Carbon Accumulation on Target Surfaces Irradiated by Protons

We show that the observed beam spot consists of accumulated carbon. The accumulation is the same on the front and back surface of a thin target, and is proportional to the bombardment time at current densities greater than ?? 2??A/cm2. In our energy range of investigation (100-300 keV), the accumulation rate is independent of energy, but does vary with the temperature of the target. For targets at ?? 15??C, the accumulation rate for an oil diffusion pumped target chamber is ?? 2??g/cm2-hr or 2??/min, which corresponds to a retention of one carbon atom per incident proton. Cooling a shield surrounding the target, or heating the target to 150??C, reduces this accumulation rate by an order of magnitude.