General considerations regarding the dosimetry of roentgen and gamma radiation

Summary

A distinction is made between the quantities “irradiation”, which characterises the roentgen radiation at a certain place as to its capacity of producing ions in air and which is measured in roentgen, and “dose”, which characterises the energy imparted to the material subjected to the roentgen radiation and which is measured in ergs per gramme or multiples of ergs per gramme.

The methods by which both quantities can be measured are analysed. A measurement of irradiation at photon energies above 3 MeV has not yet been realised.

The correlation between irradiation and dose is discussed: Irradiation is in air-equivalent or nearly air-equivalent materials at moderate photon energies, a fair measure of dose; in non air-equivalent tissue, the differences in mass absorption coefficient between this tissue and air should be taken into account; at discontinuities in the atomic composition the increased generation of secondary electrons in materials with higher atomic number will also cause an increased ionization in neighbouring tissues with lower atomic number; at photon energies above 1 MeV there is an increasing discrepancy between irradiation and dose at the same place.