Radiation dosimetry is the measurement and calculation of the absorbed dose in matter and tissue resulting from the exposure to indirect and direct ionizing radiation.
Measuring Radiation
There are several ways of measuring the dose of ionizing radiation. Workers who come in contact with radioactive substances or who may be exposed to radiation routinely carry personal dosimeters . In the United States, these dosimeters usually contain materials that can be used in thermoluminescent dosimetry or optically stimulated luminescence. Outside the United States, the most widely used type of personal dosimeter is the film badge dosimeter, which uses photographic emulsions that are sensitive to ionizing radiation. The equipment used in radiotherapy (a linear particle accelerator in external beam therapy) is routinely calibrated using ionization chambers or the new and more accurate diode technology. Internal dosimetry is used to evaluate the intake of particles inside a human being.
Ionization Chamber
This ionization chamber was used in the South Atlantic Anomaly Probe project.
Personal Radiation Dosimeter
A physician wearing a personal radiation dosimeter
Dose is reported in grays (Gy) for absorbed doses or sieverts (Sv) for dose equivalents, where 1 Gy or 1 Sv is equal to 1 joule per kilogram. Non-SI units are still prevalent as well: absorbed dose is often reported in rads and dose equivalent in rems. By definition, 1 Gy = 100 rad, and 1 Sv = 100 rem.
Biological Effects
The distinction between absorbed dose (Gy/rad) and dose equivalent (Sv/rem) is based upon the biological effects. The weighting factor (wr) and tissue/organ weighting factor (WT) have been established. They compare the relative biological effects of various types of radiation and the susceptibility of different organs.
The weighting factor for the whole body is 1, such that 1 Gy of radiation delivered to the whole body is equal to one sievert. Therefore, the WT for all organs in the whole body must sum to 1.
By definition, X-rays and gamma rays have a wr of unity, such that 1 Gy = 1 Sv (for whole-body irradiation). Values of wr are as high as 20 for alpha particles and neutrons. That is to say, for the same absorbed dose in Gy, alpha particles are 20 times as biologically potent as x-rays or gamma rays.
Dose is a measure of deposited dose and therefore can never decrease: removal of a radioactive source can reduce only the rate of increase of absorbed dose -- never the total absorbed dose.