An improved method based on the moderation of intermediate neutrons for the measurement of hydrogen in small samples is described.
With the aid of boron and cadmium filters, a space shielded from slow neutrons is set up close to an isotopic neutron source
shrouded by water moderator. A BF3 proportional counter enclosed with a sample cell is placed in this space. The neutron count rate of the counter increases
when a hydrogen-containing material is introduced into the cell, due to the moderation of intermediate neutrons passing through
the filters. With a 1.3 μg252Cf neutron source, the lower limit of hydrogen detection for 200 ml samples in 10 min count time is 0.01 wt.%. This method
is suitable for measuring the H2O content of heavy water.