We have considered the response of three detectors inside a non-multiplication media (Fig. 1) to a transient of neutron source (Fig. 2). The different answer, (P1/Diff) in Fig. 3, refers to a full treatment of the time dependence of the angular harmonics of the P1 approximation (P1) and a simplified approach (that leads to the well know diffusion time dependent equation) in which the time derivative of the first order angular harmonics is neglected (Diff).