Vertex location with Bremsstrahlung process

Particles, Track, Event, Run and Biasing
1

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_Geant4 Version:_11.2.2
_Operating System:_MacOS
_Compiler/Version:_Clang 17
_CMake Version:_3.31.0

—Dear experts,
I’m currently creating a simulation with a liquid Ar39 sphere around my detector. The Ar39 will decay and I’m trying to plot the location of vertex Ar39 that deposits energy which is contributed by Bremsstrahlung radiation in some way. However, when I try to get the position of my vertex using the following method, I kept getting location that is inside the detector, which does not make sense. In my construction file, I set my detector and the Ar39 sphere as
//building the detector

solidDetector = new G4Tubs(“solidDetector”, 0cm, 5cm, 20cm, 0deg, 360*deg);

logicDetector = new G4LogicalVolume(solidDetector, Ge76Mat, “logicalDetector”);

physDetector = new G4PVPlacement(0, G4ThreeVector(0, 0, 0), logicDetector, “physDetector”, logicWorld, false, 0, true);

fScoringVolume = logicDetector;

// Create full sphere first

G4Sphere* fullSphere = new G4Sphere(“fullSphere”, 0.cm, 40cm, 0deg, 360deg, 0deg, 180deg);

// Create the argon sphere with detector hole

solidArSphere = new G4SubtractionSolid(“solidArSphere”, fullSphere, solidDetector);

logicArSphere = new G4LogicalVolume(solidArSphere, Ar39Mat, “logicalArSphere”);

physArSphere = new G4PVPlacement(0, G4ThreeVector(0, 0, 0), logicArSphere, “physArSphere”, logicWorld, false, 0, true);
and in my macro file run.mac, I set

Set the number of threads for multi-threaded mode

/run/numberOfThreads 6

Initialize the run

/run/initialize

Set up the GPS for Ar39 decay

/gps/particle ion
/gps/ion 18 39 0
/gps/energy 0 keV

Use confined source to exclude detector volume

/gps/pos/type Volume
/gps/pos/shape Sphere
/gps/pos/centre 0. 0. 0. cm
/gps/pos/radius 40 cm
/gps/pos/confine physArSphere

Isotropic angular distribution

/gps/ang/type iso

Set the activity of the source

/gps/source/intensity 1.0

Run events

/run/beamOn 100000000.
And in my event action, I try to get the vertex position of Ar39 by using:
if (event->GetNumberOfPrimaryVertex() > 0) {
G4PrimaryVertex* primaryVertex = event->GetPrimaryVertex(0);
G4PrimaryParticle* primaryParticle = primaryVertex->GetPrimary(0);
G4ThreeVector position = primaryVertex->GetPosition();
fZpos = position.z() / CLHEP::cm;
fSpos = std::sqrt(position.x()*position.x() + position.y()*position.y()) / CLHEP::cm;
I feel like I either create my geometry wrong or the way that I try to get the vertex positio is worng.

2

Not needed and might do strange things.

G4PrimaryVertex* primaryVertex = event->GetPrimaryVertex(0);
G4PrimaryParticle* primaryParticle = primaryVertex->GetPrimary(0);
G4ThreeVector position = primaryVertex->GetPosition();

Where are you doing this? I remember having issues with using macros for primaries and then trying to read them. (0, 0, 0) is the default so I wonder if it isn’t initialized. Could you try doing it at the end of event action?

3

I put it intially at the end of event action, but I re-locate it at the beginning of the event action and now the issue is fixed. Thank you so much!

4

I thought I might have that backwards. And looking at the source code, the primary vertex is reset before the end of event action is called.