An apparatus for measuring the energy and angular distribution of secondary electrons emitted from water vapor by fast heavy-ion impact

Abstract

An apparatus has been built to measure the doubly differential cross-section of electron emission from water vapor with fast heavy-ion impact (6–25MeV/u). Ejected electrons are detected by a Chevron-type microchannel plate assembly after being analyzed by a 45° inclined parallel-plate electrostatic spectrometer, which is rotatable from 20° to 160° with respect to the incident-beam direction. The scattering chamber is made of iron, and both the electron spectrometer and the detector assembly are mounted in a μ-metal housing in order to suppress the effects of stray magnetic fields (including that of Earth). Water vapor is emitted into the interaction region from a nozzle of 1×15mm2 aperture, and is instantly frozen and trapped as ice on a stainless-steel panel, which is cooled by liquid N2. With this water-vapor generation and collection system, a stable water-vapor jet (10−2–10−3Torr) was obtained without deteriorating the pressure in the scattering chamber (∼4×10−7Torr with a vapor flow of 40.0cc/min). Using the present apparatus, we have successfully measured the energy and angular distribution (7–10,000eV and 20–160°) of secondary electrons produced in the collision of a 6.0-MeV/u He2+ ion with water vapor; binary-encounter collision peaks were clearly observed at the several keV region for angles smaller than 90°, as well as the K-LL Auger peak of oxygen at ∼500eV for all angles.