Step 6: How do they produce X-Radiation?

1. Bremsstrahlung

Literally translating to ‘braking radiation’, bremsstrahlung is the process where a high speed electron ‘brakes and sling-shots’ around an atom's nucleus, dumping its kinetic energy on one photon. An imparting electron might have an energy in excess of 60keV so some very energetic photons are be made; photons which fling off into space and become the x-rays we all know and love.

What determines the energy of the x-rays produced is the amount of voltage present on the anode. It’s quite simple actually; more voltage means more electron attraction, and more attraction results in a faster electron beam. Faster electrons are able to make higher energy photons, and thus “harder”, higher energy x-rays would be generated.

Bremsstrahlung is a continuous spectrum of radiation akin to a “white light” source. Since most electrons graze a few atoms before they have the chance to sling-shot, they often lose some energy before they make any x-radiation. A whole range of x-ray energies is thus produced.

The maximum energy that an x-ray can have is limited to the energy of electron producing it, itself directly proportional to the voltage applied on the tube’s anode. Often this energy is measured as kilovolts peak, or kVp. In reality the majority of the x-rays produced are low energy, ‘soft’ x-rays, but these are greatly attenuated by the tube’s glass wall.

2. Characteristic Production

Characteristic or k-line production is the second mode in which an electron might produce an x-ray. In this method, electrons knock others out of an atom’s lowermost shell and leave a hole which must be filled. This unstable arrangement is then promptly made stable by electrons from higher shells who jump down to fill the hole, emitting an x-ray photon during the journey.

Tungsten k-shell electrons have a binding energy of 69.5keV, so to kick these out your impacting electrons must have energies greater than 69.5keV. Typically one would need to give the anode a bit more than 72kV to accomplish this, hence the standard 75kV x-ray tube.

After a k-shell electron gets the boot, its hole will immediately be filled by an electron from tungsten's l-shell; binding energy 10.2 keV. The difference between these two energy states; 69.5 keV and 10.2 keV gives us the characteristic tungsten x-ray energy of 59.3 keV. A molybdenum anode would produce two peaks, one at 19.7keV and the other at 17.6keV.

Interestingly, this process can be used to identify elements based on their k-lines. By bombarding a sample with electrons and measuring the output spectra, an x-ray florescence analyzer can determine what elements a compound contains.

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