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Double Beta Decay

A new approach to Double-Beta Decay using CdZnTe: The COBRA experiment

Over the last 10 years various neutrino experiments have proved that neutrinos oscillate between flavour states and therefore have a non-vanishing rest mass. However, as neutrino oscillation experiments probe the difference between neutrino mass states, the absolute mass scale is still unknown.

Another important unknown is the fundamental nature of the neutrino which could be either Dirac or Majorana, and the mechanism through which neutrino mass is acquired. The neutrino might be a Dirac particle obtaining mass through the standard Higgs mechanism like other leptons in the Standard model. Alternatively, as favoured by many theoretical models, the neutrino could be a Majorana particle acting as its own antiparticle and acquiring mass through the see-saw mechanism. A golden channel for answering both the question of neutrino nature and neutrino mass is neutrinoless double beta decay.

 

Double-Beta De cay

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Double beta decay. 2 neutrino mode (left) and 0 neutrino mode (right).

Double Beta Decay is a rare process in which two nuclei in the same nucleus simultaneously beta decay, releasing two electrons and two anti-neutrinos:


     
This rare decay has a half-life of about 10 20 years but is allowed as a second order process in the Standard Model of particle physics for a handful of nuclei that cannot decay via single beta-decay. In fact, there are only 35 known nuclei that can undergo this process. Although it has not yet been observed, there is far more interest in another process in which no neutrinos are released - neutrinoless double beta decay (0vßß):

     
The lepton number violating process of neutrino-less Double-Beta Decay, is forbidden in the Standard Model and therefore, if observed, would imply New Physics. Among a considerable list of important physics topics accessible with this process, the two outstanding issues are the following: first, it is considered to be the unique channel to probe the fundamental character of neutrinos, i.e. whether they are Majorana (neutrino is its own antiparticle) or Dirac particles. Second, it provides access to the absolute mass scale of neutrinos, i.e. the half-life of this process is directly related to the effective neutrino mass.

 

The COBRA Experiment

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COBRA is based at Gran Sasso in Italy.

The COBRA experiment represents an international initiative to measure neutrino-less Double-Beta Decay (0νββ-decay). The experiment is under development at Gran Sasso. Eventually, COBRA will use 64,000 CdZnTe crystals. From August 2003 until January 2006, a four-crystal array was set-up and backgrounds and electronics were studied. Currently, a 64 crystal array (see below) is in the process of being prepared for testing.

COBRA Set-up

COBRA utilises CdZnTe room-temperature semiconductor detectors. These will be used in order to measure 0νββ-decay with an expected half-life time of above 1026 years. The signature of 0νβ-β--decay is a Gaussian peak in the energy spectrum at the Q-value of the decay.

An example of a peak at the Q-value in the energy spectrum. i.e. the signature of 0νββ-decay.

One of the main challenges is to achieve a sufficiently low background level in the various energy intervals containing a potential signal. It is this important part of the COBRA experiment on which our group at Warwick has contributed.

The COBRA experiment will make use of 64,000 of these CdZnTe detectors in order to search for neutrino-less double-beta decay. COBRA will be of a modular design in order to allow for later expansion if required. Layers of crystals will be arranged in the centre of a huge cube of shielding layers.

Advantages of COBRA

It is known that the half-life scales as:

 

With this equation in mind, CdZnTe offers a number of advantages for the experiment:

  • Of the 35 isotopes in nature allowing Double-Beta Decay, the COBRA detectors would contain nine (five for 0νβ-β--decay, four for 0νβ+β+-decay), among these two of the experimentally favoured high total decay energy (Q-value) isotopes, 130Te (Q-value: 2529 keV) and 116Cd (Q-value: 2805 keV). Additionally, COBRA could measure the complimentary 0νβ+β+-decay especially from 106Cd (Q-value 2772 keV) which offers the chance to constrain a possible neutrino mass measurement further than it is allowed by a pure 0νβ-β--decay observation.
  • 130Te has a natural abundance of ~ 34% whilst 116Cd can be enriched to 90%.
  • The scalable design allows for upgrades.
  • Room temperature operation means that there is no need for cryogenics as well as keeping costs down.
  • Energy resolution is maximised due to the semiconductor crystals being both source and detector.
  • Background can be minimised with clean material manufacture, high Q-values and the possibility of detecting multicrystal events with pixelised detectors.