Positron trapping in an electrostatic well by inelastic collisions with nitrogen molecules

Positrons from a radioactive source are slowed to electron-volt energies and accumulated and stored in a trap which uses a magnetic field for radial confinement and an electrostatic well for axial confinement. The positrons lose energy and become trapped through inelastic collisions with nitrogen molecules introduced into the trap for this purpose. The trap has three stages with progressively lower nitrogen pressure. It is found that the trapping in each stage is due primarily to electronic excitation of the nitrogen molecules, with the energy loss being approximately 9 eV per collision. Positronium formation is believed to be the dominant loss mechanism during the trapping process. Trapping efficiencies of greater than 25% have been achieved. Using a 150-mCi ${}_{}{}^{22}{}_{}{}^{}\mathrm{Na}$ source, a maximum stored number of ∼1×${10}^7$ positrons have been stored with a lifetime of 40 s, limited by the annihilation on the nitrogen gas at a pressure of 2×${10}^{\mathrm{-}6}$ Torr. The positrons cool to room temperature in a few seconds via rotational and momentum transfer collisions with the nitrogen.