PURPOSE: To simplify signal processing and visually observe atmospheric neutron distribution on a picture by accommodating a secondary electron multiplier, a phoshor and a solid-sate image pickup element in a neutron beam shielding body provided with a radiator as a window, discharging recoil protons owing to collision with these neutron beams.

CONSTITUTION: A nutron beam discharged from a neutron generating body 101 is focused by a collimater 102, penetrated into a radiator 104 and made to collide with hydrogen atoms in the radiator 104, so that recoil protons are discharged in a vacuum room 105. The recoil protons are passed through a slit 106, deflected by the electric field of an anode plate 107 at a position close to the anode plate 107 and penetrated into a microchannel plate 108. Secondary electrons generated in the plate 108 are penetrated into a scintillator 109 after repeating electronic multiplication and illuminated by a fluorescent means. The illuminated light is accumulated on a solid-state camera tube 110 adhered to the scintillator 109 and outputted to a scanning circuit 112, observing the output as a picture.