U.S. patent number RE36,201 [Application Number 08/842,512] was granted by the patent office on 1999-04-27 for high energy x-y neutron detector and radiographic/tomographic device.
Invention is credited to Thomas G. Miller.
United States Patent |
RE36,201 |
Miller |
April 27, 1999 |
High energy x-y neutron detector and radiographic/tomographic
device
Abstract
An improved fast neutron x-y detector and
radiographic/tomographic device utilizing a white neutron probe
(4). The invention includes a multiple scattering filter (44),
radiographic and tomographic imaging of the number densities of
atoms in small volume increments through a sample 32 and the
atomic, chemical and physical structure of a sample, (32), and
neural net analysis techniques, where the neural net is trained
through use of simulated volume increments. The invention detects
fast neutrons over a two dimensional plane, measures the energy of
the neutrons, and discriminates against gamma rays. In a preferred
embodiment, the detector face is constructed by stacking separate
bundles (6) of scintillating fiber optic strands (20) one on top of
the other. The first x-y coordinate is determined by which bundle
(6) the neutron strikes. The other x-y coordinate is calculated by
measuring the difference in time of flight for the scintillation
photon to travel to the opposite ends of the fiber optic strand 20.
In another embodiment, the detector is constructed of discrete
scintillator sections (48) connected to fiber optic strands (52) by
couplers (50) functioning as lens. The fiber optic strands (52) are
connected to a multi-anode photomultiplier (100) tube (56). The x-y
coordinate of a neutron interaction is determined by the row and
column of the affected scintillation section (48). Neutron energy
for both detectors is calculated by measuring the flight time of a
neutron from a point source (2) to the detector face.
Inventors: |
Miller; Thomas G. (Madison,
AL) |
Family
ID: |
46253378 |
Appl.
No.: |
08/842,512 |
Filed: |
April 23, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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964455 |
Oct 21, 1992 |
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Reissue of: |
106437 |
Aug 13, 1993 |
05410156 |
Apr 25, 1995 |
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Current U.S.
Class: |
250/390.04;
250/390.11 |
Current CPC
Class: |
G01T
3/06 (20130101); G01N 23/09 (20130101); G01V
5/005 (20130101); G01N 23/10 (20130101); G01V
5/0016 (20130101) |
Current International
Class: |
G01T
3/00 (20060101); G01T 3/06 (20060101); G01N
023/222 () |
Field of
Search: |
;290/390.04,390.11,390.12,390.02,390.03,391,370.05 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Article, "Guidelines for Preparing Responses to the Federal
Aviation Administrations Broad Agency Announcement (BAA) for
Aviation Security Research Proposals TCBAA 90-001", Federal
Aviation Administration Technical Center, New Jersey, Revision
3-Nov. 1, 1989 (Cover page & pp. 2-15). .
Article, "Determination of H, C, N, O Content of Bulk Materials
from Neutron-Attenuation Measurements", J.C. Overley Int. J.
Radiat. Isot., vol. 36, No. 3, pp. 185-191, 1985. .
"Nuclear-Based Techniques for Explosive Detection", T.Gozani,
R.Morgado, C.Seher, Journal of Energetic Materials, vol. 4, pp.
377-414 (1986). .
"Airport Tests of SAIC/FAA Explosive Detection System Based on
Thermal Neutron Activation Technique", P.Shea and T.Gozani,
American Defense Preparedness Assoc. Proceedings, Cambridge, MA,
Oct. 26, 1988. .
"Multi-dimensional Neutron-compound Tomography Using Cooled
Charge-coupled Device", IEEE Transactions on Nuclear Science, vol.
38, No. 2, Apr. 1991. .
"Element-Sensitive Computed Tomography with Fast Neutrons", by J.C.
Overley, Nuc. Instr. and Meth. In Physics Research, B24/25 (1987),
pp. 1058-1062. .
"A Pressurized Multi-Wire Proportional Chamber for Neutron
Imaging", IEEE Transactions on Nuclear Science, vol. NS-25, No. 1,
Feb. 1978, pp. 558-561. .
"PFNA Technique for the Detection of Explosives", Proc. Of First
Int. Symp. On Explosives Det. Technology, FAA Tech. Ctr., Atlantic
City Int. Airport, NJ, Feb. 1992..
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Primary Examiner: Westin; Edward P.
Assistant Examiner: Hanig; Richard
Attorney, Agent or Firm: Baker & Botts, L.L.P.
Parent Case Text
CROSS-REFERENCE
This application is a continuation in part of that certain patent
application "High Energy X-Y Neutron Detector and Radiographic
Device," Ser. No. 07/964,455, filed on Oct. 21, 1992, now
abandoned.
Claims
The embodiments of the invention and method in which an exclusive
property or privilege is claimed are defined below:
1. A method for analyzing neutrons of multiple energies which have
passed through a sample to determine the presence or absence
.Iadd.therein .Iaddend.of certain atoms .[.in specified number
densities and ratios,.]. for purposes of explosives detection or
analysis of a sample, comprising the steps of:
.Iadd.(a) .Iaddend.producing a white neutron beam (a beam of
neutrons of multiple energies);
.Iadd.(b) .Iaddend.determining .[.the.]. .Iadd.a baseline
.Iaddend.neutron .[.attenuation.]. .Iadd.intensity .Iaddend.of the
white neutron beam without a sample in the path of the beam;
.Iadd.(c) .Iaddend.directing the white neutron beam through the
sample;
.Iadd.(d) .Iaddend.reducing the multiple scattering of .[.said.].
neutrons .Iadd.by the sample.Iaddend.;
.Iadd.(e) .Iaddend.measuring the .[.attenuation.]. .Iadd.intensity
.Iaddend.of the neutrons which travel through the sample without
scattering;
.Iadd.(f) .Iaddend.comparing the baseline white neutron beam
.Iadd.intensity .Iaddend.directed onto the sample with the
.Iadd.intensity of .Iaddend.unscattered neutrons passing through
the sample, and determining .[.neutron.]. .Iadd.therefrom the
.Iaddend.attenuation .Iadd.of the white neutron beam .Iaddend.as a
function of neutron energy;
.Iadd.(g) .Iaddend.comparing the resulting attenuation with known
neutron cross-sections;
.Iadd.(h) .Iaddend.creating a radiographic or tomographic .[.image
showing.]. .Iadd.representation of .Iaddend.the number densities
.[.and.]. .Iadd.or .Iaddend.ratios of atoms throughout .[.volume.].
increments of the sample through .[.such.]. .Iadd.the
.Iaddend.comparison .Iadd.of step (g).Iaddend.; and
determining whether an explosive or other specific substance is
present in any such .[.volume.]. increment by comparing the
resulting number densities .[.and.]. .Iadd.or .Iaddend.ratios of
atoms in said .[.volume.]. increments of the sample to known number
densities .[.and.]. .Iadd.or .Iaddend.ratios of atoms in explosives
or other substance sought to be identified.
2. Apparatus for producing a radiographic/tomographic .[.view.].
.Iadd.representation .Iaddend.of a sample .[.showing.]. .Iadd.based
on .Iaddend.the number densities of atoms in
.Iadd.area/.Iaddend.volume increments through .[.a.]. .Iadd.the
.Iaddend.sample, .[.consisting of.]. .Iadd.comprising .Iaddend.a
first means for producing a beam of white neutrons and directing
said beam; a second means for conveying samples into place for
exposure to the beam; a third means for directing the beam through
the sample; a fourth means for reducing the multiple scattering of
neutrons by the sample; a fifth means .[.of.]. .Iadd.for
.Iaddend.detecting .Iadd.unscattered .Iaddend.neutrons .Iadd.passed
through the sample.Iaddend.; a sixth means .[.of.]. .Iadd.for
.Iaddend.determining the location in the sample through which
.[.such.]. .Iadd.said unscattered .Iaddend.neutrons pass; a seventh
means .[.of.]. .Iadd.for .Iaddend.measuring the intensity of
neutrons both before and after the sample is placed in the neutron
beam; an eighth means .[.of.]. .Iadd.for .Iaddend.comparing the
neutrons intensities which reach the detector means without a
sample in the neutron beam path with the .Iadd.unscattered
.Iaddend.neutron.[.s.]. .Iadd.intensities which reach the detector
means after .Iaddend.passing through the sample; a ninth means
.[.of.]. .Iadd.for .Iaddend.determining the number densities or
ratios of atoms.[., atomic, chemical or physical structure.]. of
the sample through such comparison; a tenth means .[.of.].
.Iadd.for .Iaddend.creating a
.Iadd.radiographic/.Iaddend.tomographic .[.image.].
.Iadd.representation .Iaddend.of the number densities or ratios of
atoms.[., atomic, chemical or physical structure.]. of the sample;
and an eleventh means .[.of.]. .Iadd.for .Iaddend.comparing said
.[.samples.]. .Iadd.number densities or ratios or the sample
.Iaddend.with a database of .[.samples.]. .Iadd.number densities or
ratios .Iaddend.with known features for features sought to be
identified in the unknown sample.[.s.]..
3. Apparatus .[.and means.]. of claim 2 .[.whereby.].
.Iadd.wherein.Iaddend .
.[.(A) the first means comprises a neutron accelerator producing a
pulsed beam of neutrons of multiple energies from 0.5-15
MeV;.].
.[.(B) the second means comprises a conveyor track system in which
samples are conveyed one at a time onto a turntable for purposes of
exposure to said neutron beam;.].
.[.(C) the third means comprises a sample turntable in front of a
neutron x-y detector so that all portions of said sample are
exposed to said beam of neutrons emanating from a neutron point
source produced by said accelerator;.].
.[.(D) the fourth means comprises the frustum of a cone placed
between said sample and detector, which frustum consists of a
neutron attenuating material and is constructed in a dartboard
configuration, in which sections of the dartboard alternate between
solid segments and hollow passages through such frustum, such that
the wall of each hollow passage consists of the outside edge of a
solid segment, and the hollow passages are constructed along
straight lines from said point source, which lines are
perpendicular to said detector, and means for rotating such frustum
on its axis;.].
.[.(E).]. the fifth means comprises:
(i) one or more scintillating fiber optic strands of a
predetermined geometric shape and length,
(ii) which fiber optic strands are formed into a predetermined
number of discrete bundles .[.(consisting of one or more of said
strands).]. stacked linearly one on top of the other;
(iii) with one or more scintillation sensors attached to the end of
each bundle, so that .[.all.]. .Iadd.each .Iaddend.fiber optic
strand.[.s are.]. .Iadd.is .Iaddend.coupled to a scintillation
sensor at each end of the bundle in which the fiber optic strand is
located.[.;.].
.[.(F) the sixth means comprises means of determining the specific
bundle containing a fiber optic strand in which a neutron
interaction occurs, thereby providing the first two dimensional
coordinate of the neutron interaction, and means for calculating
the other two dimensional coordinate of said neutron interaction by
measuring the difference in time which it takes a photon to travel
to opposite ends of such strand, a time delay being place in one
end to facilitate calibration;.].
.[.(G) the seventh means comprises means for calculating the energy
of a neutron by calculating the time of flight of the neutron from
the neutron point source to the interaction on said fiber optic
strand;.].
.[.(H) the eighth means comprises a means to determine the neutron
attenuation by first measuring and recording in a computer the
neutron spectrum with the sample out of the neutron beam and then
comparing such data with a measurement of the neutron spectrum with
the sample in the neutron beam;.].
.[.(I) the ninth means comprises a means to reconstruct the number
densities or ratios of atoms, atomic, physical or chemical
structure of the sample by using known total neutron cross sections
to determine which elements in the sample and their number
densities caused the measured neutron attenuation;.].
.[.(J) the tenth means comprises a means for creating a tomographic
image of the number densities of atoms, atomic, physical or
chemical structure through volume increments in the sample by
determining the neutron attenuation through the sample for several
angles through the sample;.].
.[.(K) the eleventh means comprises a database to train a neural
network to identify features of volume increments provided in a
tomographic image of an unknown sample, using said database
containing actual or simulated results of radiographic scans of
volume increments equal to the volume increments provided by said
tomographic scan, where the volume increments in the database
contain known features sought to be identified, so that the neutral
network can identify features of a volume increment of said unknown
sample when the same features appear in the volume increments of
such database.]..
4. Apparatus .[.and means.]. of claim 3, .[.whereby there is.].
.Iadd.wherein each bundle of said fifth means comprises .Iaddend.a
plurality of said fiber optic strands .[.described in the fifth
means which are.]..Iadd., said plurality of strands being
.Iaddend.routed to two or more scintillation sensors at each end of
a bundle in an alternating pattern, so that for any given
scintillating strand attached to a given scintillation sensor, all
contiguous fiber optic strands are routed to a different
scintillation sensor on the same end of the bundle, thereby
allowing discrimination of gamma rays from neutron scattering
events.
5. Apparatus .[.and means.]. of claim 3, in which the scintillation
sensor described in the fifth means is a photomultiplier tube and
the bundles are approximately 4 centimeters by 4 centimeters thick,
comprised of approximately 64 fiber optic strands per bundle, and
are greater than one meter in length.
6. Apparatus .[.and means.]. of claim 3, in which the scintillation
sensor described in the fifth means is a photomultiplier tube and
the bundles are comprised of a single bar of scintillating fiber
optic material approximately 4 centimeters by 1 centimeter thick,
and are equal to or less than one meter in length.
7. Apparatus .[.and means.]. of claim 3 is which the scintillation
sensors described in the fifth means .[.of claim 2.]. have discrete
channels.[., such as multichannel photomultiplier tubes,
microchannel plates, or CCD type detectors,.]. allowing each strand
in a bundle to be attached to a discrete anode or channel, so that
the scintillation sensor detects which of said strands registers a
scintillation.
8. Apparatus of claim 3 in which.Iadd.:.Iaddend.
the fifth means is a neutron detector .[.constructed as follows: a
neutron detector consisting of.]. .Iadd.comprising
.Iaddend.discrete sections .[.of a certain size.]. constructed of a
material which scintillates upon interaction with a neutron; each
scintillator section .[.is.]. .Iadd.being .Iaddend.connected by a
coupler to a non-scintillating fiber optic cable, which coupler is
constructed to concentrate the light, and which fiber optic cable
is connected to one anode of a multi-anode photomultiplier tube;
the photomultiplier tube .[.is.]. .Iadd.being .Iaddend.connected to
means for voltage and signaling which in turn is connected to a
specific memory bank in a computer, whereby neutrons contacting a
scintillator section cause the creation of a photon, which travels
down the fiber optic cable to said anode of the photomultiplier
tube and is recorded in the memory bank of a computer;
.[.and.].
the sixth means comprises a means for identifying the specific
scintillator section in which a neutron interaction occurs; and
the seventh means comprises a means for measuring the time of
flight of a neutron from the .[.point source.]. .Iadd.neutron beam
producing means .Iaddend.to a scintillator section, and producing
an output signal containing such .Iadd.time-of-flight
.Iaddend.information.
9. Apparatus of claim 3 .[.whereby.]. .Iadd.wherein:.Iaddend.
the fifth means comprises a high energy neutron detector .[.and
radiographic/tomographic device,.]. comprising:
(a) fiber optic scintillating strands of a predetermined geometric
shape and length, comprising a material which scintillates when a
neutron interaction occurs emitting light,
(b) one or more of said scintillating strands are fastened into a
discrete bundle of a predetermined width and depth,
(c) a predetermined number of said bundles are attached linearly
one on top of the other, whereby a detector face with two
dimensional coordinates is formed, with one set of coordinates
being the separate rows formed by the discrete bundles stacked one
on top of the other, and the other set of coordinates being the
points along the length of the scintillating strands constituting
the bundles, .Iadd.and.Iaddend.
(d) means for attaching the respective ends of the scintillating
strands in each bundle to one or more scintillation sensors
attached to each end of each of said bundles,
.Iadd.and.Iaddend.
.[.(e).]. .Iadd.the sixth means comprises .Iaddend.a means for
determining the bundle in which a neutron interaction occurs by
means of registering on one or more of said scintillation sensors
attached to each end of such bundle, thereby determining one
coordinate of the two dimensional location of the neutron on the
detector face, and .[.an apparatus combining or/sum and sum
circuits, time to amplitude converters and other common electronic
equipment such as discriminators, cabling and power supplies and a
specific location in the memory bank of a computer for storing
output information,.]. .Iadd.means .Iaddend.for measuring the
difference in time that it takes a scintillation photon in a said
fiber optic strand, caused by a neutron incident on said fiber
optic strand, to travel to the opposite ends of such fiber optic
strand, and producing an output signal containing such
.Iadd.time-difference .Iaddend.information, thereby allowing
calculation of the other coordinate of the two dimensional location
of the neutron on the detector face.[.,.].
.[.(f) the detector is constructed so that the neutrons from said
point source strike the detector face approximately perpendicular
to the lengths of one or more fiber optic strands constituting each
of such bundles; and a combination of or/sum and sum circuits, time
to amplitude converters and other common nuclear electronic
equipment such as discriminators, cabling and power supplies and a
specific location in the memory bank of a computer for storing
output information, in order to measure the time of flight of a
neutron from the point source to the detector face, and producing
an output signal containing such information, thereby allowing
calculation of the energy of the neutron.]..
10. Apparatus of claim 9, .[.whereby.]. .Iadd.wherein .Iaddend.said
fiber optic strands are routed to scintillation sensors at each end
of a bundle in an alternating pattern, so that for any given
scintillating strand attached to a given scintillation sensor, all
contiguous scintillating strands are routed to a different
scintillation sensor on the same end of the bundle, thereby
allowing discrimination of gamma rays from neutron
interactions.
11. Apparatus of claim 9 in which the scintillation sensors have
discrete channels.[., such as multichannel photomultiplier tubes,
microchannel plates, or CCD type detectors,.]. allowing a plurality
of strands to be attached to each .[.anode or.]. channel of the
sensor, or allowing the scintillation sensor to discriminate which
specific scintillating strand in a bundle registers a
scintillation.
12. Apparatus of claim 9 .[.having.]. .Iadd.wherein the third means
comprises .Iaddend.a sample turntable or similar device between the
.[.point.]. white neutron source and the detector array in order
that several neutron radiographic/spectroscopic views may be taken
through the sample .Iadd.within the range .Iaddend.from
.[.0.degree. to 180.degree. or from.]. 0.degree. to
360.degree..
13. .[.In a.]. .Iadd.A .Iaddend.device for .[.analyzing neutrons of
multiple energizer means to reduce or eliminate.]. .Iadd.reducing
or eliminating .Iaddend.the multiple scattering of .[.radiation
emanating from an object.]. .Iadd.neutrons .Iaddend.towards a
detector .Iadd.upon the passage of the neutrons through a
sample.Iaddend., .[.consisting of radiation.].
.Iadd.comprising:
a filter comprising neutron .Iaddend.attenuating material divided
into sections of .[.a specified geometric shape consisting of.].
alternating solid segments and hollow passages, which .[.device.].
.Iadd.filter .Iaddend.is placed between .[.a.]. .Iadd.the
.Iaddend.sample and .[.a.]. .Iadd.the .Iaddend.detector, and
.[.configured.].
.Iadd.means for rotating or oscillating said filter .Iaddend.so
that .[.(1).]. the said hollow passages are rotated or oscillated
so as to expose the entire detector surface, at different moments
in time, to .[.radiation.]. .Iadd.neutrons .Iaddend.proceeding
through the sample, .[.and (2).]. the dimensions of said
.[.device.]. .Iadd.filter.Iaddend., including its width and
diameter of its said hollow passages and said segments, .[.are.].
.Iadd.being so .Iaddend.constructed .[.so.]. that .[.radiation.].
.Iadd.neutrons .Iaddend.which .[.is.]. .Iadd.are .Iaddend.scattered
in said sample will not proceed through a hollow passage to the
detector .[.face.]. .Iadd.surface.Iaddend..
14. .[.Means.]. .Iadd.Apparatus .Iaddend.of claim 13 .[.for
reducing or eliminating the multiple scattering of radiation,
consisting of.]. .Iadd.wherein said filter comprises .Iaddend.the
frustum of a cone divided into alternating segments in a "dart
board" configuration around its axis, with sections .[.consisting
of.]. .Iadd.comprising .Iaddend.alternating hollow segments and
solid passages, .[.consisting of a radiation attenuating material,
and includes a means for rotating said frustum through its axis,
and is constructed so that.]. said hollow passages .[.lie.].
.Iadd.lying .Iaddend.along a straight line proceeding from .[.a
point.]. .Iadd.the .Iaddend.neutron source through a sample onto
and perpendicular to the .[.face.]. .Iadd.surface .Iaddend.of .[.a
radiation.]. .Iadd.the .Iaddend.detector .Iadd.means.Iaddend..
15. .[.Means.]. .Iadd.Apparatus .Iaddend.of claim 13 .[.for
reducing the scattering of radiation from an object onto a
detector, consisting of a means of exposing.]. .Iadd.wherein said
means for rotating or oscillating said filter exposes .Iaddend.the
entire .[.face.]. .Iadd.surface .Iaddend.of said detector at
different moments in time through the rotation or oscillation of
solid portions of .[.radiation.]. .Iadd.the neutron
.Iaddend.attenuating material and hollow passages in such material.
.Iadd.16. Apparatus of claim 2, wherein the first means comprises a
neutron accelerator producing a pulsed beam of neutrons of multiple
energies within the range of approximately 0.5-15
MeV..Iaddend..Iadd.17. Apparatus of claim 2, wherein the second
means comprises a conveyor track system in which samples are
conveyed one at a time into a position for exposure to said neutron
beam..Iaddend..Iadd.18. Apparatus of claim 17, wherein the third
means comprises:
a turntable located at said exposure position for receipt of a
sample from said conveyor track system; and
means for rotating said sample turntable so that multiple portions
of said
sample are exposed to said white beam of
neutrons..Iaddend..Iadd.19. Apparatus of claim 2, wherein the
fourth means comprises:
the frustum of a cone placed between said sample and the detector
means, which frustum comprises neutron attenuation material and is
constructed in a dartboard configuration, in which sections of the
dartboard alternate between solid segments and hollow passages
through such frustum, such that the wall of each hollow passage
defines the outside edge of a solid segment, and the hollow
passages are constructed along straight lines from the neutron
source, which lines are substantially perpendicular to said
detector means, and
means for rotating said frustum about its axis..Iaddend..Iadd.20.
Apparatus of claim 2, wherein the sixth means comprises:
means for determining the specific bundle containing a fiber optic
strand in which a neutron interaction occurs, thereby providing the
first two dimensional coordinate of the neutron interaction;
and
means for calculating the other two dimensional coordinate of said
neutron interaction by measuring the difference in time which it
takes a photon to
travel to opposite ends of such strand..Iaddend..Iadd.21. Apparatus
of claim 2, wherein the seventh means comprises means for
calculating the energy of a neutron by calculating the time of
flight of the neutron from the neutron source to the interaction on
said fiber optic strand..Iaddend..Iadd.22. Apparatus of claim 2,
wherein the eighth means comprises a means to determine the neutron
attenuation by first measuring and recording in a computer the
neutron intensity spectrum with the sample out of the neuron beam
and then comparing said recorded neutron intensity spectrum with a
measurement of the neutron intensity spectrum with the sample in
the neutron beam..Iaddend..Iadd.23. Apparatus of claim 2, wherein
the ninth means comprises a means to reconstruct the number
densities or ratios of atoms of elements of the sample by using
known total neutron cross sections to determine which elements in
the sample and their number densities caused the measured neutron
attenuation..Iaddend..Iadd.24. Apparatus of claim 23, wherein the
tenth means comprises a means for creating a tomographic
representation of the number densities of atoms through volume
increments in the sample by determining the neutron attenuation
through the sample for several angles
of said white neutron beam through the sample..Iaddend..Iadd.25.
Apparatus of claim 2 wherein the eleventh means comprises a neural
network trained to identify selected features of area/volume
increments provided in a radiographic/tomographic representation of
an unknown sample, said selected features corresponding to features
of area/volume increments of radiographic/tomographic
representations of known samples..Iaddend..Iadd.26. A method for
detecting contraband substances in luggage or other containers,
comprising:
(a) generating a pulsed white neutron beam having a spectrum of
neutron energies over a range sufficient to span the total cross
section neutron attenuation peaks characteristic of selected
constituent elements of contraband substances;
(b) directing the white neutron beam through a container to be
investigated;
(c) filtering neutrons scattered by the container;
(d) detecting unscattered neutrons passed through the
container;
(e) deriving from said detected neutrons a neutron intensity
spectrum for said container as a function of neutron energy;
(f) deriving from said container neutron intensity spectrum a
neutron attenuation spectrum for said container by comparing said
container neutron intensity spectrum with a baseline neutron
intensity spectrum derived from neutrons detected without a
container interposed in the white neutron beam; and
(g) deriving from said container neutron attenuation spectrum an
indication of the presence or absence of a contraband substance in
said
container..Iaddend..Iadd.27. The method of claim 26, wherein step
(g) comprises:
comparing said container neutron attenuation spectrum with known
total neutron cross sections for said selected elemental
constituents to derive the number densities of said selected
elemental constituents in the container; and
comparing said derived elemental constituent number densities to
the known number densities of said selected elemental constituents
in contraband substances sought to be detected..Iaddend..Iadd.28.
The method of claim 27, wherein the step of comparing said derived
elemental constituent number densities with known elemental
constituent number densities comprises:
forming ratios of said derived elemental constituent number
densities; and
comparing said elemental constituent ratios with like ratios of
said known elemental constituent number
densities..Iaddend..Iadd.29. The method of claim 28, wherein:
said selected elemental constituents comprise hydrogen (H),
nitrogen (N), carbon (C) and oxygen (O); and
said elemental constituent ratios comprise C/O, N/O and
H/C..Iaddend..Iadd. 0. The method of claim 29, wherein said derived
elemental constituent ratios are compared with said known elemental
constituent ratios by use of a neural network..Iaddend..Iadd.31.
The method of claim 26, further comprising:
directing the white neutron beam through the container at a
plurality of different angles relative to the container; and
repeating steps (c)-(g) at each of said angles..Iaddend..Iadd.32.
Apparatus for detecting contraband substances in luggage or other
containers, comprising:
(a) means for generating a pulsed white neutron beam having a
spectrum of neutron energies over a range sufficient to span the
total cross section neutron attenuation peaks characteristic of
selected constituent elements of contraband substances;
(b) means for directing the white neutron beam through a container
to be investigated;
(c) means for filtering neutrons scattered by the container;
(d) means for detecting unscattered neutrons passed through the
container;
(e) means for deriving from said detected neutrons a neutron
intensity spectrum for said container as a function of neutron
energy;
(f) means for deriving from said container neutron intensity
spectrum a neutron attenuation spectrum for said container by
comparing said container neutron intensity spectrum with a baseline
neutron intensity spectrum derived from neutrons detected without a
container interposed in the white neutron beam; and
(g) means for deriving from said container neutron attenuation
spectrum an indication of the presence or absence of a contraband
substance in said
container..Iaddend..Iadd.33. The apparatus of claim 32, wherein the
deriving means of paragraph (g) comprises:
means for comparing said container neutron attenuation spectrum
with known total neutron cross sections for said selected elemental
constituents to derive the number densities of said selected
elemental constituents in the container; and
means for comparing said derived elemental constituent number
densities to the known number densities of said selected elemental
constituents in contraband substances sought to be
detected..Iaddend..Iadd.34. The apparatus of claim 33, wherein the
means for comparing said derived elemental constituent number
densities with known elemental constituent number densities
comprises:
means for forming ratios of said derived elemental constituent
number densities; and
means for comparing said elemental constituent ratios with like
ratios of
said known elemental constituent number
densities..Iaddend..Iadd.35. The apparatus of claim 34,
wherein:
said selected elemental constituents comprise hydrogen (H),
nitrogen (N), carbon (C) and oxygen (O); and
said elemental constituent ratios comprise C/O, N/O and
H/C..Iaddend..Iadd.36. The apparatus of claim 35, wherein said
means for comparing said derived elemental constituent ratios with
said known elemental constituent ratios comprises a neural
network..Iaddend..Iadd.37. The apparatus of claim 32, further
comprising means for directing the white neutron beam through the
container at a plurality of different angles relative to the
container, and having said filtering, detecting and deriving
functions of paragraphs (c)-(g) being repeated at each of said
angles..Iaddend..Iadd.38. The apparatus of claim 32, wherein said
detecting means comprises a plurality of discrete neutron detectors
arranged in an x-y array, each of said discrete detectors being
separately coupled to the means for deriving a neutron
intensity
spectrum..Iaddend..Iadd.39. The apparatus of claim 38, wherein each
of said discrete neutron detectors comprises a scintillator which
is sensitive to fast neutrons and which is optically coupled to a
photomultiplier tube..Iaddend.
Description
FIELD OF INVENTION
This invention relates to a device and method of determining the
presence of substances in a sample through neutron
radiographic/tomographic imaging of the number densities of atoms
in the sample, and more particularly to the detection of
explosives.
BACKGROUND ART
1. Fast Neutron Radiographic and Tomographic Systems
A workable system for detecting explosives in airport luggage is
urgently needed. A very small piece of modern explosive will
destroy and airplane. These explosives are easy to hide and cannot
be detected by current systems. For example, a plastic explosive
hidden in a small radio apparently destroyed Pan American Flight
103 over Lockerbie, Soctland. The most accurate method would be a
tomographic scan which could identify the elements which make up an
explosive. For practical use in an airport, each scan would have to
be completed in seconds. A system this advanced does not exist and
is not possible under current technology.
Current methods for detecting explosives in airport luggage use
uncharged particles such as X-rays and neutrons. X-rays are
sensitive to differences in X-ray absorption coefficients in the
luggage. However, explosives have absorption coefficients similar
to many items commonly found in luggage. For this reason, detection
systems using X-rays have high false alarm rates. X-ray computed
tomography (CT) scanners are also used to inspect luggage. However,
CT scanners are also sensitive to X-ray absorption coefficients,
and so have the same problems as X-ray systems.
The most common nuclear based explosive detection methods are
thermal neutron absorption (TNA) and n, gamma pulsed fast neutron
spectroscopy. "Nuclear-Based Techniques for Explosive Detection",
T. Gozani, R. Morgado, C. Seher, Journal of Energetic Materials,
Vol. 4, pp. 377-414 (1986). The TNA detects the n, gamma reaction
on nitrogen, and so searches only for nitrogen. "Airport Tests Of
SAIC/FAA Explosive Detection System Based On Thermal Neutron
Activation", P. Shea and T. Gozani, American Defense Preparedness
Assoc. Proceedings, Cambridge, Mass., Oct. 26, 1988. TNA has an
unacceptably high false alarm rate, since many materials other than
explosives contain large amounts of nitrogen. Other problems with
TNA include that the neutrons must be thermalized, the n, gamma
cross section is in the millibarn range, it is difficult to obtain
the spatial nitrogen concentration, and the background count is
very high.
N, gamma pulsed spectroscopy detects the neutron inelastic
scattered gamma rays from nitrogen, carbon, and oxygen. Zdzisaw
Sawa and Tsahi Gozani, (PFNA Technique for the Detection of
Explosives), Proc. of First Int. Sym. on Explosives Det.
Technology, FAA Tech. Ctr., Atlantic City Int. Airport, N.J., Feb.
1992. Problems with n, gamma spectroscopy include that the cross
sections are still in the millibarn range, the background counts
are very high, determination of concentration as a function of
position has large uncertainties, and it is difficult to make a
gamma ray detector with adequate energy resolution and still
maintain high count rate capability.
TNA and n, gamma spectroscopy search for explosives in an indirect
way. Both cause a neutron interaction and then attempt to detect
the resulting gamma rays. Neither method accurately pinpoints the
location of an explosive in the luggage. A system is needed which
can probe directly for explosives through first order
interactions.
The most accurate method would be a tomographic scan which could
identify the number densities of the dements which make up
explosives in small volume increments through the luggage. Neutrons
are an ideal probe, because neutrons interact directly with the
atomic nuclie in the sample. A tomographic method which uses total
cross sections rather than partial cross sections would be optimal.
Current methods using neutron probes detect only second order
effects such as gamma rays (partial cross sections) and do not
provide tomographic images.
In conventional neutron tomography, neutrons are directed through a
sample and the result are recorded on a detector. E. W. McFarland,
R. C. Lanza and G. W. Poulos, "Multi-dimensional Neutron-computed
Tomography Using Cooled, Charged-coupled Devices," IEEE
Transactions on Nuclear Science, Vol. 38, No. 2, Apr. 1991. A
number of runs at different angles are used to create a tomographic
image. Conventional methods reconstruct the spatial distribitions
of macroscopic interaction cross sections from the attenuation of
radiation passing through the sample.
These conventional methods use monoenergetic neutron probes.
Monoenergetic probes show macroscopic cross-section variations,
while providing little information regarding atomic or chemical
structure. In contrast, a white neutron probe (fast neutrons of
multiple energies) would provide information not possible from a
monoenergetic neutron probe. Light provides a simple analogy. Under
a red light (single energy), all objects appear as shades of red.
However, a white light (multiple energies) reveals different colors
and other details. A tomographic system using a white neutron probe
would be far more advanced than any system in operation.
Unfortunately, under current technology, it cannot be built.
Problems include multiple scattering of radiation, inability to
detect and measure the energies of fast neutrons over an x-y plane,
and difficult in reducing and analyzing data. These numerous
problems would have to be solved in a single system.
Overly discussed that, under laboratory conditions, it is possible
to identify different elements in a sample by using a white neutron
beam. J. C. Overly, "Determinations of H, C, N and O content of
Bulk Materials from Neutron Activation Measurements," Int J. Radiat
Isot , Vol. 36, No. 3, pp. 185-191, 1985. Overly also discussed the
scientific principle in deducing the number densities of elements
along a neutron beam for a tomographic cut. "Element-Sensitive
Computed Tomography With Fast Neutrons" by J. C. Overly, Nuc.
Instr. and Meth. in Physics Research, B24/25 (1987) 1058-1062.
Overly required 23 hours for a single cut. A workable system for
detecting explosives must reduce this time to seconds. Even after
23 hours, Overly obtained only an estimate of number densities at a
single cut. A workable system must obtain a tomographic image of
the number densities of atoms over the entire suitcase in a matter
of seconds. As Overly acknowledged in his paper, current technology
has not advanced to the point of applying these scientific
principles in a workable system. The Federal Aviation Association
("FAA") recognized this critical need when requesting proposals for
an invention using a white neutron probe for explosives. FAA
"Guidelines for Preparing Responses to the Federal Aviation
Administration's Broad Agency Announcement For Aviation Security
Research Proposals,", p. 7, Nov. 1989. The FAA Guidelines
acknowledge that the scientific principle has not been applied to
airline security, even though it has been published for several
years. In fact, the scientific principle has not been applied to
any workable system. Neither the FAA, nor any other party, has
found a way to address all of the problems which must be solved in
order for a white neutron probe system to work. The following
sections describe these problems.
2. Multiple-Scattering Correction of Radiation
Neutron tomographic systems must detect neutrons from the source
while excluding neutrons scattered by the sample. If scattered
neutrons reach the detectors, the tomographic image will not be
accurate. X-ray systems use a detector located behind a shielded
slot only a few millimeters wide. The slot is narrow enough to
exclude most scattered radiation. This is one reason why a CAT scan
takes so long. The detector row moves slowly across the body. A
system using a large detector array could view the entire sample at
once. However, such detectors (if they existed) could not be used
for tomography until the multiple scattering problem is solved. The
problem is even more critical when designing a system using a white
neutron probe. The system must detect, measure the energies, and
catalog the location of millions of neutrons passing through
different parts of a sample each second. In order to provide
meaningful data, the spatial resolution must be as small as several
centimeters square. A workable system must eliminate multiple
scattering while completing a scan in only a few seconds. There is
a critical need for such a system.
Harding, Pat. No. 4,380,817 (1983) attempted to correct multiple
scattering for purposes other than tomography. Harding's method
measures electron density in a body through radiation which is
"single scattered" from a narrow pencil beam of radiation.
Harding's method shields the detector from the single scattered
radiation, so that the detector measures only multiple scattered
radiation. Then the multiple scattered radiation is subtracted from
the sum of the single and multiple scattered radiation. This method
is not workable for neutron tomography, which requires measuring
neutrons which are not scattered (either single or multiple
scattered). Harding's system would not work in a system using a
white neutron probe for tomography.
3. X-Y Position Fast Neutron Detectors
A tomographic system using a white neutron probe must detect fast
neutrons passing through a sample and pinpoint the location in the
sample for each neutron. The system must perform these functions
for millions of neutrons striking all portions of the sample each
second.
One solution is to place the sample in front of an x-y detector
which can record the two dimensional (x-y) coordinates of neutron
interactions. The points of interaction on the x-y detector
correspond to the locations in the sample. There are x-y neutron
detectors, but most are for thermal neutrons. (Neutrons which have
a kinetic energy of approximately 0.025 electron volts.) Many types
of x-y detectors use an element that has a large fission cross
section for thermal neutrons. The fission fragments are detected
through the ionization they produce. McFarland, cited above,
describes a detector using a sheet of .sup.6 Lif-ZnS. Lithium-6 has
a large fission cross section for thermal neutrons. A fraction of
the incident thermal neutrons interact with the Lithium-6 to
produce Lithium-7. The Lithium-7 in turn fissions into a triton and
an alpha particle, which cause a scintillation. A CCD camera
records the scintillation and its position. Another variation uses
an element that absorbs the thermal neutrons and emits X-rays or
gamma rays.
The above types of detectors have a low detection efficiency for
fast neutrons, since the fast neutron fission cross section is very
small. Also, current x-y detectors cannot perform all of the
functions needed for a tomographic system using a white neutron
probe. These functions include measuring neutron energy, achieving
high count rates, and collecting data in a way to facilitate
tomographic imaging.
Fast neutron x-y detectors do exist, but have certain drawbacks.
One type of x-y detector uses the multi-wire proportional counter
with s proton radiator. B. Director, S. Kaplin and V. Perez-Mendez,
"A Pressurized Multi-Wire Proportional Chamber for Neutron
Imaging," IEEE Tr. on Nucl. Sc., Vol. NS-25, No. 1, Feb. 1978,
558-561. The proton radiator is a thin sheet of a hydrogen rich
material such as polyethylene. A fraction of the incident neutrons
scatter from the protons in the radiator. The resulting recoil
protons enter the multi-wire proportional counter. The multi-wire
proportional counter consists of thin gas filled cells with small
wires running parallel throughout the cells. The wires are placed
at high voltage. When a proton enters a cell close to a particular
wire, a voltage pulse is created.
By recording the position of the voltage pulse, the position of the
event is known in the direction perpendicular to the wires. By
placing a second ionization chamber with wires running
perpendicular to the first set of wires, the position in the other
direction is known. The radiators must be very thin so that the
recoil protons can escape. In order to achieve reasonable
efficiency, many units must be placed in tandem. This setup cannot
efficiently count neutrons below 3 MeV, since the radiator would
require a width of nearly zero to allow the lower energy protons to
reach the first cell. Efficiency would approach zero. These
detectors do not allow measurement of neutron energy and would not
provide the high count rates or neutron energy resolution required
for a white neutron probe system.
An x-y detector for fast neutrons could be constructed from a large
number of individual photomultiplier tubes. The applicant describes
such a detector in "Contraband Detection Device", Ser. No.
07/635,996 filed on Dec. 31, 1990. One problem with this detector
is that its electronics are very complex. A complete detector
system is required for each photomultiplier tube. For example, a
detector face of only 400 cm. by 400 cm., with a spatial resolution
of 4 cm. by 4 cm., would require one hundred 4 cm. by 4 cm.
detectors to form a single row 400 cm. long. One hundred rows of
such detectors would provide a surface of 400 cm. by 400 cm. The
system would require 10,000 individual detectors and 10,000
complete electronics systems. The system would be highly complex
and expensive. Another problem is that the array is rigid and not
capable of being geometrically configured for the optimal
shape.
4. Collection, Reduction and Analysis of Data
Identifying contraband in sealed luggage is one of the most
difficult tasks facing scientists. Explosives and drugs contain the
same elements as most other items usually found in luggage. These
elements include hydrogen (H), carbon (C), nitrogen (N) and oxygen
(O). Explosives do contain characteristic ratios of these elements.
However, in order to identify these ratios, a tomographic system
would have to search an entire suitcase over small volume
increments with an uncertainty of only a few percent. These tasks
present enormous problems under current technology. Two problems,
multiple scattering and measuring radiation over an x-y plane, were
discussed above. In addition, the tomographic system would have to
gather, reduce, and analyze data for millions of neutron
interactions per second. The entire scan must be completed in
seconds. The system must distinguish neutrons from gamma rays. New
methods of data reduction and analysis would have to be developed
for such a system to function with the speed and accuracy required
for an airport system.
In summary, there are no tomographic systems which use a white
neutron probe. There are numerous problems, unsolved under current
technology, which prevent the operation of a workable system. These
problems include multiple scattering of neutrons, detecting fast
neutrons over a two-dimensional plane, calculating energies of the
neutrons, and collecting, reducing and analyzing the data, all
under circumstances when millions of neutrons will be incident on
the sample and detector each second. The entire analysis of each
sample must be completed in seconds. Such a system does not exist
and cannot be built under current technology.
Objects and Advantages
Accordingly, several objects and advantages of my invention are as
follows.
One object is to obtain a tomographic/radiographic image of the
number densities and ratios of atoms over small volume increments
in a sample.
Another object of the invention is to obtain the
tomographic/radiographic image within a matter of seconds.
Yet another object is to gather, reduce, analyze the data, and make
a determination whether an explosive or other contraband is present
in the sample, all within a matter of seconds.
A further object is to achieve high neutron count rates.
Yet another object is to eliminate or reduce multiple scattering of
neutrons from the sample.
Still another object is to determine the two dimensional location
of the unscattered neutrons passing through the sample.
Another object of the invention is to measure the energies of
unscattered neutrons passing through the sample.
A still further object is to distinguish gamma rays from
neutrons.
Additional objects, advantages and novel features of the invention
will be set forth in part in the description which follows, and in
part will become apparent to those skilled in the art upon
examination of the following, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and attained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
DRAWING FIGURES
FIG. 1 shows the entire tomographic device in its preferred
embodiment for purposes of orientation, with more details in later
figures.
FIG. 2 shows a detail of the multiple scattering filter.
FIG. 3 shows a sample on a turntable.
FIG. 4 shows the setup for the tomographic device using the strip
detector.
FIG. 5 shows the face of the strip detector with a view of the
bundles.
FIG. 6 shows the arrangement of a single bundle of the strip
detector with four photomultiplier tubes attached.
FIG. 7 shows the electronics system and circuits for one bundle of
the strip detector.
FIG. 8 shows how the tomographic device when using the strip
detector calculates the two dimensional placement and energy of a
neutron.
FIG. 9 shows a perspective view of the tomographic device when used
with the array detector.
FIG. 10 shows a perspective of the array detector.
FIG. 11 shows a detail of one scintillator section of the array
detector.
FIG. 12 shows the electronics system and circuits for one
scintillator section of the array detector.
FIGS. 13a-f show an output spectrum of the invention's neural net
analysis of a sample obtained during practice of the invention.
FIGS. 14a-b show how the tomographic device determines the x-y
position of a neutron interaction using the strip detector and
array detector.
FIG. 15 shows a flow chart providing an overview of the tomographic
imaging and analysis method.
______________________________________ Reference Numerals in
Drawings ______________________________________ 2 pulsed white
neutron source 4 pulsed white neutron beam 5 neutrons which have
passed thru sample 6 fiber optic bundle of strip detector 8
couplers for strip detector (This embodiment includes four
couplers, 8A, 8B, 8C, 8D.) 10 photomultiplier tubes for strip
detector (This embodiment includes four tubes, 10A, 10B, 10C, 10D.)
12 voltage cable for photomultiplier tubes and signaling cabling
for electronics for strip detector 14 electronics for strip
detector 15 electronics for array detector 16 computer 18 output
screen printout 20 individual scintillating fiber optic strands
inside a bundle for strip detector 21 bonding material fastening
together scintillating fiber optic strands into a bundle for strip
detector 22 second time to amplitude converter for strip detector
24 first time to amplitude converter for strip detector 26 first
or/sum circuit for strip detector 28 sum circuit for strip detector
30 second or/sum circuit for strip detector 31 turntable 32 sample
34 anticoincidence circuit for strip detector 36 time or beam
pick-off unit 37 pulsed accelerator 38 time pick-off unit for strip
detector 40 beam target 42 delay for strip detector 44 multiple
scattering filter 46 opaque cladding for scintillator section for
array detector 48 plastic scintillator section of array detector 50
scintillator - fiber optic coupler/lens for array detector 52
non-scintillating fiber optic cable for array detector 54 fiber
optic - anode coupler for array detector 56 multianode
photomultiplier tube for array detector 57 voltage divider for
array detector 58 single anode of multianode photomultiplier tube
for array detector 59 discriminator for array detector 60 signaling
cabling - array detector 61 or circuit - array detector 62 ion
source 64 radiation shield 66 luggage conveyor track with turntable
70 x-y detector (which may comprise 72 or 74) 72 array detector 74
strip detector 76 hollow passage through multiple scattering filter
78 solid segment of multiple scattering filter 80 flight path of
neutrons from point source to x-y detector face 81 x position
virtual detector of strip detector 84 time to amplitude converter
for array detector 86 time pick off unit for array detector
______________________________________
DESCRIPTION--FIGS. 1-14
Figures Incorporating the Entire Invention
FIG. 1 shows an overall view of the preferred embodiment of the
tomographic device as an airport explosives detection system. A
pulsed white neutron source 2, created by a pulsed accelerator 37,
provides a beam of white neutrons. In its preferred embodiment, the
detector surface has a radius of curvature equal to the distance
from the neutron source 2 to an x-y detector 70. The detector is a
square about one meter by one meter. The pulsed white neutron beam
4 contacts a sample 32. A conveyor track 66 conveys the sample 32
onto a turntable 31 in front of the x-y detector 70. Neutrons 5
passing through the sample contact a multiple scattering filter 44.
Neutrons which are not scattered and which pass through the filter
44 contact the x-y detector 70. A conventional computer (shown in
FIG. 4), such as an IBM 486 personal computer, stores data from the
x-y detector 70 in a specific memory bank.
FIG. 2 shows front and side views of the filter 44, which consists
of a frustum of a cone with alternating sections in a "dartboard"
configuration along is axis, with the sections alternating between
hollow passages 76 and solid segments 78. In its preferred
embodiment, the filter 44 is constructed of polyethylene or other
neutron attenuating material and is approximately 0.5 meters thick.
The hollow passages 76 lie along straight lines from the neutron
point source and perpendicular to the face of a detector.
FIG. 3 shows a sample 32 on a turntable 31. The pulsed white
neutron beam 4 strikes the sample 32. Views are taken at designated
angles from 0.degree. to 180.degree. or 0.degree. to 360.degree.
and the data stored in the memory bank of the computer 16.
Figures Incorporating the Strip Detector
FIG. 4 shows an overall view of the strip detector array. For
purposes of orientation, a pulsed white neutron source 2 provides a
beam of white neutrons 4. The strip detector consists of bundles 6
of one or more scintillating fiber optic strands 20 (shown in FIG.
5). Each bundle 6 is attached to four couplers 8, two at each end,
which attach four photomultiplier tubes 10, two at each end.
Cabling 12 is attached to the photomultiplier tubes for voltage and
signaling. The signaling cabling is directed to the electronics 14,
which in turn is connected to a specific memory bank in a computer
16 and output 18 such as a computer monitor. Electronic devices and
computer programs are readily available to process and store
information from the photomultiplier tubes 10 and electronics 14. A
common analog-to-digital converter is the PCA II available from
Tennelec Co., Oak Ridge, Tenn.
FIG. 5 shows the strip detector and a detail of the strands 20
comprising the bundles 6. A white neutron beam 4 is shown for
purposes of orientation. The bundles are 1 cm. in width by 4 cm. in
height, although the size of the bundles may vary according to the
use of the strip detector. Each strand 20 is square and can range
from 300 microns by 300 microns up to the actual size of the strip.
Each strand 20 is a light pipe. The strands 20 are constructed from
a plastic scintillator rich in hydrogen, such as BC 404. This type
of scintillator is readily available on the commercial market from
vendors including Bicron Corp., Newbury, Ohio. The strands 20
contain cladding with density less than the scintillator, which
provides internal reflections for photon scattering angles less
than the "critical angle." Each bundle is attached to another by a
bonding means 21 such as a commercially available glue. Each bundle
6 is connected to four couplers 8, which couple each end of the
individual strands 20 into one of four photomultiplier tubes 10
attached to each bundle 6. The cabling 12 attached to each
photomultiplier tube 10 supplies voltage and signaling cables to
the electronics 14 (not shown) for the detector.
FIG. 6 shows one bundle 6 and how the individual strands 20 of the
strip detector are connected to four photomultiplier tubes 10 A-D
through an alternating array. The strands 20 connected to
photomultiplier tube 10A are connected to photomultiplier tube 10D
and strands 20 connected to photomultiplier tube 10B are connected
to photomultiplier tube 10C. Each strand 20 connected to photo
multiplier tubes 10A and 10D are surrounded by four strands 20
connected to photomultiplier tubes 10B and 10D.
FIG. 7 shows two or/and circuits 26, 30 and one sum circuit 28
which are used as logic units in the circuit diagram. The or/sum
circuits 26, 30 can each be used as or circuits and sum circuits
either simultaneously or individually. Outputs from photomultiplier
tubes 10A and 10C go to the inputs of an anticoincidence circuit
34. An output from the anticoincidence circuit 34 indicates
detection of a neutron. Outputs of photomultiplier tubes 10A and
10B go to the input of or/sum circuit 26. Outputs of
photomultiplier tubes 10C and 10D go to the input of or/sum circuit
30. The sum output of or/sum circuit 26 gives the sum of the light
coming out of one end of the bundle, while the sum output of or/sum
circuit 30 gives the sum of the light coming out of the other end
of the bundle 6. The sum outputs of or/sum circuits 26, 30 are
summed by sum circuit 28, which gives the total amount of light
coming from the bundle and can be used as a side condition to
eliminate pulses that are too small or too large. The or output of
or/sum circuit 26 goes to the start input of time-to-amplitude
converter (TAC) 24. The or output of or/sum circuit 30 goes to the
stop input of TAC 24 through a delay 42. The output of TAC 24 is
used to determine the x coordinate of the neutron scattering event.
The or output of or/sum circuit 30 goes to the start input of TAC
22. The stop input for TAC 22 comes from the time pick-off 38. The
output of TAC 22 is used to determine the energy of the incident
neutrons. The outputs of TAC 22, TAC 24, sum circuit 28 and
anticoincidence circuit 34 are input into a general purpose
computer 16 into specific memory banks for recording and
processing.
FIG. 8 shows that two time-to-pulse height converters TAC 22 and
TAC 24 measure neutron energy and the position of neutron
interactions in the bundle. TAC 24 measures the time difference
between the pulses coming from the bundles 6. The tomographic
device calculates the x position of the interaction based upon this
time difference. TAC 22 measure the time of flight of each neutron
detected over a known flight path. The tomographic device
calculates the energy of the neutron based upon the time of flight
and known flight path.
Figures Incorporating the Array Detector
FIG. 9 shows an overall view of the array detector setup. A pulsed
accelerator creates a beam of white neutrons 4 from a beam target
40. The white neutron beam 4 passes through a sample 32 and
contacts the filter 44. Neutrons which are not scattered or
absorbed pass through the hollow passages 76 of the filter 44 and
contact the array detector. The array detector consists of discrete
plastic scintillator sections 48. The sides of the plastic
scintillator sections 48 are opaque to light through application of
an opaque material 46 such as electrician's tape. The entire
detector array is approximately 1 meter high and 1 meter long. Each
scintillator section 48 is attached to a fiber optic cable 52 by
means of a coupler 50. Each cable 52 is attached by means of a
coupler 54 to a multi-anode photomultiplier tube 56.
FIG. 10 shows the array detector. The face of the detector consists
of scintillator sections 48. The scintillator sections 48 are
approximately 2 cm. by 2 cm. wide and 1 cm. thick (which dimensions
may be adjusted according to the spatial resolution desired). Each
scintillator section 48 approximates a section of the inside of a
sphere with a radius of curvature equal to the flight path. Each
scintillator section 48 is connected by means of a
scintillator-fiber optic coupler 50 to a non-scintillating fiber
optic cable 52. Such coupler 50 is constructed as a lens to focus
light energy into the light pipe. Each cable 52 is a light pipe
constructed from a fiber optic cable. This type of fiber optic
cable is readily available on the commercial market. Each cable 52
is connected to one anode of a multi-anode photomultiplier tube 56.
The multi-anode tube 56 includes cabling 12 for voltage and
signaling. The signaling cabling is directed to the electronics 15,
which in turn is connected to a specific memory bank in a computer
16 and output 18 such as a computer monitor. Electronic devices and
computer programs are commercially available to process and store
information from the multi-anode tube 56 and for processing through
the electronics 15.
FIG. 11 shows one scintillator section 48 of the array detector and
how the scintillator section 48 is connected to a coupler 50
constructed as a lens to focus light energy into the cable 52,
which in turn is connected to the anode of the multi-anode tube 56.
The sides of each scintillator section 48 are covered by opaque
cladding 46.
FIG. 12 shows the electronics for the array detector. Each
scintillator section 48 is connected to the anode of a multi-anode
tube 56 via cables 52. The multi-anode tube 56 and its voltage
divider 57 convert light pulses from the scintillator sections 48
into electrical voltage pulses. The voltage pulses are fed through
connecting cables 60 to discriminators 59. Two voltage pulses are
created by each discriminator 59. One voltage pulse is fed to or
circuit 61, which in turn is fed to the start input of
time-to-amplitude (TAC) 84. The time-pick-off 86 provides the stop
input to TAC 84. The second output from the discriminator 59 is fed
to a gate input of the array detector computer 16 for storage of
that particular TAC pulse in a section of memory reserved for that
scintillator section 48.
FIG. 13 shows tomographic images created by the invention showing
explosives and drugs hidden in a suitcase. The suitcase contains
polyurethane, cocaine, and C-4 explosive. Polyurethane is used as a
decoy because it is nitrogen-rich and difficult to distinguish from
drugs or explosives. The suitcase is 60 cm. wide by 60 cm. long.
The C-4 explosive, polyurethane, and cocaine, each with dimensions
of 2 cm. by 3 cm., are in the middle of the suitcase. FIGS. 13a,
13b, 13c, and 13d show respectively the distributions of H, C, N,
and O at a tomographic cut through the suitcase at the suspected
location. FIG. 13e shows the neural network output of its
confidence limit that explosives are present at the cut. As shown
in FIG. 13e, when instructed to search only for explosives, the
tomographic device finds the explosives but ignores the
polyurethane and cocaine. FIG. 13f shows the output when the neural
network searches for drugs. As shown in FIG. 13f, when instructed
to search only for drugs, the tomographic device correctly
identifies the drugs and ignores both the explosives and
polyurethane. The tomographic device is able to search for
explosives and drugs simultaneously. As shown in FIG. 13, the
invention finds the small amounts of explosives and drugs hidden in
a suitcase even when hidden behind a nitrogen rich decoy.
FIG. 14a shows how the tomographic device determines the x-y
position of a neutron interaction using the strip detector. The
vertical (y) position is identified by the bundle 6 in which the
neutron interaction occurs. The horizontal (x) position of the
neutron interaction is determined by measuring the difference in
time which it takes a photon to travel to opposite ends of the
strand 20. The spatial resolution of the strip detector 74
constitutes a "virtual" detector 81.
FIG. 14b shows that for the array detector, the x-y position of a
neutron interaction corresponds to the row and column of the
affected scintillator section 48.
FIG. 15 provides a flowchart of the tomographic process and neural
net analysis in general terms.
Operation of Invention
1. Overview
A pulsed white neutron beam 4 is produced at a point source. The
white neutron beam 4 is incident on an x-y detector 70 (two such
detectors 72 and 74 are described in the present invention). A
computer 16 records a baseline measurement of neutron energies
without a sample in the beam. A conveyor track 66 then moves a
sample 32 onto a turntable 31 into the beam path in front of the
x-y detector 70. The white neutron beam 4 is directed at the sample
32. The filter 44 prevents neutrons scattered in the sample from
reaching the x-y detector. A portion of the non-scattered neutrons
travel through the hollow passages 76 in the filter 44 and contact
the x-y detector 70. The tomographic device records the two
dimensional positions and the energies of these neutrons. The
tomographic device compares the transmitted beam to the original
baseline beam. The tomographic device then determines neutron
attenuation as a function of neutron energy through this
comparison. The tomographic device then compares the resulting
neutron attenuation curves with known total neutron cross sections
to determine the elements/cm.sup.2 in the neutron beam path that
caused the specific neutron attenuation curves in the transmitted
beam.
If the invention uses its tomographic option, the procedure is the
same except that the invention rotates the sample 32 on the
turntable 31. The tomographic device performs multiple scans of the
sample at different angles. The tomographic device then inputs the
resulting number densities into a tomographic program to determine
the number densities of H, C, N and O per cm.sup.3 at designated
cuts along the sealed container. The tomographic device uses
information from several cuts to create three dimensional
distributions of H, C, N and O through the sample. The invention
determines the ratios of H, C, N, and O for each small volume
increment separately. The invention then applies its neural net
methods to determine whether any of the volume increments contains
an explosive. If H, C, N and O are present in the applicable
ratios, the tomographic device sounds an alarm that an explosive is
found. The following description describes certain aspects of the
invention in more detail.
2. Correction of Multiple Scattering
The tomographic device contains a multiple scattering filter 44.
The filter 44 allows neutrons to contact the detector only if the
neutrons pass through the sample 32 without scattering. In the
embodiment shown, the filter 44 is comprised of a frustum of a
solid cone constructed of polyethylene (or other neutron
attenuating material). The filter 44 is placed between the sample
32 and the x-y detector 70. The narrow end of the frustum faces the
white neutron source. The frustum is divided into a "dartboard"
configuration. The sections of the "dartboard" are alternating
solid segments 78 and hollow passages 76 through the frustum along
its axis. Each hollow passage 76 is constructed along a straight
line from the white neutron source 2 and perpendicular to the face
of the x-y detector 70. In this way, a neutron will cross a hollow
passage only by traveling directly from the point source.
The filter 44 rotates and so the sample 32 is uniformly irradiated.
In one embodiment the filter 44 is rotated by attaching a grooved
track along the outside circumference of the filter 44. A
conventional fan belt is placed in the groove and attached to a
pulley on a conventional electric motor. In the preferred
embodiment, the sample 32 will remain in the beam for 8-10 seconds.
The neutron flight time for the tomographic device is at most 400
nanoseconds, while the filter 44 rotates only a few times per
second. Hence, the filter 44 is essentially still during the flight
of any specific neutron. In this way, a neutron at the point source
"sees" either a solid segment or a hollow passage of the filter. A
neutron that "sees" a hollow passage will proceed to the sample
towards that hollow passage. If the neutron is not scattered, it
will travel through the sample, through the hollow passage, and
will contact the x-y detector.
3. Operation of Strip Detector
A. Detection of Fast Neutrons
In the manner described above, unscattered neutrons will contact a
bundle 6 of the strip detector 74. Certain neutrons will strike a
scintillator strand 20 and scatter from hydrogen (protons). The
protons will recoil in the strand 20 and emit light energy. A
wavelength shifter is mixed in the scintillator. This type of
scintillator is commercially available, such as BC 404 manufactured
by Bicron Corporation. A fraction of this light is trapped by the
strand 20 and travels (in both directions) to photomultiplier tubes
10A-D attached to each end, as shown in FIG. 6. The photomultiplier
tubes 10A-D give a voltage pulse proportional to the intensity of
the light pulse. The voltage pulse from the photomultiplier tubes
are processed by the computer 16, through storage into specific
memory banks, to yield information regarding the neutron's two
dimensional position and neutron energy. Individual bundles 6 of
the strip detector are controlled independently, each with its own
photomultiplier tubes, acquisition system and microprocessor. Among
other advantages, this construction allows more efficient
processing and would allow the system to continue to function if a
bundle becomes inoperable.
B. Calculation of Two Dimensional Location of Neutrons
As shown in FIG. 14a the strip detector 74 is constructed of
separate bundles 6 stacked in rows one on top of the other. Each
strand 20 in a bundle 6 is attached to a photomultiplier tube 10. A
neutron striking a strand 20 will cause the photomultiplier tubes
attached to the bundle to register and identify a neutron
interaction at the y (vertical) coordinate represented by the
bundle. The computer 16 registers and stores this data in a
specific memory location.
The spatial resolution of the y coordinate is determined by the
width of the bundle 6. The optimal width of a bundle will be
determined by the specific use of the detector. A wider bundle will
yield more counts but less certainty regarding the point of
interaction. A narrower bundle yields fewer counts but more
certainty. In a preferred embodiment, the bundles 6 will be about 2
cm.-4 cm. wide.
A method by which the tomographic device measures the x position of
the neutron interaction includes the following. Light travels from
the point of a neutron interaction towards each end of the strand
20. The tomographic device determines the point along the strand 20
where the photon was created by measuring the difference in time
for a photon to travel to both ends of the strand. This point is
the x coordinate of the interaction. From FIG. 5, assume the light
photons travel toward photomultiplier tubes 10A and 10B, to input
of or/sum circuit 26 whose or output is fed to the input of
time-to-amplitude converter 24. Simultaneously, photons from the
interaction travel through the strand toward photomultiplier tubes
10C and 10D, to or/sum circuit 30, whose or output is delayed by
delay 42 and goes to the stop input of time-to-amplitude converter
24. Through this measurement the tomographic device calculates the
time difference for the photon to travel to both ends of the strand
20.
The time for the pulse to get to the start input of
time-to-amplitude converter 24 would be nx/c where x is the x
position of the interaction and v is the velocity of light in the
scintillation. (Note: v=c/n where c is the velocity of light in a
vacuum and n is the index of refraction of the scintillation
material.) The time for the start pulse to get to time-to-amplitude
converter 22 would be:
where L/v is the time delay 42 that has been inserted in the start.
A delay 42 is placed in the start input of TAC 24 to normalize the
TAC 24 output so that t=0 output corresponds to x=0. Hence, the
time difference, t, as measured by time-to-amplitude converter 24
is:
This is a linear equation with a y intercept of 0 and a slope of
2/v. The tomographic device determines the distance x and hence the
point of interaction by measuring the time t through
time-to-amplitude converter 24. When x is zero, the time difference
will be 0 and when x=L, the length of the bundle, the time
difference will be 2/v. FIG. 14a shows that the resolution (or
uncertainty) of this measurement constitutes a "virtual detector"
81, as shown in FIG. 14a. For the preferred embodiment as an
explosives detection system, this resolution will be approximately
4 cm. The time x/v must be subtracted from the neutron flight time
measurement as a correction for the position of the neutron
interaction along the bundle 6.
C. Calculation of Neutron Energy
The tomographic device measures neutron energy by measuring the
flight time of a neutron over the known flight path. A method
includes the following. From FIG. 5, the flight path of each
neutron is the distance from the neutron bean target 2 to the
bundle 6 on the x-y detector. (The bundles 6 are curved so that the
flight path would be the same for all angles of the solid angle
subtended by the detector.) From the measured time and the flight
path, the tomographic device calculates velocity, v=d/t. From the
velocity, the tomographic device calculates neutron energy from
E=mv.sup.2 /2. In practice, relativistic equations are used for
greater accuracy.
From FIG. 7, time to amplitude converter 22 measures the time of
flight for the neutron. The time pick off unit 36 gives the start
signal for time to amplitude converter 22 when the neutrons stat at
the point source. The output of or/sum circuit 30 provides the stop
signal. The stop signal is a sum of the flight time of the neutron
and the flight time of the photons in the strand 20. The
tomographic device subtract the flight time of the photons in the
strand 20 (x/v) when calculating the flight time of the
neutrons.
D. Discrimination of Gamma Rays
A scintillation occurs when a neutron scatters from a proton
(hydrogen) contained in a strand 20. These recoil protons will stay
predominantly in one strand. In contrast, gamma rays will Comptom
scatter from electrons in the strand 20. These recoil electrons
will travel across several strands. In this way, gamma rays will
cause scintillations in two or more strands. FIG. 7 shows that
photomultiplier tubes 10A and 10C are fed into an anti-coincidence
circuit 34, which determines whether an event caused scintillations
in two or more strands. If so, the event was probably due to a
gamma ray. If the event caused scintillations in only one strand,
the event was probably due to a neutron. Hence, an output from the
anti-coincidence circuit signifies a neutron.
E. Increased Neutron Detector Efficiency Without Sacrificing Energy
Resolution
Under current technology, neutron detector efficiency is inversely
proportional to detector resolution. A thicker bundle 6 will count
more neutrons. However, there will be less certainty regarding the
location of a neutron interaction. By making the bundle 6 thinner
one knows to a greater accuracy the point of interaction. The
scintillator should be thin enough so that the time for a neutron
to travel the distance across the bundle is about equal to or less
than the pulse width of the neutron source, which is less than 1
nanosecond for the preferred embodiment. Hence for many purposes,
the thickness of the bundle 6 should be about 1 cm. As shown in
FIG. 3, the thickness of the strip detector 74 is 1 cm.
The following technique increases neutron detection efficiency,
while preserving the energy measuring capabilities of a thinner
detector. In the manner shown in FIG. 5, a bundle 6 is constructed
of sixty-four (8.times.8) strands 20. Each strand in the bundle is
about 1/2 cm. by 1/2 cm. Each strand 20 is attached to the anode of
a multi-anode scintillation detector such as the H4139 Hamamatsu
multi-anode photomultiplier tube. The tomographic device can
identify the specific strand 20 in which a neutron interaction
occurs by means of the anode dedicated to the strand. This detector
approximates sixty-four 1/2 cm. detectors which provide sixty-four
individual spectra. Such spectra can be normalized to a single
spectra for calculation of neutron energy. Such detector has the
increased efficiency of a 4 cm. scintillator but the increased
energy resolution of a 1/2 cm. scintillator. Hence, this detector
would require only one fourth of the time that a 1 cm. thick
scintillator would require for the same statistics.
4. Array Detector
A. Detection of Fast Neutrons
As shown in FIG. 9, unscattered neutrons passing through the sample
32 will contact one of the scintillator sections 48 of the array
detector. Certain neutrons will scatter from hydrogen protons) in a
scintillator section 48. The protons will recoil and emit light
energy. The scintillator is a fast neutron detector such as BC 404
manufactured by Bicron Corp. The sides of the scintillator sections
48 are opaque to light. A fraction of the light energy is
transmitted to the coupler 50, which is constructed as a lens to
focus the light into the attached non-scintillating fiber optic
cable 52. This light travels down the cable 52 through a fiber
optics-anode coupler 54 to an anode 58 of the multi-anode tube
56.
FIG. 12 shows a block diagram of the electronics for one row of the
array detector. The multi-anode tube 56 converts the light pulse
into a voltage pulse by means of voltage divider 57. The voltage
pulse is passed to a discriminator 59, which is dedicated to such
individual scintillator segment 48, via signal cabling 60. The
discriminator has two outputs. One output goes to the start of the
array TAC 84, which is used to measure the neutron energy. The
other discriminator output goes to a gate input of the data
acquisition and computer 16. Hence, the computer stores all events
from each scintillator section 48 in separate segments of its
memory for analysis. As shown in FIG. 12, any pulse from any
scintillator section 48 can start the TAC 84. The bookkeeping as to
which scintillator segment 48 caused the pulse is determined by the
respective gate pulse. The pulse is recorded in the dedicated
portion of the computer 16 via the gate pulses. The time pick off
86 of the pulsed accelerator provides the stop pulse for TAC 84.
The computer 16 processes these pulses, through storage into a
specific memory bank, to yield information regarding neutron
position and neutron energy. Individual rows of the array detector
72 are controlled independently, each with its own multi-anode
photomultiplier tube, acquisition system and microprocessor. This
construction allows more efficient processing of data and also
would allow the detector to continue to function if one row becomes
inoperable.
B. Calculation of Two Dimensional Placement of Neutrons
As shown in FIG. 10, the array detector is constructed of rows and
columns of discrete scintillator sections 48. The preferred
embodiment of the tomographic device includes twenty-five to fifty
individual detectors in a row and as many as fifty rows. The
vertical (y) and horizontal (x) positions of a neutron interaction
are determined by the respective row and column numbers of the
scintillator section.
The spatial resolution of the x and y coordinates are determined
respectively by the height and width of the scintillator sections
48. The optimal dimensions will be determined by the specific use
of the array detector. A larger scintillator section will yield
more counts but less certainty regarding the point of the neutron
interaction. In a preferred embodiment, the scintillator sections
48 will be about 2 cm. by 2 cm.
C. Calculation of Neutron Energy
The tomographic device determines neutron energy by measuring the
flight time of a neutron over a known distance. A method includes
the following. From FIG. 9 the flight path is the distance from the
beam target 40 to the scintillator section 48. The scintillator
sections 48 are curved so that the flight path is the same for all
angles of the solid angle subtended by the detector. All fiber
optic cables 52, which connect the scintillator sections to the
multi-anode tube 56, are the same length. The tomographic device
calculates neutron velocity, v=d/t, from the measured time and the
flight path. From the velocity, the tomographic device calculates
neutron energy from E=mv.sup.2 /2. In practice, relativistic
equations are used for greater accuracy.
From FIG. 8, the tomographic device measures the time of flight of
the neutron through time to amplitude converter 22. Time pick off
unit 38 provides the start signal for time to amplitude converter
22 at the time the neutrons start. The or output of or/sum circuit
26 provides the stop signal. The stop signal is a sum of the flight
time of the neutron and the flight time of the photons in the cable
52. The tomographic device subtracts the flight time of the photons
in the cables 52 when calculating the flight time of the neutrons.
In this way, the tomographic device measures neutron energy.
5. Use of Multi-Dimensional Neutron-Computed Tomography to
Determine Number Densities of Atoms in a Sample
In its single pass option, the invention identifies explosives or
contraband by determining the number densities (neutron/cm.sup.2)
and ratios of H, C, N and O in the neutron beam using pulsed beam
neutron time-of-flight techniques. Neutron spectra are taken with
and without the sample in the neutron beam. A method of determining
neutron attenuation is to determine the ratios of the two spectra
on an energy by energy basis (channel by channel) and then take the
natural logarithm of these ratios channel by channel. Each point on
the curve must satisfy the attenuation equation, which has four
unknowns (assuming H, C, N and O are the primary elements in the
beam). In one embodiment the tomographic device uses a multichannel
analyzer with approximately two hundred fifty channels to record
spectra of different energies. There are now two hundred fifty
equations and four unknowns. These unknowns are the number
densities in neutrons/cm.sup.2 of H, N, C, and O contained in the
neutron beam. In this way the invention is able to overstate the
problem. Regression theory or another method is applied to solve
for these unknowns and also for the standard error of these
unknowns.
The single pass option determines the number of atoms per square
cm. in the neutron beam through the entire width of the suitcase.
For example, the ratios obtained with a single pass could indicate
a small amount of explosives or a larger amount of non-lethal
material. Variability in luggage size further reduces the accuracy
of the single pass analysis. In contrast, a tomographic analysis
could determine the number densities of atoms per cubic cm. within
small incremental volumes in the luggage. For these reasons, the
invention was designed to include a tomographic option.
The tomographic device determines the number densities of atoms per
cm.sup.3 in the neutron beam path for a number of passes at
different angles through the sample. These resulting number
densities are input into the tomographic program to determine the
number densities per cubic centimeter over volume increments
through the sample. From these number densities, the tomographic
device calculates the ratios C/O, N/O and H/C for each small volume
increment in the sample.
The tomographic device does not use input directly from detectors,
as in conventional tomography. Instead, the tomographic device uses
calculated data as input. The tomographic device calculates number
densities in neutrons per cm.sup.2 for H, C, N and O for each
detector view through the sample. For a single view, there will be
as many sets of number densities as there are detectors. For n
views there will be n times the number of number densities of a
single view. From this data the invention creates four tomographic
reconstructions, one each for H, N, C and O. These reconstructions
are cuts across the sample in which the number densities of H, C,
N, and O are displayed on a two dimensional grid, with the grid
dimension being equal to the spatial resolution. Other elements
could be included.
The tomographic device was designed to gather and reduce data in
proper form for input into commercially available tomogaphraphic
programs. This decreases the cost of implementation, allow use of
the best program available, and allows the invention to take
advantage of future advances in tomographic programs.
This neutron tomographic technique applied by the invention has
several major advantages. Total neutron cross sections are used
which are in the barn range instead of the millibarn range. This
results in greatly increased count rates which give improved
statistics per given count time. The number densities can be
determined to a few percent during a run of only several seconds.
Another major advantage is that the tomographic device determines
the number densities of H, C, N, and O in neutrons per cm.sup.3
over small volume increments through the sample. This method allows
the invention to identify contraband with a high probability of
detection. Another major advantage is that the tomographic device
is constructed to facilitate tomographic imaging. This allows the
tomographic device to create tomographic images of the H, C, N and
O concentrations in the interior of a sealed container in a very
short time. The method also greatly increases sensitivity for
detecting contraband substances.
By obtaining information in this form, the invention optimizes the
neural net methods discussed below. These techniques of data
reduction, imaging, and analysis provide the tomographic system
with the speed and accuracy required for an airport security
system.
6. Neural Net Analysis
The invention creates a tomographic image of an unknown sample
through data received from its x-y detector. The tomographic image
shows the ratios C/O, N/O and H/C in small volume increments
through the sample. These volume increments are equal to a single
dimension of the x-y detector's spatial resolution cubed. The
neural network analyzes the ratios contained in each volume
increment to determine whether an explosive is present.
Assume the x-y detector has a spatial resolution of 2 cm. Assume
the invention creates a tomographic image of a suitcase which is
100 cm. by 80 cm. by 60 cm. The suitcase has a volume of 480,000
cm.sup.3 and so it would be easy to conceal a small explosive
inside. The invention provides the average number densities of H,
C, N, and O atoms over small volume increments of only 8 cm.sup.3
throughout the suitcase. The neural network analyzes this data to
identify explosives in each 8 cm.sup.3 volume increment. The
invention searches all of these small "suitcases" simultaneously,
and so the entire analysis is completed in seconds. It would be
very difficult to hide an explosive in one of these 8 cm.sup.3
suitcases.
In addition to new detection methods, the invention was also
designed to allow fast and accurate training by computer
simulation. The training takes place on a grid-by-grid basis over
the plane of the x-y detector. A training database is created which
consists of simulated neutron radiographic scans of small volumes.
These volumes are equal to one dimension of the x-y detector's
spatial resolution cubed. These simulated volumes contain specific
amounts of various substances. The substances include explosives,
drugs, and materials commonly found in luggage or which could
conceal explosives or drugs.
The density and chemical formula for each substance is obtained
from known sources such as the Handbook of Physics and Chemistry.
Based upon the density, chemical formula, and amount of the
substance, a simulated input case is created showing the number
densities and ratios of the atoms in the simulated volume. This
simulation is in the form of a radiographic scan of the substances
in the volume.
Training methods were developed to allow use of commercially
available neural net programs, such as the Explorer program by
Neuralware. In this way, the cost of implementing the invention is
reduced, a variety of programs are available, and the invention can
take advantage of advances in neural network programs. This
capability to train the invention's neural net by computer
simulation produces very accurate predictions of explosives and
allows training to occur very quickly. If a new type of explosive
is developed, the tomographic device can be trained to identify the
explosive through computer simulated runs and the trained version
implemented across the country virtually overnight.
Summary, Ramifications, and Scope
The present invention creates two and three dimensional images of
the number densities of atoms in a sample. In its preferred
embodiment, the tomographic device detects explosives and other
contraband in airport luggage. The tomographic device identifies
explosives by determining the ratios per cm.sup.3 of H, C, N and O
in small volume increments through the luggage. The present
invention also includes two new types of fast neutron detectors.
The tomographic device also solves certain critical problems in
fast neutron radiography and tomography, including detecting fast
neutrons in an x-y plane, reducing or eliminating multiple
scattering of radiation, discriminating gamma ray from neutron
interactions, and applying advanced neural net technology. FIGS.
13e-f show tomographic images of small amounts of explosives and
drugs hidden in a sealed suitcase. Clearly, the tomographic device
is far more advanced than any existing explosives detection
system.
While my above description contains many specifications, these
should not be construed as limitations on the scope of the present
invention. I have shown and described only the preferred
embodiment, simply by way of illustrating the best mode
contemplated by me of carrying out the invention. Many other
variations are possible without departing from the invention. For
example, with regard to the strip detector, the size of the bundles
and the size of the individual strands can be varied depending on
the parameters of the detector. For a strip that is one meter long,
a single strand (block) may be desirable, while for a strip
detector three meters long, several strands may be desirable.
Instead of photomultiplier tubes, other types of detectors such as
CCDs or detectors based on microchannel plates could be used. The
invention could be used for determining the composition of a
material or for detecting flaws in a sample, or for detecting other
materials than explosives, such as narcotics. With regard to the
multiple scattering filter, the size of the filter and the shape
and sizes of the hollow passages could be varied according to the
desired resolution and purposes. Depending upon the size of the
multiple correction filter and the cone of radiation from the point
source, two conveyor tracks, one on top of the other, could be used
to scan two samples at once. If fast scanning of many samples is
desired, the turntable device may be constructed to spin a sample
for a tomographic image only if the initial radiographic image
indicates uncertainty. Accordingly, the scope of the invention
should be determined not by the embodiments illustrated, but by the
appended claims and their legal equivalents.
* * * * *