Several million electron-volt (MeV) protons (singley-charged hydrogen ions) or alpha particles (doubly-charged helium ions which contain two protons and two neutrons) produced by ion accelerators are used to generate X-rays for materials analysis by particle-induced X-ray emission (PIXE). There are hundreds of these accelerators in the U.S. and world-wide located in universities and commercial laboratories. These accelerators produce a highly controlled beam of charged particles so that a known number of particles of known energy can be delivered to the sample. The analyses can be carried out in vacuum or in air. For in-air analysis the ion beam, usually protons, passes through a thin membrane into air and onto the sample. Million electron-volt protons can pass through the thin membrane and several centimeters of air without substantial energy loss.
The technique of measurement in air where the energetic ions are taken from the vacuum in the ion accelerator beam into air is referred to as external beam line measurements. Figure 4 shows the experimental set-up which is used for large or vacuum-incompatible objects.