A gold foil of mass `m= 0.20 g` was irradiated during `t=6.0` hours by a thermal neutron flux falling normally on its suface. Following `tau=12 hours` after the completion of irradiation the activity of the foil became equal to `A= 1.9.10^(7)dis//s`. Find the neutron flux density if the effective cross-section of formation of a radioactive nucleus is `sigma= 96b`, and the half-life is equal to `T=2.7 days`