US Postal Service & Irradiated Gemstones

From recent personal communication with the President of SureBeam Corporation (Titan Corporation's subsidiary which markets and operates commercial electron beam irradiators) the dose currently used on the U.S. mail facilities to protect individuals from anthrax is between 56 to 112 kiloGrays (i.e. 5.6 - 11.2 Megarads). I am assuming, but have not verified, that each of the other firms operating commercial electron beam irradiation facilities (E-Beam Services, IBA, etc.) will be using the same delivered dose range. Putting these absorbed dose values in perspective is critical to understanding this new "challenge" to the gemstone industry.

First a couple of definitions to help understand the units. These dose units are simply measures of absorbed energy. By definition, a "rad" is 100 ergs of energy deposited in one gram of any substance (i.e. 100 ergs/g). To put this in perspective, a 5-carat gemstone weighs one gram (i.e. 1 metric carat = 200 milligrams), and an erg is approximately the amount of energy given off when a mosquito collides with a screen door. Envision 20 average sized mosquitoes colliding simultaneously with a 1-carat stone and you've got a relatively good feel for the amount of energy in a rad (i.e. not much). A "megarad" (abbreviated "MRAD") is one million rads (1 MRAD = 1,000,000 rads). A newer, more internationally accepted (i.e. S.I.) unit for absorbed dose is a "Gray" (abbreviated "Gy") which is equal to 100 rads (1 Gray = 100 rads). A Gray is defined as one "Joule" of energy (1 Joule = 10,000,000 ergs) deposited in one kilogram of any substance (i.e. 1 Joule/kg). Commercial irradiation doses are normally expressed in terms of KiloGrays (abbreviated "kGy") due to the size of typical doses given such products (1 kGy = 1,000 Grays). Thus, the relationship between these two units for absorbed dose is 10 kGy = 1 MRAD, 50 kGy = 5 MRAD, etc. 

The "mean lethal dose" for humans for a one-time exposure to ionizing radiation is around 300 rads (i.e. 3 Gray), that is, 50% of the humans exposed to such an absorbed dose would be expected to die within thirty days with no medical treatment. While it varies from product to product, a typical absorbed dose for the sterilization of disposable medical products (sutures, surgical kits, operating gowns, catheters, etc.) prior to their delivery to hospitals is around 25 kGy (about 40 million times more than a typical chest X-ray, and over 8,300 times more than the human mean lethal dose). An absorbed dose of around 100 Gray (0.1 kGy) is sufficient to kill most insects and parasites, as well as inhibit plant sprouting (i.e. extend the shelf life of onions, garlic, etc.). Stepping up an order of magnitude, an absorbed dose of around 1.5 kGy --> 4.5 kGy is sufficient to kill most bacterial pathogens (except spores) in foodstuffs such as poultry, beef and spices. For example, a dose of 1.5 kGy will kill about 99.5% (and a 3 kGy dose will kill about 99.99%) of the Salmonella in contaminated poultry (the majority of which is contaminated, incidentally). Similar absorbed dose levels are sufficient for eliminating the danger of bacteria such as E. Coli 0157:H7 and Campylobacter jejune. A slightly higher dose (up to 7 kGy) is allowed for frozen meat. An absorbed dose of around 10 kGy --> 45 kGy is sufficient to inactivate bacterial spores and some viruses. The more resistant spores of Clostridium botulinum (responsible for botulism) necessitate a larger absorbed dose of 30 kGy --> 60 kGy. As previously mentioned, the current required absorbed dose for eliminating the danger of anthrax spores is 56 kGy --> 112 kGy. The US legal limits (both minimum and maximum) for commercial irradiation of various foodstuffs are set forth in Title 21, Volume 3, Parts 170-199 of the Code of Federal Regulations (21CFR3).

A quote currently circulating among gem dealers on the INTERNET attributed to a "technical guy" from Titan does very little to actually quantify this perceived problem for the gemstone dealers, although I realize it may have been taken out of context with supplementary information left off. "Glass and Gemstones - Irradiation will produce coloration of glass and gemstones, the exact color and intensity being dependent on the nature of the mineral impurities within the matrix. E-beam irradiation is currently used commercially to convert colorless topaz to deep blue using very high doses. At lower dose levels, most glass/crystal will turn a shade of gold or brown." This rather vague statement, while true, fails to point out that the coloration is also highly dependent on the total delivered dose as well as the energy level of the irradiating particles. The maximum legal energy level limit for the irradiation of medical products and foodstuffs in the US is 10 MeV for electron beam irradiators.

The gamma radiation naturally emitted from a cobalt-60 source is given off at two fixed energy levels (1.173 MeV and 1.332 MeV) due to the nature of the radioactive decay of this particular isotope. Medical X-ray machines emit particles with energy levels under 5 MeV. On a related note, there is some confusing misinformation circulating in the gem trade about the "power" level of the various commercial electron beam irradiators. This is simply a unit of measurement of the throughput capability of a particular machine (i.e. how big it is) which provides a direct indication of how much product it can treat to a give absorbed dose level within a given period of time. The power level has nothing to do with how "well" it irradiates product; that is primarily a function of the energy level and the total absorbed dose given. The power of a particular commercial electron beam irradiator is generally stated in terms of kilowatts (abbreviated "kW") of output, typically ranging from 5 kW --> 100 kW (with larger machines proposed). The power (in kW) of a particular electron beam irradiator is a function of four terms including the energy level of the emitted particles (in MeV), the particle beam current (in milliamperes), the beam pulse width (in microseconds), and the pulse repetition frequency (in pulses per second, or Hertz); that is, kW = MeV x mA x usec x pps.

The large electron beam doses typically required to enhance the color of topaz (typically 6,000 --> 12,000 Megarads) would turn paper (i.e. mail, etc.) yellowish brown and make it so brittle that it would crumble into a fine powder the first time it was touched (personal experience). Even the much smaller doses (from either electron beam or gamma irradiation) used to darken quartz and glass (50 --> 250 Megarads) have this effect on paper. I've seen profound color changes in such things as fluorite and some quartz (depends on the impurities, which is a function of the source location of the material) with 10 MeV electron beam doses as low as 20 MRADs, and visible (but less pronounced) color. Changes in various gemstone materials at absorbed doses lower than 10 MRAD. While I haven't retained quantitative records for this lower dose range (one usually doesn't bother stopping there), the threshold dose for noticeable color changes (i.e. visible to the eye) in some gemstone materials is certainly in this ballpark. Based on previous trials, I suspect such color changes could be readily reversed with the proper heat treatment.

In summary, based on the electron beam doses presently being given to some of the US mail (i.e. 5.6 --> 11.2 MRADS at a particle energy level of 10 MeV) there is indeed a likelihood of color change in some gemstones if they were irradiated under these parameters. The range of effects should be easy to confirm. If I were in charge of a gemstone laboratory, I'd start an immediate small "experimental" program in this relatively narrow-absorbed dose range to get some hard data ASAP to help preclude supposition around the gem & jewelry trade.

EXTRA NEWS: Mail fire at the US Mail facility in New Jersey (December 10, 2001) This morning about 60 lbs of mail being irradiated at a New Jersey Post Office caught fire totally destroyed due to irradiation heat produced at the mail-scanner.

Here is our comments:
That's certainly possible. There is a tremendous amount of energy in a commercial electron beam irradiator which is normally concentrated in a beam having a circular cross section of from 1 cm. --> 10 cm. in diameter. Normally, commercial product (medical products, etc.) moves on a conveyor system under the beam while the beam is also "scanned" across the product perpendicular to the direction of conveyor movement to spread out (i.e. more evenly distribute) the amount of energy that is deposited in the passing material. In past work I've inadvertently"blown" holes through solid lead bricks (similar to a cutting torch, but much more powerful) when the beam scanning accidentally stopped for a few seconds!

CREDIT: This report was written by a close associate wishing to remain anonymous.