Problem 6.286

A thin foil of certain stable isotope is irradiated by thermal neutrons falling normally on its surface. Due to the capture of neutrons a radionuclide with decay constant \(\lambda\) appears. Find the law describing accumulation of that radionuclide \(N(t)\) per unit area of the foil's surface. The neutron flux density is \(J,\) the number of nuclei per unit area of the foil's surface is \(n,\) and the effective crosssection of formation of active nuclei is \(\sigma\)