RADIOACTIVITY
DOE-HDBK-1019/1-93
Atomic and Nuclear Physics
NP-01
Page 42
Rev. 0
Radioactivity Summary
Radioactivity is the decay of unstable atoms by the emission of particles and
electromagnetic radiation.
A curie (Ci) is a unit of radioactivity equal to 3.7 x 10 disintegrations per
10
second.
A becquerel (Bq) is a unit of radioactivity equal to 1 disintegration per second.
The radioactive decay constant ( ) is the probability per unit time that an atom
will decay.
The radioactive half-life is the amount of time required for the activity to
decrease to one-half its original value.
The activity of a substance can be calculated from the number of atoms and the
decay constant based on the equation below.
The amount of activity remaining after a particular time can be calculated from
the equation below.
The relationship between the decay constant and the half-life is shown below.
Plots of radioactive decay can be useful to describe the variation of activity over
time. If decay is plotted using semi-log scale the plot results in a straight line.
Radioactive equilibrium exists when the production rate of a material is equal to
the removal rate.
Transient radioactive equilibrium exists when the parent nuclide and the daughter
nuclide decay at essentially the same rate. This occurs only when the parent has a
long half-life compared to the daughter.
