This method and apparatus are directed to baggage inspection, and more particularly, baggage inspection to locate explosives. In the preferred and illustrated embodiment, a conveyor system is utilized to transport a bag past a first magnet to achieve an initial nuclear magnetic resonance (NMR) polarization of a selected element. The element in the preferred embodiment is hydrogen, it being noted that hydrogen in explosives has a long T.sub.1 and a short T.sub.2. The first magnet provides an initial polarization. The bag continues on the conveyor belt past a second magnet. The second magnet provides the measurement field intensity and a coil contains an interacting RF forming the interrogation pulses. Echos are received. In the preferred embodiment, first and subsequent second echos are received and stored. The two signals are subtracted from one another to a null. The NMR response of elemental hydrogen in compounds typified by explosives (having a long T.sub.1 and short T.sub.2) is accented by subtraction of the two stored signals to form a response indicative of explosive materials. The apparatus includes means for adjusting the magnetic field to accommodate distortions for metal in the baggage.
Armando De Los Santos; James D. King; William L. Rollwitz; George A. Matzkanin; Phillip A. Hornung