the cobra double beta decay search experiment

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%


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:


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


(n,)-Reactions


Neutron Energy-Flux Dependence

Depletion

Feeding


Building a Multilayer Shield

Tested:• block structure sequence• layer materials in block• layer ratio’s in block• number of block repetitions


Comparison to Standard Shields


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.


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.


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.


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


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.


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


Building a Background Model: 2

Gas222Rn gas

Crystals238U, 232Th decay chains40K137Cs210Po210Pb on surface

Delrin Holder238U,232Th decay chain

40K137Cs

Chamber walls210Pb on surface


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.


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.


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.


Outlook:1


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


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


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


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


the cobra double beta decay search experiment

Documents

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double block

cdznte material

block layer ratios

energy resolution n2