Gamma ray evaluation of fast neutron irradiated on topaz from Sri Lanka by HPGe gamma ray spectrometry

Abstract

Abstract. The purpose of this study was to evaluate the radionuclide concentrations of London blue topaz after fast neutron irradiation. The London blue topaz was obtained from Sri Lanka which classified into dark and light colors in the shape of an oval and rectangle with small, medium and large size. The optical property and radionuclide concentrations of London blue topaz have been examine by UV-Visible spectroscopy and HPGe gamma ray spectrometry, respectively. The UV-absorption spectra of topaz was taken in the range of 300 to 800 nm at room temperature. The results showed that the absorption peak of topaz was observed with only broad peaks in the range of 550 to 700 nm and 630 nm that correlated to the O -center in hydroxyl sites which substitutes for fluorine in topaz structure. The radioactivity of dark and light colors in the shape of an oval and rectangle London blue topaz was in the range of 1.437  0.014 to 21.551  0.037 nCi/g (oval dark), 2.958  0.031 to 6.748  0.054 nCi/g (oval light) and 2.350  0.014 to 43.952  0.088 nCi/g (rectangle dark), 1.442  0.023 to 6.748  0.054 nCi/g (rectangle light), respectively. The decay rates of 46 Sc, 182 Ta and 54 Mn isotopes created by irradiation showed that the decay time of the radioactive element depended on the size of the topaz so increased with decreasing the size of topaz. Moreover, the size of topaz also affect the absorption coefficient. This study is applied to predict time of residue dose of topaz for enhancement colorless topaz by neutron radiation treatment.

Optimized conditions for cobalt diffusion in Sri Lankan colorless topaz and coloration mechanism elucidation through spectro-chemical investigation

“…Defects consist of missing or misplaced atoms or electrons. These defects may have originated at the time of mineral formation or may have been introduced later by external influences such as fast neutron irradiation [4]. Additionally, these colors are also affected by the presence and variations in the amounts of F, OH, and minor impurities in the material.…”

“…Among them, irradiation is the most common method used to obtain blue topaz where color centers are produced by exposing topaz either to gamma rays or neutrons or electrons, followed by low-temperature heating [8][9][10][11]. Though the method is effective, the main limitation is the radioactivity induced by irradiation [4,12]. Neutron irradiated blue topaz has to be kept aside for several months, until its residual radioactivity reaches a safer level of less than 74 Bq/g or 2 nCi/g, as defined by the Nuclear Regulatory Commission (NRC) [12].…”

“…In addition to that, the irradiation methods have significant drawbacks in terms of cost, safety, and efficacy. Further, in some developing countries such as Sri Lanka where the colorless topaz is abundant [4], facilities are inadequate to use irradiation as a value addition method.…”