POSITRON QUANTUM REFLECTION IN THIN METAL-FILMS AND EFFICIENT GENERATION OF HIGH-BRIGHTNESS LOW-ENERGY POSITRON BEAMS AT 4.2 K

High energy positrons implanted into a metal having a negative positro n affinity, phi(+), can be reemitted into vacuum with an energy equal to -phi(+) smeared by the thermal energy. Reduction of the temperature to 4.2 K to increase the brightness of the reemitted positrons is nor mally offset by a loss of emission efficiency due to the quantum mecha nical reflection at the metal-vacuum interface, approaching unity as t he temperature is reduced to zero. By using a thin moderator (0.1 mu m ), the quantum reflection is compensated by multiple encounters with t he surface, resulting in on efficient high brightness low energy posit ron moderator. Our results show that efficient accumulation of positro ns at 4.2 K necessary for the formation of antihydrogen can be done if a thin metal film is used to moderate the positrons.