Electron capture

Electron (engl. Electron capture ) is a type of radioactivity , in which a nucleus is converted to a more stable, by an electron from an inner shell ( orbital ) of its electron shell captures. One of the protons in the nucleus is converted into a neutron ; the ordinal number is reduced by one. The formula symbol for the process is EC or the Greek letter epsilon ( ). This is why electron capture is sometimes called epsilon decay . ${\displaystyle \epsilon }$

Electron capture was theoretically predicted by Hideki Yukawa in 1935 and demonstrated experimentally for the first time in 1937 by Luis Walter Alvarez . The process is mediated by the weak interaction and belongs to beta radioactivity . It transforms the respective nuclide into the same daughter nuclide as a beta-plus decay.

Electron capture plays an important role in the formation of neutron stars .

description

Electron capture, like beta plus decay, enables charge retention when a proton is converted into a neutron. The mass number of the core is retained. The number of leptons is also retained because an electron neutrino is emitted. The nucleus also gains the rest energy of the trapped electron.

${\displaystyle \mathrm {p} +\mathrm {e} ^{-}\rightarrow \mathrm {n} +{\nu }_{e}}$

Only if the conversion energy (i.e. the difference in the atomic masses of the mother and daughter nuclide converted into energy ) is at least 1022 keV does the beta-plus decay also occur as a further, alternative decay channel, in which no electron is absorbed and a positron must be generated. Conversely, electron capture also occurs with every positron-emitting nuclide.

The electrons of the K shell have the greatest probability of being at the location of the atomic nucleus. This is why the captured electron comes from this shell in around 90 percent of all electron captures. This electron capture is known as K capture. The rarer electron capture from higher shells is called L capture or M capture. Somewhat imprecisely, “K-capture” is sometimes used as a term for any electron capture; therefore instead of EC or sometimes the K is used as a formula designation. ${\displaystyle \epsilon }$

The energy released by the nuclear transformation is the equivalent of the mass change of the nucleus minus the binding energy of the trapped electron. In some cases, a part of the "gained" energy initially remains in the core (daughter core) created by the transformation as excitation energy ; the rest is distributed as kinetic energy according to the conservation of momentum (see also kinematics (particle processes) ) on the neutrino and the nucleus. Because of its very small mass, the neutrino receives almost all of the available kinetic energy.

The emitted neutrinos therefore show a discrete energy spectrum (line spectrum), depending on the energy level in which the nucleus remains. If the nucleus then returns to its basic state, the remaining energy is emitted as a photon ( gamma radiation ).

The hole in the inner shell of the electron shell created by the captured electron is reoccupied by an electron from an outer shell. There is a spontaneous emission of an X-ray photon, or the energy released is given off as the kinetic energy of an Auger electron .

Probability of decay

Like internal conversion , electron capture is also considered a type of radioactivity . However, its probability depends not only on the internal properties of the mother nucleus but also on the conditions of the shell, namely the probability of the electrons being at the location of the nucleus. Its probability of decay and thus its half-life can therefore be influenced somewhat by changing the chemical bond of the atom. Changes up to the order of percent were observed experimentally.

Examples

Electron capture besides decay: ${\displaystyle \beta ^{\operatorname {+} }}$

${\displaystyle \mathrm {{}_{13}^{26}Al} +\mathrm {e} ^{-}\rightarrow \mathrm {{}_{12}^{26}Mg} +{\nu }_{e}}$

${\displaystyle \mathrm {{}_{28}^{59}Ni} +\mathrm {e} ^{-}\rightarrow \mathrm {{}_{27}^{59}Co} +{\nu }_{e}}$

${\displaystyle \mathrm {{}_{\ 7}^{13}N} +\mathrm {e} ^{-}\rightarrow \mathrm {{}_{\ 6}^{13}C} +{\nu }_{e}}$

${\displaystyle \mathrm {{}_{\ 9}^{18}F} +\mathrm {e} ^{-}\rightarrow \mathrm {{}_{\ 8}^{18}O} +{\nu }_{e}}$

${\displaystyle \mathrm {{}_{\ 49}^{110}In} +\mathrm {e} ^{-}\rightarrow \mathrm {{}_{\ 48}^{110}Cd} +{\nu }_{e}}$

Electron capture only, no decay: ${\displaystyle \beta ^{+}}$

${\displaystyle \mathrm {{}_{\ 82}^{205}Pb} +\mathrm {e} ^{-}\rightarrow \mathrm {{}_{\ 81}^{205}Tl} +{\nu }_{e}}$

With some nuclides, electron capture occurs as a third decay channel in addition to and decay, e.g. B. with potassium -40: ${\displaystyle \beta ^{-}\!}$ ${\displaystyle \beta ^{+}\!}$

${\displaystyle \mathrm {{}_{19}^{40}K} +\mathrm {e} ^{-}\rightarrow \mathrm {{}_{18}^{40}Ar} +{\nu }_{e}\quad }$ (Share: 11%)

${\displaystyle \mathrm {{}_{19}^{40}K} \rightarrow \mathrm {{}_{20}^{40}Ca} +\mathrm {e} ^{-}+{\overline {{\nu }_{e}}}\quad }$ (Proportion: 89%)

${\displaystyle \mathrm {{}_{19}^{40}K} \rightarrow \mathrm {{}_{18}^{40}Ar} +\mathrm {e} ^{+}+{\nu }_{e}\quad }$ (Share: 0.001%)

A special case is double electron capture (analogous to double beta decay ). It was first observed in 2019:

${\displaystyle \mathrm {{}_{\ 54}^{124}Xe} +2\mathrm {e} ^{-}\rightarrow \mathrm {{}_{\ 52}^{124}Te} +2{\nu }_{e}}$

Individual evidence

1. ↑ H. Krieger, W. Petzold: radiation physics, dosimetry and radiation protection . Volume 1. 3rd edition, Teubner 1992, ISBN 978-3-519-23052-6 , page 63
2. ^ GT Emery, Perturbation of Nuclear Decay Rates, Annu. Rev. Nucl. Sci. 22 (1972) pp. 165-202
3. ↑ Nadja Podbregar: The rarest decay of the universe. April 25, 2019, accessed May 2, 2019 . 
4. ^ Robert Gast: Spectrum of Science, 18 trillion year half-life. April 24, 2019, accessed May 2, 2019 . 

See also

• Nuclear chemistry
• Nuclide map