Nanotech makes radioactive sensors obsolete

R. Colin Johnson
EE Times
(05/20/2008 12:26 PM EDT)

PORTLAND, Ore. — "Green" smoke-alarm ionizers using field-emission from nanotubes instead of radioactive isotopes could eliminate a source of dirty-bomb material, according to recipients of a U.S. Small Business Innovation Research (SBIR) contract sponsored by the Homeland Security Advanced Research Projects Agency.

Applied Nanotech Inc. (Austin, Texas) and Sionex Corp. (Bedford, Mass.) now have the funding from Homeland Security to produce a small, safe, high-performance sensor using electron field emission from carbon nanotube arrays instead the ionizing alpha rays from radioactive isotopes.

"We believe that carbon nanotube emitters can replace radioactive materials in consumer devices like smoke detectors, industrial sensors, medical equipment, homeland security applications and elsewhere," said Applied Nanotech scientist Richard Fink.

Many American households have as much as a milligram of radioactive americium-241 in the various smoke alarms and other gas-phase detectors found there. About a fifth of a milligram of americium is used to ionize the air inside a smoke detector. But just one gram of americium is dangerous for people to handle; dekagrams to hectograms are enough for "dirty" bombs, and kilograms could be used to make a nuclear bomb.

Instead of seeding our land-fills with radioactive materials like nickel-63 and americium-241, which have half-lives of 100 and 432 years, respectively, the U.S. Nuclear Regulatory Commission, the National Research Council and the Homeland Security Advanced Research Projects Agency are all investing in "green" alternatives to radioactive isotopes in smoke alarms and medical diagnostic and research equipment.

The Applied Nanotech and Sionix joint-development effort aims to provide a safe, inexpensive, high-performance alternative method of ionizing samples by using carbon nanotube emitters integrated into air-flow passages ahead of a differential mobility spectrometer. Applied Nanotech and Sionex claim to have proven in principle that carbon nanotube emitters can perform all the necessary ionization and identification steps without the use of radioactive materials.

Ionizing gas molecules

Ion mobility spectroscopy works by ionizing gas molecules as they pass through the sensor, then identifying them by their atomic weight. After ionization of the sample, the molecules are electrically attracted through a drift tube, where they spread out according to their atomic weight, allowing the location to reveal their identity to an integrated detector.

Applied Nanotech's carbon nanotube emitters perform the ionization step instead of using radioactive materials, allowing gas particles to be safely separated and detected by Sionex's integrated differential mobility spectroscopy (DMS).

Carbon nanotube emitters perform the ionization step by concentrating electrical fields in a manner similar to a lightning rod, allowing the emission of electrons at room temperature and at atmospheric pressures. As the electrons pass through the air sample they ionize the gas molecules, thus supplying the charge that enables them to be attracted to the sensors and detectors. Carbon nanotube emitters may impart either positive or negative ionization as needed for a particular sensor.

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