Surface characterization of diamond-like carbon for ultracold neutron storage

We report the characterization of diamond-like carbon (DLC) surfaces to be used for the storage of ultracold neutrons (UCN). The samples investigated were 100–300-nm-thick tetragonal amorphous carbon (ta-C) coatings produced by vacuum-arc technology on thin foils (0.1–0.2 mm aluminum, stainless steel, PET). The diamond sp3 fraction was determined by X-ray photoelectron spectroscopy (XPS) to be in the range 45–65%. Secondary-ion mass spectroscopy (SIMS) and elastic recoil detection analysis (ERDA) yielded consistent results for the hydrogen contribution (about 1×1016 cm−2 within the top 20 nm), strongly concentrated within a surface layer of 1 nm thickness. The boron contamination was found to be around 50 at. ppm. The fractional hole area of the coatings is on a level of about 1×10−4. Temperature cycling of mechanically pre-stressed samples between 77 and 380 K revealed no detrimental effect.