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

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 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
 CdZnTe provides 5 β-β-, 4 β+β+ isotopes, e.g. 130Te (2529keV, β-β-),
(2809keV, β-β-), 106Cd (2771keV, β+β+)
 T1/2 sensitivity, background limited, scales as:
Maximise Abundance, a, and
Efficiency of Detection, ε:
Maximise Target Mass and
Exposure Time:
 Natural 130Te a ~ 34%
 Scalable, modular design
 Can enrich 116Cd to 90%
 Room temperature operation
T10/2  a
Maximise Energy Resolution:
Minimise Background:
 Semiconductor
 Clean material manufacture
 Source = detector
 High Q values
 ΔE ~ 1% at 2-3 MeV possible
 Multi-crystal events, pixels
Danielle Stewart
July, 2006
Simulation of Shielding
Background Sources:
Gamma radiation from decay chains of
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
•Polyethylene (Pe)
•Pe + Bi
•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
• 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
 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
 Uranium contamination in
 Identified passivation paint on
Crystals limited to <490μBqKg-1
crystals as main source of
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 multicrystal events.
 New passivation paint: has at
least x10 lower background.
Danielle Stewart
July, 2006
Optimising Energy Resolution
1) Used CPG detectors initially due
to best known characterisation.
2) Measurement gives energy
resolution of 3% @2.8MeV.
Collecting Anode
Pulse Out
Danielle Stewart
3) Know from He et al. (NIM A388
(1997) 180):
 ΔE dependent on event depth.
 ΔE=1.29% @662keV possible.
 Not limited by CdZnTe
4) Investigating CPG improvements
 Better grid design (He and
Sturm, NIM A554 (2005) 291).
 Digital subtraction.
 Anode÷Cathode readout
 Pixelized readout (see later).
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.
2bb decay continuum ‘tail’
 Negligible, with E<2%, B2bb<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
Danielle Stewart
July, 2006
Building a Background Model: 2
222Rn gas
Delrin Holder
238U,232Th decay chain
Danielle Stewart
238U, 232Th decay chains
210Pb on surface
Chamber walls
210Pb on surface
July, 2006
The COBRA Concept
 Simulation results
analysed to give
energy spectrum.
 Reject events with
>1 crystal with
 Count events in
signal window,
 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
0bb experiments.
Danielle Stewart
July, 2006
Reducing Background: Pixels
 Pixellating CdZnTe readout enables
– Range of a ~15mm.
– Range of 2.8MeV b- ~1mm.
- g’s: separated hits.
3.2 mm
 Simulations with 200-500mm pixels
- as vetoed with 100% efficiency.
- gs vetoed with ~70% efficiency.
 Testing detectors with 16 (2x2mm)
and 256 (1.6x1.6mm) pixels.
2.8MeV electrons 1.4+1.4MeV electron pairs  Further studies of discrimination of
bb from b events underway.
Danielle Stewart
July, 2006
Conclusions from Current Status
 COBRA’s use of CdZnTe semiconductors offers many advantages for 0bb
 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
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
Second order weak decay
2νββ decay: A Z , N
A Z 2, N 2 2e 2 e
Simultaneous single beta decays – T1/2 ~ 1021 - 1024
yearsA Z , N
A Z 2, N 2 2e
0νββ decay:
Emission and re-absorption of a virtual light neutrino
Involves helicity change, observed decay rate => ν
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