CN201518007U - High-efficiency probe of lithium-6 glass scintillator neutron soil moisture meter - Google Patents

Abstract

The utility model relates to the field of soil physics, in particular to a high-efficiency probe of a lithium-6 glass scintillator type neutron soil moisture meter. The probe includes a shell, a neutron source, a lithium-6 glass scintillator detector, a photomultiplier tube, a preamplifier circuit, a cable joint and a cable, wherein the neutron source is placed in a lead box, and the lithium-6 6 Glass scintillator detectors are set 1 cm away from the neutron source. The neutron soil moisture meter of the utility model adopts the cubic regression curve instead of the original linear regression equation, which improves the accuracy of the measurement. Compared with the traditional design, the intensity of the radioactive isotope neutron source is reduced by half, and the protective container And the weight of the whole machine is reduced to one-third of the original, the volume is reduced by half, and the cost of the neutron source is reduced by one-third. Due to the reduction of the intensity of the neutron source, the level of radioactive management is lowered, and transportation, storage, waste The difficulty of source treatment is also reduced. In addition, the volume and weight of the instrument are reduced, and the operation is more convenient.

Description

High-efficiency probe of lithium-6 glass scintillator neutron soil moisture meter

technical field

The utility model relates to a design method of a lithium-6 glass scintillator type neutron soil moisture meter probe, which can be used for rapid and accurate measurement of unsaturated moisture in the soil within tens of meters underground, and can improve the measurement efficiency by 50% compared with the traditional design , belonging to the category of soil physics.

Background technique

In agriculture, forestry, hydrology, meteorology and many other fields, it is hoped to know the soil moisture content quickly and accurately. Therefore, many methods of measuring soil moisture content have been developed, such as resistance method, capacitance method, electromagnetic wave method, etc., but most of them need to dig holes to bury sensors, or take samples back to the laboratory for measurement, which destroys the original structure of the soil on the one hand. It is impossible to restore the original texture of the soil and the original movement conditions of the soil moisture, which will cause errors in the measurement results. The measurement conditions are also affected by the differences in the soil itself, and the buried pits dug cannot be too deep. There is nothing you can do. In addition, many different probes are required to work at the same time, and the differences in the probes themselves will also cause considerable inconsistency in the final results of the measurement. Many parts of the probe will be corroded and damaged, which will cause many measurement errors in the measurement. Generally, long-term observations up to decades cannot be carried out, and the accuracy of the measurement is guaranteed to remain unchanged. The measurement of soil moisture content is the result of comprehensive statistics of multiple measurement points and a large amount of measurement data. The measurement data of a small number of points cannot represent the soil moisture content in the whole field, and the data of soil moisture content can be collected quickly, in large quantities and accurately. In order to know the real water content of the soil more accurately, and to have a correct conclusion on irrigation, weather forecasting, crop growth status evaluation, etc. The principle of neutron deceleration to measure soil water content is called the neutron method to measure water. Its measurement principle is that the isotope neutron source will automatically and continuously emit high-energy fast neutrons. When these fast neutrons meet with the surrounding medium , it will collide and reduce energy and decelerate. According to the law of conservation of momentum in ordinary physics, for example, when a moving ping-pong ball hits an iron ball, the ping-pong ball only changes its direction of motion and its speed changes very little, that is, the energy The loss is very little; but when a moving table tennis ball hits a stationary table tennis ball, it will cause the stationary table tennis ball to move forward and stop itself, that is, the energy loss is very large. The mass of a hydrogen atom is very close to that of a neutron, so when a high-speed moving fast neutron collides elastically with a hydrogen atom, its energy drops sharply, and after an average of ten collisions, it decelerates and becomes a thermal neutron with low energy. At the front of the neutron moisture meter, there is an isotope neutron source, which can continuously emit fast neutrons. When the hydrogen content (that is, water content) in the soil is large, the fast neutrons are quickly slowed down into heat. Neutrons, these thermal neutrons tightly surround the neutron source like a cloud: if the soil moisture content is small, neutrons have less chance of being collided and can move to a slightly farther place, so the density of the neutron cloud sparser, and vice versa. In the vicinity of the neutron source, a thermal neutron detector is installed, which can measure the thermal neutrons entering its volume, so that there is a definite relationship between the thermal neutron count and the soil moisture content, through calibration , we can calculate the soil water content from thermal neutron counting. The neutron soil moisture meter does not need to take samples when measuring, just put the probe through the measuring conduit to the required measurement depth, the bottom of the conduit is completely sealed, groundwater and rainwater will not enter the conduit, fast neutrons pass through the conduit The wall enters the soil, and the thermal neutrons that have lost energy pass through the conduit wall and are measured by the detector. The water content value of the soil layer can be obtained in about half a minute, which is fast and accurate. Using a suitable neutron source and detector, the measurement The efficiency will not change noticeably for decades, suitable for long-term measurement, the sounding depth is not limited, and the water content value can be read directly. All measurement data comes from the same measurement probe, and the measurement efficiency of this probe will not change for decades, which avoids systematic errors caused by the inconsistency of multiple sensors. "Neutron Moisture Meter", Liu Shengkang, Atomic Energy Press, 1992. "Neutron Physics", Liu Shengkang, Atomic Energy Press, 1986. "Neutron Source and Its Application", Yuan Hanrong, Science Press, 1978.

Lithium-6 glass scintillator is a solid detector, which has the characteristics of high measurement efficiency, good layer resolution, extremely long service life, and is not easy to damage. It is an ideal sensor for neutron soil moisture meters, but isotope neutron sources will be associated with A large number of low-energy gamma rays, and there is an optimal linear distance between the measurement count and the soil moisture content, that is, there must be a certain distance between the neutron source and the detector, and the measured thermal neutron count and soil moisture content Only when there is an approximate linear relationship between them can the soil moisture content be calculated by a simple linear equation.

In the age of underdeveloped computer technology, due to the limitation of single-chip microcomputer performance, so-called smart instruments can only perform some relatively simple calculations, so the linear relationship between the two parameters of the sensor is emphasized, otherwise there will be many difficulties in programming and operation. To meet the linear requirements of the measurement response and reduce the influence of the gamma rays accompanying the neutron source, the neutron source and the Li-6 glass scintillator detector must be separated by about 6 cm, so that although the measurement linearity is better, the measurement efficiency is lower. In order to achieve a faster measurement speed, the intensity of the neutron source can only be increased, which increases the volume and weight of the protective container, and increases the difficulty in the management of the radioactive source. "Neutron Soil Moisture Meter", Department of Physics and Chemistry, Jiangsu Academy of Agricultural Sciences, Soil, 1978.2. "Neutron Moisture Meter and Its Application in my country", Liu Shengkang, Nuclear Technology, 1982.6.

The neutron source is a kind of radioactive source, which will continuously emit harmful rays to the human body. In order to ensure the health of the operator, special high-hydrogen-containing materials and a mixture of neutron-absorbing elements must be used to reduce the energy of these rays and Absorbed, and finally the radiation dose received by the operator is lower than the dose standard stipulated by the state. To achieve this, there are generally two options: reducing the intensity of the neutron source and increasing the thickness of the protective body. The latter will greatly increase the volume and weight of the whole machine, which will bring great inconvenience to transportation and use.

Utility model content

The utility model can solve the contradiction between the measurement response linearity and the measurement efficiency of the Li-6 glass scintillator detector. As a result, the intensity of the radioactive source is reduced, the volume and weight of the instrument are reduced, and the instrument is more efficient under the premise of maintaining the same measurement efficiency. practical. The neutron soil moisture meter designed and manufactured according to the utility model can still achieve the same measurement efficiency and measurement accuracy as the original design when the radiation source is reduced to half of the original one.

The high-energy neutrons emitted by the isotope neutron source collide with the elements in the measurement medium to decelerate. Therefore, in the measured medium, there is a thermal neutron cloud centered on the neutron source, dense in the middle and sparse at the edge. If the lithium-6 glass scintillator is placed as close as possible to the neutron source, more thermal neutron counts can be measured, but at the same time, low-energy gamma rays will cause background counts to increase and nonlinear problems in measurement. The utility model adopts The scheme is:

A high-efficiency probe for a lithium-6 glass scintillator type neutron soil moisture meter, comprising a housing 1, a neutron source 3, a lithium-6 glass scintillator detector 4, and a photomultiplier tube 5 arranged on a lithium-6 glass scintillation Between the body detector 4 and the preamplifier circuit 6, the shell 1 is provided with a cable joint 7 and a cable, wherein the neutron source is placed in the lead box 2, and the lithium-6 glass scintillator detector 4 is separated from the Neutron source 1 cm.

The probe of the utility model narrows the distance between the detector and the neutron source to about 1 centimeter, and the neutron source is completely wrapped with a lead box with a wall thickness of 0.5 centimeters to shield the low-energy gamma rays associated with the isotope neutron source. The distance between the neutron source and the detector has been shortened to the minimum, and the count of thermal neutrons can be measured to the maximum, that is, the efficiency of the measurement is improved, and the nonlinear problem caused by the shortened distance between the neutron source and the detector , by changing the original linear regression to polynomial regression, due to the development of computer and single-chip microcomputer technology and the popularization of data processing software, this kind of calculation has become very popular, so that the isotope neutron source can be reduced to the original Half of the time, the original measurement efficiency, measurement precision and accuracy are maintained.

What the neutron moisture meter measures is only the thermal neutron count, but the exact soil moisture content corresponding to this count must be calibrated to know. Calibration is to mark a measuring scale on a certain measuring instrument. For example, the calibration of a thermometer is to put a blank thermometer in ice water, determine the 0C°, put it in boiling water, determine the 100C° point, and then divide the two points evenly into 100 equal parts, so that it can be used It went off to measure an unknown temperature. The calibration of the neutron meter is also similar, but its reference standard value is the soil moisture content measured by the drying method. Among the dozens of measurement methods at present, the drying weighing method is the only standard reference method, and all other measurements The measurement accuracy of the method is compared with it, that is, take a sample from the whole object to be measured, weigh its wet weight, and then put the sample in an oven at 105°C for 4 hours. Then weigh the dry weight, and compare the evaporated moisture with the weight of the dry soil to calculate its weight moisture content. This moisture content value is the standard value of moisture content we believe. However, because this method requires sampling, it will destroy the original state of the soil, the operation time is long, and the calculation procedure is cumbersome. Therefore, various measurement methods have been invented, but their reference standard is still the drying method. We put the probe of the neutron meter into a certain layer of soil, measure a count value, and then use a soil drill to take out a soil sample of this layer of soil around the conduit, and use the standard drying method to measure the soil moisture content, so that we can get a After logarithmic values, and then select soils with different contents to continue measuring and sampling, we can obtain several sets of values, and obtain the regression equation through mathematical regression calculations. In the actual measurement, only the thermal neutron count value is measured, and then the water content is calculated by the regression equation, but in the actual measurement, the relationship between the thermal neutron count and the soil moisture content is not a good linear relationship, so when using linear regression When the equation calculates the water content, some ranges appear to be higher than the actual value, and some ranges are lower than the so-called systematic error of the actual value, which limits the accuracy of the measurement. When designing according to the utility model, due to the detector and neutron The distance between the sources has been shortened to the shortest, and the nonlinear phenomenon between the actual water content and thermal neutron counting is more obvious, so we will the original linear equation (one-order equation) Y (volume water content) = aX (thermal neutron counting )+b is changed into a cubic item equation, that is, Y=aX ³ +bX ² +cX ¹ +d. In this way, the regression equation changes from a straight line to a curve, and it can change with the actual measurement results, so that its correlation coefficient Improve, reduce the measurement system error, improve the measurement accuracy, even higher than the accuracy calculated by the linear equation in the original design scheme. Due to the development of computer technology, this secondary regression calculation has become very easy. A set of measured values is calculated through the calculation software to calculate the four values of a, b, c, and d, and input them into the neutron moisture meter. When this instrument When a thermal neutron count value is measured, the single-chip computer in the neutron moisture meter will automatically convert the water content and display it. The current neutron moisture meter can store more than 10 such curves for use in different soils. Choose from.

The neutron soil moisture meter designed and manufactured according to the utility model, compared with the original design method, can reduce the intensity of the radioisotope neutron source by half and reduce the weight of the protective container and the whole machine to One-third of the original, the volume is reduced by half, and the cost of the neutron source is reduced by one-third. Due to the reduced intensity of the neutron source, the level of radioactive management is reduced, and the difficulty of transportation, storage, and disposal of waste sources is also reduced. In addition, the volume and weight of the instrument are reduced, and the operation is more convenient.

Description of drawings

Fig. 1 is a structural schematic diagram of the utility model probe

Among them: 1-shell, 2-lead box, 3-neutron source, 4-detector, 5-photomultiplier tube, 6-preamplifier circuit, 7-connector, 8-cable

Figure 2 is a graph showing the relationship between thermal neutron counts and soil water content

Figure 3 is a linear regression relationship between thermal neutron counts and soil volumetric water content

Figure 4 is a three-time regression relationship between thermal neutron counts and soil volumetric water content

Detailed ways

Design drawing of the present utility model as shown in Figure 1:

1 is an alloy aluminum shell, because aluminum has a small neutron absorption cross-section, high mechanical strength, and corrosion resistance, so it is a suitable material for use. 2. The source box is made of lead. The thickness of the lead box should be more than 5 mm. It confines the low-energy gamma rays associated with the neutron source in the box, reducing the miscounting caused by the superposition of multiple small pulses in the circuit. 4. Lithium-6 glass scintillator detector, which can convert the thermal neutrons injected into the detector into optical signals, and then convert them into electrical signals by 5 photomultiplier tubes, and then record them by the counting circuit. 6 is the pre-amplification circuit of the circuit, which is electronically shielded by the aluminum shell and directly close to the photomultiplier tube, which can reduce the influence of the coupling capacitor and improve the signal-to-noise ratio of the whole circuit. 7 is the cable connector between the probe and the measuring instrument. 8 is a cable.

It can be seen from Figure 1 that the neutron source is close to the detector, with only a layer of lead shielding cover in the middle, so the detection efficiency of thermal neutrons can be increased to the maximum. Therefore, under the condition of keeping the instrument accuracy unchanged Under this condition, the intensity of the neutron source can be reduced to the minimum, the radiation source is small, and the thickness of the shielding layer can be reduced, so the volume and weight of the instrument can be greatly reduced.

The specific calibration method is to take more than 5 soil samples at the same depth of 30 cm of the measuring tube, and then place the probe of the neutron moisture meter at the same depth to measure its thermal neutron count. Use the drying method to measure their standard water content values, and then use the data statistics method to eliminate gross errors to obtain a set of mathematical expectation values of soil water content, and obtain the sample standard deviation of this set of data, so that it can be used at 95% confidence The error of the interval is not greater than 1/4 of the preset error of the neutron moisture meter to ensure the accuracy as a comparison standard. If this requirement cannot be met, the number of samples should be increased or another measurement point should be selected to repeat the above work. After obtaining a set of data, another hole is drilled nearby to bury the pipe, and the remaining measuring points are measured according to the previous method. If the water content is low, appropriate amount of water can be added to increase the water content. The principle is that each point should have a suitable gradient, from low water content to saturated water content, generally there must be more than 5 groups of points.

The following data is the measured data of a field calibration:

     Thermal Neutron Counting and Soil Moisture Calibration Data Sheet

Thermal neutron counting 2102 2874 3156 3956 4415 5421 6351

Soil moisture content 6.1 11.8 16.2 22.6 27.3 32.4 35.7

The graph drawn according to this set of data is shown in Figure 2.

Fig. 3 is a graph showing the linear regression relationship between thermal neutron count and soil volume water content, the linear regression and linear equation Y=0.0072X-7.381, and the correlation coefficient R ² =0.9665.

Fig. 4 thermal neutron count and soil volume water content 3 times regression relationship chart, its polynomial equation is Y=-0.000000000X ³ +0.0000024124X ² +0.0021060199X-6.7706806553, its correlation coefficient R ² =0.9962864065, as can be seen, with The accuracy of polynomial equation processing is better than that of linear equation processing, and the correlation coefficient is also improved.

Claims (3)

1. lithium 6 glass scintillator formula neutron soil moisture instrument are efficiently popped one's head in, comprise shell (1), be provided with neutron source (3), lithium-6 glass scintillation bulk detector (4) in the shell (1), photomultiplier (5) is arranged between lithium-6 glass scintillation bulk detector (4) and the pre-amplification circuit (6), shell (1) is provided with cable splice (7) and cable, it is characterized in that neutron source is placed in the plumbous box (2), lithium-6 glass scintillation bulk detector (4) is apart from 1 centimetre of neutron source.

2. lithium 6 glass scintillator formula neutron soil moisture instrument according to claim 1 are efficiently popped one's head in, and it is characterized in that shell (1) is the alloy aluminum shell.

3. lithium 6 glass scintillator formula neutron soil moisture instrument according to claim 1 are efficiently popped one's head in, and it is characterized in that the thickness of described plumbous box will be 0.5 centimetre.

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