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Implement optional logarithmic sampling of the photon energies #476

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Implement optional logarithmic sampling of the photon energies #476

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71872b3
Implement optional logarithmic sampling of the photon energies
Mar 6, 2024
12518ec
Photon sampling with power law proposal function
Mar 15, 2024
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4 changes: 4 additions & 0 deletions include/crpropa/module/PhotoPionProduction.h
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Expand Up @@ -43,6 +43,10 @@ class PhotoPionProduction: public Module {
// - output: (s-p^2) * sigma_(nucleon/gamma) [GeV^2 * mubarn]
double functs(double s, bool onProton) const;

double proposal_pdf(double eps, double epsMin, double epsMax) const;

double proposal_inv_cdf(double prob, double epsMin, double epsMax) const;

// called by: sampleEps, gaussInt
// - input: photon energy eps [eV], Ein [GeV]
// - output: probability to encounter photon of energy eps
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29 changes: 25 additions & 4 deletions src/module/PhotoPionProduction.cpp
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Expand Up @@ -413,6 +413,16 @@ SophiaEventOutput PhotoPionProduction::sophiaEvent(bool onProton, double Ein, do
return output;
}

double PhotoPionProduction::proposal_pdf(double eps, double epsMin, double epsMax) const {
double prob = 1 / (eps * log(epsMax / epsMin));
return prob;
}

double PhotoPionProduction::proposal_inv_cdf(double prob, double epsMin, double epsMax) const {
double eps = epsMin * pow(epsMax / epsMin, prob);
return eps;
}

double PhotoPionProduction::sampleEps(bool onProton, double E, double z) const {
// sample eps between epsMin ... epsMax
double Ein = E / GeV;
Expand All @@ -422,10 +432,21 @@ double PhotoPionProduction::sampleEps(bool onProton, double E, double z) const {

Random &random = Random::instance();
for (int i = 0; i < 1000000; i++) {
double eps = epsMin + random.rand() * (epsMax - epsMin);
double pEps = probEps(eps, onProton, Ein, z);
if (random.rand() * pEpsMax < pEps)
return eps * eV;
double eps;
double pEps;
if (sampleLog){
// convert uniformly-sampled probability to eps via inverse CDF
eps = proposal_inv_cdf(random.rand(), epsMin, epsMax);
// accept / reject based on ratio of pdfs
pEps = probEps(eps, onProton, Ein, z);
double pEps_proposal = proposal_pdf(eps, epsMin, epsMax);
if (pEps / pEps_proposal > random.rand() * pEpsMax)
return eps * eV;
} else
eps = epsMin + random.rand() * (epsMax - epsMin);
pEps = probEps(eps, onProton, Ein, z);
if (random.rand() * pEpsMax < pEps)
return eps * eV;
}
throw std::runtime_error("error: no photon found in sampleEps, please make sure that photon field provides photons for the interaction by adapting the energy range of the tabulated photon field.");
}
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