Note: Descriptions are shown in the official language in which they were submitted.
W093/02~5 2 1 1 3 5 S S PCT/US92/03117
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DETECTION OF EXPLOSIVE AND NARCOTICS BY LOW POWER LARGE
5 SAMPLE VOLUME NUCLEAR QUADRUPOLE RESONANCE (NQR)
Background of the Invention
Field of the Invention
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.
Description of the Prior Art
In order to limit the unrestricted flow of explosives and
narcotics, it is desired to detect sub-kilogram quantities of
those materials in monitoring stations. Most military
explosives and narcotics share common features: they are
crystalline solids containing nitrogen. Presently, the
explosive detections system and methods cannot reliably detect
sub-kilogram quantities of military explosives against a
background of more benign materials. In conventional vapor-
based systems, dynamites and contaminated explosives may be
detected. However, military explosives such as hexhydro-1,3,5-
trinitro-s-triazene (commonly referred to as RDX and 2,2-
bis[(nitroxy)methyl]-1,3-propanediol, dinitrate (commonly
referred to as PETN) are not reliably detected by the
conventional vapor base systems especially when countermeasures
are taken to reduce the effluent vapor and particles. Thermal
neutron systems, which are 14N detectors, can detect relevant
quantities of explosives. Unfortunately, conventional thermal
neutron analysis systems frequently alarm on nitrogen-
containing plastics. High false alarm rates are produced for
inspected bags containing a few bomb equivalents of nitrogen
in a benign form since the conventional thermal neutron
analysis systems are sensitive only to the nuclear cross
sections and not to any details of the particular chemical
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environment of the ~etecte~l nitrogen nuclear. Hence the false alarm rate is inherently high,
even with some spatial discrimination. Also, nuclear magnetic resonance (NMR) has been
considered for ~etecting explosives. Because a large magnetic field is conventionally required
for NMR, m~gn~tically recorded data would be undesirable altered and other m~gnPti7~kle
5 materials could be damaged. Furthermore, the conventional non-vapor methods and systems
are not suitable for inspect*ng people.
Buess et al., U.S. Patent 5,206,592, DETECTION OF EXPLOS~VES BY NUCLEAR
QUADRUPOLE RESONANCE, discloses a method and system for NQR detection of
explosives. Recited advantages of NQR for explosives detection are:
(i) Specificity: the NQR resonant frequency of a quadrupolar nucleus in a
crystalline solid is quite well-defined. Most explosives of interest conta*n 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 rllemir~l structure, rather
15 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.
(ii) Sensitivity: though NQR is not a very sensitive spectroscopy, the parent
disclosure describes techniques to make the response more sensitive to the des*ed explosive
20 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 subkilogram qu~ntiti~s of explosive near a brief case-sized m.o~n-lerline coil and
substantially less explosives in a small solenoidal coil of 25 mm ~ m~ter in a few seconds.
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(iii) 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. One type of surface coil, the 'meanderline'
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.
Summary of the Invention
It is an object of the present invention to safely detect
small quantities of nitrogenous explosives and narcotics within
a large volume of material to be searched using low power
techniques.
It is another object of the present invention to provide
a system for detecting nitrogenous explosives and narcotics by
nuclear quadrupole resonance over a large volume, at low power,
less intense rf fields.
These and other objects are achieved by recognizing that
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. Thus, rather than scaling
power linearly with coil size, as conventionally done to
maintain the same rf field intensity, 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.
For example, a more conventional approach would require
an rf peak power of about 6 MW for a 300 liter inspection
volume. In contrast we have achieved detection with power
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levels of 400 watts. 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.
Detailed Des~ lion of the Preferred El.ll,odi~ents
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 ~clroll,led by any means known in the art, for example, a surface coil, such
10 as a m~n~erline coil or a more conventional 'volume' 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.
As taught in the parent Buess et al. patent application, the specimen is irradiated with
a train of radio-frequency (rf) pulses whose frequency has been chosen to be near to the known
15 14N NQR frequency of the explosive or narcotic. For example, 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 prerclled irradiation sequence is the strong off-resonance comb (SORC), in which the
pulse separations are less than the spin-spin relaxation time T2, producing about one-half of the
20 equilibrium magnetization after every pulse.
Conventionally, intense rf magnetic fields are used to excite the NQR lines and
generation of such intense fields requires substantial rf power with associated possibility of
depositing unacceptable amounts of power into the scanned objects . Power deposition can have
unfortunate consequences for sc~nning of baggage and small cargo, wherein at some suitably
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high power level, damage to electronics may occur by over voltage or local heating through
electrostatic coupling of the electric field or inductive coupling to the m~n~tic
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W093/02365 2 1 1 3 5 ~ 8 PCT/US92/03117
field. For scanning people, rf power deposition, primarily by
eddy current loss, can pose a problem at these frequencies (1-5
MHz). A detailed discussion of the effects of rf power and
field strength values on articles and persons and the
acceptable levels of exposure to rf energy is unnecessary here
and beyond the scope of this disclosure. It is sufficient to
state that an advantage of the present approach that average
and peak rf power levels can be reduced by orders of magnitude
below those used in prior practice.
An rf field strength of B1 applied near the resonance
frequency nutates the spins (for a spin I=l nucleus) through
an angle of 2yB1tw, where y is the magnetogyric ratio of the
nuclear spin and t~ is the pulse width. For a fixed nutation
angle, an intense pulse has a shorter duration and,
correspondingly, excites a broader region of the spectrum.
Conventionally, one excites the NQR resonance with a pulse
sufficiently long to cause the spins to nutate through about
119~, giving a maximum magnetization. On commercial NQR
spectrometers in a laboratory setting, the pulses required to
obtain 119~ tip angle typically have widths of 20-50 ~s and
cover a bandwidth 1/t~ of 50-20 kHz. The rf field strength B1
used in such cases is therefore 10-25 gauss.
As part of the present invention, it was recognized that
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. Hence the necessary rf field
strength B1mjn is of the order of l/yT2 where T2 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. For RDX-based
explosives, the present invention has successfully utilized rf
fields as low as 0.7 G (0.07 mT). (The width of the 14N 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.)
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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. In a coil of
effective volume V and quality factor Q, a pulse of power P
creates a rf field strength B1 proportional to (PQ/Vvo)1/2, where
vO is the carrier frequency. By the principle of reciprocity,
the signal-to-noise ratio obtainable from a given amount of
sample will scale with the strength of B1 per unit current.
Hence, provided there is sufficient power to irradiate the NQR
line, a specimen of fixed size will induce a signal which
scales as (coil volume)~1/2. For example the penalty in signal-
to-noise ratio increasing coil volume by a factor of 15000 on
comparing a 20 cm3 coil to a 300 liter coil volume is about
120.
Thus, according to the present invention, one can
irradiate, for example, a volume of about 300 liters and detect
significant quantities of explosive in a reasonably short time
with an rf peak power of 400 watts, the same peak power
conventionally employed on a small 20 cm3 coil system. If one
followed the more prior art approach of maintaining the same
rf field intensity B~, one would need to scale the power
linearly with coil volume, necessitating an rf peak power of
6 MW for the 300 liter system, in contrast to the 400 watts
used according to the present invention. In the present
invention, the scaling of peak power with size can be
significantly less than linear.
As the NQR transitions are induced by rf pulses,
discussion so far has centered on the pulse or peak power which
creates the (peak) rf magnetic field intensity. Peak power
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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. Furthermore, where power dissipation in the
specimen may be a problem, there are other well-known rf
shielding techniques which can reduce the fraction of the rf
power actually dissipated in the specimen.) To appreciate the
advantages of the present invention, consider scaling up the
coil volume while keeping the rf peak power essentially fixed.
For simplicity of argument, keep the nutation angle about the
same in the large volume coil as in the small coil. Hence the
rf pulse length will need to be increased in direct proportion
to (coil volume) 1/2, and so the average power will increase by
that same factor, provided the same pulse spacing is
maintained. That is, 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. In the SORC sequence a typical rf pulse duty factor
with short pulses spaced closer than the spin-spin relaxation
time T2 might be 0.2% for a small volume coil. To maintain the
same nutation angle and the same spacing between pulses, a duty
cycle of about 25~ is then required for the 300 liter coil.
For operating conditions with a peak power of 400 w, 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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The improvement offered by the present invention to that
disclosed in the parent patent application is that very large
sample volumes can be inspected for explosives or narcotics by
NQR, without a proportional increase in peak power or average
power levels. In practice, suitcase-sized sample volumes can
be inspected at rather modest peak and average rf power levels.
Furthermore, this approach makes feasible the examination of
people by large surface coils, such as the meanderline, or even
'volume' coils such as a solenoid, as indicated below.
Obviously, many modifications and variations of the
present invention are possible in light of the above teachings.
It is therefore to be understood that, within the scope of the
appended claims, the invention may be practiced otherwise than
as specifically described.
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