Metastable-metastable collisions in the Orsay polarized electron source by helium afterglow

At Orsay the current of polarized electrons produced by the optically pumped helium afterglow does not vary as the metastable density, but as the density squared. That shows that spurious electrons are mixed to the electrons emitted by chemi-ionization reaction of the polarized metastables with CO2 gas. In this paper it is suggested that these spurious electrons are produced by metastable-metastable collisions. The contribution of these collisions to the electron current and polarization is evaluated.