CEMS Decay Scheme

A nucleus that is promoted to an excited state by gamma ray absorption can de-excite by a number of mechanisms. These fall into two categories

1. Radiative: by the emission of a gamma ray with a probability of $N(\gamma)$
2. Non-Radiative: by internal conversion and the ejection of an atomic electron with a probability of $N(e)$

The ratio of these two process is given by the internal conversion coefficient, $\alpha$, given by

$$\alpha = \frac{N(e)}{N(\gamma)}$$ (2.28)

which for $^{57}$Fe is 8.21, ie internal conversion is 8.21 times more probable than photon emission.[7]

The conversion electron is ejected from the atom with an energy $E_{c} = E_{\gamma} - E_{b}$ where $E_{\gamma}$ is the energy of the transition and $E_{b}$ is the binding energy of the electron. In $^{57}$Fe the internal conversion can occur from K, L and M shells, in order of probability. This ejection leaves a hole which can be filled by an electron from an outer shell. This releases energy in the form of an X-ray or an Auger electron, with the process continuing in this manner until all of the energy has been dissipated. The principal decay scheme is shown in Figure 2.7.

Figure 2.7: Decay scheme of $^{57}$Fe following excitation of the $14.41\ensuremath{\unskip\,\mathrm{keV}}$ state.

To exploit the decay of the resonantly excited nucleus the conversion and Auger electrons need to be detected. This is achieved by placing the sample inside a gas-flow proportional counter as shown in Section 4.1.3.