HIGH-EFFICIENCY POSITIVE (NEGATIVE) SURFACE-IONIZATION SOURCE FOR RADIOACTIVE ION-BEAM

A versatile, new concept, spherical-geometry, positive (negative) surf ace-ionization source has been designed and fabricated which will have the capability of generating both positive- and negative-ion beams wi thout mechanical changes to the source. The source utilizes a highly p ermeable, high-work-function Ir ionizer (phi = 5.29 eV) for ionizing h ighly electropositive atoms/molecules; while for negative-surface ioni zation, the work function is lowered to phi = 1.43 eV by continually f eeding cesium vapor through the ionizer matrix. The use of this techni que for negative ion beam generation has the potential of overcoming t he chronic poisoning effects experienced with LaB6 while enhancing con siderably the efficiency for negative surface ionization of atoms and molecules with intermediate electron affinities. The flexibility of op eration in either mode makes it especially attractive for radioactive ion beam applications and, therefore, the source will be used as a com plementary replacement for the high-temperature electron impact ioniza tion sources presently in use at the Holifield radioactive beam facili ty. The design features and operational principles of the source will be described in this report. (C) 1996 American Institute of Physics.