Producing X-rays & Their Spectrum

Slam fast electrons into a metal and the metal glows, not in visible light but in X-rays: photons a thousand times more energetic than light. Their spectrum has two parts: a smooth hump with a sharp edge, and spikes that fingerprint the metal.

A discovery by accident#

In 1895 Wilhelm Röntgen was studying discharge tubes when he noticed that a fluorescent screen across the room glowed even though the tube was wrapped in black card. Something invisible was coming out where the cathode rays (fast electrons) hit the glass. He called the unknown radiation X-rays. They are electromagnetic waves, like light, with wavelengths roughly from 10 nm down to 0.001 nm.

The Coolidge tube#

Modern X-ray tubes follow a design by W. D. Coolidge (1913):

• A highly evacuated glass tube, so electrons travel without hitting gas molecules.
• A filament (cathode) heated by a separate low-voltage supply gives off electrons (thermionic emission). The filament current controls how many electrons leave per second.
• A large voltage, typically tens of kilovolts, between filament and target accelerates them to high speed.
• The target is a metal with a high melting point and high atomic number (tungsten or molybdenum), set at an angle so X-rays leave through the side of the tube.
• Over 99 % of the electrons’ energy becomes heat, so the target sits on a thick copper block, often cooled by circulating oil or water.

Two kinds of X-rays#

Measure the intensity of the X-rays at each wavelength and you get the graph below: a smooth continuous curve with sharp peaks sitting on top.

Continuous X-rays (braking radiation)

An electron arriving at the target has kinetic energy eV. Inside the metal it is slowed by the electric fields of the nuclei, and a decelerating charge radiates. An electron may lose its energy in many small steps, each giving a photon, so the photons have all energies up to a maximum. This radiation is also called bremsstrahlung, German for “braking radiation”.

The most energetic photon possible takes the whole kinetic energy of one electron in one go. That sets a sharp cutoff wavelength:

Characteristic X-rays

Sometimes an incoming electron knocks an electron right out of an inner shell of a target atom, say the K shell (n = 1). An electron from a higher shell then drops into the hole, and the energy released leaves as one X-ray photon. Because the shells’ energies are fixed by the element, these photons have definite wavelengths: the sharp peaks.

• Kα: an L electron (n = 2) fills a K vacancy.
• Kβ: an M electron (n = 3) fills a K vacancy; a bigger drop, so a shorter wavelength.
• Lα, Lβ…: electrons fill an L vacancy; much less energetic.

These lines only appear if the electrons have enough energy to create the vacancy in the first place. Below that voltage you see only the continuous spectrum. A heavy element such as tungsten needs about 70 kV to show its K lines.

Energy levels of the atom

It is neatest to talk about the energy of the whole atom. Take the normal neutral atom as zero. Removing a K electron takes energy E_K, so an atom with a K vacancy has energy E_K; with an L vacancy, a smaller E_L; and so on. When an L electron fills the K hole, the vacancy moves to L and the photon carries off E_K − E_L.

Controlling a tube: intensity and quality#

X-rays of short wavelength (high energy) penetrate a lot of material and are called hard X-rays; longer-wavelength ones are soft X-rays. Doctors pick the voltage for the job: soft rays for thin tissue, harder ones for thick parts of the body.