U.S. patent application number 12/079942 was filed with the patent office on 2009-02-12 for angled-beam detection system for container inspection.
This patent application is currently assigned to ScanTech Holdings, LLC. Invention is credited to Gary F. Bowser, Mark A. Ferderer, Matthew B. Might.
Application Number | 20090041185 12/079942 |
Document ID | / |
Family ID | 36228490 |
Filed Date | 2009-02-12 |
United States Patent
Application |
20090041185 |
Kind Code |
A1 |
Might; Matthew B. ; et
al. |
February 12, 2009 |
Angled-beam detection system for container inspection
Abstract
A non-intrusive inspection system, including apparatuses and
methods, for non-intrusively inspecting containers such as, without
limitation, those employed to transport items in international
commerce. The non-intrusive inspection system is configured to
generate and scan a container with multiple bremsstrahlung, or
x-ray, beams having multiple spectra and directed at the container
in multiple directions and planes separated by one or more
angle(s). Using data collected from such scanning, software of the
non-intrusive inspection system generates three-dimensional images
of the items present in a container, calculates the volumes and
densities of such items, computes effective "Z" numbers, and
distinguishes between multiple materials, or elements, of such
items. By employing multiple bremsstrahlung beams directed upon a
container in multiple planes, the non-intrusive inspection system
reduces the number of orientations and geometries of items within a
container that might otherwise be employed to avoid the detection
of harmful materials being transported within a container.
Inventors: |
Might; Matthew B.; (Atlanta,
GA) ; Ferderer; Mark A.; (Buford, GA) ;
Bowser; Gary F.; (Auburn, IN) |
Correspondence
Address: |
COURSEY IP LAW, P.C.
POST OFFICE BOX 29509
ATLANTA
GA
30359
US
|
Assignee: |
ScanTech Holdings, LLC
Atlanta
GA
|
Family ID: |
36228490 |
Appl. No.: |
12/079942 |
Filed: |
March 28, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11258832 |
Oct 24, 2005 |
7356118 |
|
|
12079942 |
|
|
|
|
60621261 |
Oct 22, 2004 |
|
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Current U.S.
Class: |
378/57 |
Current CPC
Class: |
G01N 23/083 20130101;
G01N 2223/639 20130101; G01N 23/044 20180201; G01V 5/0041 20130101;
G01N 2223/406 20130101; G01V 5/0058 20130101; G01N 2223/1016
20130101 |
Class at
Publication: |
378/57 |
International
Class: |
G01N 23/04 20060101
G01N023/04 |
Claims
1. A method for non-intrusively inspecting a container used for the
transportation of an item therein, said apparatus comprising: (a)
scanning a container and an item therein with a first x-ray beam at
a first angle relative to the container; (b) scanning the container
and the item therein with a second x-ray beam at a second angle
relative to the container, wherein the second angle has an angular
measure different than the first angle relative to the container;
(c) producing first data representative of a portion of the first
x-ray beam that passes through the container and the item therein;
(d) producing second data representative of a portion of the second
x-ray beam that passes through the container and the item therein;
and (e) determining a characteristic of the item based at least in
part on said first data and said second data.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of priority to U.S.
provisional patent application Ser. No. 60/621,261 entitled
"Angled-Beam Detection System and Methods for Container Screening
and Inspection" and filed on Oct. 22, 2004, now pending.
FIELD OF THE INVENTION
[0002] The present invention relates, generally, to the field of
non-intrusive inspection systems and, more specifically, to
non-intrusive inspection systems and methods for inspecting
containers employed, generally, in the cargo transportation
industry.
BACKGROUND OF THE INVENTION
[0003] Today, only a small percentage of the containers that are
employed by the cargo transportation industry to transport goods in
international commerce are examined or inspected for contraband
when they enter a country on a highway or through a port of entry
such as an airport, seaport, or rail port. Such inspection is often
conducted by inspectors who physically open the containers and
visually (and/or manually) inspect items within the containers.
Alternatively, specially trained dogs may sometimes be employed to
inspect and, potentially, detect items such as drugs or explosives.
Such inspection practices are manpower intensive and take a
substantial amount of time per container, thereby making it cost
prohibitive to inspect a high percentage of the number of
containers that enter a country.
[0004] Due to recent terrorist activities and because such a small
percentage of the containers are inspected, there is heightened
concerned among citizens and government officials alike that
terrorists may place nuclear bombs, "dirty" bombs, biological or
chemical agents, or other weapons of mass destruction in such
containers in order to smuggle them into a country for subsequent
use against the citizenry. As a consequence, a number of vendors
are developing non-intrusive inspection systems for such
containers. Some of the vendors have based their systems on airport
baggage scanning systems. Unfortunately, such non-intrusive
inspection systems suffer from many difficulties, including that
many of the systems do not produce multiple views of the objects
present from multiple directions. Also, many of the systems do not
provide for the discrimination or identification of materials found
in objects present in a container, thereby making the detection of
explosives, nuclear materials, and, generally, certain weapons of
mass destruction impossible.
[0005] Other vendors have developed non-intrusive inspection
systems that provide for the discrimination or identification of
materials found in containers. However, such systems may be
"fooled" by placing items containing different elements in an
orientation relative to one another such that the combination of
the different elements appears, to such systems, as a different
and, possibly, non-harmful element. For example, a first item
containing uranium may be positioned with a second item containing
cobalt immediately behind the first item. To such
material-discriminating non-intrusive inspection systems, the items
may appear, together, as a single item containing the non-harmful
element, lead. Thus, such material-discriminating non-intrusive
inspection systems are not capable of detecting many harmful
elements that may be present in containers.
[0006] Therefore, there exists in the industry, a need for a
non-intrusive inspection system for containers, including
apparatuses and methods, that enables the discrimination of
materials within such containers independent of their placement
and/or orientation relative to one another, and that addresses the
above-described, and other, problems, difficulties, and/or
shortcomings of current systems.
SUMMARY OF THE INVENTION
[0007] Broadly described, the present invention comprises a
non-intrusive inspection system, including apparatuses and methods,
for non-intrusively inspecting containers employed to transport
items or goods therewithin, for generating three-dimensional images
of such items within such containers, for calculating the volumes
and densities of such items, for distinguishing between multiple
materials present in such items, and for detecting harmful, or
potentially harmful, materials. More particularly, the present
invention comprises a non-intrusive inspection system, including
apparatuses and methods, for non-intrusively inspecting containers
employed to transport items or goods therewithin that scans each
container with multiple bremsstrahlung, or x-ray, beams that define
one or more angle(s) therebetween and, correspondingly, define one
or more angle(s) with the container. In a further aspect of the
present invention, each bremsstrahlung, or x-ray, beam may have
multiple spectra corresponding to one or more energy levels.
[0008] Advantageously, the non-intrusive inspection system of the
present invention enables the inspection and/or screening of
containers for the presence of particular items, or objects,
therein without requiring inspection personnel to open the
containers and perform costly and time-consuming physical or manual
inspections thereof. The non-intrusive inspection system of the
present invention is, importantly, operable to generate multiple
bremsstrahlung, or x-ray, beams for direction at a container in
multiple planes that are separated by one or more angle(s). By
virtue of the beams being separated by one or more angle(s), the
non-intrusive inspection system of the present invention reduces
the number of orientations and geometries of items within a
container that might otherwise be employed to avoid the detection
of harmful, or potentially harmful, materials being brought into a
country within a container. The separation of beams by one or more
angle(s) also enables the non-intrusive inspection system of the
present invention to collect data related to the items present in a
container in multiple planes, thereby allowing the non-intrusive
inspection system to produce three-dimensional images of such items
and to calculate the volumes of such items. In addition, because
the bremsstrahlung, or x-ray, beams of the non-intrusive inspection
system of the present invention may have pulses of multiple spectra
corresponding to multiple energy levels, the non-intrusive
inspection system is adapted to distinguish between the materials
present in the items of a container and to calculate the effective
"Z" numbers (or atomic numbers) and densities of such items.
[0009] Further, the non-intrusive inspection system of the present
invention provides such capabilities and functionality using a
single, multi-energy level, charged particle accelerator that is
adapted to generate a pulsed beam of accelerated charged particles
having multiple energy levels. By employing only a single,
multi-energy level, charged particle accelerator in lieu of
multiple charged particle accelerators, the non-intrusive
inspection system of the present invention minimizes system,
operational, and maintenance costs. It should be understood,
however, that the scope of the present invention includes similar
non-intrusive inspection systems that may employ multiple charged
particle accelerators to produce one or more bremsstrahlung, or
x-ray, beams that have one or more spectra for direction at a
container at one or more angle(s) therebetween. It should also be
understood that the non-intrusive inspection system of the present
invention may be utilized to inspect the contents of containers
generally used to transport items or goods in international
commerce or to inspect the contents of other containers having
other sizes and shapes.
[0010] Other advantages and benefits of the present invention will
become apparent upon reading and understanding the present
specification when taken in conjunction with the appended
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 displays a top plan, schematic view of a
non-intrusive inspection system for inspecting the contents of a
container in accordance with an exemplary embodiment of the present
invention.
[0012] FIG. 2 displays a pictorial timing diagram of a pulsed beam
of accelerated electrons having accelerated electrons with multiple
energy levels in accordance with the exemplary embodiment of the
present invention.
[0013] FIG. 3 displays a side, elevational, schematic view of the
non-intrusive inspection system of FIG. 1.
[0014] FIG. 4 displays a partial, top plan, schematic view of the
non-intrusive inspection system of FIG. 1 more clearly showing an
angle between bremsstrahlung, or x-ray, beams alternately impinging
on a container being inspected.
DETAILED DESCRIPTION OF THE INVENTION
[0015] Referring now to the drawings in which like numerals
represent like elements or steps throughout the several views, FIG.
1 displays a top plan, schematic view of a non-intrusive inspection
system 100, according to an exemplary embodiment of the present
invention, for inspecting the contents of a container 102. The
non-intrusive inspection system 100 comprises a charged particle
accelerator 104 and first and second turning magnets 106, 108
(also, respectively, sometimes referred herein to as "accelerator
104" and "kicker magnets 106, 108"). The charged particle
accelerator 104, in the exemplary embodiment, comprises a
pulse-type, multi-energy, linear electron accelerator that is
operable to produce, or emit, a pulsed beam of accelerated
electrons 110 including a first plurality of pulses of accelerated
electrons 112 having a first energy level and a second plurality of
pulses of accelerated electrons 114 having a second energy level
different from the first energy level (see FIG. 2). Generally, the
energy levels of the pulses 112, 114 are considered to be in the
high energy range for a charged particle accelerator 104, but have
appropriate and sufficient spread therebetween such that the
bremsstrahlung, or x-ray, spectra resulting therefrom (as described
below) may be used for determining effective "Z" numbers for
materials present in items within a container 102. The individual
pulses 112 of accelerated electrons of the first plurality of
pulses 112 and the individual pulses 114 of the second plurality of
pulses 114 are emitted such that the pulsed beam of accelerated
electrons 110 includes successive pulses of accelerated electrons
having energy levels that alternate between the first energy level
and the second energy level. Thus, each pulse 112 of accelerated
electrons of the first plurality of pulses of accelerated electrons
112 having a first energy level is preceded and followed in the
pulsed beam of accelerated electrons 110 by a pulse 114 of the
second plurality of pulses of accelerated electrons 114 having a
second energy level. Similarly, each pulse 114 of accelerated
electrons of the second plurality of pulses of accelerated
electrons 114 having a second energy level is preceded and followed
in the pulsed beam of accelerated electrons 110 by a pulse 112 of
the first plurality of pulses of accelerated electrons 112 having a
first energy level.
[0016] The first turning magnet 106, as illustrated in FIG. 1, is
connected to the output port of accelerator 104 by a vacuum
electron beam guide 116 that is adapted to guide, or direct, the
pulsed beam of accelerated electrons 110 from the output port of
accelerator 104 to the first turning magnet 106 during operation of
the non-intrusive inspection system 100. The second turning magnet
108 is connected to the first turning magnet 106 by a vacuum
electron beam guide 118 that is configured to guide, or direct, the
pulsed beam of accelerated electrons 110, when received during
operation as described more fully below, from the first turning
magnet 106 toward the second turning magnet 108. The first and
second turning magnets 106, 108 are each operable to so guide, or
direct, the pulsed beam of accelerated electrons 110 when energized
at appropriate times during operation by respective energizing
pulses 117, 119 applied thereto. Notably, when no energizing pulse
117, 119 is applied to the first and second turning magnets 106,
108, the direction of travel of the pulsed beam of accelerated
electrons 110 is not changed by the first and second turning
magnets 106, 108.
[0017] The non-intrusive inspection system 100, according to the
exemplary embodiment, further comprises first and second conversion
targets 120, 122 and respective first and second collimators 124,
126. The first and second conversion targets 120, 122 are
connected, respectively, to the first and second turning magnets
106, 108 by vacuum electron beam guides 128, 130. The vacuum
electron beam guides 128, 130 are adapted to guide, or direct, the
pulsed beam of accelerated electrons 110 from the first and second
turning magnets 106, 108, respectively, toward the first and second
conversion targets 120, 122. The first and second conversion
targets 120, 122 are operable to receive pulses of accelerated
electrons 112, 114 of the pulsed beam of accelerated electrons 110
and to convert the received pulses of accelerated electrons 112,
114 into respective pulsed bremsstrahlung, or x-ray, beams 132, 134
that are output from the first and second conversion targets 120,
122 toward respective first and second collimators 124, 126.
Generally, the pulsed bremsstrahlung beams 132, 134 include
alternating spectra corresponding to the first and second energy
levels of the alternating pulses of accelerated electrons 112, 114
of the pulsed beam of accelerated electrons 110 emitted by
accelerator 104.
[0018] The first and second collimators 124, 126, generally, each
include an elongate, narrow opening (e.g., a slot) through which
respective portions of the pulsed bremsstrahlung beams 132, 134
pass to create pulsed bremsstrahlung, or x-ray, beams 136, 138
having beam shapes suitable for container inspection. Typically,
the pulsed bremsstrahlung, or x-ray, beams 136, 138 each have a fan
shape upon exiting respective first and second collimators 124,
126. The first and second collimators 124, 126 are, according to
the exemplary embodiment, mounted to and/or integrated into a wall
140 separating an accelerator room 142 in which the accelerator
104, first and second turning magnets 106, 108, and first and
second conversion targets 120, 122 reside and an inspection room
144 through which containers 102 are moved and at which the pulsed
bremsstrahlung, or x-ray, beams 136, 138 exiting the first and
second collimators 124, 126 are directed in order to inspect their
contents. During inspection, the containers 102 are, generally,
moved in a substantially linear direction of travel (e.g.,
indicated by arrow 146) along a longitudinal axis 148 of the
inspection room 144 that is substantially perpendicular to the
predominant direction of travel of the first pulsed bremsstrahlung,
or x-ray, beam 136 (e.g., indicated by arrow 150). By moving the
containers 102 relative to the pulsed bremsstrahlung, or x-ray,
beams 136, 138 and collecting data representative of the portions
of beams 136, 138 that pass through the containers 102 and the
items therein, the containers 102 and the items therein are
scanned.
[0019] The non-intrusive inspection system 100 additionally
comprises a detector subsystem 160 having a detector array 162 with
a plurality of detectors 164 that are each operable to receive
respective portions 166, 168 of the pulsed bremsstrahlung, or
x-ray, beams 136, 138 after they pass through a container 102 being
inspected within the inspection room 144 and to produce electrical
signals, or data, representative of such respective portions 166,
168. As displayed in FIG. 3, the plurality of detectors 164 of the
detector array 162 are arranged in a, generally, "L" shape with a
first portion 170 of the detector array 162 including detectors 164
oriented in a plane 172 substantially perpendicular to the
predominant direction of travel of the first pulsed bremsstrahlung
beam 136 (e.g., indicated by arrow 150) and substantially adjacent
a side of a container 102 as the container 102 travels through the
inspection room 144. A second portion 174 of the detector array 162
includes detectors 164 oriented in a plane 176 substantially
perpendicular to the plane 172 of the first portion 170 of the
plurality of detectors 164 such that the second portion 174 extends
at least partially above a container 102 as the container 102
travels through the inspection room 144. In order to enable the
reception of respective portions 166, 168 of the pulsed
bremsstrahlung beams 136, 138 that may pass through the top, or
roof, of a container 102, some of the individual detectors 164 of
the second portion 174 of the detector array 162 are oriented in a
direction substantially toward, or facing, the first and second
collimators 124, 126 as opposed to being oriented in a direction
perpendicular to the top, or roof, of a container 102 passing
through the inspection room 144.
[0020] The non-intrusive inspection system 100 further comprises a
controller 180 that is communicatively connected to the detector
subsystem 160 via bi-directional communication link 182, to the
charged particle accelerator 104 via a bi-directional communication
link 183, to the first turning magnet 106 via bi-directional
communication link 184, and to the second turning magnet 108 via
bi-directional communication link 185. Generally, the bidirectional
communication links 182, 183, 184, 185 comprise one or more
appropriate electrical signal cables having one or more electrical
signal paths. Bi-directional communication link 182 is adapted to
communicate electrical signals (including, but not limited to,
control signals and electrical signals, or data, representative of
the respective portions 166, 168 of the pulsed bremsstrahlung, or
x-ray, beams 136, 138 after they pass through a container 102)
between the detector subsystem 160 and the controller 180.
Bi-directional communication link 184 is configured to communicate
electrical signals (including, without limitation, control,
feedback, and other signals) between the controller 180 and the
charged particle accelerator 104 that are used by the controller
180 to control the operation of the charged particle accelerator
104 and to insure the generation of pulses having appropriate
energy levels at the appropriate times and in the appropriate
sequences as described herein. Bi-directional communication links
184, 185 are adapted to communicate electrical signals (including,
for example and not limitation, energizing pulses 117, 119,
control, feedback, and other signals) between the controller 180
and the first and second turning magnets 106, 108, respectively,
that are used by the controller 180 to control the operation of the
turning magnets 106, 108 in order to guide, or direct, the pulsed
beam of accelerated electrons 110 as described herein.
[0021] The controller 180, in accordance with the exemplary
embodiment of the present invention, comprises appropriate data
communication equipment and one or more computer system(s)
configured with appropriate control software and imaging and
material discrimination software that are operable when such
software is executed thereby, to control the operation of the
various components of the non-intrusive inspection system 100, to
receive the electrical signals, or data, produced and output by the
detectors 164 of the detector array 162, to determine the shapes,
volumes, and locations of items present within a scanned container
102, and to produce three-dimensional images of the items present
within a scanned container 102 (i.e., using mathematical
relationships and software methods generally known to one of
reasonable skill in the art) that are displayed to inspection
system operators. The controller 180 is also operable, when such
software is executed thereby, to receive the electrical signals, or
data, produced and output by the detectors 164 of the detector
array 162 and to calculate therefrom (i.e., using the mathematical
relationships described herein in conjunction with methods
generally known to one of reasonable skill in the art) and to
display to inspection system operators, the relative and respective
densities of the materials, or elements, present within the items
of a scanned container 102. Thus, the controller 180 and its
imaging and material discrimination software enable inspection
system operators to visibly see the locations and shapes of items
present within a scanned container 102 (i.e., on a display device
of the controller 180) and to be presented with the relative and
respective densities of the materials, or elements, of such items.
The imaging and material discrimination software may also be
configured to cause the controller 180 to generate audible and
visible alarms for inspection system operators when a particular
material, or element, is detected in an item present in a scanned
container 102. For example and not limitation, the imaging and
material discrimination software may cause the controller 180 to
generate audible and visible alarms if plutonium were detected
within in an item present in a scanned container 102.
[0022] Importantly, the accelerator 104, first and second turning
magnets 106, 108, first and second conversion targets 120, 122, and
first and second collimators 124, 126 are appropriately arranged
and oriented so that the first and second pulsed bremsstrahlung, or
x-ray, beams 136, 138 exiting the first and second collimators 124,
126 and impinging on a container 102 traveling through the
inspection room 144 during scanning and inspection define an angle,
.alpha., therebetween (i.e., as illustrated in FIG. 1 and more
clearly in the partial, top plan, schematic view of FIG. 4). The
first and second pulsed bremsstrahlung, or x-ray, beams 136, 138
lie substantially within respective planes 186, 188 having an
angle, .alpha., therebetween such that, both beams 136, 138 pass
through each point within a container 102 during scanning and
inspection of a container 102. The first and second pulsed
bremsstrahlung, or x-ray, beams 136, 138 and planes 186, 188 also
define respective angles, .theta..sub.1 and .theta..sub.2, with the
direction in which the container 102 travels through the inspection
room 144 during inspection. Generally, angle .theta..sub.1 has a
measure of ninety degrees (90.degree.) and angle .theta..sub.2 is
the mathematical complement of angle, .alpha.. Also generally,
angle .theta..sub.2 and angle .alpha. each have measures of less
than ninety degrees (90.degree.). By virtue of such arrangement and
orientation, the second pulsed bremsstrahlung, or x-ray, beam 138
passes through the container 102 (and every point therein during
scanning) in a direction (e.g., indicated by arrow 149) within
plane 188 at an angle, .alpha., relative to the first pulsed
bremsstrahlung, or x-ray, beam 136 within plane 186 and, hence, the
portion 168 of the second pulsed bremsstrahlung, or x-ray, beam 138
impinging on detectors 164 of the detector array 162 is
substantially at an angle, .alpha., relative to the respective
portion 166 of the first pulsed bremsstrahlung, or x-ray, beam 136
impinging on detectors 164 of the detector array 162. It should be
understood that the scope of the present invention includes angles,
.theta..sub.1, .theta..sub.2, and .alpha. having any measures.
[0023] By further virtue of such arrangement and orientation and
due to the pulsed bremsstrahlung, or x-ray, beams 136, 138 each
including consecutive pulses 136A, 136B, 138A, 138B (e.g., the
alpha characters designate pulses) having different spectra, the
electrical signals generated by the detectors 164 of the detector
array 162 at a particular time during inspection of a container 102
are representative of and correspond to the shapes and material
densities of the portions of the items within the container 102
lying within planes 186, 188 at such time. Thus, the non-intrusive
inspection system 100 of the present invention comprises a
"multi-plane" inspection system. Because planes 186, 188 are
separated by an angle, .alpha., the electrical signals provide
stereoscopic data corresponding to the shapes and densities of
items within the container 102 and when collected for an entire
container 102 (i.e., as the entire container 102 travels through
the inspection room 144 and through planes 186, 188) and utilized
as inputs by the imaging and material discrimination software of
the controller 180, enable the non-intrusive inspection system 100
to produce and display three-dimensional images of the items, or
contents, of a scanned container 102 and to calculate the densities
of such items or contents. Further, because planes 186, 188 are
oriented at angle, .alpha., relative to one another and because
data is collected at discrete time intervals for each plane 186,
188 as a container 102 travels through the inspection room 144
relative to the planes 186, 188, the non-intrusive inspection
system 100 makes it more difficult to pre-arrange the positions of
multiple items within the container 102 in order to "hide", render
undetectable, or indistinguishable from other items, a particular
item within the container 102 containing potentially hazardous or
dangerous materials, elements, or substances.
[0024] In operation, the accelerator 104 and the first and second
turning magnets 106, 108 of the non-intrusive inspection system 100
are appropriately controlled by the controller 180 to produce a
pulsed beam of accelerated electrons 110 and alternately direct it
at a first conversion target 120 and a second conversion target
122. The pulsed beam of accelerated electrons 110 alternately
includes pulses of accelerated electrons 112 having a first energy
level and pulses of accelerated electrons 114 having a second
energy level. Because the consecutive pulses of accelerated
electrons 112, 114 directed at the first and second conversion
targets 120, 122 alternate between respective different energy
levels, the first and second pulsed bremsstrahlung, or x-ray, beams
132, 134 produced by and exiting from the first and second
conversion targets 120, 122 include pulses of alternating first and
second spectra corresponding to the first and second energy levels
of the alternating pulses of accelerated electrons 112, 114.
Further, the first and second pulsed bremsstrahlung, or x-ray,
beams 136, 138 produced, respectively, from the first and second
bremsstrahlung, or x-ray, beams 132, 134 by first and second
collimators 124, 126 similarly include pulses of alternating first
and second spectra corresponding to the first and second energy
levels of the alternating pulses of accelerated electrons 112,
114.
[0025] More specifically, at a first time, the accelerator 104
generates a pulse of accelerated electrons 112 of a first plurality
of pulses of accelerated electrons 112 having a first energy level.
The pulse of accelerated electrons 112 is directed to the first
turning magnet 106 by vacuum electron beam guide 116. Concurrently,
no energizing pulse 117 is applied to the first turning magnet 106
by the controller 180, thereby placing the first turning magnet 106
into a de-energized state and allowing the pulse of accelerated
electrons 112 to pass through the first turning magnet 106 and on
toward the first conversion target 120 via vacuum beam guide
128.
[0026] Upon receiving the pulse of accelerated electrons 112 having
a first energy level, the first conversion target 120 converts the
received pulse of accelerated electrons 112 into a pulse of
bremsstrahlung, or x-ray, having first spectra corresponding to the
first energy level of the received pulse of accelerated electrons
112. The first conversion target 120 then emits the produced pulse
of bremsstrahlung, or x-ray, having first spectra in a direction
toward the first collimator 124. While passing through the first
collimator 124, the pulse of bremsstrahlung, or x-ray, is shaped to
produce a shaped pulse 136A of bremsstrahlung, or x-ray, having a
shape (e.g., a fan shape) suitable for inspection of a container
102.
[0027] The shaped pulse 136A of bremsstrahlung, or x-ray, having
first spectra exits the first collimator 124 traveling
substantially within plane 186 in a direction predominantly
perpendicular to the direction of travel of the container 102 and
impinges upon the container 102 as it is moved through the
inspection room 144 during scanning and inspection. The shaped
pulse 136A of bremsstrahlung, x-ray, passes through the walls of
the container 102 and items present in the container 102
substantially within plane 186. A portion 166 of the shaped pulse
136A of bremsstrahlung, x-ray, passing through the walls of the
container 102 and items present in the container 102 strikes
detectors 164 of the detector array 162. The detectors 164 then
produce and output electrical signals representative of and
corresponding to the portion 166 of the shaped pulse 136A of
bremsstrahlung, x-ray, impinging thereon and, hence, to the shapes
and material densities of the portions of the items within the
container 102 lying within plane 186 at such first time.
[0028] At a second time immediately subsequent to the first time,
the accelerator 104 generates a pulse of accelerated electrons 114
of a second plurality of pulses of accelerated electrons 114 having
a second energy level different than the first energy level. The
pulse of accelerated electrons 114 is guided to the first turning
magnet 106 by vacuum electron beam guide 116. Again, no energizing
pulse 117 is concurrently applied to the first turning magnet 106
by the controller 180, thereby maintaining the first turning magnet
106 in a de-energized state and enabling the pulse of accelerated
electrons 114 to pass through the first turning magnet 106 and on
toward the second conversion target 122 via vacuum beam guide
128.
[0029] Similar to the pulse of accelerated electrons 112, upon
receiving the pulse of accelerated electrons 114 having a second
level at the second time, the first conversion target 120 converts
the received pulse of accelerated electrons 114 into a pulse of
bremsstrahlung, or x-ray, having second spectra corresponding to
the second energy level of the received pulse of accelerated
electrons 114. Then, the first conversion target 122 emits the
produced pulse of bremsstrahlung, or x-ray, in a direction toward
the first collimator 124. While passing through the first
collimator 124, the produced pulse of bremsstrahlung, or x-ray, is
shaped to produce a shaped pulse 136B of bremsstrahlung, or x-ray,
having a beam shape (e.g., a fan shape) suitable for inspection of
a container 102.
[0030] The shaped pulse 136B of bremsstrahlung, or x-ray, having
second spectra exits the first collimator 124 traveling within
plane 186 in a direction predominantly perpendicular to the
direction of travel of the container 102 and impinges upon the
container 102 as it is moved through the inspection room 144 during
scanning and inspection. The shaped pulse 136B of bremsstrahlung,
or x-ray, passes through the walls of the container 102 and items
present in the container 102 within plane 186. A portion 166 of the
shaped pulse 136B of bremsstrahlung, or x-ray, passing through the
walls of the container 102 and items present in the container 102
then strikes detectors 164 of the detector array 162. The detectors
164 then produce and output electrical signals representative of
and corresponding to the portion 166 of the shaped pulse 136B of
bremsstrahlung, or x-ray, impinging thereon and, hence, to the
shapes and material densities of the portions of the items within
the container 102 lying within plane 186 at such second time.
[0031] Continuing, at a third time immediately subsequent to the
second time, the accelerator 104 generates a pulse of accelerated
electrons 112 of a first plurality of pulses of accelerated
electrons 112 having a first energy level. The pulse of accelerated
electrons 112 is directed to the first turning magnet 106 by vacuum
electron beam guide 116. Concurrently, an energizing pulse 117 is
applied to the first turning magnet 106 by the controller 180,
thereby placing the first turning magnet 106 into an energized
state and causing the first turning magnet 106 to direct the pulse
of accelerated electrons 112 toward the second turning magnet 108.
The pulse of accelerated electrons 112 is guided to the second
turning magnet 108 by vacuum electron beam guide 118. An energizing
pulse 119 is concurrently applied to the second turning magnet 108
by the controller 180, thereby placing the second turning magnet
108 into an energized state and causing the second turning magnet
108 to guide the pulse of accelerated electrons 112 received from
the first turning magnet 106 toward the second conversion target
122 via vacuum beam guide 130.
[0032] After receiving the pulse of accelerated electrons 112
having a first energy level, the second conversion target 122
converts the received pulse of accelerated electrons 112 into a
pulse of bremsstrahlung, or x-ray, having first spectra
corresponding to the first energy level of the received pulse of
accelerated electrons 112. The second conversion target 122 then
emits the produced pulse of bremsstrahlung, or x-ray, having first
spectra in a direction toward the second collimator 126. While
passing through the second collimator 126, the pulse of
bremsstrahlung, or x-ray, is shaped to produce a shaped pulse 138A
of bremsstrahlung, or x-ray, having a shape (e.g., a fan shape)
suitable for inspection of a container 102.
[0033] The shaped pulse 138A of bremsstrahlung, or x-ray, having
first spectra exits the second collimator 126 traveling
substantially within plane 188 in a direction at an angle, .alpha.,
relative to the predominant direction of travel of the shaped
pulses 136A, 136B of bremsstrahlung, or x-ray, produced at the
first and second times. The shaped pulse 138A of bremsstrahlung, or
x-ray, impinges on the container 102 as it is moved through the
inspection room 144 during scanning and inspection. The shaped
pulse 138A of bremsstrahlung, or x-ray, passes through the walls of
the container 102 and items present in the container 102
substantially within plane 188. A portion 168 of the shaped pulse
138A of bremsstrahlung, or x-ray, passing through the walls of the
container 102 and items present in the container 102 strikes
detectors 164 of the detector array 162. The detectors 164 then
produce and output electrical signals representative of and
corresponding to the portion 168 of the shaped pulse 138A of
bremsstrahlung, or x-ray, impinging thereon and, hence, to the
shapes and material densities of the portions of the items within
the container 102 lying within plane 188 at such third time.
[0034] Subsequently, at a fourth time immediately following the
third time, the accelerator 104 generates a pulse of accelerated
electrons 114 of a second plurality of pulses of accelerated
electrons 114 having a second energy level. The pulse of
accelerated electrons 114 is directed to the first turning magnet
106 by vacuum electron beam guide 116. Concurrently, an energizing
pulse 117 is applied to the first turning magnet 106 by the
controller 180, thereby placing the first turning magnet 106 into
an energized state and causing the first turning magnet 106 to
direct the pulse of accelerated electrons 114 toward the second
turning magnet 108. The pulse of accelerated electrons 114 is
guided to the second turning magnet 108 by vacuum electron beam
guide 118. An energizing pulse 119 is concurrently applied to the
second turning magnet 108 by the controller 180, thereby placing
the second turning magnet 108 into an energized state and causing
the second turning magnet 108 to guide the pulse of accelerated
electrons 114 received from the first turning magnet 106 toward the
second conversion target 122 via vacuum beam guide 130.
[0035] After receiving the pulse of accelerated electrons 114
having a second energy level, the second conversion target 122
converts the received pulse of accelerated electrons 114 into a
pulse of bremsstrahlung, or x-ray, having second spectra
corresponding to the second energy level of the received pulse of
accelerated electrons 114. The second conversion target 122 then
emits the produced pulse of bremsstrahlung, or x-ray, having second
spectra in a direction toward the second collimator 126. While
passing through the second collimator 126, the pulse of
bremsstrahlung, or x-ray, is shaped to produce a shaped pulse 138B
of bremsstrahlung, or x-ray, having a shape (e.g., a fan shape)
suitable for inspection of a container 102.
[0036] The shaped pulse 138B of bremsstrahlung, or x-ray, having
second spectra exits the second collimator 126 traveling
substantially within plane 188 in a direction at an angle, .alpha.,
relative to the predominant direction of travel of the shaped
pulses 136A, 136B of bremsstrahlung, or x-ray, produced at the
first and second times. The shaped pulse 138B of bremsstrahlung, or
x-ray, impinges on the container 102 as it is moved through the
inspection room 144 during scanning and inspection. The shaped
pulse 138B of bremsstrahlung, or x-ray, passes through the walls of
the container 102 and items present in the container 102
substantially within plane 188. A portion 168 of the shaped pulse
138B of bremsstrahlung, or x-ray, passing through the walls of the
container 102 and items present in the container 102 strikes
detectors 164 of the detector array 162. The detectors 164 then
produce and output electrical signals representative of and
corresponding to the portion 168 of the shaped pulse 138B of
bremsstrahlung, or x-ray, impinging thereon and, hence, to the
shapes and material densities of the portions of the items within
the container 102 lying within plane 188 at such fourth time.
[0037] Repetition of the above-described detailed operation of the
non-intrusive inspection system 100 at the first, second, third,
and fourth times at operational system speeds during the time
period necessary for the scanning and inspection of a container 102
moving through the inspection room 144 results in: the accelerator
104 producing and emitting the pulsed beam of accelerated electrons
110 repeatedly including successive pairs of pulses, with each pair
of pulses having a pulse of accelerated electrons 112 with a first
energy level and a pulse of accelerated electrons 114 with a second
energy level; the turning magnets 106, 108 repeatedly and
alternately directing the pulsed beam of accelerated electrons 110
first at the first conversion target 120 and then at the second
conversion target 122 such that successive pairs of pulses of the
pulsed beam of accelerated electrons 110 are alternately directed
at the first and second conversion targets 120, 122; and, the first
and second conversion targets 120, 122 (i) repeatedly and
alternately receiving pairs of pulses of the pulsed beam of
accelerated electrons 110, (ii) repeatedly and alternately
converting the respectively received pairs of pulses into
respective pairs of pulses of bremsstrahlung, or x-ray, of the
first or second pulsed bremsstrahlung, or x-ray, beams 132, 134 as
the case may be, with each pair of pulses of bremsstrahlung, or
x-ray, including a pulse of bremsstrahlung, or x-ray, having first
spectra corresponding to the first energy level of a received pulse
of accelerated electrons 112 and a pulse of bremsstrahlung, or
x-ray, having second spectra corresponding to the second energy
level of a received pulse of accelerated electrons 114, and, (iii)
repeatedly and alternately emitting the first and second pulsed
bremsstrahlung, or x-ray, beams 132, 134 toward the first and
second collimators 124, 126 such that the first pulsed
bremsstrahlung, or x-ray, beam 132 is directed toward the first
collimator 124 and the second pulsed bremsstrahlung, or x-ray, beam
134 is directed toward the second collimator 126.
[0038] Such repetition of the above-described detailed operation of
the non-intrusive inspection system 100 further results in: the
first and second collimators 124, 126 (i) repeatedly and
alternately receiving pairs of pulses of the first or second pulsed
bremsstrahlung, or x-ray, beams 132, 134, as the case may be, from
the first or second conversion targets 120, 122, (ii) repeatedly
and alternately shaping the respectively received pairs of pulses
of bremsstrahlung, or x-ray, of the first and second pulsed
bremsstrahlung, or x-ray, beams 132, 134 into respective shaped
pairs of pulses 136A, 136B, 138A, 138B of bremsstrahlung, or x-ray,
of respective pulsed bremsstrahlung, or x-ray, beams 136, 138 such
that the shaped pairs of pulses 136A, 136B, 138A, 138B are suitable
for scanning and inspecting the container 102 then moving through
the inspection room 144, (iii) repeatedly and alternately directing
the shaped pairs of pulses 136A, 136B, 138A, 138B of pulsed
bremsstrahlung, or x-ray, beams 136, 138 toward the container 102
such that the first collimator 124 directs shaped pairs of pulses
136A, 136B of pulsed bremsstrahlung, or x-ray, beam 136 at the
container 102 substantially within plane 186 and the second
collimator 126 directs shaped pairs of pulses 138A, 138B of pulsed
bremsstrahlung, or x-ray, beam 138 at the container 102
substantially within plane 188 and at an angle, .alpha., relative
to plane 186; the respective shaped pairs of pulses 136A, 136B,
138A, 138B of pulsed bremsstrahlung, or x-ray, beams 136, 138
alternately passing through the portions of the items of the
container 102 that lie within planes 186, 188, as the case may be;
and, the detectors 164 of detector array 162 repeatedly and
alternately (i) receiving respective portions 166, 168 of pulsed
bremsstrahlung, or x-ray, beams 136, 138 that exit the container
102 respectively and substantially in planes 186, 188, and (ii)
generating and outputting electrical signals to the controller 180
via communication link 182 such that the electrical signals are
respectively representative of the received portions 166, 168 of
pulsed bremsstrahlung, or x-ray, beams 136, 138 and, hence, of the
shapes and material densities of the portions of the items of the
container 102 that lie within planes 186, 188.
[0039] Upon repeatedly and alternately receiving electrical signals
respectively representative of the received portions 166, 168 of
pulsed bremsstrahlung, or x-ray, beams 136, 138 and of the shapes
and material densities of the portions of the items of the
container 102 that lie within planes 186, 188, the controller 180
converts the corresponding electrical signals received from the
detectors 164 into appropriate data and stores the data until it
has collected such data for the entire container 102 being scanned
and inspected by the non-intrusive inspection system 100. Thus,
once all of the data has been collected, the controller 180 has,
for its use, an entire container's worth of data for multiple
planes 186, 188 of bremsstrahlung, or x-rays, repeatedly slicing
through the container 102 (and each point therein) at an angle,
.alpha., relative to one another along the container's length at
pre-determined time intervals while the container 102 moved through
the inspection room 144. The data collected for each plane 186, 188
includes data corresponding to the first and second energy levels
of the successive pulses of the pulsed beam of accelerated
electrons 110 and, hence, to the first and second spectra of the
pulsed bremsstrahlung, or x-ray, beams 136, 138.
[0040] Using such collected data and executing its the imaging and
material discrimination software, the controller 180 generates
three-dimensional images of the contents, or items, present inside
the container 102. The controller 180, using the imaging and
material discrimination software, next calculates the position and
volume of each item present within the container 102. Because the
collected data for each plane 186, 188 includes data corresponding
to multiple energy levels of accelerated electrons, the controller
180 then computes effective "Z" numbers for each item present
inside the container 102.
[0041] In order to compute the effective "Z" numbers (or effective
atomic numbers) for each item present inside the container 102, the
imaging and material discrimination software utilizes and
implements equations, physics and mathematical analysis, and
mathematical relationships associated with multi-energy material
recognition. Generally, the determination of an effective "Z"
number for an item is based upon the physical and mathematical
relationships corresponding to the loss of intensity of a
bremsstrahlung, x-ray, beam as it travels through the various
materials or elements thereof. For each material or element
traveled through, the bremsstrahlung, or x-ray, beam looses
intensity with such loss of intensity being a function of (1) the
effective "Z" number (e.g., effective atomic number or composition)
of the material, (2) the energy of the beam, and (3) the thickness
of the material. Thus, if a bremsstrahlung, or x-ray, beam having
pulses of multiple energies (or, for that matter, multiple
bremsstrahlung, or x-ray, beams each having pulses of a single
energy different than that of the pulses of the other beams) is
directed through a number of materials and the beam's loss of
intensity is measured or determined at each energy, it is possible
to solve certain mathematical relationships, or equations, in order
to determine the effective "Z" numbers and thicknesses of each
material encountered by the beam.
[0042] If, for the sake of simplicity and descriptive purposes,
consideration is given to the determination of the effective "Z"
number and thickness of a single material through which a
bremsstrahlung, or x-ray, beam travels, the final intensity, I
(MeV), of the beam emerging from the material may be computed
by:
I(I.sub.o,.mu.,t)=I.sub.oe.sup.-.mu.t
where I.sub.o (MeV) corresponds to the intensity of the beam prior
to entering the material, .mu. (cm.sup.2/g or cm.sup.-1)
corresponds to the material's coefficient of attenuation (described
in more detail below), and t corresponds to the material's
thickness. Since the material's coefficient of attenuation is
dependent upon the material's effective "Z" number, Z, and the
energy, E.sub.ac (Joules), of the bremsstrahlung, or x-ray, the
final intensity of the beam emerging from the material may be
computed by:
I(I.sub.oZ,E.sub.ac,t)=I.sub.oe.sup.-.mu.(Z,Eac)t.
Based on this relationship, a system of two equations and two
unknowns may be obtained from two final intensities, two initial
intensities, and the two energies that produced them. The system of
two equations may then be solved to determine the material's
thickness and effective "Z" number.
[0043] Before proceeding further, it should be noted that the loss
of intensity of a bremsstrahlung, or x-ray, beam traveling through
a material results from, among other things, collisions of the beam
with the material's atoms. The loss of intensity due to such
collisions is mathematically related to the material's coefficient
of attenuation, .mu.. Physically, the material's coefficient of
attenuation, .mu., is a function of photon cross section, .sigma.,
which is the sum of four properties of the material: (1)
photoelectric cross section, .sigma..sub..tau., (2) coherent
scattering cross section, .sigma..sub.coh, (3) incoherent (Compton)
scattering, .sigma..sub.c, and (4) pair production cross section,
.sigma..sub..kappa..
[0044] The photon cross section of a particle is an expression of
the probability that an incident particle will strike it. As such,
photon cross section is strongly related to the total area of a
material and the "radius" of the particles within the material.
Typically, the photon cross section, .sigma., represents the
cross-sectional area of a single atom, and consequently, the photon
cross section is expressed in units of cm.sup.2/atom. Frequently,
however, the photon cross section is expressed in units of "barns"
instead of cm.sup.2, with one barn=10.sup.-24 cm.sup.2.
[0045] At the quantum level, the four factors of photon cross
section described above, each of which is a function of
bremsstrahlung (or x-ray) energy, E, and effective "Z" number,
comprise terms or operands when computing the photon cross section.
Thus, the photon cross section may be expressed as:
.sigma.(Z,E)=.sigma..sub..tau.(Z,E)+.sigma..sub.coh(Z,E)+.sigma..sub.c(Z-
,E)+.sigma..sub..kappa.(Z,E).
It should be noted that although each term of the above equation
may be approximated using the relationships described below, large
repositories of known photon cross section data exist for many
different materials and may be utilized in lieu of such
approximations. Interestingly, in the above equation for photon
cross section, the photoelectric cross section, .sigma..sub..tau.,
term dominates at lower bremsstrahlung, or x-ray, energies (e.g.,
<0.5 MeV). At higher bremsstrahlung, or x-ray, energies (e.g.,
>5 MeV), the pair production cross section, .sigma..sub..kappa.,
term dominates. At intermediate bremsstrahlung, or x-ray, energies
(e.g., >0.5 MeV and <5 MeV), the coherent scattering cross
section, .sigma..sub.coh, and incoherent (Compton) scattering,
.sigma..sub.c, terms dominant the equation. Consequently, material
recognition and effective "Z" number determination techniques vary
with the energy level of the utilized bremsstrahlung, or x-ray,
beam.
[0046] The photoelectric effect upon photon cross section, .sigma.,
results from an x-ray/atom collision in which the incident photon's
energy is higher than the binding energy of some electron in the
atom of the material. In such a collision, the incident photon of
the bremsstrahlung, or x-ray, beam is absorbed and in its place,
several fluorescent photons and one electron are ejected, thereby
ionizing the atom. Naturally, any bremsstrahlung, or x-ray, that is
absorbed does not exit the material and impinge upon a
detector.
[0047] The photoelectric cross section property of a material,
.sigma..sub..tau., may be crudely approximated at low energies
(e.g., several KeV to hundreds of KeV) by the following
expression:
.sigma..sub..tau.(Z,E).apprxeq.10(Z.sup.5/E.sup.3).
[0048] The coherent scattering effect upon photon cross section,
.sigma., results from an incident photon of the bremsstrahlung, or
x-ray, beam making a glancing blow off of an atom of a material,
thereby deflecting the bremsstrahlung, or x-ray, away from a
detector. For bremsstrahlung, or x-ray, wavelengths less than the
diameter of the scattering atoms, the coherent scattering cross
section property of a material, .sigma..sub.coh, may be
approximated as follows:
.sigma..sub.coh(Z,E).apprxeq.8.pi.r.sub.e.sup.2Z.sup.2(.lamda./(4.pi.aZ.-
sup.1/3)).sup.2(4/5-(.lamda./(8aZ.sup.1/3)))
where .lamda. is determined by the relationship E=hc/.lamda., h is
Planck's constant (6.626068.times.10.sup.-34 m.sup.2kg/s), c is the
speed of light (299,792,458 m/s), re is the classical electron
radius (2.817940285.times.10.sup.-15 m), and a=0.885.
[0049] The incoherent (Compton) scattering effect upon photon cross
section, .sigma., results from an incident photon of the
bremsstrahlung, or x-ray, beam knocking out a loosely bound
electron of an atom of a material and undergoing a direction change
(and energy loss) in the process. Since the direction of the
incident photon is changed, it will not impinge upon a detector.
The incoherent (Compton) scattering property of a material,
.sigma..sub.c, may be approximated by the following relationship
for bremsstrahlung, or x-ray, beams having energy levels in the
medium range:
.sigma..sub.c(Z,E).apprxeq.0.665 Z.
Notably, the above approximation of the incoherent (Compton)
scattering property, .sigma..sub.c, is not substantially effected
by the energy of the bremsstrahlung, or x-ray, beam and, thus, the
approximation does not include energy as an operand.
[0050] The pair production cross section effect upon photon cross
section, .sigma., at relativistic photon energies (E>2
m.sub.ec.sup.2--where m.sub.e represents the mass of an electron
(e.g., 9.10938188.times.10.sup.-31 kg)) results from an incident
photon of the bremsstrahlung, or x-ray, beam impacting an atom of a
material and being "consumed" entirely, thereby producing an
electron-positron pair. Thus, for relativistic photon energies, the
pair production cross section property of a material,
.sigma..sub..kappa., may be approximated proportionally as:
.sigma..sub..kappa.(Z,E).varies.Z.sup.2ln(E-2 m.sub.ec.sup.2).
At very high energies, E, the pair production cross section
property of a material, .sigma..sub..kappa., is effectively
constant.
[0051] As briefly described above, the total (linear) coefficient
of attenuation, .mu..sub.tot, for a particular material is
physically a function of photon cross section, .sigma., which is
calculated as the sum of the (1) photoelectric cross section,
.sigma..sub..tau., (2) coherent scattering cross section,
.sigma..sub.coh, (3) incoherent (Compton) scattering,
.sigma..sub.c, and (4) pair production cross section,
.sigma..sub..kappa.. Because the photon cross section, .sigma.,
depends on the effective "Z" number and the energy, E.sub.ac, of
the bremsstrahlung or x-ray beam, the total (linear) coefficient of
attenuation, .mu..sub.tot, for a particular material is also a
function of the effective "Z" number and the energy, E.sub.ac, of
the bremsstrahlung or x-ray beam and may be calculated using the
following equation:
.mu..sub.tot(E.sub.ac,Z)=.sigma.(Z,E.sub.ac).times..rho..times.N.sub.A/A
where .mu..sub.tot is measured in cm.sup.-1, .rho. is the volume
density (g/cm.sup.3) for an atom of the material, N.sub.A is
Avogadro's number (6.02252.times.10.sup.23 atom/mole), and A is the
atomic mass (g/mole) for the material. Alternatively, the total
(linear) coefficient of attenuation, .mu..sub.tot, may be
calculated in cm.sup.2/g as follows:
.mu..sub.tot(E.sub.ac,Z)=.sigma.(Z,E.sub.ac).times.N.sub.A/A.
It should be noted that as with photon cross section data, large
repositories of pre-computed coefficients of attenuation exist for
many materials and energy ranges. Thus, although the total (linear)
coefficient of attenuation, .mu..sub.tot, may be calculated or
approximated using the above equations, it may be desirable to use
a pre-computed value therefor obtained from such a repository.
[0052] With regard to the thickness, t, of a single material
through which a bremsstrahlung, or x-ray, beam travels, if the
material's length, L, with respect to the direction of travel of
the bremsstrahlung, or x-ray, beam is L cm, then t=L. However, if
not, the thickness, t, of a single material may be alternatively
defined in g/cm.sup.2 in terms of the material's length, L (cm),
and the material's density, .rho. (g/cm.sup.3), as follows:
t=L.times..rho..
[0053] As also briefly described above, a determination of the
effective "Z" number and thickness of a single material through
which a bremsstrahlung, or x-ray, beam travels may be made, as in
the exemplary embodiment, using a bremsstrahlung, or x-ray, beam
having pulses of multiple energies (or, for that matter, multiple
bremsstrahlung, or x-ray, beams each having pulses of a single
energy different than that of the pulses of the other beams) that
is directed through the material and measuring the beam's loss of
intensity at each energy. Viewed slightly differently, if a
bremsstrahlung, or x-ray, beam having alternating pulses of
multiple energies (e.g., E.sub.LO and E.sub.HI) and correspondingly
alternating intensities (e.g., I.sub.LOi and I.sub.HIi) is directed
through a single material and at a plurality of detectors, the
corresponding final intensities (e.g., I.sub.LO and I.sub.HI) are
measurable by the plurality of detectors. Then, the effective "Z"
number and thickness, t, of the material are determinable using the
following system of equations:
I.sub.LO=I.sub.LOie.sup.-.mu.tot(E.sub.LO.sup.,Z)t
I.sub.HI=I.sub.HIie.sup.-.mu.tot(E.sub.HI.sup.,Z)t.
From these equations, the following equation is obtained:
ln(I.sub.LO/I.sub.LOi)/ln(I.sub.HI/I.sub.HIi)=.mu..sub.tot(E.sub.LO,Z)/.-
mu..sub.tot(E.sub.HI,Z).
Consequently, the effective "Z" number of the material, Z, is
obtained by minimizing the following function, F:
F(Z)=(ln(I.sub.LO/I.sub.LOi)/ln(I.sub.HI/I.sub.HIi)-.mu..sub.tot(E.sub.L-
O,Z)/.mu..sub.tot(E.sub.HI,Z)).sup.2.
Using the effective "Z" number of the material, Z, the thickness,
t, of the material is then determined by backsolving either of the
following equations:
t=-ln(I.sub.LO/I.sub.LOi)/.mu..sub.tot(E.sub.LO,Z)
t=-ln(I.sub.HI/I.sub.HIi)/.mu..sub.tot(E.sub.HI,Z)
[0054] It should be noted that the above-described method of
determining the effective "Z" number and thickness, t, of a
material applies only to a single material. If, however, two or
more materials were placed in the plane of the bremsstrahlung, or
x-ray, beam as is typically encountered with a container 102, the
materials would be recognized as a material of a single element and
of a single thickness. In order to determine the "Z" numbers and
thicknesses for each material placed in the plane of the
bremsstrahlung, or x-ray, beam, it is necessary to first determine
the minimum number of scanning energies required to differentiate m
different kinds of material. If m layers of different materials are
present in the plane of a bremsstrahlung, or x-ray, beam having
pulses at multiple scanning energies and if Z.sub.i and t.sub.i
are, respectively, the atomic number and thickness of the ith
material, then the final intensities of the pulses striking
detectors of a detector subsystem may be computed by:
I(I.sub.0,{Z.sub.i},E.sub.ac,{t.sub.i})=I.sub.0.PI..sub.l.ltoreq.i.ltore-
q.me.sup.-.mu.(Z,Eac)ti
Using this equation, the minimum number of scanning energies
required for determining the "Z" numbers and thicknesses for each
material placed in the plane of the bremsstrahlung, or x-ray, beam
may be determined.
[0055] Once the minimum number of scanning energies has been
determined, principles and equations of absorption edge-based
recognition and of scattering resulting from photon-electron
collisions may be used to ascertain the "Z" numbers and thicknesses
of the m different kinds of material placed in the plane of the
bremsstrahlung, or x-ray, beam. An absorption edge is a discrete
upward spike in the coefficient of attenuation when photon energies
are near the binding energies of electrons in the shells of an atom
of a material. When the photon energy crosses the binding energy
threshold, there is a significantly higher chance that it will
ionize the atom. Note that because absorption edges are a
photoelectric phenomenon, the energy ranges at which this technique
is applicable are in the relatively low photoelectric range.
[0056] If the final intensities of the pulses of a bremsstrahlung,
or x-ray, beam striking or impinging upon detectors are measured
over a range of photon energies, a sharp downward spike will exist
at each absorption edge in a material. Because each element above
10 Z has a unique set of absorption edges, measuring final
intensities at energies just above and just below these edge
energies can yield every element in the path of the beam.
[0057] It should be also noted that photon scattering results from
a photon-electron collision and that the energy and direction of
the scattered photon may be ascertained by modeling the scattering
energy and distribution. In order to construct such a model, it is
assumed that the impinged upon electron is effectively stationary.
If E.sub..gamma. is the energy of an incident photon of a pulse of
a bremsstrahlung, or x-ray, beam and if energy and momentum are to
be conserved, the following constraints before and after the
collision must be obeyed:
E.sub..gamma.+m.sub.ec.sup.2=E'.sub..gamma.+
(m.sub.e.sup.2c.sup.4+p.sub.e.sup.2c.sup.2)
0=p'.sub..gamma.sin.theta..sub..gamma.+p'.sub.esin.theta..sub.e
E.sub..gamma./c=p'.sub..gamma.cos.theta..sub..gamma.+p'.sub.ecos.theta..-
sub.e
where E'.sub..gamma. is the photon energy after collision,
.theta..sub..gamma. is the scattering angle for the photon,
.theta..sub.e is the scattering angle for the electron,
p'.sub..gamma. is the momentum of the photon after the collision,
and p'.sub.e is the momentum of the electron after the collision.
Notably, for a photon of energy E, p=E/c and m.sub.ec.sup.2 is the
relativistic rest mass energy of an electron.
[0058] From the above, when a photon of energy E.sub..gamma.
collides with an atom of a material, the polar angle of scattering
for the photon, .theta., obeys the following constraint:
cos(.theta.)=1+(1/E.sub..gamma.-1/E'.sub..gamma.)m.sub.ec.sup.2
where in this case, E'.sub..gamma. is the new energy of the photon.
Reformulated, the final energy E'.sub..gamma. as a function of
E.sub..gamma. and .theta. is:
E'.sub..gamma.(E,.theta.)=E.sub..gamma.[m.sub.ec.sup.2/(m.sub.ec.sup.2+E-
.sub..gamma.(1-cos(.theta.))]
[0059] From this, it is possible to asymptotically bound the energy
of a back-scattered photon, even one with "infinite" energy. At its
maximal loss of energy, 180 degree (.pi. radian) recoil:
lim.sub.E.gamma..fwdarw..infin.E'.sub..gamma.(E.sub..gamma.,.pi.).apprxe-
q.0.255MeV
And, for its maximum back-scatter energy, which happens at a 90
degree (.pi./2 radian) deflection:
lim.sub.E.gamma..fwdarw..infin.E'.sub..gamma.(E.sub..gamma.,.pi./2).appr-
xeq.0.511MeV
Consequently, for worst-case calculations, a maximum photon energy
of 0.511 MeV can be used.
[0060] When the distribution of the scattering is considered, it
becomes useful to speak of the ratio of (E.sub..gamma./E'.sub.65 )
after and before collision:
P(E.sub..gamma.,.theta.)=1/(1+E.sub..gamma./m.sub.ec.sup.2(1-cos(.theta.-
)))
The above equation for final energy provides the final photon
energy for any given angle of scatter. It does not, however,
provide the probability that a photon will scatter in any one
direction. In order to determine such probability, use of the
Klein-Nishina formula of the differential cross section is
necessary:
d.sigma./d.OMEGA.=0.5r.sub.e.sup.2(P(E.sub..gamma.,.theta.)-P(E.sub..gam-
ma.,.theta.).sup.2sin.sup.2.theta.+P(E.sub..gamma.,.theta.).sup.3
where, as previously, r.sub.e is the classical electron radius. To
understand the meaning of this formula, it is necessary to
decompose cross section.
[0061] Suppose the probability that a photon scatters at all is of
not interest, but there is interest in the probability that a
photon scatters into a particular region. There is some area around
the electron that will scatter a colliding photon of a given energy
into a particular region. The particular area around the electron
is a partial cross section. If the space around an electron is
divided into mutually exclusive regions, there is a partial cross
section for each region. The sum of such partial cross sections
equals the total cross section.
[0062] The Klein-Nishina formula provides a way of knowing how the
total cross section changes as the size of the region, .OMEGA.,
measured in steradians, changes. Here, d.OMEGA.=2.pi. sin.theta.
d.theta.. Therefore, the Klein-Nishina formula may be interpreted
as "the probability that a photon of energy E.sub..gamma. will
scatter off an electron and into the region 2.pi. sin.theta.
d.theta. is d.tau./d.OMEGA.." With this formula, any possible
region into which a photon may scatter can be converted to some
part of .OMEGA.. Then, by integrating, the size of the cross
section that will knock photons into that region is determined.
Subsequently, the number of photons of a beam of photons that will
be knocked into that region may be determined.
[0063] Continuing, the ratio of the logarithmic transparencies of a
material at two energies, E.sub..gamma.1 and E.sub..gamma.2, may be
expressed as a function of the energies and Z number:
.delta.(E.sub..gamma.1,E.sub..gamma.2,Z)=ln(T.sub.1)/ln(T.sub.2)=.mu..su-
b.tot(E.sub..gamma.1,Z)/.mu..sub.tot(E.sub..gamma.2,Z)
The transparencies are determined by directing a beam of
bremsstrahlung, or x-rays, having pulses of respective energies
E.sub..gamma.1 and E.sub..gamma.2 through a material and toward
detectors. If .delta., E.sub..gamma.1, and E.sub..gamma.2 are
known, it is possible to solve for the "Z" number of the material.
Transparency, T, is the inverse of absorption and is a function of
photon energy E.sub.ac, the material's thickness, t, and the
material's "Z" number as follows:
T(E.sub.ac,t,Z)=.intg..sub.0.sup.EacdP/dE.sub..gamma.(E.sub.ac,E.sub..ga-
mma.)e.sup.-.mu.(E.gamma.,Z)tdE.sub..gamma./.intg..sub.0.sup.EacdP/dE.sub.-
.gamma.(E.sub.ac,E.sub..gamma.)dE.sub..gamma.
Thus, transparency is the ratio of radiation intensity before and
after the penetration of a barrier.
[0064] In the above equation for transparency,
dP/dE.sub..gamma.(E.sub.ac,E.sub..gamma.)=dI/dE.sub..gamma.(Eac,E.sub..g-
amma.)(1-e.sup.-.mu.det(E.gamma.)tdet).mu..sup.en.sub.det(E.sub..gamma.)/.-
mu..sub.det(E.sub..gamma.)
[0065] Given two experimental transparency measurements, T.sub.exp1
and T.sub.exp2, of a material, the material's thickness and Z
number may be determined by minimizing (in .lamda.-calculus
notation):
.lamda.(t,Z)
((T(E.sub.ac1,t,Z)-T.sub.exp1).sup.2+(T(E.sub.ac2,t,Z)-T.sub.exp2).sup.2)
Even though there may be multiple solutions to the above
expression, a solution may be obtained by trying each discrete "Z"
number and then searching for the minimal material thickness, t.
The transformation to absorption, .alpha., from a transparency, T,
is:
.alpha.(T)=(1-ln(T))
[0066] While the foregoing analysis and mathematical relationships
enable the determination of effective "Z" numbers for each item of
a container 102, they are based on the presumption that data
representative of the container's items is collected only within a
single plane of bremsstrahlung, or x-ray, passing through each
point within the container 102 in a direction substantially
perpendicular to the direction of movement of the container 102
during scanning and inspection. Thus, the foregoing analysis and
mathematical relationships do not take advantage of the
capabilities of the non-intrusive inspection system 100 of the
present invention, including its ability to collect data
representative of the container's items through the use of multiple
beams and planes of bremsstrahlung, or x-ray, passing through each
point within the container 102 at angles relative to one another.
Through the use of the additional data, the non-intrusive
inspection system 100 of the present invention provides calculated
effective "Z" numbers having improved accuracy.
[0067] In order to do so, the non-intrusive inspection system 100
utilizes a model for the effective "Z" number at each point within
a container 102 that treats the effective "Z" number as a vector
field having left (Z.sub.L), center (Z.sub.C), and right (Z.sub.R)
components such that for a given point within the container 102
denoted by the x, y coordinate pair (x.sub.T, y.sub.T), the vector
field is as follows:
Z(x.sub.T,y.sub.T)=[Z.sub.L(x.sub.T-y.sub.T/tan.theta.),Z.sub.C(x.sub.T)-
,Z.sub.R(x.sub.T+y.sub.T/tan.theta.)]
where: +x corresponds to the direction of container 102 travel
during scanning and inspection; +y corresponds to the direction
perpendicular to the direction of container 102 travel and
substantially collinear to the predominant direction of travel of
the first pulsed bremsstrahlung, or x-ray, beam 136 (e.g.,
indicated by arrow 150); +z corresponds to the direction
perpendicular to the +x and +y directions in accordance with the
right-hand rule; .theta. is the angle between the +x direction and
the bremsstrahlung, or x-ray, beams directed at the container 102;
T corresponds to the container 102 or "target"; and, Z corresponds
to an effective "Z" number function calculated for the values
identified in the parenthesis (e.g., X.sub.T-y.sub.T/tan.theta.,
x.sub.T, and x.sub.T+y.sub.T/tan.theta.) for the different
components using the above-described analysis, mathematical
relationships, and data collected by the detectors 164 of the
detector array 162 for left, center, and right bremsstrahlung, or
x-ray, beams directed at the container 102 during scanning thereof.
It should be noted that because the non-intrusive inspection system
100 of the exemplary embodiment utilizes only two bremsstrahlung,
or x-ray, beams 136, 138 (e.g., left and center beams) as seen in
FIG. 1, there is no third or right bremsstrahlung, or x-ray, beam
and, hence, there is no data collected by the detectors 164 for use
in computing a right effective "Z" number component of the
effective "Z" number vector field. While this somewhat reduces the
improvement in accuracy resulting from the use of the effective "Z"
number vector field, the reduction may be acceptable. If not, a
third (or right) bremsstrahlung, or x-ray, beam may be employed in
an alternate exemplary embodiment of the present invention using
another pair of turning magnets in order to produce a third
bremsstrahlung, or x-ray, beam having multiple spectra and provide
such data and to eliminate the reduction in accuracy present in the
current exemplary embodiment.
[0068] To use the effective "Z" number vector field to discriminate
between materials present in the contents of a container 102, it is
necessary to determine a scalar value from the vector field
generated for each point within the container 102. One approach is
to calculate the magnitude of the effective "Z" number vector
field, thereby producing a scalar value comprising a metric
average. A second approach is to utilize min Z and voxel rendering
to produce a minimal bounding volume. Using the second approach,
the accuracy of the effective "Z" number is increased with the
angle .theta. and with the inverse of the width of the target.
[0069] Using the effective "Z" numbers and the previously
calculated volumes for each item, the controller 180 then
calculates the density of each item in the container 102 using the
imaging and material discrimination software. Subsequently, when
desired by inspection system operators, the controller 180 and the
imaging and material discrimination software display or present, to
the operators, the three-dimensional images of the contents, or
items, present inside the container 102 and the calculated volumes,
effective "Z" numbers, and densities of such contents or items.
[0070] It should be noted that due to planes 186, 188 being
oriented an angle, .alpha., relative to one another, the
non-intrusive inspection system 100 is capable of generating
three-dimensional images of the contents of a container 102 and of
detecting items having materials that might, otherwise, go
undetected by similar systems if positioned behind other items in
order to, perhaps, avoid detection. It should also be noted that
due to the pulsed bremsstrahlung, or x-ray, beams 136, 138 each
including pulses of bremsstrahlung, or x-ray, having different
spectra, the non-intrusive inspection system 100 is further capable
of distinguishing between the materials of items present in a
container 102 and of calculating effective "Z" numbers and
densities for such items.
[0071] It should be understood that the scope of the present
invention includes all non-intrusive inspection systems for
inspecting containers in which, during inspection of a container, a
first bremsstrahlung, or x-ray, beam is passed through the
container at an angle relative to a second bremsstrahlung, or
x-ray, beam that is also passed through the same container.
Further, it should be understood that the scope of the present
invention includes such first and second bremsstrahlung, or x-ray,
beams comprising pulsed bremsstrahlung, or x-ray, beams, pulsed
bremsstrahlung, or x-ray, beams having pulses of different spectra,
and/or pulsed bremsstrahlung, or x-ray, beams each having
consecutive pulses with different spectra. It should be still
further understood that the scope of the present invention includes
non-intrusive inspection systems for inspecting containers having a
single bremsstrahlung, or x-ray, beam with one or more spectra that
is directed at a container at different angles relative to the
container during inspection thereof. It should be still further
understood that the scope of the present invention includes all
non-intrusive inspection systems for inspecting containers having
one or more charged particle accelerator(s) that is/are each
operable to generate one or more charged particle beam(s) having
charged particles with one or more energy level(s).
[0072] Whereas the present invention has been described in detail
above with respect to an exemplary embodiment thereof, it should be
understood that variations and modifications may be effected within
the spirit and scope of the present invention, as described herein
before and as defined in the appended claims.
* * * * *