AN ACCELERATOR BASED ELECTRON-PHOTON COINCIDENCE SPECTROMETER FOR THE STUDY OF INELASTIC ELECTRON-ATOM COLLISIONS

Inelastic electron-photon collisions can be studied with our two-photon coincidence spectrometer by replacing one of the Ge detectors with a Si(Li) detector with a 0.4 mum polymer window coupled to the target vacuum chamber. The present geometry has the detectors at +/- 45-degrees to the incident electron beam. The counting rate in the Si(Li) detector is dominated by the elastic scattered electrons and the backscattering tail of the elastic peak and is several orders of magnitude larger than the photon rate. Nevertheless, the coincidence electronics is able to separate real coincidences as a function of both photon and electron energy from the large accidental coincidence background. This permits the study of such processes as the doubly differential cross section for inner-shell ionisation and the triply differential cross section for single bremsstrahlung. This setup has a much more rapid data collection rate and yields a statistical error comparable to or better than has been previously achieved in the few experiments that have been done using a magnetic spectrometer on the electron. Preliminary examples of two dimensional energy spectra with 70 kev electrons on targets of Al, Cu, Ag, Th and Au are discussed.