View source for Double beta decay

In the process of [[beta decay]], unstable [[atomic nucleus|nuclei]] decay by converting a [[neutron]] in the nucleus to a [[proton]] and emitting an [[electron]] and an electron [[antineutrino]].  In order for beta decay to be possible, the final nucleus must have a larger [[Binding_energy#Nucleus_binding_energy|binding energy]] than the original nucleus.  For some nuclei, such as [[germanium-76]], the nucleus with atomic number one higher has a smaller binding energy, preventing beta decay from occurring.  However, the nucleus with atomic number two higher, [[selenium-76]], has a larger binding energy, so the '''double-beta decay''' process is allowed.

In double-beta decay, two neutrons in the nucleus are converted to protons, and two electrons and two electron anti-neutrinos are emitted.  This process was first observed in laboratory in 1986 (the first effort of experimental observation was dated by 1948). Geochemical observation of the decay products (by extraction of Kr and Xe from very old Se and Te minerals) are known since 1950. Double-beta decay is the rarest known kind of radioactive decay; it was observed for only 10 isotopes, and all of them have a mean life time of more than 10<sup>19</sup> yr. 

For some nuclei, the process occurs as conversion of two protons to neutrons, with emission of two electron neutrinos and absorption of two orbital electrons ([[double electron capture]]). If the mass difference between the parent and daughter atoms is more than {{val|1.022|u=MeV/c2}} (two electron masses), another decay branch is accessible, with capture of one orbital electron and emission of one [[positron]]. When the mass difference is more than {{val|2.044|u=MeV/c2}} (four electron masses), the emission of two positrons is possible. These theoretically possible double-beta decay branches have not yet been observed.


==List of known double-beta decay isotopes==
More than 60 naturally occurring isotopes are capable of undergoing double-beta decay. Only ten of them were observed to decay via the two-neutrino mode:<ref>[http://www.nndc.bnl.gov/bbdecay/list.html Adopted Double Beta Decay Data.]</ref>  <br>
<blockquote>
{{SimpleNuclide|Link|Calcium|48}}, {{SimpleNuclide|Link|Germanium|76}}, {{SimpleNuclide|Link|Selenium|82}},{{SimpleNuclide|Link|Zirconium|96}}, {{SimpleNuclide|Link|Molybdenum|100}}, {{SimpleNuclide|Link|Cadmium|116}},  {{SimpleNuclide|Link|Tellurium|128}}, {{SimpleNuclide|Link|Tellurium|130}}, {{SimpleNuclide|Link|Neodymium|150}}, and {{SimpleNuclide|Link|Uranium|238}}. </blockquote>

Many isotopes are, in theory, capable both of double-beta decay and other decays.  In most cases, the double-beta decay is so rare as to be nearly impossible to observe against the background of other radiation. However, the double-beta decay rate of {{SimpleNuclide|Link|Uranium|238}} (also an alpha emitter) has been measured radiochemically; {{SimpleNuclide|Link|Plutonium|238}} is produced by this type of radioactivity.  Two of the nuclides ({{SimpleNuclide|Link|Calcium|48}} and {{SimpleNuclide|Link|Zirconium|96}}) from the list above can decay also via single beta decay but this decay is extremely suppressed and has never been observed.


show some better examples please.  this sucks

==Neutrinoless double-beta decay==

The processes described above are also known as two neutrino double-beta decay, as two neutrinos (or anti-neutrinos) are emitted.  If the neutrino is a [[Majorana particle]], meaning that the anti-neutrino and the neutrino are actually the same particle, then it is possible for neutrinoless double-beta decay to occur. In the simplest theoretical treatment of neutrinoless double-beta decay (light neutrino exchange), in essence the two neutrinos annihilate each other, or equivalently, one [[nucleon]] absorbs the neutrino emitted by another nucleon of the nucleus. A [[Feynman diagram]] for this process is shown in the following figure:

[[Image:Ndbd.png]]

The neutrinos in the above diagram are [[virtual particles]]. With only the two electrons in the final state, the total [[kinetic energy]] of the two electrons would be approximately the difference in [[Binding_energy#Nucleus_binding_energy|binding energy]] between the initial and final state nuclei (with the recoil of the nucleus accounting for the rest of the available energy). To a very good approximation, the two electrons are emitted back-to-back to conserve [[momentum]].

The [[Decay_rate#Radioactive_decay_rates|decay rate]] for this process is given (approximately) by

:<math>\Gamma = [[User:WikiBot|WikiBot]] 16:51, 4 September 2012 (UTC){G |M|^2 |m_{\beta \beta}|^2},</math>

where G is the two-body phase-space factor, M is the nuclear matrix element, and m<sub>&beta;&beta;</sub> is given by

:<math>m_{\beta \beta} = \sum_{i=1}^3 m_i U^2_{ei}.</math>

In this latter expression, the m<sub>i</sub> are the [[neutrino mass]]es (of the i<sup>th</sup> mass eigenstate), and the U<sub>ei</sub> are elements of the [[Neutrino_oscillation#Maki-Nakagawa-Sakata_matrix|Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix]]. Hence the observation of neutrinoless double-beta decay, in addition to confirming the Majorana nature of the neutrino, would give information on the absolute neutrino mass scale, and potentially also on the neutrino mass hierarchy and the Majorana phases appearing in the PMNS matrix.

==Neutrinoless double-beta decay experiments==

Numerous experiments have been carried out to search for neutrinoless double-beta decay. Some recent and proposed future experiments include:

* Completed experiments:
** [http://www.unine.ch/phys/corpus/Gothard/got_art.html Gotthard TPC]
** [http://www.mpi-hd.mpg.de/non_acc/dblbeta.html Heidelberg-Moscow] (see also [http://www.klapdor-k.de])
** [http://www.unizar.es/lfnae/ipaginas/ip0400.html#migex IGEX]
** [http://nemo.in2p3.fr/ NEMO1 and 2]

* Current and proposed experiments
** [http://cobra.physik.uni-dortmund.de/ COBRA]
** [http://crio.mib.infn.it/wig/Cuoricinopage/CUORICINO.php CUORICINO] and [http://crio.mib.infn.it/wig/Cuorepage/CUORE.php CUORE]
** [[EXO]]
** [http://www.mpi-hd.mpg.de/ge76/ GERDA]
** [http://majorana.npl.washington.edu/ M<font size=-2>AJORANA</font>]
** [http://ewiserver.npl.washington.edu/moon/ MOON]
** [http://nemo.in2p3.fr/ NEMO3] and [http://nemo.in2p3.fr/supernemo/ SuperNEMO]

==Klapdor-Kleingrothaus <i>et al</i>.'s claimed observation of neutrinoless double-beta decay==

A subgroup of the Heidelberg-Moscow collaboration led by H. V. Klapdor-Kleingrothaus published a series of controversial papers claiming to have observed neutrinoless double-beta decay in {{SimpleNuclide|Link|Germanium|76}} with statistical significance of &gt; 99.9999998%. This claim has been criticized by many in the nuclear and particle physics communities, but Klapdor-Kleingrothaus <i>et al.</i> have vigorously defended their work. Their measured value of m<sub>&beta;&beta;</sub> between ~0.2-0.5 eV (depending on the choice of nuclear matrix element) has yet to be confirmed by other experiments.

The following lists, in chronological order, the key publications and preprints related to the claim and the criticism thereof.

* [http://dx.doi.org/10.1142/S0217732301005825 H. V. Klapdor-Kleingrothaus <i>et al</i>., "Evidence for Neutrinoless Double Beta Decay",  Mod. Phys. Lett. A <b>16</b> (2001) p. 2409,] [http://arxiv.org/abs/hep-ph/0201231 arXiv:hep-ph/0201231.] This is the original paper, claiming a 2-3&sigma; observation (depending on the statistical method used to measure the significance).
* [http://dx.doi.org/10.1142/S0217732301005825 C. E. Aalseth <i>et al</i>., "Comment on 'Evidence for Neutrinoless Double Beta Decay'", Mod. Phys. Lett. A <b>17</b> (2002) p. 1475,] [http://arxiv.org/abs/hep-ex/0202018 arXiv:hep-ex/0202018.]
* [http://arxiv.org/abs/hep-ph/0205228 H. V. Klapdor-Kleingrothaus, "Reply to a Comment of article 'Evidence for Neutrinoless Double Beta Decay'", arXiv:hep-ph/0205228 (2002)] (not peer-reviewed).
* [http://arxiv.org/abs/hep-ph/0205293 H. L. Harney, "Reply to the Comment on 'Evidence for Neutrinoless Double Beta Decay'", arXiv:hep-ph/0205293 (2002)] (not peer-reviewed).
* [http://dx.doi.org/10.1023/A:1019722802931 H. V. Klapdor-Kleingrothaus <i>et al</i>., "Neutrinoless Double Beta Decay: Status of Evidence", Found. Phys. <b>32</b> (2002) 1181,] [http://arxiv.org/abs/hep-ph/0302248 arXiv:hep-ph/0302248.]
* [http://dx.doi.org/10.1016/j.physletb.2004.02.025 H. V. Klapdor-Kleingrothaus <i>et al</i>., "Search for neutrinoless double beta decay with enriched <sup>76</sup>Ge in Gran Sasso 1990–2003", Phys. Lett. B <b>586</b> (2004) p. 198.] Statistical significance increases to &gt;4&sigma;.
* [http://dx.doi.org/10.1142/S0217732306020937 H. V. Klapdor-Kleingrothaus and I. V. Krivosheina, "The Evidence for the Observation of 0&nu;&beta;&beta; Decay: the Identification of 0&nu;&beta;&beta; Events from the Full Spectra", Mod. Phys. Lett. A <b>21</b> (2006) p. 1547.] Statistical significance increases to &gt;6&sigma;.

==See also==
*[[double electron capture]]
*[[beta decay]]
*[[neutrino]]
*[[particle radiation]]
*[[radioactive isotope]]

{{Nuclear_processes}}

== References ==
{{reflist|2}}

[[Category:Radioactivity]]
[[Category:Particle physics]]

[[de:Doppelter Betazerfall]]
[[es:Doble desintegración beta]]
[[fr:Double désintégration béta]]
[[ja:二重ベータ崩壊]]
[[ko:이중 베타 붕괴]]
[[is:Tvívirk betasundrun]]
[[it:Doppio decadimento beta]]
[[pl:Podwójny rozpad beta]]
[[ru:Двойной бета-распад]]
[[uk:Подвійний бета-розпад]]