LIMITATION OF THE POLARIZATION BY RADIATION TRAPPING IN A HELIUM AFTERGLOW ELECTRON SOURCE

A polarized electron source using an optically pumped helium afterglow was built at Orsay. Unfortunately the spin polarization decreases at high metastable densities. Calculations of the radiation trapping effe cts in a weak magnetic field are presented using the Anderson formalis m. Comparison with experimental data leads to the conclusion that thes trapping effects are one explanation of this polarization decrease. E ffects of the main parameters are studied. Some deductions for a new d esign can be made.