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1m. A(Z,N) -> A(Z+2 , N-2) + 2e + 2 ν e. COBRA Double Beta Decay Experiment D.Y. Stewart , Dr. Y.A. Ramachers, Prof. P.F.Harrison Experimental Particle Physics Group University of Warwick. Comparison of Shielding Designs:

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A(Z,N) -> A(Z+2 , N-2) + 2e + 2νe

COBRA Double Beta Decay Experiment

D.Y. Stewart, Dr. Y.A. Ramachers, Prof. P.F.Harrison

Experimental Particle Physics Group

University of Warwick

Comparison of Shielding Designs:

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. Std’s are existing designs, Prim’s are Warwick’s designs. Prim1|3 is “the best”allowing the fewest neutrons to pass through, showing our layer design is worthwhile.

107 neutrons fired into a block of material

  • What is double beta decay?
    • Second order weak decay
    • 2νββ decay: Simultaneous single beta decays –
    • 0νββ decay: Emission and re-absorption of a virtual light neutrino
    • Involves helicity change, observed decay rate => ν has mass
  • How do we “see” it?
  • 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
  • Requires good energy resolution => low background required
  • Why is it interesting?
    • 0νββ decay can probe absolute mass scales of the neutrino
    • Further sensitive to properties of ν under CP conjugation
    • If 0νββ decay is detected: ν = majorana particle
    • non-conservation of Lepton no. by 2 units

0νββ decay

2νββ decay

Liquid Scintillator Testing:

A(Z,N) -> A(Z+2 , N-2) + 2e

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

Warwick’s Involvement: Development of an optimal shielding design

ββ 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.

Experimental challenge = background control

Light-tight box

  • The current shielding design contains an active veto. The idea is to use a liquid scintillator along with avalanche photodiodes (apd’s), 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 apd’s working in coincidence.
  • 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
  • The Set-Up
  • Central detector will be 64000 array of CdZnTe crystals
  • Largest available size of crystals is 1cm3
  • Modular Design allowing for future upgrades
  • Detector array surrounded by optimised shielding
  • Currently working on 64-crystal array

Avalanche Photodiode

64-Crystal Array Set-up

  • Advantages of COBRA
  • Source = detector (9/35 possible candidates for ββ decay in Cd, Zn and Te)
  • CdZnTe has a high Q-value allowing for distinction from background radiation in the energy spectrum
  • Room temperature operation (compared to Gerda and Majorana which use Ge crystals at cryogenic temperatures)
  • Industrial Support due to medical applications of CdZnTe

Shielding Design:

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.

  • Size = 14.53 m3
  • Mass = 54166 Kg
  • Cost = £368,560

The above picture shows a side profile view of the layers of shielding in the design. This design was simulated using GEANT4 with 200 neutrons yr-1m-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!

3-Dimensional view of COBRA – simulated using GEANT4, crystals at the centre surrounded by shielding layers.