RADIOACTIVITYDOE-HDBK-1019/1-93 Atomic and Nuclear PhysicsNP-01Page 42Rev. 0Radioactivity SummaryRadioactivity is the decay of unstable atoms by the emission of particles andelectromagnetic radiation.A curie (Ci) is a unit of radioactivity equal to 3.7 x 10 disintegrations per10second.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 atomwill decay.The radioactive half-life is the amount of time required for the activity todecrease to one-half its original value.The activity of a substance can be calculated from the number of atoms and thedecay constant based on the equation below.The amount of activity remaining after a particular time can be calculated fromthe 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 overtime. 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 tothe removal rate.Transient radioactive equilibrium exists when the parent nuclide and the daughternuclide decay at essentially the same rate. This occurs only when the parent has along half-life compared to the daughter.