| 摘要 |
A method for efficiently generating thermal positions from a source of energetic positrons, consisting of a method for increasing the emission efficiency of the positron source, and a method for increasing the efficiency of a positron moderator. In an advantageous case the combined improvements lead to an about ten-fold increase in generated thermal positrons. The method for improving the source efficiency consists in reducing the self-absorption of positrons, typically emitted from radioactive atoms incorporated into a substrate by means of diffusion, by the source. This is accomplished by providing for a backing layer having a relatively small diffusion constant for the radioactive species, and a thin diffusion layer having a relatively large such diffusion constant, with the diffusion layer deposited onto the backing layer. Depositing the required amount of radioactive material onto the diffusion layer and raising the temperature of the sandwich to an appropriate diffusion temperature causes the radioactive material to diffuse into the sandwich, where it will remain concentrated mostly in the diffusion layer, thus being closer to the surface of the source than in typical prior art devices. The method for improving the moderator consists in preparing the moderator from a high-quality single crystal, of high purity, of material having a relatively short stopping distance for energetic positrons, and a relatively long mean diffusion distance for positrons. The active surface of the moderator is to be parallel to a low-index plane of the crystal, selected to have a relatively large negative positron work function. The efficiency of the moderator can be further improved by activating the active surface with about a monolayer of an appropriate chemical species having the property of making more negative the positron work function. An embodiment of method is a 58 Co source consisting of a W backing layer and a 2 mu m thick Cu diffusion layer, and a moderator consisting of 99.999 percent pure copper, with (111) active surface, activated by about a 1/3 monolayer of S.
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