Acta Chimica 21. (1959)
1. szám - PHYSIKALISCHE CHEMIE - T. Lengyel: Energy Dependence of the Back-scattering Factor of Absolute Beta Counting
52 LENGYEL: ENERGY DEPENDENCE In the case of measurements carried out with the GM tube, the three factors fw> fti and/s among the data of equation (1) may be considered as energy-depending beside the back-scattering factor. A definition for the back-scattering factor is given by the quotient (2) where I' is the observed activity by applying a back-scattering medium and II" the observed activity without applying a back-scattering medium. From relation (2) it is seen that in the case of each isotope a possibility is given to determine fB by the quotient of the results of two measurements. Since all the conditions of the measurements are identical, except for the backscattering and isotopes serving as radiation sources, any factor which does not depend on energy will be eliminated at evaluation in consequence of the division ; fw, one of the energy depending factors can be computed with the knowledge of the radiation energy and experimental conditions, fs and fH, however, are practically regarded as being equal to 1 [1, 5, 10] by choosing convenient experimental conditions, of which mention will be made later. Measuring device Data obtained with the Geiger—Müller tube with a mica end-window, which was used in the experiments, are given in Table I. Table I Technical data of the GM tube applied To the other accessories necessary for measurements, the GM tube was connected with threaded fitting parts made of plexi-glass. In each case the radiation source was applied to a 0,11 mg cm-2 thick film made of Zapon lacquer. The small nominal atomic number of the lacquer and its thinness rendered it possible to disregard practically any back-scattering, when this “infinitely thin” carrier layer was applied. To reduce the value of factor/н to 1 and to improve the geometrical conditions of measurements, the distance between the radiation source and GM tube had to be little — merely 8 mm. With absorption measurements of absorbent sheets were directly fitted to the end-window, so as to avoid secondary scattering. Also the back-scattering medium was directly fitted to the lower side of the film holding the radiation source. The back-scattering medium was made of foils with a smooth surface.* Working voltage ....................... Length of plateau...................... Slope of plateau per 100 У Thickness of end-window Dimension of the end-window Dead-time .................................. 1450 Y 300 У (1350—1650) 3% 1,9 mg cm-2 3,14 cm2 (0 = 20 mm) 130 jiisec •The applied isotopes were imported from the Soviet Union by the Atomic Energy Commission of Hungary. Acta Chim. Hung. Tomus 21. 1959