Abstract

THE two attributes for characterizing and measuring an electromagnetic ray (light, X-rays, γ-rays) are the wave length and the intensity. Two rays with equal wave length are qualitatively equal; they possess, for example, equal penetration power. The measurement of wave lengths is made by the spectrographic method. The radiation intensity is a measure of quantity; it corresponds to what is ordinarily termed quantity of radiation. In the newer definition, the radiation intensity is that radiation energy which strikes in one second upon a surface of 1 sq. cm. perpendicular to the direction of the ray. An absolute measurement of intensity for X-rays is very difficult and depends upon the very minute increase in temperature resulting from the complete absorption of the rays in any irradiated object. For all other effects of X-rays (ionization, photographic plate, fluorescent screen, selenium cell, skin erythema, etc.), the degree of action depends on the radiation intensity and also on the wave length. On this account a different fraction of the initial radiation intensity for different wave lengths is transformed in the irradiated medium into other energy forms. The action is, therefore, independent of the wave length only when the beam of given initial intensity inclusive of the secondary rays arising therefrom is completely absorbed in the irradiated material—a condition which can be realized practically only with methods based on the thermal effect. How great can be the difference in the dependence of the sensitiveness of different methods of measurement upon the wave length is demonstrated in Figures 1 and 2. If different homogeneous rays, i.e., rays which possess only a single wave length, fall simultaneously with equal intensity upon an air ionization chamber, a fluorescent screen, and photographic plate, the brightness of the screen changes much less with wave length than the ionization current (1). Upon comparison of the ionization and blackening of the photographic plate, it is especially noteworthy that the photographic sensitiveness changes with sudden jumps in two places (0.49 and 0.91 Ångström unit). Since these wave lengths coincide with the discontinuities in absorption coefficients of silver and bromine, the two effective constituents of the photographic emulsion, it must be concluded that the photographic action is closely connected with absorption. Not only for the photographic effect but also in other cases, for example ionization, the following principle holds: the action of rays changes with wave length in the same way as the intensity of the absorbed portion of the incident rays changes. Exceptions occur in the region of very short wave X-rays, when the Compton effect is appreciable, and in the case of the excitation of characteristic fluorescent rays of the irradiated material.

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