A Study of Reverse MC and Space Charge
Effect Simulation with Geant4
Reverse Physics
Study of RMC
Energy Deposition Comparison
between FMC&RMC
Simulation Efficiency Comparison
Charge Deposition Distribution
Space Charge
Effect Simulation
Dose rate Distribution
Kang Wang Beihang University
12th G4 SUW April 10th − 12th
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
If it is a valid event
Compare
Al to Sensitive
RMC Energy
Deposition
From sensitive boundary to
outside Al shielding
Spectrum outside
Al shielding
Actual process
Actual process
From sensitive
boundary to Si ball
（1/E or mono）
Reverse process
FMC Energy
Deposition
Monoenergetic Electron Deposition Spectrum
（Only ionization process considered in forward and reverse simulation）
10MeV
Monoenergetic Electron
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.
Deposition
Spectrum
of RMC
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 12MeVcentered peak
Deposition
Spectrum
of FMC
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
In the same range
FMC 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.
FMC Deposition Spectrum
in full range
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%.
Radius = 1mm
Radius = 3cm
Same shape !
Different in low energy region
Radius = 5mm
When the radius rises, the two curves
gets closer. Because electron energy 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 =
DoseRMC −DoseFMC EdepRMC −EdepFMC
=
DoseFMC
EdepFMC
Computing time /s
Simulation Efficiency of RMC&FMC
RMC computing
efficiency is 500 times
the efficiency of FMC
Smaller sensitive
detector cause
bigger difference
Radius of sensitive detector /cm
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.
Collaborators：Lihua Zhu
Beihang University
Zhenlong Zhang NSSC(CAS)
Thanks for your listening.