Positron cooling by vibrational and rotational excitation of molecular gases

Measurements of positron temperature as a function of time are presented when a positron gas, confined in an electromagnetic trap at an elevated temperature (>= 1200 K), is cooled by interactions with the molecular gases CF4, N-2 or CO at 300 K. A simple model describing positron thermalization by coupling to vibrational and rotational modes is presented, with cooling-rate predictions calculated in the Born approximation. Comparisons to the measured positron cooling-rate curves permit estimates of the magnitudes of the relevant cross sections. The results are compared with experiment for the case of vibrational excitation, where direct measurements exist; and they provide estimates of the rotational excitation cross sections where direct measurements are not currently possible. Positron cooling rates are compared for these gases at 300 K, and estimates of their effectiveness in cooling positrons to cryogenic temperatures are discussed.