What Is Neutron Activation Analysis?

Neutron activation analysis is a method of neutron bombardment of isotopes of elements in a sample with a certain energy and current intensity. By measuring the energy and intensity of the rays produced by the instantaneous gamma or radionuclide decay (mainly gamma rays) For qualitative and quantitative analysis of elements in a substance. Neutron activation analysis is mainly divided into conventional neutron activation analysis, radiochemical neutron activation analysis, and instantaneous gamma neutron activation analysis. Neutron activation analysis can measure 60 to 80 elements, and the sensitivity of most elements can reach 10-6 to 10-13 grams. Therefore, neutron activation analysis is widely used in geochemistry, space science, environmental science, archeology, life medicine, materials science with its advantages such as high sensitivity, high accuracy, non-destructive, non-reagent blank pollution, and multi-element simultaneous analysis. And forensic sciences. Leads in comprehensive analysis advantages such as multi-element, high accuracy and non-destructiveness.

Chinese name: Neutron activation analysis
Foreign name: Neutron Activation Analysis
English abbreviation: NAA

nickname: Instrument neutron activation analysis
Function: Determine the qualitative and quantitative composition of material elements
Features: Sensitivity and accuracy
Application: Environment, biology, geoscience, materials, etc.

Introduction to Neutron Activation Analysis

Neutron activation analysis, also known as instrumental neutron activation analysis, is a radioanalytical chemical method for elemental and nuclide analysis by identifying and testing the characteristic radiation of a radionuclide induced by radiation in a pattern. The basis of activation analysis is a nuclear reaction. The sample is irradiated with neutrons or protons to cause a nuclear reaction, which activates it to generate radiant energy. The spectrum is measured with a -ray spectrometer, and the sample composition is determined based on the peak analysis; quantification is based on the strength of the radiant energy. analysis. Generally, the neutron source is provided by a nuclear power plant, and the proton source uses a cyclotron or a van der Graaff-type accelerator. Activation analysis is broadly divided into five steps: sample and standard preparation, activation, radiochemical separation, nuclear radiation measurement, and data processing.

Neutron activation analysis schematic This method is characterized by extremely high sensitivity, high accuracy and precision; a wide range of measurable elements, all elements between 1 and 83 atomic numbers can be measured, and the function of simultaneous determination of multiple components, in the same In the sample, 30-40 elements can be measured simultaneously. Therefore, it is suitable for simultaneous analysis of multi-elements in environmental solid samples, such as the measurement of metallic elements in atmospheric particulate matter, industrial dust, solid waste, etc. Because the instrument is expensive, the analysis cycle is long, and the operation technology is more complicated, at present, it is still rarely deployed in China. It is one of the more sensitive multi-element simultaneous analysis methods for atmospheric particulate matter, and is widely used in foreign environmental monitoring.

In the GSR test, NAA is used to test the presence and quantity of antimony and barium on the shooter's arm. These elements are the constituents of the bottom fire of many bullets. If there are high concentrations of these elements in the hand, it means that the gun has been recently fired. Because antimony and barium may also be present in the hands of people who have not fired, the basis of NAA is to verify the presence of GSR by examining a much larger number of elements than normal. Some laboratories have collected and recorded the levels of these elements in normal human hands. This technical method uses the technique of wiping or washing to extract residues from the hands. Its main drawback is the need to add research reactors.

A brief history of neutron activation analysis

In 1936, Hungarian chemists Hevisi and H. Levi irradiated a yttrium oxide sample with a radium-beryllium neutron source (a neutron yield of about 3 × 10 neutrons per second) and reacted with Dy (n, ) Dy (The cross-section of the activation reaction was 2700 targets (en), and the half-life of the nucleation Dy was 2.35 hours.) The tritium was measured, and the quantitative analysis result was 10 g / g, completing the first neutron activation analysis in history.

Classification of neutron activation analysis

According to the energy of incident neutrons, it is usually divided into:

Cold neutron activation analysis

Thermal neutron activation analysis

Superthermal Neutron Activation Analysis

Fast neutron activation analysis

Neutron activation analysis is divided into:

Instant Gamma Neutron Activation Analysis

Conventional neutron activation analysis / instrument neutron activation analysis

Radiochemical neutron activation analysis

Cold neutron activation analysis: The incident neutron is a cold neutron activation analysis. Cold neutrons usually need special cold source facilities. Generally, liquid helium is used to cool hot neutrons to reach neutrons close to a single wavelength. Such facilities are few in the world and are usually located inside nuclear reactors. Domestic China Advanced Research Reactor (CARR) and Mianyang Research Reactor (MYRR) have established cold neutron sources.

Thermal neutron activation analysis: The activation neutron energy range is generally in the range of 0.025eV-1eV, which is also the most widely used neutron activation analysis. Thermal neutrons usually refer to certain neutrons.

Ultrathermal neutron activation analysis: The activation neutron energy range is generally 1eV-1MeV. Superheated neutron activation analysis alone is less used. In practical applications, the sample to be analyzed is usually placed in a 1 mm thick metal cadmium (Cd) box for neutron irradiation. Since cadmium has a very high absorption cross section for thermal neutrons, the role of the cadmium box is to absorb thermal neutrons. Neutrons that pass through a 1 mm piece of cadmium are called superthermal neutrons.

Fast neutron activation analysis: Activation analysis with incident neutron energy exceeding 3MeV. Fast neutron activation analysis mainly focuses on (n, p) and (n, a) nuclear reactions, and some (n, n '), (n, 2n), and (n, ng) nuclear reactions also occur. Usually, the reactor's fast The neutron or DT neutron source produces 14.7MeV neutrons.

Instantaneous Gamma Neutron Activation Analysis: An activation analysis performed by directly measuring the instantaneous gamma produced by a (n, g) reaction. The advantage of instant gamma is that the analysis speed is fast, and online analysis can be performed. The sensitivity of elements such as hydrogen (H) and boron (B) that are difficult to analyze by measuring decay gamma is very high. The disadvantage is that it is used for instant gamma. The neutron fluence rate of activation analysis is very low, so it is not as sensitive to most elements as conventional neutron activation analysis.

Conventional neutron activation analysis / instrument neutron activation analysis : Usually refers to the sample to be analyzed is directly sent to the reactor for neutron (thermal neutron, superthermal neutron, fast neutron) irradiation, and then cooled for a certain time Decay), and the samples are directly packed for measurement and analysis using a high-purity germanium detector. It is often called conventional neutron activation analysis or instrumental neutron activation analysis.

Radiochemical neutron activation analysis: Samples need to be chemically processed for neutron activation analysis. Sometimes the content of the test element in the pattern is very low or there are other factors that interfere with it. In this case, the test element needs to be separated from the sample by chemical treatment or the main interfering elements must be removed. In order not to introduce pollution during chemical treatment, people usually first irradiate the sample to be tested with neutrons to change the element to be analyzed into radioactive elements. Radiochemical separation is then carried out during the chemical treatment by adding non-radioactive carrier elements. This improves the sensitivity and accuracy of the analysis.

Neutron activation analysis principle

Neutrons are electrically neutral, so when a sample is irradiated with neutrons, there is no Coulomb repulsion between the neutron and the target nucleus, and the interaction between the neutron and the nucleus usually occurs through the nuclear force. Nuclear force is a short-range force, with a working distance of 10fm, which shows a strong attraction. When the neutron approaches the target nucleus to 10fm, it is captured by the target nucleus due to the action of the nuclear force to form a composite nucleus. The compound nucleus is generally in the excited state (indicated by *), and its lifetime is 10-12 to 10-14 seconds. It demagnetizes in various ways and can be expressed by the following formula: When a neutron collides with a target nucleus, there are three modes of action: Elastic scattering. The sum of the kinetic energy of the target nucleus and the neutron does not change before and after the scattering effect. This mode of action cannot be applied to the activation analysis. Inelastic scattering. If the sum of the kinetic energy of the target nucleus and the neutron is different before and after the action, Then this energy difference causes the excitation of the composite nucleus, which causes inelastic scattering. At this time, the heterogeneous nuclei that generate the nuclei are the target nuclei. Some of the characteristic radiations of the heteronuclide can be measured by the detector. This mode of action can be used for Activation analysis; nuclear reaction, if the target nucleus captures neutrons to form a composite nucleus and emits photons, it is called a neutron capture reaction, that is, (n, ) reaction. This is the main reaction used in neutron activation analysis. In addition (n , 2n), (n, p), (n, a) and (n, f) reactions can also be used for neutron activation analysis.

Express The radioactivity generated by a neutron irradiated sample depends on the following factors: the content of the element in the sample, strictly speaking, the content of a certain isotope that produces a nuclear reaction element; the note of irradiated neutrons the activation cross section of the element to be measured or one of its isotopes on neutrons; irradiation time, etc. (See activation analysis)

Characteristics of neutron activation analysis

Analysis of many elements : theoretically 80 types of elements can be analyzed, in fact, a pattern can generally measure 40 to 50 elements

High sensitivity : 10 to 10g for most elements

Non-destructive : The general style does not need to be destructive, but can be directly sent to the reactor for irradiation and then measured and analyzed.

Matrix independence : Because of the strong penetration of neutrons and gamma, generally it has little to do with the type of matrix. However, the pattern must not affect the safety of the reactor during the irradiation process, such as liquid, gas and other patterns need to be processed safely.

High accuracy : Due to strong neutron and gamma penetrability, the energy resolution of the high-purity germanium detector is very high, the gamma spectrum is clear, and the relative method and the K0 method are used for quantification, and standard materials are used for quality monitoring . At present, the advantages of non-destructive high-accuracy multi-element analysis are still beyond the reach of other analysis methods.

Wide sample size range : submicrogram to kilogram can be analyzed.

Low pollution : Because the pattern can be directly irradiated and measured, there is no loss and no blank reagent effect.

Automation : It can perform automatic irradiation, automatic measurement and automatic analysis.

Disadvantages : It requires a reactor, which is radioactive, and the pattern requires fractional measurement of nuclide with different half-lives. The analysis period of most elements is longer.

The NAA method is particularly suitable for elemental analysis in archeology. Compared with other elemental analysis methods, it has many advantages:

One is high sensitivity, high accuracy and precision. The sensitivity of the NAA method to more than 80% of the elements in the periodic table is generally 10-6-10-12g, and its accuracy is generally ± 5%.

Second, multi-element analysis, which can give the content of dozens of elements to a sample at the same time, especially trace elements and trace elements, can provide information inside and outside the sample at the same time, breaking through the shortcomings of many technologies limited to surface analysis .

Third, the sample size is small, which is a non-destructive analysis. It is not easy to stain and is not affected by reagent blanks. In addition, the instrument has a simple structure, convenient operation and fast analysis speed. It is suitable for rapid batch automatic analysis of similar cultural relics. The disadvantages are that it cannot detect elements and content that cannot be activated by neutrons, and it cannot measure elements with short half-lives. In addition, detection equipment is also expensive.

Some disadvantages of neutron activation analysis are as follows:

1. In general, only the content of the element can be given, and the chemical form and structure of the element cannot be determined.

2. Sensitivity varies from element to element and varies greatly. For example, neutron activation analysis has very poor sensitivity to lead and high sensitivity to elements such as manganese and gold, which can differ by up to 10 orders of magnitude.

3. Due to the statistical nature of nuclear decay and its counting, neutron activation analysis has unique analytical errors. The decrease in error is not linear with the increase in sample size.

Development Trend of Neutron Activation Analysis

The first neutron activation analysis was introduced in 1936 by Hungarian chemist Hevesy and others. They successfully measured Y with Ra + Be neutron source through Dy (n, g) Dy reaction and gas ionization detector. _The content of Dy in ₂ O ₃ is about 0.1%. With the development of NaI detectors (1948) and reactors (1951), the number and sensitivity of neutron activation analysis have been greatly improved. In the 1960s, when the first high-resolution Ge gamma spectrometer and computer-based neutron activation analysis came out, neutron activation analysis was even more sensitive, highly accurate, non-destructive, and free of reagent-free pollution and The advantages of simultaneous multi-element analysis have become the stars in the field of element analysis. It is widely used in the fields of geochemistry, space science, environmental science, archeology, life medicine, materials science and forensic science.

At present, the development of neutron activation analysis has two main aspects: First, high-sensitivity, ultra-trace and high-precision multi-element analysis based on reactor neutron sources. It is mainly used in the research of the impact of trace impurities on the performance of high-tech materials, the fixed value analysis of micro-analytical reference materials, and the analysis of 2D / 3D microbeam multi-element distribution. On the other hand, it is an on-site industrial on-line detection analysis based on isotopic neutron sources. It is mainly used for quality monitoring of industrial processes and on-site item composition detection, such as real-time batch monitoring of cement production, online inspection of coal quality in coal-fired power plants, underground mineral resource exploration, and rapid detection of hidden dangerous goods.

Extending from simple elemental analysis to determination of chemical states: With the expansion of the application field of neutron activation analysis, it is not only necessary to determine the content of elements in samples, but also to study the distribution and state of elements in depth. For example, when analyzing trace elements in water in environmental science research, an ultrafiltration pretreatment is added to decompose the water sample into low-molecular-weight components, colloids, pseudo-colloids, and particulate matter, and the neutron activation method is used to determine the Element content.

Application of instantaneous analysis: conventional neutron activation analysis cannot take advantage of the high nuclear reaction cross section to generate stable nuclear reactions, such as 113Cd (n, ) 114Cd (reaction cross section is 2 × 104 targets); Activation analysis can overcome this difficulty. The instantaneous method can be used to determine elements such as silicon, sulfur, copper, cadmium, and mercury in river sediments, all of which are difficult to determine by conventional neutron activation analysis.

Wide applications of computers: Since the 1970s, samples for neutron activation analysis have become increasingly complex, such as atmospheric particulates in environmental science, biological tissues in life sciences, meteorites in geochemistry, ceramics and porcelain in archeology, etc. It is required to provide the content of dozens of elements in hundreds of samples at the same time. The computer is used in conjunction with the automatic activation analysis device to control the irradiation time, cooling time, counting time, sample transport, analysis operation and data processing.

Application of Neutron Activation Analysis

Neutron activation analysis is mainly used in archeology to measure trace elements and trace elements in samples such as ceramics, glass, silver coins, copper mirrors, vermiculite, bone fossils, etc., and perform statistical analysis to find commonalities and differences to determine The evolution of elemental composition, origin and mineral sources. The elemental composition of ceramic soils in different regions is different, especially the difference in trace and trace element composition is greater than their fluctuations in different parts of the same clay source. Taking the study of ancient porcelain in China as an example, the ancient porcelain raw materials were collected on the spot. The trace elements contained in them are not many, and generally do not affect the quality of porcelain. The neutron activation analysis not only confirmed the ancient porcelain kiln kiln system of trace elements in ancient porcelain, but also analyzed the sources of porcelain clay in ancient kiln everywhere. The distribution of element content in enamel illustrates the difference in raw material formula. More importantly, the measurement data of neutron activation analysis were used to establish

Neutron activation analysis Quantitative element characteristic pedigrees, porcelain characteristic pedigrees are formed due to different ingredients, and the elements and content contained in them are significantly different. For example, the characteristics of the Song Dynasty and Ming Dynasty in Longquan Kiln Celadon in Zhejiang are very different. Foreign scholars have accumulated a lot of data using neutron activation analysis technology. Based on the accumulation of a large amount of data on ancient ceramics, the regional characteristics of trace and trace elements and their contents were selected, and modern ceramics and source clay were compared to further infer the year and firing place of ancient ceramics. China also uses this method for research.

Neutron activation analysis is used to measure material composition during cement production. Especially suitable for raw material batching and pre-homogenization yard. Without sampling, directly measure the bulk materials passing through the belt, give the composition results in real time, participate in automatic control, and improve the product qualification rate.