(Institute of Geophysical Exploration, Ministry of Geology)

I. Principles and Methods

Different nuclei have different absorption effects on neutrons, and the effective thermal neutron capture cross section of boron nuclei is several orders of magnitude larger than that of main rock-forming elements. Therefore, when the neutron flow passes through the boron-containing sample, its intensity is weakened, and the degree of weakening depends on the boron content in the sample. This physical phenomenon provides the possibility of determining boron content in rock samples by neutron absorption. Because the neutron characteristics of the nucleus have nothing to do with its chemical state, and the neutron characteristics of the boron nucleus are very significantly different from those of other rock-forming elements, it is not necessary to go through chemical treatment to determine the boron content of the sample by this method.

This method uses a PO-BE neutron source, the maximum energy of which is 1 1 MeV, and the energy of most neutrons is between 3 and 3-5 MeV. The energy of neutrons in PO-BE neutron source is 106 ~ 109 times higher than that of thermal neutrons. In order to obtain thermal neutrons, fast neutrons must be slowed down, and hydrogen-containing substances (such as water and paraffin) can be used as good moderators for neutrons. In this experiment, paraffin wax is used to slow down the neutron.

In neutron absorption method, the absorption of thermal neutrons by substances is mainly studied. When the slow thermal neutron flow passes through the sample, some of it is absorbed. The neutron capture cross sections of various nuclei vary greatly, which may be as low as 10- 1 or as high as 103. It can be seen from the table 1 that the neutron capture cross section of the main rock-forming elements is very small, generally not exceeding 65438+. In addition, some rare earth elements have larger neutron capture cross sections. Usually, the contents of Hg, Cd and rare earth elements in boron ore are very small, so it is certain that the absorption of neutrons by boron-containing rocks is mainly determined by boron content, so the boron content can be determined by measuring the absorption of thermal neutrons by rocks.

Table 1

Regarding the influence of the change of chemical composition of rocks (except boron) on neutron absorption, for the convenience of comparison, the absorption of thermal neutrons by elements or rocks is calculated as the equivalent percentage of boron (or B2O3):

Zhang Yujun on new methods of geological exploration.

Where, CB- equivalent boron content, CI- actual content of an element, neutron capture cross section of σb- boron nucleus, neutron capture cross section of σ I-I-I element, atomic weight of a b- boron and atomic weight of AI-I element.

Table 2

Table 2 is obtained by using the above formula, and the absorption of neutrons by oxygen is ignored in the calculation.

As can be seen from Table 2, among these main oxides, H2O, K2O3, Fe3O4 and Fe2O3 have significant effects on neutron absorption. Using this table, we can estimate the influence of the change of the main chemical composition of rocks on neutron absorption. The content of Fe3O4 in rocks is 50%, which is equivalent to 0.07% of B2O3. The above table can also be used to estimate the total neutron absorption characteristics of rocks, still in the equivalent percentage of boron:

Zhang Yujun on new methods of geological exploration.

Neutron absorption characteristics of several rocks are calculated by Formula (2), as shown in Table 3. It can be seen from the table that only iron has a great influence on the rock composition, and the change of neutron characteristics caused by lithological changes other than magnetite is less than 0.05%B2O3, so it can be considered that the systematic error caused by chemical composition changes other than iron is less than 0.05%. This systematic error can be reduce. If samples are classified according to chemical composition, this goal can be achieved by comparative method. Proper selection of standard samples can also reduce the interference of iron.

Table 3

In the above calculation, only the absorption of neutrons by rocks is considered, and the scattering effect is not considered. Because the neutron scattering cross sections of different elements are different, the change of elements with high scattering cross sections in rocks will also cause certain measurement errors. The interference of H should be considered here and must be eliminated.

Second, the measuring equipment and instruments

The instruments and equipment used in neutron absorption method are relatively simple, mainly including: PO-BE neutron source, neutron source moderator and protective equipment, slow neutron detector and recording instrument.

In the experiment, we used a PO-BE neutron source with the intensity of 3× 106 ~ 6× 106 neutrons per second. The neutron source is placed in a paraffin protective box, which is also used as a moderator. Open a hole in the middle of the protective box and plug it with a 5 cm thick paraffin cover, as shown in figure 1. The neutron flow slows down after passing through the paraffin cover and acts on the sample; Of course, some neutron flows are scattered and decelerated through the paraffin layer around the channel, and then act on the sample. In order to strengthen the protection, auxiliary protective layers are built around and above the source box with paraffin bricks, with a total thickness of 50 cm.

Two BF3 proportional neutron counting tubes for recording thermal neutrons are fixed on a pipe rack made of bakelite and plexiglass to fix the position of the sample tray at the same time. The fourth pipe rack is wrapped with aluminum plate to shield the interference of electromagnetic fields. There are paraffin protective layers at the top and both sides of the pipe rack, and only one channel is opened outward along the direction of the counter tube to replace the sample.

Figure 1 Schematic diagram of measuring device

The sample tray is made of aluminum box, and the cover of the sample tray is made of 2 mm thick plexiglass plate. Powder samples can be flattened and compacted with this plexiglass cover, which can prevent the samples from scattering. During the measurement, the relative positions of neutron source, counter tube and sample should remain unchanged.

The measuring instrument includes slow neutron detector, amplifier, discriminator, cathode output device, scaler, stable high voltage power supply and screen voltage power supply (see Figure 2). Except for the scaler, all the measuring instruments are made by ourselves.

Fig. 2 schematic block diagram of measuring instrument

Neutron detectors are two domestic boron trifluoride proportional slow neutron counting tubes. The negative pulse output by the neutron counter is amplified by two poles and transmitted to the pulse amplitude discriminator. If the output line of the counter tube is very long, it can also be transmitted to the amplifier through the cathode output device. The discriminator is used to eliminate interference pulses related to gamma rays. The negative rectangular pulse output by the frequency discriminator forms positive and negative triangular pulses through difference channel, and the positive pulse is transmitted to the scaler through the cathode output device for recording.

Two batches of neutron counting tubes with different working voltages were used in the experiment, and the working voltages were 3000-3600 volts and 2000-2300 volts respectively. The high-voltage power supply of counter tube adopts high-frequency oscillation circuit, and the stability reaches above 0.2% after long-term operation.

The screen voltage power supply of each vacuum tube is DC regulator, and its long-term working stability reaches 0.3%.

The scaler adopts 64-bit or 10-bit scaler.

III. Testing Technology and Working Methods

Three kinds of samples were determined in the experiment: standard samples, test samples and samples containing interfering elements. All kinds of samples are powder. After the sample is crushed, it passes through 80 ~ 100 mesh, and the sample is weighed with an analytical balance. If the sample is wet, it should be dried in an oven before weighing to remove surface moisture. The sample is generally 100g or 50g.

The standard sample is composed of blank mineral samples and pure boron reagents with different proportions. The so-called blank sample is a non-mineral sample collected in the mining area. Its chemical composition is similar to that of the test sample, but its boron content is very low, which should be lower than the sensitive threshold of this method. The test samples were collected from two mining areas, A and B, and each sample was chemically analyzed to compare the test results. In order to understand the interference of iron and hydrogen, two groups of samples containing interfering elements were prepared with Fe2O3 and Na2CO3 10H2O.

In order to analyze the content of boron in the sample, two values must be determined: n and nn; N is the counting rate of neutrons flowing through boron-containing samples, and Nn is the counting rate of neutrons flowing through blank samples. Use or determine the content of b in the sample.

Before starting the measurement, adjust the instrument normally, pay special attention to maintaining the stability of various working voltages, fix a blank sample as the test sample, and measure the reading of the test sample after the instrument is adjusted. Under the condition of unchanged measurement conditions, the change of the reading on different working days conforms to the attenuation law of neutron source, thus checking the normality of the instrument. Then you can start the analysis.

Put the measured sample into the sample tray, and flatten, compact and cover the sample with plexiglass plate. When changing samples, clean the sample container with a brush to prevent mutual interference. When measuring, put the sample tray into the hole and start reading, and read it twice, each reading is 2 ~ 3 minutes. The relative standard error is: where a is the number of measurements and t is the counting time.

Zhang Yujun on new methods of geological exploration.

If the counting rate is n= 10000 pulses/minute, the relative standard error under the above conditions is 0.5%. According to the error theory, 95.5% of the readings will be in the range of n 2 σ. Therefore, the repeatability of required readings is generally 65438 0%, and the maximum is no more than 2%.

During the measurement, blank samples are measured every hour to check whether the instrument works normally, and an average of ten samples can be measured every hour.

Fourth, the test results

The absorption of neutron flux increases with the increase of boron content, and the decay rate of neutron flux decreases with the increase of boron content, and the relationship between them is nonlinear, as shown in Figure 3. The sensitivity of this method is higher for low content samples than for high content samples. The test results show that the function is between 0.03% and 3% at B2O3%, which is close to a linear function. Therefore, when analyzing, the standard curve of high-content samples can use ordinary coordinates, while the standard curve of low-content samples should use double logarithmic coordinates.

Fig. 3 Standard curve

The sum calculated according to the measurement results is a relative value and does not change with the attenuation of neutron source. Because the attenuation of neutron flux has the same influence on Nn and N values, the data of different measurement dates can be explained by the same standard curve, and neutron source attenuation correction is not needed.

The interference test results of Fe and H on the determination of boron by neutron absorption show that the existence of Fe and H reduces the neutron counting rate, where n represents the reading of undisturbed element samples, and n? Indicates the reading of a sample containing interfering elements, and indicates the percentage of neutron flux weakened by the interference elements. The interference of iron increases with the increase of iron content and is stable in the range of Fe2O3 content 10% ~ 30%. When the content of Fe2O3 exceeds 30%, the interference of Fe increases rapidly with the increase of Fe2O3 content. When the content of Fe2O3 in the sample is 50%, the interference is 2.5%, which is equivalent to 0. 19%B2O3. When the content of Fe2O3 is less than 10%, the interference is small, < 1.2%. Therefore, according to the content of Fe2O3, the samples can be divided into three categories: Fe2O3 content is 0 ~10% respectively; 10~30%; 30~50%。

The interference of crystal water increases with the increase of its content, but the growth rate decreases with the increase of its content. The change of water content in samples has a very significant impact on the method, especially when analyzing exogenous boron samples.

In order to eliminate the influence of changes in chemical components such as Fe and H, samples with similar components to the tested samples should be selected as blank samples. The change of chemical composition of samples in each specific mining area has certain regularity, so representative blank samples can be used to make standard samples to reduce the interference of chemical composition change on the method.

In the experiment, the standard samples prepared from seven blank samples in two mining areas were tested. The seven groups of standard curves are in good agreement, all within the allowable measurement error range, and are very similar to the standard curves prepared from quartz powder. This shows that as long as the blank sample is properly selected, its value is only related to the boron content in the sample, but has nothing to do with the chemical composition of the sample, because the change of chemical composition has the same effect on N and N. The above results show that the same quartz powder standard curve can be used to explain the analysis results of these two mining areas.

Experiments show that this method is little influenced by the change of sample thickness and does not exceed the allowable measurement error range. Because the samples taken in the measurement have the same weight, the influence of thickness change caused by the change of sample specific gravity on the measurement results can be ignored.

The test results of samples with different weights are shown in Figure 4. The standard curve of 100g sample is intrinsically related to that of 50g sample. If the abscissa is changed to the absolute content of B2O3, the curves 1 and 2 in Figure 4 can be completely repeated. This result shows that the tested samples don't have to have the same weight, so long as the standard curve is changed to the absolute content of B2O3, samples with different weights can also use the same standard curve. The accuracy and sensitivity of this method are related to the absolute content of boron trioxide in the sample. After the sample weight is reduced, the accuracy and sensitivity are also reduced. Therefore, the weight of high-content samples should be equal to 50 grams, and in order to improve the accuracy and sensitivity of the method, it is best to use 100 grams of samples when analyzing low-content samples.

Fig. 4 Standard curves of samples with different weights

Under the above technical conditions, the sample of 100g was irradiated with a PO-BE neutron source of 1-2 Curie, and recorded with two BF3 proportional neutron counting tubes. The sensitivity threshold reached by this method is 0.005%B or 0.0 17%B2O3.

The production samples from two mining areas were determined, and the accuracy and precision of the method were tested.

The results of neutron absorption method and chemical method are well compared, as shown in Figure 5, which proves that neutron absorption method is accurate and reliable for boron determination. Only the analysis results of individual samples can not be well compared; Using the "addition method", that is, adding a known amount of boron to the sample and repeatedly measuring it, it is proved that the data with significant differences between the two methods are caused by chemical analysis errors. The comparison results of external inspection show that the coincidence rate of the two methods is 10% for the samples with B2O3 content > 1%, and 20% for the samples with B2O3 content < 1%.

Comparison chart of citrus and fruit analyzed by two analytical methods.

The accuracy of this method is verified by repeated measurement (internal inspection) of neutron absorption method for more than two times, as shown in Figure 6. Repeated measurement results have good repeatability; For the samples with B2O3 content > 1%, the accuracy of the measurement results is 3. 16% in terms of relative mean square deviation. For the samples with B2O3 content less than 1%, the accuracy of the measurement results is 9.22% in terms of relative mean square error.

Fig. 6 Comparison chart of repeated analysis results

Verb (abbreviation of verb) conclusion

The measurement method of neutron absorption boron has been successfully tested, and the self-made measuring instrument can meet the requirements of the method. The sensitivity threshold reached is 0.005%B or 0.0 17%B2O3. When B203 content is greater than 1%, the accuracy of the method is 3. 16%, and when B203 content is less than 1%, the accuracy of the method is 9.22%. This method has high determination efficiency, and each instrument can determine 80 samples in 8 hours every day. This method has low cost and can replace the boron chemical analysis of boron ore samples.

refer to

[ 1]Осгроумов Г.В.,Нейтронвыймегодаваяиза на бор руд скарнового типа.《развеэка·неэр》,no5, 196 1

[2]Хрисгианов В.К.,Установка НИХ-2дляопределениясодерх анидбора в образцах горных пород.《Передовойнаучно-техническийи производственныйопыт》 1959 г.

[3]Христианов В.К.,Панов Г.И.,Определение сопержаниябора в горных поропах метопом нейтронногоанапиза.《ж.аналцмцческоüхцmu》, 12,no 3 1957 r

[4]Христианов В.К.,Панов Г.И.чернова А.А.,Определение содержаниябора нейтронным мегодом вполевых условиях.《геохцмця》,no2, 1957г。

[5]Якубοвич А.А.，установкатипа《нейгрон》дляопрелеленияконцентрацииборавобраздахгорныхnoрол《ьюллеемньнаучно-мехнцческоüцнформаццu》,no 3，(20)， 1959 г。

НЕЙТРОННЫЙ МЕТОД ОПРЕДЕЛЕНИЯ СОДЕРЖАНИЯ БОРА В ОБРАЗЦАХ ГОРНЫХ ПОРОД

Чжан Юй-цзюнь

(инONU mymразве？ очноüzеофцзцλцмцнцсцсерсмваzеолоzццкнр)

Резюме В данной работе привелены результаты исслелований нейтронного методаопределениябора в образцах горныхпород.Основой метояаявпяетсяяислользованиев ысокогосеченияпоглощенидмедленных нейтроновяпрами бора.Длдпроведенидопыга была изготовленаспециальнаяустановка, в которую входят полонево-бериллиевый источник нейтроно вактивностью 106Нейтр./сек.、 парафиновыйзамедлигель·прибордлярегистрациимедленныхнейтронов.вкачестве·применяются·пропорциональныхсчётчика、 наполненныхтрехфгористым·оботащеннымСушность методики анализазаключаетсяв относительных измерениях степенейуменышенияинтенсивности нейтронного потокапр ипропусканиимедленых.

Journal of Geophysics, 1962, №. 1.