Giving physical meaning to the "transportation" process

Dear colleagues, in developing a simulation for a test beam I am having a hard time understanding the meaning of the Transportation process.

I have a 500 MeV electron traveling through a scintillator; my physics list is the QCCB. If I try to look at the steps in the scintillators, I see a list of steps associated to three processes: eBrem, eIoni and Transportation.

The last one always appears when the electron is crossing the border of the scintillator volume, produces no secondaries and is associated with a considerable energy loss (comparable with the other two processes). Some slides and documentation material generically refers to Transportation as a process associated with the last step of a particle in a medium; however, I cannot understand its physical meaning. Is it a non-radiative, non-ionizing energy loss? How does it differ from, say, coulombian scattering?

Here is an example of debugging lines from some events:

Scintillator, process: eBrem, deposit (MeV): 3.15843,  from ScintiPhysical1 to ScintiPhysical1, n. secondaries: 1
Scintillator, process: eBrem, deposit (MeV): 0.0662966,  from ScintiPhysical1 to ScintiPhysical1, n. secondaries: 1
Scintillator, process: Transportation, deposit (MeV): 0.515436,  from ScintiPhysical1 to Envelope, n. secondaries: 0

Scintillator, process: eBrem, deposit (MeV): 2.18718,  from ScintiPhysical1 to ScintiPhysical1, n. secondaries: 1
Scintillator, process: Transportation, deposit (MeV): 1.4499,  from ScintiPhysical1 to Envelope, n. secondaries: 0

Scintillator, process: eBrem, deposit (MeV): 0.0867602,  from ScintiPhysical1 to ScintiPhysical1, n. secondaries: 1
Scintillator, process: Transportation, deposit (MeV): 3.32824,  from ScintiPhysical1 to Envelope, n. secondaries: 0

Scintillator, process: eIoni, deposit (MeV): 3.18942,  from ScintiPhysical1 to ScintiPhysical1, n. secondaries: 1
Scintillator, process: Transportation, deposit (MeV): 0.110654,  from ScintiPhysical1 to Envelope, n. secondaries: 0

Scintillator, process: eBrem, deposit (MeV): 1.84543,  from ScintiPhysical1 to ScintiPhysical1, n. secondaries: 1
Scintillator, process: eIoni, deposit (MeV): 0.598441,  from ScintiPhysical1 to ScintiPhysical1, n. secondaries: 1
Scintillator, process: Transportation, deposit (MeV): 0.927637,  from ScintiPhysical1 to Envelope, n. secondaries: 0

Scintillator, process: eBrem, deposit (MeV): 0.0793947,  from ScintiPhysical1 to ScintiPhysical1, n. secondaries: 1
Scintillator, process: Transportation, deposit (MeV): 2.98721,  from ScintiPhysical1 to Envelope, n. secondaries: 0

Scintillator, process: eBrem, deposit (MeV): 1.58434,  from ScintiPhysical1 to ScintiPhysical1, n. secondaries: 1
Scintillator, process: eBrem, deposit (MeV): 0.260091,  from ScintiPhysical1 to ScintiPhysical1, n. secondaries: 1
Scintillator, process: Transportation, deposit (MeV): 1.42572,  from ScintiPhysical1 to Envelope, n. secondaries: 0

Scintillator, process: eBrem, deposit (MeV): 3.2917,  from ScintiPhysical1 to ScintiPhysical1, n. secondaries: 1
Scintillator, process: Transportation, deposit (MeV): 0.000797846,  from ScintiPhysical1 to Envelope, n. secondaries: 0

Scintillator, process: Transportation, deposit (MeV): 3.5226,  from ScintiPhysical1 to Envelope, n. secondaries: 0

Scintillator, process: Transportation, deposit (MeV): 3.06046,  from ScintiPhysical1 to Envelope, n. secondaries: 0

Scintillator, process: eIoni, deposit (MeV): 0.38364,  from ScintiPhysical1 to ScintiPhysical1, n. secondaries: 1
Scintillator, process: eIoni, deposit (MeV): 0.509544,  from ScintiPhysical1 to ScintiPhysical1, n. secondaries: 1
Scintillator, process: Transportation, deposit (MeV): 2.54701,  from ScintiPhysical1 to Envelope, n. secondaries: 0

Scintillator, process: eIoni, deposit (MeV): 1.47007,  from ScintiPhysical1 to ScintiPhysical1, n. secondaries: 1
Scintillator, process: Transportation, deposit (MeV): 1.78541,  from ScintiPhysical1 to Envelope, n. secondaries: 0

Scintillator, process: Transportation, deposit (MeV): 3.22091,  from ScintiPhysical1 to Envelope, n. secondaries: 0

Thank you all in advance!

Transportation is not a “physics” process at all. It is the process which takes a track starting at its PreStepPoint, and carries that track to its PostStepPoint. Transportation handles the problems of carrying the track through an electric or magnetic field. It works with the EM “continuous” processes so that, for example, the PostStepPoint at the end of multiple scattering is correct.

In /tracking/verbose output, the process name printed out is only the process which limited the step. When there was a real interaction (ionization, decay, neutron capture, whatever), then you’ll see one of those physics process named printed out.

Geant4 is set up so that whenever a particles gets to the surface of a volume, it “pauses and rests for a moment.” Specifically, a particle will never be carried directly through a boundary from one volume to another, so that other processes which specifically handle boundary interactions (like optical reflection and refraction) get the opportunity to do their thing.

That “pause and rest” is handled by Transportation. So whenever a step carries the particle right to the volume boundary, with no interaction happening first, you’ll see Transportation as the “step limiting process.”

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Transportation is the Prime Mover (the Primum Mobile) of Geant4. It is the source of inertia, so that particles in motion remain in motion :slight_smile:

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I would guess, that Mike explanations are enough and this thread may be closed.