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RadNet Glossary

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alpha particle

A positively charged particle made up of two neutrons and two protons emitted by certain radioactive nuclei. Alpha particles can be stopped by thin layers of light materials, such as a sheet of paper or the dead cells in the outer skin layer. They are not an external health threat to the body. However, they can pose a serious health threat if ingested (swallowed) or inhaled.

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ambient air

The air that surrounds us.

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background (radiation)

Natural radiation caused by sun exposure, cosmic rays from space, and radioactive elements found in the earth's crust. Radon, a radioactive gas that seeps from the ground, is an example of natural radiation. Cosmic rays include energetic protons, electrons, gamma rays, and x-rays. The primary radioactive elements found in the earth's crust are uranium, thorium, and potassium, and the radionuclides produced as they radioactively decay.


The radionuclide concentration or radiation level that is considered "normal" under routine conditions. A baseline is usually determined by averaging many measurements of background radiation. Comparing a sample's analysis results to the baseline will show if radiation levels are normal or high.

beta particles

An electron or positron emitted by certain radioactive nuclei. Beta particles can be stopped by aluminum. They can be a serious direct or external radiation threat to the body and can be lethal depending on the amount received. They also pose a serious internal radiation threat if inhaled or ingested (swallowed).

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Curie (Ci)

A measure of radioactivity based on the radioactive decay rate of approximately one gram of radium. The Curie was named in honor of Pierre and Marie Curie, pioneers in the study of radiation.

One Curie of any radionuclide has 37 billion disintegrations (transformations) in one second.

Factor Prefix Symbols Factor Prefix Symbols
1018 exa E 10-1 deci d
1015 peta P 10-2 centi c
1012 tera T 10-3 milli m
109 giga G 10-6 micro
106 mega M 10-9 nano n
103 kilo k 10-12 pico p
102 hecto h 10-15 femto f
101 deka da 10-18 atto a
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In the RadNet system, portable monitors that can be sent (deployed) quickly to a location where they are needed


The amount of radiation or energy absorbed. There are different ways to measure dose:

  • absorbed dose, the amount of energy deposited per unit mass
  • equivalent dose, a way to measure dose that accounts for the fact that different radionuclides can have more or less affect on the body (called "relative biological effect").
  • committed dose, a dose that accounts for continuing exposures over long periods of time (such as 30, 50, or 70 years).
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A term relating to the amount of ionizing radiation that strikes a surface. (This is a general definition. In health physics, exposure is specifically defined as a measure of ionization in air caused by x-ray or gamma radiation only.)

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  • Exposure Pathways
    This information describes the ways that people may be exposed to radiation.

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The splitting of a nucleus of an atom into at least two other nuclei that releases a relatively large amount of energy. Two or three neutrons are usually released during this type of transformation.

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gamma rays

Electromagnetic radiation that has high energy and a short wavelength. All gamma rays emitted from a given isotope have the same energy, a characteristic that enables scientists to identify which gamma emitters are present in a sample. Gamma rays are very similar to x-rays. They can be a serious direct or external radiation threat to the body and can be lethal depending on the amount received.

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the time required for one half the atoms of a given amount of a radioactive substance to disintegrate (radioactively decay).

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  • Half-Life
    This information explains and illustrates radioactive half-life.

health physicist

A scientist who focuses on radiation protection of humans and the environment. Health Physics uses physics, biology, chemistry, statistics and special instruments such as Geiger counters to help protect individuals from any damaging effects of radiation.

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ionizing radiation

Radiation from naturally occurring and man-made radionuclides, including alpha and beta particles and gamma rays. Ionizing radiation has enough energy to break chemical bonds.

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A nuclide of an element having the same number of protons but a different number of neutrons.

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  • Nuclides and Isotopes
    This information explains the differences and similarities between nuclides and isotopes.

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maximum contaminant level (MCL)

limits on the levels of contaminants in public drinking water systems. MCLs are "primary drinking water standards. National Primary Drinking Water Regulations (NPDWRs or primary standards) are legally enforceable standards that apply to public water systems. Primary standards protect public health by limiting the levels of contaminants in drinking water.

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The use of sampling and detection equipment to measure the levels of radiation or other materials in soil, air, or water.

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A small particle possessing no electrical charge and typically found inside an atom's nucleus. A neutron has about the same mass as a proton.

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nuclear energy

The heat energy produced by the process of nuclear reaction (fission or fusion) within a nuclear reactor or by radioactive decay.

nuclear fallout

Radioactive particles that fall to the ground after a nuclear explosion.


the positively charged mass >at the center of an atom, composed of neutrons and protons

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Minute separate particles of material such as dust that are usually captured with filters from samples of air or water.


The ways in which people are exposed to radiation or other contaminants. The three basic pathways are inhalation (contaminants are taken into the lungs), ingestion (contaminants are swallowed), and direct (external) exposure (contaminants cause damage from outside the body).

picocurie (pCi)

One one-trillionth (1/1,000,000,000,000) of a curie.

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an area of contamination as it spreads into the environment from the source of the contamination

proportional counter

A radiation detector that produces a signal which is proportional to the radiation energy that strikes it.


A small particle in an atom's nucleus, that has a positive electrical charge. Each chemical element has a particular number of protons. When an atom loses a proton during radioactive decay, it becomes an atom of a different element.

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quality assurance (QA)

A set of activities such as testing and assessment that ensure high quality in a product

radioactive decay

The spontaneous disintegration of the nucleus of an unstable atom that produces ionizing radiation, including alpha or beta particles and gamma rays


Involving radioactive material


"Versions" of chemical elements that are not stable, or in other words, are likely to radioactively decay, giving off ionizing radiation

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  • Radionuclide fact sheets
    These fact sheets provide basic information about the discovery and properties of several radionuclides.


The probability of injury or harm under specific circumstances. Risk can be expressed as a value that ranges from zero (no injury or harm will occur) to one hundred percent (harm or injury will definitely occur). Risk-based standards limit the risk that a release of a contaminant to the environment may pose rather than limiting the quantity that may be released.

spectrographic analysis

Gamma-ray spectroscopy is a technique used to identify the identities and quantities of gamma-ray-emitting radionuclides in a sample. This process uses special detectors and computer software to analyze the range (spectrum) of energies the sample emits.

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High-energy electromagnetic radiation that has high energy and a short wave length. X-rays are very similar to gamma rays.


the explosive force of a nuclear bomb. It is given in terms of the amount of TNT that would release the same amount of energy.

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