Article révisé par les pairs
Résumé : We have measured the linear polarization of the Hβ transition at 486.1 nm excited by passage of a 110 keV proton beam through perpendicular carbon foils. We have observed that the polarization depends upon the beam intensity and on the relative position of the foil and its holder. We have shown that these dependences are linked to the presence of a stray electric field at the immediate vicinity of the foil. The field is due to the potential distribution at the foil surface resulting from the electron radial flow in the high foil electric resistance (about 50 kΩ). It introduces a perturbation which in our case is more important than the temperature effect observed by Gay and Berry (Phys. Rev. A19, 952 (1979)). The field is proportional to the beam current density and is reduced for large foil and beam diameters. © 1980 IOP Publishing Ltd.