How to enable entanglement sampling in geant_v11

I am modeling positron annihilation using Na22 as my primary particle. I am trying to enable sampling of entanglement but it is showing ‘OFF’. Is there a way to do it in the I am using the following main physics lists


However, when I run my simulation, I get the following output for EM parameters,

Thank you.

_Geant4 Version: v11.1.2
_Operating System: Ubuntu
_Compiler/Version: 9.4.0
_CMake Version: 3.20.2

/process/em/QuantumEntanglement true

Note that entangle is only implemented in G4EmLivermorePolarizedPhysics, so where you instantiate your physics list:

  physList->ReplacePhysics(new G4EmLivermorePolarizedPhysics);

You may also need to allow for acollinearity, e.g:

  G4Material* water_mat = nist->FindOrBuildMaterial("G4_WATER");

This gives the positronium a small thermal motion, so the gammas in 2-gamma annihilation are not quite back-to-back. See comments in source/processes//electromagnetic/standard/src/

    // In rest frame of positronium gammas are back to back
    const G4ThreeVector& dir1 = G4RandomDirection();
    const G4ThreeVector& dir2 = -dir1;
    aGamma1 = new G4DynamicParticle(G4Gamma::Gamma(),dir1,eGamma);
    aGamma2 = new G4DynamicParticle(G4Gamma::Gamma(),dir2,eGamma);

    // In rest frame the gammas are polarised perpendicular to each other - see
    // Pryce and Ward, Nature No 4065 (1947) p.435.
    // Snyder et al, Physical Review 73 (1948) p.440.
    G4ThreeVector pol1 = (G4RandomDirection().cross(dir1)).unit();
    G4ThreeVector pol2 = (pol1.cross(dir2)).unit();

    // But the positronium is moving...
    // A positron in matter slows down and combines with an atomic electron to
    // make a neutral atom called positronium, about half the size of a normal
    // atom. I expect that when the energy of the positron is small enough,
    // less than the binding energy of positronium (6.8 eV), it is
    // energetically favourable for an electron from the outer orbitals of a
    // nearby atom or molecule to transfer and bind to the positron, as in an
    // ionic bond, leaving behind a mildly ionised nearby atom/molecule. I
    // would expect the positronium to come away with a kinetic energy of a
    // few eV on average. In its para (spin 0) state it annihilates into two
    // photons, which in the rest frame of the positronium are collinear
    // (back-to-back) due to momentum conservation. Because of the motion of the
    // positronium, photons will be not quite back-to-back in the laboratory.

    // The positroniuim acquires an energy of order its binding energy and
    // doesn't have time to thermalise. Nevertheless, here we approximate its
    // energy distribution by a Maxwell-Boltzman with mean energy <KE>. In terms
    // of a more familiar concept of temperature, and the law of equipartition
    // of energy of translational motion, <KE>=3kT/2. Each component of velocity
    // has a distribution exp(-mv^2/2kT), which is a Gaussian of mean zero
    // and variance kT/m=2<KE>/3m, where m is the positronium mass.

    // We take <KE> = material->GetIonisation()->GetMeanEnergyPerIonPair().