Hi,
   The calculation of the momentum of the recoiling nucleus on Line 647 of
G4NuclearDecayChannel (in the BetaDecayIt() method):


daughtermomentum[1] = std::sqrt(daughterenergy[1]*daughterenergy[1] +
                                2.0*daughterenergy[1] * daughtermass[1]);


can result in a 'nan' and subsequent lock of an application because the quantity
whose square root is taken can go negative due to rounding errors from previous
calculations.

A suggested fix would be to check that the quantity in the square root is
positive before taking the square root, and take it as zero otherwise:


G4double recoilmomentumsquared = daughterenergy[1]*daughterenergy[1] +
                               2.0*daughterenergy[1] * daughtermass[1];

if( recoilmomentumsquared < 0.0) {
            recoilmomentumsquared = 0.0;
}

daughtermomentum[1] = std::sqrt(recoilmomentumsquared);


I've only seen the quantity 'recoilmomentumsquared' go to slightly negative
values (typically of order 10^-8), so I don't think taking it as zero will
violate conservation of energy too much :).

Ben Morgan.