2 - The continuous X-ray spectrum

Summary

Introduction

Among the earliest established facts regarding the X-ray spectrum which is produced when an electron beam impinges upon a target were the observations that the radiation is not monoenergetic, and that its intensity is a function of the atomic number of the target material. As examples of early studies of the shape of the continuous spectrum we may cite the work of Duane and Hunt (1915), and Ulrey (1918), who examined the radiation from a target of tungsten, for several values of the electron energy. Ulrey's results are reproduced in fig. 2.1. The most prominent features of these measurements are the occurrence of a maximum in all the curves, and the existence of a short-wave limit advancing in the direction of shorter wavelengths as the applied voltage increases. The short-wave limit (known as the Duane Hunt limit) is seen to be inversely proportional to applied voltage, and is a direct consequence of the quantum nature of electromagnetic radiation – no photon can be emitted with an energy greater than that of the bombarding electrons.