Medical, Biological and Industrial Applications of Monochromatic Radiography and Microradioqraphy

Certainly there is nothing new in the idea of using an x-ray beam of essentially a single wave length (monochromatic) in contrast with the usual beam of many wave lengths (polychromatic) in the various branches of radiography, including roentgen diagnosis. In the high state of development of this science in the hands of roentgenologists to the point of delineation of extraordinarily fine detail, the question is naturally asked, what is to be gained? It seems of interest to give an account of a renewed experience of several years, particularly during the war, with monochromatic radiography. For this subject crops up anew as the result of a newly developed microradiography, in which radiographs of small objects are photographically enlarged up to 300 or 400 diameters. Therein lies the critical test of monochromatic vs. polychromatic technic. A brief review of the principles of x-ray spectroscopy will suffice as a basis for consideration of monochromatic radiography. 1. At all voltages and with x-ray tube targets of all chemical elements, a general or continuous spectrum characterizes the radiation generated. It is only above a certain critical voltage that the characteristic line spectrum of the target element is generated. Thus, at all voltages below 69,300, the tungsten target tube generates in the range of shortest wave lengths only the continuous or polychromatic beam with a short wave-length limit, λ0 determined by the voltage in accordance with the Duane-Hunt application of the Planck-Einstein equation or Ve = hc/λ0, where V is the voltage, e the electronic charge, h the Planck quantum constant, and c the velocity of light. Above 69,300 volts there appear the K-lines of the characteristic tungsten spectrum superposed on the continuous spectrum, with the result, of course, that certain wave lengths, particularly the Kα1, line at 0.2086 A.U. are greatly intensified. The much softer rays of the L, M, N series are generated at correspondingly lower voltages. To isolate this characteristic Kα1, ray would be to produce a monochromatic beam. There is only one way of accomplishing this strictly, and that is selection and reflection by a crystal, but the very great loss in energy renders the method impracticable. The more common practice is use of a characteristic filter, for which the K absorption edge lies between Kβ and Kα, cutting out the shorter rays, including Kγ and Kβ, but transmitting the Kα doublet and all longer rays. This process is also impracticable for tungsten rays, because of the high intensity of the general radiation at 70,000 volts and above, and because of the fact that some exceedingly rare element such as lutecium or ytterbium is required for the filter. Thus the attempt to isolate a single wave length from the tungsten rays is confronted with almost insurmountable obstacles, not only from a practical standpoint, but even for purely academic experimentation.