STUDY OF NEUTRON IRRADIATIONINDUCED COLOR IN TOPAZ AT

STUDY OF NEUTRON IRRADIATION-INDUCED COLOR IN TOPAZ AT THE PULSED REACTOR IBR-2 Khatchenko Yu. , Kovalenko M. , Ryabukhin O. Ural Federal University, Ekaterinburg Borzakov S. , Bulavin M. , Enik T. , Khramko K. , Kulikov S. , Tiutiunnikov S. , Utepov A. , Verkhoglyadov A. 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 Fig. 2 - Capsule in the research channel № 3 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. 3 - Sample positions in the research channel № 3 Table. 1 – Radionuclides in irradiated blue topaz 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 mode Fig. 4 demonstrates the color changes after irradiation depending on the fast neutron (>1 Me. V) 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). rator RESULTS AND DISCUSSION cm-2 1, 4· 1018 1, 4· 1017 3, 3· 1016 Nuclide Half-life Cs 134 753, 6 d Mn 54 312, 3 d Ag 110 m 249, 79 d Zn 65 244, 26 d Ta 182 114, 43 d Sc 46 83, 79 d Co 58 70, 86 d Tb 160 72, 3 d Sb 124 60, 2 d Fe 59 44, 5 d Hf 181 42, 39 d Ce 141 32, 5 d Cr 51 27, 7 d Pa 233 26, 96 d Rb 86 18, 63 d Sb 122 2, 72 d Au 198 2, 69 d Np 239 2, 35 d La 140 1, 67 d 1. Mo 99 65, 94 h 2. Sm 153 46, 28 h Na 24 14, 95 h 2, 2· 1016 3, 9· 1015 Fig. 4 - Color changes of topaz samples irradiation at the reactor IBR-2 depending on the fast neutron (>1 Me. V) 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. . de B. C. Menezes, J. -M. Spaeth: Color centers in topaz: comparison between neutron and gamma irradiation. Phys Chem Minerals (2007) 34: 437 -444