Positron cooling by vibrational and rotational excitation of molecular gases

Measurements of positron temperature as a function of time are presented\footnote{\small M. R. Natisin, J. R. Danielson, C. M. Surko, {\it J. Phys. B} {\bf 47}, 225209 (2014).} when a positron gas, confined in an electromagnetic trap at an elevated temperature ($\geq1200$~K), is cooled by interactions with the 300~K molecular gases CF$_4$, N$_2$ and CO. A simple model describing positron cooling and thermalization by coupling to vibrational modes (CF$_4$, CO), dipole-coupled rotational modes (CO), and quadrupole-coupled rotational modes (CO, N$_2$) 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. A new experiment using cryogenically cooled buffer gases is underway, and measurements of positron cooling to 50 K will be presented.