I’m trying to model a Geiger counter filled with low pressure argon (approximately 0.1 atm). As far as I understood from the user manual, in order to do so I have to use MAT-PROP card with SDUM=blank. I have several questions regarding this card:
What are the limitations for pressure lower limit? Is it possible to set it as low as I want (like 0.01 atm)?
How does ‘mean ionization potential’ work? I understand that an atom has electron shells with various binding energies. When there is an interaction with the atom, does the program account for atom shells with various binding energies or it assumes that the atom has only one ‘average’ shell with ‘mean ionization potential’?
How is mean ionization potential related to the gas pressure for this card?
thank you for your question. I am looking into it, and will let you know as soon as possible. In the meantime, if the simulation is urgent, I would suggest defining a material for your geiger counter which already has the correct density at that pressure.
it looks like you are setting the right card (MAT-PROP) for the scenario you want to simulate. Here are some of the answers to your question:
Within reasonable bounds and physical scenarios (which would most certainly include your case) there is no limit to the pressure card. I have tested it down to 1e-7 atm without issues, below this unlikely limit you might run into issues (FLUKA sets materials with densities below 1e-10 g/cm3 to vacuum). However, please be aware that the density of the material must be changed according to the pressure specified in the MAT-PROP card; i.e. you have to define the correct density in the material card, with the pressure only adding on slight gas-specific effects, but you will still need to define this.
The mean ionization potential I, also known as mean excitation energy, is defined as (see e.g. the ICRU90 report or Eq 14 in this paper:
where E is the energy transfer from a charged projectile and df/dE is the so-called optical oscillator strength, a quantity encoding atomic structure effects (i.e. shell structure) as well as the response (dynamics) to external perturbations (i.e. excitation and ionization). Thus, the mean ionization potential is not a direct average of shell binding energies, but an average excitation energy log-weighted over the response of the target system to electromagnetic perturbations.
As far as collisions of charged particles with target electrons are concerned, FLUKA does not have an explicit shell-by-shell account of electron binding (it does for photon interactions with atoms, though). Instead, an effective oscillator model accounts in an average way for electron binding, relying indeed on the mean ionization potential. Thus, you should not expect a shell-by-shell account of ionization by direct impact of charged particles, but an average overall account.
3. In principle, pressure and ionization potential are independent of one another. The ionization potential relates to a microscopic property, the pressure relates to a collective property (via the density, which has to be scaled with pressure). However, the two quantities appear together often in calculations of ionization and energy losses; hence, MAT-PROP gathers these and other quantities together.
I hope these observations are of help, and please feel free to reach out if you have any issues running your simulation.
