Double electron capture

from Wikipedia, the free encyclopedia

The double electron capture is a possibility of radioactive decay of an atomic nucleus . For a nuclide ( A , Z ) with the number A of nucleons and the atomic number Z , double electron capture is only possible if the mass of the nuclide ( A , Z  - 2) is smaller. It is related to double beta decay.

On this path of decay, two electrons from the atomic shell are captured by two protons of the atomic nucleus and two neutrons are created . Two neutrinos are released. Since the protons are converted into neutrons, the number of neutrons increases by 2 and the number of protons Z decreases by 2. The mass number A remains unchanged. By changing the number of protons, the nuclide of another element is created when the electrons are captured twice .

Example:

The probability of double electron capture is, as with double beta decay, extremely small, because here a process of weak interaction has to take place twice and because the overlap of the electron orbitals with the atomic nucleus, which is 4–5 powers of ten, is very small (i.e. the probability of a Electrons in the nucleus are extremely small, the probability that two electrons will stay at the same time is even less). The decay of 124 Xe mentioned here has a half-life of 18 trillion (1.8 · 10 22 ) years, making it the rarest decay that has ever been observed. The double electron capture can therefore only occur if the single electron capture is not possible for energetic reasons, which is the case with a total of 35 naturally occurring nuclides .

The experimental proof of double electron capture is even more difficult than that of double beta decay , because the only detectable particles here are quanta and Auger electrons , which are emitted from the atomic shell. In this energy range (a few keV ) the background noise is significantly higher. As the first and so far only double electron capture, the decay of 124 Xe could be proven beyond doubt in 2019 .

Competing β + decay

If the mass difference between the mother and daughter atom is more than two electron masses (1.022 MeV / c 2 ), the available energy is large enough to allow a combination of electron capture and β + decay . This process competes with double electron capture. The ratio of the frequencies of the two decay pathways depends on the properties of the atomic nucleus. If the mass difference is more than 4 electron masses (2.044 MeV / c 2 ), a third path of decay - double the β + decay - becomes possible. Only 6 naturally occurring nuclides can decay in all three ways. Neither of these two processes has been proven experimentally so far.

Neutrino-free double electron capture

The process described above with the capture of two electrons and the emission of two neutrinos is permitted by the standard model of elementary particle physics , because no conservation laws ( including the conservation of the lepton number ) are violated.

However, if the lepton number were not preserved, another process could occur: The energy generated is released within the core as bremsstrahlung (gamma radiation) and no neutrinos are emitted. This disintegration path has not yet been proven experimentally. It would contradict the Standard Model .

Individual evidence

  1. ↑ a b B. R. Martin: Nuclear and Particle Physics An Introduction . John Wiley & Sons, 2011, ISBN 978-1-119-96511-4 ( limited preview in Google Book Search).
  2. ↑ a b Michele Barone: Astroparticle, Particle and Space Physics, Detectors and Medical Physics Applications Proceedings of the 9th Conference: Villa Olmo, Como, Italy, October 17-21, 2005 . World Scientific, 2006, ISBN 981-256-798-4 , pp. 169 ( limited preview in Google Book search).
  3. ↑ a b Nadja Podbregar: The rarest decay of the universe. April 25, 2019, accessed May 2, 2019 .
  4. ^ A b Robert Gast: Spectrum of Science, 18 trillion year half-life. April 24, 2019, accessed May 2, 2019 .
  5. ↑ Axel Frotscher: Double beta decay of 58 Ni. Dresden 2015.
  6. ↑ C. Sáenz, E. García et al .: Results of a search for double positron decay and electron-positron conversion of 78 Kr. In: Nuclear Physics B - Proceedings Supplements. 35, 1994, p. 363, doi: 10.1016 / 0920-5632 (94) 90277-1 .
  7. ^ HV Klapdor-Kleingrothaus, I. V Krivosheina, R. Viollier: Physics Beyond the Standard Models of Particles, Cosmology and Astrophysics . World Scientific, 2011, ISBN 978-981-4460-75-0 , pp. 267 ( limited preview in Google Book search).