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moller.cxx

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// -*- C++ -*-  $Id: moller.cxx,v 1.1 2000/09/01 20:20:20 burnett Exp $
//
// This file is part of Gismo 2

// Contents -------------------------------------------------------
//
//  EGS::moller
//
// End -----------------------------------------------------------

#include "EGS.h"
#include "PEGSData.h"
#include "Random.h"

void EGS::moller(const PEGSData &mat)
{

    //                                version 4.00  --  26 jan 1986/1900
    //******************************************************************
    //   discrete Moller scattering (a call to this routine) has been
    //   arbitrarily defined and calculated to mean moller scatterings
    //   which impart to the secondary electron sufficient energy that
    //   it be transported discretely.  the threshold to transport an
    //   electron discretely is a total energy of ae or a kinetic energy
    //   of te=ae-rm.  since the kinetic energy transfer is always, by
    //   definition, less than half of the incident kinetic energy, this
    //   implies that the incident energy, eie, must be larger than
    //   thmoll=te*2+rm.  the rest of the collision contribution is
    //   subtracted continuously from the electron as ionization
    //   loss during transport.
    //******************************************************************

    float eie = this->e; // get energy in MeV
    float br;
    double peie=eie;  //precise energy of incident electron
    double prm = rm;
    double pekin=peie-prm;  //precise k.e. of incident electron
    float ekin=pekin,
          t0  =ekin/rm,
          e0  =t0+1.0,
          extrae = eie - mat.thmoll,
          e02 =e0*e0,
          betai2=e02/(e02-1.0),
          ep0 = mat.te/ekin,
          g1  =(1.-2.*ep0)*betai2,
          g2  =t0*t0/e02,
          g3  =(2.*t0+1.)/e02;

    //   H.H.Nagel has constructed a factorization of the frequency
    //   distribution function for the moller differential cross
    //   section used as suggested by butcher and messel.
    //   (H.H.Nagel, op.cit., p. 53-55)
    //   however, a much simpler sampling method which does not become
    //   very inefficient near thmoll is the following. . .
    //   let br=eks/ekin,  where eks is kinetic energy transfered to the
    //   secondary electron and ekin is the incident kinetic energy.

    //   modified (7 feb 1974) to use the true moller cross section.
    //   that is, instead of the e+ e- average given in the Rossi
    //   formula used by Nagel.  the sampling scheme is that
    //   used by messel and crawford (epsdf 1970 p.13)
    //   first sample (1/br**2) over (te/ekin,1/2) . . .

         if( g1>0 ) //THB: add this since it got stuck here ???
         for(;;)
         {   // to retry if rejected
                float rnno27 = Random::flat();
                br = mat.te/(ekin-extrae*rnno27);

                //   use messel and crawfords rejection function.
                float r=br/(1.-br);
                float rnno28 = Random::flat();
                float rejf4=g1*(1.+g2*br*br+r*(r-g3));
                if (rnno28 <= rejf4) break; //try until accepted.
         }
         else
                br=0.5;
    double pekse2=br*ekin; //precise kinetic energy of secondary electron #2
    double pese1=peie-pekse2; //precise energy of secondary electron #1
    double pese2=pekse2+prm; //precise energy of secondary electron #2

    this->e = pese1;       // reassign electron energy
    //   moller angles are uniquely determined by kinematics

    double h1=(peie+prm)/pekin;

    //   direction cosine change for 'old' electron

    double dcosth=h1*(pese1-prm)/(pese1+prm);
    float costhe=sqrt(dcosth);

    float phi = Random::flat(2.*M_PI);
    rotate(costhe, phi);

    EGS& newel = *new EGS(this);
    newel.iq = -1;
    newel.e = pese2;
    dcosth=h1*(pese2-prm)/(pese2+prm);
    costhe=sqrt(dcosth);
    newel.rotate(costhe, phi+M_PI);

}//end of subroutine moller  end;

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