Low-temperature annealing and kinetics of radiation stains in natural diamond

Green coloration in natural diamonds is generally caused by exposure to natural sources of radiation. When in direct contact with radioactive minerals or fluids, the diamonds often experience very shallow lattice damage, observed as green radiation "stains" at the locus of contact, predominantly due to alpha irradiation. With continued residence in the earth, these green stains can turn brown due to reduction of GR1 absorption. Isothermal annealing experiments on 8 diamonds in the 400-550 degrees C temperature range with radiation stains were used to determine the activation energy for this green-to-brown color transition as 1.6 +/- 0.1 eV, consistent with prior theoretical and experimental determinations for the activation energy for migration of interstitial carbon atoms. Additionally, the composition and changes of the radiation stains were surveyed using photoluminescence mapping on two diamonds as they were annealed in the temperature in the range from 375 to 500 degrees C. Photo-luminescence features that increased with annealing included the TR12, and peaks at 461.5, 464, 468.7, and 490.7 nm. PL features that showed a significant decrease with annealing included the 3H and GR1 defects along with peaks at 488.8, 492, 523.5, 542.7, and 662 nm.