| 摘要 |
<p>A slow positron source (300) having a cylindrical geometry which permits increased conversion efficiency from high energy into low energy positrons. A thin walled inner cylinder (302) is filled with a positron emitting liquid radioisotope and is separated from an outer cylinder (310) by an annular cavity (317). The outer cylinder surrounds the inner cylinder and is coated with a moderator and a high atomic number material thick enough to stop all the positrons while providing a high fraction of backscattered positrons. A positively biased pusher disk (314) is placed at one end of the cylinders, while a negatively biased extraction grid (316) is placed at the other end of the cylinders for extracting the low energy moderated positrons from the cavity so as to form a substantially monoenergetic positron beam. The positron beam is directed to impinge upon an annihilation screen (612) at selected points using standard CRT focusing and deflection arrangements (604-608). The positron beam is controlled for calibration purposes to create a desired phantom image on the annihilation screen (614).</p> |
