Development of new technology of positron annihilation defect spectroscopy under ion-bem irradiation

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A slow positron beam line was constructed at the neutron production room of HIT (High Fluence Irradiation Facility of University of Tokyo). The beam line was merged with the ion-beam line from the van de Graaf accelerator so that simultaneous irradiation of ion-beam and slow positron beam is possible. Evaluation of ion-beam induced defects is possible by measurements of Doppler Broadening of the positron annihilation radiation. Further improvement of the facility was aimed by making the slow positron beam. By chopping the DC positron beam into 5ns pulse width, and then bunching with 40MHz RF, short pulses of 500psec were created. These pulsed positrons were used to measure the formation and lifetime of ortho-positronium, whose lifetime is normally longer than 500psec. A trial to improve the pulse width shorter than 500psec has not been successful without sacrificing the counting efficiency. But further study for obtaining shorter pulses are under way.