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

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

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.

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.

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

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

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%.

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

Deviation of FMC and RMC Energy Deposition The deviation is smaller when only range 0.1-10MeV 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

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

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): 247-257.

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.

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.

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