G4SmpTerrell.cc

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// Copyright (c) 2006 The Regents of the University of California.
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// UCRL-CODE-224807
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// $Id: G4SmpTerrell.cc 67966 2013-03-13 09:38:38Z gcosmo $
//

#include <cmath>
#include "G4fissionEvent.hh"

#define TWOPI 6.283185307
#define SQRT2 1.414213562
#define BSHIFT -0.43287
#define WIDTH 1.079

G4double G4fissionEvent::G4SmpTerrell(G4double nubar) {
/*
  Description
    Sample Fission Number from Terrell's modified Gaussian distribution

    method uses Red Cullen's algoritm UCRL-TR-222526
*/

/*
  Input
    nubar    - average number of neutrons per fission
  Output
    G4SmpTerrell  - sampled multiplicity
    
*/

  G4double width;
  G4double temp1, temp2, expo, cshift;
  G4double rw, theta, sampleg;


  if (nubar < WIDTH) {
    std::ostringstream o;
    o << nubar;
    std::string errMsg = "fission nubar out of range, nubar=" + o.str();
    G4fissionerr(6, "SmpTerrell", errMsg);
  }

  width = SQRT2 * WIDTH;
  temp1 = nubar + 0.5;
  temp2 = temp1/width;
  temp2 *= temp2;
  expo = std::exp(-temp2);
  cshift = temp1 + BSHIFT * WIDTH * expo/(1. - expo);

  do {
    rw = std::sqrt(-std::log(fisslibrng()));
    theta = TWOPI * fisslibrng();
    sampleg = width * rw * std::cos(theta) + cshift;
  } while (sampleg < 0.0);

  return std::floor(sampleg);
}