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