G4UniversalFluctuation.cc

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// $Id$
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// -------------------------------------------------------------------
//
// GEANT4 Class file
//
//
// File name:     G4UniversalFluctuation
//
// Author:        Laszlo Urban
// 
// Creation date: 03.01.2002
//
// Modifications: 
//
// 28-12-02 add method Dispersion (V.Ivanchenko)
// 07-02-03 change signature (V.Ivanchenko)
// 13-02-03 Add name (V.Ivanchenko)
// 16-10-03 Changed interface to Initialisation (V.Ivanchenko)
// 07-11-03 Fix problem of rounding of double in G4UniversalFluctuations
// 06-02-04 Add control on big sigma > 2*meanLoss (V.Ivanchenko)
// 26-04-04 Comment out the case of very small step (V.Ivanchenko)
// 07-02-05 define problim = 5.e-3 (mma)
// 03-05-05 conditions of Gaussian fluctuation changed (bugfix)
//          + smearing for very small loss (L.Urban)
// 03-10-05 energy dependent rate -> cut dependence of the
//          distribution is much weaker (L.Urban)
// 17-10-05 correction for very small loss (L.Urban)
// 20-03-07 'GLANDZ' part rewritten completely, no 'very small loss'
//          regime any more (L.Urban)
// 03-04-07 correction to get better width of eloss distr.(L.Urban)
// 13-07-07 add protection for very small step or low-density material (VI)
// 19-03-09 new width correction (does not depend on previous steps) (L.Urban)
// 20-03-09 modification in the width correction (L.Urban)
// 14-06-10 fixed tail distribution - do not use uniform function (L.Urban)
// 08-08-10 width correction algorithm has bee modified -->
//          better results for thin targets (L.Urban)
// 06-02-11 correction for very small losses (L.Urban)
//

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#include "G4UniversalFluctuation.hh"
#include "G4PhysicalConstants.hh"
#include "G4SystemOfUnits.hh"
#include "Randomize.hh"
#include "G4Poisson.hh"
#include "G4Step.hh"
#include "G4Material.hh"
#include "G4DynamicParticle.hh"
#include "G4ParticleDefinition.hh"

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using namespace std;

G4UniversalFluctuation::G4UniversalFluctuation(const G4String& nam)
 :G4VEmFluctuationModel(nam),
  particle(0),
  minNumberInteractionsBohr(10.0),
  theBohrBeta2(50.0*keV/proton_mass_c2),
  minLoss(10.*eV),
  nmaxCont(16.),
  rate(0.55),
  fw(4.)
{
  lastMaterial = 0;

  particleMass = chargeSquare = ipotFluct = electronDensity = f1Fluct = f2Fluct 
    = e1Fluct = e2Fluct = e1LogFluct = e2LogFluct = ipotLogFluct = e0 = esmall 
    = e1 = e2 = 0;

}

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G4UniversalFluctuation::~G4UniversalFluctuation()
{}

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void G4UniversalFluctuation::InitialiseMe(const G4ParticleDefinition* part)
{
  particle       = part;
  particleMass   = part->GetPDGMass();
  G4double q     = part->GetPDGCharge()/eplus;
  chargeSquare   = q*q;
}

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G4double G4UniversalFluctuation::SampleFluctuations(const G4Material* material,
                                                    const G4DynamicParticle* dp,
                                                    G4double& tmax,
                                                    G4double& length,
                                                    G4double& averageLoss)
{
  // Calculate actual loss from the mean loss.
  // The model used to get the fluctuations is essentially the same
  // as in Glandz in Geant3 (Cern program library W5013, phys332).
  // L. Urban et al. NIM A362, p.416 (1995) and Geant4 Physics Reference Manual

  // shortcut for very small loss or from a step nearly equal to the range
  // (out of validity of the model)
  //
  G4double meanLoss = averageLoss;
  G4double tkin  = dp->GetKineticEnergy();
  //G4cout<< "Emean= "<< meanLoss<< " tmax= "<< tmax<< " L= "<<length<<G4endl;
  if (meanLoss < minLoss) { return meanLoss; }

  if(!particle) { InitialiseMe(dp->GetDefinition()); }
  
  G4double tau   = tkin/particleMass;
  G4double gam   = tau + 1.0;
  G4double gam2  = gam*gam;
  G4double beta2 = tau*(tau + 2.0)/gam2;

  G4double loss(0.), siga(0.);
  
  // Gaussian regime
  // for heavy particles only and conditions
  // for Gauusian fluct. has been changed 
  //
  if ((particleMass > electron_mass_c2) &&
      (meanLoss >= minNumberInteractionsBohr*tmax))
  {
    G4double massrate = electron_mass_c2/particleMass ;
    G4double tmaxkine = 2.*electron_mass_c2*beta2*gam2/
                        (1.+massrate*(2.*gam+massrate)) ;
    if (tmaxkine <= 2.*tmax)   
    {
      electronDensity = material->GetElectronDensity();
      siga = sqrt((1.0/beta2 - 0.5) * twopi_mc2_rcl2 * tmax * length
                  * electronDensity * chargeSquare);

     
      G4double sn = meanLoss/siga;
  
      // thick target case 
      if (sn >= 2.0) {

        G4double twomeanLoss = meanLoss + meanLoss;
        do {
          loss = G4RandGauss::shoot(meanLoss,siga);
        } while (0.0 > loss || twomeanLoss < loss);

        // Gamma distribution
      } else {

        G4double neff = sn*sn;
        loss = meanLoss*CLHEP::RandGamma::shoot(neff,1.0)/neff;
      }
      //G4cout << "Gauss: " << loss << G4endl;
      return loss;
    }
  }

  // Glandz regime : initialisation
  //
  if (material != lastMaterial) {
    f1Fluct      = material->GetIonisation()->GetF1fluct();
    f2Fluct      = material->GetIonisation()->GetF2fluct();
    e1Fluct      = material->GetIonisation()->GetEnergy1fluct();
    e2Fluct      = material->GetIonisation()->GetEnergy2fluct();
    e1LogFluct   = material->GetIonisation()->GetLogEnergy1fluct();
    e2LogFluct   = material->GetIonisation()->GetLogEnergy2fluct();
    ipotFluct    = material->GetIonisation()->GetMeanExcitationEnergy();
    ipotLogFluct = material->GetIonisation()->GetLogMeanExcEnergy();
    e0 = material->GetIonisation()->GetEnergy0fluct();
    esmall = 0.5*sqrt(e0*ipotFluct);  
    lastMaterial = material;   
  }

  // very small step or low-density material
  if(tmax <= e0) { return meanLoss; }

  G4double losstot = 0.;
  G4int    nstep = 1;
  if(meanLoss < 25.*ipotFluct)
    {
      if(G4UniformRand() < 0.04*meanLoss/ipotFluct)
        { nstep = 1; }
      else
        { 
          nstep = 2;
          meanLoss *= 0.5; 
        }
    }

  for (G4int istep=0; istep < nstep; ++istep) {
    
    loss = 0.;

    G4double a1 = 0. , a2 = 0., a3 = 0. ;

    if(tmax > ipotFluct) {
      G4double w2 = log(2.*electron_mass_c2*beta2*gam2)-beta2;

      if(w2 > ipotLogFluct)  {
        G4double C = meanLoss*(1.-rate)/(w2-ipotLogFluct);
        a1 = C*f1Fluct*(w2-e1LogFluct)/e1Fluct;
        if(w2 > e2LogFluct) {
          a2 = C*f2Fluct*(w2-e2LogFluct)/e2Fluct;
        }
        if(a1 < nmaxCont) { 
          //small energy loss
          G4double sa1 = sqrt(a1);
          if(G4UniformRand() < exp(-sa1))
            {
              e1 = esmall;
              a1 = meanLoss*(1.-rate)/e1;
              a2 = 0.;
              e2 = e2Fluct;
            } 
          else
            {
              a1 = sa1 ;    
              e1 = sa1*e1Fluct;
              e2 = e2Fluct;
            }

        } else {
            //not small energy loss
            //correction to get better fwhm value
            a1 /= fw;
            e1 = fw*e1Fluct;
            e2 = e2Fluct;
        }
      }   
    }

    G4double w1 = tmax/e0;
    if(tmax > e0) {
      a3 = rate*meanLoss*(tmax-e0)/(e0*tmax*log(w1));
    }
    //'nearly' Gaussian fluctuation if a1>nmaxCont&&a2>nmaxCont&&a3>nmaxCont  
    G4double emean = 0.;
    G4double sig2e = 0., sige = 0.;
    G4double p1 = 0., p2 = 0., p3 = 0.;
 
    // excitation of type 1
    if(a1 > nmaxCont)
      {
        emean += a1*e1;
        sig2e += a1*e1*e1;
      }
    else if(a1 > 0.)
      {
        p1 = G4double(G4Poisson(a1));
        loss += p1*e1;
        if(p1 > 0.) {
          loss += (1.-2.*G4UniformRand())*e1;
        }
      }


    // excitation of type 2
    if(a2 > nmaxCont)
      {
        emean += a2*e2;
        sig2e += a2*e2*e2;
      }
    else if(a2 > 0.)
      {
        p2 = G4double(G4Poisson(a2));
        loss += p2*e2;
        if(p2 > 0.) 
          loss += (1.-2.*G4UniformRand())*e2;
      }

    if(emean > 0.)
      {
        sige   = sqrt(sig2e);
        loss += max(0.,G4RandGauss::shoot(emean,sige));
      }

    // ionisation 
    G4double lossc = 0.;
    if(a3 > 0.) {
      emean = 0.;
      sig2e = 0.;
      sige = 0.;
      p3 = a3;
      G4double alfa = 1.;
      if(a3 > nmaxCont)
        {
          alfa            = w1*(nmaxCont+a3)/(w1*nmaxCont+a3);
          G4double alfa1  = alfa*log(alfa)/(alfa-1.);
          G4double namean = a3*w1*(alfa-1.)/((w1-1.)*alfa);
          emean          += namean*e0*alfa1;
          sig2e          += e0*e0*namean*(alfa-alfa1*alfa1);
          p3              = a3-namean;
        }

      G4double w2 = alfa*e0;
      G4double w  = (tmax-w2)/tmax;
      G4int nb = G4Poisson(p3);
      if(nb > 0) {
        for (G4int k=0; k<nb; k++) lossc += w2/(1.-w*G4UniformRand());
      }
    }

    if(emean > 0.)
      {
        sige   = sqrt(sig2e);
        lossc += max(0.,G4RandGauss::shoot(emean,sige));
      }

    loss += lossc;

    losstot += loss;
  }
  //G4cout << "Vavilov: " << losstot << "  Nstep= " << nstep << G4endl;
              
  return losstot;

}

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G4double G4UniversalFluctuation::Dispersion(
                          const G4Material* material,
                          const G4DynamicParticle* dp,
                                G4double& tmax,
                                G4double& length)
{
  if(!particle) { InitialiseMe(dp->GetDefinition()); }

  electronDensity = material->GetElectronDensity();

  G4double gam   = (dp->GetKineticEnergy())/particleMass + 1.0;
  G4double beta2 = 1.0 - 1.0/(gam*gam);

  G4double siga  = (1.0/beta2 - 0.5) * twopi_mc2_rcl2 * tmax * length
                 * electronDensity * chargeSquare;

  return siga;
}

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void 
G4UniversalFluctuation::SetParticleAndCharge(const G4ParticleDefinition* part,
                                             G4double q2)
{
  if(part != particle) {
    particle       = part;
    particleMass   = part->GetPDGMass();
  }
  chargeSquare = q2;
}

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