*Absolute risk (AR).*

The rate of disease among a population.

*Absorbed dose (D).*

The mean energy imparted by ionizing radiation to a medium per unit mass. Units: gray (Gy), rad. 1 Gy = 1 J/kg = 100 rads. 1 rad = 0.01 J/kg = 100 ergs/g.

*Activity.*

The amount of radioactivity defined in terms of the mean number of decays per unit time. Units: becquerel (Bq), curie (Ci). 1 Bq = 1 s^{−1} = 2.7 × 10^{−11} Ci. 1 Ci = 3.7 × 10^{9} Bq.

*Additive effect.*

When two agents do not interact, the combined effect is equal to the sum of the effects of the two agents acting alone.

*Apoptosis.*

Programmed cell death. The cell death is characterized by a distinctive fragmentation of DNA that is regulated by cellular functions.

*Attributable risk (AR).*

The estimated rate of a disease (such as lung cancer) that could, in theory, be prevented if all exposures to a particular causative agent (such as radon) were eliminated.

*Background radiation.*

The radiation to which a member of the population is exposed from natural sources, such as terrestrial radiation due to naturally occurring radionuclides in the soil, cosmic radiation originating in outer space, and naturally occurring radionuclides in the human body.

*Baseline rate of cancer.*

The annual cancer incidence observed in a population in the absence of the specific agent being studied; the baseline rate includes cancers from a number of other causes, such as smoking, occupational exposures, and so forth.

*Becquerel (Bq).*

SI unit of activity (see *Units*). 1 Bq = 1 s^{−1} = 2.7 × 10^{−11} Ci.

*BEIR V.*

Report of the fifth National Research Council Committee on the Biological Effects of Ionizing Radiation; the report was published in 1990.

*Beta particle.*

A charged particle emitted from a nucleus during radioactive decay that, if negatively charged, is identical to an electron.

*Bias.*

Factors that influence the outcome of data collection, such as causing certain measurements to have a greater chance of being included than others.

*Cancer.*

A malignant tumor of potentially unlimited growth, capable of invading surrounding tissue or spreading to other parts of the body by metastasis.

*Carcinogen.*

An agent that can cause cancer. Ionizing radiation is a physical carcinogen; there are also chemical and biological carcinogens; biological carcinogens may be extrinsic (*e.g.*, viruses) or intrinsic (genetic defects).

*Carcinoma.*

A malignant tumor (cancer) of epithelial origin.

*Case-control study.*

An epidemiologic study in which people with disease and a similarly composed control group are compared in terms of exposures to a putative causative agent.

*Cell culture.*

The growing of cells *in vitro* (in a glass or plastic container, or in suspension) in such a manner that the cells are no longer organized into tissues.

*Cohort study.*

An epidemiologic study in which groups of people (the cohort) are identified with respect to the presence or absence of exposure to a disease-causing agent, and in which the outcomes of disease rates are compared; also called a follow-up study.

*Collective effective dose*.

Number of persons times average effective dose (see *Effective dose*). Unit: person-Sv (special name used with collective dose).

*Competing risks.*

Causes other than the agent under study that contribute to the mortality rate. The mortality rate from these other causes is not included in the risk of dying from the factor under study.

*Confidence interval (CI).*

An interval estimate of an unknown parameter, such as a risk. A 95% confidence interval, as an example, is constructed from a procedure

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Glossary
Absolute risk (AR). The rate of disease among a population. Beta particle. A charged particle emitted from a nucleus
Absorbed dose (D). The mean energy imparted by ionizing during radioactive decay that, if negatively charged, is
radiation to a medium per unit mass. Units: gray (Gy), identical to an electron.
rad. 1 Gy = 1 J/kg = 100 rads. 1 rad = 0.01 J/kg = 100 Bias. Factors that influence the outcome of data collection,
ergs/g. such as causing certain measurements to have a greater
Activity. The amount of radioactivity defined in terms of the chance of being included than others.
mean number of decays per unit time. Units: becquerel
(Bq), curie (Ci). 1 Bq = 1 s–1 = 2.7 × 10–11 Ci. 1 Ci = 3.7 Cancer. A malignant tumor of potentially unlimited growth,
× 109 Bq. capable of invading surrounding tissue or spreading to
Additive effect. When two agents do not interact, the com- other parts of the body by metastasis.
bined effect is equal to the sum of the effects of the two Carcinogen. An agent that can cause cancer. Ionizing radia-
agents acting alone. tion is a physical carcinogen; there are also chemical
Apoptosis. Programmed cell death. The cell death is charac- and biological carcinogens; biological carcinogens may
terized by a distinctive fragmentation of DNA that is be extrinsic (e.g., viruses) or intrinsic (genetic defects).
regulated by cellular functions. Carcinoma. A malignant tumor (cancer) of epithelial origin.
Attributable risk (AR). The estimated rate of a disease (such Case-control study. An epidemiologic study in which people
as lung cancer) that could, in theory, be prevented if all with disease and a similarly composed control group are
exposures to a particular causative agent (such as radon) compared in terms of exposures to a putative causative
were eliminated. agent.
Cell culture. The growing of cells in vitro (in a glass or plas-
Background radiation. The radiation to which a member of tic container, or in suspension) in such a manner that the
the population is exposed from natural sources, such as cells are no longer organized into tissues.
terrestrial radiation due to naturally occurring radionu- Cohort study. An epidemiologic study in which groups of
clides in the soil, cosmic radiation originating in outer people (the cohort) are identified with respect to the
space, and naturally occurring radionuclides in the hu- presence or absence of exposure to a disease-causing
man body. agent, and in which the outcomes of disease rates are
Baseline rate of cancer. The annual cancer incidence ob- compared; also called a follow-up study.
served in a population in the absence of the specific Collective effective dose. Number of persons times average
agent being studied; the baseline rate includes cancers effective dose (see Effective dose). Unit: person-Sv (spe-
from a number of other causes, such as smoking, occu- cial name used with collective dose).
pational exposures, and so forth. Competing risks. Causes other than the agent under study
Becquerel (Bq). SI unit of activity (see Units). 1 Bq = 1 that contribute to the mortality rate. The mortality rate
s–1 = 2.7 × 10–11 Ci. from these other causes is not included in the risk of
BEIR V. Report of the fifth National Research Council Com- dying from the factor under study.
mittee on the Biological Effects of Ionizing Radiation; Confidence interval (CI). An interval estimate of an un-
the report was published in 1990. known parameter, such as a risk. A 95% confidence in-
terval, as an example, is constructed from a procedure
373

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374 GLOSSARY
that is theoretically successful in capturing the param- weighting factors and radiation weighting factors, as
eter of interest in 95% of its applications. Confidence defined by the International Commission for Radiation
limits are the end points of a confidence interval. Protection (ICRP). Unit: 1 Sv = 1 J/kg = 100 rem. Equal
Constant relative risk (CRR). A risk model that assumes the effective doses are meant to correspond—apart from
ratio of the risk at a specific dose and the risk in the age- and sex-dependent differences—to roughly the
absence of the dose remains constant after a certain time. same overall risk. For a uniform whole-body exposure
Curie (Ci). Former special unit of activity (see Units). 1 Ci = by a specified radiation type the effective dose equals
3.7 × 1010 Bq. the absorbed dose times the radiation weighting factor.
Electron volt (eV). A special unit of energy: 1 eV = 1.6 ×
DEF. The reduction in risk for low doses. 10–19 J = 1.6 × 10–12 erg; 1 eV is equivalent to the energy
DNA. Deoxyribonucleic acid; the genetic material of cells. gained by an electron in passing through a potential dif-
Deletions. Type of mutation in which sections of DNA are ference of 1 V; 1 keV=1000 eV; 1 MeV = 1,000,000 eV.
removed; term can refer to the removal of a single base Empirical model. A model that is derived from measure-
or many bases. ments in populations, as opposed to a theoretical model.
Dose. Short name for absorbed dose (1 Gy = 1 J/kg) and also Epidemiology. The study of the determinants of the fre-
for equivalent dose, effective dose, and weighted dose quency of disease in humans. The two main types of
(1 Sv = 1 J/kg). Definitions of low, medium, and high epidemiologic studies of chronic disease are cohort (or
doses vary widely in the literature. For the purposes of follow-up) studies and case-control studies.
this report, dose levels have been defined as follows: Equivalent dose. Absorbed dose multiplied by the quality
Low dose: 0-100 mGy (mSv) factor, Q, which represents, for the purposes of radia-
Medium dose: In excess of 100 mGy up to a maximum tion protection and control, the effectiveness of the ra-
of 1 Gy diation relative to sparsely ionizing radiation (see Qual-
High dose: In excess of 1 Gy up to the very high total ity factor). Units: 1 Sv = 1 J/kg = 100 rem. 1 rem = 0.01
doses used in radiation therapy (on the order of 20– Sv.
60 Gy) Etiology. The science or description of cause(s) of disease.
Dose-effect (dose-response) model. A mathematical formu- Excess absolute risk (EAR). The rate of disease in an ex-
lation and description of the way the effect (or biologi- posed population minus the rate of disease in an unex-
cal response) depends on dose. posed population. Also termed “attributable risk” or
Dose rate. The absorbed dose delivered per unit time. “risk difference.”
Dose-rate effectiveness factor (DREF). The factor by which Excess relative risk (ERR). The rate of disease in an exposed
the effect caused by a specific type of radiation changes population divided by the rate of disease in an unex-
at low doses or low dose rates (protracted or fraction- posed population minus 1.0.
ated delivery of dose) as compared to high doses deliv- Exposure. The condition of having contact with a physical
ered at high (or acute) dose rates. or chemical agent.
Dose and dose-rate effectiveness factor (DDREF). A judged
factor by which the radiation effect, per unit of dose, Fibrosis. Damage to normal tissue that results in a modifica-
caused by a given high or moderate dose of radiation tion of tissue structure but is not cancer.
received at high dose rates is reduced when doses are Fractionation. The delivery of a given dose of radiation as
low or are received at low dose rates. several smaller doses separated by intervals of time.
Dosimetric model. A method for estimating risk based on
the use of physical models for doses to target cells and Gamma radiation. Also gamma rays; short-wavelength elec-
the use of results from epidemiologic studies of expo- tromagnetic radiation of nuclear origin, similar to X-
sures to humans from other types of radiations. rays but usually of higher energy (100 keV to several
million electronvolts).
Ecological fallacy. The fact that two populations differ in Geometric mean. The geometric mean of a set of positive
many factors other than the one being evaluated and numbers is the exponential of the arithmetic mean of
that one or more of these other factors may be the under- their logarithms. The geometric mean of a lognormal
lying reason for any difference noted in their morbidity distribution is the exponential of the mean of the associ-
or mortality experience. ated normal distribution.
Ecologic study. A method of epidemiologic study in which Geometric standard deviation (GSD). The geometric stan-
rates of health effects outcome based on population dard deviation of a lognormal distribution is the expo-
rather than individual data are related to the measure of nential of the standard deviation of the associated nor-
population radiation exposure. mal distribution.
Effective dose. Sum over the absorbed doses to different or- Germ cells. Reproductive cells such as the sperm and egg
gans from different radiation types multiplied by organ and their progenitor cells.

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GLOSSARY 375
Gray (Gy). Special name of the SI unit of absorbed dose (see Linear energy transfer (LET). Mean energy lost by charged
Units). 1 Gy = 1 J/kg = 100 rads. particles in electronic collisions per unit track length.
Unit: keV/mm.
Half-life, biological. Time required for the body to eliminate High-LET radiation. Neutrons or heavy, charged particles,
half of an administered dose of any substance by meta- such as protons or alpha particles, that produce ionizing
bolic processes of elimination; it is approximately the events densely spaced on a molecular scale (e.g., L > 10
same for both stable and radioactive isotopes of a par- keV/µm; see Unrestricted LET).
ticular element. Low-LET radiation. X-rays and gamma rays or light, charged
Half-life, radioactive. Time required for a radioactive sub- particles, such as electrons, that produce sparse ionizing
stance to lose 50% of its activity by decay. events far apart on a molecular scale (e.g., L < 10 keV/
µm).
ICRP (International Commission on Radiological Protec- Restricted LET (L∆). The mean energy lost per unit track
tion). An independent international organization that length in electronic collisions with energy transfer not
provides recommendations and guidance on protection larger than ∆.
against ionizing radiation. Unrestricted LET (L) or Total collision stopping power,
ICRU (International Commission on Radiation Units and the mean energy lost per unit track length in all elec-
Measurements). An independent international organiza- tronic collisions.
tion that provides recommendations and guidance on Linear (L) model or relationship (also linear dose-effect re-
radiation quantities, units, and measurements. lationship). The linear model is a special case of the
Incidence. Also, incidence rate; the rate of occurrence of a linear-quadratic model, with the quadratic coefficient
disease within a specified period of time, often ex- equal to zero; the linear model expresses the effect (e.g.,
pressed as a number of cases per 100,000 individuals cancer or mutation) as proportional to the dose (linear
per year. function of the dose).
In utero. In the womb (i.e., before birth). Linear-quadratic (LQ) model. Also, linear-quadratic dose-
Inverse dose-rate effect. An effect in which, for a given ex- effect relationship; expresses the effect (e.g., cancer) as
posure, the probability of effect increases as the dose the sum of two components, one proportional to the dose
rate is lowered. (linear term) and one proportional to the square of the
In vitro. Cell culture conditions in glass or plastic contain- dose (quadratic term). The linear term predominates at
ers. low doses; the quadratic term, at high doses.
In vivo. In the living organism. LNT model. Linear no-threshold dose-response for which
Ionizing radiation. Radiation sufficiently energetic to dis- any dose greater than zero has a positive probability of
lodge electrons from an atom, thereby producing an ion producing an effect (e.g., mutation or cancer). The prob-
pair. Ionizing radiation includes X- and gamma radia- ability is calculated either from the slope of a linear (L)
tion, electrons (beta radiation), alpha particles (helium model or from the limiting slope, as the dose approaches
nuclei), and heavier charged atomic nuclei. Neutrons zero, of a linear-quadratic (LQ) model.
ionize indirectly by first colliding with components of Lognormal distribution. When the logarithms of a randomly
atomic nuclei. distributed quantity have a normal (Gaussian) distribu-
tion.
Kerma (kinetic energy released in material). The kinetic en- LSS (Life Span Study). Long-term study of health effects in
ergy transferred to charged particles per unit mass of the Hiroshima and Nagasaki atomic bomb survivors.
irradiated medium by indirectly ionizing (uncharged)
particles, such as photons or neutrons. Unit: gray (Gy). Mechanistic basis. An explanation derived from a knowl-
1 Gy = 1 J/kg. If all of the kinetic energy is absorbed edge of the individual stages leading to an effect.
“locally,” the kerma is equal to the absorbed dose. Meta-analysis. An analysis of epidemiologic data from sev-
eral studies based on data included in publications.
Latent period. The time between exposure and expression of Model. A schematic description of a system, theory, or phe-
the disease. After exposure to a dose of radiation, there nomenon that accounts for its known or inferred proper-
typically is a delay of several years (the latent period) ties and may be used for further study of its characteris-
before any cancer is observed. tics.
Life table. A table showing the number of persons who, of a Monte Carlo calculation. The method for evaluation of a
given number born or living at a specified age, live to probability distribution by means of random sampling.
attain successivly higher ages, together with the num- Mortality (rate). The frequency at which people die from a
bers who die in each interval. disease (e.g., a specific cancer), often expressed as the
number of deaths per 100,000 population per year.

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376 GLOSSARY
Multiplicative effects. The combined effect of two agents is extend or strengthen the validity of some elements of a
equal to the product of the effects of the two agents act- hypothesis or a model using a system that is different
ing alone. from the one that provided the basis for the hypothesis
or model.
NCRP (National Council on Radiation Protection and Mea- Proportional mortality ratio. The ratio of the percentage of a
surements). U.S. Council commissioned to formulate specific cause of death among all deaths in the popula-
and disseminate information, guidance, and recommen- tion being studied divided by the comparable percent-
dations on radiation protection and measurements. age in a standard population.
Neoplasm. Any new and abnormal growth, such as a tumor; Protraction. The spreading out of a radiation dose over time
neoplastic disease refers to any disease that forms tu- by continuous delivery at a lower dose rate.
mors, whether malignant or benign.
Nonstochastic. A description of effects whose severity is a Quadratic-dose model. A model that assumes that the excess
function of dose; for these, a threshold may occur; some risk is proportional to the square of the dose.
examples of somatic effects believed to be nonstochastic Quality factor (Q). An LET-dependent factor by which the
are cataract induction, nonmalignant damage to the skin, absorbed dose is multiplied to obtain (for radiation pro-
hematological deficiencies, and impairment of fertility. tection purposes) the dose equivalent that corresponds
Normal distribution. The so-called bell-shaped curve of ran- roughly to the absorbed dose from X- or gamma rays
domly distributed quantities; also referred to as a that causes the same degree of biologic effect. Absorbed
“Gaussian distribution.” dose (Gy) × Q = dose equivalent (Sv).
Odds ratio (OR). The odds of being exposed among dis- Rad. A special unit of absorbed dose, now replaced by the SI
eased persons divided by the odds of being exposed unit gray (see Units). 1 rad = 0.01 Gy = 100 erg/g.
among nondiseased persons. Radiation. Energy emitted in the form of waves or particles
Oncogenes. Genes that encode the potential for cancer. by radioactive atoms as a result of radioactive decay or
produced by artificial means, such as X-ray generators.
Phenotype. The genetically and environmentally determined Radioactivity. The property of nuclide decay in which par-
physical appearance of an organism. ticles or gamma radiations are usually emitted.
Photon. An electromagnetic quantum whose energy (Eph) Artificial radioactivity. Man-made radioactivity pro-
equals the product of the Planck constant (h) and its duced by fission, fusion, particle bombardment, or
frequency (n). With the convenient units eV and s, and electromagnetic irradiation.
with the wave length λ in µm: Eph = 4.136 × 10–15 Natural radioactivity. The property of radioactivity ex-
ν = 1.24/λ. hibited by more than 50 naturally occurring radio-
Pooled analysis. An analysis of epidemiologic data from nuclides.
several studies based on original data from the studies. Radiogenic. Caused by radiation.
Prevalence. The number of cases of a disease in existence at Radioisotope. A radioactive atomic species of an element
a given time per unit of population, usually 100,000 with the same atomic number and usually identical
persons. chemical properties.
Probability of causation. A number that expresses the prob- Radionuclide. A radioactive species of an atom character-
ability that a given cancer, in a specific tissue, has been ized by the constitution of its nucleus.
caused by a previous exposure to a carcinogenic agent, Relative biologic effectiveness (RBE). The ratio Dref/D,
such as radiation. where D is the absorbed dose of a specified radiation
Projection model. A mathematical model that simulta- and Dref is the absorbed dose of a sparsely ionizing ref-
neously describes the excess cancer risk at different lev- erence radiation (γ-rays or X-rays) that produces the
els of some factor such as dose, time after exposure, or same level of effect. When the magnitude of the dose D
baseline level of risk, in terms of a parametric function is not specified, the RBE is meant to be the low-dose
of that factor. It becomes a projection model when data limit of the ratio Dref/D (this low-dose RBE equals the
in a particular range of observations are used to assign low-dose effectiveness (initial slope) of the specified
values to the parameters in order to estimate (or project) radiation to that of the reference radiation).
excess risk for factor values outside that range. Relative risk (RR). The rate of disease in an exposed popu-
Promoter. An agent that is not by itself carcinogenic but can lation divided by the rate of disease in an unexposed
amplify the effect of a true carcinogen by increasing the population. Also termed “rate ratio.”
probability of late-stage cellular changes necessary to Rem (rad equivalent man). A special unit of dose equivalent,
complete the carcinogenic process. now replaced by the SI unit sievert (see Units). 1 rem =
Proof of principle. Proof-of-principle studies are those that 0.01 Sv.

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GLOSSARY 377
Retinoblastoma. An eye tumor that is an example of an in- Synergistic effect. Increased effectiveness results from an
herited malignant tumor with a dominant autosomal interaction between two agents, so that the total effect is
gene inheritance pattern. greater than the sum of the effects of the two agents
Risk. A chance of injury, loss, or detriment; a measure of the acting alone.
deleterious effects that may be expected as the result of
an action or inaction. Target cells. Cells in a tissue that have been determined to
Risk assessment. The process by which the risks associated be the key cells in which changes occur in order to pro-
with an action or inaction are identified and quantified. duce an end point such as cancer.
Risk coefficient. The increase in the annual incidence or Threshold hypothesis. The assumption that no radiation in-
mortality rate per unit dose: (1) absolute risk coefficient jury occurs below a specified dose.
is the increase of the incidence or mortality rate per unit Transformed cells. Tissue culture cells changed from grow-
dose; (2) relative risk coefficient is the fractional in- ing in an orderly pattern exhibiting contact inhibition to
crease above the baseline incidence or mortality rate per growing in a pattern more like that of cancer cells.
unit dose.
Risk estimate. The increment of the incidence or mortality Uncertainty. The range of values within which the true value
rate projected to occur in a specified exposed popula- is estimated to lie. It is a best estimate of possible inac-
tion per unit dose for a specified exposure regime and curacy due to both random and systemic errors.
expression period. Random Errors. Errors that vary in a nonreproducible
way around a limiting mean. These errors can
Sievert (Sv). Special name of the SI unit of dose equivalent be treated statistically by use of the laws of
(see Units). 1 Sv = 1 J/kg = 100 rem. probability.
SI units. Units of the International System of Units as de- Systemic Errors. Errors that are reproducible and tend
fined by the General Conference of Weights and Mea- to bias a result in one direction. Their causes can be
sures in 1960. They are the base units, such as meter assigned, at least in principle, and they can have
(m), kilogram (kg), second (s), and their combinations, constant and variable components. Generally, these
which have special names (e.g., the unit of energy, 1 J = errors cannot be treated statistically.
1 kg m2/s2, or absorbed dose, 1 Gy = 1 J/kg = 1 m2/s2 Units of dose. Also known as dosimetric units.
(see Units).
Solid cancers. Solid cancers include all malignant neoplasms Unita Symbol Conversion Factors
other than those of the lymphatic and hematopoietic tis-
Becquerel (SI) Bq 1 disintegration/s = 2.7 × 10–11 Ci
sue.
Somatic cells. Nonreproductive cells. Curie Ci 3.7 × 1010 disintegrations/s =
Specific activity. Activity of a given nuclide per unit mass of 3.7 × 1010 Bq
a compound, element, or radioactive nuclide.
Specific energy (z). The energy per unit mass actually depos- Gray (SI) Gy 1 J/kg = 100 rad
ited in a microscopic volume in a single energy deposi- Rad rad 0.01 Gy = 100 erg/g
tion event or at a given absorbed dose. This is a stochas-
tic quantity as opposed to its average, the absorbed dose, Sievert (SI) Sv 1 J/kg = 100 rem
D. The mean energy imparted by ionizing radiation to a
medium per unit mass. Unit: 1 Gy = 1 J/kg. Rem rem 0.01 Sv
Standardized morbidity ratio or Standardized mortality rate aInternational Units are designated SI.
(SMR). The ratio (multiplied by 100) of the mortality
rate from a disease in the population being studied di- NOTE: Equivalent dose equals absorbed dose times Q (quality factor). Gray
is the special name of the unit (J/kg) to be used with absorbed dose; sievert
vided by the comparable rate in a standard population.
is the special name of the unit (J/kg) to be used with equivalent dose.
The ratio is similar to a relative risk times 100.
Stochastic. Effects whose probability of occurrence in an
exposed population (rather than severity in an affected UNSCEAR (United Nations Scientific Committee on the
individual) depends on dose; stochastic effects are com- Effects of Atomic Radiation). A UN committee that pub-
monly regarded as having no threshold; hereditary ef- lishes periodic reports on sources and effects of ionizing
fects are stochastic; some somatic effects, especially radiation.
cancers, are regarded as being stochastic.
Suppressor gene. A gene that can suppress another gene such Variability. The variation of a property or quantity among
as an oncogene. Changes in suppressor genes can lead members of a population. Such variation is inherent in
to expression by genes such as oncogenes. nature and is often assumed to be random; it can then be
represented by a frequency distribution.

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378 GLOSSARY
Weighted dose (d). The dose to A-bomb survivors, roughly equal to 10 in analyses by the Radiation Effects Re-
adjusted to account for the increased effectiveness of search Foundation (RERF). Unit: 1 Sv = 1 J/kg.
the small neutron absorbed dose contribution. The
weighted dose equals the gamma-ray absorbed dose to a X-radiation. Also X-rays; penetrating electromagnetic radia-
specified organ plus the neutron absorbed dose multi- tion, usually produced by bombarding a metallic target
plied by a weighting factor that has usually been set with fast electrons in a high vacuum.