LUCIFER Marco Vignati INFN Roma XCVIII congresso SIF, Napoli, 21 Settembre 212
Neutrino nature Except for the total leptonic number the neutrino is a neutral fermion. So if the total leptonic number is not conserved neutrinos can be Majorana particles: particle and antiparticle are the same. Chirality determines the charge of the lepton produced in interactions: It is still not clear today whether neutrinos are Dirac or Majorana particles. The distinction makes sense only because they are massive. 2
Neutrinoless double beta decay Nuclear process: (A,Z) (A,Z+2) + 2 e - Can only happen if lepton number is not conserved, unlike the Standard Model allowed 2ν mode: (A,Z) (A,Z+2) + 2 e - + 2 ν The decay probability depends on the effective Majorana mass m of the neutrino exchanged between the two electron vertexes. The measurable quantity is the half-life ( Phase space factor: ~ Q 5 Nuclear Matrix element 1/2 ) of the decay: Effective neutrino mass 1 ( 1/2 ) = G(Q, Z) M nucl 2 m 2 m = F 2 N m 2 e 3
Effective Majorana mass Effective neutrino Majorana mass in terms of the measured oscillation parameters and the unknown lightest neutrino mass: 1 Current Bound m [ev] 1 1 1 2 1 3 inverted normal IS NS Cosmological Limit (arxiv:123.525) 1 4 1 4 1 3 1 2 1 1 1 m min [ev] 1 2 3 4
νdbd in Experiments Experiments measure the sum of the kinetic energies of the two emitted electrons. Signature: monochromatic line at the Q-value of the decay. Sensitivity ( S ): lifetime corresponding to the minimum number of detectable events above background at a given C.L.: Isotopic abundance (%) S = ln 2N A a A Detector mass (kg) Mt B E Measurement time (y) 1/2 detection efficiency Atomic mass Background (counts/kev/kg/y) Energy Resolution (kev) 5
Isotope choice. νdbd candidates of experimental interest: (arxiv:121.4916) In general, Q > 2615 kev isotopes are preferred because they lie above the natural radioactivity edge. However the choice has been dominated so far by technology compromises. 76 Ge 136 Xe 116 Cd 1 Mo 13 Te U and Th environmental bkg 82 Se 6
Experimental race m [ev] 1 1 1 1 2 1 3 Current Bound inverted IS normal NS Cosmological Limit Entire inverted hierarchy: M ~ 1 ton a ~ 9% B ΔE ~ 1 count/ton/year Normal hierarchy: Dream 1 4 1 4 1 3 1 2 1 1 1 m min [ev] 1 2 3 7
Bolometric technique Particle energy converted into phonons temperature variation. Crystals embedding νdbd source. Low crystal heat capacitance and low base temperature to see small temperature variations ΔT ~ E/C Heat bath ~ 1 mk Weak thermal coupling Thermometer: NTD Ge thermistor R ~ 1 MΩ Energy release Absorber Crystal C ~ 2 1-9 J/K Detector response in this configuration: ~.1 mk / MeV Resolution @νdbd ~ few kev 8
CUORE 13Te CUORE - @LNGS nat TeO2 bolometers (34% 13 Te), 75g each (ΔE =5 kev FWHM) Past: Cuoricino 62 bolometers 11 kg ( 13 Te) 2y, Bkg:.16 cpy/kev/kg T ν 1/2 > 2.8 1 24 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 > 1.6 1 26 years mββ < 4~94 mev Present: Cuore-, a CUORE-like tower. same mass of Cuoricino,.5 cpy/kev/kg. Rate [counts/ (1 kev)] 5 45 4 35 3 25 2 15 1 5 Cuoricino 6 Co γ + γ Best Fit 68% C.L. 9% C.L. νdbd 248 25 252 254 256 258 Energy [kev] CUORE: 988 bolometers 75 kg TeO2 2 kg 13 Te 9
CUORE expected sensitivity 1 m [ev] 1 1 1 2 1 3 1 4 Current Bound KK evidence - 76 Ge IS inverted normal NS m min [ev] Cosmological Limit 1 4 1 3 1 2 1 1 1 1 2 3 CUORICINO- 13 Te CUORE- 13 Te Only with - Bkg < 1 count/ton/y - 1 ton isotope 1
CUORE: the α nightmare MC: most of 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 TeO 2 TeO 2 α s TeO2 bolometers, per se, do not allow to discriminate β and α particles. α bkg partially reduced by cleaning the detector parts. β/γ smaller in CUORE thanks also to the self-shielding geometry. 11
Scintillating bolometers Scintillating crystals can be operated as bolometers. Unfortunately TeO2 does not scintillate, other compounds must be considered. The simultaneous read-out of light and thermal signals allows to discriminate the α background thanks to the scintillation yield different from β particles. Light detector Thermistor Energy Release Bolometer 12
Light detectors Germanium disks (5 cm diameter,.1-1 mm thick). Calibration with a 55 Fe source: 5.9 & 6.5 kev X-rays. ΔE @ 55 Fe: ~ 13 ev RMS ΔE @baseline: 1 ev RMS 13
Candidate #1: ZnSe DBD Isotope: Q-value [kev] isotopic abundance Light Yield [kev/mev] QF 82 Se 2995 9% 7-11 4 Largest crystal operated so far: 431g 14
QF > 1 is odd: Observed only in this compound (CdWO4 ZnMoO4 and other crystals have QF < 1). Light Yield [kev/mev] ZnSe 4 35 3 25 2 15 νdbd 1 not understood. 5 1 2 3 4 5 6 7 8 Excellent separation using light energy and signal shape Light Decay Time [ms] Light decay-time [ms] Energy [kevee] β/γ-background 14 13 12 11 α-crystal contamination 1 9 8 7 5 1 15 2 25 3 35 4 Detected Light [kev] 15
ZnSe: Light-Heat correlation Detected light [kev] 21 2 19 18 17 16 Detected light [kev] 21 2 19 18 17 16 15 15 14 14 13 13 12 259 26 261 262 263 264 Energy [kevee] 12 259 26 261 262 263 264 Decorrelated energy [kevee] 12 1 8 6 4 2 Integral 11 2 / ndf 4.35 / 13 A 1.5 ± 1.5 mean 2614 ± 1. 7.938 ± 1.116 counts / 2 kevee 16 14 12 1 8 6 4 2 Integral 11 2 / ndf 8.21 / 14 A 13 ± 1.9 mean 2615 ±.6 5.741 ±.594 ΔE@2615: 13 kev FWHM 259 26 261 262 263 264 Energy [kevee] 259 26 261 262 263 264 Decorrelated energy [kevee] 16
Operation of a tower of 32-4 Zn 82 Se crystals at LNGS. Option1: use the Cuoricino cryostat in halla (presently hosting CUORE-), if CUORE- stops in 215. Option2: use the cryostat in hallc (presently running the CUORE- and LUCIFER R&Ds). Needs cryostat update. Cuoricino cryostat: Inner shield: - 1cm Roman Pb A ( 21 Pb) < 4 mbq/kg 4 crystals per floor Plan for a ZnSe array in 215 LUCIFER External shield: - 2 cm Pb - 1 cm Borated polyethylene Nitrogen flushing to avoid Rn contamination. 17
Light detector R&D Thermistors prod. Crystal growth R&D 82 Se prod. (15 kg) Enriched crystals production Array assembly ZnSe: schedule 212 213 214 215 The most crucial part is represented by the crystal growth. The supplier (Ukraine) is presently fine-tuning the complicate procedure (that starts with metal Zn and metal Se). The request of minimizing the 82 Se waste is a complicate issue. The target is to reach > 75 % efficiency. 18
Do we need a μ-veto? Light Yield [kev/mev] 15 14 13 12 11 β/γ α ZnSe data: 1 β/γ event above the 2615 kev line in 58 hours 1 9 15 2 25 3 35 4 Energy [kevee] ~ 5x1-2 counts/kev/kg/y The event includes hits on close detectors (multi-site event) Multiple γ s produced by μ interactions in the materials close to the detector. Easy to remove in this case, but what if one has one hit only? Montecarlo simulation under development. 19
Candidate #2: ZnMoO4 DBD Isotope: Q-value [kev] isotopic abundance Light Yield [kev/mev] QF 1 Mo 334 1% 1.5.2 Largest crystal operated so far: 33g 2
Excellent discrimination using the light signal LY [kev/mev] Light Yield [kev/mev] ZnMoO4 2.4. 2.2 β/γ source 2 1.8 1.6 1.4 νdbd 1.2 1.8.6 α source.4.2 Discrimination using the shape of the heat signal! Heat signal shape parameter TVR [a.u.] 5 1 2 25 3 35 4 Energy [kev] 4 α source 2-2 νdbd -4 β/γ source -6-8 -1-12 Eur. Phys. J. C 72 (212) 2142-14 21 15 5 1 15 2 25 3 35 4 Energy [kev]
ZnMoO4 Energy resolution similar to CUORICINO (2x better than ZnSe) counts / 3 kev 9 8 7 6 5 7 6 5 4 3 2 1 2.925 FWHM= 6.3 kev Entries 11528 259 26 261 262 263 264 1.153e+4 Counts / 9 kev 2 1 1 1 source 21 2 Entries 2821 Mean 12742 RMS 1449 Integral 2821 1 1 2 3 4 5 6 7 8 9 1 Po 226 Ra 222 Rn 218 Po 4 3 214 214 Bi Po Energy [kev] 5 1 15 2 25 Energy [kev] 3 β/γ source Internal contaminations: 232 Th < 1.4 pg/g 22
2 / ndf Prob.5899 p p1-3.111e-8 ±.1678 ±.1222 ± -1.43 ± 1.143e-9 4.448e-6 Candidate #3: TeO2 TeO2 does not scintillate, however MeV β s emit Čerenkov light, unlike α s [ T. Tabarelli de Fatis, Eur. Phys. J. C 65 (21) 359]. Simulated Čerenkov emission spectrum from 1.5 MeV γ in TeO2 at low temperatures. dn/d dn/dλ [μm -1 ] 8 7 6 5 4 3 2 1 Simulated emitted Čerenkov light as a function of β/γ energy. Produced Cherenkov light[ev] Cherenkov light (ev) 1 9 8 7 6 5 4 3 2 1.2.3.4.5.6.7.8.9 1 (µm) 867 ev λ [μm] νdbd 5 1 15 2 25 Photon Energy (kev) β/γ energy [kev] 3.723 / 5 p2.428 p3.8393 23
Light energy [kev] Čerenkov from a CUORE crystal.4 2 νdbd / ndf Eth [kev].3 11.9 / 6 36.3 ± 64.47 Yield [ev/mev] 48.33 ± 2.846.2 rce u o s β/γ.1 21Po-α contamination -.1 Detected β/γ light: 48 ev/mev 15 ev @2.527 MeV -.2 -.3 1 2 3 4 5 Heat energy [kev] Detected light not sufficient to discriminate α s event by event. Needs light detector development: light collection, energy resolution. 24
CUORE + Čerenkov + enrichment y] 26 68% DBD sensitivity [1 18 16 14 12 1 8 6 4 2 γ bkg. (MC)=.1 bkg. [counts/kev/kg/y].1 with 9% enr..4 with 9% enr..1.4 1 2 3 4 5 6 7 8 Signal/Noise of light detector 25
CUORE + Čerenkov + enrichment y] 26 68% DBD sensitivity [1 18 16 14 12 1 8 6 4 2 Present: S = 15, N = 75 ev γ bkg. (MC)=.1 bkg. [counts/kev/kg/y].1 with 9% enr..4 with 9% enr..1.4 1 2 3 4 5 6 7 8 Signal/Noise of light detector 25
CUORE + Čerenkov + enrichment y] 26 68% DBD sensitivity [1 18 16 14 12 1 8 6 4 2 Present: S = 15, N = 75 ev bkg. [counts/kev/kg/y].1 with 9% enr..4 with 9% enr..1.4 1 2 3 4 5 6 7 8 Target γ bkg. (MC)=.1 Signal/Noise of light detector 25
Overview until 215 ZnSe Target: build a bolometric experiment of ~ 1 kg of 82 Se Status: Isotope in production, crystal growth under optimization. ZnMoO4 Target: build a bolometric experiment of ~ 1 kg of 1 Mo Status: MoU with IN2P3 and ITEP in consideration. TeO2 Target: develop light detectors with S/N improved by a factor 4. Status: KIDs and Neganov-Luke detectors under development. 26
Overview until 215 ZnSe baseline Target: build a bolometric experiment of ~ 1 kg of 82 Se Status: Isotope in production, crystal growth under optimization. ZnMoO4 Target: build a bolometric experiment of ~ 1 kg of 1 Mo Status: MoU with IN2P3 and ITEP in consideration. TeO2 Target: develop light detectors with S/N improved by a factor 4. Status: KIDs and Neganov-Luke detectors under development. 26
Ultimate background: β/γ So far, we do not have a measure of the β/γ background above 2615 kev with bolometric arrays à la CUORICINO, and of the CUORE β/γ background at the 13 Te Q-value. CUORICINO data 13 Te 82 Se 1 Mo so far: α s + β/γ s α s (+ β/γ s?) CUORE ( 13 Te) will measure the reduction 82 Se and 1 Mo arrays will answer 27
Conclusion: which one after CUORE? To cover the entire inverted hierarchy of neutrino masses a bolometric experiment operated in the CUORE cryostat will require: ΔE ~ 5 kev FWHM Bkg < 1 count/kev/ton/y 1 ton isotope R&D Status ΔE (kev) Bkg. reach Enrichment cost ( /g) 82 Se Isotope/crystals in preparation 13 likely 75 (contract) 1 Mo MoU to be signed 6 likely 1-13 (estimate) 13 Te (Č) CUORE experience Light Detectors 5 to be proved 1-2 (estimate) 28