This process handles the decay of unstable particles. Branching
ratios and decay modes are first accessed from the particle class
instance, then the final state particles and momenta are
generated.
Invokation:
G4Decay* theDecayProcess = new G4Decay();
pmanager ->AddProcess(theDecayProcess);
Note: even though the decay process may be assigned to neutrons,
anti-neutrons, mu+ and mu-, these particles are treated as stable by
G4Decay. To make a particle unstable (with the correct lifetime) use
the method SetPDGStable(false) of the particle.