New Adventure in Gaseous Positronics - A Cryogenic Beam

Buffer-gas-trap based beams have proven a reliable workhorse to study positron scattering and annihilation.\footnote{J. R. Danielson, et al., Rev. Mod. Phys. 87, 247 (2015).} The state of the art beam has a total energy spread $\sim 40$ meV FWHM using 300 K gas. Described here is work to create beams with narrower energy spreads (goal: total spread $\le 5$ meV FWHM using 50 K buffer gas). A Born-approximation model is used to describe cooling on vibrational and rotational excitations. Positron cooling from 1,200 K to 300 K was studied for CF$_4$, N$_2$ and CO to obtain the relevant cross sections (by fits to the model) and then predict cooling to 50 K.\footnote{M. R. Natisin, et al., J. Phys. B 47, 225209 (2014).} Using an additional cryogenic trapping stage, positrons have now been cooled to 50 K on N$_2$ and CO. Since the beam is generated in a magnetic field, the total energy spread is characterized by spreads parallel and perpendicular to the field.\footnote{ M. R. Natisin, et al., Phys. Plasmas 22, 033501 (2014).} While the perpendicular temperature is 4 meV (i.e., kT at 50 K), the parallel energy spread is larger. The currently projected total spread is $\le 10$ meV FWHM - a factor of four better than the 300 K result. Work is in progress to reach the predicted total spread at 50 K of 5 meV FWHM.