Neutrals desorbed from solid surfaces by electron beam impact can be ionized by the beam and trapped in the beam potential causing beam disruption. This can increase the beam diameter on radiographic bremsstrahlung targets degrading resolution. Measurements of the number and species of neutrals desorbed by impact of a 19.8 MeV, 1.7 kA, 60 ns electron beam on thin-foil targets are made in open and closed geometries. The study focuses on aluminum and graphite foil surfaces. A fast Bayard–Alpert ionization gauge measures the number of neutral molecules released, and a quadrupole mass spectrometer measures the species mix. At high beam current density, where thermal desorption due to target heating is expected, the dominant neutral species is H2O, and approximately one monolayer of desorbed gas is released. Other prominent species such as H2, CO, and CO2 are not thermally desorbed until current densities near foil destruction are approached. The observations are in agreement with a previous hypothesis based on the comparison of computer calculations with beam-dynamics observations.
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