Investigations of negative ion formation in Middleton type ion sources

Full text: Since 1977, sputter ion sources of the Middleton type have been the main tool used to create stable, intense negative ion beams for Accelerator Mass Spectrometry (AMS). Nevertheless, the theory behind negative ion formation is not fully agreed upon and recipes for high current output differ from lab to lab. A better understanding of the sputtering process is expected to allow further optimization of negative ion sources and thereby enhance the abundance sensitivity of AMS for several radioisotopes. As part of the work in trying to unravel the mechanisms of hard sputtering in a cesium rich environment, we have resolved the spectrum of blue light emitted from the cathode region during high current output. A ber coupled spectrometer was used to detect light in the wavelength region 200 to 1100 nm. When sputtering a carbon target, the emitted light consists almost entirely of persistent lines from neutral cesium. To our surprise, the spectra from aluminum or copper targets showed also persistent lines from Cs+. In another experiment, the effect of continuous wave laser light directed onto the cathode surface was studied. Independent from wavelength, the laser light induced a significant change in oxygen, sulfur and chlorine negative ion production from a AgCl target. Approximately 100 mW of laser light changed the elemental composition of the ion beam by one order of magnitude. This technique was successfully applied during a regular AMS measurement of 36Cl at VERA (Vienna Environmental Research Accelerator). The physics behind the effect still requires further investigations. (author)