| We report measurements of positronium (Ps) formation following positron irradiation of mesoporous SiO2 films and Ge(100) single crystals at temperatures ranging from 12-700 K. As both of
these materials generate Ps atoms via non-thermal processes, they are able to function as positronpositronium converters at cryogenic temperatures. Our data show that such Ps formation is possible provided the targets are not compromised by adsorption of residual gas. In the case of SiO2 films we observe a strong reduction in the Ps formation effciency following irradiation with UV laser pulses (lambda= 243.01 nm) below 250 K, in accordance with previous observations of radiation-induced surface
paramagnetic centers. Conversely, Ps emission from Ge is enhanced by irradiation with visible laser pulses (lambda = 532 nm) via a photoemission process that persists at cryogenic temperatures. Both mesoporous SiO2 films and Ge crystals were found to produce Ps efficiently in cryogenic environments.
Accordingly these materials are likely to prove useful in several areas of research, including Ps mediated antihydrogen formation conducted in the cold bore of a superconducting magnet, the
production of Rydberg Ps for experiments in which the effects of black-body radiation must be minimized, and the utilization of mesoporous structures that have been modified to produce cold
Ps atoms. |
