6.4 Discussion

For all three tested surfaces count rates at the CODIF between 0.3 and 3 particles per second were observed. The background signal was sufficiently low, despite the secondary electron deflection magnet having failed due to a high voltage discharge. Some complications were caused by the fact that for neutral measurements the CS had to be put at a high negative voltage (minus 19kV) to allow for sufficient acceleration of the negative ions in the extraction lens towards the TOF. The CS at high voltage attracted positive residual gas ions, e.g. from the ion gauge. This caused increased background from sputtered particles, as these particles hit the CS with an energy of 19keV. These positive ions probably account for a large part of the observed background (bottom plate in Figure 6.12). Further data analysis including the spectra obtained using nitrogen primary particles should allow the separation of this contribution to the spectra. Further analysis will also give a more detailed picture of the energy dependence of the overall detection efficiency, whereas it is already clear that the overall detection efficiency is rather high even at the lowest energies investigated (30eV).

From a preliminary point of view, all three conversion surfaces tested performed approximately equally, but differences should be visible after a more detailed data analysis.

No measurements with hydrogen were performed because of the lower energy of the hydrogen atoms at the same particle speed. The ion beam intensity from the CASYMS ion source is approximately proportional to the beam energy in the energy range used [35]. The beam intensity below 30eV was too low to get reasonable count statistics. Furthermore a rather high hydrogen background was observed due to the CS being at a negative high voltage. Thus no hydrogen measurements were performed.

Electrostatic shielding proved to be very important. After some trial and error it was concluded that every component at high voltage had to be thoroughly shielded to avoid an arbitrary deflection of the very low energy ions from the CASYMS source (Figures 6.10 and 6.11).

A successful first was the generation of a neutral beam in the CASYMS chamber. The simple design of the beam neutralizer guaranteed flawless operation.

March 2001 - Martin Wieser, Physikalisches Institut, University of Berne, Switzerland