Absolutely calibrated measurements of the neutron yield are important for the evaluation of plasma performance such as the fusion gainQin D–D operating tokamaks. The time‐resolved neutron yield is measured with235U and238U fission chambers and3He proportional counters in the JT‐60U tokamak. Theinsitucalibration was performed by moving the252Cf neutron source toroidally through the JT‐60 vacuum vessel. Detection efficiencies of three235U and two3He detectors were measured for 92 locations of the neutron point source in toroidal scans at two different major radii. The total detection efficiency for the torus neutron source was obtained by averaging the point efficiencies over the whole toroidal angle. The uncertainty of the resulting detection efficiency for the plasma neutrons is estimated to be ±11%.