5.2.2 Data Processing

The data collected at the ILENA setup consisted of two dimensional images from the position sensitive detector PSD (Figure 2.1). These images contained directly the scattering profile. To account for beam intensity variations a total of five images were taken. For three of the five images the ion deflection grid voltage in front of the MCP was turned on for a neutral particle only measurement (N), for the other two images the deflection voltage was turned off thus measuring both negative ions and neutrals (B). By interleaving the measurements in a N-B-N-B-N sequence and by taking appropriate differences and averages between these 5 images afterwards it was possible to calculate the ratio between the negative ion flux and the neutral particles flux reflected from the surface independent of primary beam intensity fluctuations. After taking the detection efficiency [12,24,25] of the MCP into account and subtraction of background the negative charge state fraction was calculated using the IDL program xeffi5 based on the description in [14]. This script also takes into account the different detection efficiency for neutral and negatively charged particles with the same energy [24,25]. Also using xeffi5 the angular distribution of the ionization yield was investigated. Any surface charging could be easily identified in this way because the ionization efficiency for conductive surfaces is approximately independent of the angle the particles were scattered by. Any asymmetry was a clear indication of surface charging. Figure 5.1 shows a screen shot of this program.

Figure 5.1: Snapshot of the xeffi5 program. The image on the left depicts the scattering properties of the neutral particles upon reflection of a 390eV per atom O \( ^{+}_{2}\) primary beam off a CVD diamond conversion surface at 82 \ensuremath{} angle of incidence. The image on the right depicts the distribution of the negative ions. The line of sight is parallel to the conversion surface in flight direction of reflected particles. The surface is located on the left border of the image. The field of view of each image is 22.5 \ensuremath{} x 22.5 \ensuremath{}. The angular distributions of neutral and negative ionized particles are the same in this example indicating that no surface charging did occur.
\resizebox*{0.8\columnwidth}{!}{\includegraphics{xeffi5.eps}}

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