Reports are flying of delaminated or melted CD discs that were sent by mail post-September 11. Although it is possible that extreme measures were implemented after the germ warfare scare, radiation levels in current use by the U.S. postal system do not impair CD interchange.
Mail is selectively irradiated by high energy, high dose electron beams. Media Sciences evaluated potential degradation by testing samples before irradiation, then securing each individual disc in a jewel case placed inside a bubble-pack envelope. Each sample was irradiated by a 10 Mev electron beam using a dose of 40 kGy. Samples were then retested, and results were compared with pre-irradiated values. Samples included one CD-ROM disc, four recorded CD-R discs with cyanine, phthalocyanine, and azo dyes, and two unrecorded CD-R discs.
Polycarbonate substrates of all samples had a slight brown tint after irradiation. Infrared reflectance at 780 nm decreased by an average of 1%. BLER and E11 error rates increased by an average of 1.13X after irradiation. Peak C1 error rates averaged over one second remained well below the ISO limit of 200. Radiation-induced errors resulted from small, random defects that were easily correctable by the cross interleaved Reed-Solomon code present in every CD. No uncorrectable errors resulted from irradiation. No correlation was observed between the above changes and CD-R dye types.
After irradiation, reflectance of the aluminum-metallized CD-ROM disc decreased by 1.8%, and C1 error rates actually decreased to 0.95X their pre-irradiated values. One recorded CD-R sample that had phthalocyanine dye and silver metal had a 1.8% decrease in Rtop with no change in C1 error rate. Another recorded CD-R sample with phthalocyanine dye and silver metal had a 1.3% drop in Rtop and 1.12X higher error rates. The recorded CD-R sample that had cyanine dye and silver metal had a 1.1% decrease in Rtop and 1.66X higher error C1 rates. One recorded CD-R sample that had azo dye and silver metal had a 0.8% drop in Rtop and 1.43X higher error rates. Values for radial tracking (push-pull), radial noise, I3, I11, asymmetry, cross talk, jitter, and effect length were not modified by irradiation, indicating that average pit geometries were not degraded.
Values for wobble amplitude, wobble carrier-to-noise, radial noise, push-pull, radial contrast, and ATIP error rates of unrecorded CD-R discs were unchanged by radiation. The sample with phthalocyanine dye had 0.8% lower Iland and 1.5% lower Igroove after irradiation. The sample with cyanine dye had 0.6% lower Iland and 0.1% lower Igroove after irradiation. After these irradiated discs were recorded, Rtop and error rate test results were comparable to those of irradiated pre-recorded CD-R discs.
CD-R samples that were irradiated separately by a 5 Mev electron beam using doses of 5 kGy and 10 kGy did not exhibit discoloration of the polycarbonate or a decrease in Rtop. C1 error rates did increase by small amounts similar to results for samples irradiated at 10 Mev and 40 kGy. These observations suggest that modest increases in C1 error rates caused by irradiation may result from a different mechanism than that responsible for the polycarbonate discoloration.
The Smithsonian Center for Materials Research has published information on the sterilization of mail sent by the USPS at http://www.si.edu/scmre/mail_irradiation.html. Comments on the interaction of radiation and materials quote opinions that 10 Mev doses of 25 kGy up to 60 kGy may be required for sterilization, causing a temperature rise in the order of 5 degrees centigrade in the irradiated material. Such temperatures should not pose any risk to CD-ROM or CD-R discs. The paper indicates that some natural organic materials such as paper or cotton may be sensitive to doses as low as 4 kGy.
Although test results of Media Sciences show that interchange of CD-ROM and CD-R discs was not degraded by high electron beam doses, packaging or organic contaminants on the surface of a disc could be affected by radiation and, in turn, might degrade CD discs.
Media Sciences thanks Titan Scan Technologies for irradiating all samples without fee. In particular, the direction and support of Michael O'Dell and Dr. Darren McKnight of Titan is gratefully acknowledged. Assistance provided by the United States Postal Service Consumer Affairs Office is appreciated.