Radiation: Quantities and Units

• Radiation quantities and units are essential for quantifying radiation and its effects.
• They provide a standardized way to measure and communicate radiation levels.
• Radiation quantities are measurable physical quantities used to characterize ionizing radiation fields or the effects of radiation on matter.
• Units provide a scale for expressing the magnitude of these quantities.
• The International System of Units (SI) is the globally recognized standard.
• There are also traditional units that may be encountered.

Activity

• Activity (A) is the rate at which a radioactive material emits nuclear radiations.
• Activity refers to the number of nuclear decays per unit time.
• The SI unit of activity is the becquerel (Bq).
• 1 Bq is defined as one decay per second.
• Traditional unit for activity is the curie (Ci).
• 1 Ci = 3.7 x 10^10 Bq.
• Activity is an intrinsic property of a radioactive substance.
• It depends only on the number of radioactive atoms present and their half-life.

Exposure

• Exposure (X) quantifies the amount of ionization produced in air by X-rays or gamma rays.
• It is defined as the electric charge of ions of one sign produced in a unit mass of air.
• Exposure applies only to photons (X-rays and gamma rays).
• The SI unit of exposure is the coulomb per kilogram (C/kg).
• The traditional unit of exposure is the roentgen (R).
• 1 R = 2.58 x 10^-4 C/kg.
• Exposure is a measure of the radiation field at a point in air.
• It does not describe energy absorbed.

Absorbed Dose

• Absorbed dose (D) is the energy absorbed per unit mass of any material by any type of ionizing radiation.
• It quantifies the energy deposited by radiation in a substance.
• The SI unit of absorbed dose is the gray (Gy).
• 1 Gy is defined as one joule of energy absorbed per kilogram of material (1 J/kg).
• The traditional unit of absorbed dose is the rad.
• 1 rad = 0.01 Gy.
• Absorbed dose is a fundamental quantity in radiation dosimetry.
• It is used to relate radiation exposure to biological effects.

Equivalent Dose

• Equivalent dose (H) accounts for the differing biological effectiveness of different types of radiation.
• It is calculated by multiplying the absorbed dose by a radiation weighting factor (wR).
• The radiation weighting factor reflects the relative biological effectiveness (RBE) of the radiation.
• The SI unit of equivalent dose is the sievert (Sv).
• H = D x wR
• The traditional unit of equivalent dose is the rem.
• 1 Sv = 100 rem.
• Equivalent dose provides a measure of the potential harm from radiation.
• It allows comparison of risks from different types of radiation.
• Radiation weighting factors (wR) are dimensionless.
• wR values are based on experimental data and expert judgment.
• Common wR values include: X-rays, gamma rays, and beta particles (wR = 1); alpha particles (wR = 20).

Effective Dose

• Effective dose (E) accounts for the varying sensitivity of different organs and tissues to radiation.
• It is calculated by summing the equivalent doses to individual organs or tissues, weighted by tissue weighting factors (wT).
• The tissue weighting factor represents the relative contribution of that organ or tissue to the overall risk of stochastic effects.
• The SI unit of effective dose is the sievert (Sv).
• E = Σ (HT x wT)
• Effective dose is an indicator of overall risk from radiation exposure.
• It is used for radiation protection purposes.
• Tissue weighting factors (wT) are dimensionless.
• wT values sum to 1 over all organs and tissues.
• Common wT values include: gonads (wT = 0.08), red bone marrow (wT = 0.12), lung (wT = 0.12), thyroid (wT = 0.04).

Quantities for Specific Applications

• Air Kerma is the kinetic energy released per unit mass of air.
• It is used in radiation therapy and detector calibration.
• Kerma stands for Kinetic Energy Released in MAtter.
• The SI unit is Gray (Gy).
• Lineal Energy is the energy deposited per unit length by ionizing radiation.
• It is used in microdosimetry.
• The SI unit is keV/μm.

Committed Dose

• Committed Equivalent Dose is the time integral of the equivalent dose rate in a particular tissue or organ after intake of radioactive material into the body.
• Committed Effective Dose is the sum of the products of the committed organ or tissue equivalent doses and the appropriate tissue weighting factors (WT).

Relationships

• Exposure is related to absorbed dose in air.
• Absorbed dose is related to equivalent dose by weighting factor.
• Effective dose is related to equivalent dose through tissue weighting factor.