[en] The author has been engaged in the development of a high-efficiency, surface-ionization type negative ion source that employs LaB₆ to measure halogens resulting from nuclear fission and offgas from reprocessing facilities. The equipment presented previously has recently improved greatly so that it can be applied to ionization of various ions and study on microscopic mechanisms of ionization caused by surface reactions. The improved equipment is used to investigate the ionization process of iodine. Solid iodine is heated and gasified iodine is introduced into the LaB₆ ionizer. Negative ions produced are taken out by imposing a voltage, accelerated, converged and subjected to ion current intensity measurement. With the iodine ion current maintained constant, the temperature of the LaB₆ is changed while the intensity of the negative iodine ion beam is measured at an imposed voltage of 0.3, 0.5 or 0.8 kV. The ion current increases exponentially with the ionizer temperature and then decreases after reaching a maximum at a certain temperature. The maximum is higher at higher voltages. Next, as the temperature is decreased, the curve shows higher current values than those observed during the heating process, indicating that temperature hysteresis is taing place. The temperature hysteresis is explained in terms of the processes of adsorption to the surface of the ionizer, diffusion over the surface and desorption as negative ions or neutral atoms. (Nogami, K.)