Radiation Theory
Radioactivity is the spontaneous breakdown of unstable nuclei
(radioisotopes) with the resulting emission of radiation. The
basic unit of radiation used in the U.S. is the curie (Ci) and is
defined as 3.7 x 10
10
disintegrations of nuclei per second. In the
“Special Form,” encapsulated sealed source used in the 3241, the
unit of measure is the millicurie (1/1,000 of a curie). The SI unit
of radiation is the Becquerel and is equal to one disintegration
per second. Therefore, one curie equals 3.7 x 10
10
Becquerels.
The strength of radioactive material is measured by its activity,
or rate of decay. This activity decreases with time. The length of
time it takes a given amount of radioactive material to decay to
half of its original strength is referred to as the “half-life.” The
half-life of an Americium-241 source is approximately 432 years.
Radiation Terminology
Various standards for the measurement of radiation exist, but
only two concern the Troxler instrument user. These units are
the curie and the roentgen equivalent man (rem). The curie,
defined as the quantity of radioactive material giving 3.7 x 10
10
disintegrations per second, is equal to the number of
disintegrations/second of one gram of radium-226. Note that
the source used in the 3241 is small, with quantities expressed
in millicurie (mCi) or Gigabecquerels (GBq).
The rad or “radiation absorbed dose,” is the unit of absorbed
dose that is equal to 0.01 Joules/kg in any medium. In order to
take into consideration the effect of various types of radiation on
biological tissue, the rem, or more appropriate for Troxler users -
the millirem - is used to measure radiation dosage. The unit rem
is derived from scaling the radiation absorbed dose (rad) by a
quality factor (QF). One rad is equal to the exposure of one rem
of photon radiation.
For example, the average neutron energy of an Americium-
241:Beryllium source is approximately 4.5 MeV. The quality
factor (QF) for this radiation is approximately 10. The absorbed
dose of 1 rad of neutron radiation produces a dose equivalent of
(absorbed dose x QF) 10 rem.
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