The curie (Ci) and becquerel (Bq) are units of activity and describe the amount of radioactivity present. A radioisotope's activity is a function of its rate of decay or number of disintegrations per second. The roentgen (R) is a unit of exposure and measures the amount of x or gamma radiation that produces a given number of ionizations in air. The roentgen is not applicable to particulate types of radiation. Many radiation survey instruments record exposure rate per hour of x or gamma radiation. The SI equivalent of roentgen is coulomb per kilogram (coul/kg). The rad (r) and gray (Gy) are units of absorbed dose and reflect the amount of energy that the radiation imparts to matter through which it passes. These units are used for photon as well as particulate radiation and are not restricted to measurement in air.
Because the different types of radiation have different magnitudes of biological effect on man, units of dose-equivalent are used to provide a common scale of measurement for the different types of radiation. The rem and sievert (Sv) are units of dose-equivalent and are obtained by multiplying the absorbed dose by modifying factors. Differentiation between units of absorbed dose and dose equivalent is most important with high LET radiation, particularly alpha radiation. Because of the large amount of energy that alpha particles deposit in a concentrated area, alpha's potential for biological damage is greater. The dose equivalent of alpha radiation is approximately 20 times the absorbed dose. In other words, one rad (or gray) of alpha radiation is approximately equivalent to 20 rem (or sievert).
For beta, x, and gamma radiation, the dose equivalent is essentially equal to the absorbed dose. In simplified terms, a rad (or gray) of beta, x, or gamma radiation is roughly equivalent to a rem (or sievert). For these types of low LET radiation, units of absorbed dose and dose-equivalent are often used interchangeably.
Body burden is used in reference to internally deposited radioactive material. Different radionuclides will deliver varying amounts of radiation to the body when internalized. The amount of radioactivity that may be present in the body for a working lifetime and pose no reasonable expectation of health risk is referred to as the Maximum Permissible Body Burden (MPBB). The amount of internally deposited radioactive material is quantified as a percentage of the MPBB that has been established for the particular radionuclide. The MPBB is based on continuous exposure in a working lifetime, thus the MPBB must be interpreted with caution in an accident setting involving acute exposure. Annual Limit of Intake (ALI) is a newer term that is also used to quantify internally deposited radioactive material.
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