Mechanisms For X-ray Production
The intensity of x rays from an x-ray tube varies with wavelength. A diagram of the wavelength spectrum from an x-ray tube shows several sharp peaks superimposed on what appears to be a continuous distribution. The peaks and the continuous region are produced by two quite different mechanisms. The continuous spectrum is produced by the incident electrons as they strike and enter the metal target. They are attracted by the positively charged nuclei of the atoms in the target and are suddenly deflected. Electromagnetic theory tells us that when electric charges are accelerated they radiate. Similarly, in the antenna of a radio or TV transmitter, electrons are made to oscillate rapidly back and forth, but in that case the accelerations are not as large and the wavelengths are much larger being measured in meters or centimeters.
A completely different mechanism produces the sharp peaks in the x-ray spectrum. These peaks are at very specific wavelengths and occur at different wavelengths for different targets. This radiation is produced when an incident electron knocks an electron out of one of the inner or low energy levels of the atom. An electron in a higher energy level falls into the vacant level and in the process an x ray is given off. The energy of this x ray is equal to the difference in energy between these two levels. Characteristic x rays have wavelengths ranging from about 10-11 m for uranium to 2.5 × 10-8 m for lithium.