Why is there energy loss at volume transportation?

Getting Started
1

I am trying to simulate a 5.4 MeV alpha particle shooting at a 50 micron thick Kapton sheet sandwiched between a pair of 5 micron thick copper sheets. The source of the alpha particles is 1 mm in front of the front edge of the copper and the intervening region is filled with air. In the results shown below Shape 2 is the front copper sheet and Shape 3 is the Kapton sheet.

The simulation is based on a modification of the detector construction code of example B1. When I run the simulation the alpha particle loses about 100 keV in the first 1 mm through ionization of the air, which seems reasonable.

But I don’t understand what is going on when it enters the copper. Shown below are examples from a couple of events. Why does the particle lose about 2 MeV when entering the Kapton and why is all the remaining energy lost in a single ionization in the Kapton? Have I mis-applied the model?

* G4Track Information:   Particle = alpha,   Track ID = 1,   Parent ID = 0
 *********************************************************************************************************

Step#    X(mm)    Y(mm)    Z(mm) KinE(MeV)  dE(MeV) StepLeng TrackLeng  NextVolume ProcName
0        0        0      -51       5.4        0        0         0    Envelope initStep
1        0        0      -51       5.4 5.49e-05 0.000429  0.000429    Envelope ionIoni
2 -9.93e-07  1.7e-07      -51      5.39  0.00751   0.0635     0.064    Envelope ionIoni
3 -2.59e-05 -7.55e-07    -50.9      5.38  0.00609   0.0588     0.123    Envelope ionIoni
4 -2.25e-05 1.17e-06    -50.9      5.38  0.00116   0.0173      0.14    Envelope ionIoni
5 8.99e-06 1.53e-05    -50.8      5.37   0.0074   0.0783     0.218    Envelope ionIoni
6 6.63e-05  8.5e-05    -50.7      5.36  0.00955    0.112      0.33    Envelope ionIoni
7 7.19e-05 0.000101    -50.7      5.36  0.00173   0.0237     0.354    Envelope ionIoni
8 7.05e-05 0.000174    -50.6      5.35   0.0082    0.118     0.472    Envelope ionIoni
9 -6.63e-05 0.000185    -50.3      5.33   0.0213    0.248     0.719    Envelope ionIoni
10 -0.000108 0.000219    -50.2      5.32  0.00324   0.0625     0.782    Envelope ionIoni
11 -0.000116 0.000236    -50.2      5.32  0.00029   0.0121     0.794    Envelope ionIoni
12 -0.000129 0.000255    -50.2      5.32 0.000851   0.0171     0.811    Envelope ionIoni
13 -0.000235 0.000382    -50.1       5.3   0.0102    0.113     0.924    Envelope ionIoni
14 -0.00016 0.000341    -50.1       5.3 0.000956   0.0191     0.943    Envelope ionIoni
15 9.95e-05 0.000222      -50       5.3  0.00416    0.057         1      Shape2 Transportation
16 0.000119  0.00021      -50      3.19      2.1  0.00501      1.01      Shape3 Transportation
17 -0.000267 -0.00173      -50         0     3.19   0.0135      1.02      Shape3 ionIoni


* G4Track Information:   Particle = alpha,   Track ID = 1,   Parent ID = 0
*********************************************************************************************************

Step#    X(mm)    Y(mm)    Z(mm) KinE(MeV)  dE(MeV) StepLeng TrackLeng  NextVolume ProcName
0        0        0      -51       5.4        0        0         0    Envelope initStep
1        0        0      -51       5.4 0.000372   0.0119    0.0119    Envelope ionIoni
2 -4.91e-07  3.6e-06    -50.8      5.38   0.0125    0.173     0.185    Envelope ionIoni
3 6.84e-06 9.24e-06    -50.8      5.38  0.00413   0.0404     0.225    Envelope ionIoni
4  0.00011 -9.72e-05    -50.4      5.34   0.0336    0.393     0.618    Envelope ionIoni
5 0.000115 0.000133    -50.2      5.32    0.023    0.231     0.849    Envelope ionIoni
6 2.52e-05 0.000411      -50      5.31  0.00909    0.151         1      Shape2 Transportation
7 2.06e-05 0.000418      -50      3.24     2.07  0.00501      1.01      Shape3 Transportation
8 -0.000129  0.00014      -50         0     3.24   0.0138      1.02      Shape3 ionIoni
2

When particles are tracked, there is some “discrete process” (where the action happens at the post-step point) which sets the length of the step. It’s this process name which is listed in the verbose output you’ve shown.

In addition, there are a number of “continuous processes” (where the action happens along the step) which contribute to the track’s total energy loss. These processes are always applied, but their names are not listed in the verbose output.

When Transportation is the limiting process, that means the track reached the boundary of a volume. The energy loss in that case is due entirely to the continuous processes mentioned above.

Does this help clarify the verbose output?

3

Thank you Michael for explaining why the energy loss shown applied to the Transportation process. As always, you are a great help.