Radiation: Quantities and Units

• Radiation quantities and units are essential for quantifying and assessing the effects of radiation.

Activity

• Activity refers to the rate at which a radioactive substance decays.
• It is defined as the number of nuclear disintegrations occurring per unit of time.
• The SI unit of activity is the becquerel (Bq), defined as one disintegration per second.
• The traditional unit of activity is the curie (Ci), where 1 Ci = 3.7 × 10^10 Bq.
• Activity is used to quantify the amount of radioactive material.
• High activity means a higher rate of decay, indicating a greater potential for radiation exposure.

Exposure

• Exposure is a measure of the ionization produced in air by X-rays or gamma rays.
• It is defined as the electric charge produced per unit mass of air.
• The SI unit for exposure is coulombs per kilogram (C/kg).
• The traditional unit of exposure is the roentgen (R), where 1 R = 2.58 × 10^-4 C/kg.
• Exposure is specifically defined for photons interacting with air.
• It does not describe the energy absorbed by the exposed material but rather the ionization in air.

Absorbed Dose

• Absorbed dose is the amount of energy deposited by ionizing radiation per unit mass of a substance.
• It is a fundamental quantity in radiation dosimetry, applicable to all types of radiation and matter.
• The SI unit of absorbed dose is the gray (Gy), defined as one joule per kilogram (1 Gy = 1 J/kg).
• The traditional unit of absorbed dose is the rad, where 1 rad = 0.01 Gy.
• Absorbed dose directly relates to the potential for biological effects in irradiated tissue.
• Different materials absorb different amounts of energy from the same radiation field.

Equivalent Dose

• Equivalent dose is a measure of the biological effect of radiation, accounting for the type of radiation.
• It is calculated by multiplying the absorbed dose by a radiation weighting factor (wR), which represents the relative biological effectiveness of different types of radiation.
• The SI unit of equivalent dose is the sievert (Sv), defined as joules per kilogram (J/kg), similar to the gray.
• The traditional unit of equivalent dose is the rem, where 1 rem = 0.01 Sv.
• Equivalent dose allows comparison of the biological effects of different types of radiation, such as alpha, beta, and gamma.
• The radiation weighting factor (wR) is dimensionless.

Effective Dose

• Effective dose is a measure of the overall risk of inducing cancer and hereditary effects from exposure to ionizing radiation.
• It takes into account both the type of radiation and the sensitivity of different organs and tissues.
• It is calculated by summing the equivalent doses to individual organs and tissues, each weighted by a tissue weighting factor (wT).
• The SI unit of effective dose is the sievert (Sv).
• The effective dose provides a single value that represents the total radiation risk to the whole body.
• Tissue weighting factors (wT) reflect the relative sensitivity of different organs to radiation-induced cancer and hereditary effects and are dimensionless.
• Effective dose is used in radiation protection to set dose limits and assess the impact of radiation exposure.
• It provides a standardized way to estimate the long-term health risks associated with radiation exposure.
• The concept of effective dose is particularly useful in situations involving non-uniform exposure of the body.