Effect Cherenkov dans le TeO2. Marco Vignati INFN Roma GDR neutrino, APC Paris, 21 June 2012

Transcription

1 Effect Cherenkov dans le TeO2 Marco Vignati INFN Roma GDR neutrino, APC Paris, 21 June 212

2 CUORE 13Te CUORE nat TeO2 bolometers (34% 13 Te), 75g each (ΔE = 5 kev FWHM) Past: Cuoricino 62 bolometers 11 kg ( 13 Te) 2 years, Bkg:.16 cpy/kev/kg T ν 1/2 > years (9% CL) mββ < 3~7 mev Future: Cuore (data taking in 215) Expected bkg:.1~.4 cpy/kev/kg Exp. T ν 1/2 > CL mββ < 4~94 mev Present: Cuore-, a CUORE-like tower same mass of Cuoricino. Rate [counts/ (1 kev)] Cuoricino 6 Co γ + γ Best Fit 68% C.L. 9% C.L. νdbd Energy [kev] CUORE: 988 bolometers 75 kg TeO2 2 kg 13 Te 2

3 Bolometers Particle energy converted into phonons temperature variation. νdbd source embedded in crystals. Low crystal heat capacitance and low base temperature to see small temperature variations ΔT ~ E/C Heat bath ~ 1 mk (copper) Weak thermal coupling Thermometer: NTD Ge thermistor R ~ 1 MΩ Energy release Absorber: TeO2 C ~ J/K Detector response in this configuration: ~.1 mk / MeV ~ 5 kev FWHM 3

4 CUORE: the α nightmare MC: the background in CUORICINO is due to degraded α particles which release only a part of their energy in the detector (surface contaminations, mainly in copper). Cu CUORICINO α s TeO 2 TeO 2 TeO2 bolometers, per se, do not allow to discriminate β and α particles. α bkg partially reduced by cleaning the detector parts. 4

5 LUCIFER Scintillating bolometers to discriminate the α background, enriched in 82Se or 1Mo. Target: define the technology for a ZERO background (<1 count/ton/year), ~1-ton isotope experiment after CUORE. Light detector: Ge bolometer Absorber bolometer: Zn82Se or Zn1MoO4 [kev] DetectedDetected lightlight [kev] 16 ZnSe 431g s α β/γ s Energy [kevee] Energy [kevee] 5

6 2 / ndf Prob.5899 p p e-8 ±.1678 ±.1222 ± ± 1.143e e-6 Cherenkov light in TeO2 TeO2 does not scintillate, however MeV β s emit Cherenkov light, unlike α s [ T. Tabarelli de Fatis, Eur. Phys. J. C 65 (21) 359]. Simulated Cherenkov emission spectrum from 1.5 MeV γ in TeO2 at low temperatures. dn/d dn/dλ [μm -1 ] Simulated emitted Cherenkov light as a function of β/γ energy. Produced Cherenkov light[ev] Cherenkov light (ev) (µm) 867 ev λ [μm] νdbd Photon Energy (kev) β/γ energy [kev] / 5 p2.428 p

7 First test: 117g TeO2:Sm crystal TeO2:Sm (3 ppb nat Sm) 3.x2.4x2.8 cm 3 VM g reflecting foil Light detector of pure Ge 66 mm diameter, 1mm thick. 7

8 α background. Entries Sm-147 Entries L1L2_Sm Mean.1123 RMS Integral / ndf / 11 const mean ± ± ± Sm ev σ 6 β/γ Th calibration Energy [[kev] Light energy kev] counts / counts / Tl <EL> [kev] EL [kev] EL [kev] 117g TeO2:Sm results JINST 6 (211) P15 Astropart. Phys. 35 (212) 558 β/γ s Entries Tl-28 L1L2_Tl28 Mean RMS.7263 Integral / ndf 1.73 / 6 const 4.77 ± 3.9 mean.1926 ± ± νdbd 2 15 α s Energy[keV] β/γ s light yield: 73 ev/mev Energy [[kev] Light energy kev] 171 MeV 8

9 CUORE crystal (5x5x5 cm 3 ) TeO2 wrapped with PTFE 4 Entries TeO2 28 Tl γ 8.5 kev FWHM Mean 2614 RMS Integral / ndf 26.4 / 21 Constant 37.8 ± 2.57 Mean 2614 ±.2 Sigma ± Energy (kev) Ge baseline Entries 2182 Mean.2722 RMS.7563 Ge ev σ Energy (kev) 9

10 Light energy [kev] CUORE crystal (5x5x5 cm3).4 2 / ndf Eth [kev] / ± Yield [ev/mev] ± β/γ s.1 21Po-α -.1 Detected β/γ light: 48 ev/mev 15 MeV Heat energy [kev] 1

11 CUORE with Cherenkov y] 26 68% DBD sensitivity [ γ bkg..1 bkg. [counts/kev/kg/y] Signal/Noise of light detector 11

12 CUORE with Cherenkov y] 26 68% DBD sensitivity [ Present: S = 15, N = 75 ev γ bkg..1 bkg. [counts/kev/kg/y] Signal/Noise of light detector 11

13 CUORE with Cherenkov y] 26 68% DBD sensitivity [ Present: S = 15, N = 75 ev bkg. [counts/kev/kg/y] Target γ bkg..1 Signal/Noise of light detector 11

14 ...and with 13 Te enrichment y] 26 68% DBD sensitivity [ bkg. [counts/kev/kg/y].1 with 9% enr..4 with 9% enr Signal/Noise of light detector 12

15 Sensitivity to ν Majorana mass 1 Bilenky and Giunti, 212 Current Bound 1 1 CUORE CUORE- 13 Te m [ev] IS inverted normal NS Cosmological Limit m min [ev] *CUORE lines indicate average of max/min 13 Te NME. 13

16 Sensitivity to ν Majorana mass 1 Bilenky and Giunti, 212 Current Bound m [ev] IS inverted normal NS Cosmological Limit CUORE CUORE- 13 Te CUORE+Cherenkov CUORE+Ch.+enrichment m min [ev] *CUORE lines indicate average of max/min 13 Te NME. 13

17 Requirement: Signal/Noise 5

18 Light collection The signal detected in bolometric tests is 1 ev, against 87 predicted. To investigate the source of losses and the signal specs we built a setup for studies at room temperature in Rome. First target: determine that the light we detect is effectively due to Cherenkov emission, not to scintillation... 15

19 Experimental setup Photonis XP297 PMT fast(~ns), UV sensitive(>2 nm) PMT Left cosmic Scintillator Top Hodoscope 2 cm thick, 4 7 cm 2 trigger: 3 ns scintillator coincidence trigger rate ~.1 Hz crystal rate:~.1hz cosϑc=1/(βn) n=2.64 at 34μm TeO2: 2.5x2.5x5 cm 3 tyvek wrapping C PMT right ϑc =68 (assuming β=1); Chromatic dispersion δθ~4 L(') = cos' (A L + B L (')) Scintillator Bottom Hodoscope A: dependent from the angle: Directional Cherenkov light. R(') = cos' (A R + B R (')) B: independent from the angle. Could be scintillation or Cherenkov light diffused. 16

20 Light direction and shape Corrected response [a.u] φmax φmax φmax -φmax R(ϕ) L(ϕ) φ = φmax Directional and flat component shapes are identical: R(φmax):mainly Čerenkov, L(φmax)=flat component hmean2copy R(φmax) Entries and 135 Mean L(φmax) RMS normalized Directional component = 1.6 x (Flat component) hmean1 Entries Mean RMS Next steps: ϕ Signal shape very simlar: flat component looks like Čerenkov ns Study the wavelength spectrum. Study light collection with different reflector configurations. 17

21 Requirement: Signal/Noise 5

22 Light detectors S/N>5: if Signal ~1 ev Noise ~ 2 ev σ Noise of Ge bolometers: ev σ Poor reproducibility: detectors used so far (7-8 ev σ) were selected among a large sample. Noise dominated by detector vibrations which induce temperature variations. Several attempts to lower the noise failed. 19

23 Possible alternatives Ge bolometers with Luke Effect: polarization of the Ge disk with electric field. Electron-hole pairs produced in interactions are boosted, inducing a higher phonon signal. Thermal noise does not see the electric field. Technique under investigation at LNGS and at Orsay. Transition Edge Sensors (TES): superconducting phonon sensors. Sensitive to athermal phonons, insensitive to vibrations. Technique proved in CRESST, but low reproducibility. Kinetic Inductance Detectors (KID): superconducting phonon sensors below the transition phase. High reproducibility, but technique to be proved. 2

24 Working group Sapienza Università di Roma and INFN Sezione di Roma F. Bellini, L. Cardani, N. Casali, I. Dafinei, F. Ferroni, M. Marafini, S.Morganti, F.Orio, D. Pinci, G. Piperno, D. Santone, C.Tomei, M.Vignati, C.Voena INFN Sezione di Milano Bicocca L. Pattavina, S. Pirro Università degli studi di Genova S. Di Domizio 21

25 Conclusions The detection of the Cherenkov light in TeO2 bolometers could improve the CUORE performances by a factor 3-6. Combined with 9% 13 Te enrichment, CUORE could cover the inverted hierarchy of neutrino masses. We detected the light, but we are still far from the required performances: light detector Signal/Noise > 5. At present: Signal ~ 1 ev Noise ~ 75 ev Studies to increase the Signal by increasing the light collection are being pursued. New low-noise light detectors are being considered. 22