Gabriele Cosmo
144 lines
6.6 KiB
C++
144 lines
6.6 KiB
C++
//
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// ********************************************************************
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// * License and Disclaimer *
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// * *
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// * The Geant4 software is copyright of the Copyright Holders of *
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// * the Geant4 Collaboration. It is provided under the terms and *
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// * conditions of the Geant4 Software License, included in the file *
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// * LICENSE and available at http://cern.ch/geant4/license . These *
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// * include a list of copyright holders. *
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// * *
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// * Neither the authors of this software system, nor their employing *
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// * institutes,nor the agencies providing financial support for this *
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// * work make any representation or warranty, express or implied, *
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// * regarding this software system or assume any liability for its *
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// * use. Please see the license in the file LICENSE and URL above *
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// * for the full disclaimer and the limitation of liability. *
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// * *
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// * This code implementation is the result of the scientific and *
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// * technical work of the GEANT4 collaboration. *
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// * By using, copying, modifying or distributing the software (or *
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// * any work based on the software) you agree to acknowledge its *
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// * use in resulting scientific publications, and indicate your *
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// * acceptance of all terms of the Geant4 Software license. *
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// ********************************************************************
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//
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//
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// GEANT4 Class file
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//
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//
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// File name: G4AllisonPositronAtRestModel
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//
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// Author: Vladimir Ivanchenko
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//
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// Creation date: 14 May 2024
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//
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// -------------------------------------------------------------------
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//
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#include "G4AllisonPositronAtRestModel.hh"
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#include "G4DynamicParticle.hh"
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#include "G4Material.hh"
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#include "Randomize.hh"
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#include "G4Gamma.hh"
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#include "G4RandomDirection.hh"
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#include "G4ThreeVector.hh"
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#include "G4PhysicalConstants.hh"
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#include "G4SystemOfUnits.hh"
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//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
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G4AllisonPositronAtRestModel::G4AllisonPositronAtRestModel()
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: G4VPositronAtRestModel("Allison")
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{}
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//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
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void G4AllisonPositronAtRestModel::SampleSecondaries(
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std::vector<G4DynamicParticle*>& secParticles,
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G4double&, const G4Material* material) const
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{
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const G4double eGamma = CLHEP::electron_mass_c2;
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// In rest frame of positronium gammas are back to back
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const G4ThreeVector& dir1 = G4RandomDirection();
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const G4ThreeVector& dir2 = -dir1;
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auto aGamma1 = new G4DynamicParticle(G4Gamma::Gamma(),dir1,eGamma);
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auto aGamma2 = new G4DynamicParticle(G4Gamma::Gamma(),dir2,eGamma);
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// In rest frame the gammas are polarised perpendicular to each other - see
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// Pryce and Ward, Nature No 4065 (1947) p.435.
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// Snyder et al, Physical Review 73 (1948) p.440.
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G4ThreeVector pol1 = (G4RandomDirection().cross(dir1)).unit();
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G4ThreeVector pol2 = (pol1.cross(dir2)).unit();
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// A positron in matter slows down and combines with an atomic electron to
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// make a neutral atom called positronium, about half the size of a normal
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// atom. I expect that when the energy of the positron is small enough,
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// less than the binding energy of positronium (6.8 eV), it is
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// energetically favourable for an electron from the outer orbitals of a
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// nearby atom or molecule to transfer and bind to the positron, as in an
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// ionic bond, leaving behind a mildly ionised nearby atom/molecule. I
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// would expect the positronium to come away with a kinetic energy of a
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// few eV on average. In its para (spin 0) state it annihilates into two
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// photons, which in the rest frame of the positronium are collinear
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// (back-to-back) due to momentum conservation. Because of the motion of the
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// positronium, photons will be not quite back-to-back in the laboratory.
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// The positroniuim acquires an energy of order its binding energy and
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// doesn't have time to thermalise. Nevertheless, here we approximate its
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// energy distribution by a Maxwell-Boltzman with mean energy <KE>. In terms
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// of a more familiar concept of temperature, and the law of equipartition
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// of energy of translational motion, <KE>=3kT/2. Each component of velocity
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// has a distribution exp(-mv^2/2kT), which is a Gaussian of mean zero
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// and variance kT/m=2<KE>/3m, where m is the positronium mass.
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const G4double meanEnergyPerIonPair = material->GetIonisation()->GetMeanEnergyPerIonPair();
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const G4double& meanKE = meanEnergyPerIonPair; // Just an alias
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if (meanKE > 0.) { // Positronium has motion
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// Mass of positronium
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const G4double mass = 2.*CLHEP::electron_mass_c2;
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// Mean <KE>=3kT/2, as described above
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// const G4double T = 2.*meanKE/(3.*k_Boltzmann);
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// Component velocities: Gaussian, variance kT/m=2<KE>/3m.
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const G4double sigmav = std::sqrt(2.*meanKE/(3.*mass));
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// This is in units where c=1
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const G4double vx = G4RandGauss::shoot(0.,sigmav);
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const G4double vy = G4RandGauss::shoot(0.,sigmav);
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const G4double vz = G4RandGauss::shoot(0.,sigmav);
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const G4ThreeVector v(vx,vy,vz); // In unit where c=1
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const G4ThreeVector& beta = v; // so beta=v/c=v
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aGamma1->Set4Momentum(aGamma1->Get4Momentum().boost(beta));
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aGamma2->Set4Momentum(aGamma2->Get4Momentum().boost(beta));
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// Rotate polarisation vectors
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const G4ThreeVector& newDir1 = aGamma1->GetMomentumDirection();
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const G4ThreeVector& newDir2 = aGamma2->GetMomentumDirection();
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const G4ThreeVector& axis1 = dir1.cross(newDir1); // No need to be unit
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const G4ThreeVector& axis2 = dir2.cross(newDir2); // No need to be unit
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const G4double& angle1 = std::acos(dir1*newDir1);
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const G4double& angle2 = std::acos(dir2*newDir2);
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pol1.rotate(axis1, angle1);
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pol2.rotate(axis2, angle2);
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}
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// use constructors optimal for massless particle
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aGamma1->SetPolarization(pol1);
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aGamma2->SetPolarization(pol2);
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secParticles.push_back(aGamma1);
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secParticles.push_back(aGamma2);
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}
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//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
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void G4AllisonPositronAtRestModel::PrintGeneratorInformation() const
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{
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G4cout << "\n" << G4endl;
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G4cout << "Allison AtRest positron 2-gamma annihilation model." << G4endl;
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G4cout << "Takes into account positronium motion in the media." << G4endl;
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}
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//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo....
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