The photographic measurement of the relative intensities of the L 1, 2, 3 lines of silver

Abstract

A knowledge of X-ray line intensity is needed in the theory of atomic structure and in chemical analysis by means of X-rays. The relative advantages of the ionization, Geiger counter, and photographic methods of measurement are discussed. The advantages of the photographic method are that (1) a permanent record is obtained, and (2) the photographic effect is cumulative, a condition which is especially valuable for lines of weak intensity. Its disadvantages are the difficulty of deducing the intensity from the blackening of the emulsion, and the care needed in development and photometry. The ionization method requires a steadier source of radiation, and difficulties arise owing to end effects, Compton scattering, and internal absorption of characteristic X-rays. The Geiger counter method counts the quanta, but the unjustifiable assumption is made that the number of electrons ejected from a metal foil by X-rays, on the side of incidence, depends only on the number of incident quanta. In the photographic method used by the author the densities, D, of the photographic images of the lines, produced by a vacuum spectrograph, are measured by means of a Moll micro-photometer. The density is a function of the exposure, the frequency, the photographic emulsion, and the development. This function is discussed. Previous work has shown that the Bunsen-Roscoe reciprocity law applies in the case of X-rays, and it is shown that for constant development, and for the range of frequencies measured, D is a function of Nt, where N is the number of quanta absorbed per second in the emulsion and t the exposure time. This function is found by varying t. Corrections are made for the overlapping of the lines. The results obtained are: Lα1: Lα2, 100: 12.0; Lα1: Lα3, from 100: 4.6 to 100: 7.3, according to the tube voltage. A value of Lα1: Lβ1, of less accuracy, is found as 100: 40.