Dear GEANT4 experts,
I'm now calculating Cerenkov radiation when a relativistic electron beam(no beam size, no divergence angle) passes through a radiator(SiO2, 1.0mm), and concern about the Cerenkov photons’ distribution in the detector plane. The experimental scheme is as follows:
Figure 0
Where, θ c
: the Cerenkov angle.
As we can expect, if the electron beam incidents the radiator(located at origin) with an angle of
θ c
, the exit Cerenkov photons will be tangential to the XZ plane in the detector plane at point(X=Y=0, Z=450mm). However, the result is very strange, as shown in figure 1.
Figure 2 shows the Cerenkov photons’ distribution in the yellow box along the Y axis, which quite deviates from Gaussian distribution.
Since I set Cerenkov Process is the only way to produce optical photons, and add none physical processes to Cerenkov photons, no absorption, no Rayleigh scattering, no MIE scattering, and even no boundary process, I think the only reason leading to this phenomenon is incorrect Step

Size. When I try to reduce the Step Size, set
“theCerenkovProcess->SetMaxNumPhotonsPerStep(1)” (the original value is 20), the odd phenomenon alleviates , as shown in figure 3 and figure 4.
When I continue to set this value small, like 0.1, 0.3, or 0.5, the result goes to another extreme, as shown in figure 5 and figure 6. The width of Cerenkov photons’ image is just the same as the thickness of SiO
2
. If we take a look at the electron distribution in the detector plane, as shown in figure 7-9(figure 8 and 9 are the electrons’ distribution along X axis and Y axis, respectively), the electrons demonstrate a Gaussian distribution because of their three Physical Processes in the radiator, i.e. G4eMultipleScattering, G4eIonisation, and G4eBremsstrahlung.
What’s the three Physical Processes’ influence on Cerenkov photons’ production??? If we take them into consideration, Cerenkov photons in the detector plane should be a Gaussian distribution along Y axis.
Eventually, I delete the line about “theCerenkovProcess->SetMaxNumPhotonsPerStep()” in
PhysicList.cc file, and use the default value, the result is the same as the situation where the value is set to 0.1, 0.3, and 0.5.
Could anyone tell me how to set the Step Size correctly? How does this odd phenomenon really come from?
I’m looking forward for your kind reply soon!
Best Regards,
Sage

1.4
1.2
1
0.8
0.6
0.4
0.2
0
-5
Figure 1
 ev
=
1.61478
Original Data
Gauss Fitting
0
Y/cm
5

Figure 2
Figure 3

 ev
=
1.49169
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
-5 0
Y/cm
Figure 4
Original Data
Gauss Fitting
5

14
12
10
8
6
4
2
0
-5
Figure 5
 ev
=
0.0383505
Original Data
Gauss Fitting
0
Y/cm
Figure 6
5

Figure 7
 ex
=
3.98596
1
0.5
2.5
2
1.5
0
-60 -40 -20
X/cm
0
Original Data
Gauss Fitting
20 40

Figure 8
 ex
=
3.98596
1
0.5
0
-60
2.5
2
1.5
-40 -20
X/cm
Figure 9
0
Original Data
Gauss Fitting
20 40