Serge Nagorny – GSSI-INFN The current status of LUCIFER experiment Serge Nagorny – GSSI-INFN on behalf of LUCIFER collaboration IAPS @ Gran Sasso – May 7th, 2015

Which nuclide is the best for such search? from experimental point of view Environmental gamma background of 238U and 232Th chains 2615 keV 48Ca 82Se 96Zr 100Mo 116Cd 150Nd

LUCIFER Low-background Underground Cryogenics Installation for Elusive Rate Demonstrator array of enriched scintillating bolometers Total enriched 82Se isotope mass is 15 kg Expect background index @ROI < 10-3 c/(keVkgy) Q, keV Useful material LY/, keV/MeV QF Zn82Se 2996 56% 6.5 4.2

Working principle of scintillating bolometers First measurement of light and heat in a bolometer made in 1992 at INFN Milano A. Alessandrello et al., “Development of a thermal scintillating detector for double beta decay of 48Ca ” Nucl. Phys. B - Proceedings Supplements 1992 Vol: 28.

Does ZnSe works as a scintillating bolometer? ZnSe, 430 g, 524 hours Excellent particle discrimination FWHM = 13.5 keV@ 2615 keV Background free ROI Natural radioactivity (U/Th/40K) on level Bq/kg

Enriched 82Se isotope Produced by URENCO company (Netherland) natural SeF6 spectroscopy on GeMPI4@LNGS 232Th chain < 0.3 mBq/kg 235U chain < 0.2 mBq/kg 238U chain < 7 mBq/kg 40K < 1.1 mBq/kg 75Se 0.20±0.06 mBq/kg cascade of centrifuges chemical conversion gas to metal ICP-MS measurements S 150 ppm Te 1 ppm others < 0.5 ppm 82Se metal distillation 15 kg of 82Se (95.5% enrichment level) delivered to LNGS

spectroscopy on GeMPI4@LNGS High purity Zinc produced by National Science Center KIPT (Ukraine) 12 kg of HP Zn, 828 hours spectroscopy on GeMPI4@LNGS 232Th chain < 0.1 mBq/kg 235U chain < 0.1 mBq/kg 238U chain < 6 mBq/kg 40K < 0.4 mBq/kg 65Zn 5.2±0.6 mBq/kg only cosmogenic activation No U/Th lines over 34 days ! ICP-MS measurements Cd 4.3 ppm others < 0.2 ppm 15 kg of HP Zinc (99,9995%) delivered to LNGS

Mechanical processing Crystal production ZnSe is well known compound… …but high quality big volume crystals are not available on the market and presently could be grown only at Institute for Scintillating Materials (Ukraine) crucial points should be solved in… Zn82Se powder synthesis non-stoichiometry due to high volatility of Se Bridgman growth low yield of crystal production nonrepeatability of crystal quality not fully controllable growth process Mechanical processing minimization of losses of enriched material recycling of crystals scraps

bkg @ ROI < 10-3 c/(keVkgy) Half-life sensitivity, 1026 y Experiment sensitivity 460 g ZnSe 30-36 crystals 17 kg of Zn82Se FWHM @ ROI = 10 keV bkg @ ROI < 10-3 c/(keVkgy) Live time, y Half-life sensitivity, 1026 y m, meV 5 0.6 65-194 10 1.2 46-138

First experimental results from low background measurements on HPGe detector Transition Decay channel Level of daughter nucleus E, keV Experimental limit T1/2, y at 68% C.L. HP Zn 82Se Previous results 64Zn64Ni +(0+2) g.s. 511  1.51021  9.41020 for 2 [1]  8.51020 for 0 [1] 2K0 1079.0  1.01022  3.21020 [1] KL0 1086.4 2L0 1093.7 82Se82Kr 2- 21+ 776.5  2.41022  2.81021 [2]  1.41021 [3]   22+ 1474.9  2.01022  1.61021 [3] 01+ 1487.6  2.81022  3.01021 [3] [1] P. Belli et al., arXiv:1110.3923v1 [nucl-ex] 18 Oct 2011. [2] Arnold et al., NP A 636 (1998) 209. [3] J. Suhonen, et al., Z. Phys. A 358 (1997) 297.

Summary Chemical and Radiopurity of HPZn & 82Se metals comply with all requirements for high quality low background ZnSe crystal production Cosmogenic activation of detector materials currently became an dominant component of internal detector background (can be minimized by using materials with well known history, by storage deep underground, by land transportation in shielded container, by minimization of production time, by depletion Zn metal on 64Zn isotope, etc.) First enriched Zn82Se crystal is planned to be grown end of June, 2015 One year is required for whole amount of Zn82Se crystal production

Typical mounting & scatter plot for ZnSe crystal QF > 1

Bolometric light detectors The light detector is a Ge thin crystal Ø=44.5 mm, h=0.175 mm 1 face is coated with 60 nm layer of SiO2 to increase light absorption These devices are calibrated through an Ionizing 55Fe source placed close to them; 55Fe shows two X-lines at 5.9 and 6.5 keV The 55Fe energy spectra evaluated on three different UMICORE Ge-crystals. σ represents the evaluated energy resolution on the peaks, while σbase represents the baseline resolution. 150 Ge Wafers are presently in our hands

Hall C – Cryostat, a very crowd place It has to be pointed out that all the measurements described are performed in the same, unique place: the Hall C CUORE R&D cryostat The setup was -slowly- changed in the last two years in order to increase by 70 % the experimental space. Main Dewar elongation New “buffles” Vacuum chamber elongation 4 5x5x5 cm3 BGO scintillating crystal (CUORE) 8 CUORE crystals (background study) 2 ZnSe + 2 CdWO4 (Lucifer) 21 different detectors

Requirements to raw material for ZnSe crystal production High performance of ZnSe crystals as scintillating bolometer strongly depends on: High perfection of crystalline structure (FWHM & Signal amplitude in Heat channel) High Light Yield (FWHM & Signal amplitude in Light channel, particle discrimination) Chemical purity K, Ti, V, Cr, Mn, Fe, Co, Ni < 1×10-6 g/g Mg, Bi, Pb, Sn, Al, Cu, Cd < 5×10-6 g/g U, Th << 1×10-9 g/g Radiopurity A(40K, U/Th& daughters) < mBq/kg no 137Cs, 90Sr, … minimization of cosmogenic nuclides activity