As mentioned in Section 4.1.2, radioactivity is usually measured not by identifying the radioactive nuclei, but by counting the emitted particles. The absolute measure­ment of radioactivity means the measurement of the total activity, i. e., all radiating particles have to be counted in 4n spatial angles.

The detectors measuring the total activity are usually gas-filled detectors (as described in Section 14.1) and measured at a direct voltage where the alpha and

beta particles can be differentiated (Figure 14.3). During the measurements, it is assumed that every radioactive decay emits one particle, which is true for most decays.

The very thin layer of the radioactive sample is placed onto a very thin plastic sample holder (at most, 10 pg/cm2), and this sample is located in the middle of two half-sphere detectors. The produced sphere is purged continuously by the mixture of argon and methane gases. These types of detectors are called “4n-counters.” The signals produced in the two half-sphere detectors are added together after preamplifying.

The other method for the measurement of the total activity is the coincidence method. This method can be used for measuring two decay types. The first one is when an alpha particle and a gamma photon, or a beta particle and a gamma pho­ton, are emitted at the same time (within 10 7 s). In the other case, two gamma photons are emitted together (also within 10 7 s). The alpha/beta particle + gamma photon, or the two gamma photons, are measured by two detectors, which are con­nected in coincidence. This means that signals (/1+2) are obtained only when sig­nals are produced on the outputs of both detectors at the same time. In addition, the alpha/beta particles (/j) and gamma photons (/2) are counted separately. Similarly, when two gamma photons are detected, two independent gamma intensities are measured.

The total activity can be calculated as follows. The radioactive intensity (/) can be expressed by the product of the activities (A) and efficiencies (k). For the sepa­rately measured alpha/beta particles:

/1 — k A

For the gamma photon:

/2 — k2A

For the coincidence signals:

/1+2 — k^A (14.7)

By expressing the total activity:

The measurement of the total activity by the coincidence method seems to be very simple. In practice, however, efficiency corrections are required even for isotopes with the simplest decay schemes.

The total activity of radioactive sources with high activities can also be measured using calorimetry. The accuracy of the activity measurements can be improved by the application of differential calorimeters.