Elementary Theory of X-Ray Production

Turning to the innermost electrons. Sections 1.4-1.6 dealt with the OUTERMOST electron of hydrogen, whose transitions between widely-spaced outer energy levels produce visible and ultraviolet light. The rest of this sub-topic turns to a very different phenomenon: transitions involving the INNERMOST electrons of a HEAVY atom (many-electron, high atomic number), whose energy levels are held far more tightly by the much larger nuclear charge -- so tightly that transitions between them release photons of vastly higher energy, in the X-ray region of the electromagnetic spectrum, first discovered (accidentally) by Wilhelm Röntgen in 1895.

The Coolidge tube. X-rays for laboratory and medical use are produced in a device called a Coolidge tube (developed by William Coolidge in 1913), an evacuated glass or metal tube containing two electrodes:

• A cathode, consisting of a tungsten filament, heated to incandescence by a separate low-voltage supply so that it emits electrons by thermionic emission -- exactly as the heated filament of a vacuum-tube diode does.
• An anode (the target), a block of a heavy metal with a very high melting point -- commonly tungsten or molybdenum -- positioned facing the cathode.

A large potential difference $V$, typically tens of kilovolts, is applied between the cathode and the anode, accelerating the thermionically emitted electrons to very high speed before they strike the target. On impact, the electrons are abruptly brought to rest (or greatly slowed) within the target material; only a very small fraction, of order $1\%$, of their kinetic energy is converted into X-ray photons (by the two mechanisms Section 1.8 describes) -- the remaining $99\%$ or so is dissipated as heat in the target, which is why the target must be a metal with a high melting point and is often actively cooled (with cooling fins, circulating oil, or, in demanding applications, spun rapidly as a rotating anode) to prevent it melting under continuous use. …