In this case, the number n₀ of particles of A, deposited per second from the source, is equal to the number of particles of A which change into B per second, and of B into C, and so on. This requires the relation
where P₀, Q₀, R₀ are the maximum numbers of particles of the matter A, B, and C when a steady state is reached.
The values of P, Q, R at any time t after removal of the source are given by equations of the same form as (3) and (5) for a short exposure. Remembering the condition that initially
it can readily be shown that
where
