K. Zuber, Uni. SussexIDEA Meeting, Milano 9. Nov. 2006

Status of COBRA

COBRA

Use large amount of CdZnTe Semiconductor Detectors

Array of 1cm3

CdTe detectors

K. Zuber, Phys. Lett. B 519,1 (2001)

Isotopes

Zn70 0.62 1001 ß-ß-Cd114 28.7 534 ß-ß-Cd116 7.5 2805 ß-ß-Te128 31.7 868 ß-ß-Te130 33.8 2529 ß-ß-Zn64 48.6 1096 ß+/ECCd106 1.21 2771 ß+ß+Cd108 0.9 231 EC/ECTe120 0.1 1722 ß+/EC

nat. ab. (%) Q (keV) Decay mode

COBRA: CdZnTe semiconductors

Advantages

• Source = detector

• Semiconductor (Good energy resolution, clean)

• Room temperature

• Tracking („Solid state TPC“)

• Modular design (Coincidences)

• Industrial development of CdTe detectors

• Two isotopes at once

• 116Cd above 2.614 MeV

COBRA collaboration

University of Dortmund

University of Sussex

Laboratori Nazionali del Gran Sasso

University of Warwick University of BirminghamUniversity of Liverpool

University of York

Rutherford Appleton Laboratory

University of Bratislava

Washington University at St. Louis

University of Surrey (UK), University of Hamburg (Germany),Jagellonian University (Poland), University of Prague (Czech Republik),Louisianna State University (USA)

Material Research Centre Freiburg

The 2x2 prototype

4 naked 1cm3 CdZnTe

4.3 kg x days of data accumulated, stopped March 2006

Setup installed at Gran Sasso Underground Laboratory

PertinaxCopper CZTWax

Physics - 113Cd

T1/2 = (8.2 ± 0.2 (stat.) +0.2-1.0 (sys)) 1015 yrs

113Cd one of only three 4-fold forbidden -emitters known in nature

C. Goessling et al., Phys. Rev. C 72, 064328 (2005)

Latest Limits

worldbest

First COBRADouble beta results

T. Bloxham et al.,submitted

The background model

Input: Pertinax (grid, base), detectors, paint and copper

Limiting background: Passivation paint on detectors

Coincidences – 214Bi

T1/2 = 162 ± 19s

Can exclude backgrounds through timing coincidence214Bi 214Po 210Pb

7.7MeV alpha half-life = 164.3s

Beta withendpoint 3.3MeV

„self-calibrating“

Strategies

• We started a long term R& D with eV-Products to find another passivation (they have alternatives)

• Alternative providers not using any paint(Freiburg Material Research Centre)

The 64 detector arrayThe next step towards a large scale experiment,Scalable modular design, explore coincidences

Worldwide largestexperiment of 1cm3

CPG detectors

Mass factor 16 higher,about 0.42 kg CdZnTe

Physics: - Can access2ECEC in theoreticallypredicted region-Precision measurement of 113Cd- New limits70 detectors in total available/characterised

Redesigned prototype

Delrin holder and kapton foilScalable design for larger masses

The first layer

Installed at LNGS in april 2006

New passivation (4 detectors)Sample measurement at ICP-MS @ LNGS:U238: 1.4 mBq/kg (before 2.1 Bq/kg)Th232: 0.61 mBq/kg (before 1.1 Bq/kg)

Improvement of about 3 orders of magnitude

Monte Carloexpectation

Paint contribution at 2.8 MeV: about 0.2 counts/keV/kg/yr

New passivation

Very preliminary: At least a factor 10 better, lot of construction workaround COBRA at LNGS, no coincidences, no nitrogen flushing...

Around 10 counts/keV/kg/yr

Raw dataCd116

2 - decay

€

F =8Q(ΔE /Q)6

me

= 3.7*10−10

S. Elliott, P. Vogel, Ann. Rev. Nucl. Part. Sci. 2002

Energy resolution (FWHM) important semiconductor

Fraction of 2 in 0 peak:

Signal/Background:

4331

02/1

22/1 ==

TT

FBS

yrsT 1922/1 102.3 ×=

yrsT 2602/1 102×=

2 is ultimate, irreducible background

Energy Resolution

• Only electron signal read out (CPG technology)• Possible improvements: cooling, new grids• Better detectors are available

E = 1.9% @ 2.8MeV

=2.9% @ 662keV

Resolution of =0.8% at 2.8 MeV

Back of the envelope

1/2 = ln2 • a • NA• M • t / N (T) ( Background free)

50 meV implies half-life measurements of 1026-27 yrs

1 event/yr you need 1026-27 source atoms

This is about 1000 moles of isotope, implying 100 kg

Now you only can loose: nat. abundance, efficiency, background, ...

Cd116- Matrix elements

<m>=0.4eV

V. Rodin et alV. Rodin et al., nucl-th/0503063, Nucl Phys. A 2006nucl-th/0503063, Nucl Phys. A 2006

Phase space: 116Cd/76Ge = 5 116Cd/130Te=1.7

Sensitivity

50 meV

€

T1/ 2 ∝ M × t /ΔE × B

Dimension it right!

116Cd

116In

116Sn

A real time low-energy solar neutrino experiment?

Threshold energy: 464 keV

7Be contribution g.s. alone: 227 SNU

= 14s

K. Zuber, Phys. Lett. B 571,148 (2003)

e

ee

Current idea: 40x40x40 CdZnTe detectors = 420 kg, enriched in 116Cd

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., acc by Nucl. Inst. Meth A

detectorsdetectors

Monte Carlo

Sophisticated MC basedon GEANT4, written in C++Signal (DECAY0) and background

And many more things going on

Red = 24°C

Blue = 10°C

T-measurement

Pulse shape analysis

Thermal n-capture

PL: Zn-content

nm

The solid state TPCEnergy resolution Tracking

Pixellated CdZnTe detectors

• Massive backgroundreduction• Positive signal information

Pixellisation - I• Massive BG reduction by particle ID , 200m pixels (example simulations):

• eg. Could achieve nearly 100% identification of 214Bi events (214Bi 214Po 210Pb).

00

1-1.5mm1-1.5mm

~15~15mm

3 MeV 3 MeV

7.7MeV life-time = 164.3s

Beta withendpoint 3.3MeV

= 1 pixel, and = several connected pixel, = some disconnected p.

Pixellisation - II Running 256 pixel det with ASIC, 1.6mm pixel size

crystal

ASIC readout

122 keV

136 keV

Pixellated detectors

2D - Pixelisation on both electrodes

Solid state TPC

Rejection power of pixels

0 1000 2 0 0 0 3000 4 0 0 0 5000 6 0 0 0 7000 8 0 0 0 9000 1 0 0 0 0

0.00001

0 . 0 0 0 1

0.001

0 . 0 1

0.1

1

R aw signa l

B eta V eto

A lpha and B eta V eto

A ll vetoes

First (very preliminary) look on rejection power

Suggests a background reduction of 1000!

Nobody said it was going to be easy, and nobody was right

George W. Bush

Summary

• COBRA plans to use a large amount of CdZnTe semiconductors for double beta searches

• Collaboration of about 30 people established• Currently preparing a 64 detector array (about 0.5

kg), first 16 installed at LNGS april 2006• Design changed to allow easy upgrade to larger

scales• Work on signal enhancer/active veto and pixellated

detectors has started• Progress is fast