System and method for contraband detection using nuclear quadrupole resonance including a sheet coil and RF shielding via waveguide below cutoff

A system for detecting a target substance within a class of explosives and narcotics containing quadrupolar nuclei in a specimen employing the phenomenon of nuclear quadrupole resonance (NQR), said system comprising:

• a sequence controller having means for providing timing and programming pulses to said system;
• a radio frequency (RF) subsystem comprising a variable frequency RF source to provide pulsed RF excitation at a frequency generally corresponding to predetermined characteristic nuclear quadrupolar resonant frequency of the specimen, wherein nuclear quadrupolar resonant frequency is the frequency of nuclear precession due to quadrupolar interaction with molecular electric field gradients;
• a detection head subsystem comprising:
• a single turn distributed RF coil sheet shaped and configured to define a cavity of predetermined volume therein and to receive the specimen within the cavity defined by said RF coil, said cavity having a first end and a second end, the RF signal from said RF source being transmitted within said cavity and being uniformly applied to the specimen within said RF coil cavity and generating a uniform field within said cavity, said RF coil also functioning as the pickup coil for the NQR signals from the specimen and providing an NQR output signal;
• apparatus for tuning said RF coil to about the desired characteristic nuclear quadrupolar resonant frequency for the specimen under test;
• and
• an electrically conductive RF shield surrounding and spaced from and electrically isolated from said RF coil, said RF shield being shaped and configured to provide electromagnetic interference and radio frequency interference (EMI/RFI) shielding from external noise and to prevent RF and magnetic flux from escaping from said RF coil cavity and RF shield combination, said RF shield configuration being longer than said RF coil, thereby extending beyond both said first end and said second end of said coil, said RF shield being an electrically integral part of said RF coil to improve the Q and the efficiency of said RF coil and contributing to the uniformity of flux field applied to the specimen within said RF coil, said RF coil and RF shield together forming a scanner;
• a signal capture and data processing subsystem having a digital signal processor and comprising:
• means for receiving the NQR output signal from said RF coil;
• memory means storing characteristics of NQR signals from at least one target substance in the class explosives and narcotic compounds;
• means for processing the NQR output signal from said RF coil;
• and
• means for comparing characteristics of the processed NQR output signal with the characteristics in memory and emitting a final output signal;
• and
• a display device receiving the final output signal from said signal capture and data processing subsystem and, in response thereto, said display device selectively indicating the presence of the target substance, the absence of the target substance, and an intermediate result when conditions of the received signal from the specimen indicate that further testing is necessary.