WLS fiber simulation

Jailbone · December 9, 2020, 7:10pm

Hello,

I am trying to simulate my first detector geometry which should consist of a plastic scintillator with embedded wavelength shifting fibers. My problem is that I haven’t figured out a way of proper implementing wls fibers. I am wondering if I should expect to see an optical photon being absorbed by the fiber, and then photons being reemitted isotropically, if I shoot one optical photon into the fiber in GUI mode. Until now this is not what happens. My optical photon simply gets refracted when entering the fiber, in and then out of the fiber.

The properties look like this at the moment:

// Core (Polystyrene)
  //

  G4double refractiveIndexWLSfiber[] =
  { 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60,
    1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60,
    1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60,
    1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60,
    1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60, 1.60};

  assert(sizeof(refractiveIndexWLSfiber) == sizeof(photonEnergy));

  G4double absWLSfiber[] =
  {5.40*m,5.40*m,5.40*m,5.40*m,5.40*m,5.40*m,5.40*m,5.40*m,5.40*m,5.40*m,
   5.40*m,5.40*m,5.40*m,5.40*m,5.40*m,5.40*m,5.40*m,5.40*m,5.40*m,5.40*m,
   5.40*m,5.40*m,5.40*m,5.40*m,5.40*m,5.40*m,5.40*m,5.40*m,5.40*m,1.10*m,
   1.10*m,1.10*m,1.10*m,1.10*m,1.10*m,1.10*m, 1.*mm, 1.*mm, 1.*mm, 1.*mm,
    1.*mm, 1.*mm, 1.*mm, 1.*mm, 1.*mm, 1.*mm, 1.*mm, 1.*mm, 1.*mm, 1.*mm};

  // assert(sizeof(absWLSfiber) == sizeof(photonEnergy));

  G4double emissionFib[] =
  {0.05, 0.10, 0.30, 0.50, 0.75, 1.00, 1.50, 1.85, 2.30, 2.75,
   3.25, 3.80, 4.50, 5.20, 6.00, 7.00, 8.50, 9.50, 11.1, 12.4,
   12.9, 13.0, 12.8, 12.3, 11.1, 11.0, 12.0, 11.0, 17.0, 16.9,
   15.0, 9.00, 2.50, 1.00, 0.05, 0.00, 0.00, 0.00, 0.00, 0.00,
   0.00, 0.00, 0.00, 0.00, 0.00, 0.00, 0.00, 0.00, 0.00, 0.00};

  assert(sizeof(emissionFib) == sizeof(photonEnergy));

  // Add entries into properties table
  G4MaterialPropertiesTable* mptWLSfiber = new G4MaterialPropertiesTable();
  mptWLSfiber->
           AddProperty("RINDEX",photonEnergy,refractiveIndexWLSfiber,nEntries);
  // mptWLSfiber->AddProperty("ABSLENGTH",photonEnergy,absWLSfiber,nEntries);
  mptWLSfiber->AddProperty("WLSABSLENGTH",photonEnergy,absWLSfiber,nEntries);
  mptWLSfiber->AddProperty("WLSCOMPONENT",photonEnergy,emissionFib,nEntries);
  mptWLSfiber->AddConstProperty("WLSTIMECONSTANT", 0.5*ns);

  Polystyrene->SetMaterialPropertiesTable(mptWLSfiber);

  //
  // Cladding 1 (PMMA)
  //

  G4double refractiveIndexClad1[] =
  { 1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49,
    1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49,
    1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49,
    1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49,
    1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49, 1.49};

  assert(sizeof(refractiveIndexClad1) == sizeof(photonEnergy));

  G4double absClad[] =
  {20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,
   20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,
   20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,
   20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,
   20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m,20.0*m};

  assert(sizeof(absClad) == sizeof(photonEnergy));

  // Add entries into properties table
  G4MaterialPropertiesTable* mptClad1 = new G4MaterialPropertiesTable();
  mptClad1->AddProperty("RINDEX",photonEnergy,refractiveIndexClad1,nEntries);
  mptClad1->AddProperty("ABSLENGTH",photonEnergy,absClad,nEntries);

  PMMA->SetMaterialPropertiesTable(mptClad1);

  // Cladding 2
  //

   G4double refractiveIndexClad2[] =
   { 1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42,
     1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42,
     1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42,
     1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42,
     1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42, 1.42};

   assert(sizeof(refractiveIndexClad2) == sizeof(photonEnergy));

  // Add entries into properties table
  G4MaterialPropertiesTable* mptClad2 = new G4MaterialPropertiesTable();
  mptClad2->AddProperty("RINDEX",photonEnergy,refractiveIndexClad2,nEntries);
  mptClad2->AddProperty("ABSLENGTH",photonEnergy,absClad,nEntries);

  FP->SetMaterialPropertiesTable(mptClad2);

Mostly taken from the optical examples. I haven’t specified surface properties yet. What am I missing or not understanding right?

dsawkey · December 15, 2020, 4:15pm

I suggest trying to get WLS to work using the extended example OpNovice2. There you just need to adjust macro parameters. There’s a lot that needs to be configured correctly.

As a guess: what photon energy do you use, what is the WLS absorption length at that energy, and how does that compare to the fibre size.

Jailbone · December 18, 2020, 10:48am

Hi, I am a total beginner so I do not understand where the fiber properties are defined in this OpNovice2 example. But I found the extended example “wls” quite helpful. That is where I found the property definitions from my entry post. In addition I specified the electron energies to be:

  G4double photonEnergy[] =
            { 2.034*eV, 2.068*eV, 2.103*eV, 2.139*eV,
              2.177*eV, 2.216*eV, 2.256*eV, 2.298*eV,
              2.341*eV, 2.386*eV, 2.433*eV, 2.481*eV,
              2.532*eV, 2.585*eV, 2.640*eV, 2.697*eV,
              2.757*eV, 2.820*eV, 2.885*eV, 2.954*eV,
              3.026*eV, 3.102*eV, 3.181*eV, 3.265*eV,
              3.353*eV, 3.446*eV, 3.545*eV, 3.649*eV,
              3.760*eV, 3.877*eV, 4.002*eV, 4.136*eV };

also like in the “wls” example. The core of my fiber has a diameter of 1.48 mm for which i defined the wls absorption length like above.

Strangely, when I shoot an optical photon into the fiber the wavelength stays the same, so does not get shifted at all.

dsawkey · December 18, 2020, 1:49pm

Yes, but my question was, what is the energy of the optical photon that you shoot?

Jailbone · December 18, 2020, 2:58pm

G4ParticleTable* particleTable = G4ParticleTable::GetParticleTable();
  G4ParticleDefinition* particle = particleTable->FindParticle("opticalphoton");

  fParticleGun->SetParticleDefinition(particle);
  fParticleGun->SetParticleTime(0.0*ns);
  fParticleGun->SetParticlePosition.....;
  fParticleGun->SetParticleMomentumDirection....;
  fParticleGun->SetParticleEnergy(2.8*eV);

dsawkey · December 19, 2020, 3:15pm

I’m trying to point out that because the wls interaction length is about 5000x greater than the diameter of the fibre, there will be few wls interactions. As a test, try setting the wls interaction length smaller than the size of the fibre.

NB, in OpNovice2, the parameters are set in the macro.

Jailbone · December 20, 2020, 7:09pm

I am sorry, what exactly is the wls interaction length?

And I can’t find any parameters set for the wls fibers in the macro files of OpNovice2. Only some properties of the scintillator for example.
Thanks for your patience.

Jailbone · January 3, 2021, 5:47pm

anybody can help? Please

jazminstewart · January 17, 2023, 5:19pm

I am also working on this problem, any advancements @Jailbone ?

dsawkey · January 31, 2023, 9:36pm

I’m not sure what the question is here. Wls properties are set in the macro wls.mac for the OpNovice2 example. The wls absorption length is the mean distance an optical photon will travel before it undergoes wls process.