An available supply of high-specific-activity radioisotopes was identified by the U.S. Department of Energy as a critical priority in the development and eventual deployment of next-generation medical diagnostic and cancer therapy tools. A radioisotope mass separator, located at the Missouri University Research Reactor Center, was developed to provide radioactive ion beams for the separation and production of high-specific-activity lanthanides used in radiopharmacology. Experiments characterizing the ionization efficiency of a thermal ion source supporting the production of high-specific-activity 153Sm are reported. With the goal of maximizing ion current while maintaining beam stability, experiments with the test ion source expand upon previously reported data from a titanium prototype ion source. Experimental results of various factors affecting ion source performance, such as diameter of the extraction orifice, ionizer geometry, ionizer temperature, and sample evaporation rate, are presented.