A new integrating photometer for X-ray crystal reflections, etc

Abstract

In a previous communication it was shown how a new principle in photometry could be applied to the construction of a simple “α-ray photometer” with possible integrating properties. It will be the object of the present paper to describe how the theoretical and practical development of this principle has now led to the design of an integrating photometer which has been found experimentally to give correct measurements of the total intensity in X-ray crystal reflections, and which also seems capable of adaptation to the measurement of the integrated intensity of other types of radiation. Briefly, the new method uses a carbon print of a negative instead of the negative itself, and α-rays instead of light. In ordinary photometric procedure the density of the photographic image is found by transmitting through it a beam of light which then falls on some light-sensitive receiver such as a photo-electric cell. In the procedure about to be described, the density of the negative is not measured, but, by using the “stopping power" for α-rays of a carbon tissue positive, ionisation in an α-ray electroscope is produced in direct proportion to the original X-ray intensity which formed the negative. Carbon printing-paper or “tissue” consists of paper supporting a uniform gelatine film, impregnated with an insoluble colloidal pigment, and sensitised by immersion in dilute aqueous potassium dichromate. The gelatine then possesses the property of becoming insoluble on exposure to light, so that after printing in the usual manner, development involves only the washing away in warm water of the still soluble portions of the film. It is a comparatively simple matter to obtain the insoluble film stripped from all support and free to be placed as an absorbing layer in the path of the α-particles from a small deposit of polonium. The stronger the original X-rays, the blacker the photographic image and the weaker the light which falls on the tissue during printing. This means that the stripped tissue is a film of varying thickness— during development, in fact, the relief is easily seen. It is thinnest where the original X-rays were the most intense, so that when it is used as an absorbing layer for α-rays, the ionisation in the a-ray electroscope increases with increasing X-ray intensity, and, in fact, can be made directly proportional to it. The total ionisation can thus be made a measure of the total X-ray intensity whatever its distribution over the photographic image.