© Copyright QinetiQ 2006 G4 C-S Biasing Based on the G4WrapperProcess Fan Lei Geant4 Biasing Mini...

© Copyright QinetiQ 2006

G4 C-S Biasing Based onthe G4WrapperProcess

Fan LeiGeant4 Biasing Mini Workshop, SLAC21 March 2007

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© Copyright QinetiQ Plc 2006

Contents

• Introduction

• G4WarpperProcess

• Two Algorithms

• G4XWrapperProcess classes

• Status & Examples

• Discussions

3


Introduction

• C-S biasing is a technique to artificially enhance/reduce the interaction cross-section of a particular process, in order to improve the statistics of a simulation (variance reduction) .

• Use cases:– Thin layer interactions (enhancement)– Thick layer shielding penetration (reduction)– ...

• Other similar variance reduction techniques: forced collisions; path length stretching

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C-S biasing in Geant4: Background

• Started the discussion in 2001, at the SLAC mini-workshop

– Identified the wrapper approach as the implementation model

– A virtual class G4WrapperProcess was created by H. Kurahige

• Prototype G4XWrapperProcess class created by F.Lei (2003 Triumf workshop)

– Applicable to all discrete processes

• GHAD Implementation by H-P Wellish (2003 Triumf workshop)

– for photo-nuclear processes only

• Further work done to G4XWrapperProcess in ESA REAT-MS project (2006)

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G4WrapperProcessDoIT ////////////

– virtual G4VParticleChange* PostStepDoIt

– virtual G4VParticleChange* AlongStepDoIt

– virtual G4VParticleChange* AtRestDoIt

GPIL //////////////

– virtual G4double AlongStepGetPhysicalInteractionLength

– virtual G4double AtRestGetPhysicalInteractionLength

– virtual G4double PostStepGetPhysicalInteractionLength

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C-S Biasing: Algorithms

• No “standard algorithm”. It depends on the application requirements

• Simple application cases: (right)– True PHS (simple)– True Secondary

Radiation (simple)– Both PHS & Secondary

radiation (complicated)

Dete

ctor/sh

ieldIncident

radiation

Edeposition/PHSSecondary radiation

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Algorithm-1At each step:If the biased process is being selected, the weights of current track and

the secondaries are changed to

w’’ = (1 – exp(-s/x))/(1-exp(-f*s/x) * w

If the process is not being selected, the weight of the current track is changed to

w’ = exp(-(1-f)*s/x) * w

where w: the original parent particle weight

s: the step length

x: the unbiased current interaction length of the process

f: the enhancement factor

Normal scoring for both PHS and fluence

s

wW’’

W’’

Selecteds

w

Unselected

W’

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Algorithm-2At step level, if the process is selected:

The primary is not killed, but continues its flight with a reduced weight:

The weight of the secondary is

where w: the original parent particle weight

nl: the number of interaction length traversed

f: the enhancement factor

Special scoring is required for PHS

wff

nl

fw ))

11(exp(

1''

wnl

nlf

nl

w)exp(1

)exp()exp(

W’’

W’’

W’

w

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New Classes

• G4XWrapperProcess– G4XWrapperProcess(const G4String& aName = "X-",G4ProcessType

aType = fNotDefined )

– void SetEnhanceFactor( G4double )

– void SetBiasMode2(G4bool mode)

• G4XProcessManager– void AddDiscreteProcess(G4VProcess*);

– void AddProcess(G4VProcess*, G4int, G4int, G4int);

• G4XProcessManagerMessenger – /xsbias/setProcess LowEnCompton 10 false

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G4XWrapperProcess – example (GPIL)

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G4XWrapperProcess – example (DoIt)

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Impact to Physics List

The only change required

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Current Status

• Available from http://reat.space.qinetiq/xsbias

– G4XWrapperProcess, G4XProcessManager & G4XProcessManagerMesseneger

– Biased physics list version of

• Exrdm (extended/radioactivity/exrdm)

• MULASSIS

• GRAS

• Limitations:

– Not working for Std-EM after g4.7.0 ( see VI’s presentation)

– Not yet Region/Volume sensitive

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1 GeV p + 1mm Si Slab

Emission Neutron Spectrum

1.00E-06

1.00E-05

1.00E-04

1.00E-03

0.00E+00 2.00E+05 4.00E+05 6.00E+05 8.00E+05 1.00E+06 1.20E+06

Energy (keV)

neu

tro

ns/

ch/e

ven

t

Biased x 10

Unbiased

PHS

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

1.00E-01

1.00E+00

0.00E+00 5.00E+04 1.00E+05 1.50E+05 2.00E+05 2.50E+05

Energy (keV)

Co

un

ts/c

h/e

ven

t

Biased x 10

Unbiased

2hrs CPU time for both biased/unbiased simulationsProton elastic/inelastic cross-section x10

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100 eV n + 1mm Si Slab

2hrs CPU time for both biased/unbiased simulationsNeutron elastic, inelastic and capture cross-sections x10

Emission Gamma Spectrum

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

0.E+00 2.E+03 4.E+03 6.E+03 8.E+03 1.E+04 1.E+04

Energy (keV)

gam

mas

/ch

/eve

nt

Biased x 10

Unbiased

PHS

1.E-11

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

0.E+00 2.E+02 4.E+02 6.E+02 8.E+02 1.E+03 1.E+03

Energy (keV)

Co

un

ts/c

h/e

ven

t

Biased x 10

Unbiased

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100 keV gamma + 0.1mm Si Slab

2hrs CPU time for both biased/unbiased simulationsBoth photo-e & Compton cross-sections x10

Emission Gamma Spectrum

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

1.00E-01

1.00E+00

0.00E+00 2.00E+01 4.00E+01 6.00E+01 8.00E+01 1.00E+02 1.20E+02

Energy (keV)

gam

mas

/ch Biased x 10

Unbiased

PHS

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

0.00E+00 2.00E+01 4.00E+01 6.00E+01 8.00E+01 1.00E+02 1.20E+02

Energy (keV)

Co

un

ts/c

h

Biased x 10

Unbiased

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Discussions-1:• Simple implementation based on the wrapper

approach

• No change to existing process required and easy to use

• Minimum impact to existing Physics Lists

• Same approach can be used for Forced Interaction biasing

• No special scoring required for algorithm-1 implementation

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Discussions-2:• Not working with current version of Standard EM

– See Vladimir’s presentaion

• No activation/deactivation for region

• Process based only

– most hadron physics are model based

– expand the biasing mechanism in G4HadronicProcess

• Possible multiple interactions in the volume of interest

– Optimal enhancement factor

– Move the un-interacted part to the next boundary

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