The neutron slowing down process is frequently studied by means of a pulsed-electron beam from a Linac impinging on a heavy metal target. The resulting pulses of photoneutrons are thermalized and the differential spectrum is measured via the time-of-flight method. If the thermalizing medium contains deuterium or beryllium, a secondary distributed photoneutron source is produced by the gamma-ray flash from the Linac. The magnitude of this secondary source in D2O was measured by foil activation and it was shown that the effect of the secondary source can be accurately evaluated.