Cf-252 has a fission BR of 3% and so 97% of the time your simulation is just fruitlessly following alphas. If you search these forums you will find that using RD biasing does not work for fission sources (this specific point was why I first came to these forums).
For starters and to avoid that as a potential reason why you don’t have many events put the attached file somewhere and then a macro command somewhere as:
/process/had/rdm/setRadioactiveDecayFile 98 252 z98.a252_modify.txt
That file will force every event to be a fission event. If you use RD biasing with fission sources you will simply not get any neutrons so what you are seeing is maybe electrons from gamma interactions in the walls. The average energy of Cf-252 neutrons, like most SP fission sources, is around 2 MeV. I doubt that 1 meter of air would thermalize them enough for capture, but maybe a foot of plastic. Also what pressure are you using? He3 tubes tend to be very high pressure because they have very low efficiency.
z98.a252_modify.txt (284 Bytes)