A gold foil of mass m = 0.20 g was irradiated during
= 6.0 hours by a thermal neutron flux falling normally on its
surface. Following i = 12 hours after the completion of irradiation
the activity of the foil became equal to A = 1.9.107 dis/s. Find
the neutron flux density if the effective cross-section of formation
of a radioactive nucleus is a --= 96 b, and the half-life is equal
to T = 2.7 days.