|Diagnostics of diamonds treated by irradiation and annealing|
When irradiating a diamond, carbon atoms shift from the positions they occupied in the crystal lattice of the diamond, leaving some vacancies. These vacancies change the light-absorbing properties of the diamond and therefore modify its color.
The main feature of the absorption spectra of irradiated diamonds is the presence of an absorption band at 741 nm, which is because the irradiation creates neutral vacancies called GR1 centers. This absorption band is responsible for the appearance of blue-green coloration.
Since a neutron is approximately 2000 times as massive as an electron, it destroys the crystal lattice of the irradiated diamond along its path. Thus, in addition to the GR1 center formation, some disordered regions associated with this destruction appear in the diamond structure. Such regions cause a high level of background absorption of visible light in neutron-irradiated diamonds. As a result, an irradiated diamond looks darker than a non-irradiated one.
Annealing destroys the GR1 centers formed during the irradiation and creates new color centers in the irradiated diamond. At a temperature of about 500°C, the vacancies (GR1 centers) become mobile. As a result, other nitrogen defects capture the GR1 centers to form H3 (503 nm absorption band) and Í4 (496 nm absorption band) centers, as well as some others (575, 595, and 637 nm absorption bands).
The type and concentration of the new centers depend on the physical type of the diamond and the annealing temperature. For example, annealing of irradiated type Ib yellow diamonds at a temperature of 700-800°C causes 637 nm absorption band to appear in the optical absorption spectra of the diamond (although 503, 575, and 595 nm absorption bands may also arise). This absorption band is responsible for pink or violet coloration of the treated stones.
Thus, the main method used in diagnostics of diamonds treated by irradiation (and subsequent annealing) is optical spectroscopy, and the key diagnostic features are absorption peaks at 503, 575, 595, 637 nm, observed in the measured spectra. However, some of the above-mentioned spectroscopic features can be found in non-treated natural diamonds. Therefore, to make the diagnostics unambiguous, it is necessary to further develop the in-depth studies of the processes that occur in the structure of a diamond exposed to ionizing radiation (and annealed after that).