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Washington D.C. CUORE September 19-21 2002
a low temperature technique approach to neutrinoless double beta decay and dark matter
Oliviero Cremonesi INFN - Sezione di Milano (Italy) on behalf of the CUORE collaboration
LBL – Firenze U. – Gran Sasso – Insubria U. – Leiden U. INFN Legnaro – Milano – USC – Zaragoza U.
♦ Neutrino oscillations and neutrinoless Double Beta Decay (0ν DBD) ♦ A low temperature approach to 0ν DBD (Mi DBD experiment) ♦ Detector concept and performance ♦ From Mi-DBD to CUORICINO ♦ CUORE description and sensitivity Double Beta Decay § § § ¨ ¨© ¢
¦ ν → ν § § § ¢ ¨ ¨©
Three main ν → § § § ¢ ¨ ¨©
decay modes χ → χ
♦ IF Neutrinos are massive DIRAC particles: 0ν-DBD Helicities can be accommodated thanks to the finite mass, BUT Lepton number is rigorously conserved is forbidden ♦ IF Neutrinos are massive MAJORANA particles: 0ν-DBD Helicities can be accommodated thanks to the finite mass, is allowed AND Lepton number is not relevant
SM allowed
¨ ¨© ν New neutrino properties: ¢ ν = (ν)c Helicity ≠ matching Neutrino is a massive mν 0 Majorana particle DBD & Neutrino Properties Theoretical description … Nuclear matrix element: Phase space: high Q (A,Z) Relevant uncertainty source (h)
− 0 1 2 2 e ( ) = W T1/2 ∑Gk (Q,Z)Mk k r r m m s l s l s s a a a a k i c i c i c i c e n e n i i l l s s s a s a c y c y y h y h c h c h u u
h Physical mechanism h e e P W P 2 N N P P e 2 m m G(Q, Z)M m Neutrino mass
±1 if CP conserved (A,Z+2) 2 α ≡ = 2 = i ek mν mee ∑Uek mk ∑ Uek e mk k k U U U Pontecorvo-Maki-Nakagawa-Sakata e e1 e2 e3 1 mixing matrix: = U1 U2 U3 2 U1 U2 U3 3 0νν-DBD and neutrino flavor oscillations (1) ♦ what do we presently know from neutrino flavor oscillations?
oscillations do occur neutrinos are massive ∆ 2 2 -4 M12 (eV ) = [0.2 – 2] x10 Small ν ν ν solar e oscillate to µ, τ ∆ 2 several experiments – last decisive SNO approximate measurements of two Mij ∆ 2 2 -3 M23 (eV ) = [2 – 6] x10 Big atmospheric νµ oscillate to ντ SuperKamiokande reactor exp. approximate measurements and/or constraints on Ue1, Ue2, Ue3 Small solar ν − Big ♦ what will we never know from neutrino flavor oscillations?
neutrino mass hierarchy
absolute neutrino mass scale
α hierarchy (M1 0ν-DBD can complete the information coming from oscillations even for vanishing M , m is within the ♦ 1 ee fix the absolute mass scale reach of next experiments in case of ♦ determine DIRAC or MAJORANA nature inverse hierarchy or degeneracy ♦ give information on CP-phases 2 2 2 mee ~ Ue1 M1 + Ue2 M2 + Ue3 M3 Log[mee(eV)] Log[mee(eV)] best fit present sensitivity 90% c.l. fit 76 to oscillation data to oscillation data ( Ge experiments) degeneracy next generation inverse hierarchy sensitivity (CUORE) technically feasible direct hierarchy sensitivity Log[M1(eV)] Log[M1(eV)] Experimental search for DBD ♦ → - → - ν We are looking for: (A,Z) (A,Z+2) + 2e (A,Z) (A,Z+2) + 2e + 2 e ♦ More common strategy: Detect the two electrons and measure their energy ♦ Signature: shape of the two electron high energy sum energy spectrum resolution two neutrino DBD low background continuum with maximum at ~1/3 Q - underground neutrinoless DBD operation - shielding peak enlarged only by the detector energy resolution sum electron energy / Q - low radioactivity of materials e- e- detector ♦ Two approaches: source e- e- detector Source ≡ Detector Source ≠ Detector (calorimetric technique) + event shape reconstruction + high energy resolution - low energy resolution - no event topology DBD Experimental sensitivity Experimental evaluation of a decay lifetime: N = number of decaying nuclei under observation τ = ln2 • N • T / N N 1/2 N S NS = number of observed decays (τ>>T) T = measure live time “Zero Bkg” (No counts): Ns= k Sensitivity = Lifetime corresponding to the minimum detectable number of events above background at a given C.L. Background fluctuations: ε 1/2 S = ln2 · NA · n · a · · T / (R · W) Ns=(N) NA = Avogadro Number 1 FWHM R = Background level (keV-1 y-1) n = Mole number a = Isotopic abundance Background ∝ detector mass e = Detection efficiency W = Energy resolution (FWHM) T = Live time ε S = ln2 · NA · a / A · m · T / (R’ · W) Linear dependence! 0νν-DBD: choice of the candidate nuclide ♦ 1/τ 2 2 Life time: = G(Q,Z) |Mnucl| mee phase space ∝ Q5 high Q is required (> 2 MeV) Some candidates are better than others for nuclear dynamics Large source of uncertainty (a factor ~3 is usually estimated) Isotopic enrichment is ♦ Isotopic abundance must be high - not always feasible - often very expensive nuclides with natural high isotopic abundance are preferred ♦ In the calorimetric approach, the candidate or a chemical compound of its must be a suitable material for detector construction 76Ge → large Germanium semiconductor detector 136Xe → ionization in gas or liquid Xe – scintillation in liquid Xe many candidates → low temperature detectors (bolometers) Properties of 130Te as a DBD emitter excellent feature for future reasonable-cost expansion of Double Beta Decay experiments 130Te presents several nice features: large phase space, ♦ high natural isotopic abundance (I.A. = 33.87 %) lower background (clean window ♦ high transition energy ( Q = 2528.8 ± 1.3 keV ) between full energy and Compton edge ♦ encouraging theoretical calculations for 0ν−DBD lifetime of 208Tl photons) ♦ already observed with geo-chemical techniques m ≈ 0.1 eV ⇔ τ ≈ 1026 y τ × 21 ee ( 1/2 incl = ( 0.7 - 2.7 ) 10 y) Comparison with other candidates: 0ν−DBD half-life (y) for mee = 0.1 eV Isotopic abundance (%) Transition energy (MeV) (different calculations) 40 5 1030 4 20 1027 3 0 2 1024 48Ca 76Ge 82Se 96Zr 100Mo116Cd130Te136Xe150Nd 48Ca 76Ge 82Se 96Zr 100Mo116Cd130Te136Xe150Nd 48Ca76Ge 82Se 96Zr 100Mo116Cd130Te136Xe150Nd Low T Detector concepts Te dominates in mass the compound Excellent mechanical Energy absorber and thermal properties Calorimetric approach TeO crystal 2 Good energy resolution ≅ ≅ C 2 nJ/K 1 MeV / 0.1 mK No limit to material choice Heat sink ≅ T 10 mK Thermometer NTD Ge-thermistor R ≅ 100 MΩ Thermal coupling dR/dT ≅ 100 kΩ/µΚ G ≅ 4 nW / K = 4 pW / mK ♦ Temperature signal: ∆T = E/C ≅ 0.1 mK for E = 1 MeV ♦ Bias: I ≅ 0.1 nA ⇒ Joule power ≅ 1 pW ⇒Temperature rise ≅ 0.25 mK ♦ Voltage signal: ∆V = I × dR/dT × ∆T ⇒ ∆V = 1 mV for E = 1 MeV ♦ Signal recovery time: τ = C/G ≅ 0.5 s ♦ Noise over signal bandwidth (a few Hz): V = 0.2 µV In real life signal about rms a factor 2 - 3 smaller Energy resolution (FWHM): ≅ 1 keV Structure & Evolution of the detectors Common points: 1997 Thermistor: NTD Ge chip glued with epoxy Heat sink: Cu plates, frames and bars Holding method and thermal contact: Teflon elements ♦Crystal mass: 340 g - 760 g ♦ Elementary module: 4 detectors ♦ Small amount of Teflon ♦ Crystal surfaces: lapped by us with radio-pure power Mi DBD - I ♦ Crystal mass: 340 g 2001 ♦ Elementary module: 1 detector ♦ Large amount of Teflon ♦ Crystal surfaces: lapped in China with 238U-contaminated power Mi DBD – II CUORICINO CUORE Phases of the experiment Past, present and future: 1997 The first large mass array of bolometers was operated. 20 crystals × 340 g = 6.8 kg (Mi DBD - I experiment) 1998-2001 Tests on larger crystals (760 g) were successfully carried on aiming at more powerful experiments 2001 The 20 crystal array is rebuilt with improved BKG features (Mi DBD - II experiment) 2001 A new, larger mass array is in preparation. 44 crystals × 760 g ≈ 40 kg (CUORICINO experiment, approved) 18 crystals × 340 g 2002 Full data taking of CUORICINO 2003-2006 If approved, construction of a second generation array 1000 crystals × 760 g = 760 kg (CUORE experiment) The Mi DBD - II: experimental set-up (a general test for the CUORICINO set-up) 5 modules, 4 detector each, are arranged in a tower-like compact structure (6.8 kg) The tower is mounted inside a dilution refrigerator Coldest point and cold finger The tower is surrounded by an inner lead shield, (Roman lead) and all the refrigerator by a 20 cm thick outer lead shield Mi DBD – II @ LNGS Laboratori Nazionali del Gran Sasso Two dilution refrigerators: Hall A Hall C (R&D) Mi DBD II: results Total statistic: single 340 g detector + 0ν2β 4 × array + 20 × array (I and II) + enriched crystal = 4.3 kg y τ × 23 1/2 > 2.08 10 y @ 90% c.l. DBD Q-value (M.L. assuming flat BKG + 208Tl and 214Bi peaks) 60 238U + 232Th calibration spectrum ) similar to 340 g crystal V e thanks to improved detector design k 214 Bi 2 . 1 / ( s 228 Performance of CUORICINO-type t Ac n detectors (5×5×5 cm3 - 760 g): u 40K 208 o Tl C ♦ Detector base T: ~ 7 mK 10 ♦ Detector operation T: ~ 9 mK ♦ Detector response: ~ 250 mV/ MeV ♦ FWHM resolution: ~ 3.9 keV @ 2.6 MeV 0.6 1.6 2.6 Energy [MeV] The background in Mi DBD experiment History of the background (BKG) BKG obtained with single 340 g detector BKG in the range 1.0 - 2.8 MeV 0.41 kg y 40K 208Tl Operation in clean conditions BKG obtained with Mi DBD - I 3.10 kg y Polishing, etching and passivation of copper Lapping of crystals with ultra-pure powder BKG obtained with Mi DBD - II 0.45 kg y The background in the region of 0νν-DBD BKG in the range 2.3 - 2.8 MeV DBD Q-value Counts keV kg y 10 1 0.1 Energy (keV) 2400 2600 2800 BKG levels 7 Counts DBD Q-value Before crystal and Cu treatment keV kg y 5 0.59 ± 0.06 c/kev/kg/y 3 After crystal and Cu treatment 1 0.33 ± 0.11 c/kev/kg/y 2500 2556 Energy (keV) Discussion on the BKG (1) We have identified 4 possible sources for the residual BKG in the DBD region: Excluded since adding B-polyethilene shield had no effect ♦ Neutrons ♦ Degraded alphas from TeO2 surface The alpha continuum extends ♦ Degraded alphas from Cu frame and plate surface down to the DBD region ♦ Energy degraded 2615 keV photons A Montecarlo simulation seems to show that the alpha BKG is determined mainly Coincidences between adjacent detectors show that by a superficial contamination of U-Th in Cu plates and frames. crystal surfaces emit alphas belonging to 238U and 232Th series. This contamination explains Crystal contamination determines peaks more than continuum. the alpha region BKG, the DBD region BKG and the 208Tl peak. 208Tl -2615 keV peak Straight lines correspond to full energy shared by two detectors Alpha region (continuum) Experimental spectrum Alpha region Montecarlo (peaks) Discussion on the BKG (2) Further proof that the BKG in the DBD region is presently dominated by degraded alpha particles (surface effect): Clear correlation between: BKG in the DBD region ⇔ BKG in the region 3-4 MeV free from alpha and gamma peaks Mi DBD - I Mi DBD - II ± ± BKG 2500-2556 keV (c/keV/kg/y) 0.59 0.06 0.33 0.11 ± ± BKG 3000-4000 keV (c/keV/kg/y) 0.66 0.01 0.27 0.03 Background: MC estimates 5x5x5 TeO crystals Geant4 2 Gendec Mibeta I 3x3x6 TeO crystals 2 CUORE Mibeta II Background: MC estimates (2) Cryostat, shields and detector details Mi DBD Background Model Experiment/Montecarlo comparison: • reliability • quantitative contributions from different sources Experimental MC: bulk Mounting box GEANT4 Crystals s s t t n n u u o Mi DBD I o C C Mi DBD II External Cuoricino Cuore MC: Surface (crystals+mounting box) Energy (keV) Reliable estimate of experimental sensitivity Limit on 0νν-DBD – Panoramic 0ν Experiment Isotope T1/2 (y) mee (eV) You Ke et al. 1998 48Ca > 9.5 × 1021 (76%) < 8.3 Klapdor-Kleingrothaus 2001 76Ge > 1.9 × 1025 < 0.35 Aalseth et al 2002 > 1.57 × 1025 < 0.33 - 1.35 Elliott et al. 1992 82Se > 2.7 × 1022 (68%) < 5 Ejiri et al. 2001 100Mo > 5.5 × 1022 < 2.1 Danevich et al. 2000 116Cd > 7 × 1022 < 2.6 Bernatowicz et al. 1993 130/128Te* (3.52 ±0.11) × 10-4 < 1.1 – 1.5 Bernatowicz et al. 1993 128Te* > 7.7 × 1024 < 1.1 – 1.5 Mi DBD – 2002 130Te > 2.1 × 1023 < 0.85 – 2.1 Luescher et al. 1998 136Xe > 4.4 × 1023 < 1.8 – 5.2 Belli et al. 2001 136Xe > 7 × 1023 < 1.4 – 4.1 De Silva et al. 1997 150Nd > 1.2 × 1021 < 3 Danevich et al. 2001 160Gd > 1.3 × 1021 < 26 H.V. Klapdor et al. claim: 0.11 - 0.56 eV (0.39 eV c.v.) skepticism Mod. Phys. Lett. A 16 (2001) 2409 in the DBD community Sensitivity limits 2nd generation experiments common features: • Large amount of source material (isotopic enrichment) • Background reduction (R&D required) • Intermediate scale steps (0.1-0.5 eV sensitivity on CUORICINO = tower of 13 modules, 4 detector (790 g) each M = 41 kg Coldest point New configuration: 2 planes will consist of 340 g detectors arranged in a 3×3 matrix Cold finger Plane section Tower Lead shield Same cryostat This detector will be completely and similar surrounded by active materials. structure Substantial improvement as Mi DBD in BKG reduction Prospects for the BKG in CUORICINO Now (Mi DBD - II): BKG ∼ 0.3 counts/keV/kg/y Proof that it is a surface effect Final lapping by us with radio-pure powder on crystals that have never seen Improvement in surface cleaning the Chinese cerium-oxide contaminated powder surface mass 5 × 5 cm3 340 g We will gain anyway a factor: × = 0.62 3 × 6 cm3 760 g verified in our test set-up with 5 × 5 × 5 cm3 crystals Just for geometry, we expect a BKG in the range 0.1- 0.2 counts/keV/kg/y A BKG of ∼0.1 counts/keV/kg/y is within the reach of the CUORICINO set-up CUORICINO sensitivity Sensitivity: Lifetime corresponding to the minimum detectable number of events above background at a given C.L. Detector mass (kg) Running time (y) Isotopic abundance 1/2 Detector efficiency a M T F0ν = 4.17 × 1026 × × ε A b Γ Atomic mass BKG (counts/keV/kg/y) Energy resolution (keV) Reasonable: b = 0.1 - Γ = 5 keV Pessimistic: b = 0.3 - Γ = 10 keV F0ν = 6.85 × 1024 T1/2 F0ν = 2.8 × 1024 T1/2 3 years mee < 0.11 – 0.69 eV mee < 0.17 – 1.08 eV 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 mee (eV) H.V. Klapdor et al. claim: 0.11 - 0.56 eV (0.39 eV c.v.) CUORICINO: criostat & wiring CUORICINO: cold electronics CUORICINO: thermistors and heaters CUORICINO: anti-Rn Lapping machine Gluing box CUORICINO: 4 crystal plane Copper surface treatment TeO2 crystal surface treatment New improved mounting design The CUORE set-up CUORE = closely packed array of 1000 detectors 25 towers - 10 modules/tower - 4 detector/module M = 760 kg Cubic structure, ideal for active shielding no more inert Cu plates facing crystals Each tower is a CUORICINO-like detector Special dilution refrigerator CUORE: R&D Long term stability: • Cryogenics • Detector optimization Large number of detectors: • reproducibility • read-out & calibration Background suppression: • materials selection • setup materials • cleaning/preparation materials • surface tratment improvement • radiation shields improvement Expected energy resolution: 5 keV FWHM @ 2.615 MeV ββ2ν contribution: < 10-4 counts/keV/kg/y CUORE: main issues Mass & isotopic abundance are obvious (not principle) limits Rn contribution are important only in the assembly phase: • Rn-free volume: external shield (anti-Rn box: Nitrogen flux) Floor-space minimal requirements: • 60 m2 for experimental area (at least 5 m height) • large recovery tank for LHe Shareble clean room for detector assembly Safety issues: Cryogenics • contained amounts of LHe (100-200 l) • gas bottles • liquid Nitrogen Laboratory depth: µ reaction induced activity can contribute to background (e.g. 208Pb) but environmental radioactivity is the major concern Prospects for the BKG in CUORE Unlike CUORICINO, we have not a small scale, experimentally-tested structure from which to extrapolate a possible BKG for CUORE We will have precious indications from the two crystals completely surrounded by active materials We have to rely on Montecarlo simulations, which assume typical contaminations in the main CUORE components From the Montecarlo simulations it can be seen that: 1. the high granularity of CUORE 2. the minimum amount of inert material between neighboring crystals allow to achieve large BKG suppression factors with the coincidence method CUORE sensitivity Montecarlo simulation Summarizing the BKG contributions: ♦ Bulk contamination is not a problem ⇒ ∼ 0.001 counts/keV/kg/y ♦ Surface contamination from passive materials is potentially dangerous, but the amount of Cu facing the detector will be reduced by a factor 10 -100 with respect to now ⇒ ∼ 0.01 - 0.001 counts/keV/kg/y 1 y sensitivity: Pessimistic estimation: b = 0.01 - G = 5 keV 0ν × 25 × 1/2 × 1/4 F = 9.4 10 ( T[y ] ) mee < 40 - 249 meV ( T[y ] ) (27-167 meV in 5 years) Optimistic estimation : b = 0.001 - Γ = 5 keV 0ν × 26 × 1/2 × 1/4 F = 2.96 10 ( T[y ] ) mee < 22.4 - 140.6 meV ( T[y ] ) (15-94 meV in 5 years) Sensitivities of next generation projects 5y Experiment Author Isotope Detector description T 1/2(y) mee * COBRA Zuber 2001 130Te 10 kg CdTe semiconductors 1 x 1024 0.71 Arnaboldi et al CUORICINO 130 40 kg of TeO bolometers 1.5 x 1025 0.19 2001 Te 2 10 kg of bb(0n) isotopes (7 kg Mo) with NEMO3 Sarazin et al 2000 100 4 x 1024 0.56 Mo tracking Arnaboldi et al. CUORE 130 760 kg of TeO bolometers 7 x 1026 0.027 2001 Te 2 EXO Danevich et al 2000 136Xe 1 t enriched Xe TPC 8 x 1026 0.052 Zdesenko et al 1 t enriched Ge diodes in liquid nitrogen + GEM 76 7 x 1027 0.018 2001 Ge water shield Klapdor- GENIUS Kleingrothaus et al 76Ge 1 t enriched Ge diodes in liquid nitrogen 1 x 1028 0.015 2001 MAJORANA Aalseth et al 2002 76Ge 0.5 t enriched Ge segmented diodes 4 x 1027 0.025 DCBA Ishihara et al 2000 150Nd 20 kg enriched Nd layers with tracking 2 x 1025 0.035 116 26 CAMEO Bellini et al 2001 Cd 1 t CdWO4 crystals in liquid scintillator > 10 0.069 several tons of CaF crystal in liquid CANDLES Kishimoto et al 48 2 1 x 1026 Ca scintillator 2 t Gd SiO :Ce cristal scintillator in liquid GSO Danevich 2001 160 2 5 2 x 1026 0.065 Gd scintillator 34 t natural Mo sheets between plastic MOON Ejiri et al 2000 100 1 x 1027 0.036 Mo scintillator Caccianiga et al Xe 136 1.56 t of enriched Xe in liquid scintillator 5 x 1026 0.066 2001 Xe Moriyama et al XMASS 136 10 t of liquid Xe 3 x 1026 0.086 2001 Xe (5 y operation) * Staudt, Muto, Klapdor-Kleingrothaus Europh. Lett 13 (1990) 31 CUORE cost estimation Item Cost (in today USA dollars) Crystals 4.300.000 Refrigerator 600.000 Cryogenic equipment (liquefier, dewars etc) 300.000 Chemical materials (grinding powders etc) 200.000 Shielding materials (lead, neutron shield etc.) 600.000 Electronics 600.000 Installation 400.000 Contingency 1.000.000 TOTAL 8.000.000 CUORE: temptative schedule 2003 2004 2005 2006 2007 2008 Proposal Detector Crystals Materials selection Procedures settling Growth/Preparation Sample test (technical+radiopurity) Thermistors & heaters Preparation & characterization Cryostat Materials selection Design Order & construction Installation and test Underground Laboratory Design Construction CUORE Final Assembly CUORE Start Conclusions ♦ Neutrino physics is a hot field today: we know from oscillations that neutrinos are massive, but we need other experiments to fix the absolute mass scale and the neutrino nature ♦ 0ν-DBD can provide the lacking information about neutrinos ♦ LT detection of particle is a powerful method to study 0ν-DBD (Mi DBD experiment) ♦ Detector performances are highly compatible with a sensitive DBD search (5 keV resolution at DBD energy already proved - possible improvements) ♦ CUORE (preceded by the intermediate step CUORICINO) is a new generation DBD experiment based on an array of 1000 LT detectors (total mass: 790 kg) ♦ CUORE sensitivity to neutrino masses will be ~ 25 meV, allowing a significant test of the neutrino mass spectrum