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Abstract
Although the experimental determination of the absolute efficiency is by far the most accurate method, it can sometimes be difficult or impossible. Indeed, accurately calibrated y-ray sources are difficult to be prepared for all the geometrical arrangements used in y-ray spectroscopy. Several methods have been proposed and applied to overcome the above problem. The basic concept of these methods is essentially the same and is composed of two steps: the precise determination of the absolute full peak efficiency for a point source and a numerical (mainly Monte Carlo) correction for the source extended dimensions (including self-absorption).
This work introduces two new concepts to determine the full-peak efficiency of the gamma spectrometers for analyzing volumetric samples with: (1) known elemental compositions (CONCEPT I) and (2) unknown elemental compositions (CONCEPT II). The latter concept is important when the elemental composition of the sample is unknown especially at low energy photon detection where self absorption is strongly material dependent. In CONCEPT I, a formula is developed to give a correction factor to the detector efficiency when the detector is calibrated with a point source. In CONCEPT II, another formula is developed to give a correction factor to the detector efficiency when the detector is calibrated with a standard water source. The main advantages in evaluating the detector efficiency using the proposed concepts are: (1) to eliminate the need for volumetric standard sources in labs, and, (2) to eliminate the need for knowing the elemental composition of the sample to be measured.