Radiation Protection Quantities and Units

Introduction to Dosimetry Quantities

In the field of radiation protection, dosimetry is used to quantify the radiation dose absorbed by tissues or organs. The basic dosimetry quantity is the mean organ or tissue dose, denoted as \( D_T \), which is used to measure the energy imparted to a specific organ or tissue. This quantity is given by the following equation:

\[ D_T = \frac{ \epsilon_T }{ m_T } \]

Where:

• \( m_T \) is the mass of the organ or tissue \( T \),
• \( \epsilon_T \) is the total energy imparted by radiation to that tissue or organ.

The unit of the mean organ dose is the joule per kilogram (J/kg), which is also known as the gray (Gy) in the International System of Units (SI).

Types of Radiation Protection Quantities

Different types of ionizing radiation vary in their effectiveness at causing damage to human tissue at the same dose. As such, it is necessary to introduce additional quantities to account for these differences in biological effectiveness. The primary quantities used for this purpose are:

• Equivalent Dose – This quantity accounts for the biological effectiveness of different types of radiation by multiplying the absorbed dose by a radiation weighting factor \( w_R \), specific to each radiation type. It is used to quantify the potential harm caused by radiation exposure to human tissues.
• Effective Dose – This takes the equivalent dose a step further by weighing the equivalent doses to different organs or tissues by a tissue weighting factor \( w_T \), which reflects the relative sensitivity of each tissue or organ to radiation.

Since these quantities cannot be measured directly, the International Commission on Radiation Units and Measurements (ICRU) has defined a set of operational quantities for radiation protection. These include:

• Ambient Dose Equivalent – A measure of the radiation dose received by an individual from external sources in their environment.
• Directional Dose Equivalent – This is used to account for the directional nature of radiation exposure, often relevant in specific monitoring situations.
• Personal Dose Equivalent – This quantity is used to monitor the dose received by an individual from external radiation sources, typically in personal dosimetry.

Internal Exposure and Committed Dose Quantities

In the case of internal exposure, such as exposure to radionuclides that enter the body, the equivalent and effective doses are not only influenced by the physical properties of the radiation but also by how the radionuclide is biologically processed in the body. These biological factors include the retention time of the radionuclide in the body and its rate of turnover within tissues.

The ICRU takes these factors into account by defining committed dose quantities, which refer to the dose an individual is expected to receive over time following the intake of a radionuclide. These are expressed as:

• Committed Equivalent Dose – The dose that will be received by a specific organ or tissue over a specified period of time following the intake of a radionuclide.
• Committed Effective Dose – This is the total effective dose to the individual from the intake of a radionuclide, accounting for the different sensitivities of various tissues and organs.

These quantities are critical for evaluating the long-term health effects of radionuclide exposure and are used for assessing the risks associated with internal radiation sources.

Summary of Radiation Protection Quantities and Units