Status of COBRA

6th SNOLAB Workshop, 22.8.2007Picture courtesy farine@snolab.ca

COBRA

Use large amount of CdZnTe Semiconductor Detectors

Array of 1cm3

CdZnTe 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 2809 ß-ß- Te128 31.7 868 ß-ß- Te130 33.8 2529 ß-ß- Zn64 48.6 1096 ß+/EC Cd106 1.21 2771 ß+ß+ Cd108 0.9 231 EC/EC Te120 0.1 1722 ß+/EC

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

COBRA: CdZnTe semiconductors

V.K.B. Kota, priv. comm.

116Cd comes of best...

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 Surrey (UK), University of Hamburg (Germany),Technical University Prague (Tschech Rep.), Jagellonian University (Poland),Louisianna State University (USA)

University of Dortmund

University of Sussex

Laboratori Nazionali del Gran Sasso

University of Warwick

University of Birmingham

University of Liverpool

University of York

Rutherford Appleton Laboratory

University of Bratislava

Washington University at St. Louis

Material Research Centre Freiburg

More welcome

University of Jyvaskyla

University of La Plata

COBRA at LNGS

COBRASince april 20072004-march 2007

worldbest

First COBRADouble beta results

T. Bloxham et al., arXiv:0707.2756,,Phys. Rev. C 76, 025501 (2007)

PertinaxCopper CZTWax

Based on 4 detectors, total of 4.34 kg x days

Redesigned prototype

Delrin holder and kapton foilScalable design for larger masses

The 64 array - the next step4x4x4 detector array = 0.42 kg CdZnTe semiconductors

Zur Anzeige wird der QuickTime™ Dekompressor „Cinepak“

benötigt.

The first layer

Installed at LNGS in april 2006, world wide largest array of this type of detector, remaining 48 will be installed autumn 2007

The first layer - some spectraCd-113 beta decaywith half-life of about 1016 yrs

The first layer - Coincidences

Just starting to analyse/understand the power of that..

Preliminary

Coincidences around Det 7

Example:3-coincidence

Powerful tool!!!

Coincidences

Example: 130Te 1st excited state

1. 1 Detector with 2530 keV2. 1 Detector with 1994 keV3. 2 Detectors (One with

1994 keV, one with 536 keV))4. 2 Detectors (One with

1994 keV, two sum up to 536 keV)

€

130Te→130Xe∗+ 2e− + γ (536keV )

Have performed a search for mode 3 in 4.2 kg x days and foundno event !

New passivation (4 detectors)

Sample measurements at ICP-MS @ LNGSsuggest improvement of about 3 orders of magnitude

Monte Carloexpectation

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

Major background so far: Red paint on detector surface

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

Sensitivity

50 meV

€

T1/ 2 ∝ M × t /ΔE × B

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

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

The solid state TPCEnergy resolution Tracking

Pixelated CdZnTe detectors

• Massive backgroundreduction• Positive signal information

Pixelisation - 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.

Rejection power of pixels

First look on rejection power

Suggests a background reduction of 1000!

T. Bloxham, M. Freer,Nucl. Inst. Meth. A 572, 722 (2007)

Likelihood using:

-Number of pixels

- spatial separation

- energy loss (dE/dx)

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

crystal

ASIC readout

122 keV

136 keV

Single pixel 57Co spectrum

Additional 16 pixel detectorwith conventional readout running

Pixel detectors - next step

64 pixel detectors 2x2x0.5 cm3

Pixel electrodes will be replaced by 200 m pixels, mask in hand

By end 2007 we‘ll have two high resolution pixel detectors

Pixellated detectors

2D - Pixelisation on both electrodes

Solid state TPC

COBRA for solar neutrinos - King COBRA

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

Summary

• COBRA is a new approach to double beta decay using CdZnTe semiconductors

• Follows two lines: Background identification and reduction (LNGS) and development of high resolution pixel-CZT

• Two major items for 2007:Finish the 64 array at LNGS in new locationTwo high resolution pixel detectors

• Revived interest in neutrinoless double EC• Possible interest to come to SNOLAB early 201x• Progress is fast....