James's primary responsibility is running the Australian Positron Beamline Facility at the ANU, a central part of the research activities of the Australian Centre of Excellence for Antimatter-Matter Studies (CAMS). This facility features two positron beamlines that use the radioactive decay of sodium-22 and a Malmberg-Penning trap to produce high intensity and energy resolution positron beams for reaction studies. Typical experiments feature positron collisions with atoms and molecules and also materials using positron annihilation lifetime spectroscopy (PALS), a nondestructive technique which allows for measurements of porosity or defects in solids.
These studies at ANU are in many respects similar to the electron scattering experiments in Morty and Leigh's lab, except the reactions are initiated by an anti-electron, i.e. a positron, instead of an electron. However, the simple difference in the sign of the charge of the scattering particle introduces new physics into the reactions, one notable example being the possibility of positronium formation, where a positron and electron bind to form a stable subsystem. Positronium is often termed an 'exotic atom' as it shares many similarities to atomic hydrogen, but significantly less mass since the proton in hydrogen is replaced by a positron.
Besides overviewing the technological side handling and studing positrons, James presented recent data which showed evidence of 'cusp' effects in the scattering cross sections of noble gas atoms, viz. small peaks in the elastic cross section centred around the onset of positronium formation. This effect was observed in all noble gas atoms on the periodic table from helium to xenon, although to date no theory has been able to predict these effects. James also showed recent data for positron scattering from water, which is of particular applied relevance as positron-water interactions are the dominate process responsible for thermalizing positrons in human beings during a PET scan, a technique commonly used to identify cancerous tumors. Positron-water scattering cross sections are required to better understand and hopefully improve upon the spatial resolution of PET.
Thanks James for visiting the department and a talk so close to Thanksgiving.