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
107 neutrons fired into
a block of material
What is double beta decay?
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
0 decay: Emission and re-absorption of a virtual light
Liquid Scintillator Testing:
A peak at the Q-value is the signature of 0 decay
The Q-value corresponds to released energy in nuclear
A(Z,N) -> A(Z+2 , N-2) + 2e
Involves helicity change, observed decay rate => has
Half-life varies as Q5
Requires good energy resolution => low background required
What is COBRA? - Cadmium-Telluride 0-neutrino double-Beta
The aim of the COBRA experiment is to search for neutrinoless double beta (0)
as a path to new Physics beyond the Standard Model
Testing currently taking place at LNGS, Italy
decays of interest typically have energy release of > 2MeV but there are
many forms of radiation which have similar signatures. The result is background
U/Th from LAAPD's (Large Area Avalanche PhotoDiodes)
Low energy neutrons
Modular Design allowing for future upgrades
High energy neutrons
Largest available size of crystals is 1cm 3
Gamma radiation from decay chains of 238U and 232Th
Central detector will be 64000 array of CdZnTe
: = background control
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
How do we see it?
Why is it interesting?
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)
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
configurations of 2 apds
Industrial Support due to medical applications of CdZnTe
3-Dimensional view of COBRA simulated using GEANT4,
crystals at the centre surrounded by shielding layers.
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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