Crossed beam measurement of absolute cross sections: an alternative method and its application to the electron impact ionisation of He+

A new experimental method is reported for the crossed beam measurement of absolute cross sections. It essentially consists in sweeping one of the beams across the other in a linear 'see-saw' motion. The method is applied to one case of electron-ion collisions. A description is given of an electron gun designed to realise a linear and distortionless motion of a ribbon electron beam. Measurements are reported regarding the electron impact ionisation of He+ in the energy range 55-74 eV. (author)