 Underground tests of Mo-containing scintillating bolometers
3.1 Mo-containing scintillating bolometers
The bolometers were fabricated from the crystal scintillators listed in Table 1. Each scintillating crystal was equipped with one or two epoxy-glued Neutron Transmutation Doped (NTD) Ge temperature sensors Haller:1994 (), whose resistance exponentially depends on temperature as . and are two parameters depending on the doping, the compensation level and on the geometry in the case of . In our samples, is derived to be 0.5. In the present work we used high resistance (HR) and low resistance (LR) sensors with typical parameter values = 4.8 K, = 2.2 and = 3.9 K, = 1.0 , respectively. Therefore, HR NTDs have a resistance of 10 M at 20 mK working temperature, while an order of magnitude lower resistance is typical for LR NTDs. The NTD Ge thermistors, biased with a constant current, act as temperature-voltage transducers. The thermal link to the bath was provided by Au bonding wires which give also the electrical connection with the NTD Ge sensors. In addition, each crystal was supplied with a small heater made of a heavily-doped Si Andreotti:2012 (), through which a constant Joule power can be periodically injected by a pulser system to stabilize the bolometer response over temperature fluctuations Andreotti:2012 (); Alessandrello:1998b ().
The detectors were assembled according to either LUMINEU or LUCIFER standard schemes (see Table 3). The mechanical structure and the optical coupling to the crystal scintillators are designed to optimize the heat flow through the sensors and to maximize the light collection. The standard adopted by LUMINEU for the EDELWEISS-III set-up implies the use of a dedicated copper holder where the crystal scintillator is fixed by means of L- and S-shaped PTFE clamps Armengaud:2015 (); Poda:2016 (); Poda:2015 (). The holder is completely covered internally by a reflector to improve the scintillation-light collection. For the prototype of the LUMINEU suspended tower, shown in Fig. 2, the holders were slightly modified to make the array structure able to pass through the holes in the copper plates of the EDELWEISS set-up. In case of the LUCIFER R&D standard, the crystal is fixed to a copper frame by S-shaped PTFE pieces and copper columns, as well as side-surrounded by a plastic reflective film (e.g. see in Beeman:2012a ()). This frame is thermally anchored to the mixing chamber of the dilution refrigerator.
Thin bolometric light detectors (see Table 4) were coupled to the scintillating crystals to register the scintillation light. All of them are based on high purity Ge wafers and their typical size is 44–45 mm in diameter and 0.17–0.3 mm thickness, but two detectors have slightly lower area and tens m thickness. Some light detectors were constructed according to the LUMINEU standard described in Tenconi:2015a (), with the additional deposition of a 70 nm SiO antireflecting coating on one surface of the Ge wafer to increase the light absorption Mancuso:2014 (). Another type of light detectors used in the present study was developed by the LUCIFER group Beeman:2013b (). One bolometer was assembled according to CUPID-0 mounting standard Artusa:2016 (). In all these cases, the Ge wafer is held by PTFE clamps. The last type of used light detectors is the state-of-the-art optical bolometer developed at IAS (Orsay, France) Coron:2004 (). The suspension of the Ge wafer is carried out by Nb-Ti wires in this case. All the light detectors were equipped with one NTD Ge thermistor.
|Standard||Light||Ge size (mm)||Coating||NTD sensor|
|detector ID||Type||Mass (mg)|
3.2 Underground cryogenic facilities
In the present investigations, we used two cryogenic set-ups: CUPID R&D and EDELWEISS-III located at Gran Sasso National Laboratories (LNGS, Italy) and Modane underground laboratory (LSM, France), respectively. The general description of these facilities is given in Table 5. Some features are related to the specific applications: the CUPID R&D is mainly oriented on the R&D of bolometers (including scintillating bolometers) for searches, with ROI at a few MeV, while the EDELWEISS-III set-up was conceived to perform direct dark-matter searches with the help of massive heat-ionization bolometers, with a ROI in the tens-of-keV range.
|Pirro:2006b (); Arnaboldi:2006 (); Arnaboldi:2004 ()||Armengaud:2015 (); Hehn:2016 (); EDW-performance ()|
|Location||Underground lab||LNGS (Italy)||LSM (France)|
|Rock overburden (km w.e.)||3.6||4.8|
|Type||wet||wet and dry|
|Experimental volume (L)||8||50|
|Outside mechanical decoupling||no||yes|
|Inside mechanical decoupling||yes||yes (since 2016)|
|Base temperature (mK)||7||10|
|Shield||Low activity lead (cm)||20||18|
|(external)||Roman lead (cm)||no||2|
|Boron carbide (cm)||1||no|
|Shield||Roman lead (cm)||5.5||14|
|Readout||Electronics||Cold + Room-Temp.||Cold|
|and DAQ||Dual readout channels||10 + 8||48|
|ADC digitization (bit)||18||16 or 14|
|Sampling rate (kSPS)||up to 250/N||up to 1|
|Data taking mode||trigger and/or stream||trigger or stream|
|Exceptional||K, Cs, AmBe||Th, K, AmBe|
|Pulser system||yes||yes (since 2015)|
As one can see from Table 5, an efficient suppression of the cosmic-ray flux is provided by a deep underground location of both set-ups. The EDELWEISS-III is larger and can host up to 48 scintillating bolometers with a copper holder size of 8060 mm each. The reversed geometry of the EDELWEISS-III cryostat does not allow to decouple mechanically the detectors plate from the mixing chamber, as it was done by two-stage damping system inside the CUPID R&D set-up Pirro:2006b (). The external damping system (pneumatic dampers) of the EDELWEISS-III is adapted to the operation of tightly held massive EDELWEISS detectors, and not to scintillating bolometers. In particular, thin light detectors are very sensitive to the vibrations induced by the three thermal machines of the set-up. Therefore, an internal damping inside the EDELWEISS-III has been implemented through a mechanically-isolated suspended tower (see Fig. 2). Dilution refrigerators of both set-ups are able to reach a base temperature around 10 mK.
The EDELWEISS-III set-up is surrounded by a significantly massive passive shield against gamma and neutron background. The absence of an anti-radon system as that used in the CUPID R&D is somehow compensated by a deradonized (below 20 mBq/m) air flow. The radon level is monitored continuously. An important advantage of the EDELWEISS-III set-up is a muon veto system with about 98% coverage (however, no clock synchronization with scintillating bolometers has been implemented yet).
All the EDELWEISS-III readout channels utilize a cold electronics stage, while only about half of those of CUPID R&D have this feature. The EDELWEISS-III readout system uses AC bolometer bias modulated at a frequency of up to 1 kHz, which is also kept for the demodulation procedure applied to the data sampled with a 100 kSPS rate (the modulated data can be also saved). Higher resolution without a significant enlargement of the data size is available for the CUPID R&D case, which envisages DC bolometer bias. In contrast to DC current, there are difficulties in the operation of high resistance NTDs with AC bias (e.g. unbalanced compensation of nonlinearities related to the differentiated triangular wave applied for NTD excitation — see details in EDW-performance (); Gaertner:1997 ()).
|Set-up||Run ID||Detectors||Sampling (kSPS)||Data|
An important difference between the set-ups is in the calibration procedure and the related policy. The CUPID R&D is well suited for a regular control of the detector’s energy scale in a wide energy range up to 2.6 MeV. On the contrary, a periodical calibration with the EDELWEISS-III set-up is available only with a Ba source (’s with energies below 0.4 MeV), while the insertion of a Th source, as well as a few other available sources, requires lead/polyethylene shield opening, which is not supposed to be done frequently. Also, there is prohibition of the in-situ use of Fe sources for the light detectors calibration, which is not the case for the CUPID R&D case. Finally, the control of the detector thermal response by a pulser system connected to the heaters is available for both set-ups.
3.3 Low-background measurements and data analysis
The list of the low-background bolometric experiments and the main technical details are given in Table 6. The bias currents of the order of a few nA were set to maximize the signal-to-noise ratio resulting in a working-point thermistor resistance of a few M.
An optimum filter technique Gatti:1986 (); Radeka:1967 () was used to evaluate the pulse height and shape parameters. It relies on the knowledge of the signal template and noise power spectrum; both are extracted from the data by averaging about 2 MeV energy signals (40 individual pulses) and baseline waveforms (5000 samples), respectively. For those detectors that were equipped with two temperature sensors, the data of the thermistor with the best signal-to-noise ratio were analyzed. The light-detector signal amplitude is estimated at a fixed time delay with respect to the heat signals as described in Piperno:2011 (). Due to spontaneous temperature drifts, the amplitudes of the filtered signals from the crystal scintillator are corrected for the shift in thermal gain by using the heater pulses333The data of Run308 have been stabilized by using events of Po from crystal contamination and it gives results similar to those of the heater-based stabilization method Armengaud:2015 ()..
The heat response of the scintillating bolometers is calibrated with quanta of Th (238.6, 338.3, 510.8, 583.2, 911.2, and 2614.5 keV), K (1460.8 keV) and/or Ba (356.0 keV) sources. The light detectors in the CUPID R&D set-up were calibrated with the Mn X-ray doublet (5.9 and 6.5 keV) of the Fe source.
4 Performance of Mo-containing scintillating bolometers
4.1 Time profile of the pulses
The rising edge of the bolometric signal depends on the sensitivity of the sensor to athermal and/or thermal phonons created by a particle interaction and has a characteristic time ranging from microseconds (dominant athermal component) to milliseconds (dominant thermal component). Since the NTD sensors are sensitive mainly to thermal phonons, the rise time of the tested detectors given in Table 7 is within the expectation. The heat detectors have longer leading edge (tens of ms) than that of light detectors (few ms) due to the larger volume and therefore to the larger heat capacity of the absorber.
The decaying edge time constant of the bolometric signal represents the thermal relaxation time, which is defined by the ratio of the heat capacity of the absorber to the thermal conductance to the heat bath. Therefore, it strongly depends on the material, the detector coupling to the heat bath and on the temperature. As one can see in Table 7, the variation of the decay time is even larger than that of the rise time, but again it has the typical values normally observed in light detectors (tens of ms) and massive bolometers (hundreds of ms). The improved coupling of the NTD sensor to the heat bath of B297 and B304 light detectors Coron:2004 () leads to shorter decay time at the level of a few ms.
The only exception in the signal time constants of massive detectors is evident for both ZnMoO crystals, the largest of all tested samples, which exhibit signals faster by about a factor 2 than the other devices, in particular those tested in the same set-up and at similar temperatures. It is interesting also to note that the bottom crystal is twice faster than the top one, as it was observed also in the test of the 60 g ZnMoO detectors Barabash:2014 (). The fast response of the enriched ZnMoO bolometers has no clear explanation, but it is probably related to crystal quality. However, a fast detector response is crucial for a separation of the Mo events pile-ups Chernyak:2012 (); Chernyak:2014 (); Chernyak:2017 (). Thus, this feature of ZnMoO scintillators (e.g. below 10 ms) can lead to a better capability to discriminate random coincidences by heat pulse-shape analysis than that considered in Ref. Chernyak:2014 ().
4.2 Voltage sensitivity
In a bolometric detector, the response to a nuclear event is a temperature rise directly proportional to the deposited energy and inversely proportional to the detector heat capacity. A thermistor converts the temperature variations to a voltage output, digitized by the readout system. Therefore, a bolometric response is characterized by a voltage sensitivity per unit of the deposited energy.
A signal pulse height of the order of few tens to hundreds nV/keV is typical for NTD-instrumented massive bolometers. This figure corresponds to what is observed in all the tested crystals (see Table 7).
The reduced size of both the absorber and the sensor of light detectors (see Tables 3 and 4) leads to lower heat capacities and therefore to higher sensitivites, which are in the range 1–2 V/keV for a good-performance
|Size (h mm)||5040||6040||6040||4040||5040||5040||4440||4444|
|Light detector ID||M3||Lum12||GeB||GeT||B297||GeB||B304||Lum11||GeOld|
|Size (h mm)||440.17||440.17||450.30||450.30||400.045||450.30||250.030||440.17||450.30|
|(eV)||X-ray Mn, 5.9 keV||–||–||787(3)||289(1)||334(4)||555(5)||504(4)||–||303(2)|
|(keV)||Ba, 356 keV||3.7(1)||5.1(1)||–||–||–||–||–||2.54(4)||–|
|Ac, 911 keV||4.4(7)||10(1)||5.6(7)||9(2)||2.0(3)||3.9(6)||3.1(6)||3.1(5)||3.1(2)|
|K, 1461 keV||–||7.9(2)||6.7(6)||14(1)||–||4.2(3)||4.4(3)||4.1(2)||–|
|Tl, 2615 keV||9(1)||12(1)||9.1(7)||22(2)||3.8(6)||6(1)||4.7(7)||6.3(6)||5.0(5)|
|Po, 5407 keV||8.8(1)||9.0(2)||47||100||7(2)||9(1)||9(2)||5.4(3)||–|
|* — Estimations are based on rough calibrations by scintillation light (see Section 4.2).|
|** — Low light yield is caused by non-optimal light collection conditions of the measurements (see Section 4.4).|
detector. This is the case for all the tested light detectors444In order to estimate the performance of the light detectors operated in the EDELWEISS-III set-up, we roughly calibrated them by using the heat-light data of Th runs and assuming that the light yield (see Sec. 4.4) for events is equal to 1.0 and 0.77 keV/MeV for ZnMoO and LiMoO detectors, respectively. The assumption about the scintillation yield of ZnMoO is based on early investigations of the similar size detectors. In the case of LiMoO, we expect similar scintillation properties of the samples produced from the same boule. (see in Table 7), except the ones with even smaller size (B297 and B304) and subsequently sensitivity enhanced by up to one order of magnitude.
4.3 Energy resolution
Most of the used light detectors have similarly good performance also in terms of energy resolution, in particular their baseline noise is 0.14–0.5 keV FWHM (see in Table 7). The only exceptions are detectors with enhanced sensitivity (B297 and B304) for which the baseline noise is below 0.05 keV FWHM. However, they also exhibit a strong position-dependent response, therefore the energy resolution measured with an uncollimated Fe source is near to that obtained with the other light detectors (FWHM 0.3–0.8 keV at 5.9 keV).
As it was mentioned above, better than 10 keV FWHM energy resolution at the ROI is one of the most crucial requirements for cryogenic double-beta decay detectors. This goal was successfully achieved with both natural and Mo-enriched ZnMoO and LiMoO based bolometers555The operation of the ZMO-t bolometer, a twin of the ZMO-b, was severely affected by an insufficient tightening of the PTFE elements, therefore we omit quoting its performance. (see Table 7). Below we discuss the obtained results.
The LiMoO detectors exhibit twice better energy resolution than the ZnMoO ones and the achieved values of 4–6 keV FWHM at 2615 keV are at the level of the best resolutions ever obtained with massive bolometers Artusa:2014a (); Artusa:2014b (); Alduino:2016 (). In particular, the energy resolution of LiMoO bolometers is comparable to the performance of the TeO cryogenic detectors of the CUORE-0 experiment (the effective mean FWHM at 2615 keV is 4.9 keV with a corresponding RMS of 2.9 keV Alduino:2016 ()). This is mainly due to the fact that LiMoO, as TeO, demonstrates a low thermalization noise, i.e. a small deviation of the energy resolution from the baseline noise width. The results of the ZnMoO and LiMoO detectors show possible improvement of the energy resolution by lowering of the temperature, as it is expected thanks to increased signal sensitivity. A dependence of the performance on the sample position in the ZnMoO boule, observed early with small Barabash:2014 () and now with large samples, is also evident. It could be related to the degradation of the crystal quality along the boule. Thanks to the higher crystal quality, no such effect is observed for LiMoO crystals.
The energy spectra of a Th source measured by the Mo-enriched bolometers (enrZMO-t and enrLMO-b) and the corresponding energy-dependance of the heat-channel resolution are illustrated in Fig. 3. The chosen data of ZnMoO and LiMoO detectors represent the typical energy resolution for bolometers based on these materials in case of optimal experimental conditions (Table 7). Using the fitting parameters for the curves shown in Fig. 3 (right), the expected energy resolution of the enrZMO-t and enrLMO-b cryogenic detectors at of Mo is 9.70.1 keV and 5.40.1 keV, respectively. Thus, the energy resolution of the ZnMoO detectors is acceptable but needs still an optimization, while LiMoO bolometers already meet the resolution required for future generation bolometric experiments Beeman:2012 (); Beeman:2012a (); Artusa:2014a (); CUPID ().
4.4 Response to s and particle identification capability
4.4.1 Scintillation-assisted particle discrimination
By using coincidences between the heat and the light channels, one can plot a light-vs-heat scatter plot as the ones presented in Fig. 4. The heat channel of all data shown in Fig. 4 is calibrated by means of quanta of the calibration sources and it leads to 10% heat miscalibration for particles due to a so-called thermal quenching, common for scintillating bolometers (e.g. see results for different scintillators in Beeman:2012 (); Cardani:2013 (); Artusa:2016 (); Arnaboldi:2010 ()). Therefore, in order to present the correct energy of the events, an additional calibration based on the peaks identification is needed.
As it is seen in Fig. 4, the light-vs-heat scatter plot contains two separated populations: a band of ()’s and a distribution of events associated to decays. This is due to the fact that the amount of light emitted in an oxide scintillator by particles is quenched typically to 20% with respect to quanta ( particles) of the same energy (see, e.g., Ref. Tretyak:2010 ()). Therefore, the commonly used particle identification parameter for scintillating bolometers is the light yield (), that we will define as a ratio of the light-signal amplitude measured in keV to the heat-signal amplitude measured in MeV.
The data of all detectors with directly calibrated light channel (all measurements at LNGS) have been used to determine the and values of () and events selected in the heat-energy range 2.5–2.7 MeV and 2.5–7 MeV666The interval for the selection of s is reduced to 2.5–3.5 MeV for the measurements performed with a smeared source (enrZMO-t, enrZMO-b, and LMO-2 detectors)., respectively. In spite of the quite evident constancy of the in a wide energy range (as it is seen from the slop of s in Fig. 4), the event selection was applied above 2.5 MeV, because the same distributions have been used to calculate / discrimination power (see below) close to the ROI of Mo. The values extracted from the present data are given in Table 7.
The light yields for events measured with both ZnMoO scintillating bolometers are in the range 1.2–1.3 keV/MeV, similar to the results of previous investigation of natural (see Beeman:2012b (); Berge:2014 () and references therein) and Mo-enriched Barabash:2014 () ZnMoO detectors. Thanks to the progress in the development of high quality lithium molybdate scintillators — as documented in the present work and recently in Ref. Velazquez:2017 () — the values for LiMoO and LiMoO scintillation bolometers, which lay in the range 0.7–1 keV/MeV, become comparable to the light yields of the ZnMoO detectors. The improvement of the with respect to the early investigations with LiMoO detectors Barinova:2010 (); Cardani:2013 () is of about a factor of 2. One LiMoO bolometer (LMO-3) was viewed by a light detector with a significantly lower area implying a reduced light collection and consequently a rather small value of 0.12 keV/MeV.
The ratio of the parameters for s and ()s gives the quenching factor of the scintillation light signals for particles: . An absolute light detector calibration is not needed to calculate this parameter. As it is seen in Table 7, the results for ZnMoO and LiMoO detectors are similar showing 20% quenching of the light emitted by particles with respect to the () induced scintillation.
The efficiency of discrimination between and populations can be characterized by the so-called discrimination power parameter defined as:
where () denotes the average value (width) of the or () distribution. The value is estimated for () and events selected for the determination (see above).
As reported in Table 7, the achieved discrimination power for all the tested detectors is = 8–21, which implies a high level of the /() separation: more than 99.9% rejection while preserving practically 100% signal selection efficiency. The separation efficiency is illustrated in Fig. 5 for the scintillating bolometer enrZMO-t with the lowest achieved due to the modest performance of the GeB light detector. It is to emphasize the 0.1 keV/MeV obtained with the LMO-3 detector, which would not allow effective particle identification by using a standard-performance light detector with 0.2–0.5 keV FWHM baseline noise777This is the case for Cherenkov light tagging in TeO bolometers; e.g. see Ref. Artusa:2017 ().. However, the performance of the B304 optical bolometer — which featured 0.02 keV FWHM baseline noise — was high enough to provide highly-efficient particle identification even with this detector ( = 11).
4.4.2 Peculiarities in particle identification
Fig. 4 illustrates observed peculiarities of some detectors which could affect the particle identification capability. These peculiarities are originated either by a noise-affected detector performance or by a feature of the detector’s response to s, which exhibits classes of events with more quenched or enhanced light signals. Below we will discuss briefly these observations and their impact on background in a decay experiment with Mo.
High vibrational noise in a light detector affects the precision of the light-signal amplitude evaluation, especially for events with a low scintillation signal ( events and ()s below 1 MeV). This was an issue of the measurements with ZnMoO detectors in Run308 and Run309, and this effect is apparent in Fig. 4 (top left). The problem can be solved by using a mechanically isolated system inside a cryostat (see Table 5 and e.g. Refs. Pirro:2006b (); Lee:2017 (); Olivieri:2017 ()). In particular, a stable and reliable light-channel performance of the ZnMoO scintillating bolometer in the suspended tower (Run310) is evident from Fig. 4 (top right).
The data of natural and Mo-enriched ZnMoO bolometers contain some events that have more quenched light output and enhanced heat signals; e.g. see “dark hot ” in Fig. 4. In the past, the same effect was observed in bolometric tests of small ZnMoO Gironi:2010 () and ZnMoO Mancuso:2016 () crystals which also exhibit defects and macro inclusions. A major part of such events is distributed close to Po ( structures at around 6 MeV in electron-equivalent energy in Fig. 4), the main contamination of the investigated ZnMoO crystals. Only a short-range () interaction in the crystal bulk exhibits this anomaly, because it is not evident either for interactions at the crystal surface (energy-degraded events) or for () events which have longer mean path in the crystal than the bulk ’s. This phenomenon is probably related to the thermal quenching, as suggested by the pronounced anti-correlation between light and thermal signals in the response. The effect is more evident for the enriched crystals, which contain more inclusions than the natural ones: e.g. about 40% of Po events acquired by the enrZMO-b detector are attributed to the “dark hot ”, while four times lower amount of such events is observed in the ZMO-b bolometer. Thereby, the origin of this anomaly in the response to interactions is probably related to the crystals imperfections. Taking into account that two electrons are expected in the of Mo, we expect that the signal is unaffected by this anomaly. Furthermore, it does not affect the detector’s capability to identify and reject the -induced surface events, which constitute the most challenging background in a bolometric experiment without particle identification. Negative effects are only expected on the precision of the spectroscopy, which however is important not only to build a background model through radiopurity determination, but also for the off-line rejection of - delayed events from decays of Bi-Tl Beeman:2012 (); Pirro:2006 (); Beeman:2012b ().
The light-vs-heat data of several scintillating bolometers contain also events with an enhanced light signal with respect to those of the prominent distribution. As it is seen in Fig. 4, these events belong to three families: BiPo events, surface events with escaped nuclear recoils hitting the light detector, and the so-called “bright ”.
The first family consists of unresolved coincidences in Bi-Po decays (BiPo events in Fig. 4). Due to the slow bolometric response, the decays of Bi ( = 2254 keV) and Bi ( = 3272 keV) overlap with subsequent decays of Po ( = 8954 keV, = 0.3 s) and Po ( = 7833 keV, = 164 s), respectively. Therefore, they are registered as a single event with a heat energy within 8–11 MeV range and a light signal higher than that of a pure event of the same energy. Since the BiPo events are distributed far away from 3 MeV, they have no impact on the ROI of Mo.
In a case of an decay on a crystal surface, a nuclear recoil (or an particle) can escape from the scintillator and hit the light detector. Such events belong to the second family indicated in Fig. 4. Taking into account that only a few keV energy-degraded recoil can mimic a light signal of ZnMoO or LiMoO bolometer, the heat energy release has to be close to the nominal -value additionally enhanced due to the thermal quenching. Therefore, independently on the surface activity of radionuclides from U/Th chains (4–9 MeV -values), they cannot populate the ROI of Mo. Among other natural -active nuclides, a probable contaminant is Pt ( = 3252 keV Wang:2017a ()) due to the crystal growth in a platinum crucible. However, even in such case the expected heat signal is about 0.5 MeV away from the of Mo, as well as the Pt bulk contamination in the studied crystals is expected to be on the level of a few Bq/kg Armengaud:2015 (). We can therefore conclude that also this class of events does not play a role in the search for the decay of Mo.
The last family — consisting of “bright ” events in Fig. 4 — stem from the documented scintillation properties of the reflecting film. Specifically, an energy deposition in this film can take place for surface-originated decays, which can produce a heat and a light signal in the scintillating crystal but also a flash of scintillation light from the reflecting film, which adds up to that of the crystal scintillator. This results into an enhanced light signal. Consequently, the population of energy-degraded events can leak to the ROI of Mo in the heat-light scatter plot, prviding an unavoidable background. To check the scintillation response of the 3M film, we have performed a test using a photomultiplier and a Pu source. The observed scintillation is at the level of 15%–34% relatively to NE102A plastic scintillator (depending on the side of the film facing the photomultiplier). Therefore, such a feature of the reflector spoils the particle discrimination capability of the detector. In order to solve this issue, a reflecting material without scintillation properties has to be utilized or the reflecting film has to be omitted888The results of the recently-completed Run311 in the EDELWEISS-III set-up, in which both LiMoO detectors enrLMO-t and enrLMO-b were operated without the reflecting foil, demonstrate the capability of particle discrimination at the level of 9 sigma in spite of half light-collection efficiency resulting in 0.4 keV/MeV light yield Poda:2017 ()..
4.5 Response to neutrons
The ZMO-b, LMO-1, and enrLMO-t detectors were also exposed to neutrons from an AmBe source. The results for Li-containing bolometers are illustrated in Figs. 6 and 7 (left). The () band exceeds the natural Tl end-point because of the prompt de-excitation ’s following Be(,n)C reaction. The cluster of events in the region is caused by the reaction Li(n,t) (-value is 4784 keV ENDF ()). The Li has a natural abundance of 7.5% Meija:2016 (), and the large cross section for thermal neutrons (940 barns ENDF ()) gives rise to the clear distribution at a heat energy of around 5 MeV. In the energy scale, the distribution is shifted by about 7 with respect to the 4784 keV total kinetic energy released in the reaction. The energy resolution (FWHM) on the peak was measured as 7.7(3) and 5.9(2) keV for the LMO-1 and enrLMO-t detectors, respectively. This is an unprecedented result obtained with Li-containing detectors (e.g. compare with the results of Li-containing cryogenic detectors in Refs. Cardani:2013 (); Martinez:2012 (); Gironnet:2009 () and references therein). A second structure at higher energy is attributed to the non-thermal neutrons, in particular to the resonant absorption of 240 keV neutrons. A linear fit to the less prominent lower band, ascribed to nuclear recoils induced by fast neutron scattering, gives a light yield of 0.07(2) keV/MeV.
4.6 Particle identification by heat signals
As it was shown before, particles exhibit a higher heat signal than ()s of the same energy. Even if a clear interpretation of this effect is lacking, this is probably related to the details of the phonon production mechanism in the particle interaction, which can lead to phonon populations with different features depending on the particle type. Therefore, one could expect some difference also in the shape of the heat signals between and () events and hence a pulse-shape discrimination capability of scintillating bolometers 999And vice versa, the negligible, if any, difference in the thermal response to s and ()s, e.g. reported for TeO Bellini:2010 (), is probably responsible for the lack of a particle identification by the pulse-shape of non-scintillating bolometers, as it is the case of the TeO bolometers..
Previous measurements with ZnMoO detectors demonstrated the possibility of pulse-shape discrimination by using only the heat channel Beeman:2012a (); Gironi:2010 (); Beeman:2012b (). However, the discrimination ability strongly depends on the experimental conditions and sometimes can fail Cardani:2014 (). No indication of this possibility has been claimed so far for LiMoO bolometers.
A tiny difference between and () heat pulses of the enrZMO-t detector (about 3% in the rising edge) allows us to perform an event-by-event particle identification using only the heat signals (e.g. = 3.8 was obtained for 2.5–3.5 MeV data). The data of the enrZMO-b bolometer, more affected by noise, show partial pulse-shape discrimination. It is worth noting that these results were obtained in spite of a low sampling rate (1 kSPS) and in one of the worst noise conditions among all the tested detectors.
LiMoO-based bolometers also demonstrate the possibility of the pulse-shape discrimination by a heat-signal shape analysis. Unfortunately, the data of most detectors were acquired with a low sampling rate (1 kSPS) and/or do not contain a large statistics of and radiation in the same energy range, essential condition to investigate precisely this remarkable feature. However, significant results have been obtained by the analysis of the neutron calibration data (2 kSPS) of the LMO-1 detector. An example of the tiny difference in the time constants of () and heat pulses (less than 0.5 ms, i.e. a bin for the 2 kSPS sampling) is reported in Fig. 7 (right). By exploiting the rise and decay time parameters, we evaluated a between s in the 2.5–7 MeV and -triton events in the 5–7 MeV range as 5.4 and 8.1, respectively. These results could probably be improved by using other pulse-shape parameters, as it was demonstrated with ZnMoO detectors Beeman:2012a (); Gironi:2010 (). However, due to a few per mille difference of the thermal signals induced by ()s and s, the pulse-shape discrimination of scintillating bolometers is expected to be less efficient in comparison to the light-assisted particle identification which exploits an about 80% difference in response (an exception for ZnMoO has been reported in Beeman:2012a ()). This is also the case for the LMO-1 detector, for which the double read-out allows to reach about twice better discrimination power. However, the requirement of 99.9% rejection of -induced background (with a acceptance larger than 90%) is achieved even for 3, therefore pulse-shape discrimination with the heat signals only could allow to simplify the detector structure and to avoid doubling the read-out channels in a CUPID-like experiment.
5 Backgrounds and radiopurity of Mo-containing scintillating bolometers
5.1 Alpha background
The spectrum measured by the ZMO-b detector in Run308 can be found in Armengaud:2015 (); Poda:2015 (), therefore the illustration of other spectra of the ZnMoO bolometers is omitted. The background spectra of events accumulated by the natural LiMoO and all the enriched detectors are shown in Fig. 8 and Fig. 9, respectively. The anomaly (“dark hot ”) in the response to s in the ZnMoO bolometers was corrected by using the results of the fit to the Po events distribution in the -vs-heat data. The Th calibration data (168 h) of the enrLMO-b detector were combined with the background data to increase the statistics.
All the crystals exhibit a contamination by Po, however we cannot distinguish precisely a surface Po pollution from a bulk one. Furthermore, most likely the observed Po is due to Pb contamination of the crystals, as this is the case for the ZnMoO scintillator (ZMO-b) Armengaud:2015 (). The LMO-3 crystal, produced from the LiCO compound strongly polluted by Ra (see Table 2), is contaminated by Ra too. There is also a hint of a Ra contamination of the other natural ZnMoO and LiMoO crystals (ZMO-t, ZMO-b, LMO-1, and LMO-2), but the low statistics does not allow to estimate Ra activity in the crystals. In addition to Po and Ra, both ZnMoO crystals demonstrate a weak contamination by U and U.
The spectra were analyzed to estimate the activity of radionuclides from the U/Th chains and Pt. Determination of Pt activity in the detectors operated with the smeared sources (enrZMO-t, enrZMO-b, and LMO-2) is difficult. We assumed that the energy resolution of the peaks searched for is the same as the resolution of the Po peak present in the spectra of all detectors. The area of the peaks was determined within energy interval, where is a standard deviation of the Po peak. If no peak observed, the Feldman-Cousins approach Feldman:1998 () was applied to determine upper limits at 90% C.L. A summary of the radioactive contamination of the natural and Mo-enriched ZnMoO and LiMoO crystals is given in Table 8.
The measured a