balance reveals a discrepancy which can be accounted for if the emission of a further particle – the
neutrino, v is postulated. Overall, positron emission can be summarized in a general equation
K-electron Capture
An unstable, low n:p ratio may also be adjusted by the capture of an orbital electron, which would
naturally involve the nearest shell, K-shell, of electrons
The vacancy left in the low-energy K-shell will be filled by an electron from a higher level with the
resultant emission of radiations of extra nuclear origin, X-rays, which are distinguished from the
accompanying nuclear γ radiation. It should be noted that the n:p criterion is the same for both positron
and electron capture processes and it is not unusual to find both occurring with different atoms of the
same nuclide.
Internal Conversion
In some limited circumstances, surplus nuclear energy may be transferred directly to an orbital electron.
The electron is then expelled from the atom and superficially resembles a negatron. Usually such
'conversion electrons' will have relatively low energies, but the important difference from the negatron
is their extra nuclear origin. The process of energy transfer is ill-understood but it should be emphasized
that it involves the direct transfer of energy from a nuclear orbital to an extra nuclear electronic orbital
and does not involve a γ-photon as was at one time believed. This reaction is commonly observed with
unstable isomeric nuclear states which also decay by the emission of low energy γ-photons.
Gamma Decay (Isomeric Transition)
The emission of electromagnetic γ-radiation is the natural accompaniment of most nuclear processes
and provides the route for an excited nuclear isomer to decay to its ground state. A typical decay
sequence is set out in Figure 10.4.
The γ-energies are characteristic of the nuclear energy levels and transitions. Figure 10.12 shows the
appearance of a γ-ray spectrum. Such spectra are of immense value in the identification and
determination of radionuclides in a radioactive mixture.
The Kinetics of Decay Reactions
The rate at which radioactive atoms decay is unaffected by the chemical or physical form of the
nucleide and depends only on the number N of atoms present and the decay constant λ/s–^1 for that
particular nuclide. In a single