Because protons and neutrons are clustered together in the
nucleus, the mass of an atom is concentrated in the nucleus. The
atom in Figure 13 has two protons and two neutrons; therefore,
it is a helium atom. The atomic weight of an atom is the sum of
the protons and neutrons.
Mass
Atomic Weight Scale)
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 sources used in the a 3400
series gauge, 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 cesium-137 is 30 years, while that of
americium241:beryllim is 432 years.
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