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Search for Double Beta Decay of 106Cd with an Enriched 106 Cdwo4 Crystal Scintillator in Coincidence with Cdwo4 Scintillation Counters

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Search for Double Beta Decay of 106Cd with an Enriched 106 Cdwo4 Crystal Scintillator in Coincidence with Cdwo4 Scintillation Counters Article Search for double beta decay of 106Cd with an enriched 106 CdWO4 crystal scintillator in coincidence with CdWO4 scintillation counters P. Belli1,2 , R. Bernabei 1,2* , V.B. Brudanin3 , F. Cappella4,5 , V. Caracciolo1,2,6 , R. Cerulli1,2 , F.A. Danevich7 , A. Incicchitti4,5 , D.V. Kasperovych7 , V.R. Klavdiienko7 , V.V. Kobychev7 , V. Merlo1,2 ,O.G. Polischuk7 , V.I. Tretyak7 and M.M. Zarytskyy7 1 INFN, sezione di Roma “Tor Vergata”, I-00133 Rome, Italy 2 Dipartimento di Fisica, Università di Roma “Tor Vergata”, I-00133 Rome, Italy 3 Joint Institute for Nuclear Research, 141980 Dubna, Russia 4 INFN, sezione Roma “La Sapienza”, I-00185 Rome, Italy 5 Dipartimento di Fisica, Università di Roma “La Sapienza”, I-00185 Rome, Italy 6 INFN, Laboratori Nazionali del Gran Sasso, 67100 Assergi (AQ), Italy 7 Institute for Nuclear Research of NASU, 03028 Kyiv, Ukraine * Correspondence: Dipartimento di Fisica, Università di Roma “Tor Vergata”, I-00133 Rome, Italy. E-mail address: [email protected] (Rita Bernabei) Received: date; Accepted: date; Published: date Abstract: Studies on double beta decay processes in 106Cd were performed by using a cadmium tungstate 106 106 scintillator enriched in Cd at 66% ( CdWO4) with two CdWO4 scintillation counters (with natural Cd composition). No effect was observed in the data accumulated over 26033 h. New improved half-life limits were set on the different channels and modes of the 106Cd double beta decay at level of 20 22 106 lim T1/2 ∼ 10 − 10 yr. The limit for the two neutrino electron capture with positron emission in Cd + 106 2nECb ≥ × 21 106 to the ground state of Pd, T1/2 2.1 10 yr, was set by the analysis of the CdWO4 data in coincidence with the energy release 511 keV in both CdWO4 counters. The sensitivity approaches the 21 22 theoretical predictions for the decay half-life that are in the range T1/2 ∼ 10 − 10 yr. The resonant neutrinoless double-electron capture to the 2718 keV excited state of 106Pd is restricted at the level of 0n2K 21 T1/2 ≥ 2.9 × 10 yr. Keywords: Double beta decay; 106Cd; Scintillation detector; Low background experiment 1. Introduction Observations of the neutrino oscillations suggest that the neutrinos are massive, which calls for arXiv:2010.08749v1 [nucl-ex] 17 Oct 2020 extension of the Standard Model of particles and fields (SM). However, oscillation experiments cannot determine the neutrino mass and the neutrino mass hierarchy. One of the most promising tools to determine the absolute neutrino mass scale and the neutrino mass hierarchy, the nature of the neutrino (Dirac or Majorana particle?), to check the lepton number conservation is double beta (2b) decay of atomic nuclei, a process in which two electrons (or positrons) are emitted simultaneously and nuclear charge changes by two units: (A,Z)!(A,Z±2)[1–3]. The neutrinoless mode of the decay (0n2b) violates the lepton number conservation law and it is possible if the neutrinos are Majorana particles (particle is equal to its antiparticle). Being a process beyond the SM, the 0n2b decay has the potential to test the SM [4–6]. Moreover, the Majorana nature of the neutrino might shed light on the Universe baryon asymmetry problem [7,8]. The two-neutrino 2b decay (2n2b) is a radioactive process allowed in the SM with the longest half-lives ever observed: 1018 – 1024 yr. The 2n2b− decay mode has been detected in several nuclides [9]. 2 of 16 The 0n2b decay is not observed. The most sensitive 2b−-decay experiments quote half-life limits at level of 24 26 T1/2 > (10 − 10 ) yr, which correspond to Majorana neutrino mass limits in the range hmni < (0.1 − 0.7) eV. Probing the inverted hierarchy region of the neutrino mass requires improved sensitivities of 2b− 27 28 experiments at level of hmni ∼ (0.02 − 0.05) eV (i.e. half-life sensitivity in the range: T1/2 ∼ 10 − 10 yr). The sensitivity of the experiments in the search for “double beta plus” processes: double electron capture (2EC), electron capture with positron emission (ECb+) and double positron decay (2b+) is substantially lower, while the physical lepton-number violating mechanisms of the neutrinoless 2EC, ECb+ and 2b+ processes are considered essentially the same as for the decay with electrons emission. At the same time, there is a motivation to search for the 0nECb+ and 0n2b+ decays owing to the potential to clarify the possible contribution of the right-handed currents to the 0n2b− decay rate [10], and an interesting possibility of a resonant 0n2EC process [11–14]. As for the allowed two-neutrino mode of the double beta plus decay, there are claims of positive results (indication) for the 2n2EC radioactivity of three nuclides. The 2n2EC decay of 130Ba was claimed in two geochemical experiments where anomaly in the isotopic concentrations of daughter xenon traces in old 21 barite (BaSO4) minerals was interpreted as the sought effect with the half-life T1/2 = (2.16 ± 0.52) × 10 20 yr [15], and with T1/2 = (6.0 ± 1.1) × 10 yr in [16]. In the analysis [17] the disagreement was explained by a possible cosmogenic contribution with a conclusion that the result of [15] is a more reliable one. An 78 +5.7 21 indication on the 2n2EC process in Kr with the half-life T1/2 = 9.2−2.9 × 10 yr was obtained with a proportional counter with a volume of 49 lt filled by gas enriched in 78Kr to 99.81% [18]. The value was +1.3 22 124 then updated to 1.9−0.8 × 10 yr in [19]. Recently a detection of the 2n2EC of Xe with the half-life (1.8 ± 0.5) × 1022 yr was claimed in [20]. However, the indications of 130Ba 2EC decay should be confirmed in direct counting experiments, while the results for 78Kr and 124Xe need to be confirmed with bigger statistics and very stable experiments. Other allowed 2n decay channels with decrease of the nuclear charge by two units, 2nECb+ and 2n2b+, are not observed yet. The nuclide 106Cd is one of the most appealing candidates to search for 2EC, ECb+ and 2b+ decays with a long history of studies (a review of the previous investigations reader can find in Ref. [21]). 106 The interest to Cd can be explained by one of the biggest decay energy Q2b = 2775.39(10) keV [22], comparatively high isotopic abundance d = 1.245(22)%[23] and possibility of gas centrifugation for enrichment, existing technologies of cadmium purification, availability of Cd-containing detectors to realize calorimetric experiments with a high detection efficiency. At present there are three running experiments searching for the double beta decay of 106Cd: COBRA, TGV-2 and the present one. The COBRA collaboration utilizes CdZnTe semiconductor detectors at the Gran Sasso underground laboratory (Laboratori Nazionali del Gran Sasso, LNGS). The experiment started with one Cd0.9Zn0.1Te detector with mass of '3 g, and one CdTe detector ('6 g) [24]. CdZnTe detectors are used in the current stage of the experiment [25,26]. The measurements resulted in the half-life limits for several channels of 106Cd double beta decay at level of ∼ 1018 yr. The main goal of the TGV-2 experiment, located at the Modane underground laboratory, is search for 2n2EC decay of 106Cd (a decay channel expected to be the fastest one) with the help of 32 planar HPGe detectors with a total sensitive volume ≈ 400 cm3. In the first stage of the experiment, foils of cadmium enriched in 106Cd to (60–75)% were used [27–29]; now 23.2 g of cadmium sample enriched in 106Cd to 99.57% are installed in the set-up [30]. The experiment gives the strongest limit on the 2n2EC decay: 20 20 T1/2 > 4.7 × 10 yr. For other decay modes and channels the sensitivity is at level of 10 yr [31]. 106 106 A cadmium tungstate crystal scintillator from cadmium enriched in Cd to 66% ( CdWO4) was developed in 2010 [32]. The experiments with that detector are carried out at the LNGS in the DAMA/CRYS, DAMA/R&D set-ups, and in an ultra-low background GeMulti HPGe g spectrometer of 3 of 16 1+ 106 + 47 Ag 0 - 2,3 2236(100) 2748 106 + Cd 4 2741(100)2229(51) 2741 48 1160(100) 2718 2EC, ECβ+, 2β+ + 0 1766(100)1150(9)716(29) 2278 + 0 873(100)439(3) 2001 + 0 1194(100)578(15) 1706 + 2 1562(10)1050(100)434(1)429(4) 1562 + 3 1046(100)430(44)328(4) 1558 + 4 717(100) 1229 + 0 622(100) 1134 + 2 1128(54)616(100)1128 + 2 512 512 0+ 106 46Pd Figure 1. Simplified decay scheme of 106Cd [36] (levels with energies in the energy interval (2283–2714) keV are omitted). Energies of the excited levels are in keV. Relative intensities of g quanta are given in parentheses. the STELLA (SubTErranean Low Level Assay) facility [33] at the LNGS. The first stage of the experiment 106 106 20 with the CdWO4 detector gave the half-life limits on 2b processes in Cd at level of ∼ 10 yr [21]. 106 In the second stage the CdWO4 scintillator was installed between four HPGe detectors (with volume ' 225 cm3 each) of the GeMulti HPGe g spectrometer to detect g quanta expected in the most of the 106Cd decay channels, including the annihilation g’s emitted in decay modes with positron(s) emission (a simplified decay scheme of 106Cd is presented in Fig.1). The experiment improved the 106Cd half-life 20 21 limits to the level of T1/2 ≥ (10 − 10 ) yr [34].
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