(Institute of Geophysical Exploration, Chinese Academy of Geological Sciences)

1. What is neutron activation analysis? What are its advantages?

Neutron activation analysis is a physical analysis method. Its basic principle is that stable isotopes are irradiated by neutrons, and nuclear disintegration produces radionuclides. By measuring the radiation characteristics of radionuclides (such as the energy and intensity of γ and β rays, half-life, etc.), the elements in irradiated samples are qualitatively and quantitatively inferred.

Quantitative measurement can be relative measurement or absolute measurement. Relative measurement is usually used for activation analysis of geological samples. The formula for calculating the content of elements to be measured in the sample during absolute determination is as follows:

Zhang Yujun on new methods of geological exploration.

Where: g is the content of the analyzed element;

M—— the atomic weight of the analyzed element;

A—— measurement radioactivity of reaction products of elements to be measured;

δ-reaction cross section;

φ-neutron flux;

θ-isotope abundance;

N0- Avogadro constant;

T 1- irradiation time;

T2—— cooling time, that is, the time interval from stopping irradiation to starting measurement;

λ-attenuation constant;

; Tl/2 is the half-life;

η-detection efficiency of detector;

α —— Absolute abundance of detection radiation measurement (i.e. decay network factor).

The radiation intensity of active radionuclides is directly proportional to the content of analyzed elements, reaction cross section, neutron flux and isotope abundance.

Neutron activation analysis has the advantages of high sensitivity, simple operation, automatic analysis with a small electronic computer, simple sample preparation and simultaneous analysis of multiple elements. Because of these advantages, neutron activation analysis has developed rapidly in recent years and has been widely used.

Now take vanadium as an example to illustrate the sensitivity of neutron activation analysis. Vanadium has two stable isotopes: and. Its natural abundance is 0.24% and 99.76% respectively. After neutron irradiation, after (n, y) reaction, V50 becomes V5 1 (reaction cross section δ = 250 target). V5 1 is a stable isotope; V5 1 becomes V52 (reaction cross section δ = 4.5 target), and V52 is a radioactive isotope of β decay, with β ray energy of 2.73 MeV, γ ray energy of 1.45 MeV and half-life of 3.8min.. The existence of V52 can be determined by measuring these radioactivity, and the content of vanadium in the sample can be inferred by quantitative determination of 1.45 MeVγ-ray. In the neutron activation analysis of vanadium, when neutron flux is 1× 10 13 neutron /cm 2.s, irradiation is 1 min and cooling is 1 min, and its sensitivity is 2.0×10. Irradiation for 30 minutes, cooling 1 min, and its sensitivity is 3.4×10-2g; When irradiated to saturation, its sensitivity is 2.9× 10- 12g. This sensitivity can't be achieved by other methods.

Neutron activation analysis takes γ -ray energy spectrum measurement as the main means to judge and determine the analytical indicator nuclides of more than 70 elements. The measurement of β particles is seldom used. Sometimes radiochemical purification and half-life determination are needed.

Neutron flux directly affects the sensitivity of neutron activation analysis. It is hoped that higher neutron flux will be used in activation analysis, and the reaction with larger cross section and radioactive products with appropriate half-life (neither too long nor too short) will be selected.

The sensitivity of activation analysis is also related to the complexity of sample components, which often interfere with each other. In the past, chemical separation and coordination were often needed, which was a very slow process. Since the appearance and application of Ge(Li) semiconductor detector, the resolution of energy spectrometer has been greatly improved, which makes it possible to analyze dozens of low-content elements simultaneously through one irradiation with little or no chemical treatment.

Thermal neutrons (produced by reactors, accelerators or isotope neutron sources) and 14 MeV neutrons (produced by T(D, n)He4 reaction of accelerators) are often used in neutron activation analysis. In some cases, 3 MeV neutrons (produced by D(D, n)He8 reaction of accelerator) are also used.

Tables 1 to 3 give the sensitivity or detection limit of single element in thermal neutron, 14 mev neutron and 3 mev neutron activation analysis respectively.

Table 1 thermal neutron activation analysis sensitivity (g)

Table 1 shows the sensitivity (g) of thermal neutron activation analysis. The neutron flux is 1× 10 13 neutrons/cm for 2 seconds. Detected with a 4 ×3 inch NaJ(T 1) crystal scintillation counter. The irradiation time of the sample is 65438 0 minutes and 30 minutes respectively, and the cooling time is 65438 0 minutes. The products of phosphorus (P), lead (Pb), thallium (T 1) and yttrium (y) after (n, γ) reaction are all pure β radioactivity, which are not listed in the table.

Table 2/sensitivity of kloc-0/4mev neutron activation analysis (g)

Table 2 shows the sensitivity (g) of 14 mev neutron activation analysis. The neutron flux obtained by T(D, n)He4 reaction is 1× 10 10 neutrons/cm2.s. It is detected by a 3-inch× 3-inch NaJ(T 1) crystal scintillation counter. When the half-life of the product is less than 1, the irradiation time is three times half-life, the cooling time is one time half-life, and the counting time is three times half-life. When the half-life of the product is greater than 1, the irradiation time is 5 minutes, the cooling time is 1 minute, and the counting time is 5 minutes.

Table 3 Sensitivity of 3 MeV Neutron Activation Analysis (G)

Table 3 shows the sensitivity (g) of 3 MeV neutron activation analysis. The neutron flux is 1× 108 neutron/cm2.s. Carbon (C), nitrogen (N), oxygen (O), sulfur (S) and bismuth (Bi) can't be analyzed, and other elements not listed in the table can't meet the requirements put forward in the minutes of 1960 analysis meeting.

Second, the development of neutron activation method in foreign geological exploration

The application of neutron activation in geological exploration has a history of 20 years from early experiments to now, but the production application time is not long. In recent years, due to the application of new technologies such as semiconductor detectors, new neutron sources, microcomputers and multi-channel energy spectrometers, foreign countries have paid more attention to the development of in-situ neutron activation analysis and achieved remarkable results in production.

195 1 year, American F. E. Senftle started the neutron activation test of silver with Ra-Be neutron source and Geiger counter. 1954, T.C. Vazjieyiqikov (тсвозеников) of Sverdlov Institute of Mining in the Soviet Union began to conduct neutron activation logging tests in copper mines. After 1957, several units of the Soviet Union, such as the Institute of Geophysics of Ural Science Branch, the Institute of Nuclear Physics of Uzbek Academy of Sciences, and the Moscow All-Soviet Institute of Nuclear Geochemistry and Geophysics, conducted frequent activation logging tests on bauxite, manganese, fluorite and copper mines, using Po-Be neutron source, NaJ(T 1) crystal and single-channel energy spectrometer (also useful vacuum tube) However, due to the low flux of the neutron source, the low resolution of the detector and the huge size of the energy spectrometer, although the test results have been reported and it is believed that some coring drilling can be carried out in these mines, these tests have not been reported in production for more than ten years.

Zhang Yujun on new methods of geological exploration.

Around 1964, the Soviet Union adopted neutron activation method, X-ray fluorescence analysis method and neutron beryllium determination method as rapid analysis methods for non-radioactive minerals and non-radioactive elements, and achieved good results, especially after the first symposium on activation analysis was held, which expanded the application scope of nuclear technology in geological exploration.

Around 1968, due to the development of atomic energy industry, nuclear technology, electronic technology and semiconductor technology, the application of neutron activation analysis in geology and exploration has changed obviously. The important new technologies that cause these changes are: the high-resolution Ge(Li) semiconductor detector has been used in geological work, and its resolution is dozens of times higher than that of NaJ(T 1), which is not conducive to nondestructive analysis and exploration. A portable small mineral analyzer was successfully developed by using a small multi-channel energy spectrometer. Using a small electronic computer to process data, the analysis results can be obtained in a few minutes; The application of the new neutron source Cf262 in geology provides new possibilities, which they think is a "great breakthrough". Cf252 releases a large number of neutrons through spontaneous fission, and 0. 1 mg Cf252 generates 2.34× 108 neutrons per second, which is equivalent to a small neutron generator. Cf252 is small in size, low in gamma-ray dose and simple to use. This source can be used for direct determination of various elements in field ores and rock outcrops, direct prospecting by submarine neutron activation, lunar exploration, etc.

Since 1968, foreign nuclear technology has been widely used in geology and exploration, and the United States, Canada and Britain have held professional conferences and published monographs. 1965 and 1968 hold international professional conferences; The Soviet Union held another symposium in Tashkent in 1970, and vigorously promoted neutron activation and X-ray fluorescence analysis. 1968165438+10 in Argentina and1969 65438+in Poland in February, respectively. In the compilation of technical documents of the Subcommittee on Nuclear Technology and Mineral Resources held in Poland on 1969, the Canadian combined flow analysis device of neutron activation and X-ray fluorescence analysis, the application of nuclear technology in the exploration and mining of metallic minerals, the exploration and development of non-petroleum mineral resources, geochemical research and mineral processing flow analysis are introduced. The Philippines also uses neutron activation analysis to circle the mining area of the deposit; India and Spain introduced plans to exploit mineral resources by nuclear technology.

Since 1970, the important progress of neutron activation analysis in geology abroad is that the Ge(Li) probe for well has been successfully developed and tested in the field, and the Ge(Li) probe for logging with an outer diameter of 89 mm and a length of 1600 mm has been installed in Sweden. The volume of Ge(Li) detector used is 22 cm 3, and liquid nitrogen cooling can work 10 hour. The energy resolution of the probe is 2.3 keV (for 122 keV photon) and 2.6 keV (for 133 keV photon). The probe has been used in the ground test and drilling model of Lanstad uranium mine to study neutron activation spectrum. In addition, some progress has been made in submarine geological mapping and neutron activation direct prospecting experiments.

Third, a few examples

1. Neutron activation analysis is a powerful supplementary means for other analysis methods, which can solve some difficulties of other methods in many cases.

(1) microanalysis of precious samples;

Some geological samples, such as meteorites, moon samples and samples collected by mountaineers. The sample size is very small, but it is extremely precious, and the content of some target elements is very low, so it is difficult to analyze this kind of samples routinely.

1970 reports that thermal neutron activation analysis technology is used to analyze trace elements in rocks and meteorites on earth. In this study, the following elements in three kinds of chondrites were analyzed by using Ge(Li)γ energy spectrum measurement and computer data processing to minimize chemical procedures: As, Au, Co, Cs, Ga, Ge, Hg, Mo, Os, re and Sb.

In the same year, a neutron activation analysis method for analyzing 39 elements in precious geological samples weighing only 0.5g was also reported. After irradiation, the samples were divided into 12 groups by chemical method, and the radioactivity was measured by a NaJ(T 1) crystal and two Ge(Li) detectors. The analytical accuracy of half of the 39 elements is above 5%, and the analytical accuracy of elements less than 1/5 is less than 25%.

(2) Comprehensive analysis with little or no chemical separation:

There are many reports on comprehensive neutron activation analysis of geological samples in laboratory, and one of the important research purposes is to reduce or even not use chemical treatment.

For example, in 1970, a neutron activation analysis method is reported to determine 32 elements in rock samples by only one chemical treatment. Seven American standard samples containing various components were measured and analyzed by high resolution Ge(Li)γ energy spectrum. The analysis process is divided into three steps: firstly, short-lived nuclides (10 "~1') are determined, and Sc, Hf, Dr, Mg, Al, Co, Ti, V (and Na) are determined; Then, the activated sample is dissolved in antimony pentoxide aqueous solution to remove Na24, and then the medium-life nuclides are determined to obtain the contents of elements such as K, Cu, Zn, Ga, Sr, Ba, La, Eu, Sm (and Mn). Finally, the long-lived nuclides are determined and the elements such as Sc, Cr, Fe, Co, Zr, Rb, Sb, Cs, Ba, Ce, Eu, Yb, Tb, Lu, Hf, Ta, Th are analyzed. This method uses only one radiochemical treatment for Na24.

(3) Non-destructive determination of rare earth content in rocks:

Because of the difficulty in chemical analysis of lanthanum group elements, many people have studied neutron activation analysis of rare earth groups, and one of these reports is particularly interesting. In 1967, Cobb successfully used Ge(Li) detector to determine the content of lanthanum group in rocks without any chemical treatment. Most of the measured rare earth contents are in good agreement with the data obtained by chemical separation method, and eight different rock types are considered in the experiment.

2. Field mobile laboratory

In order to reduce the round-trip time of sample delivery and solve the difficulty of analysis conditions caused by being far away from reactors, accelerators and central laboratories, mobile neutron activation analysis equipment has been successfully developed abroad in recent years, and the effect is very good. This paper takes a set of neutron activation analysis equipment in Canada that can be transported to the site for use as an example to illustrate this problem.

Canada has a large area, and most minerals are distributed in remote areas. Because it takes too much time to transport samples from remote areas to the reactor site for analysis, it can't meet the requirements of geological work, so a set of neutron activation analysis equipment installed on the vehicle was manufactured in 1966, which is equivalent to a mobile neutron activation analysis laboratory. To 1968, I have been to 25 places and done a lot of analysis. This device is very useful for geological work.

This equipment adopts Sb-Be isotope neutron source, and Sb 124 is 6000 curies. The Sb source is placed in the BE block, with water reflecting layer and 43 cm neutron and γ -ray shielding. The diameter of the whole neutron source is about 1 m, weighing 5 tons, and the neutron flux is 2× 108 neutron/cm2 s, with four irradiation positions. Because the half-life of Sb 124 is only 60 days, the source of Sb 124 should be changed two or three times a year.

This neutron source is installed on a trailer with a length of12m. Other equipment on the trailer includes a generator with power of12kW, an air compressor, a small computer, 1024 channel analyzer, and Ge(Li) with lead shield (inner diameter of 25 cm, thickness of 10cm, total length of1.2m).

The distance from the neutron source to the detector is 9 meters, and the sample is transported by compressed air. The irradiation, measurement and data processing of samples are automatically controlled by electronic computer.

In addition to lead, silicon, phosphorus and sulfur, elements considered important in minerals can also be analyzed with sufficient accuracy.

More than 200 samples can be analyzed every 24 hours. The determination of single element is faster, including silver 7000, gold 200, copper 5000 and zinc 4000. Although this equipment costs an average of $30,000 per year, due to its high production efficiency, the cost of each sample is less than 1 USD, and the cost of each determination is even lower.

Geological applications include:

Determination of uranium and thorium in water 1× 10-8g/g

Gold halo diagram 1× 10-8g/g

Determination of arsenic in ores 1× 10-4g/g

Determination of Hafnium in Zirconium Sand 3× 10-5g/g

Determination of rare earth elements by U*** 1× 10-2g/g

Correlation analysis of rock/kloc-0 /×10-6 ~/kloc-0 /×10-4g/g.

Ore source identification/kloc-0 /×10-7 ~/kloc-0 /×10-3g/g

The determination of uranium in water is to determine the relationship between lakes or rivers and uranium deposits; Zirconium with low hafnium content is very important to the atomic energy industry. Because it is difficult to separate hafnium from zirconium, it is particularly important to find zirconium sand with low hafnium content. Arsenic is an important harmful component. High sensitivity analysis is also very useful for geological structure.

The mobile neutron activation device in the Soviet Union mainly uses the Po-Be neutron source, and the neutron yield is usually 107 neutron/sec, while the flux in the tunnel is even lower, only 102 ~ 104 neutron/cm2 sec. This has an impact on the geological effect.

A neutron device with a flux of 9× 107 neutron /cm2.s was installed in the United States with a source of 12 AM 24 1-CM 242-Be of about 4000 curies. It is reported that 73 elements can be analyzed. F.E. Senftel and R.W.Perkin used Cf252 and Ge(Li) detectors near 1970, and most economically significant elements below 0. 1 ~ 0.0L can be determined within 1 ~ 2 minutes.

3. Ground neutron activation direct exploration.

1964, in order to verify the possibility of direct prospecting by neutron activation method in the outcrop of the earth's surface, USGS installed two automatic neutron activation devices with accelerators, which can generate neutron current with current intensity of 108 neutron /cm2. The whole design can make the neutron flow focus on the ground for irradiation. After irradiation, the NaJ(Tl) scintillation probe can be replaced and connected to the multichannel analyzer within a few seconds. Experiments with this device show that neutron activation technology is possible as a field ground exploration method for at least 30 elements. The main disadvantage of this method is that the penetration thickness is not satisfactory, only about 65438 0 feet. Besides, the equipment is too heavy. 1967 published the test of precious metals and semi-precious metals using the above device. The activation radioactive intensities of Ru, Rh, Pd, Ag, Re, Os, Ir, Pt and Au irradiated by thermal neutrons of 108 /cm2 s are calculated, and the higher ones are Rh, Ag and Ir. The field test of silver was carried out in Aminius mine, and the activation determination was carried out in pyrite enrichment zone. The silver content was about 65438 0.5 oz/ton. The determination of Ag 1 10 is based on its energy (0.66 MeV) and half-life (23.5 seconds). It is estimated that the sensitivity of this method can be improved to 0. 1 ~ 0.2 oz/ton. The measuring time only takes a few minutes. The model and field test of gold are also carried out, but the result is not as good as that of silver. The experiment of precious metals shows that the in-situ neutron activation method of silver and gold is a feasible prospecting method.

The Soviet Union measured F and Cu by ground neutron activation method. The former is to delineate F halo to find fluorite, apatite and rare earth minerals. When the surface is activated, the topsoil is excavated to a depth of 25 cm and a square of 20×20 cm. The neutron source is a PO-BE neutron source with 3-6 curies, and one point is measured every 10 meter. A team of two people can get a score of 100 ~ 120 in each class. The sensitive threshold of fluorine determination is 0.05%. In addition, the F value of mine engineering is determined. In the field ground neutron activation analysis of copper, 0.2% ~ 0.4% of copper can be detected with 2× 107 neutron /s Po-Be source, and the relative error is 10% ~ 15%.

From around 1970, the US Geological Survey has done a lot of research work and tried to use Cf252 for neutron activation on the ground and seabed. In particular, the latter is an important dynamic in the strategy of competing for sea areas.

Translated collection of geochemical analysis methods, 1977.