Uranium X-Ray Grids

Abstract

The x-ray grid is a device used in diagnostic roentgenology with the purpose of preventing radiation scattered by the object examined from reaching the detector used in the examination. Most grids conventionally constructed are composed of a number of lead strips, approximately 0.002 inch thick, separated by material relatively transparent to x-rays. The grid is characterized by the number of lines per unit length and by its “ratio,” i.e., the ratio of distance between two adjacent lead strips to their height. The purpose of this paper is to study the advantages which can be gained by replacing lead in otherwise conventional x-ray grids with uranium and to report on the practical improvement thus achieved. If one considers the most desirable requirements for a grid constructed according to conventional design, one reaches the conclusion that ideally the x-ray-opaque strips should be infinitely thin. On the other hand, they must absorb a large fraction of the scattered radiation. The combination of these two conflicting requirements calls for the use of a material with the highest possible linear x-ray absorption coefficient for the scattered radiation present. Most conventional grids are constructed with lead strips. X-ray absorption, in the energy range under consideration, takes place in elements of high atomic number, mostly by photoelectric effect. It appears, therefore, that considerable gain in x-ray absorption might be expected if lead were replaced by an element of higher atomic number. An examination of the periodic table of elements indicates that uranium should serve this purpose admirably. It has one of the highest densities and the highest atomic number among naturally occurring elements. Its radio activity is insufficient to be a troublesome factor for the contemplated purpose. Chemically, uranium is relatively inert. Physically, it is harder than lead. Thanks to its importance in reactor technology, its metallurgy is well known, and thin uranium foil is available. Figure 1 is a plot of the linear x-ray absorption coefficients of lead and uranium for radiation energies from 10 to 150 kev. The curves were drawn on the basis of the figures calculated by Charlotte M. Davisson and published in Beta and Gamma Ray Spectroscopy, edited by Kai Siegbahn. These curves exhibit the “K” and “L” peaks characteristic of photoelectric absorption. Between 22 and 88 key, uranium absorbs more than twice as much radiation as an equal thickness of lead. This energy range comprises the majority of scattered radiation energies encountered in conventional roentgenologic technics.