A polished MgO surface (10mm x 10mm) with a (100) orientation was used. The surface had a maximum surface roughness R of less than 5nm (1nm typically) as specified by the supplier Mateck . The sample was stored in a sealed container as provided by the supplier. After incorporation into the JUSO apparatus a pressure in the 10mbar range was achieved. The sample was also heated to 150 C for several hours to clean the surface. No sputtering of the surface with heavy noble gases was done.