EHS > Radiation
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.
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.
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.
x 106 = uCi
Units of Exposure
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
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.
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
dose = rad dose x QF x other modifying factors
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.
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.
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.
Effects of Radiation
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