Void coefficient measuring device

The device of the present invention measures the void coefficient of a hydrogen-containing fluid easily at a high accuracy by utilizing fast neutrons. Namely, a fast neutron source is disposed at the outer side of a pipeline in which the hydrogen-containing fluid flows. A neutron detector having the circumference being surrounded by neutron moderators is disposed to a position opposite to the source. Neutron shielding materials are disposed at the circumference of the pipeline. Then, only fast neutrons passing the hydrogen-containing fluid in the pipeline among fast neutrons emitted from the neutron source are formed into thermal neutrons by the neutron moderators and detected efficiently. The amount of the detected neutrons depends on the void coefficient in the fluid in the pipeline. Accordingly, the void coefficient in the hydrogen-containing fluid can be measured. The device of the present invention can measure the void coefficient efficiently by using the radiation source at an intensity not undergoing legal regulation. (I.S.)