COBRA Double Beta Decay Experiment Comparison of Shielding

COBRA Double Beta Decay Experiment Comparison of Shielding

COBRA Double Beta Decay Experiment

Comparison of Shielding Designs:

D.Y. Stewart, Dr. Y.A. Ramachers, Prof. P.F.Harrison
Experimental Particle Physics Group
University of Warwick

1m

107 neutrons fired into
a block of material

What is double beta decay?
e

e

e
e
n1

n1

n2

n2

Further sensitive to properties of under CP conjugation
If 0 decay is detected: = majorana particle
non-conservation of Lepton no. by 2 units

Second order weak decay
2 decay: Simultaneous single beta decays
A(Z,N) -> A(Z+2 , N-2) + 2e + 2e

NEW PHYSICS
MODEL

BEYOND

0 decay: Emission and re-absorption of a virtual light
neutrino

THE

STANDARD

Liquid Scintillator Testing:
A peak at the Q-value is the signature of 0 decay
The Q-value corresponds to released energy in nuclear
transition

A(Z,N) -> A(Z+2 , N-2) + 2e
Involves helicity change, observed decay rate => has
mass

Half-life varies as Q5
Requires good energy resolution => low background required

What is COBRA? - Cadmium-Telluride 0-neutrino double-Beta

Research

Apparatus
The aim of the COBRA experiment is to search for neutrinoless double beta (0)
decay
as a path to new Physics beyond the Standard Model
UK-led initiative
Testing currently taking place at LNGS, Italy

Warwicks Involvement:

optimal

decays of interest typically have energy release of > 2MeV but there are
many forms of radiation which have similar signatures. The result is background
interference.

U/Th from LAAPD's (Large Area Avalanche PhotoDiodes)
Low energy neutrons

Modular Design allowing for future upgrades

High energy neutrons

by

an

shielding design

Largest available size of crystals is 1cm 3
surrounded

of

Gamma radiation from decay chains of 238U and 232Th

Central detector will be 64000 array of CdZnTe
crystals

array

Development

Experimental Sources
challenge
Background
: = background control

The Set-Up

Detector
shielding

A number of configurations of materials eg. type of material, ordering
of materials and thickness, for shields were simulated using MCNPX
based on previous simulations. These designs were then compared to
existing shielding designs in use by other experiments. Stds are
existing designs, Prims are Warwicks designs. Prim1|3 is the best
allowing the fewest neutrons to pass through, showing our layer
design is worthwhile.

0 decay can probe absolute mass scales of the
neutrino

0 decay

2 decay

How do we see it?

Why is it interesting?

optimised

Currently working on 64-crystal array

Advantages of COBRA

64-Crystal Array Setup

Source = detector (9/35 possible candidates for decay in Cd, Zn and Te)

Shielding Design:

CdZnTe has a high Q-value allowing for distinction from background
radiation in the energy spectrum

A number of designs and combinations of materials were tested using the simulation
packages MCNP and GEANT4. The overall shielding design is to be optimised for shielding
against neutrons. A variety of neutron absorbers were tested using MCNP so a comparison
between them could be made, the results are shown below.

Room temperature operation (compared to Gerda and Majorana which use
Ge crystals at cryogenic temperatures)

The current shielding design contains an
active veto. The idea is to use a liquid
scintillator along with avalanche photodiodes
(apds), which detect scintillations. This is a
new idea and has never been done before so
will need to be thoroughly tested in order to
have an understanding of how they will work.
Testing will be done in a light-tight box in
order to simulate the conditions within
COBRA. The design of the box is shown
above.
It
allows
for
a
number
of
configurations of 2 apds
working in
coincidence.

Light-tight box

Avalanche
Photodiode

Industrial Support due to medical applications of CdZnTe

Size
14.53 m3
Mass
3-Dimensional view of COBRA simulated using GEANT4,
crystals at the centre surrounded by shielding layers.

=
=

54166 Kg

Cost
=
The above picture shows a side profile view of the368,560
layers of shielding in the design. This design
was simulated using GEANT4 with 200 neutrons yr-1 m-2, the high energy neutron flux at Gran
Sasso. 50,000 high energy neutron events were simulated (equivalent to 52.27 years of running)
and only 3x108 counts/year/KgKeV were detected by the crystals. This corresponds to less than 1
neutron per year!

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