Activity and Counting Rate

The activity (disintegration rate) A as defined in section 4.2 is a property of radioactive matter and can be measured by various devices which give a certain counting rate I, which depends on the activity A, the overall counting efficiency rj of the device and the background counting rate /q  

Usually, the counting rate is measured in counts per minute (cpm). / = / - /q is the (net) counting rate caused by the radioactive sample to be measured. Iq is the reading in the absence of a radioactive sample. It is due to the radiation emitted by the surrounding material and cosmic radiation. At low radioactivity of the sample, the background counting rate Iq may contribute appreciably to I, and special measures are taken to minimize /q. 

The overall counting efficiency depends on the properties of the radionuclides and the measuring device. The various factors contributing to t will be discussed in detail later in this section. 

At constant rj, the net counting rate / = / — /q is proportional to the activity A, and for many purposes, such as determination of half-lives or application of radionuclides as tracers, measurement of the relative activity, given by / at constant t], is sufficient. 

In Fig. 7.1 the logarithm of the net counting rate / is plotted as a function of time t. The curve obtained is called the decay curve and is used for determination of half-lives. 

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