Proton microbeam analysis in air

A technique has been developed to permit proton induced X-ray analysis (PIXA) to be carried out on biological materials in air, rather than in vacuum. Air convection can then dissipate some of the heat generated in the specimen by the proton beam. A microbeam system of magnetic quadrupoles developed for a 3MeV accelerator was used in the production of a proton beam focused to a spot of diameter less than 4μm. The beam then emerged into air through a 7.5μm kapton foil (a DuPont polyamide film, highly resistant to radiation damage) and in order to preserve positional resolution the specimens were mounted directly on the outside of the beam exit foil. The test specimen consisted of Chinese hamster lung cells washed in isotonic sucrose and plated onto the kapton. The beam was scanned in a line at comparatively high frequency (500 Hz) across the target and the X-rays emitted from the specimen detected in a 10 mm2 Si-Li detector with a resolution of 158 eV at 5.89 keV. P, S, Cl, K and Ca were found with differing positional distributions, together with Ar from the air and Si of unknown origin, by a scan across a single cell. Although the method has important intrinsic advantages, further investigations are required to show that the technique gives reasonable estimates of elemental distributions without too much perturbation of the concentration by the analytical procedure. (U.K.)