The external ion beam facility is necessary for the investigation of samples, which are unstable under vacuum conditions (e.g. soft organic tissue or liquids) or which are to large to be mounted in a vacuum chamber (e.g. historical artefacts, bones, papers, paintings etc.), with ion beam analytic methods (e.g. PIXE, PIGE, IL).

This facility was established at the 2 MV van de Graaff accelerator and is used with protons only.

The system has no exit window. The ion beam is transferred from the vacuum system of the accelerator into the air via a small hole with a diameter of 100 microns (see Figure 1).  Because of this leak a differential pumping technique is used to maintain a good vacuum inside the beam tube and accelerator.

The exit port of the external ion beam facility consists of an alu-nose with two diaphragms (400 microns and 100 microns respectively). The diaphragms are made of carbon to avoid undesired X-ray emission.

The diameter of the external ion beam ist defined by the 100 microns diaphragm. Due to scattering processes the beam spot diameter increases with increasing distance from the exit port. At a distance of 7 mm the beam spot is about 250 microns in diameter (at an ion energy of 1650 keV).

Figure 1: Construction diagram of the external ion beam facility