Prior to the coating of a sample to a filament surface, a Joule-heating treatment of the filament was performed to remove any impurities from the filament and to minimize the influence of the background in thermal ionization mass spectrometry. In this work, the effect of the surface condition of the tungsten filament induced by the Joule-heating treatment on an uranium isotopic (235U/238U) measurement has been investigated. By the Joule-heating treatment, crystal grains were formed on the filament surface, and the filament surface became smooth. It was found that by extending the Joule-heating time, the surface became smoother and the precision of uranium isotopic (235U/238U) measurement was improved.
