4. METHOD OF SOLUTION
The time dependent variations of the concentrations are described by a system of linear ordinary differential equations of first order with constant coefficients (if the flux is held constant). This system is especially easily solved in the case of a triangular matrix of the coefficients. Such a triangular form is attainable by:
(1) Restriction to reactions of the following kind:
(b) (n,gamma) reaction (capture)
(c) beta decay
(d) gamma decay of the excited states of the nuclei
(2) Arrangement of the isotopes in a special order.
This means neglecting reactions of the kind (n,alpha), (n,2n), emission of neutrons (formation of delayed neutrons) and similar processes which are in general of less importance.