The experiments on recording the radiation decay signal of the GIR2 reactor core in the range from 0 to 40 µs when exposed to an external intense neutron pulse with a duration of ∼1 µs are performed. The reactivity interval from subcritical (−7.5 β) to critical (+0.28 β) states of the reactor is studied. It is found that the signal of the detector with a plastic scintillator depends on the reactivity and is proportional to the change in the density of thermalized neutrons in the reactor converter. The results show that the reactivity of the GIR2 reactor can be determined on the basis of measurement of the signal decay of a scintillation detector and the well-known expression α∞=βΛ(ρ−1) to calculate the reactivity values only for states with a reactivity greater than ∼−3 β.