The COBRA Double Beta Decay Search Experiment Danielle Stewart July, 2006

To Follow…… What is COBRA? Shielding work at Warwick Current status of COBRA Future Plans University of Warwick, University of Liverpool, University of Birmingham, University of Sussex, University of York, University of Dortmund Danielle Stewart July, 2006

Why use CdZnTe detectors for a 0νββ search? The COBRA Concept Cadmium-Tellurium 0-neutrino Beta decay Research Apparatus A 64,000 array of 1cm3 CdZnTe semiconductor crystals based at Gran Sasso (K. Zuber, Phys. Lett. B 519,1 (2001)). Why use CdZnTe detectors for a 0νββ search? Danielle Stewart July, 2006

Advantages of CZT T1/2 sensitivity, background limited, scales as: CdZnTe provides 5 β-β-, 4 β+β+ isotopes, e.g. 130Te (2529keV, β-β-), 116Cd (2809keV, β-β-), 106Cd (2771keV, β+β+) T1/2 sensitivity, background limited, scales as: Maximise Abundance, a, and Efficiency of Detection, ε: Natural 130Te a ~ 34% Can enrich 116Cd to 90% Maximise Target Mass and Exposure Time: Scalable, modular design Room temperature operation Maximise Energy Resolution: Semiconductor Source = detector ΔE ~ 1% at 2-3 MeV possible Minimise Background: Clean material manufacture High Q values Multi-crystal events, pixels Danielle Stewart July, 2006

Simulation of Shielding Background Sources: Gamma radiation from decay chains of 238U and 232Th U/Th from LAAPD's (Large Area Avalanche PhotoDiodes) Low energy neutrons High energy neutrons MCNP – Design Strategy (Monte Carlo N Particle transport code) GEANT4 – Realism Danielle Stewart July, 2006

Standard Neutron Attenuation Materials: Water Polyethylene (Pe) Pe + Bi Premadex Pe + B (30%) Pe + B (5%) Pe + Li Danielle Stewart July, 2006

(n,g)-Reactions Danielle Stewart July, 2006

Neutron Energy-Flux Dependence Depletion Feeding Danielle Stewart July, 2006

Building a Multilayer Shield Tested: block structure sequence layer materials in block layer ratio’s in block number of block repetitions Danielle Stewart July, 2006

Comparison to Standard Shields Danielle Stewart July, 2006

Comparison to Standard Shields Danielle Stewart July, 2006

Shielding Results Multilayer shields outperform standard shielding structures. Best results in this study for Metal, Moderator, Capture Material combination. Best Ratio 3:8:4 Best materials: Lead, Pe-Bi and Pe-Li Iron for Lead as cheaper metal against neutrons outside Best fine-tuned full shield: Single or double block in clamp of outer Pe moderator and inner Lead layer. Danielle Stewart July, 2006

Current Status: Prototype R&D Prototype: 4 x 1cm3 CZT eV PRODUCTS crystals August 2003 – January 2006, Gran Sasso Studied Background and Electronics Uranium contamination in Crystals limited to <490μBqKg-1 from 214Bi β-α coincidence. Identified passivation paint on crystals as main source of background Danielle Stewart July, 2006

The COBRA Concept: 64 Array Installing now at Gran Sasso R&D on: Energy resolution (N2 cooling). Backgrounds: measurement of contamination levels. Background reduction via multi-crystal events. New passivation paint: has at least x10 lower background. Danielle Stewart July, 2006

Optimising Energy Resolution Used CPG detectors initially due to best known characterisation. Measurement gives energy resolution of 3% @2.8MeV. Know from He et al. (NIM A388 (1997) 180): ΔE dependent on event depth. ΔE=1.29% @662keV possible. Not limited by CdZnTe material. Investigating CPG improvements Better grid design (He and Sturm, NIM A554 (2005) 291). Digital subtraction. Anode÷Cathode readout Pixelized readout (see later). ΔV~30V Collecting Anode Difference Pulse Out Cathode~-2kV Danielle Stewart July, 2006

Background Reduction “King Cobra” – preliminary design for sensitivity to mee~50meV. 418kg mass in 64000 1cm3 CdZnTe crystals with 90% 116Cd. Sensitive to 50meV if B<10-3keV-1kg-1yr-1, ΔE<2% at 2805keV (116Cd). Have to study contribution of potential sources to signal window to find requirements for shielding and acceptable contamination levels. Danielle Stewart July, 2006

Building a Background Model: 1 Flexible Geant4 framework, Venom, developed for COBRA simulation. 2nbb decay continuum ‘tail’ Negligible, with DE<2%, B2nbb<2x10-7kg-1yr-1keV-1 Neutrons and Muons: Simulation of shielding shows these to be negligible. Ultimately left with a,b,g sources: Explore simple model initially, using Geant4 Radioactive Decay Module. Danielle Stewart July, 2006

Building a Background Model: 2 Crystals 238U, 232Th decay chains 40K 137Cs 210Po 210Pb on surface Gas 222Rn gas Delrin Holder 238U,232Th decay chain 40K 137Cs Chamber walls 210Pb on surface Danielle Stewart July, 2006

The COBRA Concept Simulation results analysed to give energy spectrum. Reject events with >1 crystal with Edep>10keV. Count events in signal window, 2805±28keV. Convert counts to event rate as function of contamination. Resultant levels conservative – no active veto around crystals. U/Th major contributors: O(mBqkg-1) acceptable – same as for other 0nbb experiments. Danielle Stewart July, 2006

Reducing Background: Pixels Pixellating CdZnTe readout enables tracking: Range of a ~15mm. Range of 2.8MeV b- ~1mm. - g’s: separated hits. Simulations with 200-500mm pixels indicate - as vetoed with 100% efficiency. - gs vetoed with ~70% efficiency. Testing detectors with 16 (2x2mm) and 256 (1.6x1.6mm) pixels. Further studies of discrimination of bb from b events underway. 3.2 mm 2.8MeV electrons 1.4+1.4MeV electron pairs Danielle Stewart July, 2006

Conclusions from Current Status COBRA’s use of CdZnTe semiconductors offers many advantages for 0nbb searches. 64 crystal array being installed. Reduced major paint background, limited U contamination in CdZnTe <490mBqkg-1. Detector development underway to optimize energy resolution. Detailed study of backgrounds underway. U/Th at mBqkg-1 levels acceptable, use 64-array to begin contamination measurement. Development of pixellated readout offers further background reduction via discrimination of as and gs from bs through tracking. Danielle Stewart July, 2006

Outlook:1 Background Model – Paper in preparation by Ben (Collection of all background work done) 3 detectors to “play” with! New Warwick responsibility. New passivation methods New readout schemes (home-made pre-amplifier already) Not usual CPG technique Surface characterisation – Chris McConville Pre-amplifier Danielle Stewart July, 2006

Outlook:2 Liquid Scintillator project Set up for avalanche photo diode (apd) readout Try to replace apd’s with home-made light detectors -Thick GEM’s (Gas Electron Multipliers) with photocathode Mechanical shielding design Light tight box Danielle Stewart July, 2006

New Physics beyond Standard Model Introduction Oscillation experiments => non-zero neutrino mass 0νββ decay can probe absolute mass scales If 0νββ decay is detected: ν is majorana particle non-conservation of Lepton no. by 2 units New Physics beyond Standard Model Postgraduate Seminar – November 2005

Double Beta Decay Isotopes on left/right decay by β-/(β + and EC) decay Parabola split due to nuclear pairing energy Single beta decay is forbidden Neighbouring odd-odd nucleus becomes virtual intermediate state Only 35 isotopes have necessary ground state configuration Postgraduate Seminar – November 2005

Double Beta Decay Possibilities 2νββ 0νββ Second order weak decay 2νββ decay: Simultaneous single beta decays – T1/2 ~ 1021 - 1024 years 0νββ decay: Emission and re-absorption of a virtual light neutrino Involves helicity change, observed decay rate => ν mass Postgraduate Seminar – November 2005

Energy Spectrum A peak at the Q-value is the signature of 0νββ decay The Q-value corresponds to released energy in nuclear transition Half-life varies as Q5 Require good energy resolution Postgraduate Seminar – November 2005