1. The COBRA Experiment: Future Prospects
Ben Morgan
On Behalf of the COBRA Collaboration
Particle Physics 2006, Warwick, April 2006

2. 1: Neutrino Physics with 0nbb
Observing 0nbb implies neutrinos Majorana in nature.
Measuring 0nbb half-life measures absolute neutrino mass.
Phase Space
Nuclear Matrix
Element 2 1 / ) , ( ee F GT m M Z Q G T n - =
Effective Majorana Mass
Current limits: mee~200meV.
Heidelberg-Moscow (Ge) evidence now stronger.
Oscillations: no absolute mass, but expect eigenstate of 10-50meV.
Next goal in 0nbb: sensitivity to mee~50meV and inverse hierarchy of mass eigenstates.
HDMS(?)
CUORICINO

3. 2: Towards 50meV With COBRA
mee~50meV =>T1/2~1026yr
Or ~1event per 100kg per year!
T1/2 sensitivity, background limited, scales as:
Maximise target mass M, exposure time t.
Scalable, modular design.
Minimize background B.
Radiopure semiconductor (7N grade raw material).
Multicrystal events, pixels.
High energy resolution DE
CdZnTe semiconductor.
Maximise abundance a, efficiency of detection e.
Enrich 116Cd to 90%.

4. 3: The Crystal Array
“King Cobra” – preliminary design for a (40x40x40) array
418kg of 1cm3 CdZnTe crystals(90% 116Cd).
Sensitive to 50meV if B<10-3keV-1kg-1yr-1, DE<2% at 2805keV.
Must therefore study potential sources of background.
1cm

5. 4: Building a Background Model
Many sources of background, but several distinct classes:
2nbb decay ‘tail’ – Have excellent energy resolution so negligible!
Neutrons and Muons:
Reduced in underground lab, but also need good shielding.
See poster by Danielle Stewart.
Ultimately left with a,b,g sources:
Contamination of crystals and surrounding materials by natural radioisotopes.
Creation of radioactive isotopes in materials by cosmic rays above ground prior to installation.
Detector simulation required to understand the contribution of these sources to the signal region.

6. 5: The Venom Framework
Flexible framework for COBRA simulations built around Geant4 and ROOT toolkits.
Geometry, Event Generator, Actions all user definable at runtime through Factory interfaces.
Radioactive source event generators interfaced to Geant4 RDM
Accurate modelling of decay chains, products and timing.
Modelling of cosmic ray activation (under development).

7. 6: Natural Radioactivity
Very simple model studied at present:
Decay products tracked fully through geometry, energy deposited in crystal(s) recorded.
Crystals
238U decay chain
232Th decay chain
40K
137Cs
210Po
210Pb on surface
Gas
222Rn gas
Delrin Holder
238U decay chain
232Th decay chain
40K
137Cs
Chamber walls
210Pb on surface

8. 7: Acceptable Contamination
Simulation results analysed to produce energy spectrum with:
Cut events where 2 or more crystals have Edep>10keV.
DE=1% at 2805keV.
Events summed in signal window 2805±28keV.
Calculate background rate as function of source contamination
Comparison with maximum permitted background gives acceptable contamination.
Numbers conservative – no active veto around crystals.
U/Th major contributors: O(mBqkg-1) acceptable – same as for other 0nbb experiments.
Current limits on U/Th from COBRA prototype <0.3mBqkg-1.

9. 8: Reducing Background: Timing
Previous slide assumed ‘raw’ uncoincident decays.
Jeanne showed that 70% of the 2-3MeV background from 238U due to fast b-a decay:
With ‘real time’ data can veto these with at least 40% efficiency.
Acceptable contamination for U can be relaxed by similar fraction.
In addition, 57% of the 2-3MeV background from the 232Th decay chain is from the a-b sub-chain:
Although half-life is longer, may also be able to veto these events.
Studies now underway to determine efficiency for vetoing these fast coincidences.
T1/2=164.3ms
214Bi214Po210Pb
T1/2~180s
212Bi208Tl208Pb

10. 9: Cosmogenic Backgrounds
Background from cosmic ray production of radioisotopes depends on
Time spent on surface (activation).
Time spent underground before starting measurement (cooling off).
Calculate average activity over 1yr after 50 days activation plus:
‘A’: No cooling off period.
‘B’: 1month cooling off.
Activities very low at mBqkg-1, but simulate decays to determine background in signal window. p
124Sb
130Te
124Sb b- g
Surface
Underground

11. 10: Acceptable Activations
Results from simulations processed as for natural backgrounds to find background as function of source contamination.
Can tolerate few 100mBqkg-1, 2-3 orders of magnitude greater than expected in both activation scenarios.
Expected from modelling of activation

12. 11: Reducing Background: Pixels
Readout from CdZnTe crystal can be pixellated.
Enables tracking:
Range of a ~15mm.
Range of 2.8MeV b- ~1mm.
g’s: Compton scattering, multiple separated hits.
Unique signatures => eliminate backgrounds with ~100% efficiency.
Testing detectors with 16 (2x2mm) and 256 (1.6x1.6mm) pixels.
Work in progress to determine efficiency of background rejection.
3.2 mm
2.8MeV electrons
MeV electron pairs

13. 12: Conclusions
Target for neutrinoless double beta decay is sensitivity to neutrino Majorana masses of ~50meV
Challenging target, but use of CdZnTe semiconductors by COBRA offers many advantages.
Preliminary study of backgrounds in crystal array performed.
U/Th at mBqkg-1 levels acceptable, use 64array to begin measurement of contaminations in components.
Cosmogenic activation: Preliminary results suggest minimal source of background, more detailed study underway.