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

2. 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

3. 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

4. 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

5. 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

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

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

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

9. 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

10. Comparison to Standard Shields
Danielle Stewart July, 2006

11. Comparison to Standard Shields
Danielle Stewart July, 2006

12. 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

13. 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

14. 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

15. Optimising Energy Resolution
Used CPG detectors initially due
to best known characterisation.
Measurement gives energy
resolution of
Know from He et al. (NIM A388
(1997) 180):
ΔE dependent on event depth.
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

16. Background Reduction
“King Cobra” – preliminary design for sensitivity to mee~50meV.
418kg mass in cm3 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

17. 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

18. 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

19. 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

20. Reducing Background: Pixels
Pixellating CdZnTe readout enables tracking:
Range of a ~15mm.
Range of 2.8MeV b- ~1mm.
- g’s: separated hits.
Simulations with mm 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
MeV electron pairs
Danielle Stewart July, 2006

21. 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

22. 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

23. 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

24. 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

25. 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

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

27. 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