Determination of Absolute Disintegration Rates

As indicated in section 7.1, measurements of relative and absolute activities are to be distinguished. For determination of relative activities, the overall counting efficiency Tj in eq. (7.3) must be constant, but it need not be known, whereas ri must be known exactly for determination of absolute activities. The overall counting efficiency can either be calculated or be determined by calibration. In both cases all factors in eq. (7.3) must be considered. 

In many detectors the internal counting efficiency depends on the energy of the radiation. This must also be taken into account, if sources containing other radionuclides are used for calibration. Generally, calibration curves = f E) are deter- 

Finally, the influence of the dead time D in eq. (7.3)) has to be taken into account, particularly if the dead time of the detector is high (as in the case of Geiger Muller counters) and if the counting rates of the sample and the calibration source are markedly different. 

With respect to the influence of the factors S, B and W in eq. (7.3), determination of the absolute activity A of a and emitters may cause problems, and thin samples and windowless counters are preferable for these radiations. Thin samples are obtained by electrical or vapour deposition on thin metal sheets or thin polymer foils. By use of windowless 2n- or 47c-proportional counters the influence of W can be neglected. Determination of absolute activities of y-ray emitters involves fewer problems, because the influence of the factors S, B and IF is, in general, negligible. 

Absolute activities of radionuclides may also be determined by coincidence measurements, provided that the decay scheme is relatively simple, e.g. only one f transition followed immediately by emission of one or more y-ray photons. The principles of the use of coincidence circuits are discussed in the following section. 

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The determination of absolute disintegration rates is of great importance in all areas of nuclear chemistry, tracer work, age calculation, etc. Numerous methods have been employed, many using techniques described above, as track counting, liquid scintillation measurements, 47t proportional counters, etc. If the nuclei decay through 0-y emission, the absolute rate may be obtained by two detectors placed close to each side of a thin sample, one detector jS-sensitive and the other y-sensitive. 

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