Internal Conversion - When The Process Is Expected

When The Process Is Expected

Internal conversion is favoured when the energy gap between nuclear levels is small, and is also the primary mode of de-excitation for 0+→0+ (i.e. E0) transitions. These occur where an excited nucleus has zero spin, and thus is unable to rid itself of energy by gamma emission, but also has insufficient decay energy to decay by internal pair creation. Internal conversion is also the predominant mode of de-excitation whenever the initial and final spin states are the same (but with other different quantum numbers); however the multi-polarity rules for non-zero initial and final spin states do not necessarily forbid the competing de-excitation by emission of a gamma ray in such cases.

The tendency towards internal conversion can be expressed by the internal conversion coefficient, which is empirically determined by the ratio of de-excitations that go by the emission of electrons to those that go by gamma emission.

The internal conversion process competes with gamma decay. This competition is quantified in the form of the internal conversion coefficient which is defined as where is the rate of conversion electrons and is the rate of gamma-ray emission observed from a decaying nucleus. For example, in the decay of an excited state of the nucleus of 125I, 7% of the decays emit energy as a gamma ray, while 93% release energy as conversion electrons. Therefore, this excited state of 125I has an internal conversion coefficient of . Internal conversion coefficients are observed to increase for increasing atomic number (Z) and decreasing gamma-ray energy. As one example, IC coefficients are calculated explicitly for 55Fe, 67Ga, 99mTc, 111In, 113mIn, 115mIn, 123I, 125I, 193mPt, 201Tl and 203Pb by Howell (1992) using Monte Carlo methods (for 55Fe the IC coefficient is zero).

The energy of the emitted gamma-ray is regarded as a precise measure of the difference in energy between the excited states of the decaying nucleus. However, this is not true in the case of conversion electrons. The energy of a conversion electron is given as where and are the energies of the nucleus in its initial and final states, respectively, while is the binding energy of the electron.