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Internal Conversion Coefficient – ICC

Internal Conversion - ICE plus Auger
Internal conversion followed by Auger electron emission.

Internal conversion is an electromagnetic process by which an excited nuclear state decays by the direct emission of one of its atomic electrons. Internal conversion competes with gamma emission, but in this case, the electromagnetic multipole fields of the nucleus do not result in the emission of a gamma-ray. Instead, the fields interact directly with atomic electrons.

The internal conversion coefficient (ICC), α, characterizes the competition between internal conversion and gamma-ray emission. In some cases, internal conversion is favored over gamma decay; in others, it may be completely negligible. The internal conversion coefficient is defined as the ratio of the number of internal conversions decays to the number of gamma decays. This ICC is defined for each electron shell (i.e., the K, L, and M shells, etc), such that the total ratio, αtotal, is the sum of the ICCs for each shell as:

αtotal = αK +  αL + αM = number of IC / number of gamma decays

For example, in the decay of the excited state at 35 keV of 125Te (which is produced by the decay of 125I), 7% of the decays emit gamma-ray, while 93% emit a conversion electrons. Therefore, an internal conversion coefficient of this excited state (125Te) is ICC = 93/7 = 13.3.

Using the Band-Raman Internal Conversion Coefficient calculator, the ICCs can be calculated using principles of atomic physics since it depends primarily on the density of the atomic electrons at the center of the nucleus. Internal conversion coefficients are observed to increase for increasing atomic number (Z) and decreasing gamma-ray energy.

References:

Radiation Protection:

  1. Knoll, Glenn F., Radiation Detection and Measurement 4th Edition, Wiley, 8/2010. ISBN-13: 978-0470131480.
  2. Stabin, Michael G., Radiation Protection and Dosimetry: An Introduction to Health Physics, Springer, 10/2010. ISBN-13: 978-1441923912.
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  9. Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.

See above:

Internal Conversion