About the author: Wang Ying, member of the Third Artificial Products Professional Committee of insurance institute of china, and associate researcher of the Institute of Low Energy Nuclear Physics of Beijing Normal University.

I. Introduction

Natural topaz presents many colors in nature, such as colorless, blue, yellow, pink and so on. All these different colors are basically caused by the existence of color centers (Nassau, 1984). However, the natural topaz produced in alluvial boulders in many places in China is almost colorless by naked eye observation. Using radiation technology to improve the color of natural colorless topaz is of great significance for improving the value and quality of middle and low-grade gems and making full and rational use of gem resources.

At present, there are 60Co-γ radiation source devices, electron accelerators and reactors used for topaz discoloration. Because the color of topaz irradiated by 60Co-γ radiation source is too light, there are two commonly used irradiation discoloration methods: reactor fast neutron method and electron beam irradiation method generated by accelerator. Compared with the method of changing the color of topaz by fast neutron radiation in reactor, the advantages of changing the color of topaz by electron beam radiation generated by accelerator are: no radioactivity, short treatment period and bright color. The physical and chemical properties of irradiated crystals are the same as those of natural topaz (Wang et al., 1996). The disadvantage is that the color is too light to reach the colors of Swiss blue and London blue.

Second, materials and methods

1) The test materials are topaz finished torus with different specifications, mostly round, oval, emerald and irregular, with a diameter of 6mm× 8mm ~12mm×14mm.

2) Electron beam irradiation is carried out in BF-5 electron linac of Institute of Low Energy Nuclear Physics of Beijing Normal University (Figure 1). The energy of the machine is 3 ~ 5 MeV, the average beam current is 200μA, the scanning width is 60cm, the beam uniformity is 6%, and it is equipped with a cooling water device.

Figure 1 BF-5 electron linac

3) Our method combines accelerator with 60Co-γ ray irradiation. 60Co-γ-ray irradiation was conducted in Beijing Radiation Application Research Center and Institute of Atomic Energy of China Academy of Agricultural Sciences.

4) Heat treatment: put the irradiated topaz sample into a "muffle furnace", quickly raise the temperature to a predetermined temperature, keep the temperature for a certain time, take out the sample and cool it.

Three. Results and discussion

1. radiation dose

Irradiation dose refers to the amount of electron beam energy absorbed by a unit mass substance (in gy), which plays an important role in the discoloration effect of irradiated topaz. The irradiation dose is also related to the electron accelerator itself. With the increase of irradiation dose, the effect of topaz discoloration also increases, that is, the depth and stability of color are directly proportional to irradiation dose. However, if the radiation dose is increased to 1× 108Gy, the changed topaz will still not reach the color of Swiss blue, which will increase the cost of discoloration, so it is very important to determine the appropriate radiation dose.

In the process of irradiating colorless topaz, we found that colorless topaz will turn into different shades of yellow-brown after irradiation at a very low irradiation dose (about 6kGy), and the color will deepen with the increase of irradiation dose. When the total radiation dose reaches about 5× 107Gy, the yellow-brown color becomes slightly lighter, and then the radiation dose increases to about 1× 108Gy, and a considerable part of topaz appears light green tone. The color comparison of topaz under different irradiation doses (table 1) shows that the appropriate irradiation dose is 5×107 ~/kloc-0 /×108 gy under the conditions of our electron accelerator equipment. When 60Co-γ radiation is combined with electron beam irradiation, the color of discolored topaz is slightly brighter than that treated by electron beam irradiation alone. Topaz irradiated by 60Co-γ radiation source can't reach the quality of commercial gemstones because of its light color and gray tone.

Table 1 comparison of topaz discoloration under different irradiation sources

In addition, in the process of irradiating topaz with electron beam, because the penetration ability of electron beam depends on the energy of electron beam and the density of irradiated substance, under a certain irradiation dose, the greater the beam current and the power of electron beam, the greater the heat generated, which is not conducive to the formation of topaz color center. Therefore, the necessary under-beam cooling device is necessary.

2. Heat treatment

Heat treatment is an important link in the discoloration process of topaz. The purpose of heat treatment is to eliminate noise and fully display the required blue color. When the heat treatment temperature is too low or the time is too short, the color of the changed topaz is not bright and gray. If the temperature is too high, all previous efforts may be wasted, and the irradiated topaz will fade to colorless; This process is reversible.

In the case of low radiation dose, the yellow-brown color of topaz is unstable, and in the case of strong sunlight, it usually fades after several hours or 1 day. When the total radiation dose reaches above 5× 107Gy, the discolored topaz will not fade under natural conditions, that is, sunlight and normal temperature. The author has a piece of topaz that has changed color 16 years. Up to now, the color is still very stable, and no discoloration can be observed by naked eyes. However, when the temperature exceeds 450℃, the color of blue topaz will completely fade under the irradiation of electron beam and 60Co-γ radiation source.

Four. conclusion

Colorless topaz was treated by electron beam radiation generated by 3 ~ 5 MeV electron linac. The color change of topaz under different irradiation doses was studied, and the appropriate irradiation dose was selected, which should be not less than 5× 107Gy.

Accurately controlling the heat treatment conditions of irradiated yellow-brown topaz is the key to obtain bright sky-blue topaz.

refer to

King, man. 1996. Experimental study on topaz optimization. Jewelry technology, (2): 18 ~ 2 1.

Nassau K. 1984。 Gem enhancement. Butterworth, London