kiloelectron volt

The kiloelectronvolt is the X-ray unit, and it has a property no conversion table conveys: the machine's voltage dial reads directly in it. An X-ray tube accelerates electrons through its set voltage, and no photon can carry more energy than one electron brought in, so a CT scanner running at 120 kV emits a spectrum that cuts off at exactly 120 keV. The cutoff is called the Duane-Hunt limit, and it means a radiographer choosing kilovoltage is literally choosing the keV endpoint of the beam. Dental sets run near 60 to 70 kV, mammography at 25 to 30 because soft-tissue contrast is best at low energy, and industrial radiography pushes higher to get through steel.

Anchors across the scale: copper's Kα line at 8.05 keV is the standard illumination of X-ray crystallography, the technetium-99m gamma at 140.5 keV is the workhorse photon of nuclear medicine imaging, and the electron rest mass is 511 keV, the energy of each photon in the annihilation pairs a PET scanner detects.

The wavelength relation is compact in these units: energy in keV times wavelength in nanometres equals 1.2398, so a 1 keV photon has a wavelength of 1.24 nm and a 12.4 keV photon reaches the 0.1 nm scale of atomic spacing. That coincidence of scales is exactly why X-rays diffract off crystal lattices and visible light cannot.