Track in magnetic field

Geometry, Fields and Transportation
1

When tracking particles inside a vacuum volume with magnetic field, in the stepping action I get the pre and post step positions as two points on the boundaries of the volume. How can I get the full curved path that the particle followed inside the volume?

2

You should be able to see it. Try

/vis/scene/add/trajectories smooth

Not sure the best way to get those intermediate points. Have a look in G4SmoothTrajectory. It looks like the step has a method GetPointerToVectorOfAuxiliaryPoints. Give it a try.

3

Thank you @allison
I am trying to access it in the SteppingAction with
step->GetPointerToVectorOfAuxiliaryPoints()
but it always return nullptr
I’ve added a command
/tracking/storeTrajectory 2 also with 3 and 4
still the same

4

I can confirm that /vis/scene/add/trajectories smooth works. Please try that rather than got to /tracking/storeTrajectory directly.

But…that might not help. I do not understand why step->GetPointerToVectorOfAuxiliaryPoints() returns a null pointer.

Please continue to dig and we will solve this.

5

Thank @allison, thats good to know that through the GUI it works, so I can find out how it does it.
In my application am not using the GUI so I cannot issue the /vis/scene/add/trajectories command.
I would like to access it through the SteppingAction to perform custom scoring.

6

Dear Allison,

I found out that the step->GetPointerToVectorAuxiliaryPoints() will always return a null pointer if the Transportation process used is ‘G4CoupledTranportation’.

You can see at the end of the method ’ G4CoupledTransportation::AlongStepDoIt’ where the following lines are commented:

// Add smooth curved trajectories to particle-change
//
//fParticleChange.SetPointerToVectorOfAuxiliaryPoints
//(fFieldPropagator->GimmeTrajectoryVectorAndForgetIt() );

In the case of G4Transportation these lines are not commented and it is possible to get the auxiliary points.

Is there a reason to have commented these lines for G4CoupledTransporation?

Thanks!
André

7

Dear Andre,

These lines should also work fine in G4CoupleTransportation.

So if you need this capability, please uncomment them!

Cheers,
John

8

Should this be filed to Bugzilla, so that it is fixed in the next release?

9

Perfect!,
thanks for your reply @japost

Best,
André