content using an integration time of 10 s. The versions of NQR spectrometers However, he says, "I have come to believe designed by Garroway and his colleagues that there is no NQR 'explosive simulant.' at NRL are similar to the ones used until Each material has its specific properties, the mid-1970s, at least as far as the basic so detecting a small quantity of our com hardware is concerned. The sequence of pound is useful only to demonstrate that operation is basically the same: The spec NQR will work with small samples in a trometers transmit and receive rf signals large-volume coil. It doesn't imply that that are then demodulated by radio tech similar sensitivity should be expected for niques at audio frequency and digitized any other compound." for a computer readout. But the new prototypes incorporate adaptations of ad vanced pulse techniques and computer Getting personal capabilities developed since the 1980s for Garroway and his colleagues are also NMR. starting to test a modified coil designed to scan the passengers themselves for explo "We now have a better understanding sives or contraband. The "meander line" of spin dynamics in complicated rf pulse or surface rf coil is flattened out in a ser sequences, in terms of both the theoreti pentine shape. That permits the instru cal aspects and the parameters for optimi ment to scan a wide zation," Garroway surface area at a says. "Understandι controllable limited ing the require depth to avoid ments for the rf ex harming the person citing field is crucial being scanned. The for scaling up to peak signal power very large systems used with the mean like the luggage der coil will be 10 W scanner." or less and, as with The bigger the the luggage scan coil, the bigger the ners, each excita transmitter needed tion pulse lasts less to get a reasonable than a millisecond. signal. Garroway's prototype operates The group at in the range of 1-5 NRL has envisioned MHz, just above the a hand-held mean™~~">~κ~~--™-™» der coil-based in top of the AM band, •— " ' strument, similar to with peak power of the metal detector wands now in use, that 1 kW. This level is somewhat lower than can be passed over the subject to locate the magnitude used in medical NMR im explosives or contraband on the subject's agers. The pulse sequences used in the body. Given that NQR is affected by the prototype's pulse programmer have been mobility of the analytes, however, sample adapted from modern NMR techniques or instrument movement may be a prob and used to optimize the signal-to-noise lem. Garroway says his group's designs ratio. For small packages, a prototype with are based on the assumption that luggage a smaller rf coil than that of the luggage and package scanners will have a con scanner improves detection because it veyor belt with a stop/start function so scans closer to the packages. that the items are stationary during the As for a practical detection limit for scan time. Achieving perfect subject im explosives in the field, Garroway says the mobility will be more difficult with the prototype detects sub-kilogram samples, passenger scanner. The group is begin but he prefers not to mention specific de ning to examine the effects of slight move tection limits; following the Lockerbie ment on the signal to see whether that incident, such information is considered kind of immobilization is necessary. classified. Kroll says his prototype at Martin Bomb detection experts are increas Marietta has detected a 1-lb sample of an ingly concerned about the sophistication RDX-like simulated explosive with 40% Ν with which bombs are being smuggled
The group is beginning to examine the effects of subject movement on the NQR signal.
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Analytical Chemistry, Vol. 66, No. 5, March 1, 1994 323 A
