Radioactivity (A, also known “absolute activity”) is defined as the number of decompositions in a unit time. Radioactivity is in proportion to the initial quantity of the radioactive nuclei:

A = — dN = AV = AN0 e-At = A0 e-At (4.12)

It is important to note that the activity—time function (Figure 4.12) is analogous to the number of the radioactive nuclei—time function (Eq. (4.8)).

The unit of radioactivity is the becquerel (Bq), which describes the number of decomposition/disintegrations that take place in 1 s (1 Bq = 1 dps = 1 disintegrations per second). An earlier unit of radioactivity was the curie (Ci), which is the number of decompositions in 1 g of radium in 1 s. The relation between the two activity units is 1 Ci = 3.7 X 1010 Bq. Besides these two, dpm (which means “disintegrations per minute”) is frequently used for practical purposes.

Radioactivity is usually measured not by identifying the radioactive nuclei but by counting the emitted particles. In theory, to achieve accurate activity measure­ment, all particles emitted in 4n spatial angles should be taken into consideration. In practical applications, however, it is more common that the radioactive intensity (I), a quantity proportional to the radioactivity, is measured. The proportionality factor is the measuring efficiency (k):

I = kA = kAN (4.13)

The intensity—time function, of course, is similar to the activity—time function (shown in Eq. (4.12)):

I = I0e-At (4.14)

Obviously, this relation is valid so long as the measuring efficiency (k) stays constant for all measurements.

The units of intensity are as follows:

• cpm means counted particles per minute,

• cps is counted particles per second.