What is Radiation?
Radiation means ionizing radiation: alpha particles, beta particles, gamma rays, X-rays, neutrons, high-speed electrons, high-speed protons, and other particles capable of producing ions. Radiation, as used in this document, does not include non-ionizing radiation, such as radio waves or microwaves, or visible, infrared, or ultraviolet light.
What is Radiation Dose?
Radiation Dose refers to the amount of energy deposited in matter by ionizing radiation per unit mass. When ionizing radiation interacts with atoms, it can dislodge electrons, creating charged particles and potentially causing biological damage. Quantifying this energy deposition is fundamental for assessing radiation risks and implementing appropriate safety measures.
Absorbed Dose
Absorbed dose measures the amount of energy imparted by ionizing radiation to a unit mass of irradiated material. The standard unit for absorbed dose is the gray (Gy), where 1 gray represents the absorption of 1 joule of radiation energy per kilogram of matter. Absorbed dose provides a basic understanding of the physical impact of radiation on a material or tissue.
Equivalent Dose
Equivalent dose considers not only the absorbed dose but also the biological effectiveness of different types of radiation. It is calculated by multiplying the absorbed dose by a radiation weighting factor, reflecting the radiation type’s relative biological effectiveness (RBE). The unit of equivalent dose is rem or sievert (Sv), with 1 sievert equal to 1 gray multiplied by the radiation weighting factor. This adjustment accounts for the varying abilities of different radiation types to cause biological damage.
For external dose, the equivalent dose to the whole body is assessed at a depth of 1 cm in tissue; the equivalent dose to the lens of the eye is assessed at a depth of 0.3 cm in tissue, and the equivalent dose to the extremity and skin is assessed at a depth of 0.007 cm in tissue.
Effective Dose
Effective dose considers both the type and amount of radiation received and the sensitivity of the irradiated tissues or organs. It is calculated by multiplying the equivalent dose for each tissue or organ by a tissue weighting factor, reflecting the relative sensitivity of that tissue to radiation-induced damage. The sum of these weighted equivalent doses estimates the overall risk of radiation exposure to an individual.
Units of Dose
Gray (Gy) is the SI unit of absorbed dose. One gray is equal to an absorbed dose of 1 Joule/kilogram(100 rads).
Sievert is the SI unit of any of the quantities expressed as dose equivalent. The dose equivalent in sieverts is equal to the absorbed dose in grays multiplied by the quality factor (1 Sv=100 rems).
Rad is the special unit of absorbed dose. One rad is equal to an absorbed dose of 100 ergs/gram or 0.01 joule/kilogram (0.01 gray).
Rem is the special unit of any of the quantities expressed as dose equivalent. The dose equivalent in rems is equal to the absorbed dose in rads multiplied by the quality factor. (1 rem = 0.01 sievert = 1000 mrem)
Dosimeters: Monitoring Radiation Exposure
A dosimeter typically measures the amount, type and energy of ionizing radiation an individual is exposed to. While small amounts of ionizing radiation exposure are typical (even outside of the workplace), the more exposure you have to ionizing radiation, the more likely it is to become dangerous over time. It’s important that workplaces with potential for ionizing radiation exposure are monitoring this on a regular basis.
For more information about occupational dose limits, download our guide here.
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