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Desorption Yield Measurements of Copper Characterized using UHV-ERDA ∗

Ion-induced desorption is a serious luminosity limitation for high current and low charge state heavy ion accelerators [1]. In order to find low desorbing materials for the loss regions of SIS18 a dedicated experimental program was started measuring desorption yields of different materials, which are in-situ well characterized using UHV Elastic Recoil Detection Analysis (ERDA). Two targets, with a size of 50 x 50 mm, were cut out of the same piece of 99.95% oxygen free high conductivity (OFHC) copper. Target (A) was lapped in the target laboratory of GSI using a ceramic (Al2O3) paste and cleaned in HNO3 acid in order to remove a possible oxide layer and polishing grains. After preparation the sample was mounted in the UHV-ERDA chamber with minimal exposure to air. Target (B) was polished with a standard polishing paste (Fe2O3, grain size ≤ 10μm in a Stearin matrix) which is used for copper coated accelerator cavities. After polishing the target was cleaned in an ultrasonic bath, flushed with demineralized water and dried under atmosphere for 4 h at 250◦ C. The desorption yield and ERDA measurements were performed at the high charge state injector (HLI) of GSI using a 1.4 MeV/u Xe beam. From the raw ERDA spectra, compare [2], element specific depth distributions can be derived using the KONZERD code [3] as shown in Fig. 1. Here the depth distributions of copper, oxygen and carbon for the both samples are shown: whereas sample (A) is a pure metallic copper with only minor oxygen contamination on the surface, target (B) has a highly oxidized surface. For both targets the carbon contaminations is below 1 %. The thickness of the Cu2O oxide layer on target (B) is around 450 nm. The corresponding pressure rise under ion bombardment is shown in Fig. 2. A significant higher pressure rise was observed for target (B). At the end of each desorption yield measurement we have applied a positive or negative bias voltage to the target in order to measure the influence of secondary, charged particles (electrons and ions) to the pressure rise. A pressure increase is clearly visible and is slightly more pronounced for a positive bias voltage. Here secondary ions emitted from the target gain 2000 keV kinetic energy in the electric field and are accelerated towards the chamber wall resulting in a low energetic ion stimulated desorption. The effective desorption yield for sample (A) was measured to be ηeff ≈ 360 and for sample (B) ηeff ≈ 1500 taken from the peak maximum. For more details see [4]. Hence, the desorption yield of the oxidized copper is