Brief introduction of the first author: Gao Xiuqing, vice chairman of the Third Artifacts Professional Committee of China Insurance Association, and former senior engineer of China Institute of Atomic Energy.
I. Introduction
With the development of science and technology and the accumulation of social wealth, people's demand for jewelry is increasing. However, natural mineral resources are limited. Due to long-term exploitation, natural gemstone resources are decreasing day by day, and high-grade gemstones with good color, texture and luster are even more scarce. In order to meet the market demand, people have studied how to improve the quality of gemstones. All kinds of defective gemstones with poor color and transparency have been artificially treated by various optimized treatment processes, so that their appearance characteristics such as color, transparency and cleanliness have been improved, and the potential beauty of natural gemstones has been fully displayed, thus improving their aesthetic value and commercial value. At the same time, the natural resources of gemstones are fully utilized.
With the rapid development of gem optimization technology in the world, many countries and departments have set up special research institutions, and have a group of experts and advanced technical equipment, specializing in the research and commercial production of natural gems. And our country needs to strengthen in this respect.
Second, the classification of gem optimization processing technology
The application of gemstone optimization processing technology depends on the characteristics of gemstones and the goals we want to achieve. For example, the improvement of color can be achieved by heat treatment or by multi-process combining radiation and heat treatment. Gemstone optimization processing technology can be roughly divided into five categories.
1. heat treatment technology
Heat treatment technology is the earliest, oldest and most widely used technology. The heat treatment process requires a variety of process conditions, such as ① the selection of different temperature ranges from low to high, from150 ~ 2000℃; (2) Selection of heating and cooling rate, temperature gradient, maximum temperature and constant temperature time; (3) Furnace atmosphere, oxidation or reduction atmosphere or inert gas protection, etc. ④ Vacuum, vacuum control, etc. Ruby, sapphire, aquamarine and blue tanzanite sold in the market are all heat-treated; ⑤ Ventilation and pressurization.
2. Radiation heat treatment technology
Irradiation is to make high-energy particles enter the gem crystal, and through energy exchange, a large number of lattice defects and dislocation atomic defects are produced in the crystal, forming color centers. The energy of the color center is high or low, which forms a mixture of different colors and makes the colors ugly. However, the low-energy color center is unstable, so heat treatment can destroy the low-energy color center, that is, remove the stray color and fix the color beautifully. This is the irradiation coloring and heat treatment fixation technology. This method does not change the physical and chemical properties of the gem itself, but makes the color bright in appearance and improves the transparency to some extent. According to China's national standards, this method belongs to the category of "treatment" in optimization treatment.
At present, many natural gemstones in the international market are artificially processed, which has been recognized by the industry and welcomed by consumers.
In order to make full use of natural gem resources and improve the value of gems, we have carried out research on the optimization treatment technology of gems, mainly heat treatment and irradiation treatment to optimize gem crystals. Different kinds of gems and different goals should be achieved by different technological conditions. Topaz, crystal, emeralds, diamonds, tourmalines, etc. are best treated by this technology. The irradiation technology and main equipment will be introduced in detail later.
3. Chemical treatment technology
By means of chemical reaction, chemical diffusion and chemical precipitation, coloring elements are infiltrated into crystals, or colorants are precipitated in cracks or pores, thus improving their appearance color.
4. High temperature and high pressure diamond bleaching technology
Light brown diamonds are treated into white by special equipment with high temperature and high pressure, and some of them can reach D-color beauty diamonds.
5. High temperature and high pressure discoloration technology of diamond.
Light brown diamonds are processed into colored diamond with bright colors such as yellow, yellow-green, blue-green, blue and pink.
Three, artificial irradiation discoloration method and main equipment
There are many kinds of radiation sources for gem irradiation, which can be divided into: ① γ -ray irradiation, mainly using 60Co radiation source; Accelerators, such as linear accelerators and cyclotrons, are mainly used for high-energy charged particle irradiation; (3) Fast neutron irradiation, which uses the special irradiation device of nuclear reactor for irradiation treatment.
1)γ -ray irradiation: After being treated, the coloring power of the gemstone is weak and the diamond cannot be colored. For topaz treatment, it can only reach sky blue, and its advantage is that it will not induce radioactivity.
2) High-energy charged particle irradiation: Gemstones are optimized with high-energy charged particles (such as high-energy electrons and protons) generated by accelerators. Generally speaking, the higher the energy of charged particles, the better the discoloration effect; If the energy can reach more than 20MeV, it is better as a radiation source, with the advantage of low residual radioactivity. However, due to the narrow beam, long irradiation time and high irradiation cost. Topaz, for example, is generally light blue after being irradiated by high-energy electrons.
3) Fast neutron irradiation: When the fast neutron produced by the reactor irradiates gemstones, due to its high energy and strong penetration, fast neutrons (E≥ 1MeV) can cause lattice damage, produce a large number of point defects, appear vacancies and dislocated atoms, form new color centers and change colors, so the improvement effect is ideal. The thermal neutrons in the reactor bombard the gem crystal at the same time, and the trace impurity elements in the gem will produce (n, γ) nuclear reaction, which will be "activated" to generate artificial radionuclides with different half-lives. When using reactor neutron irradiation to treat gemstones, the improvement effect is good, but thermal neutrons also induce a certain amount of residual radioactivity. In order to reduce the activation of impurity elements in gemstones, special thermal neutron shielding devices should be adopted to reduce the radioactivity of gemstone crystals after irradiation as much as possible. This method has been successfully studied and applied to the irradiation modification of topaz, and its discoloration effect is very ideal.
Fourth, heat treatment after irradiation
Take the irradiation treatment of topaz as an example: the color of topaz crystal after irradiation is not single, and many colors can be observed, such as gray blue, blue-green, light brown, dark brown, light brown, dark brown, etc., and occasionally purplish red and orange. The purpose of heat treatment is to eliminate the noise, make the required color fully appear and make its color stable, which is called "fixing color" treatment.
The color change of topaz during irradiation and heat treatment is shown in figure 1.
Figure 1 Color change of topaz after irradiation and heat treatment.
The temperature and holding time of heat treatment are very important, and the heat treatment conditions are suitable. The treated crystal presents gorgeous colors, charming luster, crystal clear and fascinating. The heat treatment conditions are not suitable, and the blue color contains dark gray, which is dense and dull.
This work adopts comprehensive treatment technology, and the treated topaz, crystal, beryl, etc. All of them have been obviously improved, especially after topaz treatment, the colors are bright and the refraction effect is good after carving, and there are many colors such as "sky blue", "Swiss blue" and "London blue" (table 1).
Table 1 Color effect of irradiated gemstones
5. Radioactivity of neutron irradiated gemstones
1. radioactive source
When the gem is irradiated by neutrons, it will be activated by the nuclear reaction of thermal neutrons, and it will last for a long time. With the extension of time, the activity intensity will gradually decrease.
For example, topaz is basically an oxide of aluminum and silicon, in addition, it also contains trace impurity elements. Neutron activation analysis shows that most topaz contains tantalum, chromium, iron, manganese, cesium, cobalt and other elements. After neutron irradiation, these elements are "activated" into nuclides with artificial radioactivity. The 7-ray and β-ray emitted by these artificial radioactivity have different energies and half-lives, ranging from 100 days to only a few minutes.
For nuclides with half-life, such as 28Al, 3 1Si, 18O, etc. They can decay within a week, and the matrix elements that make up topaz all produce the above artificial radionuclides with short half-lives, so they have little impact on people. Trace impurity elements contained in topaz not only have high-energy 7 rays after activation, but also have a long half-life, so the radioactivity mainly comes from impurity elements in topaz.
2. Intensity and duration of radioactivity
After neutron irradiation, the gem is radioactive for a certain period of time, and its activity intensity decays regularly with time, and finally decays to an exemption value.
The intensity and duration of radioactivity are determined by the irradiation treatment method, the types and concentrations of matrix elements and impurity elements of gem materials. The types and contents of impurities vary greatly with the origin and metallogenic conditions. Therefore, it is necessary to select gem raw materials with suitable core characteristics of matrix elements and impurity elements for irradiation discoloration. Only in this way can the irradiated material not contain long-lived nuclides.
Six, radioactive measuring instruments
The residual activity of irradiated gemstones must be monitored, classified and screened by instruments, and strictly managed to ensure safety.
There are many kinds of nuclear detectors with different functions, and Nal crystal detectors can be used. Ge(Li) detector (high sensitivity, high resolution) can be combined with computer to quickly give specific activity data of various nuclides. In addition, α, β and γ surface pollution instruments can be used for field measurement and grading screening. This instrument can not give absolute strength, but can only show relative strength, which is suitable for rough measurement and screening. In our work, the method of combining instrument screening and grading with instrument measurement is adopted to track and detect each batch of materials until they are qualified.
Seven. Duty-free value
At present, there is no uniform regulation on the residual radioactivity exemption value of irradiated gemstones in the world, and there is no corresponding standard in China. According to the above situation, we adopt 74Bq/g(2nCi/g) as the specific activity limit of radioactive material exemption, based on the relevant provisions in the Rules for the Safe Transport of Radioactive Materials stipulated by the International Atomic Energy Commission and the relevant provisions in the document Basic Standards and Practices for the Exemption Management of Radioactive Sources issued by the State Bureau of Technical Supervision of China. Solids below 74Bq/g can be managed as non-radioactive substances.
Inspection-free value is a management regulation formulated by weighing various factors. Substances that do not exceed the exemption value will not cause harm to human body, because this value is much lower than the risk of 10-6 ~ 10-7ci/g, and the residual activity of irradiated gemstones decreases gradually with time. As long as it is placed for a long time and treated according to its rules, it will not cause harm to transportation, processing and wearer.