the cobra double beta decay search experiment danielle stewart july, 2006
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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 DortmundDanielle 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)).
Danielle Stewart July, 2006
Why use CdZnTe detectors for a 0νββ search?
Advantages of CZT
EBMt
aT
02/1
Maximise Target Mass and Exposure Time:
Scalable, modular design
Room temperature operation
Minimise Background:
Clean material manufacture
High Q values
Multi-crystal events, pixels
Maximise Energy Resolution:
Semiconductor
Source = detector
ΔE ~ 1% at 2-3 MeV possible
Maximise Abundance, a, and Efficiency of Detection, ε:
Natural 130Te a ~ 34%
Can enrich 116Cd to 90%
Danielle Stewart July, 2006
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:
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
Standard Neutron Attenuation
Materials:•Water•Polyethylene (Pe)•Pe + Bi•Premadex•Pe + B (30%)•Pe + B (5%)•Pe + Li
Danielle Stewart July, 2006
(n,)-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
Identified passivation paint on
crystals as main source of
background
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
Collecting Anode
Difference
Pulse Out
Cathode~-2kV
1) Used CPG detectors initially due to best known characterisation.2) Measurement gives energy resolution of 3% @2.8MeV.
3) Know from He et al. (NIM A388 (1997) 180):
ΔE dependent on event depth. ΔE=1.29% @662keV possible. Not limited by CdZnTe
material.
4) Investigating CPG improvements Better grid design (He and
Sturm, NIM A554 (2005) 291). Digital subtraction. Anode÷Cathode readout Pixelized readout (see later).
ΔV~30V
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
Flexible Geant4 framework, Venom, developed for COBRA simulation.
1) 2 decay continuum ‘tail’ Negligible, with E<2%, B2<2x10-7kg-1yr-1keV-1
2) Neutrons and Muons: Simulation of shielding shows these to be negligible.
3) Ultimately left with sources: Explore simple model initially, using Geant4 Radioactive Decay
Module.
Building a Background Model: 1
Danielle Stewart July, 2006
Building a Background Model: 2
Gas222Rn gas
Crystals238U, 232Th decay chains40K137Cs210Po210Pb on surface
Delrin Holder238U,232Th decay chain
40K137Cs
Chamber walls210Pb 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(Bqkg-1) acceptable – same as for other
0 experiments.
Danielle Stewart July, 2006
Reducing Background: Pixels
3.2 mm
2.8MeV electrons 1.4+1.4MeV electron pairs
Pixellating CdZnTe readout enables tracking:– Range of ~15m.– Range of 2.8MeV - ~1mm.
’s: separated hits.
Simulations with 200-500m pixels indicate
s vetoed with 100% efficiency.
s vetoed with ~70% efficiency.
Testing detectors with 16 (2x2mm) and 256 (1.6x1.6mm) pixels.
Further studies of discrimination of from events underway.
Danielle Stewart July, 2006
Conclusions from Current Status
COBRA’s use of CdZnTe semiconductors offers many advantages for 0 searches.
64 crystal array being installed. Reduced major paint background, limited U contamination in CdZnTe <490Bqkg-1.
Detector development underway to optimize energy resolution.
Detailed study of backgrounds underway. U/Th at Bqkg-1 levels acceptable, use 64-array to begin contamination measurement.
Development of pixellated readout offers further background reduction via discrimination of s and s from s through tracking.
Danielle Stewart July, 2006
Outlook:1
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
Pre-amplifier
Outlook:2
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
Introduction
New Physics beyond Standard Model
Oscillation experiments => non-zero neutrino mass
0νββdecay can probe absolute mass scales
If 0νββ decay is detected:ν is majorana particle
non-conservation of Lepton no. by 2 units
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
e
en
1 n2
e e
n1
n2
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
A Z , N A Z 2,N 2 2e 2e
A Z , N A Z 2,N 2 2e
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