Ion source brightness and nuclear microprobe applications

The ion source used to provide beams for nuclear microprobe systems must ideally satisfy several demands. Of primary importance is the beam brightness. It is clearly desirable to employ the brightest possible source in order to focus the smallest possible probe size on the specimen, with the highest possible beam current. Also important is the need for minimal maintenance, particularly for ion sources used inside single ended accelerators. We report here on measurements conducted on the beam from the RF ion source in our 5U Pelletron accelerator. We have found that the beam brightness is highly heterogenous, with the paraxial rays about an order of magnitude brighter than the surrounding rays. This is desirable for nuclear microprobe operation because probe forming lens systems optimised for large demagnification magnitude can exploit the high brightness of the paraxial region. We find several other accelerators around the world also exhibit this characteristic. (C) 1997 Elsevier Science B.V.