Detecting Gamma Energy Deposition

Particles, Track, Event, Run and Biasing
1

_Geant4 Version:11.2.1
_Operating System: MacOS

I am attempting to get the Energy Deposition by gammas in an Ionization Chamber (air) and Silicon detector. I use “FTFP_BERT_EMV” physics List with “G4EmStandardPhysics_option3” as well. Through my Hits and SensitiveDetector, that has worked perforctly for Protons and Carbon Ions, it is simply refusing to work for gammas.

The PDG of the hits are registering at 22 for gammas, but the energy deposition is always zero.

I am setting step limiters as such in DetectorConstruction:

    // Setting Step Limit for Ionization Chamber
    G4double maxStep_I = 0.001*cm; // Example maximum step length
    G4UserLimits* stepLimit_I = new G4UserLimits(maxStep_I);
    logicShape2->SetUserLimits(stepLimit_I);
    
    
    // Setting Step Limit for Silicon Detector
    G4double maxStep_Si = 0.01 * um; // Example maximum step length
    G4UserLimits* stepLimit_Si = new G4UserLimits(maxStep_Si);
    logicShape1->SetUserLimits(stepLimit_Si);

And then Registering it in the physicsLists:

G4PhysListFactory factory;
    G4VModularPhysicsList* physicsList = factory.GetReferencePhysList("FTFP_BERT_EMV");
   // Setting User Limits
    G4StepLimiterPhysics* stepLimitPhys = new G4StepLimiterPhysics();
    physicsList->RegisterPhysics(stepLimitPhys);
    G4VPhysicsConstructor* emPhysics = new G4EmStandardPhysics_option3();
    physicsList->RegisterPhysics(emPhysics);
    G4ProductionCutsTable::GetProductionCutsTable()->SetEnergyRange(0.0001*eV,10.*MeV);

I wanted to ask if anyone has gone through the issue of energy deposition of gammas and knows what might be the issue here.
Thank you :slight_smile:

2

Dear Khalil Elachi,

My first guess is that if a gamma interacts in the ionization chamber, then its energy is transferred to a secondary electron which ultimately deposits the energy. Thus, the actual energy deposition is due to an electron, not to a gamma. Actually, gammas (and neutrons) are classified as indirectly ionizing radiation because of this.

Also, I would ensure that the probability of interaction of the gamma beam inside the sensitive volume is large enough. In this sense, I suggest you to check the value that the energy deposition scorer gives in your case. It is likely that you will need larger statistics with gammas as compared against protons and carbon ions.

Hope this helps,

Miguel

3

Hello,
Thank you for the help. I was getting the Energy Deposition from G4Step pertaining to PDG 22, but when I switch it to PDG 11 I am getting the nominal values that I was expecting.

Thank you, much appreciated!