Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/441—Nuclear Quadrupole Resonance [NQR] Spectroscopy and Imaging

Definitions

• the present invention is directed generally to a method and an improved system for detecting nitrogenous explosives or narcotics by nuclear quadrupole resonance (NQR) , and more specifically, to a lower power method for detecting those materials.
• NQR nuclear quadrupole resonance
• NQR resonant frequency of a quadrupolar nucleus in a crystalline solid is quite well-defined.
• Most explosives of interest contain nitrogen and are crystalline solids.
• Most nitrogen found in the contents of airline bags is in a polymeric form, with associated broad, weaker NQR resonances and generally at frequencies other than the characteristic frequencies of the explosive.
• NQR is sensitive to the chemical structure, rather than just the nuclear cross-section, as in the thermal neutron analysis approaches. For NQR, false alarms from other nitrogenous materials will be far less of a problem than in nuclear-based detection techniques.
• Sensitivity though NQR is not a very sensitive spectroscopy, the parent disclosure describes techniques to make the response more sensitive to the desired explosive and less sensitive to interfering signals. Sensitivity is a function of coil geometry and coil size. The invention described in the parent disclosure has demonstrated sensitivity to detect the equivalent of sub- kilogram quantities of explosive near a brief case-sized meanderline coil and substantially less explosives in a small solenoidal coil of 25 mm diameter in a few seconds.
• Localization one of the novel features of the NRL approach is to localize the transmitting field and the receiver by a specialized surface coil, never previously used for NQR.
• the -meanderline 1 coil localizes the sensitive inspection region to a well- defined region. Furthermore, the electrical and magnetic fields of the meanderline coil fall off very rapidly with ' distance, so that a person can be scanned by an NQR detector without depositing substantial rf power into the body.
• the strength of the applied rf field need only be at least equal to the strength of the local magnetic field due to dipole-dipole interactions.
• a corollary of this principle is that the signal-to-noise ratio of a signal induced by a specimen of fixed size decreases by only the square root of the coil size, and using this recognition in the detection of explosives and narcotics by NQR.
• the power can be increased significantly less. Specifically, the power need only be increased by the square root of the increased coil size to assure maintenance of the same signal to noise ratio. This approach permits the use of larger coils than previously used.
• the approach is useful for both volume coils and surface coils.
• a 5 watt meanderline coil NQR explosives detector is feasible for use on people: the prior approach would have necessitated about a peak power of about 30 kW.
• the technique utilized according to the present invention is pure nuclear quadrupolar resonance as taught in the previously mentioned Buess et al. patent application. Excitation and detection may be performed by any means known in the art, for example, a surface coil, such as a meanderline coil or a more conventional 'volume 1 coil such as a cylindrical or rectangular solenoid, a toroid, or a Helmholtz coil. Pure NQR is typically performed in zero magnetic field: no magnet is required.
• the specimen is irradiated with a train of radio-frequency (rf) pulses whose frequency has been chosen to be near to the known 1 N NQR frequency of the explosive or narcotic.
• rf radio-frequency
• RDX has resonance lines near 1.8, 3.4 and 5.2 MHz, while
• PETN-s NQR resonances are near 0.4, 0.5, and 0.9 MHz.
• Any irradiation sequence useful in NQR processes may be used according to the present invention.
• One preferred irradiation sequence is the strong off-resonance comb (SORC) , described in lainer et al., J. Mole. Struct. , 58 , 63 (1980), (the entirety of which is incorporated herein by reference) in which the pulse separations are less than the spin-spin relaxation time
• ⁇ is the magnetogyric ratio of the nuclear spin
• t H is the pulse width.
• an intense pulse has a shorter duration and, correspondingly, excites a broader region of the spectrum.
• the pulses required to obtain 119° tip angle typically have widths of 20-50 ⁇ s and cover a bandwidth l/t H of 50-20 kHz.
• the rf field strength B., used in such cases is therefore 10-25 gauss.
• the magnitude of the rf field strength need only be larger or equal to the magnitude of the local magnetic field strength due to dipole-dipole contributions.
• the necessary rf field strength B 1rn ⁇ n is of the order of l/ ⁇ T 2 where T 2 is the spin-spin relaxation time due to dipolar decoupling. Therefore, for example, the strong off resonance comb excitation will work quite satisfactorily at such low rf intensity.
• the present invention has successfully utilized rf fields as low as 0.7 G (0.07 mT) .
• the width of the U N NQR line is also partly determined by inhomogeneous interactions due to distribution of the quadrupolar coupling constants, induced by strain, impurities and variations in temperature. Such an inhomogeneous contribution to the width is not as important as the homogeneous contribution from the dipole- dipole coupling.
• SUBSTITUTE SHEET Therefore, although the prior art applies an rf field of a strength that is at least 100 times greater in magnitude than that of the local magnetic field, the present invention achieves successful NQR detection of nitrogenous explosives and narcotics by using a rf field strength to local field strength ratio of from 1 or about 1 to about 50, preferably as close as possible to 1. Typically, a ratio of about 2 to about 30, more ' typically a ratio of about 2 to about 20, and most typically a ratio of about 2 to about 10 is used.
• a second, related aspect of the present invention is the use of large volume sample coils: since only rather modest rf field strengths are required, a fixed rf power can irradiate a much larger volume by the present method.
• a pulse of power P creates a rf field strength B 1 proportional to (PQ/Vv 0 ) 12 , where v 0 is the carrier frequency.
• B 1 the signal-to-noise ratio obtainable from a given amount of sample will scale with the strength of B., per unit current.
• a specimen of fixed size will induce a signal which scales as (coil volume) "12 .
• the penalty in signal- to-noise ratio increasing coil volume by a factor of 15000 on comparing a 20 cm 3 coil to a 300 liter coil volume is about 120.
• SUBSTITUTE SHEET dictates not only the necessary power requirement of the rf transmitter, but also determines the peak voltages induced in the specimen. One must also be concerned with average power which also places some requirements on the rf transmitter and also determines the maximum power which is deposited into the scanned object. (It must be noted that most of the rf energy is dissipated in the coil by resistive losses, with only a ' fraction dissipated in the specimen through dielectric or eddy current losses.
• the scaling of average power with size can be significantly less than linear, and in particular, may be between as low as proportionate to the square root of coil volume without significantly decreasing the signal to noise ratio.
• a typical rf pulse duty factor with short pulses spaced closer than the spin-spin relaxation time T 2 might be 0.2% for a small volume coil.
• a duty cycle of about 25% is then required for the 300 liter coil.
• the average power dissipated in the small coil would be 0.8 W and, by the present invention, only 100 W in the 300 liter coil, far less than the 6 kW average power which would be dictated by maintaining the large rf magnetic field in the large sample coil. While the above description has focused on "volume coils", for the sake of simplicity, other types of coils, such as the circular surface coil, the pancake coil, the meanderline and other variants, may be successfully used in conjunction with the principles of the present invention.

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• Physics & Mathematics (AREA)
• Spectroscopy & Molecular Physics (AREA)
• High Energy & Nuclear Physics (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Geophysics And Detection Of Objects (AREA)
• Other Investigation Or Analysis Of Materials By Electrical Means (AREA)
• Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)

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Citations (4)

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• 1992
  • 1992-04-24 WO PCT/US1992/003117 patent/WO1993002365A1/en not_active Application Discontinuation
  • 1992-04-24 CA CA 2113558 patent/CA2113558C/en not_active Expired - Lifetime
  • 1992-04-24 EP EP92916035A patent/EP0746774A1/en not_active Ceased
  • 1992-04-24 JP JP50276593A patent/JP2002509601A/ja active Pending

Patent Citations (4)

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