Cardinal Surveys Company Glossary

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Absorbed Dose [rad]:
The rad is the unit of absorbed dose (D) measuring the energy imparted by ionizing radiation to matter.
1 rad = .01 Joule/kilogram (or: 100 ergs/gm)

Activity [Curie]:
The activity of a radioactive substance is often designated by the Curie [Ci]. The Curie is not a measure of dose; it merely states the amount of a radioactive disintegrations per unit time. The Curie is a unit of measurement defined as the activity of a radioactive substance disintegrating at a rate of: 3.7 x 1010 disintegrations per second.
Activity Units
Millicurie1/1,000 Ci[mCi]
Microcurie1/1,000,000 Ci[uCi]
Nanocurie1/1,000,000,000 Ci[nCi]
Picocurie1/1,000,000,000,000 Ci[pCi]
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ALARA concept:
The philosophy inherent in any program of radiation safety is to reduce exposure, whether internal or external, to a minimum:

"As Low As is Reasonably Achievable"

Whenever it is impossible or impractical to remove a source of radiation, other means must be considered for purposes of personnel protection. Three factors which determine the total exposure one receives in a given radiation field are:

1. Time of exposure.
2. Distance from Source.
3. Amount of shielding present.

The following is an excerpt from the Federal Register / Vol 51, No. 6 / Jan 9, 1986:

(Page 1126):

Dose control terms:

(1) "ALARA" (acronym for "As low as is reasonably achievable") means making every reasonable effort to maintain exposures to radiation as far below the dose limits in this part as is practical: (i) Consistent with the purpose for which the licensed activity is undertaken, (ii) taking into account the state of technology, the economics of improvements in relation to benefits to the public health and safety, and other societal and socioeconomic considerations, and (iii) in relation to utilization of nuclear energy in the public interest.

(Page 1130):

20.102 As low as is reasonably achievable levels of exposure.

(a) Each licensee shall ensure that the dose to individuals receiving occupational doses and to members of the public is as low as is reasonably achievable (ALARA) and does not exceed the appropriate limits.

Procedures and engineering controls based on sound radiation protection principles and practices should be used, to the extent practical, to reduce potential exposures.

Alpha particle:
A positively charged nuclear particle identical with the nucleus of a helium atom. It consists of two protons and two neutrons and is ejected at high speed from the nucleus of an atom in certain radioactive transformations.

Alpha radiation:
Alpha particles can be stopped by very thin absorbing materials, e.g.: a few sheets of paper, or, 1/64th inch of aluminum foil. Since alpha particles travel only short distances in air, and alpha particles up to 7.5 MeV are absorbed by the outer layer of skin (dead tissue), alpha radiation is not considered an external exposure problem.

Smallest particle of an element which is capable of entering into a chemical reaction.

Beta particle:
An electron or positron ejected from the nucleus of an atom during radioactive decay.

Beta radiation:
Though Beta particles have a greater penetration in any absorber than alpha particles, they can still be stopped by thin absorbing materials, e.g.: 1 inch of wood, or, 1/4 inch of Lucite. However, bremsstrahlung must be considered in shielding beta radiation: the higher the atomic number (Z) of the absorber, the greater the percentage of bremsstrahlung. Combination shields are effective, e.g.: for transport containers; a low-Z absorber can be used to stop the betas, followed by a high-Z absorber to attenuate the bremsstrahlung.

The secondary photon radiation (X-ray) produced by the deceleration of charged particles (especially beta particles) as they pass through matter (from German for "braking radiation").

Deposition of radioactive material in any place where it is not desired, particularly where its presence may be harmful.

Units of measurement (see Activity). One curie is that quantity of a radioactive nuclide disintegrating at the rate of 3.700 x 1010 atoms per second.

Units Of Activity
UnitsDisintegrations / Second
microcurie3.7 x 104
millicurie3.7 x 107
picocurie3.7 x 10-2

Decontamination factor:
The ratio of the amount of undesired radioactive material initially present to the amount remaining after suitable processing steps have been completed.

Dose Equivalent [rem]:
The dose equivalent concept allows the scaling up of the absorbed dose in order to better compare the effect of different types of radiation on human systems. Dose equivalent (DE) is expressed in rem (roentgen equivalent man) and is defined as the product of absorbed dose (D) in rads and other necessary modifying factors.

In normal protection work the product of absorbed dose and quality factor (QF) expresses the irradiation in terms of a common scale for all ionizing radiations.

DE = D x QF ... [rem]
Examples of QF are:
Type of RadiationQuality Factor(QF)
X-ray, gamma ray, beta particles1
Alpha particles10
Heavy recoil atoms20
Neutrons2 to 10.5

In regard to protection purposes the term QF replaces the former term RBE (relative biological effectiveness); in order to talk about RBE one must define exposure conditions as well as the effect studied.

Much effort has been directed toward the problem of calculating the dose which a person receives as a result of internally deposited radionuclides. Among the many factors of main concern that enter into such a calculation are: the shape of the organ, the type of radiation and the distribution of the deposit. The distribution factor (DF) is used to correct for non-uniform distribution in the case of internally deposited radionuclides. Thus, the dose equivalent becomes:

DE = D x QF x DF ... [rem]

Exposure [Roentgen]:
As a beam of photons passes through air, the interactions which take place produce electrons, which then lose energy by creating ion pairs. The exposure is measured by collecting these pairs. Thus, the exposure concept is based on the ability of photons to produce ionization. The special unit of exposure is the Roentgen [R]:

1 R = 2.58 x 10-4 Coulomb/kilogram of air
Milliroentgen1/1000 R[mR]

(This unit is numerically equal to the older definition for Roentgen: 1 R = 1 esu/cc of air; an exposure to X- or gamma- radiation such that the associated corpuscular emission per 0.001293 grams of air produces, in air, ions carrying one electrostatic unit or quantity of electricity of either sign.)

Exposure Rate [R/hr]:
The exposure (X) is defined in terms of ionization produced in a volume of air. The exposure rate is given by the quotient of exposure and time (t).

Exposure rate = X / t... [R/hr]

The special unit of exposure rate is Roentgen per hour, or milliroentgens:

Milliroentgen per hour: 1/1000 R/hr... [mR/hr]

A simple formula for use in Health Physics applications to estimate the exposure rate at a distance of one meter from a known isotropic point source of activity C [Ci] and energy E [MeV] for an energy range form .2 MeV to 2 Mev in air is:

Exposure rate = 0.53 x C x E [R/hr]

Film Badge:
A pack of photographic film which measures radiation exposure for personnel monitoring. The badge may contain two or three films of different sensitivity and filters to shield parts of the film badge from certain types of radiation.

Gamma Ray:
A photon emitted spontaneously from the nucleus of an atom of a radioactive substance. It is not a particle but a form of electromagnetic radiation, similar to light.

Geiger - Mueller Detector:
In very simple terms, the Geiger-Mueller is a metal tube that is filled with an inert gas. There is a wire running through the center and a wire mesh lining the inside of the tube. The mesh and the wire have opposite applied voltages. The charges are insulated from each other by the inert gas. When the gas is bombarded by radiation it looses its insulating properties and allows a sudden avalanche of electron and ion flow. This is registered as one count.

The tube then goes into a quenching mode. Basically it turns off and returns to its normal state so that it can start over again. While the quenching mode can be measured in milliseconds, this lag time reduces its efficiency as compared to the scintillation detector.

Advantages of the Geiger-Mueller are that it is very rugged and inexpensive, as compared to other detector types.

The physical or radioactive half-life is the time required for the activity of a given isotope to decay to one-half of its initial value. In evaluating the effects of radioactive substances deposited in the human system we need to address two additional half-lifes:

1. The biological half-life: It is the time required for the body to eliminate one-half of the amount of a radioactive substance internally deposited by excretion, exhalation and perspiration.

2. The effective half-life: It is defined as the time required for the radioactivity from a given amount of radioactive substance deposited in the tissues or organs to diminish by 50 % as a result of the combined action of radioactive decay and loss of the material by biological elimination. The effective half-life is usually experimentally determined.

Half-value Layer:
The half-value layer is the thickness of a substance which reduces the intensity of a beam of radiation to one-half of its initial value. The half-value layer is a function of the energy of the gamma and the composition of the shield or absorber. Examples:

Half-Layer Values
Radioactive MaterialLeadSteelConcrete

Health Physics:
A science and profession devoted to the protection of man and his environment from unnecessary radiation exposure.

Inverse Square Law:
The radiation field decreases with distance from the source. When considering a point source in air, the decrease will follow the inverse square law, which states that the amount of radiation at a given distance from a source is inversely proportional to the square of the distance.

I/i = d2/D2


I x D2 = i x d2

(Where I = intensity at a distance (D) from a point source, and i = intensity at a distance (d) from the same source).
Example: If the exposure rate at 1 meter equals 100 mR/hr then the exposure rate at 2 meters equals 25 mR/hr.

Exposure Rates VS. Distance - 100 mci Sources
Radioactive IsotopemR/hr @ 3'mR/hr @ 6'mR/hr @ 9'

Atomic particle, atom or chemical radical bearing an electrical charge, either positive or negative. Ionization: The process by which a neutral atom or molecule acquires a positive or negative charge.

Ionizing Radiation:
Any electromagnetic or particulate radiation capable of producing ions, directly or indirectly, in its passage through matter.

A neutron is an elementary nuclear particle with a mass slightly larger than a proton and a net charge of zero.

Photons are much more penetrating than alpha and beta particles. X and gamma radiation is never completely absorbed; however, we can choose a shield composition and thickness which will reduce the intensity to non-hazardous levels (see also: Half-value Layer). Generally, high density materials are best suited for gamma shielding.

Radiation Detection:
Radiation cannot be detected with the unaided senses, an instrument must be used to identify presence, type or intensity of radiation. All instruments consist of a detector and a measuring apparatus (some substance that responds to the radiation and a system to measure the extent of the response). Some detection systems use the ionization produced in them, and other systems depend upon excitation. Chemical and photographic detection principles are also used.

The property possessed by some elements (e.g.: uranium) of spontaneously emitting Alpha, Beta or Gamma Rays by disintegration of the nuclei of the atoms. An element is said to be radioactive if it can spontaneously decay or be transformed into another element. This transformation is always accompanied by emission of nuclear radiation.

Record of radiation from radioactive material in an object, made by placing the object in close proximity to a photographic emulsion.

Isotopes are species of atoms of a chemical element with the same number of protons but differing in the number of neutrons, resulting in the same atomic number but different atomic masses. Some isotopes are unstable, releasing energy in the form of radioactivity, they are called radioisotopes.

Roentgen (R):
The basic unit of exposure to X or gamma radiation. The special unit of exposure. One R = 2.58 x 10-4 coulombs/kgm. of air. One milliroentgen (mR) is equivalent to 1/1000 of one Roentgen.
See Dose Equivalent (Roentgen's Equivalent to Man)

Scintillation Detector:
The scintillation detector is composed of a sodium iodine crystal and a photo multiplier that are usually glass encased. Every time a gamma ray passes through the crystal, a small flash of light is emitted. The photo multiplier picks up this flash and magnifies it to be registered as one count. This makes for a very sensitive detector. For all practical purposes there is no down time like that found in the Geiger - Mueller detector.

Another advantage of the scintillation detector is that it can differentiate energy levels in the counts. Thus it can be calibrated for use in spectrum analysis.

On the down side, scintillation detectors are expensive as compared to Geiger - Mueller's and they are not very rugged.

Shielding is a method of radiation protection. A shield is a body of material positioned to prevent or reduce the passage of radiation. The effectiveness of a shield is determined by the interaction between the incident radiation and the absorbing medium. See Half-value Layer.

A photon emanating from outside the nucleus of an atom. It is not a particle but a form of very short wave electromagnetic radiation, similar to gamma rays but originating outside the nucleus.

Penetrating electromagnetic radiation whose wave lengths are shorter than those of visible light. They are usually produced by bombarding a target (metallic) with fast electrons in a high vacuum. In nuclear reactions, it is customary to refer to photons originating in the nucleus as Gamma Rays and those originating in the extranuclear parts of the atom as X-rays. They are sometimes referred to as Roentgen rays after Wilhelm Conrad Roentgen, the discoverer. The only difference between X and gamma radiation is the source.

See Dose Equivalent (Roentgen's Equivalent to Man)

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