LUCIFER: A Scintillating Bolometer Array for the Search of Double - PowerPoint PPT Presentation

LUCIFER: A Scintillating Bolometer Array for the Search of Double Beta Decay Fabio Bellini “Sapienza” Università di Roma & INFN Roma DBD 2011 Osaka 17/11/2011

Bolometers for DBD search Well established technology ‣ DBD source embedded in a crystal cooled down at few mK Heat bath ‣ (Only) energy measured via temperature variation Δ T =E/C Weak thermal coupling induced by particle energy release Thermometer ‣ Need very low heat capacity Absorber Crystal (dielectric, diamagnetic): TeO 2 : Δ T ~0.1 mK/MeV = DBD source Energy release ‣ TeO 2 : excellent energy resolution (~0.3% @ 2-3 MeV) and massive detector ‣ low background ~few ⋅ 10 -2 cts/keV/kg/y Need ~10 -3 cts/keV/kg/y to access inverted hierarchy 2

The isotope choice The possibility to use different candidates depends on: ‣ capability to grow large radio-pure crystals with good mechanical and thermal properties ‣ isotopic abundance and cost/easiness enrichment All isotopes tested as bolometer in crystalline form with the exception of 136 Xe and 150 Nd 76 Ge 130 Te 116 Cd 100 Mo 82 Se Environmental
 “underground”
Background: 238 U
and
 232 Th
trace
 contamina<ons Gain ~ 100 if Q ββ > 2615 keV common highest γ line ( 208 Tl) with BR ~36% in Th chain 3

The α problem Bolometers are fully sensitive, up to detector surface ⇒ no dead layer Surface contamination of the bolometers themselves or of the materials surrounding them emitting α particles gives a continuum background in the Region of Interest Very difficult to reduce this background below 0.05 cts/keV/kg/y below and above 2615 keV ‣ need α rejection >98% to reach 10 -3 cts/keV/kg/y 4

The solution S cintillating bolometers: use different α / γ light emission for background discrimination The light detector: a thin opaque bolometer facing a polished side of the main bolometer Light detector Thermistor Bolometer Energy Release The experimental basis of this technique was the R&D activity performed by S.Pirro at LNGS in the framework of the Bolux(INFN), ILIAS-IDEA (EC WP2-P2) program 5

LUCIFER Low ‐ background Underground Cryogenics Installation For Elusive Rates ERC ‐ 2009 ‐ AdG 247115 ¡ Principal ¡Inves.gator: ¡ Lucifer is a Latin word (from the words lucem F. ¡Ferroni ferre ), literally meaning "light-bearer", which in Co-­‑ ¡Inves.gator: ¡ that language is used as a name for the dawn A.Giuliani appearance of the planet Venus, Coordinator: ¡S.Pirro heralding daylight. Bringing light underground 6

The candidates: CdWO 4 Q ββ Useful material LY QF (keV) (% weight) (keV/MeV) CdWO 4 2809 32 ~17 ~0.16 Pro: 80 Astropart.Phys. 34:143 ,2010 ‣ ~0.5 kg crystal successfully tested 208 Light [keV] 60 Tl ‣ very good crystal quality ‣ high light yield 40 40 K 210 Po Q ββ 234 U 238 U 20 180 W 0 Cons: 400 1000 2000 3000 4000 5000 Energy (Heat) [keV] ‣ only 32% of useful material ‣ 113 Cd (huge neutron cross section) ⇒ (n, γ ) reaction ⇒ possible continuum γ background 7

The candidates: ZnMO 4 Q ββ Useful material LY QF (keV) (% weight) (keV/MeV) ZnMO 4 3034 44 ~1 ~0.2 Pro: good pulse shape discrimination on main (heat) bolometer Astropart.Phys. 34:797 ,2011 Cons: poor light yield ,only small crystals (~30 g) up to now JINST 5:P11007,2010 . Q ββ 8

The candidates: ZnSe Q ββ Useful material LY QF (keV) (% weight) (keV/MeV) ZnSe 2995 56 ~7 ~4 Astropart.Phys. 34:344 ,2011 Pro: ‣ ~340 g crystal successfully tested ‣ good light yield and radio-purity Light Detector(Ge) ‣ pulse shape discrimination on light detector ‣ the most mass effective ZnSe bolometer Cons: ‣ inverted Quenching Factor! ‣ crystal production: not yet solid protocols and reproducibility issue 9

The candidates: ZnSe No explanation for the inverted Quenching Factor. Light Discarded hypotheses: Detector(Ge) ‣ ZnSe self-absorption ‣ Light collection efficiency ZnSe bolometer ‣ Light detector transparent to certain wavelengths Arbitrary Units 1 α ’s from 238 U e 234 U sources 0.8 Light detector Light Energy (a.u.) 800 0.6 Shape Indicator 0.085 0.4 700 0.08 0.2 600 0.075 0 500 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.07 Time (s) 400 γ ’s (n, γ ) from AmBe source 0.065 300 α 0.06 200 0.055 100 β , γ 0.05 0 0 1000 2000 3000 4000 5000 6000 7000 8000 0 100 200 300 400 500 600 700 800 Light Energy (a.u.) Heat Energy (keV) 10

The scintillating candidates Q ββ Useful material LY QF (keV) (% weight) (keV/MeV) CdWO 4 2809 32 ~17 ~0.16 ZnMO 4 3034 44 ~1 ~0.2 ZnSe 2995 56 ~7 ~4 Baseline crystal for LUCIFER: ZnSe 11

ε1 ¡= ¡1.28 ¡eV ¡ ¡ ¡ ¡ ¡ ¡ λ1 ¡= ¡970 ¡nm ε2 ¡= ¡1.92 ¡eV ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡;Ϳ λ2 ¡= ¡645 ¡nm ε3 ¡= ¡2.03 ¡eV ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡;Ϳ λ3 ¡= ¡610 ¡nm ε4 ¡= ¡2.70 ¡eV ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡;Ϳ λ4 ¡= ¡460 ¡nm ZnSe Luminescence properties well known ng 2 are dge. Crystal growth known: the pale ‣ Bridgman technique at 1525° C ‣ high twining tendency ‣ high volatility: stoichiometry control Fig. 3 X-ray excited luminescence measured at room Effort focused on: ‣ enrichment 5 cm ‣ ZnSe synthesis ‣ efficient crystal growth 12

ZnSe production 1$ raw$(elemental)$Se$ 3$ SeF 6 $synthesis$ Need radio-chemical pure Se 2$ cer0fica0on$ ‣ ICPMS measurements 4$ SeF 6 $enrichment$ 5$ Enrichment (URENCO)>95% Se$conversion$ 6$ Se$beads$produc0on$$ ‣ Chemical problems in Se conversion 8$ (mainly reagent contamination) 7$ enriched$Se$ Zn$elemental$ elemental$ Beads (powder not good for crystal) 9$ purifica0on$ ‣ require dedicated instruments 10$ cer0fica0on$ (HPGe gamma spectroscopy) ‣ Purification (99.999%) ⇒ zone refining 11$ ZnSe$synthesis$ 12$ cer0fica0on$ Synthesis of ZnSe 15$ 13$ ZnSe$crystal$ recovery$and$ growth$ ‣ High or low temperature method (yield optimization) recycling$ 14$ Growth of ZnSe crystal mechanical$ processing$ ‣ Avoid twining and reach reproducibility 16$ package$and$ 26# shipment$ 13

Light detectors Light Detectors are generally pure Germanium disks (thickness 0,3-1 mm) Performances are evaluated on the 55 Fe doublet: 5.9 & 6.5 keV x-Ray ‣ Good energy resolution: σ ~130 eV theoretical resolution σ ~80 eV 14

LD test: TeO 2 Cerenkov light TeO 2 bolometers don’t scintillate: detection of Cerenkov light Cerenkov threshold: 50 keV for β , α below threshold ⇒ particle discriminati on Light detector of pure Ge TeO 2 :Sm (30 ppb nat Sm) 66 mm diameter, 1mm thick. 3.0x2.4x2.8 cm 3 VM2002 One side coated with SiO 2 to increase 116.65 g reflecting foi l absorption of μ m wavelengths. 147 Sm: α decay at 2310 keV 15

Results arXiv:1106.6286 submitted to Astropart. Phys. L1L2_Sm147 L1L2_Sm147 Entries 851 200 [keV] Mean 0.01123 counts / 0.05 0.5 background RMS 0.08898 180 0.4 Integral 851 160 L 0.3 2 � / ndf E 13.55 / 11 140 0.2 const 192 ± 8.6 0.1 120 mean 0.008343 ± 0.003042 0 � 0.08702 0.00249 ± 100 -0.1 80 -0.2 60 -0.3 40 [keV] 232 0.5 Th calibration 20 0.4 0 L 0.3 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 E 0.2 Energy [keV] 0.1 L1L2_Tl208 L1L2_Tl208 Entries 185 0 Mean 0.1969 counts / 0.05 45 -0.1 RMS 0.07263 40 -0.2 Integral 185 2 -0.3 / ndf 10.73 / 6 � 35 const 40.77 3.09 ± > [keV] 30 mean 0.1926 ± 0.0058 0.20 73 eV/MeV 25 � 0.08702 0.00000 ± 0.15 184 eV @2.527 MeV 20 L <E 0.10 15 0.05 10 0.00 5 -0.05 0 0 1000 2000 3000 4000 5000 6000 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 Energy[keV] Energy [keV] ~2 σ separation R&D on going on 5x5x5 cm 3 TeO 2 crystal: light collection optimization 16

LUCIFER Detector Single module: 4ZnSe -1light detector Tower: 12 single modules Preliminary Hosted @ Laboratori Nazionali del Gran Sasso ‣ Equivalent vertical depth relative to a flat overburden: ~ 3.1 ± 0.2 km.w.e ‣ Gamma flux:~0.73 γ /(s cm 2 ) ‣ Neutron flux: ~4 · 10 -6 n/(s cm 2 ) below 10 MeV ‣ Muon flux: (2.58±0.3) · 10 -8 μ /(s cm 2 ) 17

Conclusions The main challenge for a 0 ν DBD next generation bolometer experiment is the α background rejection to ~10 -3 cts/keV/kg/y The scintillating bolometer is a promising technique ‣ the LUCIFER goal is to demonstrate the feasibility of this technique on a reasonable large scale ‣ but has a remarkable physics reach by itself ZnSe 82 Se weight(kg) Half life(10 26 y) m ββ (meV) baseline 17.6 2.3 51-65 assuming Δ E ~ 10keV, live time ~ 5 y, bkgd~10 -3 cts/keV/kg/y J.Mendez et al. arXiv:0801.3760; F.Simkovic et al. Phys.Rev. C77 045503,(2008); J.Suhonen et al. Int.J.Mod.Phys E17 1 (2008) Data taking foreseen in 2014 18