1. A Study of Reverse MC and Space Charge Effect Simulation with Geant4
Reverse Physics
Energy Deposition Comparison between FMC&RMC
Study of RMC
Simulation Efficiency Comparison
Charge Deposition Distribution
Space Charge Effect Simulation
Dose rate Distribution
Kang Wang Beihang University
12 th G4 SUW April 10 th − 12 th

2. The Verification of Reverse Physics Process
Geometry：
Si Sensitive Sphere(Radius 9.5cm) （in order to get energy full deposited）
Spherical Al shielding(Radius 10 cm Thickness 1mm)
Initial spectrum
（1/E or mono）
Reverse process
Spectrum outside Al shielding
From sensitive boundary to outside Al shielding
From sensitive boundary to Si ball
FMC Energy Deposition
Actual process
Al to Sensitive
Actual process
If it is a valid event
RMC Energy Deposition
Compare

3. Monoenergetic Electron Deposition Spectrum （Only ionization process considered in forward and reverse simulation）
The energy of electrons gains during the reverse process. However there still exists a fluctuation in the gain energy. Hence making the spectrum of outgoing particles obtain a 12MeV-centered peak
10MeV
Monoenergetic Electron
Deposition Spectrum of RMC
Deposition Spectrum of FMC
Conclusion：The center value of the peak in final spectrum and monoenergetic spectrum seem to be in consistency. It’s the fluctuation of both energy gain and loss that cause the energy distribution.
Set this spectrum as the new primary spectrum for electrons. Electrons lose energies during the forward process. Again there exists a fluctuation in the loss energy. The spectrum of outgoing particles has a 10MeV-centered peak.

4. Deposition  of Electron with 1/E Primary Spectrum （Only ionization process considered in forward and reverse simulation）
RMC and FMC deposition spectrum is in consistency, which proves the reverse ionization process does match the actual ionization process.

5. Ionization & Multiple Scattering
RMC Deposition Spectrum
The shapes of the two spectra in the full range are a little similar in some way: Shows a Exponential decline. But when it comes to the rang of 0-1.1MeV, they are quite different, which may indicate that the reverse Multiple Scattering do not match the real multiple scattering process so perfectly.
In the same range
FMC Deposition Spectrum
in full range
FMC Deposition Spectrum

6. Ionization & Compton Scattering
RMC Deposition Spectrum
The curve seems to have a widening in FMC deposition spectrum compared to RMC. And the deviation between two peak also cannot be ignored. But they do have a similarity in the full range. So in conclusion the reverse Compton Scattering matches the actual process but not completely.
In the same range
FMC Deposition Spectrum
in full range
FMC Deposition Spectrum
deviation

7. Energy Deposition Comparison between FMC&RMC (All the physics process included)
Geometry：
Si Sensitive Sphere(Radius from 1mm-8cm)
Cubic Al shielding(Length 20 cm Thickness 3mm)
Spectrum：
FMC: Uniform Spectrum 1keV-10MeV;Electron
RMC: Uniform Spectrum 1keV-10MeV;Electron
(Outside Spectrum normalized to Uniform spectrum)
Compare the curve and deviation of FMC and RMC deposition when ∆Edep/Edep reaches 1%.

8. Different in low energy region
Radius = 1mm
Same shape !
Different in low energy region
Radius = 5mm
When the radius rises, the two curves gets closer. Because electron energy nearly fully deposit.
Radius = 3cm
Radius = 8cm

9. Deviation of FMC and RMC Energy Deposition
The deviation is smaller when only range MeV is considered. Yet it also means that the low energy region of RMC and FMC differ more.
∆dose= Dose RMC −Dose FMC Dose FMC = Edep RMC −Edep FMC Edep FMC

10. Simulation Efficiency of RMC&FMC
Computing time /s
Radius of sensitive detector /cm
RMC computing efficiency is 500 times the efficiency of FMC
Smaller sensitive detector cause bigger difference

11. Space Charge Effect Simulation
Motivation: There is a urgent need for the charge simulation of satellite with a complex inner structure. The existing Monte Carlo simulation is no longer appropriate according to its low efficiency, and it fails to meet the needs of space missions. In order to improve the computation efficiency, we need to find a rapid simulation for electron transport.
Plan: Reverse MC under Geant4 is a rather good option since it can reduce significantly the computing time by backward tracking from the sensitive region till the external source.
Desorgher L, Lei F.NIM in PRA, 2010, 621(1):

12. Charge Deposition Distribution
In order to calculate Charge Deposition Distribution and Dose Distribution, we divide sensitive target into small parts.
The terminal points of the electron decide which part those electron finally stop in. And by counting the number of stopping electron, we can obtain the average charge increment per unit time. For example, the electron stops in PartABC. So the charge of PartABC -1e.
By track the secondary particles ,the Compton Scattering and δ-rays is also taken into consideration. For example, if secondary is produced at E, then PartABC’s charge +1e.The part in which secondary particle stops in -1e.

13. Dose rate Distribution
For the dose rate distribution, there is a little bit different. Because we care about the dose rate near a certain point or a area, small parts is no longer suitable in this case. So we create a sphere covering the point.
By calculating the inflow and Outflow energy of the sphere per unit time, we can obtain the average dose rate at or near Point B.

14. Thanks for your listening.
Collaborators：Lihua Zhu Beihang University
Zhenlong Zhang NSSC(CAS)
Thanks for your listening.