The COBRA Double Beta Decay Experiment Brian Fulton University of York, England On behalf of the COBRA collaboration DBD07, Osaka
Contents Who we are The experimental concept What has been achieved so far The next steps
Who we are
COBRA collaboration �������������������� ��������������"��� ��������������������� ��������������������� �� �������������������!��� #�� �������$!!���������������� ���������������������� .��������#������ �-������/������� ����������������������������������� ������������������������ ��� �����������������������%������ ���������������������&�'()���������������*�������&�������() +����������������������&,�����()���������������,������&-��� �#�!�����() ����������������������������&��$(
The experimental concept
The COBRA Concept K. Zuber Zuber, Phys. , Phys. Lett Lett. B 519,1 (2001) . B 519,1 (2001) K. CZT 0-neutrino Beta-decay Research Apparatus CdZnTe semiconductor detectors Build up a large array of CdZnTe (9 double beta decay isotopes) 1 cm 3 CPG Detector
Isotopes ���%���%�&A( B�&��C( ���������� 0�12� 2%34� 5225� 6767� -�558� 49%1� :;8� 6767� -�553� 1%:� 492<� 6767� =�549� ;5%1� 939� 6767� =�5;2� ;;%9� 4:4<� 6767� 0�38� 89%3� 52<3� 6>?@-� -�523� 5%45� 4115� 6>6>� -�529� 2%<� 4;5� @-?@-� =�542� 2%5� 5144� 6>?@-� �
Advantages D �������E �������� D ��������������&�����������������������F5A( D #�������!������� D .��������������&-�����������( D �������������!�������������������������� D G��������� ������!������� -�=����������� D 553 -� ������4%358�.�C D =��������&������������=,-(
Experimental Requirements �� � ν ∝ � ε 0 ∆ �� 1 / 2 • 64,000 1cm 3 crystals = 418 kg Cd 116 Cd • 90% enriched in 116 • Backgrounds < 0.001 count keV -1 kg -1 year -1 • Energy Resolution < 2%
Energy Resolution Resolution of σ =0.8% at 2.8 MeV ∆ E = 1.9% @ 2.8MeV =2.9% @ 662keV • Only electron signal read out (CPG technology) • Possible improvements: cooling, new grids • Better detectors are available
What has been achieved so far?
Proof of concept Stage (2004-2006) 4 detector set up in Gran Sasso
0.5cm 3 , surface 1 cm 3 , Gran Sasso, no shielding 1 cm 3 , Gran Sasso, with shielding Cd113 half-life (4-fold forbidden decay) C. Goessling et al. Phys. Rev C, 72, 064328 (2005)
First COBRA Double beta results T. Bloxham et al. Phys. Rev C, in press World best limits on 64 Zn and 120 Te
Samples measured at LNGS Activities (mBq/kg) Main problem is passivation paint used on detectors New Passivation Paint Decrease x10 Had expected x10 3 Next level of background (Rn?)
Test Stage (2006-present) 64 detector set up in Gran Sasso
The first layer Installed at LNGS in summer 2006
The first layer - some spectra Cd-113 beta decay with half-life of about 10 16 yrs
The first layer - Coincidences Coincidences Coincidences around Det 9 Preliminary Powerfu l tool!!! Example: 3-coincidence Just starting to analyse/understand the power of this
Simulation of energy deposition in a 5 x 5 detector array for a 2614 keV gamma starting from in central detector
Spatial Coincidences 116 Cd 0 νββ is single crystal event ~64% of the time β – γ from natural background Beta and gamma generally in different crystals Reduce 232 Th chain events from crystals by >50% β + β + decay 106 Cd ������������������� ��������� �����������������������
Spatial Coincidences Can also identify decays to excited states (may give handle on physics mechanism) 553 -� -� → 553 → 116 116 Sn (2 + , 1294keV) Sn (2 + , 1294keV) β � ���� �������� β β β ���������������������� γ γ γ γ
Timing Coincidences The major contribution to 238 U spectrum at 2 − 3MeV is the fast β−α decay: → 214 → 210 Bi → Po → 214 Bi 214 Po 210 Pb 214 Pb α α α α α α β β α α β β β β β β endpoint 3.3MeV, accounts 7.7MeV alpha µ s 164.3 µ for >70% events in 2-3MeV half-life = 164.3 s region from 238 U chain >40% efficiency for tagging 214 Bi events originating inside the crystals
H�������������� 458 ��������� χ χ / ndf / ndf 7.883 / -2 7.883 / -2 2 2 Constant Constant 4.129 4.129 ± ± 0.195 0.195 Slope Slope -4270 -4270 ± ± 504.7 504.7 Counts 35 = 5?4 E�534�I I 5< 5< µ µ � � = 5?4 E�534� 30 25 20 15 10 5 0 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 0.0008 0.0009 0.001 Time between events (Seconds)
The next steps
Two main approached to double beta decay Energy resolution Tracking Pixellated CdZnTe detectors
Pixel CZT- A solid state TPC Massive BG reduction by particle ID , 200 µ m pixels (example simulations): Total E=2805 Total E = 2805 Y pixel 450 32 2 νββ νββ 2 30 2500 α α 400 30 350 28 2000 300 28 26 1500 250 26 200 24 1000 150 24 57 75%:�� 5%:�� F5: µ µ � � 5 F5: 100 22 500 22 50 20 0 20 0 20 22 24 26 28 30 32 34 20 22 24 26 28 30 X pixel α = 1 pixel, β and ββ = several connected pixel, γ = some disconnected p. (or different detector) 7.7MeV α with Beta with eg. Could achieve nearly 100% identification of 214 Bi events endpoint life-time = ( 214 Bi → 214 Po → 210 Pb) 164.3 µ s 3.3MeV
Rejection power of pixels T. Bloxham, M. Freer, Nucl. Inst. Meth. A (2007) Simulation for 3mm thick detector with 16 x 16 200 µ m pitch pixels 232 Th and 238 U chains 1. One/two electrons 2. ….plus alpha rejection ………plus β−α time correlation 3. Suggests a background reduction of 1000!
Tests of 16×16 1.6mm pixel detectors Detector ASIC Readout �������,����� :1 -���!������ ������ Two detectors with 200 µ m pixillation ������ being produced Looking at new generation ASICs for readout of these
Other current activities
Monte Carlo Sophisticated MC based on GEANT4, written in C++ Signal (DECAY0) and background 200 GeV muon
Shielding and Veto • Simulated LNGS neutron flux ~3x10 -7 counts/year/kg/keV in the crystals. • • • <1 neutron per year! <1 neutron per year! (in 64000 detectors) ��������� ��������� D. Stewart et al., accepted by Nucl. Inst. Meth. A
Understand n-capture backgrounds Thermal neutron capture on 113 Cd
Digital pulse shape readout (improved resolution and position from induced signals)
First results from CZT detectors Event near anode Event near cathode
Materials studies to improve detectors
SUMMARY • We have established a potential approach for neutrinoless double beta decay offering some advantages • We have a test setup at Gran Sasso which will allow us to improve backgrounds and explore the advantage of concidences • Starting a major programme to develop pixillated CZT detectors which would provide a tracking capability to give an enormous background reduction Eventual goal would be a 64,000 detector experiment And we have dreams…..
Solar neutrinos with COBRA - KING COBRA A real time low-energy solar neutrino experiment? e Threshold energy: 464 keV ν e e 7Be contribution g.s. alone: 227 SNU 116In τ = 14s 116Cd 116Sn K. Zuber, Phys. Lett. B 571,148 (2003) Signal: Coincidence between two electrons in a single detector
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