//fluorine 
    G4int Z = 9, A = 18;
    G4double ionCharge   = 0.*eplus;
    G4double excitEnergy = 0.*keV;
    
    G4ParticleDefinition* ion
       = G4IonTable::GetIonTable()->GetIon(Z,A,excitEnergy);
    fParticleGun->SetParticleDefinition(ion);
    fParticleGun->SetParticleCharge(ionCharge);