A quantitative analysis of ionization growth times measured in pure helium between plane parallel silver electrodes (d = 0.817 cm) for 5.07 < po < 38.51 mmHg, in the range 8.61 < Es/po < 38.08 v cm^-1 mmHg^-1, for overvoltages up to 6% has been made. This shows that the dominant secondary ionization process at the higher pressures is delayed photoelectric emission due to the incidence at the cathode of collision-induced non-resonance radiation produced in the volume destruction of atoms in the 2 ¹S metastable state in two-body collisions in the gas. The cross section computed for this process is about 3 × 10^-20 cm²/atom. At the lower pressures used secondary electron emission due to the incidence of helium positive ions plays an increasingly important role, contributing as much as 75% of the secondary emission at p = 5.07 mmHg.