Gray (Gy).

Special name of the SI unit of absorbed dose (see Units). 1 Gy = 1 J/kg = 100 rads.

Half-life, biological.

Time required for the body to eliminate half of an administered dose of any substance by metabolic processes of elimination; it is approximately the same for both stable and radioactive isotopes of a particular element.

Half-life, radioactive.

Time required for a radioactive substance to lose 50% of its activity by decay.

ICRP (International Commission on Radiological Protection).

An independent international organization that provides recommendations and guidance on protection against ionizing radiation.

ICRU (International Commission on Radiation Units and Measurements).

An independent international organization that provides recommendations and guidance on radiation quantities, units, and measurements.


Also, incidence rate; the rate of occurrence of a disease within a specified period of time, often expressed as a number of cases per 100,000 individuals per year.

In utero.

In the womb (i.e., before birth).

Inverse dose-rate effect.

An effect in which, for a given exposure, the probability of effect increases as the dose rate is lowered.

In vitro.

Cell culture conditions in glass or plastic containers.

In vivo.

In the living organism.

Ionizing radiation.

Radiation sufficiently energetic to dislodge electrons from an atom, thereby producing an ion pair. Ionizing radiation includes X- and gamma radiation, electrons (beta radiation), alpha particles (helium nuclei), and heavier charged atomic nuclei. Neutrons ionize indirectly by first colliding with components of atomic nuclei.

Kerma (kinetic energy released in material).

The kinetic energy transferred to charged particles per unit mass of irradiated medium by indirectly ionizing (uncharged) particles, such as photons or neutrons. Unit: gray (Gy). 1 Gy = 1 J/kg. If all of the kinetic energy is absorbed “locally,” the kerma is equal to the absorbed dose.

Latent period.

The time between exposure and expression of the disease. After exposure to a dose of radiation, there typically is a delay of several years (the latent period) before any cancer is observed.

Life table.

A table showing the number of persons who, of a given number born or living at a specified age, live to attain successivly higher ages, together with the numbers who die in each interval.

Linear energy transfer (LET).

Mean energy lost by charged particles in electronic collisions per unit track length. Unit: keV/mm.

High-LET radiation.

Neutrons or heavy, charged particles, such as protons or alpha particles, that produce ionizing events densely spaced on a molecular scale (e.g., L > 10 keV/μm; see Unrestricted LET).

Low-LET radiation.

X-rays and gamma rays or light, charged particles, such as electrons, that produce sparse ionizing events far apart on a molecular scale (e.g., L < 10 keV/μm).

Restricted LET (LΔ). The mean energy lost per unit track length in electronic collisions with energy transfer not larger than Δ.

Unrestricted LET (L) or Total collision stopping power, the mean energy lost per unit track length in all electronic collisions.

Linear (L) model or relationship (also linear dose-effect relationship).

The linear model is a special case of the linear-quadratic model, with the quadratic coefficient equal to zero; the linear model expresses the effect (e.g., cancer or mutation) as proportional to the dose (linear function of the dose).

Linear-quadratic (LQ) model.

Also, linear-quadratic dose-effect relationship; expresses the effect (e.g., cancer) as the sum of two components, one proportional to the dose (linear term) and one proportional to the square of the dose (quadratic term). The linear term predominates at low doses; the quadratic term, at high doses.

LNT model.

Linear no-threshold dose-response for which any dose greater than zero has a positive probability of producing an effect (e.g., mutation or cancer). The probability is calculated either from the slope of a linear (L) model or from the limiting slope, as the dose approaches zero, of a linear-quadratic (LQ) model.

Lognormal distribution.

When the logarithms of a randomly distributed quantity have a normal (Gaussian) distribution.

LSS (Life Span Study).

Long-term study of health effects in the Hiroshima and Nagasaki atomic bomb survivors.

Mechanistic basis.

An explanation derived from a knowledge of the individual stages leading to an effect.


An analysis of epidemiologic data from several studies based on data included in publications.


A schematic description of a system, theory, or phenomenon that accounts for its known or inferred properties and may be used for further study of its characteristics.

Monte Carlo calculation.

The method for evaluation of a probability distribution by means of random sampling.

Mortality (rate).

The frequency at which people die from a disease (e.g., a specific cancer), often expressed as the number of deaths per 100,000 population per year.

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