The equation of radionuclide production is as follow dN/dt = k - λN All my textbook assumes that k is a constant vale. http://jol.liljenzin.se/KAPITEL/CH15NY3.PDF for the equation to be valid, (iii) the flux is not decreased ... . .. Corrections must then be introduced. Please advise me that is there any reference that doing this correction. What form of the equation will be, is there analytic solution ?