Samples of blue topaz produced by electron beam irradiation

Topaz, a semi-precious stone, is used extensively in the jewellery market, particularly in the Far East.  As mined, topaz comes in a wide range of colours, however, most of the material is colourless.  One of the most desired colours is “London Blue”.  Colour centers – essentially atomic lattice displacements – can be introduced into the stone with nuclear particle irradiation.  The first attempts of irradiating topaz at CRNL for commercial contracts were made at NRU with neutrons.  These were successful, however, trace contaminants in the gemstones were also activated and there was typically a 1-2 year “decay” period between irradiation and release to the commercial market to allow the induced radioactivity to decay to near-background levels.  Only a modest amount of the order of 1-2 kg was irradiated in total.

Electrons can also produce colour accelerators and at energies of 10-12 MeV no neutrons are present and essentially no radioactivity is induced in the gemstones, thereby allowing immediate shipping to the customers.  The powerful electron beams of the PHELA and later IMPELA accelerators at CRNL were used to irradiate large quantities of topaz under commercial contracts.

The gemstones, which arrived precut and polished, were arranged in irradiation trays mounted vertically in front of a beam that was scanned over the tray surface.  The trays had a thin (typically aluminum) window at the front and a thick aluminum backing at the rear.  Water was flowed through the tray to remove the heat deposited (typically 3-4 kW for PHELA, and 40-50 kW for IMPELA) and prevent overheating.  This is important to prevent cracking of the gems as well as to maintain the gems below the temperature where the colour centers are heat-annealed out.  The thickness of the gap in the tray containing the gems is determined by the beam energy.  Electrons should be able to pass through the gems and produce the colour centers but their range should be greater than the product thickness to prevent charges from building up in the gemstones by fully stopping the electrons.  A large charge buildup within the gem can lead to a discharge and cracking of the gems.

To induce the “sky-blue” or “London-blue” colours desired by the jewellery market, white or colourless topaz requires a massive irradiation dose in the range of 30-40 x 10⁶ Gy.  Some non-desirable “brown” or smoky colour centers may also appear in some of the gemstones at this level of irradiation.  There are a number of proprietary heat-treatment sequence used to anneal unwanted colour venters from the stones.

PHELA was used to irradiate 10s of kg of gemstones whereas close to 1000 kg were irradiated under commercial contract with IMPELA in the period 1992-1996.  Commercial contracts for topaz irradiation provide significant income for a number of irradiation centers, including ones that have installed IMPELA accelerators.