Energy loss of electrons in scintillator at 1 GeV

For the Beamline for Schools experiment, we are simulating an electron at 1 GeV going through 1 cm of scintillator (G4_PLASTIC_VINYLTOLUENE). The result is that an electron looses a few MeV in the scintillator. Thîs doesn’t agree with the Estar value ~24 MeV:

G4WT1 > Step# X(mm) Y(mm) Z(mm) KinE(MeV) dE(MeV) StepLeng TrackLeng NextVolume ProcName
G4WT1 > 0 -1.35 -1.05 -4.5e+03 752 0 0 0 World initStep
G4WT1 > 1 -1.35 -1.04 -4.43e+03 751 0.0112 65.1 65.1 World eIoni
G4WT1 > 2 -1.35 -1.05 -4.42e+03 751 0.00192 11 76.1 World eIoni
G4WT1 > 3 -1.37 -1.09 -4.36e+03 751 0.0125 62 138 World eIoni
G4WT1 > 4 -1.45 -1.22 -4.22e+03 751 0.0298 146 284 World eIoni
G4WT1 > 5 -1.53 -1.3 -4.13e+03 751 0.0161 88.1 372 World eIoni
G4WT1 > 6 -1.54 -1.31 -4.12e+03 751 0.00214 8.62 381 World eIoni
G4WT1 > 7 -1.65 -1.4 -4e+03 751 0.0256 119 500 Scint_PV Transportation
G4WT1 > 8 -1.65 -1.4 -4e+03 750 0.362 2.35 502 Scint_PV eIoni
G4WT1 > 9 -1.64 -1.4 -3.99e+03 749 1.29 7.65 510 World Transportation
G4WT1 > 10 -1.66 -1.41 -3.97e+03 749 0.0024 15.3 525 World eIoni
G4WT1 > 11 -1.86 -1.61 -3.8e+03 749 0.0349 175 700 FrontChamber_PV Transportation
G4WT1 > 12 -1.87 -1.62 -3.79e+03 749 0.00483 10.8 711 FrontChamber_PV eIoni

When running the TestEM5 with electrons at 1 GeV through 1 cm of plastic the result is:

G4WT3 > * G4Track Information: Particle = e-, Track ID = 1, Parent ID = 0
G4WT3 > *********************************************************************************************************
G4WT3 >
G4WT3 > Step# X Y Z KineE dEStep StepLeng TrakLeng Volume Process
G4WT3 > 0 -7.5 mm 0 fm 0 fm 1 GeV 0 eV 0 fm 0 fm World initStep
G4WT3 > 1 -5 mm 0 fm 0 fm 1 GeV 1.09e-19 eV 2.5 mm 2.5 mm World Transportation
G4WT3 > 2 5 mm 0 fm 0 fm 975 MeV 24.9 MeV 1 cm 1.25 cm Absorber Transportation

So, the result is correct: 24.9 MeV.

We don’t understand the difference between the results. In both cases the ‘eBrem’ process is in the physics list. The documentation says that the FTFP_Bert list contains the EM Standard physics list?

Would TestEM5 run correctly with a FTFP physics list?
What could be reason for the difference?

BTW: why are the track dump slightly different? They are both produced via /tracking/verbose 1



ESTAR value is mean unrestricted stopping power. In reality secondary delta-electrons may left scintillator slab , so visible energy deposition will be different.


1- what are your cut values in TestEm5 and Beamline ?
2- G4_PLASTIC_VINYLTOLUENE does not exist in G4 materials. Please, double check your material definitions in Beamline.
3- As many others examples, TestEm5 has its own SteppingVerbose, instanced in the main and ActionInitialization.

Bonjour Michel,
ad 1. I don’t know! You may be touching an important point here. Our beamline simulation was derived from the B2a test program, but I commented out some lines about UserLimits
That’s maybe not a good idea? One could force a small step size in the scintillator?
But I thought that Geant4 would know when the electron crosses the boundary to the World and do the energy loss calculation correspondingly?
Where would stepping cuts be used in TestEm5?
ad 2. sorry, I typed it off my head, it should be G4_PLASTIC_SC_VINYLTOLUENE.

If, in, the line
runManager->SetUserInitialization(new PhysicsList());

is replaced by
G4VModularPhysicsList* physicsList = new FTFP_BERT;
physicsList->RegisterPhysics(new G4StepLimiterPhysics());

would it work?
( I could try, of course …)


PS. forgot to say, I’m using the dedx1.mac in TestEM5.

Following your suggestion, I had a closer look at cuts. In the macro dedx1.mac there is a line

/run/setCut 1 km

Removing this line, the computed energy loss by the electron is lower by a factor 10!
Inversely, adding this line to the macro used in our simulation leads to agreement with TestEM5 and the values from Estar.
Maybe an explanation of this in the README would be useful?
In addition, I tried to use the FTFP_BERT physics list in TestEM5, removing /testem/phys/addPhysics local from the macro and this doesn’t change the energy loss calculation: this physics list contains the EM Standard.

Sorry for late response; I was not free yesterday. Few words of explanation :

This “cut” has nothing to do with step limitation. It is the threshold for the creation of secondaries. It is expressed in term of minimal range of these secondaries rather than their kinetic energy.

In each step, the total energy lost by incident particle is shared between stopping power of this particle (called restricted dE/dx) and kinetic energy carried by secondaries (d-ray or brem). But the sum of the 2 contributions must remain more on less constant, eg independent of the cut; this sum is called full dE/dx.

You may have a look into Geant4 Physics Reference Manual.
See also :˜maire/tutorials/index.html
It you use Geant4 10.4 or 10.5, you may play with TestEm18, and study carefully its output.

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After your answer and looking at the Geant4 Applications Guide my understanding is the following:
. when the bremsstrahlung (steppingManager?) creates a photon with a certain energy, the range of the photon is computed
if the range is less than that defined by the cut (1 km), the photon won’t be tracked(generated)
so this is a way of suppressing the generation of secondary photons
. BUT the electron track will ‘remember’ the energy loss. Therefore the total energy loss will ‘stay’ with the electron and can be computed by just ‘looking’ at the electron after the step.
. IF the cut is small, a number a photons will be generated and the electron ‘transfers’ the energy to those. Therefore, in that case one would have to ‘look’ at all the photons and sum their energies

Does that sound sort of correct.? In any case the simulation now works.


PS. we do this simulation to try to understand if the energy(momentum) loss of the electron in a scintillator could be measured by tracking using a magnet behind the scintillator. For that the energy loss is of course an important parameter.

You are basically correct.
Secondary may be photon (by brems) or electron (by ionisation)

Thanks, also for the pointer to your tutorials.

I think, we understand the story and may close this thread.If needed we reopen.