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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.

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The COBRA Double Beta Decay Search Experiment

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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

The COBRA Concept

Cadmium-Tellurium 0-neutrino Beta decay Research Apparatus

  • A 64,000 array of 1cm3CdZnTe 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

  • MaximiseTarget Mass and Exposure Time:

  • Scalable, modular design

  • Room temperature operation

  • MaximiseAbundance, a, and Efficiency of Detection, ε:

  • Natural 130Te a ~ 34%

  • Can enrich 116Cd to 90%

  • MinimiseBackground:

  • Clean material manufacture

  • High Q values

  • Multi-crystal events, pixels

Advantages of CZT

  • CdZnTe provides5β-β-, 4β+β+isotopes, e.g. 130Te (2529keV,β-β-), 116Cd (2809keV, β-β-), 106Cd (2771keV, β+β+)

  • T1/2 sensitivity, background limited, scales as:

  • MaximiseEnergy Resolution:

  • Semiconductor

  • Source = detector

  • ΔE ~ 1% at 2-3 MeV possible

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


Danielle Stewart July, 2006

Neutron Energy-Flux Dependence



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


  • Identified passivation paint on

    crystals as main source of


  • Uranium contamination in

    Crystals limited to <490μBqKg-1

    from 214Bi β-α coincidence.

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 limitedby CdZnTe material.

  • Investigating CPG improvements

    • Better grid design (He and Sturm, NIM A554 (2005) 291).

    • Digital subtraction.

    • Anode÷Cathode readout

    • Pixelized readout (see later).


Collecting Anode


Pulse Out


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


238U, 232Th decay chains




210Pb on surface


222Rn gas

Chamber walls

210Pb on surface

Delrin Holder

238U,232Th decay chain



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


  • 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


Danielle Stewart July, 2006


  • 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

  • Oscillation experiments => non-zero neutrino mass

  • 0νββdecay can probe absolute mass scales

  • If 0νββdecay is detected:

    • νis majorana particle

    • non-conservation of Lepton no. by 2 units

Postgraduate Seminar – November 2005


New Physics beyond Standard Model

  • 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

  • Second order weak decay


    • Simultaneous single beta decays – T1/2 ~ 1021 - 1024 years


    • Emission and re-absorption of a virtual light neutrino

    • Involves helicity change, observed decay rate => ν mass

Postgraduate Seminar – November 2005

Double Beta Decay Possibilities



Postgraduate Seminar – November 2005

Energy Spectrum

A peak at the Q-value is the signature of 0νββdecay

The Q-value corresponds to released energy in nuclear transition

Half-life varies as Q5

Require good energy resolution

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