Double electron capture (2 ε- capture, εε- capture, ECEC decay) is one of the types of double beta decay of atomic nuclei , in which the nucleus captures two electrons from the atomic electron shell. If the electron shell ( K , L , M , etc.) with which electrons are captured is specified, then we speak of double K- capture, etc. Theoretical predictions indicate a higher, other things being equal, probability 2 K - capture than capture from higher shells; capture of two electrons from different electron shells, for example, K and L, is also possible.
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Radioactive decay
Nucleosynthesis
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Decay Characteristics
Two modes of double electron capture are distinguished - two-neutrino and neutrino-free. In the case of two- neutrino decay allowed by the well-known conservation laws, the nucleus captures two orbital electrons and emits two electron neutrinos . The nuclear charge decreases by two units (two protons turn into two neutrons ). If the decay occurs in the ground state of the daughter nucleus, then almost all of the energy released in the decay (equal, up to a factor of c 2 , the mass difference of the parent and daughter atoms ) is carried away by the neutrino, with the exception of the part of the energy spent on creating vacancies in the electron shell.
In the case of a hypothetical neutrinoless 2 ε capture, which is forbidden by the Standard Model and changes the lepton number by two units, the main part of the released energy is carried away by the gamma quantum of the internal bremsstrahlung or the electron of internal conversion . During capture with the transition of the nucleus not to the ground, but to the excited level, a cascade of gamma rays / conversion electrons must also be observed accompanying the transition of the daughter excited nucleus to the ground state. For the existence of neutrinoless 2 ε capture (as for the neutrinoless modes of all other types of double beta decay), it is necessary that the electron neutrino is mixed with the electronic antineutrino by one mechanism or another, or, as an equivalent statement, the Majorana mass of the electron neutrino (parameter, the given value of this mixing) was nonzero. The main mechanism of neutrinoless 2ε capture considered in the literature is the exchange of massive Majorana neutrinos, but a number of other mechanisms have been proposed - right currents in weak interaction (this requires the presence of a hypothetical supermassive W boson that provides weak interaction of right currents), supersymmetry with violation of R parity , exchange leptoquarks and so on. d. Thus, the search neutrinoless 2ε-capture allows you to limit the parameters of a number of theories, introducing "new physics" beyond the Stan Artney model.
2ε transitions, according to the theory, are resonantly amplified if the parent atom in mass is close enough to the daughter atom with the nucleus in the ground or excited state and two electron vacancies in the shell. Several isotopes (for example, gadolinium-152 in the case of KL I capture) approximately satisfy this condition. A number of experimental works are devoted to the search for resonance transitions and the accurate measurement of the difference in masses of atoms participating in 2ε capture on Penning traps .
In all modes of double electron capture, two (and, upon emission of a conversion electron, three) vacancies are formed on the lower electron shells of the atom. These vacancies are quickly filled with electrons from higher shells, and the energy released during this transition is carried away by Auger electrons and / or characteristic x-ray radiation .
If the available decay energy (the difference between the masses of the parent and daughter atoms) exceeds the doubled electron mass (2 m e c 2 ≈ 1022 keV) , then double electron decay can be accompanied by a competing double beta process - electron capture with positron emission. If the available decay energy exceeds the quadruple mass of the electron (4 m e c 2 ≈ 2044 keV) , another competing decay channel — double positron decay — is switched on. Of all the nuclides that exist in nature, only six have available decay energy exceeding 2044 keV and, therefore, all three types of double beta decay are allowed with a decrease in the nuclear charge.
Experimental Observations
Unlike double neutrino double beta decay with increasing nuclear charge, where the decay has been reliably identified for more than 10 isotopes, there are no unambiguously recognized by the community experimental observations of double electron decay in either the two-neutrino, and even more so in the neutrino-free mode. However, there are a number of indications of the observation of double electron capture, which need independent confirmation [1] . A geochemical analysis of ancient samples of barite (BaSO 4 ) with an age of 170 Ma indicates the decay of the barium-130 isotope caused by double electron capture
with a half-life of T 1/2 = (2.2 ± 0.5) ⋅10 21 years. [2] . In this case, the decay product, xenon -130, accumulates in the sample. Excess xenon-130 with respect to other xenon isotopes is an indication of the presence of a process leading to its appearance. Although the geochemical method does not distinguish between a two-neutrino decay mode and a non-neutrino one, it is assumed that the observed excess of xenon-130 is due to a two-neutrino allowed decay. However, this result contradicts both the earlier work [3] , which established the lower limit for the half-life at the level of 4⋅10 21 years, and the later one [4] , in which a barite sample of 3.5 billion years old was used and three times more shorter than the first work [2] , the half-life of 130 Ba: T 1/2 = (6.0 ± 1.1) × 10 20 years. Due to large discrepancies in the results that may be caused by some unaccounted-for background process, the existence of 130 Ba double electron capture is not yet considered reliable.
In another experiment [5] , a sample of gaseous krypton enriched in krypton-78 was studied in a low-background proportional chamber located at the Baksan neutrino observatory at a depth of several kilometers underground. A peak was detected in the detector spectrum accumulated over 8400 hours , which can be interpreted as a manifestation of a two-neutrino double K capture
with half-life T 1/2 = (9.2 +5.5
−2.6 (stat.) ± 1.3 (syst.)) × 10 21 years.
In 2019, a double electron capture of xenon-124 [6] with a half-life of T 1/2 = (1.8 ± 0.5 (stat.) ± 0.1 (syst.)) × 10 22 years was discovered.
Notes
- ↑ The experiments that reveal indications of the presence of an effect, which were subsequently refuted in more sensitive experiments, are not considered here.
- ↑ 1 2 AP Meshik, CM Hohenberg, OV Pravdivtseva, and Ya. S. Kapusta, Phys. Rev. C 64 (2001) 035205. DOI : 10.1103 / PhysRevC.64.035205
- ↑ AS Barabash, RR Saakyan. Experimental limits on 2β + , Kβ + , and 2K processes for 130 Ba and on 2K capture for 132 Ba // Phys. Atom Nucl. - 1996. - Vol. 59. - P. 179–184.
- ↑ M. Pujol, B. Marty, P. Burnard, P. Philippot. Xenon in Archean barite: Weak decay of 130 Ba, mass-dependent isotopic fractionation and implication for barite formation // Geochimica et Cosmochimica Acta. - 2009. - Vol. 73. - P. 6834–6846. - DOI : 10.1016 / j.gca.2009.08.08.002 .
- ↑ Yu. M. Gavrilyuk, AM Gangapshev, VV Kazalov, VV Kuzminov, SI Panasenko, SS Ratkevich. Indications of 2ν2K capture in 78 Kr // Physical Review C. - 2013. - Vol. 87. - P. 035501. - DOI : 10.1103 / PhysRevC.87.035501 .
- ↑ Aprile, E. et al. Observation of two-neutrino double electron capture in 124 Xe with XENON1T (Eng.) // Nature : journal. - 2019 .-- Vol. 568 . - P. 532-535 . - DOI : 10.1038 / s41586-019-1124-4 .