the first prototype for a scintillating bolometer double beta decay experiment.

Transcription

1 the first prototype for a scintillating bolometer double beta decay experiment. On behalf of CUPID-INFN collaboration

2 Cuore Upgrade with Particle IDentification In the middle of 2015 INFN decided to support CUPID activity to develop: CUPID-0: a scintillating bolometer experiment as small scale demonstrator Some R&D to optimize cryogenic light detector performances (see L. Cardani talk)

3 ββ Decay Searches

4 Bolometric ββ Decay Experiments (I) Cuoricino/CUORE0 CUORE CUPID Past Present Future Dell Oro et all., Phys. Rev. D 90, (2014) T 0ν 1/2 1/m 2 ββ To improve the sensitivity it is necessary to reduce the experimental background. - Estimated background for CUORE 10-2 counts/(kev kg year) - Designed background for CUPID 10-4 counts/(kev kg year)

5 Bolometric ββ Decay Experiments (II) To reduce experimental background α/β discrimination is needed Energy-degraded alpha background A careful selection of the ββ isotopes 2615 kev 208 Tl Help in reducing β/ɣ background But isotope enrichment is needed

6 Scintillating Bolometers Light Reflector Thermal Bath (10 mk) Thermal Sensor Light Absorber Thermal Sensor Thermal Conductances Absorbing crystal With the simultaneous read-out of Photons and Phonons High energy resolution: as bolometer High discrimination capability: as scintillator

7 CUPID-0 Experiment CUPID-0 will be installed in the Cuoricino/CUORE-0 dilution refrigerator placed in the Hall A of LNGS 24 (22+2) Zn 82 Se bolometers 400 g each, 95% in 82 Se 2 ZnSe bolometer 400 g each, not enriched in 82 Se Q ββ ( 82 Se) = 2996 kev Light detectors made with high purity Ge wafers Thermal sensors made with NTD thermistors Detector assembled in 5 towers in Cuoricino/CUORE-0 cryostat Total active mass of the detector 11 kg ( 5.3 kg 82 Se) Expected ROI 10-3 count/(kev kg year) Expected FWHM energy ROI 10 kev

8 ZnSe Scintillating Bolometers ZnSe crystals show: - Excellent light/heat discrimination - α scintillates more then β - Excellent pulse shape discrimination - especially on light signal For CUPID-0 experiment 82 Se URENCO (96%) Zn 82 Se ISMA (Ukraine) Final enrichment 95% in 82 Se

9 Zn 82 Se Crystals Characterization (I) 3 Zn 82 Se crystals in a single tower (1.32 kg total mass) 3 ZnSe crystals in a single tower (1.3 kg total mass) crystals were not completely polished on surfaces Light read out with 4 Ge wafer per tower with antireflecting coating Smeared α source to determine the discrimination potential Measurement in LNGS Hall C cryostat (used for R&D activities) Due to cryogenic problem, Hall C cryostat reached 20 mk (instead of 10 mk) Operating conditions of the detector tower not optimal Energy resolution worst then planned for CUPID-0 detector But sufficient for all the requested tests on Zn 82 Se crystals

10 Zn 82 Se Crystal Characterization (II) All Zn 82 Se bolometers worked properly All Zn 82 Se crystals showed similar performances Ge light detectors worked very well To discriminate α respect to β/ɣ: - Cuts on light signal amplitudes - Cuts on light signal shapes Zn 82 Se Excellent discrimination potential was measured for all the Zn 82 Se bolometers

11 Eur. Phys. J. C76 (2016) 7, 364. Zn 82 Se Crystal Background Background measurement: 530 h live time Considerations on α background spectrum: - U and Th chains equilibrium - Energies of α peaks indicate internal contamination Po seems to be also on surfaces (5.3/5.4 MeV peaks) - Contribution outside the peaks seems not critical All Zn 82 Se crystals show comparable internal contamination Crystal production seems to be reproducible We can extrapolate the final background for CUPID-0

12 0νββ Decay Energy Region Eur. Phys. J. C76 (2016) 7, 364. Final spectrum obtained after: - α background discrimination - Multi event coincidence rejection No 82 Se ROI for 0νββ decay Few events are visible above 2.6 MeV but - Insufficient Hall C cryostat shielding - Inefficient Rn removal around the cryostat For the CUPID-0 experiment we will add: - Clean polished Zn 82 Se crystal surfaces - Detector assembly in a low Rn environment - New and more cleaner copper structure (CUORE like) - Better shielded cryogenic apparatus (Hall A cryostat)

13 Extrapolation to CUPID-0 Experiment Based on our MC simulation model of the CUPID-0 detector and assuming: - Energy resolution = 30 kev FWHM - Discrimination potential = 12 - Measured contamination as crystal internal background It is possible to evaluate the expected background in the ROI of 82 Se 0νββ decay Eur. Phys. J. C76 (2016) 7, 364. T 1/2 (0νββ 82 Se) = 7 x years (90% cl) (x 1 year data taking, 0 background approx.)

14 CUPID-0 Time Schedule August 2016 September 2016 October 2016 November 2016 December /18 - All CUPID-0 detector parts - CUPID-0 tower completely mounted - Reconfiguration of Hall A cryostat completed - CUPID-0 detector installed in Hall A cryostat - Hall A cryostat cool down - CUPID-0 detector characterization - Start of data taking of CUPID-0 experiment - CUPID-0 data taking CUPID-0 experiment will take data for about 2 years A final sensitivity around y for the half lifetime of 82 Se is expected

15 Conclusions The production of all the CUPID-0 components was completed Preliminary test of Zn 82 Se crystals shows very promising performaces Very high discrimination potential for α background rejection was demonstrated Estimated CUPID-0 background around 10-3 counts/(kev kg y) CUPID-0 detector completely assembled by end of summer 2016 Data taking will be start before the end of looking forward to CUPID experiment