A neutron position detector according to an embodiment includes a tubular enclosure used as a cathode, an anode located at an axial center inside the enclosure, and a gas that includes a .sup.3He gas and an additive gas and is sealed inside the enclosure. The additive gas includes nitrogen as a quenching gas, and argon as a gas that reduces the ranges of reaction products as neutron and .sup.3He gas.

A neutron position detector according to an embodiment includes a tubular enclosure used as a cathode, an anode located at an axial center inside the enclosure, and a gas that includes a .sup.3He gas and an additive gas and is sealed inside the enclosure. The additive gas includes nitrogen as a quenching gas, and argon as a gas that reduces the ranges of reaction products as neutron and .sup.3He gas.