Ordinary CCDs Survive Radiation Environments
Brent D. Johnson
Charged with the responsibility of cleaning up one of the country's most dangerous toxic waste dumps, CH2M Hill Hanford Group began looking for inexpensive video camera systems that could resist flammable gas and radioactive environments.
The Hanford Nuclear Reservation in Washington state was built in the 1940s to produce materials for US nuclear weapons. The site comprises 177 underground tanks that measure up to 75 feet wide and 30 to 50 feet high. They collectively contain approximately 53 million gallons of radioactive waste. Some have leaked, prompting a massive cleanup.
To provide a virtual pair of "eyes" in the tanks for cleanup efforts, CH2M Hill collaborated with R.J. Electronics on imaging alternatives. They faced the difficulty of producing a camera that could withstand high-energy gamma and beta radiation as well as hydrogen and ammonia.
This combination required some special features. For example, the internal parts of the RCS-2017 camera they designed are purged and pressurized with instrument-quality air. This isolates the electrical components and filters out oil, moisture and particulates.
One challenge was to develop an effective radiation-hardened camera that would fit through a 4-in. hole in the top of a tank. The designers miniaturized the RCS-2017 to 2.9 in. in diameter with full functionality, but the size limitation precluded the use of shielding.
The lack of shielding caused them to do a lot of radiation testing with CCD chips. They found that some CCDs stand up much better than others in the harsh environmental conditions. CH2M Hill engineer David Smet had experimented with a charge-injection-device chip that appeared to have better radiation durability, but he complained that viewing was not its strong suit and that it was not as reliable as the CCD.
The camera uses a Sony 1/4-in. color CCD, which works for up to 120 hours -- three to four hours at a time -- in the hazardous environment. The 1-lx CCD produces 460 lines of horizontal resolution.
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