ABMP Medical Health Physics key terms

The SI unit of activity, equal to one nuclear transformation per second. Because medical radionuclide quantities are large, activity is commonly expressed in megabecquerels or gigabecquerels.

The number of radioactive decays occurring per unit time in a sample, usually stated in becquerels or curies. Activity depends on the number of atoms present and the decay constant of the radionuclide.

The time required for the activity of a radionuclide to decrease to one half of its initial value. Physical half-life describes decay alone, while effective half-life also accounts for biological removal from the body.

The probability per unit time that an individual radionuclide atom will decay. It equals the natural logarithm of two divided by the physical half-life and defines the exponential decay curve.

The combined half-life resulting from both radioactive decay and biological clearance of a substance from the body. It is calculated as the product of the physical and biological half-lives divided by their sum.

The time needed for a living system to eliminate half of an administered substance through physiological processes such as excretion or metabolism, independent of radioactive decay.

A traditional unit of activity equal to 3.7 times ten to the tenth decays per second, originally based on one gram of radium-226. One curie equals 37 gigabecquerels.

High-energy electromagnetic radiation emitted from an excited atomic nucleus during radioactive decay. Gamma rays are highly penetrating and are the principal radiation of concern in nuclear medicine imaging and many therapy sources.

Electromagnetic radiation produced when energetic electrons interact with matter, either through bremsstrahlung or characteristic inner-shell transitions. X-rays originate outside the nucleus, unlike gamma rays.

Radiation produced when a charged particle, typically an electron, is decelerated by the electric field of an atomic nucleus, converting kinetic energy into photons. It dominates the x-ray spectrum at higher tube potentials.

The thickness of a specified material that reduces the intensity of a narrow beam of radiation to half its original value. It is a practical measure of beam quality and shielding effectiveness.

The reduction in photon beam intensity as photons are absorbed or scattered while passing through matter. Attenuation depends on photon energy, material density, and atomic number.

A gas-filled detector operated at high voltage in which each ionizing event produces a large avalanche, giving a count but no energy information. It is useful for contamination surveys and radiation area checks.

A gas-filled detector operated at a voltage where collected charge is proportional to the ion pairs produced, allowing accurate measurement of exposure or dose rate without avalanche amplification.

A portable radiation detection instrument used to measure dose rate or count rate in a work area. Selection of the detector depends on the radiation type and energy expected.

A passive personal monitoring device using a crystalline material that stores energy from radiation and releases it as light when heated. The light output is related to the dose received.

A passive dosimeter that stores radiation energy in a crystalline detector and releases it as luminescence when stimulated by light. It offers good sensitivity and is widely used for occupational monitoring.

A device worn by an occupationally exposed worker to record cumulative radiation dose for regulatory and ALARA purposes. It is typically worn on the torso and may include extremity or collar badges.

The mean energy imparted by ionizing radiation to a unit mass of material, measured in gray. It is a physical quantity that does not by itself account for biological effectiveness.

The SI unit of absorbed dose, equal to one joule of energy deposited per kilogram of material. One gray equals one hundred rad.

A radiation protection quantity obtained by multiplying the average absorbed dose in a tissue by a radiation weighting factor that reflects the biological effectiveness of the radiation type. It is expressed in sieverts.

A whole-body protection quantity that sums equivalent doses to individual tissues after weighting each by its tissue sensitivity factor. It allows comparison of stochastic risk from different exposure patterns.

The SI unit for equivalent dose and effective dose, equal to one joule per kilogram with appropriate weighting factors applied. One sievert equals one hundred rem.

A measure of the ionization produced in air by x-rays or gamma rays, historically expressed in roentgen. It applies only to photons in air and is not a measure of dose in tissue.

A biological effect whose probability increases with dose while its severity does not, typically arising from DNA damage in a single cell. Cancer and heritable effects are considered stochastic.

A tissue reaction that occurs only above a threshold dose and whose severity increases with dose, caused by cell killing or functional damage. Examples include skin erythema and cataract formation.

A radiation protection assumption that stochastic risk increases in direct proportion to dose with no threshold, even at very low doses. It underlies conservative dose limits and ALARA practice.

The principle of keeping radiation doses as low as reasonably achievable, taking economic and social factors into account. It requires optimization of protection rather than merely meeting regulatory limits.

Regulatory maximum permitted radiation doses for occupational workers and members of the public, excluding background and medical exposures. Limits apply to effective dose and to specific organs such as the lens of the eye.

A structured institutional effort combining management commitment, training, planning, and dose tracking to minimize occupational and public exposure beyond simple compliance with numerical limits.

An authorization issued by a regulatory authority permitting possession, use, or transfer of specified radionuclides under defined conditions, including training, security, and recordkeeping requirements.

A qualified individual named on a license who is responsible for implementing the radiation protection program, including procedures, training, audits, and oversight of licensed activities.

A multidisciplinary group that oversees the institution's radiation protection program, reviews policies and incidents, and advises administration on safety and regulatory matters.

A defined work area where occupational exposure is possible and access is limited and supervised for radiation protection purposes. Posting and monitoring requirements apply within it.

An area where access is controlled to protect individuals from exposure to radiation or radioactive materials. In medical facilities it commonly includes imaging and therapy rooms and radionuclide storage spaces.

An accessible area where radiation levels could result in a person receiving a specified dose in one hour, requiring appropriate posting. It is one of several posted area classifications defined by regulation.

In shielding design, the total radiation output or activity used per week in a room, expressed in appropriate units such as milliampere-minutes per week. It is a primary input to barrier thickness calculations.

The fraction of the workload during which the primary beam is directed at a particular barrier. It is used in shielding calculations to account for the direction of beam use.

The fraction of time that an area beyond a barrier is expected to be occupied by persons during the operating schedule. It adjusts shielding requirements for adjacent spaces of differing use.

The three fundamental methods for reducing radiation exposure. Minimizing time near a source, maximizing distance, and interposing absorbing material all reduce dose independently.

The relationship stating that photon fluence and dose rate from a point source decrease in proportion to the square of the distance from the source. Doubling distance reduces dose rate to one quarter.