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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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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
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

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