Definitions

(a) (1) "Dose," as used in sections 19-24-1 to 19-24-14, inclusive, means the quantity of radiation absorbed, per unit of mass, by the body or by any portion of the body. When a dose during a period of time is specified, the dose means the total quantity of radiation absorbed, per unit of mass, by the body or by any portion of the body during such period of time. Several different units of dose are in current use. The definitions are set forth in subdivisions (2) and (3) below. (2) The "rad," as used in sections 19-24-1 to 19-24-14, inclusive, is a measure of the dose of any ionizing radiation to body tissues in terms of the energy absorbed per unit mass of the tissue. One rad is the dose corresponding to the absorption of one hundred ergs per gram of tissue. (One millirad (m rad) = 0.001 rad.) (3) The "rem," as used in said sections, is a measure of the dose of any ionizing radiation to body tissue in terms of its estimated biological effect relative to a dose of one roentgen (r) of x-rays. (One millirem (mrem) = 0.001 rem.) The relation of the rem to other dose units depends upon the biological effect under consideration and upon the conditions of irradiation. For the purpose of this regulation, any of the following is considered to be equivalent to a dose of one rem: (A) A dose of one roentgen due to x- or gamma radiation; (B) A dose of one rad due to x-, gamma, or beta radiation; (C) A dose of one-tenth rad due to neutrons or high energy protons; (D) A dose of five-hundredths rad due to particles heavier than protons and with sufficient energy to reach the lens of the eye. If it is more convenient to measure the neutron flux, or equivalent, than to determine the neutron dose in rads, as provided in subparagraph (C) above, one rem of neutron radiation may for purposes of sections 19-24-1 to 19-24-14, inclusive, be assumed to be equivalent to fourteen million neutrons per square centimeter incident upon the body; or, if there exists sufficient information to estimate with reasonable accuracy the approximate distribution in energy of the neutrons, the incident number of neutrons per square centimeter equivalent to one rem may be estimated from the following table: Neutron Flux Dose Equivalents Neutron energy (Mev) Number of neutrons per square centimeter equivalent to a dose of 1 rem (neutrons/cm2) Average Flux to deliver 100 millirem in 40 hours (neutrons/cm2 per sec.) Thermal . . . . . . . . 970 x 106 670 0.0001 . . . . . . . . 720 x 106 500 0.005 . . . . . . . . 820 x 106 570 0.02 . . . . . . . . 400 x 106 280 0.1 . . . . . . . . 120 x 106 80 0.5 . . . . . . . . 43 x 106 30 1.0 . . . . . . . . 26 x 106 18 2.5 . . . . . . . . 29 x 106 20 5.0 . . . . . . . . 26 x 106 18 7.5 . . . . . . . . 24 x 106 17 10 . . . . . . . . 24 x 106 17 10 to 30 . . . . . . . . 14 x 106 10 (4) For determining exposure to x- or gamma rays with energies up to three Mev, the dose limits specified may be assumed to be equivalent to the "air dose." "Air dose" means the dose as measured by a properly calibrated appropriate instrument in air at or near the body surface in the region of highest dosage rate. (b) Units of radioactivity. (1) Radioactivity is commonly, and for purposes of sections 19-24-1 to 19-24-14, inclusive, shall be measured in terms of disintegrations per unit time or in curies. One curie (c) = 3.7 x 1010 disintegrations per second (dps) = 2.2 x 1012 disintegrations per minute (dpm). A commonly used submultiple of the curie is the microcurie (µc). One µc = 0.000001 c = 3.7 x 104 dps = 2.2 x 106 dpm. (2) For purposes of said sections it may be assumed that the daughter activity concentrations in the following table are equivalent to an air concentration of 10-7 microcuries of Radon 222 per milliliter of air in equilibrium with the daughters RaA, RaB, RaC, and RaC. Maximum time between collection and measurement (hours) 1 Alpha-emitting daughter activity collected per milliliter of air Microcuries/cc Total alpha disintegrations per minute per cc. 0.5 . . . . . . . . . . . 7.2 x 10-8 0.16 1 . . . . . . . . . . . 4.5 x 10-8 0.10 2 . . . . . . . . . . . 1.3 x 10-8 0.028 3 . . . . . . . . . . . 0.3 x 10-8 0.0072 1 The duration of sample collection and the duration of measurement should be sufficiently short compared to the time between collection and measurement, as not to have a statistically significant effect upon the results. (3) Natural uranium and natural thorium. (A) The purposes of sections 19-24-1 to 19-24-14, inclusive, the sum of 3.7 x 1010 disintegrations per second from U-238 plus 3.7 x 1010 disintegrations per second from U-234 plus 9 x 108 dis/sec from U-235. Also, a curie of natural thorium (thorium-natural) means the sum of 3.7 x 1010 dis/sec from Th232 plus 3.7 x 1010 dis/sec from Th228. (B) For the purposes of said sections, one curie of natural Uranium (U-natural) is equivalent to 3,000 kilograms, or 6,615 pounds of natural uranium; and one curie of natural thorium (thorium natural) is equivalent to 9,000 kilograms or 19,850 pounds of natural thorium.