Limits and performances of a BaWO4 single crystal

Nuclear Inst. and Methods in Physics Research, A 901 (2018) 150–155

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Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima

Limits and performances of a BaWO4 single crystal V. Caracciolo a, *, F. Cappella b,c , R. Cerulli d,e , A. Di Marco d,e , M. Laubenstein a , S.S. Nagorny f , O.E. Safonova g , V.N. Shlegel h a

INFN, Laboratori Nazionali del Gran Sasso, I-67100 Assergi (AQ), Italy INFN sezione Roma, I-00185 Rome, Italy Dipartimento di Fisica, Università di Roma ‘‘La Sapienza’’, I-00185 Rome, Italy d INFN sezione Roma ‘‘Tor Vergata’’, I-00133 Rome, Italy e Dipartimento di Fisica, Università di Roma ‘‘Tor Vergata’, I-00133, Rome, Italy f Department of Physics, Queen’s University, Kingston, ON K7L 3N6, Canada g V.S. Sobolev Institute of Geology and Mineralogy of the Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russia h Nikolaev Institute of Inorganic Chemistry, 630090 Novosibirsk, Russia b c

ARTICLE Keywords: Scintillator BaWO4 Double beta decay Rare events

INFO

ABSTRACT A Barium Tungstate single crystal (BaWO4 ) was produced using the low thermal gradient Czochralski technique. In this paper the results concerning its light emission and radioactive contaminants are presented. The aim of this work is to investigate the possibility to realize BaWO4 crystals with suitably features to study the double beta decay of 130 Ba and 132 Ba isotopes by the ‘‘source=detector’’ approach. The results show the limitations of a BaWO4 crystal as a scintillator and give some idea on how to overcome them in order to take profit from the potentiality of this single crystal.

1. Introduction The double beta decay with emission of two neutrinos (2𝜈2𝛽) [1] is a nuclear transition allowed in the Standard Model (SM) of particle physics and observable in many even–even nuclei. This transition has so far been measured for several nuclei [2]. Moreover, neutrinoless double beta (0𝜈2𝛽) decay [3] is forbidden in the theoretical framework of the SM; it is, however, predicted in several theoretical SM extensions [4–7]. Therefore, 0𝜈2𝛽 decay is a very suitable approach to investigate some of the major unresolved issues in particle physics, such as lepton number conservation, Majorana nature of neutrinos, and neutrino masses. In addition, experimental data on the 2𝜈2𝛽 decay could be very useful to improve theoretical calculations of the decay probability related e.g. to the nuclear matrix elements [5]. Natural barium contains two potentially 2𝛽 active isotopes,130 Ba (Q2𝛽 = 2618.7(2.6) keV) and 132 Ba (Q2𝛽 = 844.0(1.1) keV) [8,9]. The 130 Ba isotope is of particular interest because its double beta decay is not yet detected unambiguously; at present there is only an indication of a multichannel weak decay (2𝛽 + , 𝜖𝛽 + and 2𝜖) in 130 Ba by geochemical experiments with determined values of the half-life of 𝑇1∕2 = (2.2 ± 0.5) × 1021 y [10] and 𝑇1∕2 = (6.0 ± 1.1) × 1020 y [11]. The first direct laboratory search for 2𝛽 decays of 130 Ba was performed by using a BaF2 *

Corresponding author. E-mail address: [email protected] (V. Caracciolo).

https://doi.org/10.1016/j.nima.2018.06.005 Received 16 May 2018; Accepted 1 June 2018 Available online 15 June 2018 0168-9002/Crown Copyright © 2018 Published by Elsevier B.V. All rights reserved.

crystal scintillator [12], where only 𝑇1∕2 limits were obtained at the level of ≈ 1017 yr. In this frame, the development of a Ba-containing crystal scintillator is a very powerful tool to investigate through the lowbackground ‘‘source = detector’’ approach the two-neutrino and zeroneutrino double-beta decay in Ba isotopes. There are more than 60 Ba-containing compounds that have been investigated as crystal scintillator by different groups. A detailed list of these compounds and their main operational properties is reported in [13]. In Table 1, instead, we have listed just the most prospective ones from the point of view of light yield and radiopurity. In this paper we report on recent progress in the production of high quality, large volume BaWO4 single crystals using the Low Thermal Gradient (LTG) Czochralski method with highly purified material—WO4 powder [14– 20]. The combination of these factors looks very promising for the production of high quality BaWO4 given the long history of success producing WO4 -based crystal [19,21–24,14]. 2. Development of a commercial BaWO𝟒 crystal One possible way to produce low-background high quality crystals is to use the LTG Czochralski technique [55–57], which operates with a closed Pt-crucible. An additional and very important advantage of this

V. Caracciolo et al.

Nuclear Inst. and Methods in Physics Research, A 901 (2018) 150–155 Table 1 Inorganic Ba-based scintillating compounds and some of their optical properties [13]. Formula(dop.)

Light yield (photons/keV)

𝜆 Emission peak (nm)

Reference

Ba2 GdCl7 (Ce) Ba2 Si3 O8 (Eu) Ba2 SiO4 (Eu) Ba3 (PO4 )2 (Eu) Ba3 P4 O13 (Eu) Ba5 Si8 O21 (Eu) BaBr1.7 I0.3 BaBr2 BaBr2 (Ce) BaBrCl(Eu) BaBrI(Eu) BaCl2 (Eu) BaClBr(Eu) BaClI(Eu) BaF2 BaFI(Eu) BaGdCl5 (Ce) BaHfO3 (Ce) BaKPO4 (Eu) BaSi2 O5 (Eu) Cs2 BaBr4 (Eu) Cs2 BaCl4 (Eu) Cs2 BaI4 (Eu) CsBa2 Br5 (Eu) CsBa2 I5 (Eu) CsBa2 I5 (In) CsBa2 I5 (Na) CsBa2 I5 (Tl) CsBa2 I5 (Yb) K2 BaI4 (Eu) KBa2 I5 (Eu) NaBaPO4 (Eu)

30 35 22; 40 27 25 20 112 19.3 ∼10.3 ∼45 85 19; 52 52 54 1.3–63.9 55 35 40 35 30 ∼25 ∼30 ∼17 ∼70 ∼90 35 33 40 54 63 90 20

355; 377 505 502; 505; 525 42 440 511 414 425; 475 345; 370 405 413 ∼400

[25] [25] [25,26] [25] [25] [25] [27] [28] [29,30] [31,32] [33,34,27,35] [28,30,36,37] [38] [38] [39–46] [47] [25] [48] [25] [25] [38] [38] [38] [38,49] [50,34,35,51] [52] [52] [52] [53] [54] [54] [25]

∼200; 310; ∼360 405 363; 389 400 425 520

435 540 430

448 444 450; 610

Fig. 1. (Left) BaWO4 boule (about ø45 mm × 45 mm). (Right) BaWO4 single crystal used in the present paper (ø28 mm × 23 mm). No inclusions, visible defects and cracks into the crystal volume have been observed.

technique is the small temperature gradient at the level of about 1 K/cm, which is a one to two orders of magnitude lower than the conventional Czochralski method where it can reach 60–100 K/cm. Due to the low temperature gradient, and therefore low evaporation of material from the melt, the losses do not exceed 0.5% of the initial charge. Last, but not the least, the LTG Czochralski technique allows to crystallize up to 90% of the loaded charge (already achieved for BGO [58], cadmium tungstate [14,59] and zinc molybdate [60] crystals). This feature is crucial in case of costly materials, containing enriched isotopes or highly purified initial components. The BaWO4 single crystal we used in this study was grown using the LTG Czochralski technique at the V.S. Sobolev Institute of Geology and Mineralogy of the Siberian Branch of the Russian Academy of Sciences (Novosibirsk, Russia). To produce the BaWO4 crystal, a furnace with resistive heating and a configuration similar to the one described in [61] were used. This configuration with a three-zone heater allows to change the thermal conditions within wide

limits and to ensure an almost flat front of crystallization. The best results were obtained with a slightly convex crystallization front and a crystallization rate of 1–1.5 mm/h. The tungsten oxide powder (WO3 ), which was additionally purified similar to that used for the production of ZnWO4 [62] and 116 CdWO4 [63] crystals, and commercially available barium carbonate (BaCO3 ) of 99.5% purity grade were used as initial components for the crystal production charge. The stoichiometric mixture of the initial components was loaded into a platinum crucible ø70 mm × 150 mm, and gradually heated with a rate of 100 K/h up to the melting point (about of 1500 ◦ C) [64]. The total duration of the homogenization and synthesis processes was about 10 h. Despite the relatively high temperatures applied, the platinum crucible remained in the air atmosphere without significant changes in geometry and weight. The total mass of loaded BaWO4 charge in the crucible was 550 g, obtaining eventually a BaWO4 single crystalline boule – without visible 151

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Fig. 2. Energy spectra taken with the BaWO4 sample over 89.17 h (dark blue) and without the sample over 1167 h (orange). The red-marked peaks are related to the radionuclides listed in Table 2. Table 2 Radioactive contaminations of the BaWO4 single crystal. The upper limits are given at 95% C.L., and the expanded standard uncertainties with an expansion coefficient of 𝑘 = 1.

defects, inclusions or cracks – of 380 g (about of ø45 mm × 45 mm) (see Fig. 1 left side). Thus, the yield was 69%. From this single crystalline boule a cylindrical sample with dimensions of ø28 mm × 23 mm and mass of 89.4 g was cut and optically polished (see Fig. 1 right side). The mass concentration of Ba isotopes in the crystal structures is ∼36%.

Chain

3. Radioactive contaminations

Nuclide

Activity (Bq/kg)

40

K 137 Cs

<0.39 <46 ⋅ 10−3

228 Ra

1.12 ± 0.11 0.700 ± 0.065

232 Th

The BaWO4 crystal scintillator was measured for 89.17 h with an ultra-low background high purity germanium (HPGe) 𝛾 ray spectrometer in the STELLA (SubTerranenan Low Level Assay) facility of the Laboratori Nazionali del Gran Sasso (LNGS) [65]. The detector has a volume of 433 cm3 and 98.7% relative efficiency. This detector has a rather thin Cu window of 1 mm thickness, that improves the gamma detection efficiency at low energies. The passive shield of the detector consists of 5 cm of electrolytic copper and 20 cm of graded lead. The set-up is sealed in an air-tight stainless steel box continuously flushed with high purity nitrogen gas to avoid the presence of residual radon. The STELLA facility is located deep underground at the LNGS (average overburden of 3600 m water equivalent) [66]. In Fig. 2 the measured energy spectra of the background and of the sample are shown. In order to determine the radioactive contamination of the sample, the detection efficiencies were calculated using a Monte Carlo simulation based on the GEANT4 software package [67]. Peaks in the spectra due to the naturally occurring daughter radionuclides of the uranium and thorium chains are observed. Contaminations of 228 Ra, 228 Th, 226 Ra and 234𝑚 Pa at the level of 1.12(11) Bq/kg, 0.700(65) Bq/kg, 24.4(1.0) Bq/kg, 7(2) Bq/kg respectively have been measured while only upper limits have been set for other radionuclides (see Table 2). Broken radioactive secular equilibrium in the U and Th chains was observed, as it typically occurs due the crystal growing process. Since the same WO3 powder was previously used for the ZnWO4 and 116 CdWO4 crystal production [68,20], where such level of contamination was not observed, we can conclude that BaCO3 is the main origin of such radiocontamination. This assumption, moreover, is supported considering the radioactive contamination of the BaF2 crystal scintillator developed in [12]. Thus, further purification of BaCO3 powder is strongly required.

228 238

Th

U 226

Ra Th 234𝑚 Pa 235 U 234

235 U

24.4 ± 1.0 <38 7±2 <0.31

PL was excited using a nitrogen laser (180 and 350 nm wavelength). Emission spectra of the crystal are presented in Fig. 3. A complex PL band structures is observed: (i) a PL band in the ultra violet region with a maximum at 350 nm; (ii) a second PL band in the visible region with different maxima in the range (400–500) nm; (iii) a more modest PL band in the range (560–700) nm, in agreement with the results obtained by other authors for BaWO4 crystals [70]. In Fig. 3 a strong emission in the blue region is observed for 180 nm and 350 nm excitations, and a modest emission in the green region is shown (see Fig. 3, left). In Fig. 3 (right), the peak at 700 nm is the transmission of the second-order diffraction of the excitation at 350 nm wavelength [69]. Moreover, an ultra-violet luminescence, peaked at ∼330 nm, is prominently observed (see Fig. 3, left). These results are similar to the case of blue luminescence observed at room temperature using other WO4 compounds as CdWO4 , CaWO4 , SrWO4 , PbWO4 [14,71]. In fact, the blue emission from the scheelite tungstate crystals is known to be due to radiative transitions within (WO4 )2− molecular complexes [72]. 5. Transmittance properties of the BaWO𝟒 crystal The transmittance of the BaWO4 scintillation crystal was measured in the spectral range (300–700) nm using a SPECORD UV–Vis spectrophotometer along the 𝑧-axis of the crystal. The results of the optical transmission measurements are shown in Fig. 4, and demonstrate that the sample exhibits good transmission properties in the relevant wavelength range (430–700) nm of the BaWO4 emission spectrum. As shown in Fig. 4 a clear absorption band around 400 nm is present, that could be responsible for the PL spectrum shape around the same wavelength values in the region 300–500 nm (see Fig. 3, left).

In conclusion, to achieve lower levels of internal radioactive contamination of the crystal, new growing and handling protocols are necessary, as well as a thorough selection and purification of the initial materials used for the BaWO4 crystal growth. 4. Luminescence properties of the BaWO𝟒 crystal The photo-luminescence (PL) spectrum of the BaWO4 crystal was measured with a single monochromator MDR-23 at room temperature. 152

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Fig. 3. Emission spectra of the BaWO4 crystal under ultraviolet excitation. Photoluminescence was excited by a ultraviolet light at 180 nm wavelength (left) and 350 nm wavelength (right). The peak at 700 nm (right) is the transmission of the second-order diffraction of the excitation at 350 nm wavelength [69]. Table 3 The acquisition configurations used during the scintillation test of the BaWO4 . Configuration name

PMT

Digitizer

A B C

EMI D631 FLB EMI 9302 FLA EMI 9302 FLA

CAEN DT5720 CAEN DT5720 LeCroy WavePro 735Zi

However, the pulse shape of the events seems to be similar to noise events, contrarily to what is expected for tungstates compounds (see below). To evaluate better the pulse shape of the events, a sampling of 20 GSamples/s has been used with configuration C. The data acquired in this configuration are compatible with the results of configuration B and no additional information could be extracted. The intrinsic emission of tungstates is generally attributed to the radiative recombination of self-trapped Frenkel excitons localized at the WO2− molecular ions [73]. Typically, the decay time of the intrinsic 4 luminescence of tungstates is at least 10 μs.1 A possibility is that, due a very high exciton mobility towards the defect centers, this intrinsic emission – 100 μs of acquisition window – is not present [74]. Nevertheless, in our opinion, the effective light emission is either too weak at room temperature to be clearly distinguished from defect luminescence or it does not exist in BaWO4 .

Fig. 4. The optical transmission curve of the BaWO4 crystal, measured with a SPECORD UV–Vis spectrophotometer.

6. The scintillation test of the BaWO𝟒 crystal The BaWO4 crystal was fixed in direct contact with the light detector (LD). In two different measurement campaigns two types of photomultiplier tubes (PMT) were used, EMI D631 FLB (PMT1) and EMI 9302 FLA (PMT2). The spectral sensitivity of the EMI D631 FLB PMT, with a MgF2 window, is in the ultraviolet region; the EMI 9302 FLA spectral sensitivity is optimized, instead, in the visible region. The use of these two types of PMTs was useful to test a reasonably wide spectral region of light emission of the BaWO4 . The surface of the crystal was covered with a diffusive PTFE tape to improve the light collection. The signal from the PMTs were acquired using a CAEN DT5720 digitizer operating at 250 MSamples/s and a LeCroy Oscilloscope, model WavePro 735Zi, operating at 20 GSamples/s. The signals from the PMTs were recorded in a time window of 100 μs. An event-by-event data acquisition system stored the pulse profiles of the events. In Table 3 the acquisition configurations are reported. The tests of the BaWO4 crystal were performed at LNGS using the experimental facilities of the DAMA collaboration. The crystal was irradiated with 𝛾 quanta of 60 Co,137 Cs and 22 Na calibration sources. In Fig. 5 the spectral areas acquired in the configurations A and B normalized in time are reported. The black histograms are the background energy spectra, the colored histograms are, instead, the energy spectra obtained when the crystal is irradiated by gamma quanta. The B configuration was moderately more performing than the configuration A (in terms of response), probably due to the better matching between the spectral sensitivity of the PMT used and the emission spectra of the BaWO4 (𝜆 at quantum efficiency peak is respectively 330 nm and 500 nm). Some indication of a faint scintillation light emission is shown.

7. Limits and perspectives of BaWO𝟒 crystal for rare events investigations The good properties of an ideal inorganic scintillator have been frequently underlined: the physical characteristics (high detection efficiency, physical form, chemical and mechanical stability) and the luminescent characteristics (emission wavelength, high light yield, good linearity, suitable decay time and in some also good radiation hardness). The possibility to have them together with high radio-purity and enriched material in target nuclei has made this kind of detectors a suitable solution to search for rare processes (using the ‘‘source = detector’’ approach). The investigation of a BaWO4 single crystal has shown interesting intrinsic luminescent and transmission characteristics. However, the effective light emission, at room temperature, seems to be too weak. In fact, one needs to be very cautious to draw conclusions about the properties of the scintillator only on the basis of the photo-luminescence spectrum, that does not reflect the mechanism of energy transfer and thermalization inside the medium. Moreover, considering the strong 1 Due to the forbidden nature of the transition: the exciton ground state of WO2− molecular ions has a triplet nature, with the lower lying energy level 4 separated from the higher lying double degenerate level by a delta energy, which is attributed to the spin–orbit interaction.

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Fig. 5. The spectral pulse-area acquired in the A (left) and B (right) configuration and normalized in time. (left) In black the background, in blue the spectrum acquired irradiating the crystal by using a 22 Na source and in red by a 137 Cs. (right) In black the background, in blue the spectrum acquired irradiating the crystal by using a 22 Na source, in red by a 137 Cs and in green by a 60 Co source. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

References

emission in the UV region, as observed in Fig. 3, and the relatively good transmittance, additional and better understanding of the physical processes involved in the development of materials has to be obtained to be able to improve, eventually, the scintillation light emission. In fact, considering that localization and delocalization of excitations are strongly affected by the positions of the luminescent centers and their energy levels relative to the valence and conduction bands of the lattice atoms, a crucial role could be the use of appropriated dopants in the BaWO4 compound. Further dedicated R&D activity is in progress, to study also the light emission properties at lower operation temperatures down to −100 ◦ C. Furthermore, in the investigation of rare processes a high radiopurity is mandatory for these detectors. Many sources of background exist: (i) radionuclides of cosmogenic origin; (ii) natural radioactivity; (iii) radon trapped inside the detectors during the growth and/or the assembly; (iv) possible permeability of some materials to radon and residual radon traces near the detectors; (v) eventual presence of residual contaminants; etc. In the measured crystal an high content of natural radioactivity, especially radium, is the main problem. R&D on barium purification method from radium is in progress at the LNGS. Moreover, a more stringent protocol for crystal growth and handling is under development.

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8. Conclusion The large interest in 2𝛽 decay gave and is still giving the rise of a multitude of dedicated experiments in nuclear and particle physics. The case of 2𝛽 decay of 130 Ba and 132 Ba in particular, is motivated by recent positive, but indirect, indications. Taking advantage of the recent progress in the development of WO4 compound crystal scintillators, a Ba-based single crystal was developed and produced by LTG Czochralski technique. In particular the PL, transmittance, radioactive contamination and light emission were investigated. In spite of the relatively good photo-luminescent and transmission properties, the measured effective light emission is either too weak to be clearly distinguished from defect luminescence or it may even not exist in BaWO4 at room temperature. Nevertheless, the different response observed between the A and B configurations gives moderately optimistic prospects. Thus, in order to improve its performance and radio-purity the following research activities are planned: (i) to investigate the light emission at lower operational temperatures and contemporarily try to use photosensors with a better response in the UV region. (ii) to consider doping the BaWO4 . (iii) to apply adequate radio-purification procedures to the raw materials before growth and to fix a more stringent protocol for growing and handling. 154

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