Quality Determination of the Roentgen Rays

THE recent interest created by the introduction of a standard unit of measuring the intensity of the roentgen rays again brings to the forefront the importance of creating a uniform method of quality determination. Our interest in the matter has been expressed in former communications and herewith we present the results of our continued experimentation along these lines. (1) The Half Value Layer as a Preferential Quality Measure The adoption of a single method for determining and expressing the quality appears to us to be just as important as the acceptance of a standard unit of intensity. The first part of our present work consisted of the comparison of several of the more recently proposed methods for measuring the quality. In Chart I we have tabulated our experimental results. The arrangement is such that the relative advantages of each method can easily be determined. An important point not evident on the chart is the practical difficulty encountered with some of the methods. This is especially true where an absorbing medium of high atomic number is employed in measuring the rays of long wave length. We have reference to the handling of fine thicknesses of metal requiring micrometric measurements below one-tenth millimeter (Method B, Chart I), as well as to the difficulties presented in measuring small quantities of transmitted intensity (C, E, and F). The percentage of variation of the transmitted intensity with the shorter wave length is very small as applied in Method E. From the foregoing it is evident that heavier absorbing mediums, suitable for testing strongly filtered rays, offer serious difficulties in measuring rays of long wave length. Conversely, fixed lighter absorbing mediums give too narrow a range with rays of short wave length. Further, it may be worthy of mention that in existing tables the number of Ångström units are indicated with homogeneous radiation (1). On the other hand, the practical roentgen therapist is mostly dealing with a heterogeneous beam. Therefore rather wide variations occur, depending upon the method employed, as indicated in Scales E and F compared with the effective wave length given in G. A further study of the chart reveals the fact that the half value method, especially with aluminum, can be used over the full range of wave lengths with the least number of the objections heretofore mentioned. The determination of the half value layer in copper with the rays of shorter wave length has certain scientific advantages; there is, however, either the difficulty of handling extremely thin layers (as previously mentioned) or the necessity of substituting another medium for measuring the soft rays, thus introducing a double standard. The advantages of using the half value layer in aluminum as a quality measure appear to be: 1. The thickness of the absorbing medium is quite proportional to the quality.