Using a Neutron Flaw Detector to Increase Sensitivity in Automatic Moisture Measurement
Abstract Generated abstract
This paper examines automatic moisture measurement by fast neutrons for materials in which slow-neutron methods are affected by elements with high neutron absorption cross sections. It argues that a scintillation fast-neutron detector, used without preliminary moderation, can reduce neutron losses, simplify the moisture-meter design, and lower the required source activity compared with systems that moderate fast neutrons before detection. Experiments on wood specimens using a stilbene detector, water phantoms, and gravimetric calibration showed good agreement between methods and indicated moisture determination with about 1.3% accuracy under the reported conditions. Tests with cadmium and indium foils suggested that the approach is insensitive to impurities that strongly absorb slow neutrons, supporting its possible use for soils and other heterogeneous materials.
Full Text
USING A NEUTRON FLAW DETECTOR TO INCREASE SENSITIVITY IN AUTOMATIC MOISTURE MEASUREMENT
V. I. GORBUNOV, V. B. ELAGIN, G. Sh. PEKARSKII
One of the principal rapid methods for measuring moisture is the method of measuring moisture by means of neutrons [1—4]. The method is based on the fact that water contains the light element hydrogen, and in elastic collisions with light nuclei a large fraction of the energy is lost per collision. The large cross section for elastic interaction with hydrogen nuclei makes the method especially sensitive to changes in the amount of water in a sample.
The slowing down of fast neutrons in a substance occurs owing to acts of interaction with the nuclei of the substance. One of the main processes is elastic collision of neutrons with nuclei. The energy of a neutron after an elastic collision can be determined from the expression [5].
\[ \frac{E_2}{E_1}=\frac{1}{2}\left[(1+\alpha)+(1-\alpha)\cos\psi\right], \]
where \(E_1\) is the energy of the neutron before the collision;
\(E_2\) is the energy of the neutron after the collision;
\(\psi\) is the scattering angle in the center-of-mass system.
The quantity \(\alpha\) is defined as
\[ \alpha=\left(\frac{A-1}{A+1}\right)^2, \]
where
\(A\) is the atomic number of the nucleus.
It follows from the formula that a decrease in the atomic number of the substance leads to an increase in the energy loss per collision. The value of the average energy of neutrons after one elastic collision is determined as
\[ E_2=E_1\left[1-\frac{2A}{(A+1)^2}\right]. \]
The average energy loss by neutrons is
\[ \Delta E=\frac{1}{2}E_1. \]
Neutrons lose the maximum energy in a single collision when colliding with hydrogen nuclei.
Large average losses per collision with a hydrogen nucleus and the considerable cross section for elastic scattering on hydrogen lead to a ra-
to a sharp increase in the number of neutrons slowed down when the hydrogen content in the substance increases. The number of slowed-down neutrons is related to the number of hydrogen nuclei per unit volume of the substance under investigation and, consequently, to the moisture content of the substance. The increase in the flux of slow neutrons is recorded by slow-neutron counters. However, there is a whole series of substances that contain elements with a large absorption cross section for slow neutrons, and the instrument readings will depend strongly on changes in the composition of the substance. Even microimpurities of such elements as cadmium, hafnium, boron, samarium, and others, which have anomalously large capture cross sections for slow neutrons, will strongly affect the instrument readings. In such materials, moisture can be measured only from the change in the counting rate of fast neutrons, i.e., from the remainder of the initial flux of fast neutrons. L. K. Tatochenko [1] describes the operating principle of such a moisture meter using fast neutrons. To register the transmitted flux of fast neutrons, a slow-neutron counter surrounded by a layer of moderator is used. To eliminate the influence on the instrument readings of slow neutrons formed in the object being monitored, the counter is separated from the object by a layer of material with a large absorption cross section for slow neutrons (for example, cadmium 1–2 mm thick). The best measurement results are obtained when the substance is large in the direction perpendicular to the axis of the neutron beam.
The optimal thickness of the object under investigation is selected from the condition of minimum relative error and is \(X = 8 \div 10\) cm. The activity of the Po + Be source required for measuring moisture with a relative error of 10% is, for \(X = 10\) cm, a time constant \(\tau = 1\) sec, and an accuracy of thickness measurement \(X = 1.2\) mm, 13–20 curies, i.e., 15–20 times higher than when measuring moisture using slow neutrons with the same error. This comparison can be made if the substance under investigation does not contain elements with a large absorption cross section for slow neutrons.
Such an increase in source activity when measuring moisture by registering fast neutrons is explained by the very low efficiency of counters for neutrons. The efficiency of a counter for slow neutrons is 5%. When a slow-neutron counter is used to register fast neutrons, a large number of neutrons are lost in the moderator, and the efficiency becomes still lower.
As a result, practical implementation of the method for determining moisture by measuring the transmitted flux of fast neutrons with their preliminary moderation before registration is difficult.
The scintillation detector of fast neutrons that has been developed, with a registration efficiency of 30% for neutron energies of 3–4 MeV, makes it possible to eliminate the stage of preliminary moderation of fast neutrons before registration, which is associated with a large loss in the flux of fast neutrons that have passed through the object and carry useful information about the object under investigation. This makes it possible, while retaining all the advantages of the moisture-control method using fast neutrons, to simplify the design of the moisture meter by eliminating the intermediate moderation operation and to significantly reduce the source activity required to obtain the same accuracy in measuring moisture.
Figure 1 shows the scheme for measuring moisture using fast neutrons. Also shown there are the scheme for measuring moisture using thermal neutrons (1,a) and the scheme for measuring moisture using fast neutrons with preliminary moderation of the fast neutrons before registration (1,b).
The designations in the figure are: 1 — fast-neutron source; 2 — specimen under test; 3 — moderator; 4 — neutron-slowing detector; 5 — fast-neutron detector.
Experimental investigations were carried out in solving a complex problem of monitoring wood, conducted at the Scientific Research Institute of Electronic Introscopy. The measurements were performed with a flaw detector using a stilbene crystal \(30 \times 50\). A collimator 30 mm in diameter was made of paraffin with the addition of boric acid. The height of the collimator was 200 mm. In the experiment a block 177 mm thick, made of absolutely dry wood with a density of \(0.54\ \text{g}/\text{cm}^3\), was used, together with a set of Plexiglas phantoms filled with water, of thickness 8, 16, 25, 50, and 96 mm. The moisture content \(W\), calculated with the introduction of each phantom, corresponded to the data given in Table 1.
Table 1
Calculated moisture content of wood specimens when phantoms of different thickness are introduced into the measured volume
| Thickness, mm | 8 | 24 | 50 | 100 | 195 |
|---|---|---|---|---|---|
| \(W\), % | 8.7 | 26 | 54.5 | 107 | 210 |
Fig. 1. Scheme for measuring moisture content using fast neutrons. 1 — fast-neutron source, 2 — specimen under test, 3 — moderator, 4 — thermal-neutron detector, 5 — fast-neutron detector
The possibility of simulating moisture distributed throughout the volume of wood by means of a water phantom consists in the fact that, in wood and in water, which are close in hydrogen content, slowing down occurs as a result of identical interaction events. Therefore, replacing a homogeneous mixture by a heterogeneous one does not lead to a change in the spectral and angular distributions behind the materials. To check this assumption, experiments were carried out with the water phantom arranged on the source side of the detector. Within the accuracy of the experiment, the results coincided.
The sensitivity of the method depends on the type of wood, the thickness of the specimens, and the overall moisture level. At an overall moisture level of about 15%, with a specimen thickness up to 300 mm, the moisture content is determined with an accuracy of 1.3% when the instability of the apparatus is 5% (including the statistical error). In addition, calibration of the instrument was carried out according to the moisture content of specimens determined by successive weighings of a moist specimen and of one dried at a temperature above 100°C. The measurement results are in good agreement with the results of moisture measurements using phantoms.
To check the influence of the content of impurities with a large absorption cross section for slow neutrons, foils of indium and cadmium were introduced into the measured volume. Introducing the foils does not affect the measurement results. This makes it possible to conclude that the instrument can determine the moisture content of soils and other materials when elements with a large interaction cross section for slow neutrons are present in the object under study.
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