An ion source using the surface ionization process is described. Elements whose ionization potentials are below or around 6 eV may be ionized in the source. The element vapour to be ionized is passed through one or more rhenium grids on which surface ionization occurs. The source is tested for different elements—lithium, indium, rare earths, uranium—the latter corresponding to the upper limit of the ionization potential range. The properties and performance of the source are studied. Currents from many tens of &mgr;A to many mA have been extracted, depending on the source element.