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Radiation Units

Two types of units are used for radiation, units of activity and units of exposure (dose). Units of activity quantify the amount of radiation emitted by a given radiation source. Units of exposure quantify the amount of radiation absorbed or deposited in a specific material by a radiation source.

In the world today, two sets of units exist. They are the Special units (Curie, Roentgen, Rad and Rem) and the SI or International Units (Becquerel, Gray and Sievert). In the United States, the Special units must be used as required by Federal law. Therefore, in our discussions the units used will always be the Special units. SI units are defined and described in the appendix on units.

Units of Activity

The unit of activity for radiation is the Curie, abbreviated Ci. Most laboratory facilities use only millicurie (mCi, 0.001 Ci) or microcurie (uCi, 0.000001 Ci) amounts of radioactive materials, since reliable data can only be obtained using low levels of activity for a given isotope. The Curie is an amount of radioactive material emitting 2.22 x 1012 disintegrations (particles or photons) per minute (DPM). (The international, or SI, unit for radioactivity is the Becquerel, defined as one disintegration per second.) Activity can be measured with an appropriate radiation detection instrument. Most of these measurements are made with a liquid scintillation counter, gamma well counter or Geiger-Mueller (GM) survey meter with appropriate detection probes. These instruments detect a percentage of the disintegrations and display in counts per minute (CPM).

It is important to note that the CPM readings from survey instruments are not the true amount of radiation present, since there are factors which decrease the detection capability of even the most sensitive instruments. Two factors influence radiation detection sensitivity: the geometry of the counting system and the energy of the radionuclide being measured. Lower energy radionuclides are detected with lower efficiencies than higher energy radionuclides. Detection instruments are calibrated with known sources with different energy levels to determine the efficiency of the instrument in order to account for these variables.

Therefore, in order to correct for the above sources of error in the measurements, a calculation must be made to standardize the units of activity used in all facilities licensed by the NRC. It is required by NRC law that all records relevant to NRC licensed activities must be maintained in units of DPM or microcuries. To make the necessary conversion to the microcurie unit, the following formula must be used in all records of surveys, waste materials or radioactive solutions generated within the facility.

CPM/Efficiency = DPM

DPM/2.22 x 106 = uCi

Units of Exposure

The Roentgen, abbreviated as "R", is the unit for measuring the quantity of x-ray or gamma radiation by measuring the amount of ionization produced in air. One Roentgen is equal to the quantity of gamma or x-radiation that will produce ions carrying a charge of 2.58 x 10-4 coulombs per kilogram of air. An exposure to one Roentgen of radiation with total absorption will yield 89.6 ergs of energy deposition per gram of air. If human tissue absorbs one Roentgen of radiation, 96 ergs of energy will be deposited per gram of tissue. (The international units do not include the Roentgen, but simply use the amount of energy deposited in air as the descriptive term.)

The Roentgen is easy to measure with an ion chamber, an instrument that will measure the ions (of one sign) produced in air by the radiation. The ion chamber has a readout in Roentgen per hour or fractions thereof, and is an approximation of tissue exposure. EHS, National Superconducting Cyclotron Laboratory (NSCL) and other departments with the potential for external radiation exposures use ion chambers for measuring exposure potential. It is a useful instrument for gamma radiation; however, it is not quantitatively accurate for alpha, beta or neutron radiation.

The rad and the rem are the two main radiation units used when assessing radiation exposure. The rad (radiation absorbed dose), is the unit of absorbed dose, and refers to the energy deposition by any type of radiation in any type of material. (The international unit for absorbed dose is the Gray; it is defined as being equal to 100 rads.) One rad equals 100 ergs of energy deposition per gram of absorber.

The rem (radiation equivalent man) is the unit of human exposure and is a dose equivalent (DE). (The international or SI unit for human exposure is the Sievert, which is defined as equal to 100 rem.) It takes into account the biological effectiveness of different types of radiation. The target organ is important when assessing radiation exposures, and a modifying factor is used in radiation protection to correct for the relative biological effectiveness (RBE or quality factor). Also, the chemical form of the radiation producing the dose is of critical importance in assessing internal doses, because different chemicals bind with different cell and/or organ receptor sites.

Additionally, some types of radiation cause more damage to biological tissue than other types. For example, one rad of alpha particles is twenty times more damaging than one rad of gamma rays. To account for these differences, a unit called a quality factor (QF) is used in conjunction with the radiation absorbed dose in order to determine the dose equivalent in rem:

rem dose = rad dose x QF x other modifying factors

Tissue weighting factors, Wt, are used for incorporating the actual risk to tissues for different radioisotopes and tissues in dose calculations. These weighting factors assign multiplication factors for increasing or decreasing the actual biological risk to a given tissue.

Another way to evaluate risk to an individual for internal intakes of radioactive material is the use of body retention class, D, W or Y. These classes stand for Days, Weeks or Years of retention time in the human body and are specified in the Title 10 CFR 20 limits in the Appendix B. This classification is based on the chemical form of the radioactive material, which affects the biochemical pathway and resultant target organ, therefore determining the retention time.

The dose rate is proportional to the radiation flux (number of particles or photons/square centimeter/second) and is expressed in rem/hour or mrem/hour. (Radiation dosimeter readings are reported in mrem units). The dose rate can be estimated by using an ion chamber when the radiation source is a gamma or x-ray emitter. The ion chamber is useful in estimating dose for beta radiation, but special detectors for alpha or neutron radiation are required.

The provisions of the MSU Exposure Control Plan provide protection to employees who have occupational exposure to human blood or other potentially infectious materials (OPIM). Established human cell lines1 which are characterized2 as free of contamination from human hepatitis viruses, human immunodeficiency viruses, and other recognized bloodborne pathogens, are not to be considered as OPIM and are not covered by the bloodborne pathogens standard and the Exposure Control Plan.

Biological Effects of Radiation

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