The Source for Polarized 6-Lithium Ions at the Heidelberg En-Tandem

The source was developed at the University of Hamburgl,2,3) and installed at the Heidelberg EN-Tandem4). The principle is that of an atomic beam source, but the usually used electron bombardement ionizer is replaced by a surface ionizier with an efficiency close to one. The outline of the source is displayed in fig.1. An intense atomic beam is formed by evaporating 6Li from an oven through a Laval nozzle (0.5 mm o). After that the core of the beam is skimmed by a conical and a plane heated orifice (1 and 1.5 mm o respectively). The beam is polarized with respect to the elctron spin by passing a permanent 6-pole magnet. Than it enters a weak field transition. The transition unit, which had an efficiency of 90% 4) has been redesigned in some parts and reaches now full efficiency. The surface ionizer consists of a 10×25 mm2, 0.1 thick tungsten strip heated to about 1000° C. A monolayer of oxygen which increases the work function of tungsten to values higher than the ionization energy of lithium5) is provided by an oxygen leak near the tungsten strip. In this way a nearly 100% efficiency of the ionizer is achieved. Since the surface absorption time is known to be 20 to 100 ms for temperatures around 1000°C 6) a strong magnetic field (B/B c=4) has to be provided for the ionization region in order to avoid depolarization effects and to define the direction of polarization.