Answer to Question #8362 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
I have (by accident) bought an irradiated smoky quartz crystal. What I usually do with crystals is hold them in my hands for prolonged lengths of time, for "cuddling" if you will. Is this in any way a little harmful? I think this is because crystals have the ability to hold, enhance, and emit energy—maybe also radiation? I am seriously concerned about this.
There is no radiation hazard associated with irradiated smoky quartz.
Irradiating a crystal will cause some energy to be stored in the crystal structure, which can change the optical properties, typically observed as a change in color. Using radiation to change the color of gemstones has been studied almost as long as radioactivity has been known. An early study, published within 10 years of the discovery of radioactivity, placed diamonds in radium powder to give a green color. As the use of radiation has become more sophisticated, the ability to treat gemstones with radiation in order to create color changes has become more prevalent.
With irradiated gemstones, there is a possibility that activation may occur, which means that some of the nonradioactive atoms in the crystal could be made radioactive. Generally, for this to occur the irradiation process would need to either remove or add a neutron from the atom. Neutrons can be removed by a reaction called photoneutron production. This occurs when the energy of the radiation striking the atom is greater than the energy required to release a neutron from the nucleus. The threshold energy is different for each element, and the radiation energy must be above the threshold for the specific element being irradiated. If neutrons are available, they can be absorbed into an atom to make it radioactive. Neutrons may be generated from the photoneutron production reaction or directly from a nuclear reactor.
There are three types of radiation sources used to color gemstones. Each has a different probability of activation:
- Gamma irradiators typically use sources containing cobalt-60 (60Co) to supply energy (photons) that will interact with the crystal structure and create the color change. The photon energy from 60Co is below the threshold energy for the photoneutron production reaction in all elements. This means that no gemstone irradiated with 60Co gamma photons will become radioactive. Gamma irradiators are used to create smoky quartz.
- Electron beam irradiators involve a particle accelerator to create a narrow stream of electrons that are directed at the gemstones. For accelerators with electron energy below about 12 MeV, there are very few neutrons produced in the irradiation process, and the probability of activation is quite low. With higher electron energy, the probability increases so that accelerators operating at more than about 15 MeV invariably create some activation. However, the half-lives of these activation products tend to be short, so that the radioactivity decays rapidly to background. For example, high-energy electron beam irradiation of topaz creates fluorine-18 (18F) with a half-life of 1.8 hours, which decays to background levels within a day or two. The irradiator facility will hold these gemstones until the radioactivity has decayed to levels below the exempt concentrations defined in regulations. Generally, this holding time is on the order of a few days for topaz, but may be significantly longer for gemstones like beryl, where activation products can have a longer half-life.
- Small nuclear reactors, designed for research and development rather than electric power production, are used to irradiate gemstones with neutrons. Neutron irradiation produces a darker and richer color in the gemstone than electrons or photons, but invariably creates some radioactivity in the gemstone. Many of these activation products have short half-lives, which decay rapidly to background, but a few may be created that have longer half-lives measured in months or years. The holding times for neutron-irradiated gemstones can be significantly longer than for electron-irradiated gemstones, but the regulations of the US Nuclear Regulatory Commission prohibit distribution of gemstones until the radioactivity has decayed to levels below the defined exempt concentrations.
There are several publications available that discuss gemstone irradiation and the potential for activation. A good general discussion of theory and practical consideration is "Gemstone Irradiation and Radioactivity," by Charles E. Ashbaugh III, in the winter 1988 issue of Gems and Gemology. In addition, the January 2008 HPS News cover article is a report by Andy Karam on regulatory issues with importing irradiated gemstones.
Mark A. Smith, PhD, CHP