Khatchenko Yu. , Kovalenko M. , Ryabukhin O. RESULTS AND DISCUSSION Study of neutron irradiation-induced color in topaz at the pulsed reactor IBR-2 Borzakov S. , Bulavin M. , Enik T. , Khramko K. , Kulikov S. , Tiutiunnikov S. , Utepov A. , Verkhoglyadov A. Khatchenko Yu. , Kovalenko M. , Ryabukhin O. Ural Federal University, Ekaterinburg Joint Institute for Nuclear Research, Dubna INTRODUCTION Blue topaz is very popular in the world as jewelry, but it is quite rare in nature. For this reason, various types of radiation is widely used to enhance these gemstones [1]. The irradiation forms the color centers in the crystal structure, which change the mineral's color and, thereby, increase its consumer value. Reactor neutron irradiation is commonly applied to create an attractive «London Blue» color (fig. 1, 5) in near-colorless topaz. Usually, the most developed thermal reactors are used for this purpose. However, it’s established that the production of «blue» centers is due to fast neutrons [2]. Thermal and resonance neutrons lead to radioactivity from nuclides of trace-element impurities in topaz. The use of a fast reactor IBR-2 makes it possible to achieve effective color modification with a minimum of induced activity. This work is devoted to the experimental selection of the optimal fluence and the energy spectrum of neutrons for economic coloration topaz technology. Fig. 1 - Topaz, neutron-treated at the reactor IBR-2 . IRRADIATION Topazes of different origin were irradiated by neutrons at the pulsed reactor IBR-2. Samples were placed in the research channel № 3 in an aluminum capsules, filled with boron carbide to remove thermal neutrons. Fluence magnitude varies by position the samples at different distances from the moderator (fig. 2, 3). The activity of irradiated samples was аnalyzed on gamma spectrometer with HPGe detector. Fig. 2 - Capsule in the research channel №3 Fig. 3 - Sample positions in the research channel №3 RESULTS AND DISCUSSION Fig. 4 demonstrates the color changes after irradiation depending on the fast neutron (>1 MeV) fluence magnitude. It’s seen that to obtain «London blue» color, the fast neutron fluence should be greater than 1017 cm-2. Various annealing modes at temperatures above 500 °C give any shade of blue color (the optical absorption band is centered at 620 nm). Achieved color remains stable at room temperature for a long time. Gamma-ray spectroscopy revealed the variable presence of few radionuclides in the irradiated topazes (table 1). moderator Table. 1 – Radionuclides in irradiated blue topaz cm-2 1,4·1018 1,4·1017 3,3·1016 2,2·1016 3,9·1015 Nuclide Half-life Cs134 753,6 d Mn54 312,3 d Ag110m 249,79 d Zn65 244,26 d Ta182 114,43 d Sc46 83,79 d Co58 70,86 d Tb160 72,3 d Sb124 60,2 d Fe59 44,5 d Hf181 42,39 d Ce141 32,5 d Cr51 27,7 d Pa233 26,96 d Rb86 18,63 d Sb122 2,72 d Au198 2,69 d Np239 2,35 d La140 1,67 d Mo99 65,94 h Sm153 46,28 h Na24 14,95 h Fig. 5 - Color changes of topaz: a) before and b) after neutron irradiation at the reactor IBR-2 Fast neutron fluence was amount to 1018 cm-2 Fig. 4 - Color changes of topaz samples irradiation at the reactor IBR-2 depending on the fast neutron (>1 MeV) fluence magnitude CONCLUSION The optimal sample position in the channel №3 (with fast neutron fluence ~1018 cm-2) to obtain «London blue» topaz is determined. Based on activation analysis data a special filter system to reduce activation can be made. REFERENCES Wang Ying, Gu yong-bao: Research on radiation-induced color change of white topaz. Radiation Physics and Chemistry 63 (2002) 223-225 K. Krambrock, L. G. M. Ribeiro, M. V. B. Pinheiro, A. S. Leal, M. Â. de B. C. Menezes, J.-M. Spaeth: Color centers in topaz: comparison between neutron and gamma irradiation. Phys Chem Minerals (2007) 34:437-444