Eur. Phys. J. C manuscript No. (will be inserted by the editor) Search for double β-decay modes of 64Zn using purified zinc F. Bellini1,2, M. Beretta3,4,12, L. Cardani2, P. Carniti3,4, N. Casali2, E. Celi5,6,a, D. Chiesa3,4, M. Clemenza3,4, I. Dafinei2, S. Di Domizio7,8, F. Ferroni5,2, L. Gironi3,4, Yu.V. Gorbenko9, C. Gotti3,4, G.P. Kovtun9,10, M. Laubenstein6, S. Nagorny6,5,13,b, S. Nisi6, L. Pagnanini5,6,c, L. Pattavina6, G. Pessina4, S. Pirro6, E. Previtali3,4, C. Rusconi11,6, K. Sch¨affner5,6, A.P. Shcherban9, D.A. Solopikhin9, V.D. Virich9, C. Tomei2, M. Vignati2 1Dipartimento di Fisica, Sapienza Universit`adi Roma, Roma I-00185 - Italy 2INFN - Sezione di Roma, Roma I-00185 - Italy 3Dipartimento di Fisica, Universit`adi Milano - Bicocca, Milano I-20126 - Italy 4INFN - Sezione di Milano - Bicocca, Milano I-20126 - Italy 5Gran Sasso Science Institute, I-67100, L'Aquila - Italy 6INFN - Laboratori Nazionali del Gran Sasso, Assergi (L'Aquila) I-67100 - Italy 7Dipartimento di Fisica, Universit`adi Genova, Genova I-16146 - Italy 8INFN - Sezione di Genova, Genova I-16146 - Italy 9National Science Center \Kharkov Institute of Physics and Technology", 61108 Kharkov, Ukraine 10Karazin Kharkov National University, Kharkov 61022, Ukraine 11Department of Physics and Astronomy, University of South Carolina, Columbia, SC 29208 - USA 12Present Address: University of California, Berkeley, CA 94720, US 13Present Address: Queen's University, Physics Department, K7L 3N6, Kingston (ON), Canada Received: / Accepted: Abstract The production of ultra-pure raw material is well as limits on naturally occurring radionuclides are a crucial step to ensure the required background level set to less than mBq/kg. The excellent radiopurity of in rare event searches. In this work, we establish an the zinc sample allows us to search for electron capture innovative technique developed to produce high-purity with positron emission and neutrinoless double electron (99.999%) granular zinc. We demonstrate the effective- capture of 64Zn, setting the currently most stringent + arXiv:2012.05873v1 [physics.ins-det] 10 Dec 2020 ness of the refining procedure by measuring the internal εβ 21 lower limits on their half-lives, T1=2 > 2:7 × 10 yr contaminations of the purified zinc with a high-purity 0ν2" 21 (90% C.I.), and T1=2 > 2:6 × 10 yr (90% C.I.), re- germanium detector at the Laboratori Nazionali del spectively. Gran Sasso. The total activity of cosmogenic activated nuclides is measured at the level of a few mBq/kg, as aemail: [email protected] Keywords double electron capture · electron capture bemail: [email protected] with positron emission · zinc · material purification · cemail: [email protected] radiopurity · cosmogenic activation 2 PACS 23.40.-s β decay; double β decay; electron and purification (Sec.2), validating the whole production muon capture · 27.50.+e mass 59 ≤ A ≤ 89 · 29.30.Kv through mass spectrometry (Sec.3) and γ-spectroscopy X- and γ-ray spectroscopy (Sec.4). The excellent radiopurity achieved allows us to search for double β-decay of 64Zn, in particular neutri- noless and two-neutrino modes of electron capture with 1 Introduction positron emission (εβ+), as well as neutrinoless dou- ble electron capture (0ν2"), setting the most stringent The search for rare events such as dark matter inter- lower limits on their half-lives to date (Sec.5). actions [1] or neutrinoless double-beta decay [2] has an important role in the investigation of physical processes not predicted by the standard model. The common ex- perimental effort for next-generation experiments fo- 2 A new process for zinc purification cuses on the reduction of detector contamination, to reach an extremely low background index. In this sce- The raw zinc, chosen with purity level higher than 99.98%, nario, the purification of the initial materials used for is prepared for the purification process trough the fol- the detector construction acquires a pivotal relevance. lowing steps: (i) cutting of initial metal ingot in order to Zinc-containing crystals are widely used in rare event get pieces of about 2 kg of mass, (ii) etching of Zn pieces physics as low-temperature calorimeters (ZnSe in CUPID- in the nitric acid solution (0.1 M HNO3), (iii) washing 0 [3], ZnMoO4 in LUMINEU [4]), room-temperature of etched Zn pieces in HP water, (iv) drying of washed scintillators (ZnWO4 [5{7]) or semiconductor detectors Zn pieces, and (v) sampling for tracing of contamina- (CdZnTe in COBRA [8]). To understand the impor- tion level by means of mass-spectrometry. The actual tance of the purity of the initial materials, we consider purification process presented here consists then of two the specific case of ZnSe [9]. The most common pro- phases: (i) the refining of the initial zinc by filtration duction method for these crystals is the growth by melt and distillation under vacuum, and (ii) the formation crystallization under high inert gas pressure, known as of granules of (3 - 5) mm in diameter by dripping of Bridgman-Stockbarger technique [10]. With this method molten metal in the coolant agent. This last step mod- large volume crystals can be grown, reaching up to 60 ifies the material in a suitable form for the subsequent mm in diameter and 2 kg in mass. The downside of stages of crystal production. The innovative method this technique is the lack of perfection in the produced used to implement the first step has been developed crystalline structure. During crystallization, a fraction at the National Science Center \Kharkov Institute of of the ZnSe compound in the liquid phase dissociates, Physics and Technology" (Kharkov, Ukraine). The pro- transferring part of the single components (Zn and Se) cedure involves the filtration of the molten metal and its to the cold zone. The transfer is due either to dif- subsequent distillation by condensing the metal steam fusion through the semi-penetrable graphite crucible into the solid phase [11]. The distillation process is car- o walls, or to evaporation from the free surface of the ried out at a temperature TD = TM + (40 − 50) C, melt. Because of differences in diffusion coefficients, the while the condensation process at a temperature TC = o o melt results enriched in one of the components, caus- TM − (30 − 40) C, being TM = 419 C the melting ing deviations from the stoichiometry that can be sig- temperature. The distillation set-up is made from high- nificantly larger than 1%. Such a process leads to the purity high-density graphite, characterized by chemical formation of intrinsic point defects (IPD) in the crys- inertness to zinc and a minimal content of impurities. talline structure, affecting both the scintillating and To further increase the graphite purity, the setup was bolometric crystal properties, as well as the detector thermally treated under vacuum at (1000 - 1100) oC radiopurity. Since this deviation from stoichiometry in- before use. The total concentration of all possible con- creases together with the contamination of the raw ini- taminants in graphite has been therefore reduced be- tial materials, their chemical purity impacts directly on low 1.0 ppm. The distillation set-up was assembled of the detector performance. This is true in particular for two identical pieces, as shown in Fig1: the bottom one bolometric detectors, where defects of the crystalline serves as a crucible, while the upper part is a collector structure affect the phonon scattering in the crystal. for the condensed Zn metal. The small outlet hole in the As a result the thermal conductivity of the detector de- lateral wall of the condenser is intended for evacuation creases, causing a loss in energy resolution. It is there- in order to remove volatile impurities. The crucible is fore mandatory to use initial materials with superior heated by the heater, while the condenser is heated in- chemical and radio-purity to obtain high-performance directly by the thermal radiation of the crucible, as well scintillating bolometers based on zinc-containing crys- as by the heat transfer from the stream of the distill- tals. In this work, we introduce a novel method for zinc able metal. The crucible volume was designed to hold 3 2.0 kg of loaded initial zinc. The operating pressure dur- Final purified ing of the zinc refining was about (10−3 − 10−2) Pa. At zinc Residual the first stage, the purification process was carried out zinc against volatile impurities (like a Na, K etc.) by their Crucible condensation on the inner surface of the condenser (see Outer hole Fig1, left). A part of them is also removed continuously pumping the crucible volume through the outer hole. In order to remove oxides and other slags, the molten Initial metal was filtered, through a plate with a small conic- zinc ity and a hole in the middle. At the end of this stage, a ~ ~ ~ ~ ~ ~ Filtered certain amount of zinc metal that contains a high con- zinc centration of the volatile impurities is removed from the ~ ~ ~ ~ ~ ~ Residual coldest part of the condenser. The second stage is car- ~ ~ ~ ~ ~ ~ zinc ~ ~ ~ ~ ~ ~ ried out in order to remove the non-volatile impurities (like a Cu, Fe, Si, Ni, Co, V, Cr, Al, Tl, Bi, Mn etc.) Heater by distillation of the zinc metal, poured in the crucible Fig. 1 Scheme of the set-up for zinc purification: a) stage of after filtration. After evaporation (up to 95% of loaded filtration and refining against of the volatile impurities (left); material), the pure zinc is condensed on the inner part b) stage of purification against of the non-volatile impurities (right). We melt the initial zinc using a heater, obtaining of the condenser, while the non-volatile impurities re- the filtered zinc.