1020 - people@roma2
Transcript
1020 - people@roma2
First results of double beta decay experiment with 106CdWO4 crystal scintillator P. Belli1, R. Bernabei1,2, R. S. Boiko3, V. B. Brudanin4, F. Cappella5,6, V. Caracciolo7, R. Cerulli7, D. M. Chernyak3, F. A. Danevich3, S. d'Angelo1,2, A. E. Dossovitskiy8, E. N. Galashov9, A. Incicchitti5,6, V. V. Kobychev3, S. S. Nagorny3, F. Nozzoli1, B. N. Kropivyansky3, V. M. Kudovbenko3, A. L. Mikhlin8, A. S. Nikolaiko3, D. V. Poda3,7, R. B. Podviyanuk3, O. G. Polischuk3, D. Prosperi5,6, V. N. Shlegel9, Yu. G. Stenin9, J. Suhonen10, V. I. Tretyak3, Ya. V. Vasiliev9 1 INFN sez. Roma Tor Vergata, Rome, Italy 2 Dip. di Fisica, Università di Roma ”Tor Vergata”, Rome, Italy 3 Institute for Nuclear Research, Kyiv, Ukraine 4 Joint Institute for Nuclear Research, Dubna, Russia 5 INFN, sez. Roma, Rome, Italy 6 Dipartimento di Fisica, Università di Roma La Sapienza, Rome, Italy 7 INFN, Laboratori Nazionali del Gran Sasso, Assergi (AQ), Italy 8 Joint Stock Company NeoChem, Moscow, Russia 9 Nikolaev Institute of Inorganic Chemistry, Novosibirsk, Russia 10 Department of Physics, University of Jyväskylä, Jyväskylä, Finland 1 Double beta decay of 106Cd 106Cd natural abundance is 1.25(6)% 2 Development of 106CdWO4 scintillator [1] • • • • • Enriched 106Cd – 66% Crystal growth: Low-thermal-gradient Czochralski technique Mass of 106CdWO4 charge – 265 g Mass of single crystal boule – 231 g (87.2%) Non-recoverable losses – 0.78 g (0.3%) Boule 106CdWO4 (231 g) Scintillator 106CdWO4 ≈∅27×50 mm (215.4 g) 3 [1] P. Belli et al., NIMA 615 (2010) 301 3 Detector with 106CdWO4 DAMA R&D set-up at LNGS (Italy) Size, mm Mass, g Live time, h ∅27×50 215.4 3929 (1320 with Eγ > 1.8 MeV) Polystyrene Light-guide 3’’ PMT EMI9265 106CdWO 4 Quartz Light-guide 4 DAMA R&D set-up [1] Laboratori Nazionali del Gran Sasso Copper Detector Paraffin Lead Hermetic Plexiglass box Cadmium 5 [1] R. Bernabei et al., Nuovo Cim. A 112 (1999) 545 Calibration by γ quanta Calibration sources: 22Na, 60Co, 131Ba, 137Cs, and 228Th ZnWO4 ∅41×27 mm, 239 g FWHMγ=(11.2 Eγ)1/2 6 Background spectrum of 106CdWO4 106CdWO 4 1320 h 7 Pulse-shape discrimination with 106CdWO4 106CdWO 4 ∅27×50 mm, 216 g, 2400 h Overlapped pulses Overlapped pulses Overlapped pulses 8 Pulse-shape discrimination with 106CdWO4 106CdWO 4 1320 h 9 Time-amplitude analysis 106CdWO 4 4183 h 220Rn ↓α 216Po (T = 145 ms) 1/2 ↓α 212Bi 10 Radioactive contaminations of the 106CdWO4 crystal Source Activity, mBq/kg 116CdWO 4 CdWO4 106CdWO 215 g 0.053(9) b 0.039(2) c ≤ 0.026 b ≤ 0.014 c ≤ 0.1 b 0.053(5) c ≤ 0.6 b ≤ 0.5 b ≤ 0.004 b ≤ 0.045 b ≤ 0.18 b ≤ 0.018 b ≤ 0.063 b ≤ 0.3 b ≤ 0.8 b ≤ 0.3 b ≤ 0.3 b Σα 1.40(10) b 0.26(4) b 2.1(1) b 40K 0.3(1) a 91(5) a 0.43(6) a ≤5a 558(4) a ≤ 3.4 a ≤ 11 a 174 112(5) Bq/kg a 1.3(3) a 232Th 228Th 238U + 234U 230Th 226Ra 210Po 113Cd 113mCd 207Bi a – Fit of background spectra. b – Pulse shape discrimination. c – Time-amplitude analysis. 11 Simulated response functions to 2β processes in 106CdWO4 scintillator 12 Resonant double electron capture of 106Cd Resonant LK capture? Resonant 2K capture? 2742(7) 2721(7) 13 Resonant enhancement of 0ν2ε capture in 106Cd T1/ 2 (yr) = 5.561× 10 23 x + 9.42 eV 2 mν 2 2 14 Half-life limits on 2β processes in 106Cd Decay channel Level of 106Pd Experimental limit on T1/2 at 90% C.L. Present work Best previous limits 0ν2ε g.s. ≥ 3.6 × 1020 ≥ 8.0 × 1018 2νεβ+ g.s. ≥ 7.2 × 1019 ≥ 4.1 × 1020 2+1 512 keV ≥ 9.0 × 1019 ≥ 2.6 × 1020 2+1 1128 keV ≥ 3.2 × 1020 ≥ 1.4 × 1020 0+1 1134 keV ≥ 3.5 × 1020 ≥ 1.6 × 1020 0νεβ+ g.s. ≥ 2.1×1020 ≥ 3.7 × 1020 2ν2β+ g.s. ≥ 2.5 × 1020 ≥ 2.4 × 1020 2+1 512 keV ≥ 3.2 × 1020 ≥ 1.7 × 1020 0ν2β+ g.s. ≥ 2.1 × 1020 ≥ 2.4 × 1020 Resonant 0ν2K 2718 keV ≥ 1.4 × 1020 ≥ 1.7 × 1020 [*] Resonant 0νKL 2741 keV ≥ 3.2 × 1020 ≥ 1.6 × 1020 15 Conclusions and perspectives • An experiment using a cadmium tungstate crystal scintillator enriched in 106Cd up to 66% is in progress in the DAMA R&D set-up at the Laboratori Nazionali del Gran Sasso of INFN. • The total α activity of the 106CdWO4 scintillator is on the level of ≈ 2 mBq/kg. The main components of background of the detector are β active 113mCd (112 Bq/kg) and 207Bi (1.3 mBq/kg). • Lower limits on half-lives for 2β processes in 106Cd were set on the level of 1020 yr. • A possible resonant enhancement of 0ν2ε processes was estimated in the framework of QRPA approach. • A sensitivity of the experiment to different 2β processes in 106Cd after ≈ 3 yr of measurements is expected to be on the 16 level of ~ 1021 yr.