Note: Descriptions are shown in the official language in which they were submitted.
Background of the Disclosure
The present disclosure is directed to detection ap-
paratus for elements. It contemplates the use of NMR
techniques. It particularly finds application in an appa-
ratus used in the detection and quantitative measurement of
hidden or secreted explosives. The present invention can be
readily adapted for de~ection of other materials in a wide
range of circumstances Explosives are presenting more and
more problems to airlines, postal authorities, packing and
shipping clerks, and many other people. Explosives in
letter bombs and other hidden explosives have created severe
problems, causing serious injuries and sometimes death, as
radical groups, including political extremists, resort to
the use of explosives and secreted bombs to settle their
grievances. Detection of explosives hidden in packages,
letters and elsewhere has become a problem. They are easy
to detect with metal detectors if they incorporate substan-
tial quantities of metal. Known techniques will locate such
explosives by detecting the presence of metal. However, it
is possible to make nonmetallic explosive bombs. The so-
called letter bomb is a good example of this. It is rela-
tively lightweight, not susceptible to conventional metal
detection techniques, and is otherwise quite dangerous. It
is dangerous because it typically does not explode until
someone attempts to open it, resulting in serious injuries
to the hands and upper portions of the body. It is so small
63~
that it can hardly be detected in and amo~g a typical batch
delivery of mail.
The detection technique must obtain a unique indication
of the explosive. The unique nature of the response is thus
tied to the fact that the primary and basic constituent of
any type of secreted bomb or explosive device is, in fact,
explosive materi~l itself. As a consequence of the presen
invention, enhanced nuclear magnetic resonance detection
apparatus has been provided which is able to detect ex-
plosives without requiring direct physical contact betweenthe ~uspected material and the detection apparatus. The
present invention thus utilizes easily achieved electro-
magnetic fields which penetrate into the package or sus
pected parcel. Indeed, the magnetic field will penetrate
the earth and other nonmetallic low conductivity materials.
Such materials include wood, plastics, glass and the like.
For instance, a letter bomb can be easily detected using the
present invention. This invention can-also detect explo-
sives in baggage or parcels. A land mine which is buried in
the ground and which is formed solely of explosives and
plastic parts can also be detected. Metal detectors, in-
cluding beat frequency oscillators, are unable to detect
bombs ma~e without metal.
Nuclear magnetic resonance is defined as the resonance
achieved whereby energy is transferred ~etween an RF mag-
netic field and a nucleus placed in a constant magnetic
field sufficiently strong to at least partly decouple the
nucleus from its orbital electrons. ~he relationship be-
tween frequency at which maximum energy is absorbed by the
atomic nuclei of the element, the resonant frequency and the
magnetic field intensity is a clue to identification of the
--3--
i3~
particular element involved. The NMR detection technique,
in general, is old and is found in various detector systems.
The difficulty with it lies in par~ in the scale factors.
For instance, significant quantities of material must be
present to concentrate the element of interest so that a
sizable response is obtained. The signals obtained by NMR
are ordinarily very small which requires high quality detec-
tion equipment. To the extent that they are larger for some
elements, they typically are smaller for other elements.
This is particularly true for some elements where the iso-
tope of the element of interest is available only in minute
quantities. Moreover, close coupling has been required to
improve the NMR signal.
Explosive detection typically is based on detecting
several elements together. Fortunately, the elements to be
detected come in different ratios for different explosives
which therefore have different signatures. Typically,
explosive compounds include hydrogen, nitrogen, carbon and
oxygen. The relative amounts of each element varies, and,
in some explosives, one of these elements may not be present.
The response of hydrogen to NMR techniques is maximum com-
pared to nitrogen. Regretably, hydrogen is typically always
a constituent o~ the materials near or surrounding the
specimen of interest. Thus, orre may have to look to the
responses of several combinations of the elements in the
suspected explosive.
The pre3ent invention, in the preferred embodiment,
uses the transient response to yield enhanced detection and
to overcome the problems o~ a steady state detection appa-
ratus. The problems include a lack of sensitivity in thedetector, the difficulties of obtaining adequate magnetic
63~
field strength and homogeneity at the suspected specimen,
and the difficulty of separating signals from hydrogen
nuclei resident in supporting materials such as wood,
plastic, soil, etc. The use of transient apparatus reduces
the necessity for high quality homogeneous magnetic fields.
This lowers the size, cost and complexity of the apparatus.
~loreover, since the coupling between nuclei or nuclei and
the lattice relates to the relaxation time, the transient
NMR signal may be more easily analyzed to delineate hydrogen
nuclei in a solid (perhaps the explosive) from hydrogen
nuclei in plastic or fluid materials, typically water or
pulpy materials such as wood, paper or cloth.
~ ne scale factor which presents great difficulties in
N~ techniques utilizing transient or steady state response
is the extremely large values of the so-called longitudinal
or spin-lattice relaxation time often observed in many
compounds. These times can measure tens of minutes, some-
times hours, in solids. Detection of the NMR response from
such materials requires that they remain in a polarizing
magnetic field, undisturbed for a time comparable to the
spin-lattice relaxation time prior to testing and observa-
tion. The relaxation time is so unduly large in such mate-
rials that NMR detection and measurement cannot be used
other than for laboratory investigations. Practical ap-
plications are forbidden as a result of this scale factor.
The longitudinal relaxation time (hereinafter referred
to as Tl) for selected compounds may be reduced in certain
conditions by the present invention. It has been discovered
that it is possible to adjust the polarizing magnetic field
applied to the specimen of interest so that two atomic
elements in the specimen are interacted. As an easy example,
--5--
3~
consider an explosive material which has nitro~en and
hydrogen. It is possible to adjust the polarizing magnetic
field so that the sèparation between Zeeman energy levels
for the proton (hydrogen nuclei) coincides with that between
the quadrupolar energy levels for the nitrogen spin system.
In certain compounds, the hydrogen and nitrogen are situated
relative to the lattice such that the hydrogen Tl is re~uced
as a result of the transfer o~ energy between the nitrogen
nuclei and the hydrogen nuclei. This transfer is enhanced
when the NMR frequency of the hydrogen coincides with the
NQR frequency of nitrogen.
The present invention is further capable of discrimi-
nating the NMR response of the same type nuclei in a differ-
ent material. As a simple example, the NMR resp~nse of
hydrogen nuclei in a solid is typically dif~erent from that
of hydrogen nuclei in a liquid. As another example, the NMR
response of hydrogen in some explosives may be discriminated
~rom that of many nonexplosive materials. This is helpful
in discriminating between different types of materials as in
the detection of secreted explosives.
The NMR response has a second time constant descriptive
of it which is the transverse time response or spin-spin
relaxation time constant, or T2 hereinafter. It has been
found highly desirable to seek the lon~itudinal time re-
sponse, or Tl, of most elements in contrast to detection of
T2. The present invention is uniquely successful in that it
is able to modify and reduce Tl in selected materials to a
smaller value and thereby obtain a more rapid response.
This serves to distinguish the NMR response of various
materials from other materials. This enables prompt and
rapid recognition of the unique signature of various e~plo-
sive materials.
~ 3~
In an alternate form not using the NM~-NQR between two
different types of nuclei, the magnetic field is held steady,
and the time be~ween successive NMR responses elicited from
the sample is varied. Compounds having different relaxation
times Tl can be discriminated by this form of the invention.
For a given element in a particular compound, the response
will vary dependent on the elapsed time between successive
observations of the response time.
Brief Description of the Preferred Embodiment
The apparatus and method of the present invention are
directed to an enhanced NMR detection technique. In one
embodiment, first and second elements in the presence of one
another are tested in a specimen which is potentially an
e~plosive. Through the testing of the specimen having two
elements in it, a magnetic field is imposed on the test
specimen. If the first element has a nuclear magnetic
dipole moment, then it will have a nuclear magnetic reso-
nance at a frequency which is proportional to an externally
applied magnetic field. If that magnetic field is made to
be of such an intensity that the NMR frequency of the first
element coincides with the NQR frequency of the second
element, coupling between the species nuclei will be greatly
enhanced, and Tl will be measurably reduced. Under these
conditions, energy passes more freely and rapidly between
the two elements. This typically will reduce Tl of one of
the elements and sometimes of both.
This invention makes use of this characteristic to
reduce the time required for detection of an NMR response
and to provide a means for separating the NMR response
produced by nuclei by certain selected materials from the
3~
NMR response produced by the same type nuclei in different,
and usually more common, materials. It should be remembered
that the amplitude of the NMR response is dependent upon the
quantity or concentration of nuclei, the type nuclei and
other scale factors~ The response is also a function of the
amount of time (relative to Tl) that the sample has been al-
lowed to remain in an appropriate magnetic field prior to
being tested. Some time is required to allow the nuclei to
become aligned with the magnetic field such as is necessary
to produce the largest NMR effects. Increased time normally
ali~ns more of the nuclei with the polarizing field. The
alternating electroma~netic field produced by the transmitter
in the process of obtaining the NMR response from the sample
causes the nuclear alignment to be disturbed. The disturbance
can be substantial. Realignment as necessary to obtain an
appreciable NMR response in subsequent tests is limited by
the time constant of the nuclei. If attempts are made to
repeatedly test the N~ response of the sample separated by
a time interval between tests that is short compared to Tl,
the NMR output signal is greatly reduced. The apparatus of
this invention varies the time constant of the nuclei in a
controlled manner to reduce the time required Eor obtaining
an NMR response of a useful amplitude and yielding enhanced
response from nuclei in a selected material. When the
sample is placed in a magnetic field of such an intensity
that the NMR frequency of the type nuclei to be detected
coincides with the NQR frequency of a second type nuclei in
the same compound, the Tl of the first nuclei may be reduced
by an appreciable factor. In this invention the magnetic
field applied to the compound to be tested is varied in a
manner to cause the material to be subjected to a field
--8--
3~
intensity such that the coincidence of NMR and NQR fre-
quencies occurs in the compound. For maximum effect, this
field intensity is maintained for a period of time that is
long compared to the shortened Tl of the compound. The NMR
response in the selected nuclei of the compound is then
tested. Following exposure of the compound to a different,
second, magnetic intensity for a time period that is short
compared to the Tl of the nuclei in that field intensity,
the NMR response in the selected nuclei is again tested.
The NMR response obtained following the e~posure to the
first field intensity is compared to the NMR response ob-
tained following the exposure to the said second field
intensity. If the compound has coincident NMR and NQR
~L~equencies at either field intensity, then this will be
revealed by a difference between the first and second NMR
responses made apparent by comparison.
An alternate approach is to hold the magnetic field
steady and inte~rogate the sample with RF pulses at a varied
rate. The relaxation time Tl will vary for some compounds,
~0 and the variation in time between tests will indicate the
presence of a particular compound.
Brief Description of the Drawin~s
Fig. 1 shows a sample testing apparatus in accordance
with the teachings of the present invention;
Fig. 2 is a timing chart showing how an enhanced signal
can be obtained;
Fig. 3 is a detailed schematic block diagram circuit
illustrating a means for analyzing data;
Fig. 4 is a graph of signal output versus time for
various chemicals illustrating various relaxation times;
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63~
Fig. 5 is a graph of magnetic field strenyth
versus time showing how a time-variant magnetic field tests
for various explosive materials; and
Fig. 6 is a graph of frequency versus field intensity
showing several frequencies at which coincidence will occur.
Detailed Descri~tion of the Disclosed Embodiment
Attention is first directed to Fig. 1 o~ the drawings,
where the numeral 10 identifies an NMR detection apparatus
in accordance with the teachings of the present invention.
The testing apparatus incorporates a sample holder 12 which
is surrounded by a coil 14. The coil 14 is connected in a
circuit communicating with a coupling network 16. The coil
and coupling network typically handle RF signals.
~ transmitter 18 is connected -to the coupling net-
work 16. A receiver 20 for the requencies of interest is
likewise connected to the coupling network. The receiver 20
forms an output signal which is communicated to a discrimi-
nator 22 which, in turn, is connected to a display 24. The
timed operation of all the equipment is determined by a
sequencer ?5, It forms a signal which is supplied on a
conductor to the transmitter 18, causing it to fire and form
an output pulse. This timed event is also communicated to
the discriminator 22 and the display 24. The sequencer 26
is also connected to a magnet controller 30. It forms a
suitable DC level which forms a magnetic field across the
poles of a large magnet 32. The magnet 32 has a coil or
winding 34 which is connected to the controller 30. Cùrrent
through the winding establishes a specified magnetic field
between the opposed or facing poles of the magnet.
--10--
_ ..
- ~4~3~
The operation of the ~evice is best described by
reference to certain timing charts and the signals shown on
them. As a beginning point, the magnet 32 establishes a
fixed magnetic field. It is adjustable to various levels,
but it is a DC field. It is typically a low level magnetic
field, typically in the range of up to a few thousand gauss.
It has been observed that the amplitude of the NMR signal
response depends on the duration of magnetization which is
inflicted on the specimen. As previously defined, NMR
phenomena occurs in a fixed magnetic field, and that is the
field provided by the magnet 32.
NMR output additionally requires an RF magnetic field
at right angles to the fixed or constant magnetic field. To
this end, the coil 14 has an axis approximately perpendicular
to the lines o~ magnetic flux between the two poles of the
magnet 32.
The rate at which alignment of the nuclei is achieved
in tne sample is indicated by the time constant Tl. Thus,
after the sample is placed in the magnetic field, and the
magnetic field is turned on, the amplitude of the prospec-
tive NMR response increases as a function of duration. Full
amplitude output is attained only after continued exposure
to the magnetic field for a period in excess of several fold
of Tl.
Dependent on the closeness of coupling of the element
to the lattice in which it is located, there is a time-
variant alignment with the field. Closely bound elements
align slowly and require hundreds of seconds to obtain one
time constant (63%) alignment. Moreoever, each interroga-
tion has a disrupting effect. The RF field initiatesprecession toward the RF lines of force from randomly
63~
achieved azir,luthal positions of the element nuclei pre-
viously magnetically aligned. Thus, each RF pulse is a
disturbance upsettïng alignment, and, therefore, excessive
pulsing with RF pulse bursts is counterproductive.
Sampling o~ the NMR response signal is occasioned by
transmitter bursts through the use of the transmitter 18
connected to the coil 14. However, each transmitted burst
substantially disrupts previously achieved nuclear align-
ment, and, hence, realignment must thereafter be restarted
to prepare for another RF pulse. This extends the time in
which a full amplitude NMR response (proportional to a-
lignment) can be reached. Accordingly, excessive sampling
is self-defeating in that the time to obtain a high degree
of alignment is extended. So to speak, the alignment pro-
cess must start all over again as a result of the distur
bance to alignment caused by the transmitted energy burst
applied to the coil 14.
There is a relationship between the magnetic field of
the magnet 32 and the frequency of the field formed by the
~0 coil 14. This is given by the relationship of equation 1.
Freq = k x H
where Freq = transmitter frequency
k = a constant
H = the static magnetic field strength.
By choosing a value of magnetic field strength, a particular
frequency for the NMR excited element is achieved. The
magnetic field strength is adjusta~le to vary the NMR fre-
quency. The adjustment will benefit the test provided the
field intensity adjustment is made with a view of finding
and matching the frequency of the second sc~mple element in
the NQR mode.
12 ~_
- (
0~3~
Under the assumption that the first element is present
with a second element subject to NQR, the two frequencies
are matched at a common frequency. The NQR mode oE excita-
tion is not universal to all elements. It is limited to
those having a nuclei spin number greater than l/2, and
includes isotopes of chlorine, iodine, nitrogen and others.
It is eventually a fi~ed frequency phenomena. The NQR
frequency can be varied slightly by external magnetic
fields, but it cannot be widely tuned by external means as
can the NMR frequency. It is pre existent, and the fre-
quency is dependent upon internal electric fields in the
molecular structure of the material. Therefore, the mag-
netic field is varied to vary the NMR frequency. The NQR
frequency of the second element, present in close proximity
to the first element in the lattice, is fixed, and the NMR
frequency is tuned to achieve a match-up. Coupling between
the first and second elements is achieved such that energy
is exchanged between elements to accelerate alignment of the
first element. The frequency match-up need not be perfect,
~0 but the rate of alignment is improved as the match-up is
improved. The NQR is intrinsic to the material of the
lattice and is primarily independent of external stimulation.
~hen the NMR mode of excitation in the first element is
achieved, there is an interchange between the two elements,
thereby transferring energy between them and modifying the
longitudinal relaxation time of the first element. This
time will be represented hereinafter as T3. T3 is thus the
modified longitudinal relaxation time.
Consider an example of the two element relationship.
For a sample of the explosive RDX, the nitrogen 14 has three
frequency groupings where NQR occurs, one being in the range
-13-
..~,
3~
of 1.830 to 1.733 megahertz, a second frequency of about
3.359 to 3.410 megahert~, and a third of about 3.192 to
5.240 megahertz. The NMR frequency of hydrogen in the
explosive RDX corresponding to these three NQR frequency
ranges was achieved at magnetic field intensities of about
400, 800 and 1200 gauss, respectively. This data has been
obtained for hydrogen and nitrogen in the presence of one
another in the expIosive RDX, using the nitro~en isotope
having a molecular weight of 14. As will be observed, in
the explosive RDX, each frequency is not a single resonant
frequency, but it is a collection of several closely grouped
frequencies. As an example, the frequencies mentioned above
are ranges, there being at least two frequencies or more in
each grouping. While higher frequencies may exist at which
the NMR of one element matches the NQR of another element,
it may be easier to use the lower frequencies listed above,
but higher crossover frequencies provide an improved NMR
response.
As will be observed from the foregoing data, multiple
frequencies exist in the explosive RDX at which the hydrogen-
nitrogen energy transfer occurs. The relationship between
the NMR frequency of hydrogen in RDX and magnetic field
strength is thus shown in Fig. 6, along with the crossover
regions where coincidence occurs with the NQR frequencies of
nitrogen 14. Spreading of the NQR lines is a result of the
2eeman effect caused by the magnetic field intensity.
Attention is next directed to Fig. 2 of the drawings.
In Fig. 2 of the drawings, several timed events are shown.
Fig. 2 is a timing chart. The numeral 40 identifies a first
magnetic level applied to the specimen from the magnet 32.
Preferably, a constant magnetic field i5 achieved for the
-14-
` ~4~63~
moment. The transmitter 18 is operated to form a first RF
burst 42 of a specified length. After a pause, another
burst 44 is applied from the transmitter. Typically, the RF
burst lengths may be on the order of 10 microseconds and the
pause between bursts of similar duration. After the ap- -
plication of the two bursts, the receiver 20 forms an output
pulse 46 which occurs after the second pulse. This pulse 46
is indicative of the NMR echo signal from a single element
of the material present in the field. To this juncture, the
NQR effect of the second element has not come into play.
It is presumed that the pulses 42 and 44 have a fixed
and common frequency duration and amplitude. Thereafter,
the following excitation is applied to the specimen. The
level 48 identifies a different magnetic field intensity
level. This different, fixed field acts on the sample which
has first and second elements in it which are intimately
comingled with one another. This is a magnetic field level
which brings tha NMR frequency of the first element to a
frequency matching the NQR frequency of the second element.
The field intensity is again returned to the level 40
and the NMR echo is obtained by the transmitted interroga-
tion pulses identified by the burst 50 and a second pulse
burst identified at 52. The pulses 50 and 52 are the same
as the pulses 42 and 44 in frequency, power level spacing
and length. The receiver output is an enhanced or enlarged
NMR signal 54, if material of characteristics described in
the immediately preceding paragraph is present in the sample
under test. It is enhanced by the coupling between the
first and the second element at a field level 48 which
reduces Tl to T3, and thus allows greater nuclear alignment
or polarization of the first element to occur within the
-15-
;)63~
time period separating ~he burst pair ~2 and 44 from the
pair 50 and 52 than occurred during the time period between
the first application of the field 40 and the first burst
pair 42 and 44. The larger amplitude is indicative of the
enhanced NMR echo amplitude.
The timing chart of Fig. 2 thus shows an enhanced
received signal. The enhancement is the result of the
greater polarization achieved in the first element within
the a~ailable time as a result of shortened relaxation time
which results from the match-up of the NMR-NQR frequencies.
The NQR frequency of the second element and the NMR fre-
~uency of the first element are matched, and energy then
easily transfers between the two elements. It should be
noted that the NMR frequency is variable dependent on field
intensity. By and large, the NQR frequency is only slightly
variable by external stimuli and is fixed by the molecular
structure of the element.
The detectable N~ amplitude is quite small at the
beginning of the magnetic field because there is very little
initial alignmen-t among the nuclei within the field. The
rate at ~hich aligmnent occurs relates to the definition of
the spin-lattice relaxation time Tl. Because the initial
amplitude is small, an NM~ signal at this time may be
difficult to detect.
In Fig. 2, the magnetic field is dropped back to the
level 40. Again, two more transmitter bursts are applied to
the coil 14. These are the pulse bursts 56 and 58 in
Fig. 2. The receiver will again provide an o~tput pulse 60.
It is shown to have reduced amplitude. This is the result
of the small nuclear realignment achieved in the short time
interval compared to the relaxation time, elapsed since the
-16-
33~
last disturbance, the pulse pair 50 and 52. It should be
noted that during the time period be-tween pair 50 and 52 and
pair 56 and 58 the magnetic field intensity is such that
- coincidence of the NMR-NQR frequencies does not occur, and
the relaxation time is not reduced.
It should be noted that the time (tl) between the pulse
pair 42 and 44 and pulse pair 50 and 52 may be the same as
the time period, t2, between pulse pair 50 and 52 and pulse
pair 56 and 58. Durin~ the time period, tl, the nuclei
attain greater alignment or polarization because the time
constant Tl is reduced to T3. This reduction is achieved by
the enhanced coupling that occurs between the nuclei of the
first element and the nuclei of the second element when the
magnetic field intensity is such that the NMR frequency of
the first element coincides with the NQR frequency of the
second element as previously described. T3 may be much
shorter than Tl, and nuclear alignment will then occur at a
much faster rate with the shorter time constant than is the
case with the longer time constant. By choosing the times
Of tl and t2 to be short compared to Tl but lon~ compared to
r3, the nuclear alignment that occurs durin~ the interval tl `
will be much greater than that which occurs during the time
t2. This causes the N~ echo 54 to be larger than the NMR
echo 60 when the material contains a compound wherein field
intensity 48 causes a reduction of Tl as previously described.
The two NMR signals 54 and 60 obtained from materials which
do not contain a compound with these characteristics will be
of very nearly equal amplitudes. A comparison of the ampli-
tudes of these two signals yields information on the presence
of the compound of interest in the material undex test.
-17-
Attenti~n is next directed to Fig. 3 of the drawings
where the discriminator is shown in greater detail. It is
triggered by the sequencer 26. It has an input signal from
the receiver 20 which is connected to three similar, even
identical, sample and hold amplifiers. Each amplifier is
turned on by a pulse generator. The pulse generator 62, the
generator 64 and generator 66 are respectively connected to
amplifiers 72, 74 and 76. The first and second amplifiers
are connected to a first comparator 68. A second compara-
tor 70 is connected to the second and third amplifiers.They measure the difference in the signals from the sample
and hold amplifiers and provide outputs to first and second
signal shapers 78 and 80. They, in turn, drive indica-
tors 82 and 84. Returning now to Fig. 2 of the drawings,
the sequencer 26 triggers the sample pulse generators to
take samples in the timed sequence indicated by the timed
wave forms 86, 88 and 90 o~ Fig. 2. These signals are the
input signals for the comparators. From the timed operation
of the sample and hold amplifiers, the signals are delivered
for use in comparing with known criteria to identify the
presence of a particular compound in the test specimen.
For various and sundry explosives~ it will be appre-
ciated that the signal provided is dependent on the chemical
and crystalline makeup of the explosi~es. The relaxation
time of several explosives is rather long. This is illus-
trated in Fig. 4 of the drawings. Fig. 4 thus illustrates
how the response will differ. The ordinate of the plot is
the pea~ amplitude of the hydrogen free induction decay
nuclear magnetic response which follows a single burst of
appropriate RF energy from the transmitter. A similar plot
would be applicable to th~ NMR echo following a double pulse
-18-
63~
burst as previously described. Fig. 4 thus shows the manner
in which the hydrogen NMR response increases as a func~ion
of time. Time is the time after first exposure of the
sample to the magnetic field or the elapsed time following
the prior disorienting transmitter burst. For the explosive
material RDX, it is also plotted on a tenfold scale in
Fig. 4. As will be appreciated, its response is so slow
that time will not ordinarily permit the use of the NMR
detection techniques without the enhanced response taught by
the present invention. In other words, the enhancement
taught herein is almost essential to detect RDX in any
reasonably short period of time.
Fig. S shows a timed and shaped pulse for the magnetic
field (level 48 in Fig. 2) which assures NMR frequency level
crossings between the relatively fixed NQR responses of
various explosives which are comprised of at least hydrogen
and nitrogen in compounds. As shown in Fig. 5, the magnetic
field is measured in gauss, and it is stepped or varied to
the indicated levels. As it is varied, it passes through
various intensities indicated on the decay curve where the
hydrogen NMR ~requency is equal to the nitrogen NQR fre-
quency for the indicated explosive compounds. The curve
thus shows how the NMR frequency of the hydrogen nuclei is
made equal to the NQR frequencies of the coupled nitrogen
n~clei in the compound and where within a short interval the
nuclei become aligned to enable detection.
Returning now to Fig. 2 of the drawings for explanation
of another means of discrimination where the relaxation time
of the element of interest is not altered, attention is
directed to the response of the receiver shown on the chart.
Assume that the sample includes an element in it which is to
--19--
3~
be detected. The element has a specified relaxation time
which is relatively long compared to that of interfering
materials which may be present. The time between the first
doublet 42 and 44 and the second doublet 50 and 52 is made
long in comparison to the relaxa~ion time. The time between
the second doublet burst and the third doublet burst is
shorter than the first time and preferably shorter than the
Tl of the material to be tested. The amplitude of the NMR
response following the second doublet burst is maximum while
the amplitude following the third doublet burst may be
relatively small. The two differing received responses
provide a basis for discrimination.
The apparatus shown in Fig. 3 is used for obtaining
this measure. The ~requency selected for the doublet burst
from the transmitter is selected such that the nuclei to be
detected is resonant when the magnetic ~ield is at the
level 40 shown in Fig. 2. The different magnetic field in-
tensity 48-is not re~uired for this discrimination technique.
The d~-scribed apparatus and method of the presènt
invention finds application primarily in the detection of
explosives, but it can be used to detect the presence of
elements in other types of compounds. It functions quite
nicely with inorganic materials. Organic materials present
no difficulty either. An example of a detectable nonexplo-
sive material exemplifying another hydrogen-nitrogen coupling
is hexamethylenetetramine.
The present invention provides output data which can be
compared with the signature of selected chemical compounds.
~hile there might be some ambiguity, in the sense of detect-
ing explosives, the ambiguity presents no problem. Thus,the explosive RDX may have a signature similar to a
-20-
63~
nonexplosive compound. Wnen used in inspection for bombs
and the like, it is wise to treat the nonexplosive compound
as an e~plosive. This occurs out of an abundance of pre-
caution, and, to that extent, the ambiguity might be incon-
venient but certainly not dangerous. More importantly, this
ambiguity is highly unlikely in inspecting packages, letters
and other mail. Therefore, the existence of possible ambi-
guities in the data is not meaningful. What is meaningful
is that RDX has a characteristic signature in parameters of
the NMR-NQR cross coupling between the hydrogen and nitrogen
in the explosive. Needless to say, other elements of the
materials can also be excited and tested. It is not neces-
sary to test only for hydrogen and nitrogen. The tests can
be xun for hydrogen and nitrogen, subsequently rerun for
hydrogen-chlorine interaction and so on. In each instance,
a different signature can be developed and compared with
standards obtained from laboratory measurements.
Representative test data for the hydrogen NMR fre~uency
equal to the nitrogen 14 ~QR frequency for several materials
20 is as follows:
Frequency
ChemicalField in Gaussin Megahertz
RDX explosive 1220 5.2
RDX explosive 790 3.4
RDX explosive 420 1.8
PETN 210 0.9
PETN `120 0~5
PETN 104 0.4
TNT 204 0.87
HMT 185 0.79
-21-
`` ~ ;31
..
The explosives listed above can be scanned by the time
shaped magnetic pulse of Fig. 5 which is representative of
the range of variations or intensity levels. The variations
- of field intensity interrogates for the listed explosives.
HMT (or hexamethylenetetramine) is not an explosive, and it
is included to show the response of a nonexplosive. Indeed,
the signature of a two element (one isotope NQR responsive
element) compound or mixture can be analyzed. The signature
is quickly obtained, and it is readily compared to the
expected data. In these tests, the NMR of hydrogen at 587
gauss is a frequency of about 2.5 megahertz. The frequency
is not critical for scanning for other coupled NQR elements,
and, hence, the frequency can be any value, say 2.0 to 5.0
megahertz. For best discrimination, it should not be
selected to coincide with the NQR frequency of a material to
be detected. Where a reduction of the relaxation time only
is desired, it may be selected to coincide with the NQR
frequency.
The foregoing is directed to the preferred embodiment,
~0 but the scope thereof is determined by the claims which
follow.
