GRAS Validation
and
GEANT4 Electromagnetic Physics Parameters
R. Lindberg, G. Santin;
ronnie.lindberg@esa.int
Space Environment and Effects Section, ESTEC
Presentation Outline



Introduction
A few Words About GRAS and MULASSIS
GRAS Internal Validation


GEANT4 Electromagnetic Physics



Comparison with MULASSIS
Tuning the parameters in GRAS
GRAS applied to complex geometry: ConeXpress
Conclusions
2
Introduction

ConeXpress radiation analysis

ESABASE


Ray-tracing and SHIELDOSE-2 curve
GEANT4

Ray-tracing (SSAT) and SHIELDOSE-2 curve

Used following tools for comparison

SSAT




Developed by Qinetiq
Ray-tracing (a.k.a sector
shielding analysis)
GRAS


MULASSIS


Developed by Qinetiq
1D multi-layer geometry.

Developed by G. Santin and
V. Ivantchenko
Uses GDML geometry;
modular physics
Modular analysis driven via
script
3
ConeXpress Results


GEANT4 SSAT ray-tracing results agree with ESABASE
However, GEANT4 GRAS full Monte Carlo gives very different
results (orders of magnitude)


Uses same geometry model as SSAT analysis
First validation attempt

GEANT4 internal comparison
 GRAS ↔ MULASSIS


Shows discrepancy of ~20 % for a semi-infinite slab case
Greatest difference in lower energy range (≤ 2 MeV) for electrons
4
Understanding the Problem (1/3)
Average Dose per Event
(MeV)
GRAS vs. Mulassis 0.25 - 2.75 MeV

1.00E+00
1.00E-01 0
0.5
1
1.5
2
2.5
3
1.00E-02
1.00E-03

1.00E-04
1.00E-05
GRAS
1.00E-06
GRAS gamma
1.00E-07
GRAS e-

The geometry setup
used was the semiinfinite slab case
2 mm Silicon target
3 mm Aluminium shield
1.00E-08
Electron Energy (MeV)


Dose in energies below 1.5
MeV comes from gamma
radiation
e--contribution starts to
dominate around 1.5 MeV
3
5
Understanding the Problem (2/3)
Average Dose per Event (MeV)
Dose (MeV) for Electron Energies 0.25-2.5 MeV
1.00E+00
1.00E-01
1.00E-02
Total dose GRAS
1.00E-03
Total dose Mulassis
GRAS e- contr.
1.00E-04
Mulassis e- contr.
1.00E-05
GRAS gamma
Mulassis gamma
1.00E-06
1.00E-07
1.00E-08
0
0.5
1
1.5
2
2.5
3
Electron Energy (MeV)



GRAS analysis was inserted into MULASSIS to obtain e- and gamma cont.
Gamma contribution agrees well between the two.
Simulations show that there were differences in the e- contributions between
GRAS and MULASSIS
6
Understanding the Problem (3/3)
Dose Ratio GRAS/MULASSIS
250%
Total Dose
Ratio
200%
e- Contribution
150%
100%
50%
0%
0
0.5
1
1.5
2
2.5
3
Electron Energy (MeV)





Dose from gamma-contribution is the same but...
…e- contribution differs and…
…statistical errors are small (<1%) compared to total dose value, so
difference is not due to statistical error, furthermore…
…the difference in dose between GRAS and MULASSIS is largest at
“threshold energy”, so…
…what’s the catch?
7
Electron EM Processes and Fine Tuning

Same EM physics used in GRAS and MULASSIS

Cause of different results was due to “fine tuning” of the
electromagnetic energy loss modelling
Several parameters influence the modelling of GEANT 4 EM:
 facRange:



Integral:


If true, dE(step) is obtained with integral of dE/dx curve
Cuts:


Maximum fraction of kinetic energy that particle can loose in a step
Is the production cuts for secondary electrons
StepMax:




Is one of the most important.
Limits the maximum step length.
“Process” in GRAS.
This parameter is not available in MULASSIS
8
Internal Validation Conclusion (1/2)
GRAS vs. Mulassis in Slab, e-, MaxTheta=0 deg

GRAS gives near perfect
agreement with
MULASSIS when using
the same EM physics
parameter
Average Dose per Event
(MeV)
1.4
1.2
1
0.8
0.6
Dose, StepFunction=1.0
Dose, StepFunction=0.2
Mulassis
0.4
0.2
0
0
1
2
3
4
5
6
7
8
Particle Energy (MeV)
GRAS / MUL.
101.0%
GRAS / MUL.
Ratio
100.5%

100.0%
99.5%

99.0%

0
1
2
3
4
5
Electron Energy (MeV)
6
7
8
Integral set to true
facRange set to 1.0
stepMax set to 100 mm
(similar to not having stepMax at all)
9
Internal Validation Conclusion (2/2)
Ratio GRAS / MULASSIS, sphere geometry
Several runs were conducted
to verify correlation
2-7 MeV e-
E.g.



105.0%
Sphere case,
maxtheta=90,
protons and electrons,
Ratio

110.0%
100.0%
95.0%
90.0%
0
2
4
6
8
Particle Energy (MeV)
Notice
the scale.
Ratio GRAS / MULASSIS, sphere geometry
30-400 MeV protons
104.0%
102.0%
Ratio

100.0%
98.0%
96.0%
0
100
200
300
400
500
Particle Energy (MeV)
10
EM Physics Tuning – Parametric Study

Parametric study to look at effects of different settings

Parameter ranges:

facRange: 0.2 - 1.

Integral: Boolean – true or false

Cuts: between 0.01  100 mm

StepMax: between 0.01  100 mm (100 mm ~ no step limiting)
11
Parameter Comparison (1/2)
Average Dose for Particle Energy of 1.50 MeV
0.0006
Cuts= 0.01 mm
0.0005
Cuts=0.10 mm
0.0004
Cuts=1.00 mm
0.0003
Cuts=100.00 mm
0.0002
0.0001
0
0.01
Avr. Dose per Event
(MeV)
Avr. Dose per Event (MeV)
Average Dose for Particle Energy of 1.25 MeV
0.0012
Cuts= 0.01 mm
0.001
Cuts=0.10 mm
0.0008
Cuts=1.00 mm
0.0006
0.0002
0
0.1
1
10
StepMax (mm)
0.01
100
0,016
Cuts= 0.01 mm
Cuts=0.10 mm
0,012
Cuts=1.00 mm
0,008
Cuts=100.00 mm
0,004
0
0,1
1
StepMax (mm)
10
100
0.1
1
StepMax (mm)
10
100
Average Dose for Particle Energy of 2.00 MeV
Avr. Dose per Event
(MeV)
Avr. Dose per Event
(MeV)
Average Dose for Particle Energy of 1.75 MeV
0,01
Cuts=100.00 mm
0.0004
Dose differs
2.5x depending
on StepMax
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
0.01
Cuts= 0.01 mm
Cuts=0.10 mm
Cuts=1.00 mm
Cuts=100.00 mm
0.1
1
10
StepMax (mm)
100
12
Parameter Comparison (2/2)
1.5 MeV Particle Energy, Cuts=0.01 mm
False, 1.0
False, 0.2
0.0009
True, 1.0
True, 0.2
0.0008
0.0007
0.0006
0.0005
0.01
0.1
1
StepMax (mm)
10
100
Avr. Dose per Event
(MeV)
Avr. Dose per Event (MeV)
0.001
1.5 MeV Particle Energy, Cuts=100.00 mm
0.0008
False, 1.0
False, 0.2
0.0007
True, 1.0
0.0006
0.0005
0.01
0.1
1
10
100
StepMax (mm)
13
Tuning Effect with Space Env. Spectra
Ran simulations in GRAS for
different spectra and Al shielding
thickness:





e- GTO
e- MEO (Galileo)
e- GEO
p+ GEO
MULASSIS simulated by using
StepMax=100.00 mm and
StepFunction=1.0
Trapped Electron Spectra
MEO
GTO
GEO
1,00E+19
Fluence (/cm2/MeV)

1,00E+17
1,00E+15
1,00E+13
1,00E+11
1,00E+09
1,00E+07
0
2
4
6
8
Energy (MeV)
14
Tuning Effect with Space Env. Spectra
Trapped e- GTO spectrum
Trapped e- MEO spectrum
Average dose per event (MeV)
Average dose per event (MeV)
Al. thick.,
mm
GRAS
MULASSIS
GRAS/MUL
Al. Thick.
mm
GRAS
MULASSIS
GRAS/MUL
3
0,05391
0,04625
117%
3
0,04824
0,04066
119%
4
0,01736
0,01399
124%
4
0,01458
0,01167
125%
5
0,00591
0,00456
130%
5
0,00474
0,00363
131%
10
0,00047
0,00049
96%
10
0,00045
0,00046
97%
Trapped e- GEO spectrum
solar proton GEO spectrum
Average dose per event (MeV)
Average dose per event (MeV)
Al. thick.
mm
GRAS e-
MULASSI
S e-
GRAS/MUL
3
0,02983
0,02410
124%
3
1,88
1,88
99,8%
4
0,00818
0,00653
125%
4
5,58
5,57
100,1%
5
0,00281
0,00228
123%
5
4,12
4,13
99,9%
10
0,00038
0,00041
95%
10
3,33
3,34
99,8%
Al. thick.
mm
GRAS
e-
MULASSI
S e-
GRAS/MUL
15
Next Step – Complex Geometry


Currently conducting analysis on complex geometry –
ConeXpress
Use radiation spectra from SPENVIS


Run each particle spectra separate and combine to obtain total
ionised dose.
Presents different problems than simple geometry

Number of simulated events has to be very high due to thick
shielding generated by subsystems, especially for electrons
16
Next Step – Complex Geometry
GDML model of ConeXpress
17
Conclusions

Internal validation (GRAS ↔ MULASSIS) successful


GRAS Parametric study of EM physics parameters shows difference







Up to 30%, using a space environment spectra
Up to 2.5 times, using mono-energetic beam particle source
Tentative set of parameters chosen as


Earlier difference due to different physics parameters
facRange to 0.2
Integral set to true
Cuts around 0.01 mm
StepMax around 0.1 mm – trade-off between CPU time and small step size
 impacts radiation analyses results
Suggested implementation of StepMax and facRange in MULASSIS
18