Questions on counting primary particle ionizations

I want to count the number of times that an electron ionizes a region filled with gas. I have approached this 2 ways. In the first approach I modified the TestEm1 example slightly by adding a material consisting of a 70/30 mix of Ar/CO2. I then simulated 2.3 MeV electrons hitting a 16 mm thick volume filled with the Ar/CO2 mixture. If I am understanding this example the histogram of the number of primary particle steps should be equivalent to the number of ionizations. I then executed the runs.mac file suitably modified to use the correct particle, energy, and detector size.

The run produces 2 histograms for the number of primary particle steps, one with a cut size of 1mm and one with a cut size of .01mm. The resulting histograms for the run are attached as text files. testEm1hist1mm.txt (198 Bytes) testEm1hist10um.txt (269 Bytes) If I am understanding these histograms, number of times there were 1, 2, or 3 steps is on line 9 and the number of times there were 3-6 steps is on line 10, etc. But why are there so many more steps with the 10um cut file? Is it that the larger cut value results in fewer ionizations contributing to a step because they do not meet an energy threshold?

I also tried to use a modification of example B1 to look for primary particle ionizations. I still don’t know enough about geant4 to be able to add a histogram to that example so I passed the simulation output through some filters, first restricting results to those with an eIoni process in the volume with the gas mixture and then eliminating secondary particle ionizations by eliminating steps with energy less than half the input energy and those with a step ID of 1.

When I did that and looked at the results I counted about 2100 events with step IDs 2-4, but no events with a step ID greater than 4 (which I equated to a 3rd ionization by a primary electron). In addition, When I attempted to change the step cut in the same way as done in TestEm1, there was no significant change in the result.

My first question is: Is my understanding that counting primary particle steps is equivalent to counting ionizations correct? Is my understanding of the reason that changing the cut value changes the histogram correct? The next question is what is wrong with my reasoning regarding counting ionizations by filtering steps to eliminate secondary ionizations?

Hi mluntz,

For each of the examples that you’ve stated, the electromagnetic physics lists available in the files are condensed history physics models. Condensed history models involve bundling multiple ionizations into a single step point. Which means that counting the number of steps does not yield the number of ionizations, as the energy deposition at each step point is the result of many ionizations. I would refer you to the article “Lazarkis P, Incerti S, Ivanchenko V et al. Investigation of track structure and condensed history physics models for applications in radiation dosimetry on a micro and nano scale in Geant4”. Which should help explain condensed history and track structure physics models. If you look at Figure 1 of that article you can see that the mean number of ionizations from condensed history models can be much less than that of track structure.

If you want to be able to count the number of ionizations, I can think of a couple of approaches such as: 1.) measuring the energy deposition in your volume and with knowledge of the mean ionization potential for your material taking the ratio of (energy deposition per track)/(mean ionization potential) to approximate the mean number of ionizations you expect to observe. 2.) Alternatively, you could work with track structure physics models (such as Geant4 DNA) in which the number of simulated ionizations will correspond more closely to the true number of ioniziations.