A=lN (3.2)From a knowledge of the decay constant and radioactivity of a radionu-
clide, one can calculate the total number of atoms or mass of the radionu-
clides present (using Avogadro’s number 1 g · atom =6.02 × 1023 atoms).
Because Eq. (3.1) is a first-order differential equation, the solution of this
equation by integration leads to
Nt=N 0 e−lt (3.3)where N 0 and Ntare the number of radioactive atoms at t=0 and time t,
respectively. Equation (3.3) is an exponential equation indicating that the
radioactivity decays exponentially. By multiplying both sides of Eq. (3.3) by
l, one obtains
At=A 0 e−lt (3.4)The factor e−ltis called the decay factor. The decay factor becomes e+ltif the
activity at time tbefore t=0 is to be determined. The plot of activity versus
time on a linear graph gives an exponential curve, as shown in Figure 3.1.
However, if the activity is plotted against time on semilogarithmic paper, a
straight line results, as shown in Figure 3.2.
Half-Life
Every radionuclide is characterized by a half-life, which is defined as the
time required to reduce its initial activity to one half. It is usually denoted
22 3. Kinetics of Radioactive Decay
Fig. 3.1. Plot of radioactivity versus
time on a linear graph indicating an
exponential curve.