INVESTIGATION OF POSITRON MODERATOR MATERIALS FOR ELECTRON-LINAC-BASED SLOW POSITRON BEAMLINES

Positron re-emission properties were studied on moderator materials in order to improve the positron moderation system of electron-linac-bas ed intense slow positron beamlines. The re-emitted positron fraction w as measured on tungsten, SiC, GaN, SrTiO3, and hydrogen-terminated Si with a variable-energy pulsed positron beam. The results suggested tha t tungsten is the best material for the primary moderator of the posit ron beamlines while epitaxially grown n-type 6H-SiC is the best materi al for the secondary moderator, Defect characterization by monoenerget ic positron beams and surface characterization by Auger electron spect roscopy were carried out to clarify the mechanism of tungsten moderato r degradation induced by high-energy electron irradiation. The charact erization experiments revealed that the degradation is due to both rad iation-induced vacancy clusters and surface carbon impurities. For the restoration of degraded tungsten moderators, oxygen treatment at simi lar to 900 degrees C is effective. Furthermore, it was found that oxyg en at the tungsten surface inhibits positronium formation; as a result , it can increase the positron re-emission fraction.