Marshall Islands Dose Assessment and Radioecology Program
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Glossary of Terms

Activity Activity is the rate of transformation or decay of a radioactive material. The SI unit of activity is the reciprocal second (s¹) and its special name is the Becquerel. Federal and state agencies in the United States use conventional units where activity is given in curies (Ci); 1 Ci = 3.7 x 10^10 Bq.
Alpha Particles Alpha particles are one of the primary types of radiation associated with radioactivity and exist as energetic nuclei of helium atoms, consisting of two protons and two neutrons. Alpha rays are heavy, slow moving charged particles that travel only one or two inches in air, and can be stopped by a piece of paper or the outer dead layer of human skin.
Background Radiation The average person in the United States receives about 3.6 mSv (360 mrem) of ionizing radiation every year. About 3 mSv (300 mrem) per year comes from natural background radiation including cosmic radiation, radiation emitted by naturally occurring radionuclides in air, water, soil, and rock, and radiation emitted by natural radionuclides deposited in tissues of organs; and about 0.60 mSv (60 mrem) from man-made sources, such as exposures to diagnostic x-rays and consumer products (e.g., from smoking tobacco). The general worldwide contribution from radioactive fallout contamination is less than 0.3% of the average total annual dose. Exposures to natural background radiation vary depending on the geographic area, diet, and other factors, such as the composition of materials used in the construction of homes. The natural background radiation dose in the Marshall Islands is around 1.4 mSv (140 mrem) per year and is significantly less than what most people receive around the world.
Baseline We have all been exposed to some level of worldwide fallout contamination. In the United States, the general population receives up to 0.015 mSv (1.5 mrem) (0.3% of the average total annual dose) from exposure to worldwide fallout contamination from atmospheric nuclear weapons testing and about 0.005 mSv (0.5 mrem) (or 0.1% of the average total annual dose) from operations related nuclear power generation. Similarly, people living in the Marshall Islands will have very small quantities of internally deposited fallout radionuclides, such as cesium-137, strontium-90, and plutonium, in their bodies from worldwide contamination of food, air, water, and soil. Assessments of possible increases in radiation exposure from elevated levels of fallout contamination in the northern Marshall Islands can only be made on the basis of comparisons with residual systematic burdens of radionuclides acquired from previous exposures to global fallout contamination. Under the Marshall Islands Radiological Surveillance Program, efforts are being made to improve on the reliability of measurements of background urinary excretion rates of plutonium from Marshallese populations against which the results of future bioassay measurements can be compared to accurately assess the impacts of resettlement on radiation exposure and dose.
Becquerel (Bq) A Becquerel (abbreviated as Bq) is the International System (SI) unit for activity of radioactive material. One Bq of radioactive material is that amount of material in which one atom is transformed or undergoes one disintegration every second. Whole body counting and plutonium bioassay measurements are usually reported in activity units of kBq (kiloBecquerel) (1000 × 1 Bq) and µBq (microBecquerel) (1 × 10^6 × 1 Bq), respectively.
Biokinectic The word biokinectic is used here to describe the adsorption (uptake), distribution, and retention of elements in humans.
Calibration Calibration is the process of adjusting or determining the response or reading of an instrument to a standard.
Committed Dose Equivalent The committed dose equivalent is the time integral of the dose-equivalent rate in a particular tissue that will be received by an individual following an intake of radioactive material into the body by inhalation, ingestion, or dermal absorption. For adults the committed dose is usually the dose received over 50 years. For children, the committed dose is usually calculated from the age of intake to age 70 years. For these age groups the term integrated dose equivalent is used.
Committed Effective Dose Equivalent (CEDE) The committed dose equivalents to various tissues or organ in the body each multiplied by an appropriate tissue-weighing factor and then summed. The conventional unit for committed effective dose equivalence (CEDE) used by federal and state agencies within the United States is the rem. The international scientific (SI) unit of committed effective dose equivalent is called a sievert (Sv). One Sv is the same as 100 rem.
Critical Level The amount of a count (Lc or final measurement of a quantity of an analyte at or above which a decision is made that the analyte is definitely present (Lc≈ MDA/2).
Default Assumptions (used in assignment of dose) The largest dose contributions attributable to exposure to residual nuclear fallout contamination in the Marshall Islands result from either (1) internal exposure from intakes of radionuclides through ingestion, inhalation, and/or absorption through the skin, or (2) external exposure from radionuclides distributed in the soil. External exposure rates can be measured directly using instrument surveys of the radiation field. The assignment of dose to internally deposited radionuclides is much more complicated. Biokinectic and dosimetric models developed by the International Commission on Radiological Protection (ICRP) are used to convert whole body burdens (from whole body counting or from in vitro bioassay tests, such as urinalysis) into dose. In the case of a chronic exposure, organ and body burdens continue to build up over time until a steady state is reached where losses due to decay and excretion are balanced by intake and absorption. Cesium-137 has an effective half-life in an adult of about 110 days, and under chronic exposure conditions reaches a maximal dose contribution after about 2 years. By contrast, plutonium absorbed from the gastrointestinal or respiratory tract enters the blood stream and deposits in liver and bone with an effective half-life of 20 to 50 years. Only a small fraction of plutonium entering the blood stream is excreted in urine with the long-term excretion rate approaching 2 x 10^5 of the systemic body burden per day. Knowledge of excretion rates and time of exposure are important when interpreting urinalysis data. A more detailed discussion of the dose calculation methodology is given elsewhere (see under Daniels et al., 2007).
Direct Bioassay The measurements of radioactive material in the human body utilizing instrumentation that detects radiation emitted from radioactive material in the body (synonymous with in-vivo measurements).
Dose Assessment The scientific process used to determine radiation dose and uncertainty in the dose.
(Exposure) Dose Assessment A quantification of the magnitude, duration, and timing of radiation exposures, and the resulting doses from such exposures, based on all possible types of radiological agents involved and their primary pathways and routes of exposure.
Dose Equivalent The dose equivalent is the adsorbed dose at a point in tissue multiplied by a biological effectiveness factor or quality factor for the particular types of radiation to cause biological damage. The conventional unit of dose equivalents used by federal and state agencies in the United States is the rem. A dose of 100 rem to an adult normally produces some clinical signs of radiation sickness and requires hospitalization. The international scientific unit for dose equivalent is the joule per kilogram (J kg¹) and is called the sievert (Sv). One Sv is the same as 100 rem.
Effective Dose Equivalent The effective dose equivalent for the whole body is the sum of dose-equivalents for various organs in the body weighted to account for different sensitivities of the organs to radiation. It includes the dose from radiation sources internal and/or external to the body. The effective dose equivalent is usually expressed in units of millirem (mrem). The international scientific unit for dose equivalent is the joule per kilogram (J kg-1) and is called the sievert (Sv). One Sv is the same as 100 rem.
Exposure Pathway The physical route a hazardous substance takes in leading to the exposure of an organism.
External Dose or Exposure or Radiation That portion of the dose equivalent delivered by ionizing radiation originating from a source outside the body of an organism (e.g., also know as direct radiation).
Fission Track Analysis During neutron irradiation heavy nuclei, such as uranium and plutonium, undergo nuclear fission with release of large fission fragments. This property has led to the development of a number of measurement techniques, such as delayed neutron activation analysis and fission-track analysis. Fission-track analysis is a measurement technique commonly employed in plutonium urinalysis (bioassay) monitoring programs. Urine samples are chemically treated to remove plutonium. The plutonium is then mounted in contact with a special plastic or quartz slide known as solid-state nuclear track detector (SSNTD). The slide along with the sample is then irradiated in a reactor where neutron-induced fission of plutonium-239 (or uranium-235) causes emission of energetic fission fragments. Some of the fragments penetrate into the SSNTD damaging the integrity of the material before coming to rest. The SSNTD is separated from the sample and chemically etched to expose the damaged areas (known as fission tracks) on the detector surface. The fission tracks are then counted under an optical microscope. The amount of plutonium (and/or uranium) present in the sample is a function of the total number of tracks and the neutron flux.
Gamma-rays Gamma rays are electromagnetic waves produced by spontaneous decay of radioactive elements during de-excitation of an atomic nucleus. Sunlight also consists of electromagnetic waves, but gamma rays have a shorter wavelength and much higher energy. High-energy gamma rays, such as those produced by decay of cesium-137, may penetrate deeply into the body and affect cells. Gamma rays from a cobalt-60 source are often used for cancer radiotherapy.
Half-lifeThe period of time in which the activity of a radioactive material decreases by half.
High-End Health RiskUse of the term high-end health risk usually relates to the maximally exposed individuals in a population.
In VitroIn-vitro measurements are synonymous with indirect bioassay techniques, such an plutonium urinalysis.
In Vivo In-vivo measurements are synonymous with bioassay techniques, such whole body counting.
Indirect Bioassay Measurements to determine the presence of and/or the amount of a radioactive material in the excreta, urine, or in other biological materials removed from the body (synonymous with in-vitro measurements).
Individual An individual is any human being.
Internal Dose or Exposure or Radiation That portion of the dose equivalent delivered by ionizing radiation originating from a radiation source inside the body of an organism (e.g., from intakes of radionuclides by ingestion, inhalation, or dermal adsorption).
Isotope Atoms with the same number of protons but different numbers of neutrons are called isotopes of that element. We identify different isotopes by appending the total number of nucleons (the total number of proton plus neutrons in the nucleus of an atom) to the name of the element, e.g., cesium-137. Isotopes are usually written in an abbreviated form using the chemical symbol of the element.
Minimum Detectable Amount (MDA) The minimum datable amount (MDA) is the smallest activity or mass of an analyte in a sample or person that can be detected with an acceptable level of uncertainty.
plutonium Plutonium (Pu) is a radioactive, metallic element. It is the element used in most modern nuclear weapons. It can be derived from natural uranium.
Quality Assurance All those planned and systematic actions necessary to provide adequate confidence that an analysis, measurement or surveillance program will perform satisfactorily.
Quality Control Those actions that control the attributes of analytical process, system, or facility according to predetermined quality requirements.
Radiation Dose (or mrem)A generic term to describe the amount of radiation a person receives. Dose is measured in units of thousands of a roentegen equivalent man (rem). The millirem (normally abbreviated as mrem) is the preferred unit used by federal and state agencies in the United States. Dose is a general term used to assist in the management of exposure to radiation. The common International System (SI) unit for dose is the millisievert (mSv). One mSv is the same as 100 mrem.
Radioactivity A natural and spontaneous process by which unstable atoms of an element emit energy and/or particles from their nuclei and, thus change (or decay) to atoms of a different element or a different state of the same element.
Radiological Monitoring Radiological monitoring is the measurement of radiation levels or individual doses, and the use of the results to assess radiological hazards in the environment or workplace, or the potential and actual doses resulting from exposures to ionizing radiation.
Remediation Remediation is the actions taken to reduce risks to human health or the environment posed by the presence of radioactive or hazardous materials.
RiskThe probability of harm from the presence of radionuclides or hazardous materials taking into account (1) the probability of occurrences or events that could lead to an exposure, (2) probability that individual or populations would be exposed to radioactive or hazardous materials and the magnitude of such exposures, and (3) the probability that an exposure would produce a response.
Total Effective Dose Equivalent (TEDE)The sum of the deep-dose equivalent (for external exposures) and the committed effective dose equivalent for external from intakes of radionuclides as described by the U.S. Nuclear Regulatory Commission under 10 CFR Part 20.1003.
ValidationDefining the process of the method capability and determining whether it can be properly applied as intended.
Whole Body For the purposes of external exposure includes the head, trunk, the arms above and including the elbow, and legs above and including the knee.

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