Ionization of He3 Gas

Physics Processes, Models and Cross Sections
1

Geant4 Version: 11.2.1
Operating System: Linux Mint
Compiler/Version: gcc version 9.4.0
CMake Version: 3.16.3

Hi there!
I’m new to geant4 and had a question come up while working on a project involving the simulation of thermal neutrons interacting with He3 gas. I am using QGSP_BERT_HP physics, which works well for the interaction of neutrons with He3, I get the expected Q value for
neutron + He3 -> proton + triton + 764 keV
So from this point on I am interested in the ionization of He3 by the proton and triton.
After setting

G4ProductionCutsTable::GetProductionCutsTable()->SetEnergyRange(10*eV, 100.*MeV)

I see secondary electrons produced by the Proton passing through He3. So here comes my question: Why can i not observe ionized He3?

I have defined the gas with

  G4double He3_pressure = 4.8 * bar;
  G4double temperature = 293 * kelvin;
  G4double molar_constant = CLHEP::Avogadro * CLHEP::k_Boltzmann;

  // Helium-3 definition:
  G4double He3_aMass = 3.016   * g / mole; 
  G4int protons = 2, neutrons = 1;
  G4int nucleons = protons + neutrons;
  G4Isotope *he3 = new G4Isotope("He3", protons, nucleons, He3_aMass);
  G4Element *He3 = new G4Element("Helium3", "He3", 1);
  
  He3->AddIsotope(he3, 100 * perCent);
  G4double He3_density = (He3_aMass * He3_pressure) / (temperature * molar_constant);
  Helium3 = new G4Material("Helium3", He3_density, 1, kStateGas, temperature, He3_pressure);
  Helium3->AddElement(He3, 100 * perCent);

I have looked for secondaries in the output of a highly verbose trackingManager and only found electrons undergoing eIoni processes.
Thanks in advance!

2

I may be wrong about this, but my understanding is that Geant4 “automatically” creates any trackable nucleus (i.e., the G4Ions object you’d see reported in /tracking/verbose) to be a neutral atom, implicitly collecting or giving up whatever electrons might be needed to the surrounding material.

3

Thanks for the fast reply!

Unfortunately I do not see G4Ions in tracking. I have tried changing the Material from He3 to Hydrogen (G4_H from nistmanager). Then shooting 500 keV protons, I would expect secondary electrons / protons (H+). Also in this simplified scenario, I only see electrons as secondaries.
Could you maybe help me by providing an example, where a beam ionizes a material and secondary Ions are tracked (if this is part of geant4 examples)? Any comment would also be helpful!

4

Are you setting production cuts as part of your configuration (either via macro commands, or in your code)? Recoil nuclei generally have an extremely short stopping distance, so a macroscopic production cut (e.g., 1 mm) will suppress all the nuclei.

The rdecay01 example, with some tweaking, might show you how to get and see nuclear recoils.

5

Hi,

Just to complete the other answer, few further clarifications:

  • only particles above a given threshold are tracked by Geant4 (I think is around 1 eV), because the main goal of Geant4 is to calculate deposited energy. There are other codes, such as Garfield++, which can keep track of these ions further.

  • some processes check the so-called production threshold, so they only add secondary particles (to the stack of particles to be tracked) if that secondary has more energy than its corresponding threshold. This threshold is known as production cut. Here it is the C++ code to change it.. UI commands may be used as well, eg /run/setCutForAGivenParticle proton 0.0 mm

  • The production cut for “Protons” actually applies to charged ions as well, so you may want to set it to zero to see the nuclear recoils (because the nuclear stopping power seems to be important only at low energies)

  • the stopping power may be electronic or nuclear, and the nuclear seems to contribute orders of magnitude less than the electronic, only being comparable at very low energies. In the picture below the electronic, nuclear and total stopping power of protons in helium is shown (source, PSTAR database by NIST).

    Screenshot at 2024-06-02 19-24-04
    Screenshot at 2024-06-02 19-24-04778×665 20.4 KB

  • the nuclear stopping power is included in Geant4, and it may lead to nuclear recoils, but the energy of the recoil may be below the production threshold. The electronic stopping power formula (Bethe 1930) assumes frozen target nucleus.

Please, let us know if you managed to see those recoil 3He :slight_smile:

Alvaro
PS : if someone notices some errors, please let me know.

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