#### Learning Objective

• Define the terms used to define radiation exposure, the gray (Gy) and sievert (Sv)

#### Key Points

• Radiation dosimetry is the measurement and calculation of the absorbed dose in matter and tissue resulting from exposure to indirect and direct ionizing radiation.
• Dose is reported in gray (Gy) for matter or sieverts (Sv) for biological tissue.
• There are several ways of measuring doses from ionizing radiation, including personal dosimeters and ionization chambers.

#### Terms

• grayThe derived SI unit of radiation dose defined as the absorption of one joule of radiation energy by one kilogram of matter. Symbol: Gy.
• sievertThe derived SI unit of radiation dose that is a measure of the health effect of low levels of ionizing radiation on the human body. Symbol: Sv.
• dosimetryThe measurement of doses, especially of ionizing radiation.
• jouleThe derived SI unit of energy, work, and heat; equal to the energy of one watt of power for a duration of one second; symbol: J.

Radiation dosimetry is the measurement and calculation of the absorbed dose in matter and tissue resulting from exposure to indirect and direct ionizing radiation. It is a scientific subspecialty in the fields of health physics and medical physics that is focused on the calculation of internal and external doses from ionizing radiation.

## Gray vs. Sieverts

Dose is reported in gray (Gy) for matter or sieverts (Sv) for biological tissue, where 1 Gy or 1 Sv is equal to 1 joule per kilogram. Non-SI units are still prevalent as well, where dose is often reported in rads, and dose equivalent in rems.

Radiation dose refers to the amount of energy deposited in matter and/or biological effects of radiation. It should not be confused with the unit of radioactive activity, the becquerel (Bq). Exposure to a radioactive source will give a dose that is dependent on the activity, time of exposure, energy of the radiation emitted, distance from the source, and shielding. The equivalent dose is then dependent upon these weighting factors. Dose is a measure of deposited dose, and therefore can never go down.

## Tools for Measuring Ionizing Radiation

The worldwide average background dose for a human being is about 3.5 mSv per year, mostly from cosmic radiation and natural isotopes in the earth. There are several ways of measuring doses from ionizing radiation, including personal dosimeters and ionization chambers. There are a number of types of wearable dosimeters: quartz fiber, film badge, thermoluminescent, and solid state. People use these dosimeters to keep track of exposure, typically if they expect to come in contact with radioactive substances as part of their jobs.