Optical reflectivity measurements on thorium metal samples

Thorium samples, the surfaces of which were mechanically polished, electropolished and as grown from the van Arkel process, were systematically studied by optical reflectivity measurements. These optical data were compared with published measurements showing significant discrepancies. Increasing surface roughness was found to lower the reflectivity considerably and probably to induce surface plasmons indicated by a deep minimum in reflectivity at 4 eV. Best reflectivity data were obtained only for electropolished surfaces. Well chosen conditions for the van Arkel process give also good results