Inner conversion

This article deals with the nuclear physical decay. For the photochemical effect see internal transformation .

As internal conversion (engl. Internal conversion , IC ) is a special case of in physics radioactivity , respectively. In atomic nuclei , it occurs in an excited state and, in addition to gamma decay, is a possibility of leaving the excited state. However, no gamma quantum is emitted, but a shell electron to which the energy was transferred. The energy that is released when the nucleus moves into a lower excited state is transferred to a shell electron (the “conversion electron ”) through direct electromagnetic interaction . The electron leaves the atom with the transferred energy reduced by its binding energy. As a result, the electron has a different energy depending on the shell from which it comes.

The affected shell is refilled by pushing up shell electrons from higher shells. The binding energy released is either emitted as characteristic X-rays or through the ejection of Auger electrons . In both cases the atom remains as a (singly or doubly) positively charged ion .

Type of interaction

The internal conversion is not a two-stage process in which the nucleus first emits a gamma quantum and this then transfers the energy to a shell electron by impact. This is shown experimentally in the fact that conversion electrons are also observed at transitions where gamma emission is not possible ("forbidden") due to the conservation of angular momentum. Examples are the doubly magic nuclides oxygen-16 and calcium-40, in which the first excited state, like the ground state, has nuclear spin zero and positive parity so that no gamma quantum can be emitted.

Energy spectrum of the conversion electrons

Decay scheme of ²⁰³ Hg: β ^- decay with 214 keV and γ decay with 279 keV

Electronic spectrum of ²⁰³ Hg
(Wapstra et al., Physics 20 (1954) 169)

The kinetic energy E _{e of} the emitted conversion electron is the difference between the energy E _γ transferred by the nucleus and the original binding energy E _{B, shell of} the ejected shell electron according to its shell:

${\displaystyle E_{\mathrm {e} }=E_{\gamma }-E_{\mathrm {B,Schale} }}$   with shell = K, L, M, ...

The conversion electrons show, unlike electrons from beta decay , a line spectrum with several discrete lines. Depending on whether the electron was bound in the K, L etc. shell, one speaks of K, L etc. conversion.

Example: ²⁰³ Hg

As you can see from the decay scheme on the right, ²⁰³ Hg decays in two stages: First a beta decay into an excited nucleus, then the nucleus normally releases its remaining energy as a gamma quantum. The beta decay generates a continuous beta spectrum with a maximum energy of 214 keV and leads to an excited state of the daughter nucleus ²⁰³ Tl. This decays in 2.8 · 10 ⁻¹⁰  s by emitting a gamma quantum of 279 keV to the ground state of ²⁰³ Tl.

${\displaystyle {\begin{array}{lllll}{}_{80}^{203}\mathrm {Hg} &\to &{}_{81}^{203}\mathrm {Tl} ^{*+}+\mathrm {e} ^{-}\mathrm {+} \ {\overline {\nu }}_{e}&,\ \Delta E=\mathrm {214\,keV} ,&\mathrm {(\beta ^{-})\ \ und} \\\\{}_{81}^{203}\mathrm {Tl} ^{*}&\to &{}_{81}^{203}\mathrm {Tl} ^{+}+\mathrm {e} ^{-}&,\ \Delta E=\mathrm {279\,keV} ,&\mathrm {(IC)\ \ oder} \\{}_{81}^{203}\mathrm {Tl} ^{*}&\to &{}_{81}^{203}\mathrm {Tl} \ \ +\gamma &,\ \Delta E=\mathrm {279\,keV} ,&\mathrm {(IT).} \\\end{array}}}$

The electron spectrum measured with the help of a magnetic spectrometer can be seen on the right. It shows the continuous beta spectrum on the one hand , and the K, L, and M lines of the internal conversion on the other. Since the binding energy of the K electrons in the ²⁰³ Tl is around 85 keV, the K line is at 279 keV - 85 keV = 194 keV; the L and M lines are due to the lower binding energy to be overcome at 258 keV and 270 keV, respectively. Because of the limited energy resolution of the spectrometer, the L and M lines are not separated, but can only be recognized by the asymmetrical curve shape of the peak .

Probability of decay

Conversion coefficients for E ₀ transitions for Z = 40, 60, and 80 according to the tables by Sliv and Band, as a function of the transition energy

Since the internal conversion occurs as an alternative to the gamma emission, the total decay probability of the initial core state per unit of time is the sum of the two individual probabilities: ${\displaystyle \lambda }$

${\displaystyle \lambda =\lambda _{\gamma }+\lambda _{\mathrm {e} }}$

The ratio is called the conversion coefficient . As the picture shows, it increases with increasing atomic number Z and decreasing energy. ${\displaystyle \lambda _{\mathrm {e} }/\lambda _{\gamma }}$

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

Individual evidence

1. ^ Bernard L. Cohen: Concepts of Nuclear Physics. New York, etc .: McGraw-Hill 1971, p. 298.
2. ↑ LA Sliv and IM Band, Table of Internal Conversion Coefficients, in: Alpha-, Beta- and Gamma-Ray Spectroscopy, ed. By Kai Siegbahn, North-Holland Publishing (1966), Vol. 2, Appendix.
3. ^ GT Emery, Perturbation of Nuclear Decay Rates, Annu. Rev. Nucl. Sci. 22 (1972) pp. 165-202.

literature

• Theo Mayer-Kuckuk: Nuclear Physics , Teubner Study Books , Stuttgart 1992, ISBN 3-519-43021-5 .
• H. Krieger: Fundamentals of radiation physics and radiation protection , BG, Teubner Verlag, 2007, ISBN 3-519-00487-9
• Hans G. Bucka : Nukleonenphysik (594 pages), Walter de Gruyter, Berlin 1981, ISBN 978-3-11-005751-5