The reported alpha-particle detector consists of eight flat proportional chambers connected in pairs. It is designed for measurement of the (n,α) reaction on resonance neutrons from a pulsed neutron source with a pulse duration of ∼50 ns. The target’s total area is 10 000 cm2, which allows the use of large amounts of investigated materials (up to 100 g). The detector is used as a low-background counter of alpha particles. Its background is <3 × 10−4 count/cm2·s at a neutron flux of >106 n/cm2·s. For an integral neutron flux of ∼1012 n/cm2, stable operation of the detector was obtained.