An intermediate neutron moderation method for measurement of moisture and/or hydrogen contents of small samples is presented.
The sample is placed on the top face of a neutron howitzer, with a cadmium sheet between. Thermal neutrons resulting from
intermediate neutron moderation in the sample are detected with a3He proportional counter placed on the sample, by a cadmium difference method. With a 500 mCi Am-Be neutron source, the limit
of moisture detection for a 10×20×1.8 cm3 asbestos plate in 1 min count time is 0.5 wt.%. The precision of measuring the hydrogen contents of 250 ml hydrocarbons containing
112 mg H/ml is 0.9% under the same conditions.
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.
A method of measuring heavy water concentration is described based on intermediate neutron moderation. With a 1.6 g252Cf neutron source and a BF3 proportional counter, for 500 ml samples, accuracies of the measurements are within ±0.02 mol% in the concentration range of 0100 mol% with 210 minute counting. To evaluate the practical effectiveness of the method, a model apparatus was built for routine use in laboratory and on-stream measurements. It was successfully applied to observe the substitution process between light water and heavy water in an ion exchange resin tower at a nuclear power station.
For boron concentration in the range of 010%, a 600 g252Cf neutron source was used for thermal neutron irradiation. As for the track counting technique, a visual observation method by human eye and an automatic counting method with an image scanner showed a good agreement between the determinations by both methods. The latter is useful for improving accuracy in elemental determination by using the nuclear track technique because it gives reproducible count values which are free from human error in comparison with the former.