PURPOSE: To lengthen the life of a core by providing a mesh-shaped hollow insulating tube enveloping the core in a cylinder body closed by end plates.

CONSTITUTION: A mesh-shaped hollow insulating tube 11 is mounted between insulating terminals 6, 7 so as to envelop a core 5. When high voltage is added to the core 5, electrons are gas-amplified so as to output a pulse signal whose size is in compliance with neutrons. At this time, fluorine gas generated in the process of gas amplification reacts upon the core 5, and a fluorine compound with sublimation property is generated so that the core 5 is gradually consumed and becomes thinner. However, tension is not added to the core 5 and the core 5 is supported in the hollow insulating tube 11, and thereby the core 5 does not break receiving the tension. With this arrangement, the life of the core 5 can be lengthened.