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Description  |
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TECHNICAL FIELD
This invention relates to a method and device for detecting concealed
substances such as explosives or drugs, for example in luggage at an
airport.
Explosives are occasionally concealed in luggage and parcels by terrorists
for example, and smuggled through airports despite the efforts of customs
officers. These devices are often not found because the daily volume of
luggage and cargo is such that manually searching every item is simply
impractical.
The Lockerbie disaster in 1988 highlighted the danger posed by Semtex, a
plastic explosive that is difficult to detect by conventional means,
particularly when formed into thin sheets. It is believed that the bomb
contained only 1/2 kg of Semtex which was packed into a portable radio.
The U.S. Federal Aviation Administration (FAA) is searching for a means of
detecting concealed explosives and has set minimum requirements for such a
detection system.
A balance must be struck between the risk of not detecting an explosive and
the delay and disruption caused by searching and false alarms since while
the detonation of an explosive device in an airport or aircraft is a rare
event, delays and disruptions are daily concerns.
BACKGROUND ART
Most explosives are characterized by high nitrogen and oxygen content and
low carbon and hydrogen content. They are also usually of high density.
Aware of this, the FAA began funding tests of thermal neutron activation
(TNA) in 1985. TNA involves the use of a radioactive source such as
Californium-252 which emits neutrons. The neutrons are slowed or moderated
in materials high in hydrogen such as polyethylene (at which stage the
neutrons are "thermalized") and are then absorbed by the object of
interest. The absorption leads to the emission of gamma rays which are
characteristic of the elements present.
Analysis of these rays provides information as to the nitrogen content of
the object bombarded. While explosives characteristically have a high
nitrogen content, so do other materials such as certain plastics, silk and
nylon which are commonly contained in luggage. Unfortunately TNA screening
devices cannot distinguish between explosive materials and these
non-explosive materials and so false alarms are often raised which can
cause considerable delays. Also, TNA scanning devices require a very
intense neutron source and extreme measures are needed to shield airport
staff and travellers from the radiation.
The sensitivity of these devices is also less than desirable but improving
that would increase the incidence of false alarms. Furthermore, TNA
scanning devices are about the size of a small car, can weigh of the order
of 10 tonnes and cost about $1 million each. Also, it has been suggested
that between 300 and 700 such units would be required to deal with the
demands of large international airports in the United States at a cost of
$500 million for the machines plus housing and operating costs of at least
$92 million a year.
Dual beam X-ray machines are being field tested. They can detect organic
materials, such as explosives, with one beam and inorganic materials, such
as metals, with the other beam.
Hand-held vapour sniffers are also being tested. These take in air and
identify molecules in terms of their vapour pressure, atomic weight and
liquid solubility by chromographic means.
Plastic explosives such as Semtex however have a lower vapour pressure than
TNT so they can be difficult to detect by such means.
Computerised tomography or CT scanning commonly used in medical diagnosis
and research has been applied to the problem by scanning for an object's
density, total mass and indicating its atomic number and composition. But
as Dr. Grodzin of MIT explained at an international meeting (International
Conference on Accelerators in Industry and Research, Denton, Tex., Nov.
5-9, 1990): "More than a dozen nuclear-based techniques have been proposed
for rapidly scanning airport luggage to find hidden explosives by
measuring their elemental distributions. In most almost every scheme, the
technological challenge is the accelerator, which must produce its intense
beams of neutrons and photons . . . in an airport environment, perhaps
even in an airport concourse".
DISCLOSURE OF THE INVENTION
The object of the invention is to provide an improved or at least
alternative method and apparatus for detecting concealed substances
including particularly explosives.
In broad terms the invention comprises a method for detecting the presence
of a number of substances in a container, comprising irradiating the
container with fast neutron and gamma source radiation, measuring the
extent to which each species of the source radiation is transmitted
through the container, and analysing the measurements with reference to
the known characteristic attenuation coefficients and density properties
of the substances for each species of the source radiation.
The invention also comprises apparatus for detecting the presence of a
number of substances in a container, which comprises means for irradiating
the container with fast neutron and gamma source radiation, means for
measuring the extent to which each species of said radiation is
transmitted through the container, and means for analysing the
measurements with reference to the known characteristic attenuation
coefficients and density properties of the substances for each species of
the source radiation.
In this specification and claims "container" includes suitcases, bags,
packages, boxes, parcels, freight containers and containers of any type
for carrying luggage or goods or the like, and in particular "containers"
that are moved through airports, rail and shipping terminals, postal
distribution centres and the like.
Also, in this specification "substance" is intended to include any
substance that is desired to be detected if present in a container, but
the method and device of the present invention will be described in terms
of detecting explosives or drugs only.
Preferably the neutron and gamma radiation are emitted substantially
simultaneously and they have different flight times to the detectors, or
different responses in the detectors so they can be readily distinguished.
Preferably the source radiation is obtained from a radioactive isotopic
source such as .sup.252 Cf or Am--Be, perhaps by means of a particle
accelerator/target system.
Preferably the device of the present invention has means for conveying a
number of containers between the irradiating means and the transmission
measuring means.
Preferably the transmission measurements are analysed by a data processor
and the results are displayed on a monitor.
Preferably the transmission measurements are combined with data collected
by TNA measuring equipment and/or x-ray measuring equipment.
Preferably the device has an alarm system that is triggered when one of the
substances is detected.
When the substance for detection is an explosive substance qualitative
analysis or detection can be made on the basis that explosive substances
characteristically have a high nitrogen and oxygen content but a low
carbon and hydrogen content. The thickness or the amount of explosive
substance is not required to be known, thus Semtex sheets are no less
easily detectable than Semtex sticks or lumps.
Preferably, neutron and gamma radiation are simultaneously transmitted
through the container and the reduction of intensities of the two
radiations together is analysed to yield information of material thickness
and mass attenuation coefficients. We refer to this method as the "NEUGAT"
(trademark) method (is neutron/gamma transmission). Preferably a computer
is employed to display such information graphically for example by
plotting characteristic explosive composition against density so that
action can be taken if an explosive-like substance is detected.
Each scan can be analysed as a combination of hypothetical explosive and
non-explosive material or, drugs and non-drugs material. This is possible
since on average the density of the contents of a suitcase is about 0.2
g/cm.sup.3 whereas the densities of explosive substances and drugs are
typically about 1.6 g/cm.sup.3 and about 1.0 g/cm.sup.3 respectively.
Complementary techniques can also be incorporated. For example, positron
emission tomography and x-ray scanning can be used in addition to a
nuclear technique with neutrons.
The neutron and gamma radiation can be provided from any suitable source,
for example a radioactive isotope source such as .sup.252 CF or Am--Be or
a nuclear particle accelerator target system. All sources can be kept well
within the limits required for operation in a public work-place. It also
provides sufficient precision when detecting explosives since the neutron
absorption coefficients vary by only approximately 20%.
New Zealand patent specification 213777/214666 discloses a method and an
apparatus for quantitatively analysing a mixture of two or more
components, such as meat and fat, using neutron/gamma transmission
scanning technology. The disclosure of that specification is to be
incorporated by reference into the specification of the present
application.
According to New Zealand patent specification 213777/214666, to obtain
quantitative analysis of an n-component system, at least n distinguishable
species of radiation are required and a mathematical analysis is made by
solving the n simultaneous equations describing the extent of transmission
of each species of radiation to find the weight fraction of each
component. This is described in more detail as follows:
Radiation is passed through the object being measured and the transmitted
radiation intensities are measured simultaneously by a suitable detector
or array of detectors that is/are connected to an electronic measuring and
computing apparatus. In a layered system, a more complex analysis can be
made by considering the known length of material through which the
radiation passes and the densities of the layered components so that the
depths of each layer can be measured.
Each component of the object mixture has a different known mass attenuation
coefficient for each species of source radiation. The attenuated
intensities (countrates) of neutron and gamma radiation must also be
measured when there is no object in the beam to account for background
radiation.
When there is an object in the beam the intensities of the neutron and
gamma radiation received by the detector(s) can be described in terms of a
two-component system for example as follows:
I.sub.n =I.sub.no exp [-(u.sub.na M.sub.s +u.sub.nb M.sub.b)](A)
I.gamma.=I.gamma..sub.o exp [-(u.gamma..sub.a M.sub.a +u.gamma..sub.b
M.sub.b)] (B)
wherein I.sub.n and I are the countrates of neutron and gamma rays after
passing through the object; I.sub.no and I.gamma..sub.o are their
unattenuated countrates; .mu..sub.na, .mu..sub.nb, .mu..gamma..sub.a and
.mu..gamma..sub.b are the neutron and gamma ray mass attenuation
coefficients of components a and b, for the source radiation employed; and
M.sub.a, M.sub.b are the mass thicknesses (mass per unit area in the beam)
of the components a and b.
Solving these two simultaneous equations (A and B) will give the values of
M.sub.a and M.sub.b and the weight fraction, for example, of component a,
is then simply M.sub.a /(M.sub.a +M.sub.b).
It should be noted that it is not necessary to know the thickness of the
object to make this simple calculation. And the measurement can be
integrated over the object volume sensed by the detector or detector array
to give the overall weight fraction for that volume.
DESCRIPTION OF THE DRAWINGS
A preferred form of apparatus of the invention is diagrammatically
represented in the accompanying drawings by way of example only and they
are not intended to limit the scope of protection sought which is defined
by the claims. In the drawings:
FIG. 1 shows the preferred form apparatus for detecting the presence of
substances such as explosives or drugs in suitcases, travel bags and other
containers of luggage that often pass through the customs section of an
airport.
FIG. 2 is a cross-sectional view through the preferred form apparatus of
FIG. 1.
FIGS. 3 and 4 represent time of flight spectra in the monitor and detectors
of the preferred form apparatus of the invention.
FIG. 5 is a graphical representation of the pulsing of radiation by means
of a pulsed accelerator.
FIG. 6a represents the calculated composition expressed as a weight percent
(wt %), W, against density, RHO, and shows an operational line called the
"NEUGAT" line which in general will be a curve.
FIG. 6b is similar to FIG. 6a but shows a hypothetical separation of
explosives, drugs and clothes signals on which basis a customs officer can
decide whether a suitcase should be manually searched.
FIG. 7 represents a graphical representation similar to that shown in FIG.
6 except the nitrogen gamma count rate is additionally shown by means of a
third axis.
The preferred form apparatus 1 shown in FIGS. 1 and 2 has a detection unit
5 through which suitcases, travel bags and other containers 10 can be
passed by means of a conveyor 15 in an airport, rail or shipping terminal,
or the like.
The detection unit 5 houses means for irradiating a container 10 within the
detection unit 5 with simultaneous fast neutron and gamma radiation.
Sources 6 of neutron and gamma radiation are surrounded by radiation
shielding within the detection unit 5. A number of detectors 7 are below
the conveyor 15 carrying containers such as luggage as shown. The
detectors 7 are supported by detection equipment 20 which includes photo
multipliers 21. The detectors and the detection equipment 20 enable each
species of said radiation to be detected after transmission thereof
through the container 10 that is being analysed. The detection equipment
20 is conveniently located below the detection unit 5 and the containers
10 pass through the detection unit 5 one by one between the irradiating
means and the detectors.
Data processing facilities such as a computer 25 are connected to the
detection unit 5 so that data collected by the detectors of the detection
equipment 20 is processed by the computer 25 and displayed by a monitor 30
in a form an operator stationed at the detection unit 5 can readily
interpret. If the operator having considered the information displayed by
the monitor 30, suspects the suitcase or other container 10 contains an
explosive substance the operator simply stops the conveyor 15 and takes
whatever action is required.
The use of the apparatus 1 avoids the delay and disruption caused by
manually searching all or selected containers. The apparatus 1 may be
fully automated so that an operator is alerted when the computer 25,
programmed with appropriate software, recognises an explosive-like
substance in a suitcase or other container 10.
The detection unit 5 contains NaI(T1) [sodium iodide] or B90 [bismuth
germanate] scintillation detectors.
The detection equipment 20 may conveniently comprise, for example, a number
of Nuclear Enterprises NE213 liquid scintillators or Bicron BC 501 liquid
scintillators (an organic scintillator utilizing proton recoil and pulse
shape discrimination), in a cell 15 cm in diameter and 15 cm in thickness,
coupled to Philips XP2041 or RCA 8854 photomultiplier tubes. Standard
electronic measuring equipment can be used. Such a system, used in
conjunction with an Am--Be source of about 10 curie or a 25 .mu.g
Californium-252 source.
The time of flight spectra in the monitor 30 and detectors are represented
in FIGS. 3 and 4 of the drawings.
The preferred form apparatus of the present invention operates by scanning
the container with fast neutrons and gamma rays to measure the
transmission properties, simultaneously measuring the characteristic
.gamma.-rays from neutron capture on nitrogen and combining information on
container mass and volume. In this way, regions of the scan that are
characteristic of explosive substances can be identified.
Lower neutron fluxes are used in comparison to present machines based
wholly on the .sup.14 N(n,.gamma.).sup.15 N reaction. This eases the
problem of using intense radiation sources in an airport environment. The
strength of the source used should be about 1% of those presently used.
An important feature of the device 1 is that it measures a key quantity,
W.sub.R or W (described below) which is independent of the thickness of
the material within the container that is scanned.
Am--Be or 252-Cf or a pulsed accelerator are suitable for use as sources of
radiation but other radioactive materials known to one skilled in the art
can be used. The pulsed accelerator involves an energetic deuteron beam
(e.g. 2 MeV) incident for example on a deuterium gas target, a tritium gas
target, a Beryllium or Lithium target producing reactions respectively
.sup.2 H(d,n).sup.3 He; .sup.3 H(d,n).sup.4 He; .sup.9 Be(d,n).sup.10 B;
.sup.7 Li(d,n).sup.8 Be, which are all neutron producing reactions.
The pulsed beam allows the separation of neutrons and gamma ray events in
time since neutrons take longer times to travel to the detectors than the
.gamma.-rays. The beam can pulse as illustrated in FIG. 5 wherein the
separation of pulses is about 250 ns.
During the short burst periods fast neurons and .gamma.-rays are detected.
During the 100 .mu.s period the .gamma.-rays from neutron thermalisation
and capture by .sup.14 N are detected. Detection during the 100 .mu.s
period minimises interference from the direct deuteron beam which is off
in this period.
Detection of Drugs and Explosives
Several variations are possible:
Equations:
Neutron transmission through container;
I.sub.n /I.sub.no =exp-{.mu..sub.n1 m.sub.1 +.mu..sub.n2 m.sub.2 }(1)
Gamma transmission through container:
I.gamma./I.gamma..sub.o =exp-{.mu..gamma..sub.1 m.sub.1 +.mu..gamma..sub.2
m.sub.2 } (2)
Thickness of contents if pure explosive;
##EQU1##
density of contents of container if pure explosive;
##EQU2##
Contents "factor"
##EQU3##
the data collected can be graphically represented as shown in FIGS. 6a and
6b.
Objects are scanned and moment by moment measurements made. Each
measurement involves the following:
The results of the analysis of the transmissions of the neutrons and the
gamma rays as counted in the detector are represented in a two (or more)
dimensional region called a W-RHO plane which is defined by the software.
This plane has certain mathematical characteristics which will now be
described.
A mixture of two standard materials (which may be fictitious) are
characterized by certain mass attenuation coefficients. The real time
measurements of this mixture can be represented as follows:
The composition is calculated from equation 5 (usually expressed as a
weight percent (wt %), W) by multiplying the result from equation 5 by
100. These wt % values define the Y values in the W-RHO plane. The X
values are calculated from the quantity given by equation 4 wherein x is
the effective thickness for the particular measurement. The effective
thickness is determined from the attenuation of the gamma ray component
alone. The expression:
1n(I.sub..gamma.o /I.sub..gamma.)=.mu..sub..gamma.1 M1+.mu..sub..gamma.2 M2
(6)
is used to estimate the amount of material in the beam. A value of the wt %
is a "working value" that is adopted solely for the purposes of the
thickness calculation. The masses per unit area of the two components are
indicated here by M1 and M2 (large letters) to distinguish them from the
NEUGAT estimates m.sub.1 and m.sub.2 deduced from equations 1 and 2.
Now, wt % W=(M1/(M1+M2))*100 (7)
and equations 6 and 7 can be used to solve for M1 and M2. The effective
thickness, x, can be calculated from:
x=(M1/rho1)+(M2/rho2) (8)
wherein rho1 is the density of pure material 1 and rho2 is the density of
pure material 2. It is this value "x" which is substituted into equation 4
and defines the X axis of the W-RHO plane.
An operational line can be defined on the W-RHO plane but which is only
relevant when mixtures of the "standard" materials 1 and 2 are being
analyzed. This operational line is called the "NEUGAT line" and is
analogous to the load line used to represent the range of values which can
be taken up by electronic devices such as amplifiers. In general the line
will be a curve defined by the equation:
##EQU4##
The end points for NEUGAT measurements are:
when RHO=rho1 then W=100 (10)
when RHO=rho2 then W=0 (11)
The NEUGAT line is shown in FIG. 6a. The region where data for a sample
consisting of 30 wt % material 1 (ml) is also indicated in that Figure. If
mixtures of the "standard" materials 1 and 2 are being analysed the data
will fall in a group for a certain mixture somewhere along the NEUGAT
line.
The thicknesses of material in the beam can vary considerably so real time
corrections are preferably made to the mass attenuation coefficients for
each measurement in a sequence based on the effective thickness for that
measurement. For example:
(i) The effective thickness of the material is estimated from equation 8.
(ii) The effective thickness value is used to correct the mass attenuation
coefficients based on a predetermined algorithm (usually the coefficients
reduce quadratically with increasing thickness).
(iii) The values of m1 and m2 are estimated using NEUGAT (equations 1 and
2).
(iv) The composition W is determined using equation 5 and expressed as a
weight percent.
(v) The results of (i) and (iii) are used to determine RHO.
(vi) The precision of the measurements of W and RHO can be calculated using
a standard statistical analysis such as that described by Tominaga et al
in International Journal of Applied Radiation and Isotopes, 34, 429
(1983).
The procedure for making real time measurements of mixtures of the
"standard" materials 1 and 2 using NEUGAT alone has been described thus
far, however, the importance of the W-RHO plane becomes apparent when
different mixtures are used.
The procedure that is now to be described enables real time measurements of
mixtures of any chemical combination to be made and interpreted. Every
chemical combination will have a grouped response somewhere on the defined
W-RHO plane but of course that is now not confined to the NEUGAT line
since that was defined solely with respect to the "standard" materials 1
and 2.
The NEUGAT response is unique to each chemical so it can be thought of in
terms of a signature or fingerprint. Similar chemicals give similar
responses and in fact the distance between the (x,y) values for each
chemical grouping is a measure of how similar certain chemicals are.
Explosives and drugs have characteristic features and thus distinctive
signatures that enable their detection by the invention method and device
of the present invention. For example, explosives have characteristically
high densities of nitrogen and hydrogen, and while drugs have lesser
densities they usually have high hydrogen concentrations. Experimentation
will reveal the regions on the W-RHO plane that characterise other groups
of chemically similar compounds such as prohibited materials.
Some regions of the W-RHO plane may overlap and so to enable accurate
determinations the W-RHO plane can be extended to three or more dimensions
by introducing other constraints such as neutron activation.
FIG. 6b shows a possible separation of explosives, drugs and clothes using
the W-RHO plane NEUGAT approach and FIG. 7 shows an extension to three
dimensions by including neutron activation data. Four or more dimensions
can be used if necessary if a number of complementary methods are are
brought to bear on the problem.
The presence of nitrogen can be monitored in addition to neutron/gamma
transmission using the .sup.14 N(n,.gamma.).sup.15 N reaction. This is
graphically illustrated in FIG. 7. C.sub.R represents the nitrogen gamma
count rate and:
I.sub.n =transmitted neutrons;
I.sub..gamma. =transmitted gamma rays;
I.sub.no =incident neutrons;
I.sub..gamma.o =incident neutrons;
.mu..sub.n1 =mass attenuation coefficient for reference material 1
(neutrons);
.mu..sub.n2 =mass attenuation coefficient for reference material 2
(neutrons);
m.sub.1 =areal density of reference material 1;
m.sub.2 =areal density of reference material 2;
.mu..sub..gamma.1 =mass attenuation coefficient for reference material 1
(gamma rays);
.mu..sub..gamma.2 =mass attenuation coefficient for reference material 2
(gamma rays);
RHO=density;
rho1=density of reference material 1;
rho2=density of reference material 2;
.mu..sub..gamma.exp =mass attenuation coefficient for pure explosive (gamma
rays);
o.sub.exp =density of explosive; and
The reference materials are chosen so that materials such as explosives,
drugs are conveniently portrayed. The reference materials, which may be
hypothetical, are specified by the choice of the mass attenuation
coefficients .mu..sub.1n, .mu..sub.2n, .mu..sub.1.gamma. and
.mu..sub.2.gamma..
The positions of the "scan event points" depend on the "relative amounts"
of the reference materials present. In actuality, other materials may be
present with different mass attenuation coefficients, but particular
materials will be grouped in definite regions of the scan event graphs.
The foregoing describes the invention including a preferred method and
device thereof. Alterations and modifications as will be obvious to one
skilled in the art are intended to be incorporated within the scope of the
invention which is defined by the following claims.
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