TM 5-6635-386-12&P
2-2. COMMON RADIATION TERMS AND VALUES
Radiation is similar to light. It increases in intensity by a
factor of FOUR each time the distance from the source
is cut in HALF.
The radiation level drops by a factor of FOUR each time
you move TWICE as far away from the source.
Certain terms are used to describe radiation factors
important to tester users. Be familiar with the following
terms and values:
CURIE: A term used to describe the size of a radioactive
source. It represents a quantity of material disintegrating
at the rate of 3.7 x 10 disintegrations per second, or the
same rate as one gram of Radium. This is not an index
of how dangerous the source might be, but only an index
of quantity of the material in question.
MILLICURIE: One thousandth (1/1000) of a curie.
MICROCURIE: One millionth (1/1, 000, 000) of a curie.
ROENTGEN: A term describing the amount of radiation
accumulated or exposed to, by standing near a large
radioactive, unshielded source for a short time or near a
small, unshielded source for a long time.
REM (rem): This is a better term for measuring human
exposure accumulation than Roentgen because it has
been corrected to provide a common base for radiation
effects on people. Some radiation is highly penetrating
and would be more potentially dangerous than other
forms. The descriptions become equal when they are all
corrected to the common rem base.
MILLIREM (mrem): One thousandth (1/1000) of a rem.
Tester radiation levels are commonly measured in these
very small units.
MILLIREM/HOUR (mrem/hr): A term used to describe
the "brightness" of a radioactive gamma source. It is the
strength
of
the
radiation
field
at
the
point
of
measurement. This term is similar to footcandles of light
when discussing light.
The brightness of the radiation field will be dictated by
the type of radioactive material involved, the size of the
source, the amount of shielding present, and the
distance from the source. The total amount of radiation
accumulated would then become a factor of how much
time was spent in the radiation field.
NOTE
Since the tester uses small, well shielded
sources, operators will be involved with only
millirems of radiation and with levels which are
only in the mrem/hr range.
100 mrem: Weekly allowed dose (5 rem/yr is max annual
work dose).
5 mrem/hr: Average radiation dose at tester surface.
1/2 mrem/hr: Average gamma dose at 2’ from tester
(arm’s length).
1/3 mrem/hr: Average neutron dose at 3’ from tester
(source to midtrunk distance when carrying tester).
1/2 mrem: Average heavy workweek accumulation of
radiation for a tester user (1/200th of allowed dose).
2-2
