BaF2 Scint Sim - No slow component?

Particles, Track, Event, Run and Biasing
1

Hi experts,

I’ve been trying to made a model of a BaF2 detector. However, I can’t see the slow component on the output, just the fast component. The slow component, from what I understand should be more intense, which is what I have in my code, but I get no signal around 630ns.

I’ve tried changing all the parameters (larger and smaller), testing other particles and energies but no luck. I also registered G4OpticalPhysics, just in case but no luck. I looked over otehr forum posts as well and it looks like I am on the right path here.

My main sim is 14 MeV neutrons on a graphite block. Sorry if a made an error copying the code because I had to change it around a little because if-statements. Future apologies if this is a simple oversight.

_Geant4 Version:_11.0.1

    energy[0] = 1.239841939*eV/0.220;
    energy[1] = 1.239841939*eV/0.310;
    rindex[0] = 1.54;
    rindex[1] = 1.50;
    absorbLength[0] = 72.15*cm;
    absorbLength[1] = 749.68*cm;
    fastComp[0] = 1.;
    fastComp[1] = 0.;
    slowComp[0] = 0.;
    slowComp[1] = 1.;

    G4MaterialPropertiesTable *mpt = new G4MaterialPropertiesTable();
	mpt->AddProperty("RINDEX",energy,rindex,2);
    mpt->AddProperty("ABSLENGTH", energy, absorbLength, 2);
    mpt->AddProperty("SCINTILLATIONCOMPONENT1", energy, fastComp, 2, true);
	mpt->AddProperty("SCINTILLATIONCOMPONENT2", energy, slowComp, 2, true);
    mpt->AddConstProperty("SCINTILLATIONYIELD", 11800./MeV);
	mpt->AddConstProperty("RESOLUTIONSCALE", 1.0); 
	mpt->AddConstProperty("SCINTILLATIONTIMECONSTANT1", 0.9*ns,true);
	mpt->AddConstProperty("SCINTILLATIONTIMECONSTANT2", 630*ns,true);
	mpt->AddConstProperty("SCINTILLATIONYIELD1", 1);
	mpt->AddConstProperty("SCINTILLATIONYIELD2", 5);
	sintChoice->SetMaterialPropertiesTable(mpt);
2

Your energy vector is in descending order of energy. I think it is required to be in ascending order. I do not know the effect of having it backwards and so this may not solve your problem, but you should change it (and, of course, also the corresponding vectors that are energy-dependent).

3

Hi John,
Thanks for the assistance.

I changed the energy vector to be ascending, but no luck.
The time of energy deposit didn’t change much. Nothing really around ~630ns.

I also tried beaming the detector directly (and a graphite block) with different energies and particles.

Below is typically what I see. x-axis is global time (time from beginning of event) in nanoseconds. y-axis is log.
If I understand how this works, there should be a peak or bulge around 630ns, larger than the 0.9ns peak (ratio of 1:5), for the slow component for energy deposited in the BaF2 scint.

G4Forums_GlobalTimeOfEnergyDepInScint
G4Forums_GlobalTimeOfEnergyDepInScint1680×864 20.7 KB

Thanks for the assistance!

4

If I understand how this works, there should be a peak or bulge around 630ns, larger than the 0.9ns peak (ratio of 1:5), for the slow component for energy deposited in the BaF2 scint.

No, there shouldn’t be. The scintillation response is the sum of two exponential decay components. At large times (>> tau1 = 0.9ns), which is pretty much most of the graph you show, you will have an exponential decay with a single time constant = tau2 = 630ns. I did a few pencil and ruler measurements on the graph and indeed the time response from ~100ns to ~2000 ns is an exponential (straight line decay on a log plot) with a time constant of ~600ns.

5

Thanks a bunch for that!

I thought it may have been my own ignorance here and I’m very happy you showed that to be the case!

Kind regards,
MJ

6

You’re welcome. It’s an easy mistake to make if you are not thinking of the correct model. Good luck.

John