Less than expected number of secondary electrons being generated due to alpha particle ionizing the air media

Hello, I am working on simulating a free air ionization chamber (parallel plate with seperation between the plates being 5 cms) irradiated by a single alpha particle (created using GPS) and recording the parameters like generation time, initial velocities, track ID of the secondary electrons getting generated in the process. I am employing ‘PreUserTracking()’ for recording the mentioned data and from the ‘track ID’ , I am able to see that approx. 300 electrons are being generated from a 5 MeV alpha particle. However, the literature suggests that since 35 eV is required for an electron ion pair to be generated (in the given context of alpha particle), so the number of electrons being generated should actually be close to 10^5.
I have tried using the production cuts and have set energy range as (1.0eV, 1.0GeV) but that didn’t produce any significant change.
The physics list I am using is EmStandardPhysics_option4 and the electric field present between the plates is 8000*V/m.
I would be grateful for any help or suggestion on the issue.
Thank you