A photomultiplier simultaneously detects gamma and neutron radiation. The detector includes an external scintillator sensitive to neutron radiation, which is optically coupled to an internal scintillator sensitive to gamma radiation; a solid-state silicon photomultiplier capable of simultaneously registering optical signals from both the external scintillator and internal scintillator, and of transforming the signals into electrical pulses; and a signal processing unit. The external scintillator is a lamination/paint or film. The entire surface of the internal scintillator is covered, except for an area that is adjacent to the input window of the solid-state silicon photomultiplier. The signal processor includes a preamplifier, a spectrometric amplifier, and a pulse shape analyzer. The photomultiplier has improved neutron sensitivity due to the external scintillator covering almost the entire edge surface of the internal scintillator. A solid-state silicon photomultiplier reduces size and power consumption, and improves resistance to mechanical and magnetic impacts.