Goodmorning everyone. I’m new in Geant4 simulation and I’m trying to simulate the response of a liquid scintillator in a small cylinder hit by gammas or neutrons. The geometry is an aluminum box with inside a glass cylinder with the liquid scintillator sourrounded by an aluminum foil and two PMTs for the readout. As an output I obtain a ROOT file with different Trees for the scintillation and for the the PMTs. As for the scintillation Tree I am recording the deposited energy per event but at least 20% of the events have a deposited energy greater than the energy of the shot particle. Is it normal? I don’t think so!
What I do in my SteppingAction file is:
I would guess that you are also recording the energy deposition of the secondary particles, i.e. your gamma is converted into an electron which also deposits its energy in the scintillator and is recorded. If you only want the energy deposition of the primary I would add another if statement:
G4Track *track = step→GetTrack();
if (track→getTrackID() == 1) {fEventAction->AddEnergyDeposit(stepEnergy);}
This if statement should limit your scoring to energy deposition of your primary only.
I’ve been thinking about it, but what I really want is the total energy deposited per event. For example, let’s suppose we shoot a gamma into the scintillator. It can undergo the photoelectric effect, Compton scattering, or pair production.
If we consider Compton scattering, the final state will include an electron and a scattered gamma photon. This secondary gamma can in turn undergo another Compton scattering (or other interactions), so the total deposited energy also depends on secondary and even tertiary particles.
Therefore, if I use the condition if (track->GetTrackID() == 1), I’ll only register the energy deposited by the primary gamma, and not by its secondaries. Is that correct, or am I misunderstanding?
Forgive me if I am wrong, but are you trying to generate a detector response function, i.e. the pulse height spectra you expect from your scintillator for a given source? As I understand it the common method of generating a detector response function is to tally the energy deposition of the primary particle, and then apply a statistical broadening function based on measured data of your existing system. If I was not wrong, this topic comes up every now and then here is someone who might have had a similar question as you, Detector Response Function .
A sensitive detector would make this easier but your code should work. What immediately comes to mind is when you see these events with energy greater than the initial energy are these runs that include neutrons? Neutron capture reactions can do that because some rest mass will be converted to kinetic energy.
My idea is to get a spectrum of how much energy is deposited in the scintillator per event (counts on the y-axis and deposited energy on the x-axis) and I thought that registering all the energy deposition was the best way because in principles the sum of all the little energy depositions should be less or equal to the initial particle energy.
Thank you for all the suggestions, I’m going to read the similar questions and the article to get other details.
I have actually a SD associated to the scintillator.
I am currently generating gammas and neutrons and in both cases I have some events with an energy deposition grater than the initial particle genergy. Actually in the gammas case they are very few events (more or less from 0.25% to 0.60%) while for neutrons it depends on the enegry. For 4 MeV neutrons it’s the 15%, for 3 MeV neutrons the 18% and for 2 MeV neutrons 24%. Is it consistent with the physics?
What do these spectra look like? Doppler broadening can make photopeak events appear to have “more energy” than the initial energy because of the blurring. As for the neutrons, it depends on the composition of your scintillator. The threshold for neutron capture is 0 MeV. So higher energy neutrons might just be escaping more easily. The signature of such captures is… gamma ray peaks or beta spectra from the resulting excited nucleus.
You do not say what kind of liquid scintillator you are using. But if it is a typical organic liquid, its constituents are H, C and possibly O. If we consider incident neutron and gamma energies of < 20MeV, prompt neutron capture gamma rays can have energies greater than the incident neutron energy. For instance, neutron capture on 1H yields 2.22MeV gamma rays. Capture on 12C yields a number of gamma rays with a total energy of several MeV (Q-value is 4.95MeV and prominent lines are at 3.68 and 1.26MeV). Neutron capture on 16O behaves similarly to 12C. However, gamma rays in the same energy range cannot deposit more energy than their initial energy.
