Speaker
Description
We have adapted one of the ANU positron beamlines, which use a Surko buffer gas trap and a strong magnetic field, to enable direct measurements of reaction products from atomic collision experiments. An effusive gas jet was added to the beamline, which allowed us to cross a helium beam with the high-resolution, pulsed positron beam. Long-lived (metastable) neutral excited helium atoms formed in the positron collisions were detected by a strategically positioned channel electron multiplier (CEM).
Helium has two long-lived metastable states (2$^3$S and 2$^1$S), though only the 2$^1$S state is directly accessible to positrons. Excitation of the 2$^3$S state requires a spin-flip from the ground state and since positrons do not have access to the exchange interaction like electrons, they instead require the spin-orbit interaction, which is both weaker for positrons than electrons and weak for helium in general (spin-orbit scales roughly with atomic number Z$^4$). Thus, we expect only 2$^1$S excitation.
In the experiment, a pulsed positron beam crosses a He beam and metastable atoms are detected with high efficiency (~ 90%) by a CEM. Time-of-flight techniques and electrostatic retardation are used to separate the relatively slow He atoms from faster reaction products (ions, positrons, electrons, positroniums). The 2$^1$S state has a lifetime of ~ 19 ms, which is far longer than the average flight time of the atoms from the collision volume to the detector (~ 40 $\mu$s).
Excitation of the He 2$^1$S state as a function of incident positron energy will be presented and compared with previous measurements using conventional gas-cell arrangements and with theory.
