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This section of RADNET includes definitions and acronyms pertaining to biologically significant radionuclides, their dosimetry and health physics impact, source term release, site release criteria and a checklist of biologically significant radionuclides.  The previous section, Definitions and Conversion Factors includes terms other than the ones found in this list.

ABSORBED DOSE: the energy imparted by ionizing radiation per unit mass of irradiated material. the units of absorbed dose are called the gray (Gy). (Toxicological Profile for Ionizing Radiation, pg. 305).

ACTION LEVEL: a derived media-specific radionuclide-specific concentration or activity level of radioactivity that triggers a response such as seizure of contaminated foodstuffs following a nuclear accident (see FDA DIL). In the MARSSIM, the action level is called the investigation level and would trigger the response of further investigation or site cleanup if the release criterion is exceeded.

ANNUAL LIMIT ON INTAKE (ALI): the derived limit for the amount of radioactive material taken into the body of an adult worker by inhalation or ingestion in a year. For a given radionuclide, ALI is defined as the smaller of the intakes that would result in a committed effective dose equivalent of 5 rems and a committed dose equivalent of 50 rems to any individual organ or tissue. (Toxicological Profile for Ionizing Radiation, pg. 306).

AS LOW AS REASONABLY ACHIEVABLE (ALARA): thereduction of exposure to ionizing radiation so as to reduce collective doses as far below regulatory limits as is reasonably possible.

BIOLOGICALLY SIGNIFICANT RADIONUCLIDES: radioactive substances such as plutonium, cesium, strontium, radioiodine, and tritium, etc. which provide the most significant health hazards to humans among all nuclides released from anthropogenic sources. Biological significance is a result of a combination of high decay energy, biogeochemical availability, efficient energy transfer to biological systems, and ubiquitous production during nuclear accidents and from industries. In this Website, biologically significant radionuclides are noted as indicator nuclides and are used to characterize inventories and pathways of nuclear effluents in the biosphere.

The biological significance of radiation results from the enormous amount of energy contained in each emission. Visible light has an energy range of 1.77 to 4.13 electron volts (ev). Most chemical changes occur within a range of 5 to 7 electron volts (ev). Biologically significant radiation levels range from 18,610 ev (0.01861 Mev) for the weak beta emitting tritium (1/2T = 12.346 yr.) to 511,630 ev (0.51163 Mev) for the ubiquitous cesium-137 (1/2T = 30.174 yr.) to 5,155,400 ev (5.1554 Mev) for the highly radiotoxic plutonium-239 (1/2T = 24,131 yr). These highly energetic emissions carry enough energy to tear electrons from neutral atoms and molecules. In delicate biological tissues, the impact of introducing radiation containing hundreds of thousands to millions of electron volts "can only be described as chemical and biological mayhem" (Gofman, 1981, p. 22). For example, the alpha radiation resulting from the decay of plutonium-239 has little penetrating power due to its large mass, but, if inhaled and deposited in the lung, is among the most radiotoxic of nuclides since its 5,155,000 ev (5.155 Mev) will be distributed within the area of only a few cells.

The weaker beta radiation of tritium (3H) is slightly more penetrating than alpha radiation; its biological significance comes from its ubiquitous production during the fission process, its tendency to follow the water cycle in nature, and its ability to become tissue bound in humans and the biotic environment. Cesium-137, a beta emitter with a gamma component, is biologically significant due to its energy level, its long half-life, its ubiquitous production during the fission process, and its tendency to follow the potassium cycle in nature, giving a whole body dose to those who ingest it.

COMMITTED EFFECTIVE DOSE EQUIVALENT (CEDE): the effective dose equivalent is the summation of the products of the dose equivalent received by specified tissues of the body and a tissue-specific weighting factor (HE50 = summation(WTHT50)). It is a risk-equivalent value, expressed in Sv or rem, that can be used to estimate the health-effects on an exposed individual. It is used in radiation safety because it implicitly includes the relative carcinogenic sensitivity of the various tissues. (MARSSIM, pg. GL-3). (Toxicological Profile for Ionizing Radiation, pg. 307).

COMMITTED DOSE EQUIVALENT (HT50): the dose equivalent to organs or tissues of reference (T) that will be received from an intake of radioactive material by an individual during the 50-year period following the intake. (Toxicological Profile for Ionizing Radiation, pg. 307).

CONTAMINATION: the presence of residual radioactivity in excess of levels which are acceptable for release of a site or facility for unrestricted use. (MARSSIM, pg. GL-4). RADNET note: this is a particularly controversial definition of contamination in that it is predicated upon arbitrary release criteria which in effect allow significant levels of contamination to remain in a remediated or decommissioned site.

CRITICAL GROUP: the group of individuals reasonably expected to receive the greatest exposure to residual radioactivity for any applicable set of circumstances. (MARSSIM, pg. GL-4).

DATA QUALITY INDICATORS: measurable attributes of the attainment of the necessary quality for a particular decision. Data quality indicators include precision, bias, completeness, representativeness, reproducibility, comparability, and statistical confidence. (MARSSIM, pg. GL-5).

DERIVED AIR CONCENTRATION (DAC): the concentration of a given radionuclide in the air which, if breathed by the reference man for one working year (2,000 hours) under conditions of light work, results in an intake of one ALI. (Toxicological Profile for Ionizing Radiation, pg. 308).

DERIVED CONCENTRATION GUIDELINE LEVEL (DCGL): A derived, radionuclide-specific activity concentration within a survey unit corresponding to the release criterion. The DCGL is based on the spatial distribution of the contaminant and hence is derived differently for the nonparametric statistical test (DCGLw) and the Elevated Measurement Comparison (DCGLEMC). DCGL's are derived from activity/dose relationships through various exposure pathway scenarios. (MARSSIM, pg. GL-5).

DERIVED CONCENTRATION GUIDES (DCGs): the concentration that would result in a radiation dose equal to the DOE public dose limit of 100 millirems per year. "The DCGs consider only the inhalation of air, the ingestion of water, or submersion in air." The DOE DCGs raise the question that, if individuals receive a dose equal to the DCG for a particular nuclide, wouldn't they also be receiving substantial exposure for the other nuclides listed in the guide? The DCGs have nothing to do with ground deposition or dietary intake of radionuclides which provide an additional source of exposure following a nuclear accident. See RAD 6: Radiation Protection Guidelines.

DERIVED INTERVENTION LEVEL (DIL): A protection action guideline issued in draft form only by the Food and Drug Administration pertaining to contamination of human foodstuffs and based upon a committed effective dose equivalent of 5 mSv, or a committed dose equivalent to individual tissues and organs of 50 mSv, whichever is more limiting. The FDA DLL's are listed in RADNET Section 6: Radiation Protection Guidelines: Accidental Radioactive Contamination of Human Food and Animal Feeds: Recommendations for State and Local Agencies. Typical intervention levels expressed in Becquerels / kilogram (1 Bq = 1 disintegration per second = 27 picocuries) of contaminated foodstuffs are 131I: 167 Bq/kg (1 year old child), 137Cs: 1360 Bq/kg (adult), 239Pu: 2.2 Bq/kg (3 month old infant), 241Am: 2.0 Bq/kg (3 month old infant) The new FDA guideline is especially noteworthy in extending the DIL's to include a variety of radioisotopes not considered to be of much importance until after the Chernobyl accident (see table E6) e.g. 129I: 56 Bq/kg (10 year old child).

DOSE ASSESSMENT: an estimate of the radiation dose to an individual or a population group usually by means of predictive modeling techniques, often supplemented by the results of measurement. (Toxicological Profile for Ionizing Radiation, pg. 309).

DOSE COMMITMENT: the dose that an organ or tissue would receive during a specified period of time (e.g., 50 or 70 years) as a result of intake (as by ingestion or inhalation) of one or more radionuclides from a given release. (MARSSIM, pg. GL-4).

DOSE CONVERSION FACTOR: a factor (Sv/Bq or rem/Ci) that is multiplied by the intake quantity of a radionuclide (Bq or Ci) to estimate the committed dose equivalent from radiation (Sv or rem). The dose conversion factor depends on the route of entry (inhalation or ingestion), the lung clearance class (D, W or Y) for inhalation, the fractional uptake from the small intestine to blood (f1) for ingestion, and the organ of interest. EPA provides separate dose conversion factor tables for inhalation and ingestion, and each provides factors for the gonads, breast, lung, red marrow, bone surface, thyroid, remainder, and effective whole body. (Toxicological Profile for Ionizing Radiation, pg. 309).

DOSE EQUIVALENT (DE): a quantity used in radiation protection. It expresses all radiation on a common scale for calculating the dose for purposes of radiation safety. It is defined as the product of the absorbed dose in rads and certain modifying factors. (The unit of dose equivalent is the rem. In SI units, the dose equivalent is the sievert, which equals 100 rem). (Toxicological Profile for Ionizing Radiation, pg. 309).

EXPOSURE RATE: the amount of ionization produced per unit time in air by X-rays or gamma rays. The unit of exposure rate is roentgens/hour (R/h); for decommissioning activities the typical units are microroentgens per hour (µR/h), i.e. 10-6R/h. (MARSSIM, pg. GL-7).

GRAY (Gy): the SI unit of the absorbed dose. One Gy equals the absorption of 1 joule of energy (about 1/4 of a calorie) per kilogram of absorber. One gray equals 100 rad. (Toxicological Profile for Ionizing Radiation, pg. 311).

High-LET: the characteristic ionization patterns by alpha particles, protons or fast neutrons having a high relative specific ionization per unit path length. (Toxicological Profile for Ionizing Radiation, pg. 311).

LINEAR ENERGY TRANSFER (LET): Another key concept in determining biological effectiveness and significance, LET expresses the combination of charge and speed in effecting the efficiency of ionizing radiation. LET describes "the amount of energy transferred per unit of path traveled by the ionizing particle (electron, alpha particle or other)" (Gofman, 1981, p. 28). Alpha particles have twice the charge of a beta particle and, therefore, four times the efficiency of ionizing radiation per collision. Alpha radiation is much slower than beta or gamma radiation; therefore, it is much more efficient than the faster radiation, causing more ionizations per millimeter of distance traveled (See Gofman, p. 26-28). The efficient LET of alpha isotopes such as 239Pu combine with their high decay energies to form the basis of their biological effectiveness. High radiotoxicity and great biological significance accompany these long-lived anthropogenic radionuclides in the environment.

MULTI-AGENCY RADIATION SURVEY AND SITE INVESTIGATION MANUAL (MARSSIM): a controversial publication issued by the EPA, NRC and DOE which delineates the release criterion pertaining to the annual radiation dose that maximally exposed members of the public can receive, as a condition for decommissioning or remediating nuclear power plants or other NRC or DOE facilities. The MARSSIM is of particular importance now that the NRC has set 25 mrem/yr TEDE as the release criteria for decommissioning nuclear power plants under its jurisdiction.

NATURALLY OCCURRING RADIOACTIVITY (NOR): RADNET does not cite or annotate most research articles on NOR. Check Section 13: RADLINKS for NORM and the Uranium Institute. Their Websites will bring you to comprehensive information sources on this important source of exposure to ionizing radiation.

QUALITY FACTOR (Q): The linear-energy-transfer-dependent factor by which absorbed doses are multiplied to obtain (for radiation protection purposes) a quantity that expresses the biological effectiveness of the absorbed dose on a common scale for all ionizing radiation. (Toxicological Profile for Ionizing Radiation, pg. 315).

RADON: RADNET does not cite or annotate most research articles on radon. Check Section 13: RADLINKS for the EPA's National Radon Proficiency Program, University of Maine's Physics Department,NORM, OncoLink, the Environmental Measurements Laboratory and the Uranium Institute. Their Websites will bring you to comprehensive information sources on this important source of exposure to ionizing radiation.

RELATIVE BIOLOGICAL EFFECTIVENESS (RBE): A key component of the biological significance of radiation, RBE expresses the phenomenon that one kind of radiation is more effective (damaging) than another. Gofman (1981, p. 47) notes, "the RBE for alpha particles may be 10 for one biological effect, whereas it may be 1 or 2 for some other biological effect... the RBE for one radiation compared to another is not a fixed quantity."

RADIATION EQUIVALENT MAN (rem): the conventional unit of dose equivalent. The corresponding International System (SI) unit is the Sievert (Sv); 1 Sv = 100 rem.

SIEVERT (Sv): the special name for the International System unit of dose equivalent. 1 Sv = 100 rem = 1 Joule per kilogram.

SOMATIC EFFECTS: effects of radiation limited to the exposed individual, as distinguished from genetic effects, which may be expressed as µCi/gram, Bq/m3, etc. (Toxicological Profile for Ionizing Radiation, pg. 317).

SOURCE TERM RELEASE: radioactive waste inventories discharged from a particular nuclear accident orsource point, e.g. Chernobyl, Sellafield, weapons tests, etc. Each plume is characterized by a unique fingerprint of radioactive emissions which can be identified by a particular series of isotopic ratios. Weapons testing fallout was high in radiostrontium, low in cesium-134, and, thus, differed from the Chernobyl source term which had much less radiostrontium and a higher ratio of cesium-134 to cesium-137 than weapons test fallout. Eisenbud (1987) and most early reports on the Chernobyl accident, in a classic example of misinformation, based the source term for Chernobyl upon Russian data which only included inventories of radionuclides deposited on Russian soil. Further research indicated that the source term release for Chernobyl included larger quantities of radioactive emissions than initially estimated and much higher levels of contamination than expected in locations which were a great distance from Chernobyl. An important study of the pre-Chernobyl sources of radioactivity, including naturally occurring, industrial, atomic power, weapons testing, and fuel reprocessing sources is the UNSCEAR Text (1982) (See RADNET Section 14); important U.S. and Russian military source points are excluded. A more detailed summary of the impact of the Chernobyl accident is contained in Section 10 of this website.

SOURCE TERM RELEASE DURATION: the source term release duration can vary from a few seconds for a weapons test explosion to hundreds of years or more for chronic discharges from source points such as military weapons production facilities. For example, the January, 1968, crash of a United States bomber carrying nuclear weapons, into the ocean near Thule, Greenland, released an estimated inventory of 1 TBq 239,240Pu as well as smaller quantities of 238Pu and 241Am (Aarkrog, 1994: see RAD: 11 Part 12). The duration of this source term release was a matter of a few minutes; the duration of the plume movement is a function of the long radioactive half-lives of the isotopes in the source term release. The geographic magnitude of the plume pulse is a function of the chemical forms of the released isotopes and the biogeochemical cycles which may aid their spread in the biosphere: in the case of the Thule accident, the plutonium will tend to remain localized on the ocean sea bed unless it undergoes a change in chemical form from plutonium oxide to a form of plutonium more susceptible to bioaccumulation and transport by natural processes. The source term release duration from Chernobyl was measured in weeks; the biogeochemical cycling of the longest lived radionuclides within the source term pulse will be measured in millennia. These nuclear accidents have an obvious presence which contrasts with the much longer release duration of the source terms of less obvious accidents such as are now occurring at the Rocky Flats Technology site or the Hanford or Savannah River Reservations. We do not yet know when the slow chronic release of plutonium from the Rocky Flats weapons production facility, especially the existing buildings with their contaminated duct work, piping etc., will terminate. The effectiveness of the proposed environmental remediation of this site and its contaminated buildings and soil will determine the source term release duration of this accident which could continue for decades or centuries. The same paradox applies to any nuclear facility which is a source point of uncontained anthropogenic radioactivity; the source term release duration will continue for as long as anthropogenic radioactivity is released from that particular location. This raises a question for an accident such as Chernobyl: while the principal source term release occurred over a period of a few weeks, what is the duration of the secondary, chronic leakage of radioactivity from this unsecured source point? (See the source term citations at the end of this section.)

STOCHASTIC EFFECTS: health effects that occur randomly and for which the probability of the effect occurring, rather than its severity, is assumed to be a linear function of dose without threshold (e.g. hereditary effects, cancer, etc.) (10 CFR 20.1003).

(TE)-NORM: "Technologically-Enhanced Naturally Occurring Radioactive Material, which are large-volume, low-activity waste streams produced by industries such as mineral mining, ore benefication, production of phosphate fertilizers, water treatment and purification, and oil and gas production. The majority of radionuclides in TENORM are found in the uranium and thorium decay chains. Radium and its subsequent decay products (radon) are the principal radionuclides used in characterizing the redistribution of TENORM in the environment by human activity. ... TENORM is found in many waste streams; for example, scrap metal, sludges, slags, fluids, and is being discovered in industries traditionally not thought of as affected by radionuclide contamination. Not only the forms and volumes, but the levels of radioactivity in TENORM vary." (http://www.normis.com/nrm101.htm)

TOTAL EFFECTIVE DOSE EQUIVALENT (TEDE): the sum of the deep dose equivalent (from external exposures) and the committed effective dose equivalent (from internal exposures). (Toxicological Profile for Ionizing Radiation, pg. 318).


Cesium-137 is the nuclide of choice used in this Website to characterize changing patterns of the dietary intake of artificial radionuclides*. Other biologically significant radionuclides and their sources of production include the following:

*Prior to the first nuclear explosion at Alamogordo, New Mexico on July 16, 1945, the dietary intake of radiocesium was zero.

Naturally Occurring Radionuclides:
Isotope Name Half-life Principle Decay Mode Maximum Energy Product of:
Radium-226 1599.0 y alpha 4.78450 Mev Natural Source 238U decay scheme
Radon-222 3.82351 d alpha 5.48966 Mev Same
Polonium-210 138.3763 d alpha 5.30451 Mev daughter 210Bi in radium decay scheme
Artificially Produced Radionuclides which also exist Naturally*:
Tritium 12.346 y beta 0.018610 Mev 6Li
Carbon-14 5730 y beta 0.155 Mev 14N
Krypton-85 10.701 y beta 0.672 Mev 84Kr
Artificial Radionuclides Produced by the Fission Process:
Strontium-89 50.55 d beta 1.488 Mev 88Sr 
Strontium-90 28.82 y beta 0.546 Mev fission
Iodine-129 157 x 107 y beta 0.150 Mev fission
Iodine-131 8.040 d beta 0.6065 Mev fission
Cesium-134 2.062 y beta 1.454 Mev 133Cs
Cesium-137 30.174 y beta 0.51163 Mev fission
Transuranic Nuclides Produced by the Fission Process:
Neptunium-237 2.14 x 106 y alpha 4.2 Mev 241Am
Plutonium-238 87.71 y alpha 5.49921 Mev 238Np
Plutonium-239 2.4131 X 104 y alpha 5.1554 Mev 235mU
Plutonium-241 14.355 y alpha 5.17 Mev Multiple n-capture from 238U, 239Pu
Americium-241 432.0 y alpha 5.48574 Mev 241Pu
Curium-242 162.76 d alpha 6.1129 Mev same as 241Pu
Curium-244 18.099 y alpha 5.80496 Mev same as 241Pu
Other Important Fission Products Include:
Molybdemum-99 67 hr beta 1.23 Mev
Technetium-99 2.12 x 105 y beta 0.292 Mev
Ruthenium-103 39.8 d beta 0.70 Mev
Ruthenium-106 1 y beta 0.039 Mev
Silver-110m 252 d gamma 0.74 Mev
Tellurium-132  78 hr. beta 0.22 Mev
Barium-140 12.8 d beta 1.02 Mev
Cerium-144 290 d beta 0.31 Mev
Europium-154 8.2 y beta 0.70 Mev
Important Activation Products Include:
Nickel-63  100 y beta 0.067 Mev
Nickel-59 76,000 y electron capture 1.06 Mev
Cobalt-58 0.194 y e.c. and gamma 0.474 Mev
Cobalt-60 5.2719 y beta 0.31788 Mev
Iron-55 2.68 y e.c.
Manganese-54 0.855 y gamma 0.835 Mev
Niobium-95 35 d beta 0.160 Mev
Zirconium-95 0.175 y gamma 0.396 Mev

Note: Most beta emitters have gamma emissions as a secondary mode of decay and vice versa (Exceptions: tritium, strontium-90, ruthinium-106).

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