Internal conversion

[1][2] Thus, in internal conversion (often abbreviated IC), a high-energy electron is emitted from the excited atom, but not from the nucleus.

The atom thus emits high-energy electrons and X-ray photons, none of which originate in that nucleus.

The atom supplies the energy needed to eject the electron, which in turn causes the latter events and the other emissions.

Since primary electrons from IC carry a fixed (large) part of the characteristic decay energy, they have a discrete energy spectrum, rather than the spread (continuous) spectrum characteristic of beta particles.

Whereas the energy spectrum of beta particles plots as a broad hump, the energy spectrum of internally converted electrons plots as a single sharp peak (see example below).

Ratios of K-shell to other L, M, or N shell internal conversion probabilities for various nuclides have been prepared.

The decay scheme on the left shows that 203Hg produces a continuous beta spectrum with maximum energy 214 keV, that leads to an excited state of the daughter nucleus 203Tl.

The figure on the right shows the electron spectrum of 203Hg, measured by means of a magnetic spectrometer.

It includes the continuous beta spectrum and K-, L-, and M-lines due to internal conversion.

The 0+→0+ transitions occur where an excited nucleus has zero-spin and positive parity, and decays to a ground state which also has zero-spin and positive parity (such as all nuclides with even number of protons and neutrons).

In such cases, de-excitation cannot take place by emission of a gamma ray, since this would violate conservation of angular momentum, hence other mechanisms like IC predominate.

This also shows that internal conversion (contrary to its name) is not a two-step process where a gamma ray would be first emitted and then converted.

The competition between IC and gamma decay is quantified in the form of the internal conversion coefficient which is defined as

For example, calculated IC coefficients for electric dipole (E1) transitions, for Z = 40, 60, and 80, are shown in the figure.

[5] In this type of decay, an electron and positron are both emitted from the atom at the same time, and conservation of angular momentum is solved by having these two product particles spin in opposite directions.

In IC, however, the process happens within one atom, and without a real intermediate gamma ray.

Electron capture also involves an inner shell electron, which in this case is retained in the nucleus (changing the atomic number) and leaving the atom (not nucleus) in an excited state.