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Idea Re: Radioactive decay of tritium. 

Forum: Hadronic Processes
Re: Question Radioactive decay of tritium. (Antanas Bukartas)
Date: 14 Apr, 2016
From: Michael H. Kelsey <Michael H. Kelsey>

On Thu, 14 Apr 2016 11:42:49 GMT, Antanas wrote:
> I am trying to model a liquid scintillation detector with a liquid
> tritium source. For the modeling of the source i am using radioactive
> decay libraries of Geant 10.02. I succeeded in implementing everything I
> need to the OpNovice example. And it works for any other nuclei (for
> example Carbon 14, Cobalt 60, etc..) except tritium. In the viewer I can
> see a dot (generic ion) being created, but there is no secondary
> particles. I tried setting the cuts with
> // SetCutsWithDefault();
>   G4ProductionCutsTable::GetProductionCutsTable()->SetEnergyRange(120*eV,
> 1*GeV);
> in the physics list, as the energy of tritium beta is 18.6 keV. But
> still, no secondaries.

Unfortunately, the default behaviour in Geant4 is for tritium to be stable. You have to modify the physics associated with tritium "by hand" to have it decay. For SuperCDMS, I wrote a "TritiumPhysics" builder which I attach to whatever active physics list we are using:

  physList->RegisterPhysics(new CDMSTritiumPhysics);

where I define

void CDMSTritiumPhysics::ConstructProcess() {
  // Make tritium unstable, so it can be decayed
  G4ParticleDefinition* tr = G4Triton::Definition();
  tr->SetPDGStable(false);

  // Remove G4Decay process, which requires a registered decay table
  G4VProcess* decay = 0;
  G4ProcessManager* pman = tr->GetProcessManager();
  G4ProcessVector* pvec = pman->GetAtRestProcessVector();
  for (G4int i=0; i<pvec->size() && decay==0; i++) {
    if ((*pvec)[i]->GetProcessName() == "Decay") decay = (*pvec)[i];
  }
  if (decay) pman->RemoveProcess(decay);

  // Attach RDM, which is a rest-discrete process
  tr->GetProcessManager()->AddProcess(new G4RadioactiveDecay(), 1000, -1, 1000);
}

The middle part of this is needed because we don't have a built-in "G4Decay" table for tritium in Geant4, only the radioactive decay file.

  -- Michael Kelsey

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