What is radiation dose?

When radiation interacts with matter, it deposits energy.  Radiation dose is defined as the energy deposited by radiation per unit mass of material. There is a special unit for this quantity: the gray (often abbreviated as “Gy”), which is equal to one joule per kilogram.

The same amount of radiation dose will not necessarily have the same effect on different parts of the body because not all organs are equally sensitive to radiation. Some organs that are more sensitive to radiation are the gonads, colon, bone marrow, lung, and stomach.

The effects of radiation on humans depend not only on dose, but also on the type of radiation involved. The quantity “dose equivalent” takes this into account. The unit for dose equivalent is also joules per kilogram, but is given the name sievert (Sv) to indicate that its value has been adjusted to account for effects due to the type of radiation.

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How is radiation measured?

There are a number of different detectors that can be used for radiation measurements.  The type of detector chosen depends on the purpose of the measurement, the nature of the radiation, and the sensitivity requirements.

Survey meters are often used for radiation protection surveys.  Examples include ionization chambers, Geiger-Muller (GM) counters, scintillation detectors, and neutron rem detectors.  Each class of detectors exhibits a level of response and sensitivity that depends on the design of the detector and the detailed physics of how radiation interacts with the device. In most cases the devices measure light or electrical charge produced when radiation deposits energy in the detector. The radiation dose can then be determined from the device reading.

Wearable detectors are often used for monitoring radiation dose to people who work closely with radiation. Examples of personnel monitors include film, thermoluminescent dosimeters (TLDs), optically stimulated luminescence dosimeters (OSLDs), and pocket dosimeters (a type of ionization chamber).

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