I am working on calculating total ionizing doses for space missions. I distinguish ionizing events by checking process subtypes for each hit. I am getting very low event rates at roughly 1000 ionizing events on a 50 million particle input run. Are such low ionizing event rates expected? Is there another method for calculating TID that I could use?

Hi, to count ionizations when using condensed history approach physics models is not appropriate as many interactions are condensed in a single step. Why donâ€™t you simply integrate the edep over the SV volume?

Could you elaborate on what you mean by this? The goal was to create a dose map over the test plate to determine where electronics should be placed to receive a certain dose. Would your approach work for something like that?

you can definitively calculate dose maps. That is fine. Depending on your geometry, you may need to reduce the cut and fix a maximum step to obtain more accurate results in terms of energy deposition.
What is not possible to do is to calculate the number of ionizations that produces that dose with Condensed History Approach. The edep is associated with the pre-step point and many real ionization events are condensed in a single step. You can count the number of ionizations using a track structure code (e.g. geant4-DNA). You may look into the TS model for silicon (here I make the assumption you use silicon) which are available for electrons. The context is different, but this concept is explained in Lazarakis et al, https://ro.uow.edu.au/cgi/viewcontent.cgi?article=2347&context=eispapers1
Again you can calculate dose maps, but not the number of ionizations associated with that dose. You can do it by a software point of view, but this does not represent the real number of ionization events, which will be a lot higher.

How would I go about getting the energy deposition that contributes to ionization? Would that just be step->GetTotalEnergyDeposit() - step->GetNonIonizingEnergyDeposit(), or is it more complicated than that?

integral energy deposition at a step is the best method to compute dose in a media. An important moment how to choose an adequate voxel structure for scoring. In your case it should be sensitive volume of your electronic device.