nuclear reactor in-vessel neutron detector has a gas-tight ionization chamber consisting of tubular concentric inner and outer electrodes (2,4) supported by ceramic end members (8,10) with a portion of the space within the inner electrode (4) sealingly partitioned off to reduce the space occupied by a chamber-filler gas, either helium or argon, which fills the spaces that remain within the outer electrode via an exhaust-cum-gas filler pipe (14) supported by an end plate (12), to reduce the number of atoms of the ionization gas. Coated on at least one of the inner surface (20) of the outer electrode and the outer surface (22) of the inner electrode is a coating (24) of enriched uranium, substantially 90% 235 U, as a sensitive material, the ratio of the number of atoms of which to the number of atoms of the filler gas is selected to be an optimum, being between 0.45 and 1.8 for helium and 2.3 and 9.0 for argon, so that inert gas atoms formed from the 235 U accumulate in the filler gas to increase the detector sensitivity to offset the depletion of the 235 U.