14
Internal Radiation Dosimetry
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Radiation can cause detrimental effects on human tissues, and these effects
depend on various factors, such as dose, dose rate, time of exposure and so
on. This chapter describes the method of calculating absorbed doses in
various organs from radionuclides ingested internally either purposely (e.g.,
medical procedures) or accidentally.
Radiation Units
Three units of measure are related to radiation: the roentgen (R) for expo-
sure, the rad (radiation absorbed dose) for absorbed dose, and the rem
(roentgen equivalent man) for dose equivalent.
The roentgenis the amount of x- or g-radiation that produces ionization
of one electrostatic unit of either positive or negative charge per cubic cen-
timeter of air at 0°C and 760 mm Hg, standard temperature and pressure
(STP). Because 1 cm^3 air weights 0.001293 g at STP and a charge of either
sign carries 1.6 × 10 −^19 C or 4.8 × 10 −^10 electrostatic units, it can be shown
that
1R =2.58 × 10 −^4 C/kg (14.1)It should be noted that the roentgen applies only to air and to x- or g-
radiations. Because of practical limitations of the measuring instruments,
the R unit is applicable only to photons of less than 3 MeV energy.
The radis a more universal unit. It is a measure of the energy deposited
per unit mass of any material by any type of radiation. The rad is specifi-
cally defined as
1 rad =100 ergs/g absorber (14.2)Since 1 joule (J) = 107 ergs,
1 rad = 10 −^2 J/kg (14.3)