ABSTRACT
X-rays are photons. They have more energy than UV light, and less than gamma rays. Very roughly, the lower energy limit of X-rays is about 10 eV, and the upper limit is 100 keV X-rays arise in nature as a result of various processes, mainly cosmological. Controlled terrestrial generation of X-rays almost always depends on bombardment of a target (usually metallic) with medium-energy electrons. Two processes are at work. In the first, the atomic electrons are initially excited into higher-energy states by the primary electrons. The excited atoms (strictly, ions) then relax back to their lowest energy state (the ‘ground’ state), shedding their excess energy in the form of X-rays. Since, as we shall shortly see, the energies of these X-rays are specific to the atom that generated them, they are known as ‘characteristic X-rays’. The second process leads to the production of X-rays with a range of energies from zero up to the energy of the primary beam, and for reasons which we shall describe later, they are called ‘bremsstrahlung Xrays’.
