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1 August 2002

Quadrupole assumes role

Washington—In tests that concluded early last month, the Transportation Security Administration (TSA) said checkpoint screeners at 32 of the nation's largest airports botched the detection of fake weapons such as guns, dynamite, and bombs in about 25% of their chances.

If it's any consolation to the frequent-flying reader, that's an improvement. Similar tests conducted in February showed the failure rate to be nearly one half of the smuggling attempts.

The most promising technology under development for airport use is nuclear quadrupole resonance spectroscopy. This science will allow the authorities to bulk screen luggage.

The June 2000 InTech reported on the higher-tech X-ray devices at work and installing in U.S. airports that provide image, density, and atomic number of potentially explosive materials.

In July, InTech reported on the near-future technology, ion mobility spectroscopy, that works at some airports now and promises to proliferate as soon as possible, according to the TSA.

The most promising technology under development for airport use is nuclear quadrupole resonance (NQR) spectroscopy. This science will allow the authorities to bulk screen luggage.

NQR's roots

NQR is the predecessor to the techniques of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). It was developed and used widely back in the 1950s but was eclipsed by the emergence of NMR spectroscopy. Only recently have researchers begun to reconsider NQR for the detection of land mines and remote detection of other compounds. It offers a highly specific method of detecting and identifying target materials.

Unlike NMR and MRI, NQR does not require an external magnet to align the nuclear spins. Instead, the valence electrons align the nuclear spins along preferred directions. By applying a radio frequency (RF) pulse with a frequency corresponding to the energy associated with the alignments of nuclear spins, the populations depart from thermal equilibrium.

Following the RF pulse and nuclear alignment, an NQR signal return pulses when the nuclei realign to their initial orientation. This realignment produces its own weak radio signal that registers on a detector. These resonance frequencies produce signals that range from 0 to 6 MHz. So far, NQR has examined 10,000 chemical compounds, and each compound has produced its own unique signal, depending on the molecular structure.

The weakness of the return signal was a problem because it could and tended to get lost in the surrounding noise. The Naval Research Laboratory (NRL) solved this signal-to-noise ratio problem by developing a special detection coil.

The geometry of the coil causes electrical and magnetic noise to be canceled, providing a larger signal-to-noise ratio in the NQR signal from the object being tested. Because of its immunity to external electrical and magnetic environmental noise, an NQR detection system with the NRL-patented coil does not require external RF shielding and is practical and suitable for field use.

The American Chemical Society reported that Quantum Magnetics (QM) is manufacturing a detector using the NRL detection coil and that a small number have been deployed in U.S. airports for field testing.

QM has two different sized instruments on the market that screen carry-on and checked baggage at 300–400 bags per hour. The QM technology also has uses, and is developing to certain stages, in land mine detection, narcotics detection, nondestructive sensing, and industrial process monitoring. IT

InTech senior technical editor Nicholas Sheble edits the Sensors department. Write him at nsheble@isa.org.

Screening with nuclear quadrupole resonance