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 surface. Following τ = 12 hours after the completion of irradiation the activity of the foil became equal to A = 1.9.107 dis/s. Find the neutron flax density if the effective cross-section of formation of a radioactive nucleus is (σ = 96 b, and the half-life is equal to T = 2.7 days.