PrimaryGeneratorAction2.cc

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// $Id: PrimaryGeneratorAction2.cc 99721 2016-10-03 08:11:44Z gcosmo $
// 
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#include "PrimaryGeneratorAction2.hh"
#include "PrimaryGeneratorAction.hh"

#include "G4Event.hh"
#include "G4ParticleGun.hh"
#include "G4ParticleTable.hh"
#include "G4ParticleDefinition.hh"
#include "G4PhysicalConstants.hh"
#include "G4SystemOfUnits.hh"
#include "Randomize.hh"

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PrimaryGeneratorAction2::PrimaryGeneratorAction2(G4ParticleGun* gun)
: fParticleGun(gun)
{    
  // energy distribution
  //
  InitFunction();
}

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PrimaryGeneratorAction2::~PrimaryGeneratorAction2()
{ }

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void PrimaryGeneratorAction2::GeneratePrimaries(G4Event* anEvent)
{
  //cosAlpha uniform in [cos(0), cos(pi)]
  G4double cosAlpha = 1. - 2*G4UniformRand();
  G4double sinAlpha = std::sqrt(1. - cosAlpha*cosAlpha);
  G4double psi      = twopi*G4UniformRand();  //psi uniform in [0, 2*pi]  
  G4ThreeVector dir(sinAlpha*std::cos(psi),sinAlpha*std::sin(psi),cosAlpha);

  fParticleGun->SetParticleMomentumDirection(dir);
  
  //set energy from a tabulated distribution
  //
  //G4double energy = RejectAccept();
  G4double energy = InverseCumul();  
  fParticleGun->SetParticleEnergy(energy);    

  //create vertex
  //   
  fParticleGun->GeneratePrimaryVertex(anEvent);
}
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void PrimaryGeneratorAction2::InitFunction()
{
  // tabulated function 
  // Y is assumed positive, linear per segment, continuous
  //
  fNPoints = 16;
  const G4double xx[] = 
    { 37*keV, 39*keV, 45*keV,  51*keV,  57*keV,  69*keV,  71*keV,  75*keV, 
      83*keV, 91*keV, 97*keV, 107*keV, 125*keV, 145*keV, 159*keV, 160*keV }; 
      
  const G4double yy[] =
    { 0.000,  0.077,  0.380,  2.044, 5.535, 15.077, 12.443, 14.766,
     17.644, 18.518, 17.772, 14.776, 8.372,  3.217,  0.194,  0.000 };
  
  //copy arrays in std::vector and compute fMax
  //
  fX.resize(fNPoints); fY.resize(fNPoints);
  fYmax = 0.;
  for (G4int j=0; j<fNPoints; j++) {
    fX[j] = xx[j]; fY[j] = yy[j];
    if (fYmax < fY[j]) fYmax = fY[j];
  };
     
  //compute slopes
  //
  fSlp.resize(fNPoints);
  for (G4int j=0; j<fNPoints-1; j++) { 
    fSlp[j] = (fY[j+1] - fY[j])/(fX[j+1] - fX[j]);
  };
  
  //compute cumulative function
  //
  fYC.resize(fNPoints);  
  fYC[0] = 0.;
  for (G4int j=1; j<fNPoints; j++) {
    fYC[j] = fYC[j-1] + 0.5*(fY[j] + fY[j-1])*(fX[j] - fX[j-1]);
  };     
}

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G4double PrimaryGeneratorAction2::RejectAccept()
{
  // tabulated function 
  // Y is assumed positive, linear per segment, continuous
  //  
  G4double Xrndm = 0., Yrndm = 0., Yinter = -1.;
  
  while (Yrndm > Yinter) {
    //choose a point randomly
    Xrndm = fX[0] + G4UniformRand()*(fX[fNPoints-1] - fX[0]);
    Yrndm = G4UniformRand()*fYmax;
    //find bin
    G4int j = fNPoints-2;
    while ((fX[j] > Xrndm) && (j > 0)) j--;
    //compute Y(x_rndm) by linear interpolation
    Yinter = fY[j] + fSlp[j]*(Xrndm - fX[j]);
  };
  return Xrndm;
}

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G4double PrimaryGeneratorAction2::InverseCumul()
{
  // tabulated function
  // Y is assumed positive, linear per segment, continuous 
  // --> cumulative function is second order polynomial
  
  //choose y randomly
  G4double Yrndm = G4UniformRand()*fYC[fNPoints-1];
  //find bin
  G4int j = fNPoints-2;
  while ((fYC[j] > Yrndm) && (j > 0)) j--;
  //y_rndm --> x_rndm :  fYC(x) is second order polynomial
  G4double Xrndm = fX[j];
  G4double a = fSlp[j];
  if (a != 0.) {
    G4double b = fY[j]/a, c = 2*(Yrndm - fYC[j])/a;
    G4double delta = b*b + c;
    G4int sign = 1; if (a < 0.) sign = -1;
    Xrndm += sign*std::sqrt(delta) - b;    
  } else if (fY[j] > 0.) {
    Xrndm += (Yrndm - fYC[j])/fY[j];
  };
  return Xrndm;
}

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