Outline of Presentation Motivation and General Considerations for 0 - PowerPoint PPT Presentation

ββ 0 ν Majorana The Majorana Neutrinoless Double Beta Experiment Kevin T . Lesko Lawrence Berkeley National Laboratory for the Majorana Collaboration 17 September 2005

Outline of Presentation Motivation and General Considerations for 0 ν DBD Experiments Majorana Approach and Goals Backgrounds and Mitigation Plans Current Status Conclusions

Recent Neutrino Successes Massive neutrinos Reduced Parameter space by 7 orders of magnitude, LMA ➻ confirmed for solar ➻ No dark side Strong evidence for MSW Evidence for Oscillations from Super-K and KamLAND Maximal Θ 23 , Large but non- maximal Θ 12

Outstanding Problems for Neutrinos Neutrino Mass Scale MNSP Matrix Elements θ 13 - size of angle θ 12 - unitarity of matrix Mass hierarchy Verify Oscillations Sterile Neutrinos? LSND affect? CP Violation Neutrino Nature (Dirac or Majorana)

Neutrinoless Double Beta Decay Oscillation experiments indicate ν s are massive, set relative mass scale, and minimum absolute mass. β decay + cosmology set maximum for the absolute mass scale. One ν has a mass in the range: 45 meV < m ν < 2200 meV 0 νββ experiments can determine the absolute mass scale and only way to establish if neutrinos are Dirac or Majorana 0 νββ can establish mass hierarchy Even negative results are now interesting

Decay Rates, Signal, and Sensitivity Decay Rate: [T 0 ν 1/2 ] -1 = G 0 ν (E 0 ,Z)  〈 m ν 〉  2  M 0 ν F - (g A /g V ) 2 M 0 ν GT  2 G 0 ν (E 0 ,Z) = 2-body phase factors M 0 ν F = Fermi Matrix Elements M 0 ν GT = Gamow-T eller Matrix Elements  〈 m ν 〉  = Effective Majorana Electron Neutrino Mass  〈 m ν 〉  ≡  U Le1  2 m 1 +  U Le2  2 m 2 e i φ 2 +  U Le3  2 m 3 e i φ 3  ln2 [T 0 ν 1/2 ] -1 = N ββ / ε N source t exp

ln2 [T 0 ν 1/2 ] -1 = N ββ / ε N source t exp T wo Limits to Experimental Reach with Background 〈 m ββ 〉 ~ [A/ax ε G 0 ν  M 0 ν  2 ] 1/2 [b Δ E/Mt exp ] 1/4 without Background 〈 m ββ 〉 ~ [A/ax ε G 0 ν  M 0 ν  2 ] 1/2 [1/Mt exp ] 1/2 A= Molecular weight a= isotopic abundance x = # isotope nuclei per molecule ε = efficiency

Masses Hierarchy and 0 νββ } Cosmology & Klapdor } } 1000 Degenerate Degenerate 100 Effective ββ Mass (meV) Inverted Inverted 10 Normal Normal 2 2 1 U e1 = 0.866 δ m sol = 70 meV 2 2 U e2 = 0.5 δ m atm = 2000 meV U e3 = 0 0.1 2 3 4 5 6 7 2 3 4 5 6 7 2 3 4 5 6 7 1 10 100 1000 Elliott Minimum Neutrino Mass (meV)

With Background 〈 m ββ 〉~ [A/ax ε G 0 ν  M 0 ν  2 ] 1/2 [b Δ E/Mt exp ] 1/4 〈 m ββ 〉 ~ 1/[  M 0 ν  (G 0 ν T 1/2 )] to get the scales right: 〈 m ββ 〉 ~ 10 meV to 100 meV T 1/2 ~ 10 27 years t exp ~ years & M ~ 100kg four factors to focus on: backgrounds, energy resolution, mass, and stability

Energy Resolution Radioactive backgrounds signals Instrumentation effects 2 νββ backgrounds Perfect Experiment Two Neutrino Spectrum Two Neutrino Spectrum Zero Neutrino Spectrum Zero Neutrino Spectrum 1% resolution 1% resolution Γ (2 (2 ν ) = 100 * ) = 100 * Γ (0 (0 ν ) 0.0 0.5 1.0 1.5 2.0 Sum Energy for the Two Electrons (MeV) Sum Energy for the Two Electrons (MeV) U. Zargosa

Detector Mass & Purity Isotopic enrichment, chemical purity, & inactive detector elements all effect experimental sensitivity added wrong mass ⇒ risk of additional backgrounds, hidden background sources, non-probeable (i.e. dead) detector elements Want experimental mass to be all the correct isotope and all to be “active” detector elements to probe degenerate mass range ~ 50 - 100 kg to probe inverted mass range ~ 500 - 1000 kg to probe normal mass range ~ multi-ton range

Backgrounds Internal Radioactive Contamination Isotopes of concern are a function of the Q-value: for 76 Ge 2039 keV, U, Th chains External Radioactive Contamination Neutrons (fission, CR-generated, reaction) Instrumental Issues (cross talk, noise, etc.)

Stability Need stable and dependable operation for years High live-time fraction Low maintenance

The Majorana Experiment Majorana is scalable, permitting expansion to ~ 1000 kg scale – Reference Design (180 kg) to address first goals • 171 segmented, n-type, 86% enriched 76 Ge crystals. • 3 independent, ultra-clean, electroformed Cu cryostat modules. • Enclosed in a low-activity passive shielding and active veto. • Located deep underground ( ~ 5000 mwe). – Background Specification in the 0 νββ ROI 1 count/t-y – Expected 0 νββ Sensitivity(3 y or 0.46 t-y 76 Ge exposure) T 1/2 ≥ 5.5 x 10 26 y (90% CL) 〈 m ν 〉 < 100 meV (90% CL) ([Rod05] RQRPA matrix elements) or a 10% measurement assuming a 400 meV value.

Why Germanium? 76 Ge offers an excellent combination of capabilities and sensitivities: ready to proceed with demonstrated technologies without proof-of-principle R&D. Favorable nuclear matrix element • Excellent energy resolution — 0.16%  M 0 ν  =2.4 [Rod05], 2.68±0.06 (QRPA) at 2.039 MeV yielding ROI of ~ 4 keV (G 0 ν = 0.30x 10 -25 y -1 ev -2 ) • Powerful background rejection. Reasonably slow 2 νββ rate Segmentation, granularity, timing, ( Τ 1/2 = 1.4 × 10 21 y) pulse shape discrimination Demonstrated ability to enrich from 7.44 • Well-understood technologies to 86% – Commercial Ge diodes ∴ High fraction of Ge is both source & – Existing, well-characterized large Ge arrays (Gammasphere, active detector Gretina) Elemental Ge further maximizes the Best limits on 0 νββ used Ge • source-to-total mass ratio Τ 1/2 > 1.9 × 10 25 y (90%CL) Excellent History of Intrinsic high-purity Ge diodes with high purity

Detector Model 57 crystal module • Conventional vacuum cryostat made with electroformed Cu. Three-crystal stack are individually removable. Vacuum jacket Cap Cold Plate T ube (0.007” wall) Cold Ge Finger (62mm x 70 mm) 1.1 kg Crystal T ray (Plastic, Si, etc) Thermal Shroud Bottom Closure

Allows modular deployment and operation contains up to eight 57-crystal modules (M180 populates 3 of the 8 modules) 40 cm bulk Pb, 10 cm ultra-low background shield T op view Veto Shield Sliding Monolith LN Dewar Inner Shield 57 Detector Module

• Sensitivity to 0 νββ decay is ultimately limited by Signal-to- Background performance • Our specification for backgrounds is 1 cnt/t-y in 0 νββ ROI. The specification is based on existing assay limits plus demonstrated techniques for impurity reduction Bkg T arget Activation Demonstrate Purity Issue Ref. Location Exposure Rate Spec. d Rate Ge Crystals 68 Ge & 60 Co 100 d 1 atom/kg/d 1 atom/kg/d [Avi92] T arget Purity Achieved T arget Mass Spec. Assay Inner Mount 2 kg [Arp02] 2-4 μ Bq/ Cryostat 38 kg & 1 μ Bq/kg <8 μ Bq/kg kg 232 Th in Cu ongoing Cu Shield 310 kg work Small Parts 1 g/crystal 1 mBq/kg 1 mBq/kg 1 mBq/kg [Mil92]

Klapdor-Kleingrothaus H V, Krivosheina I V, Dietz As discussed by John A and Chkvorets O, Phys. Lett. B 586 198 (2004). KKDC: total of 10.96 kg of mass and 71 kg-years of data. Τ 1/2 = 1.2 x 10 25 y 0.24 < m v < 0.58 eV (3 sigma) Expected signal in Majorana (for 0.46 t-y) 135 counts With a background of Specification: < 1 total count in the ROI (Demonstrated < 8 counts in the ROI)

Reducing & Mitigating Backgrounds • Reduce internal, external, & cosmogenic-created activities – Minimize all non-source materials – Use of ultra-pure materials – Clean passive shield & active veto shield – Go deep — reduced µ induced activities 0 νββ - a single site phenomenon Many backgrounds - multiple site • Invoke background rejection techniques – Use of discrete detectors to reject scattered background events – Single Site Time Correlated events (SSTC) – Energy resolution – Advanced signal processing – Single site event selection – Event Reconstruction 3-D – Segmented Detectors (finer multiplicity) – Pulse shape analysis

Cuts Efficiency & Background Estimates 2039 keV ROI + Analysis cuts discriminates 0 νββ from backgrounds Only known activities that occur ~ 2039 keV are from very weak branches, with corresponding strong peaks elsewhere in the spectrum 16 0 νββ Crystals Inner Mount 14 Cryostat Backgrounds signal Cu Shield Small parts 12 External For T 1/2 = 3 x 10 26 y 0vbb 10 8 6 4 2 0 Raw 0vbb Final 0vbb Raw Granularity PSD SSTC Segmentation

Influence of Depth on Backgrounds The total background target is met at ~ 5000 mwe, at 6000 mwe ~ 15-20% of the expected background will be from μ -induced activities in Ge and the nearby cryostat materials (dominated by fast neutrons). Mei and Hime 2005 See Mei, this workshop