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 positron 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 normally offset by a loss of emission efficiency due to the quantum mechanical reflection at the metal-vacuum interface, approaching unity as the temperature is reduced to zero. By using a thin moderator (0.1 μm), the quantum reflection is compensated by multiple encounters with the surface, resulting in an efficient high brightness low energy positron moderator. Our results show that efficient accumulation of positrons at 4.2 K necessary for the formation of antihydrogen can be done if a thin metal film is used to moderate the positrons.