1. 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)
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: