1 Recent results of the Kyiv group in 2  decay researches V.I. Tretyak Institute for Nuclear Research, MSP Kyiv, Ukraine AMORE Collaboration Workshop, Korea,

22 Content: 1.Introduction 2.Search for 2  decays of 96 Ru, 104 Ru 3.Double  processes in 64,70 Zn, 180,186 W with ZnWO 4 scintillator 4.New observation of 2  2 decay of 100 Mo to excited level of 100 Ru 5.Investigation of Li 2 MoO 4 as bolometer 6.Investigation of ZnMoO 4 as bolometer 7.Development of 106 CdWO 4 scintillating crystal 8.Conclusion

33 1.Introduction Few our experiments performed during ~1-1.5 last years will be reviewed.

44 2. Search for 2  decays of 96 Ru, 104 Ru with HP Ge spectrometry [P. Belli et al., Eur. Phys. J. A 42 (2009) 171] 96 Ru: Q 2  =2718 keV,  =5.54% - one of only 6 potential 2  + decayers KL electron capture: energy release 2695  8 keV – close to E exc ( 96 Mo)=2700 keV Only 1 experiment [E.B. Norman, PRC 31 (1985) 1937]: T 1/2 > yr Our measurements: LNGS (3600 m w.e.) low background HP Ge 468 cm 3 nat Ru 99.99% g 158 h of data taking

55 Results: 1.Radiopurity (mBq/kg): 232 Th, 235 U < ~5, 60 Co, 137 Cs < ~2 238 U =7, 40 K = 3400, 106 Rh = 24 2.Limits on 2  processes in 96 Ru: excess in 511 keV peak due to 40 K (main contribution), 106 Rh, 208 Tl T 1/2 for 2  +,  +, 2  decays to the ground state and all possible excited levels of 96 Mo > yr - ~3 orders of magnitude higher than Norman’ Ru  104 Pd(556 keV): T 1/2 > 3.5×10 19 yr (first limit) 4.Improved statistics during last time to 2162 h (few times better limits; presented at NPAE’2010) 5.Plans to purify Ru

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77 3. Search for 2  decays of 64,70 Zn, 180,186 W with ZnWO 4 crystal scintillators [P. Belli et al., Nucl. Phys. A 826 (2009) 256] [L.L. Nagornaya et al., IEEE TNS 56 (2009) 994] Continuation of our previous studies at LNGS [P. Belli et al., PLB 658 (2008) 193]. Additional motivation: indication on  + decay of 64 Zn by Yugoslavian physicists with T 1/2 =1.1×10 19 yr [I. Bikit et al., Appl. Radiat. Isot. 46 (1995) 455].

88 ZnWO 4 properties: density 7.8 g/cm 3, light yield ~13% of NaI(Tl), emission max at 480 nm, refractive index , average decay time at RT 24  s, chemically inert, non-hygroscopic, melting point 1200 o C. Used crystals – 117, 169, 699 g; grown by Czochralski method by In-t Scint. Mat. (Kharkiv, Ukraine); low background DAMA R&D set-up at LNGS (3600 m w.e.); >10,000 h of measurements; FWHM % at 662 keV. Data analysis: (1)time-amplitude analysis – search for fast decay subchains in U/Th chains; (2)pulse-shape discrimination – distinguishing  from  (also from noise, …); (3)fit of exp. spectrum by MC models.

99 pulse-shape discrimination fit of exp. spectrum by MC models

10 ZnWO 4 is very pure scintillator

11 All possible 2  processes in 64 Zn, 70 Zn, 180 W, 186 W were simulated with GEANT4 (and initial kinematics given by DECAY0 event generator). Limits on T 1/2 were obtained by fitting the exp. spectrum by sum of response functions for the background components and 2  processes.

12 Obtained limits on  + decays of 64 Zn  yr for 0 and 7.0  yr for 2 - completely rule out Yugoslavian indication with 1.1  yr. Only two nuclei among 2  + /  + /2  potential decayers – 40 Ca and 78 Kr - were studied athe the similar level of sensitivity. Limits on T 1/2 for 70 Zn, 180 W, 186 W are mostly better than those obtained in previous experiments.

13 4. New observation of 2  2 decay of 100 Mo to excited level of 100 Ru [P. Belli et al., NPA 846 (2010) 143] Few previous results on decay to ( keV) 100 Ru level: T 1/2 from 5.7  to 9.3  yr However, only limit in one experiment [PLB 275 (1992) 506]: Modane Laboratory, 1 kg 100 Mo, HP Ge 100 cm 3 T 1/2 >1.2  y at 90% CL

14 Experiment: g of 100 MoO 3 (enrichment in 100 Mo – 99.5%) - Underground conditions of LNGS (3600 m w.e.) - Massive shielding - Low-background setup with 4 HP Ge detectors 225 cm 3 each - 18,120 h of data taking

15 2-d experimental spectrum of events with multiplicity = 2 Coincidence events when energy of one of  quantum is fixed as: E 1 = 609 keV ( 214 Bi) E 1 = 2615 keV ( 208 Tl) Other lines of 214 Bi and 208 Tl are evident

16 Coincidence events in experimental spectrum when energy of one of  quantum is fixed at the value expected for 2  2 decay 100 Mo  100 Ru * : E 1 = 540  2 keV E 1 = 591  2 keV Coincident events with energy E 2 = 591 keV or 540 keV are observed; T 1/2 = (6  2)  y To check the effect, window is shifted to E 1 = 545  2 keV Simulations with GEANT4 and EGS4: consistent results for efficiencies

17 1-d spectrum Both peaks, corresponding to energies expected for 2  2 decay of 100 Mo  100 Ru *, are observed S(540 keV) = 310  54 counts  T 1/2 =  1.0  yr S(591 keV) = 255  51 counts  T 1/2 =  1.2  yr Joining the results, one gets: T 1/2 =  1.1  yr Mo 540 keV 100 Mo 591 keV

18 By-product - limit on charge non-conserving (CNC)  decay of 100 Mo:  CNC > 4.5  yr at 90% CL Usual (CC)  decay: (A,Z) → (A,Z+1) + e  + e СNС  decay: (A,Z) → (A,Z+1) + e + e e In CNC  decay, it is supposed that some uncharged massless particle ( e, γ, Majoron, etc.) is emitted instead of an electron. In result, you have additional 511 keV (usually spent on electron rest mass) that gives possibility for transitions (to ground state or excited levels of daughter nucleus) that are normally energetically forbidden.

19 5. Investigation of Li 2 MoO 4 as a bolometer [O.P. Barinova et al., NIMA 613 (2010) 54] Work is motivated by attempts to find good material able to act as scintillating bolometer for searches for 2  0 decay of 100 Mo. Li 2 MoO 4 compound was obtained from MoO 3 and Li 2 CO 3 powders with subsequent recrystallization. Crystals up to  2.5×3.5 cm were grown by Czochralski technique. Density – 3.0 g/cm 3, melting point – 701 o C, material is weakly hygroscopic. No luminescence at 300 K, but it exists at 10 K (max of emission spectrum at 580 nm) in [O.P. Barinova et al., Perspective Materials 4 (2008) 34]. However, we observe luminescence even at higher temperatures (up to 450 K) – probably because of higher sensitivity of the set-up. Test at 10 mK: CUORE R&D set-up at LNGS, Li 2 MoO 4  2.5×0.9 cm (1.3 g), 40 h. Light output - 7% comparing with CdMoO 4, 20% - with CaMoO 4 Quenching factor for  ’s ~0.3

Pb  0.3 Bq/kg Li 2 MoO 4 (34 g) was also tested with HP Ge; purity (mBq/kg): 226 Ra < 20, 228 Th < 30, 137 Cs < 4, 60 Co < 8, 40 K = 170. Could be applied in searches for 2  processes in 92 Mo, 98 Mo, 100 Mo; also in searches for 7 Li solar axions (by resonant capture on 7 Li in the detector with excitation of 478 keV level).

21 6. Investigation of ZnMoO 4 as a bolometer [L. Gironi et al., arXiv: [nucl-ex]] [L.L. Nagornaya et al., IEEE TNS 56 (2009) 2513] 100 Mo: Q 2  =3035 keV,  =9.67% - one of the most promising candidate in 2   searches Inorganic scintillators containing Mo: CaMoO 4 (drawback – background from 2  2 of 48 Ca,  =0.187%) CdMoO 4 (drawback –  decay of 113 Cd + high cross section for n capture) PbMoO 4 (drawback – radioactivity of 210 Pb+ 210 Bi, low mass fraction of Mo) Li 2 MoO 4 (drawback – low light yield) Li 2 Zn 2 Mo 3 O 12 (drawback – low light yield) ZnMoO 4 (grown only recently) [L.I. Ivleva et al., Crystallogr. Rep. 53 (2008) 1087] [L.L. Nagornaya et al., IEEE TNS 56 (2009) 2513]

22 ZnMoO 4 properties: inert material melting point o C density g/cm 3 wavelength of emission max nm (at RT)

23 Test of ZnMoO 4 as a bolometer: hexagonal shape – diagonal 2.5  1.1 cm 14 mK, CUORE R&D set-up at LNGS  rise – 12 ms  decay – 60 ms FWHM – 4–6 keV LY – 1.1 keV/MeV (at 2615 keV line; 17.4 keV/MeV for 510 g CdWO 4

24 Important feature:  and  events can be distinguished by different thermal pulses. It allows to use only heat signals without registering light signals. Thus, with not so perfect crystals, very encouraging results are obtained. ZnMoO 4 could be very promising material for future search for 2  0 decay of 100 Mo.

25 7. Development of 106 CdWO 4 scintillating crystal [P. Belli et al., NIMA 615 (2010) 301] 116 Cd: Q 2  =2770 keV,  =1.25% - one of only six potential 2  + decayers Long history of investigations starting from 1952 which includes, in particular: - measurements in the Solotvina underground lab ( nat CdWO CdWO 4 ), - COBRA experiment at LNGS (several 1 cm 3 nat CdZnTe), - TGV experiment at Modane ( 106 Cd foils 75% enrichment between 32 HP Ge) Best current T 1/2 limits – on the level of yr (in dependence on mode) Theoretical predictions – on the level of yr (the most optimistic) KINR possesses ~100 g of 106 Cd (66.4% enrichment) Growth of 106 CdWO 4 crystal scintillators could be important step on the way to observe 2  + /  + /2  decays of 106 Cd (efficiency ~100%) Main stages: - purification of Cd metal, - production of CdWO 4 powder, - crystal growing - control of radiopurity of all initial materials and all involved materials.

26 Purification of Cd metal – by vacuum distillation in KIPT (Kharkiv, Ukraine). Factor of purification – up to ~10 2 for some elements (Pb, Cr, …). Irreplaceable losses – 1.8%. Production of CdWO 4 powder – NeoChem (Moscow, Russia). Reagents of only high purity grade + additional distillation for water, acids, ammonia. Result: g of 106 Cd metal were converted in g of CdWO 4 compound (which contains g of 106 Cd, i.e. 97.2%). Purity: mostly only limits on the level of ppm for all unwanted admixtures. Crystal growing – NIIC (Novosibirsk, Russia), by low-thermal-gradient Czochralski technique. Result: from initial charge of 265 g, a boule of 231 g was grown. Scintillation element: g (  2.7×5.0 cm). Total irreplaceable losses of 106 Cd - 2.3%.

27 FWHM=10.0% at 662 keV The described 106 CdWO 4 is fourth scintillating crystal made of enriched isotope in the history of 2  researches. Previous ones: 40 CaF 2 (Eu) and 48 CaF 2 (Eu) by E. der Mateosian and M. Goldhaber in 1966, and 116 CdWO 4 by KINR group in 1989.

28 Measurements of this crystal are in progress now in the DAMA R&D set-up at LNGS. Background spectrum for 1320 h The total  activity is on the level of ~2 mBq/kg. The main components of background of the detector are  active 113m Cd (112 Bq/kg) and 207 Bi (1.3 mBq/kg). Lower limits on half-lives for 2  processes in 106 Cd were set on the level of yr. A sensitivity of the experiment to different 2  processes in 106 Cd after  3 yr of measurements is expected to be on the level of ~ yr.

29 8. Conclusions As one can see, many efforts were made by the KINR group (together with our Italian, Ukrainian, Russian collaborators) in studies of 2  decays during last years: - 96 Ru, 104 Ru, 100 Mo with HP Ge detectors (for 100 Mo – observation of the effect); - 64,70 Zn, 180,186 W with ZnWO 4 scintillators (with mass up to 0.7 kg); - development/investigation of Li 2 MoO 4 and ZnMoO 4 as bolometers (ZnMoO 4 is very promising material); - development of enriched 106 CdWO 4 scintillating crystal. Obtained T 1/2 limits are mostly the best to-date, exceeding previously known results up to 3 orders of magnitude.

30 Thank you for attention! 30