U.S. patent application number 11/893961 was filed with the patent office on 2007-12-13 for x-ray diffraction-based scanning system.
Invention is credited to Michael C. Green.
Application Number | 20070284533 11/893961 |
Document ID | / |
Family ID | 32926645 |
Filed Date | 2007-12-13 |
United States Patent
Application |
20070284533 |
Kind Code |
A1 |
Green; Michael C. |
December 13, 2007 |
X-ray diffraction-based scanning system
Abstract
An x-ray diffraction-based scanning method and system are
described. The method includes screening for a particular substance
in a container at a transportation center using a flat panel
detector having a photoconductor x-ray conversion layer to detect
x-rays diffracted by a particular substance in the container. The
diffracted x-rays may be characterized in different ways, for
examples, by wavelength dispersive diffraction and energy
dispersive diffraction.
Inventors: |
Green; Michael C.; (Palo
Alto, CA) |
Correspondence
Address: |
BLAKELY SOKOLOFF TAYLOR & ZAFMAN
1279 OAKMEAD PARKWAY
SUNNYVALE
CA
94085-4040
US
|
Family ID: |
32926645 |
Appl. No.: |
11/893961 |
Filed: |
August 17, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10379271 |
Mar 3, 2003 |
7065175 |
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11893961 |
Aug 17, 2007 |
|
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11409250 |
Apr 21, 2006 |
7274768 |
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11893961 |
Aug 17, 2007 |
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Current U.S.
Class: |
250/370.09 ;
378/101; 378/103 |
Current CPC
Class: |
G01N 23/20 20130101;
G01T 1/247 20130101; G01V 5/0025 20130101; G21K 5/10 20130101; G01T
1/2018 20130101; G01T 1/244 20130101; H01J 35/26 20130101; H01J
35/06 20130101 |
Class at
Publication: |
250/370.09 ;
378/101; 378/103 |
International
Class: |
G01T 1/24 20060101
G01T001/24; H05G 1/10 20060101 H05G001/10; H05G 1/24 20060101
H05G001/24 |
Claims
1. An x-ray scanning system, comprising: an x-ray source; and means
for increasing available power input to an x-ray source when the
x-ray source is operating at less than 100% duty cycle.
2. The x-ray scanning system of claim 1, wherein the means
comprises an energy storage device.
3. The x-ray scanning system of claim 2, wherein the energy storage
device comprises a flywheel.
4. The x-ray scanning system of claim 2, wherein the energy storage
device at least doubles the input power to the source during source
on-time.
5. An apparatus, comprising: an x-ray source; and means for
increasing available power input to an x-ray source when the x-ray
source is operating at less than 100% duty cycle.
6. The apparatus of claim 5, wherein the means comprises an energy
storage device.
7. The apparatus of claim 6, wherein the energy storage device
comprises a flywheel.
8. The apparatus of claim 6, wherein the energy storage device at
least doubles the input power to the source during source
on-time.
9. A flat panel detector, comprising: a substrate; an amplifier
layer disposed above the substrate; an electrode layer disposed
above the amplifier layer; a conversion layer disposed above the
electrode layer.
10. The flat panel detector of claim 9, wherein the conversion
layer is a direct conversion layer.
11. The flat panel detector of claim 10, wherein the conversion
layer comprises a semiconductor material to convert x-rays to
electric charges directly.
12. The flat panel detector of claim 11, wherein the conversion
layer comprises a semiconductor material to convert x-rays to
electric charges without an intermediate process of converting
x-rays to visible light.
13. The flat panel detector of claim 10, wherein the conversion
layer comprises CZT.
14. An x-ray diffraction scanning system, comprising: a flat panel
detector, comprising: a semiconductor layer; a first set of
conducting lines coupled to the semiconductor layer; and a second
set of conduction lines coupled to the semiconductor layer; a pulse
measurement circuit coupled to the first and second set of
conducting lines.
15. The x-ray diffraction scanning system of claim 14, wherein the
pulse measurements circuit comprises: a first amplifier coupled to
the first set of conducting lines; a second amplifier circuit
coupled to the second set of conducting lines; a timing correlator
coupled to the first and second amplifiers; a pulse shaping
amplifier coupled to the timing correlator; and a peak detector
coupled to the pulse shaping amplifier.
16. The x-ray diffraction scanning system of claim 14, wherein the
flat panel detector further comprises a substrate and wherein the
first and second sets of conducting lines are disposed in the
substrate.
17. The x-ray diffraction scanning system of claim 14, wherein the
flat panel detector further comprises a continuous contact layer
disposed above the semiconductor layer.
18. An apparatus, comprising: a flat panel detector, comprising: a
semiconductor layer; a first set of conducting lines coupled to the
semiconductor layer; and a second set of conduction lines coupled
to the semiconductor layer; a pulse measurement circuit coupled to
the first and second set of conducting lines.
19. The apparatus of claim 18, wherein the pulse measurements
circuit comprises: a first amplifier coupled to the first set of
conducting lines; a second amplifier circuit coupled to the second
set of conducting lines; a timing correlator coupled to the first
and second amplifiers; a pulse shaping amplifier coupled to the
timing correlator; and a peak detector coupled to the pulse shaping
amplifier.
20. The apparatus of claim 18, wherein the flat panel detector
further comprises a substrate and wherein the first and second sets
of conducting lines are disposed in the substrate.
21. The apparatus of claim 18, wherein the flat panel detector
further comprises a continuous contact layer disposed above the
semiconductor layer.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. patent application
Ser. No. 10/379,271 filed Mar. 3, 2003, now U.S. Pat. No. 7,065,175
issued Jun. 20, 2006, and U.S. patent application Ser. No.
11/409,250 filed Apr. 21, 2006 entitled, "X-Ray Diffraction-Based
Scanning System."
FIELD
[0002] This invention pertains to the field of x-ray scanning
systems and, in particular, to x-ray diffraction-based scanning
systems.
BACKGROUND
[0003] The events of Sep. 11, 2001 forced recognition of an urgent
need for more effective and stringent screening of airport baggage.
The need for security expanded from the inspection of carry-on bags
for knives and guns to the complete inspection of checked bags for
a range of hazards with particular emphasis upon concealed
explosives. The demonstrated willingness of terrorists to die in
the pursuit of their aims meant that 100% passenger-to-bag
matching, which could be put in place rapidly, was not sufficient
to counter an attempt to conceal explosives in checked baggage and
bring down an airliner. Successful screening for the presence of
explosives presents numerous technological challenges, many of
which are not met in present systems. X-ray imaging is the most
widespread technology currently employed for screening. Last year
approximately 1100 x-ray explosives detection systems incorporating
computerized tomography (CT) scanners were purchased by the
Transportation Security Agency (TSA) in an accelerated procurement
program directed toward a goal of 100% screening of checked bags by
Dec. 31, 2002.
[0004] Existing x-ray baggage scanners, including CT systems,
designed for the detection of explosive and illegal substances are
unable to discriminate between harmless materials in certain ranges
of density and threat materials like plastic explosive. Thus,
depending upon the level of the sensitivity setting, they either
pass through a percentage of threat material, "missed detection" in
security parlance, or they generate a high rate of false positives.
CT scanner-based explosives detection systems are able to overcome
problems of superimposition effects that arise in line scan
systems. CT measures average x-ray absorption per voxel in slices
projected through suspect regions of a bag. This parameter is not
sufficiently specific to distinguish explosives from many other
common materials. Items implicated in false positives include
candy, various foodstuffs (e.g., cheese), plastics, and toys. Much
attention has attended the deployment of CT-based explosives
detection systems and their high false positive rate of around 30%
in real world operating conditions is now well publicized in the
media and has been acknowledged by the TSA. Concerns have been
expressed about the resultant need to open and hand search a
substantial portion of the checked bags, out of sight of the owner
of the luggage. This is time consuming and expensive for the
airlines and the prospect of airport delays and the potential for
theft is a source of concern to the traveling public.
[0005] Moreover, CT scanners are unable to detect the presence of
explosive material that is formed into thin sheets because CT
averages the x-ray absorption coefficient over each voxel.
Pentaerythritoltetranitrate (PETN), for example, will readily
detonate when in the form of a sheet 1 mm thick. The density of
PETN is 1.77 g/cc and a sheet 50 cm.times.50 cm.times.1 mm, easily
incorporated into the skin of a suitcase, weighs approximately 442
grams, or almost 1 pound, which is sufficient to cause a powerful
explosion.
[0006] Identification systems based on X-ray diffraction techniques
provide enormously improved discrimination of materials. Such
systems measure the d-spacings between the lattice planes of
micro-crystals in materials. This form of energy-selective
diffraction imaging has been employed in a type of medical
tomography and in the non-destructive examination of pigments in
works of art. X-ray diffraction provides a substance-specific
fingerprint that greatly increases the probability of specific
material detection and concomitantly reduces the incidence of false
positives. Its applicability to explosives detection and the
detection of other illicit substances has been demonstrated by
Yxlon International of Germany with a prototype diffraction-based
system.
[0007] Prior x-ray diffraction-based security systems for
explosives detection and baggage scanning are not yet highly
developed. These systems, such as Yxlon's system as illustrated in
FIG. 1, are based upon work done by Bomsdorf and Muller at the
University of Wuppertal in Germany. The Yxlon system utilizes
small-area, single-crystal germanium (Ge) detectors. A divergent
tight cluster of collimated x-ray pencil beams, originating from an
effective point source, is directed through the bag under
examination and sensed by the high-purity Ge detector cooled to
liquid nitrogen temperature of -196 degrees C. However, such a
system suffers from a number of fundamental constraints. First, the
high-purity Ge detector is too expensive to use in large area
sensors. Second, the requirement for liquid nitrogen cooling is
cumbersome and expensive to maintain in an airport environment. In
addition, they can examine only a small area of a bag at one time
due to the small detector size. This requires multiple passes of
the beam through the bag being screened in which the beam is
meander-scanned (zig-zag, back-and-forth pattern) through the bag
in order to inspect the entire contents of the bag. This is too
slow for volume applications like routine baggage scanning. Tests
done by the Canadian Customs on detecting concealed samples of
heroin and cocaine, which for a diffraction system is an equivalent
task to identifying an explosive compound, have indicated scan
times of the order of 1.5 minutes, far longer than the desired 6
seconds per bag.
SUMMARY OF THE INVENTION
[0008] An x-ray diffraction-based scanning method and system are
described. In one embodiment, the method includes providing a flat
panel detector having a photoconducting x-ray conversion layer and
screening for a particular substance in a container at a
transportation center using the flat panel detector.
[0009] Other features and advantages of the present invention will
be apparent from the accompanying drawings, and from the detailed
description, which follows below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The present invention is illustrated by way of example and
not intended to be limited by the figures of the accompanying
drawings in which like references indicate similar elements and in
which:
[0011] FIG. 1 illustrates a prior art x-ray diffraction scanning
system.
[0012] FIG. 2A is a cross-section view illustrating one embodiment
of an x-ray diffraction system.
[0013] FIG. 2B is a perspective view, respectively, illustrating
one embodiment of an x-ray diffraction system.
[0014] FIG. 2C illustrates one embodiment of a pair of collimators
having orthogonal collimation planes with respect to each
other.
[0015] FIG. 3 shows a plot of diffraction angle against x-ray
photon energy for the six strongest diffraction lines of
trinitrotoluene (TNT).
[0016] FIG. 4A illustrates one embodiment of an x-ray diffraction
system having a movable detector assembly.
[0017] FIG. 4B is a top view along the primary axis of the x-ray
sheet beam of a diffraction system illustrating one embodiment of a
detector assembly.
[0018] FIG. 5A illustrates one embodiment of components of a flat
panel detector.
[0019] FIG. 5B illustrates one embodiment of a flat panel detector
having a direct conversion layer.
[0020] FIG. 5C illustrates an alternative embodiment of a flat
panel detector having a direct conversion layer.
[0021] FIG. 6 is an exemplary figure showing the x-ray sensitivity
of a polycrystalline HgI.sub.2 detector layer operating at room
temperature with 80 KV(p) photons.
[0022] FIG. 7 illustrates one embodiment of electronic components
of the scanning system.
[0023] FIG. 8 illustrates one embodiment of an x-ray generator.
[0024] FIG. 9 illustrates an alternative embodiment of a flat panel
detector that may be used with an energy dispersive mode of scan
operation.
DETAILED DESCRIPTION
[0025] In the following description, numerous specific details are
set forth such as examples of specific components, processes, etc.
in order to provide a thorough understanding of the present
invention. It will be apparent, however, to one skilled in the art
that these specific details need not be employed to practice the
present invention. In other instances, well known components or
methods have not been described in detail in order to avoid
unnecessarily obscuring the present invention.
[0026] The terms "above," "below," and "between" as used herein
refer to a relative position of one layer or component with respect
to another. As such, one layer deposited or disposed above or below
another layer, or between layers, may be directly in contact with
the other layer(s) or may have one or more intervening layers.
Moreover, one component in front, behind, below or between another
component may be in physical contact with the other component or
may have one or more intervening components, or may be otherwise
indirectly coupled with other component. The term "coupled" as used
herein means connected directly to or connected indirectly through
one or more intervening components or operatively coupled through
non-physical connection (e.g., optically).
[0027] A diffraction-based x-ray system is described. The
diffraction-based x-ray system may be used to detect and measure
atomic layer spacing in crystalline and microcrystalline materials
to provide a means of specific material identification. Classical
x-ray diffraction takes place from the atomic layer planes in
crystals. In accordance with the Bragg equation, constructive
interference takes place when: N.lamda.=2dsin .theta.
[0028] where, .lamda. is the x-ray photon wavelength, d is the
atomic layer plane spacing, and .theta. is the diffraction angle.
The d-spacings of the atomic layer planes are substance-specific
and diffraction data provides a non-contact x-ray fingerprint for
identifying crystalline materials.
[0029] In one particular embodiment, the x-ray system may be used
for transportation center (e.g., an airport, train station, etc.)
container (e.g., bags, luggage, boxes, etc.) scanning for the
detection of a particular substance, for example, an explosive. The
majority of explosive compounds are well-crystallized solids at
room temperature. Even "plastic explosive" is composed of a high
explosive such as PETN or RDX in powder form, which is highly
crystalline on a micro-scale, dispersed in a soft binder of
polyurethane and wax.
[0030] The sensitivity of the diffraction technique is enhanced by
the nature of the typical baggage contents, e.g., predominantly
garments, paper and plastics, which are amorphous materials which
do not interact with the incident x-ray beam to give strong
coherent diffraction signals. Metals are crystalline (in
particular, microcrystalline) and efficiently diffract x-rays, but
the atomic layer d-spacings in the compact crystal lattices of
metals are small, e.g., in the range 0.5-2.5 Angstroms (.ANG.),
compared to those of larger crystalline organic compounds which are
typically in the range 2-10 .ANG.. Therefore, x-ray diffraction
lines that originate from metal materials are distinguishable from
those from most other materials, including explosives.
[0031] The x-ray diffraction system may use conventional
wavelength-dispersive (WD) diffraction or fixed-angle,
multi-wavelength diffraction for improved throughput. In addition,
the use of a large-area flat-panel x-ray detector having a two
dimensional array of pixels, or multiple smaller area flat-panel
detectors, coupled with an elongated x-ray source may permit a full
three-dimensional (3D) volumetric x-ray diffraction scan of a
container in a single pass and, thereby, improve the throughput of
the screening system. Further, orthogonal collimators may placed in
front of the flat panel detector to limit the acceptance angle of
x-ray photons entering the flat panel detector and also ensure that
each small block of pixels of the flat panel detector(s) views a
separate area of the sheet beam, thereby dividing it into volume
elements.
[0032] Although the diffraction-based x-ray system may be described
at times in relation to the detection of explosives for baggage
screening for ease of discussion, the system is not so limited. In
alternative embodiments, the diffraction-based x-ray system
described herein may be used to detect other substances and for
other purposes.
[0033] FIGS. 2A and 2B are a cross-section view and a perspective
view, respectively, illustrating one embodiment of an x-ray
diffraction system. The x-ray diffraction system 200 includes an
x-ray generator 210 having an x-ray source, a pair of collimators
275, and a two dimensional (2D) flat panel detector (FPD) array
276. The x-ray generator 210 is composed of an x-ray tube with a
longitudinally extended target and one or more x-ray beams to
generate an x-ray sheet beam 220, as further illustrated in FIG. 8.
In one embodiment, the x-ray sheet beam 220 is composed of a
continuous highly collimated x-ray sheet. In an alternative
embodiment, the x-ray sheet beam 220 is composed of multiple close
parallel-collimated sub-beams generated by source collimator
215.
[0034] As illustrated by FIG. 8, source collimator 215 may include
collimator blocks 217 that operate to limit beam divergence in the
x-direction and collimator foils 218 that operate to limit beam
divergence in the y-direction. A linear electron gun 213 may be
used to generate an electron beam 219 using cathode 212. The
electron beam 219 strikes the surface 209 of an elongated (e.g., in
the range of 1 mm to 2 meters) rotating x-ray target (anode) 211 to
generate x-rays that emanate through window 216. In one particular
embodiment, target 211 has a width of approximately 1 meter. As
discussed above, collimator 215 may be used to collimate the x-rays
to produce x-ray sheet beam 220. The shape of the beam may be
configured by altering the shape of x-ray target 211. In one
embodiment, for example, cuts may be made in surface 209 to produce
a "picket-fence-like" x-ray sheet beam 220 composed of multiple,
individual beams. It should be noted that alternative
configurations for the x-ray generator 210 may be used to produce
x-ray sheet beam 220.
[0035] In one embodiment, the x-ray generator 210 has a large
target 211 surface 209 area and can, therefore, operate at high
peak and average power. If provided with adequate heat removal
capacity, for example liquid cooling of the target 211 via a
ferrofluidic seal, it can operate continuously with .gtoreq.100 KW
of input power. The rate at which a diffraction system can acquire
data depends upon the detector efficiency. However, once the
detector efficiency has been optimized the data acquisition rate
scales directly with the available input x-ray power.
[0036] The limit on the throughput of the above diffraction system
in a transportation center operating environment is likely to be
set by available wall plug power. If this is, for example 50 KW,
and the duty cycle for tube operation is less than 100% then an
energy storage system (for example a compact flywheel unit) drawing
a continuous 50 KVA can be use to increase the available power
input to the tube during its on time. If the tube operates with a
50% duty cycle it could be operated with 100 KW of input power.
[0037] Referring again to FIGS. 2A and 2B, the x-ray sheet beam 220
is directed to a container 240 on a conveyor 250 as the conveyor
moves the container 240 in direction 243 through the axis of the
x-ray sheet beam 220. The x-ray sheet beam 220 passes vertically
through the container 240 to be scanned as it moves along conveyer
250 in direction 243. Alternatively, the container 240 to be
scanned need not be on a conveyor 250, but may be positioned under
the x-ray sheet beam 220 through other means. In one embodiment,
the width of the x-ray beam 220 is selected to cover, for example
the whole width (e.g., 242) of container 240. Alternatively, the
x-ray beam 220 may have a width 249 greater than the width of
conveyor 250 (as illustrated in FIG. 2B). The x-ray sheet beam 220
may have widths, for example, in the approximate range of 2 mm to 2
meters as determined by the width of the x-ray target 211.
[0038] If the x-ray sheet beam 220 intercepts a crystalline
material in the container 240 (e.g., a plastic explosive), x-ray
photons are diffracted at an angle (.theta.) 264 to the incident
x-ray beam. The angle 264 depends upon the d-spacing of the atomic
planes in the material. The trajectories of the diffracted photons
lie on cones (e.g., cone 260) with half angle .theta. centered on
the beam axis. The diffracted x-rays 265 are detected and their
properties measured by a detector assembly 270 located below the
conveyer 250 and displaced laterally from the path of the primary
sheet beam so that the detector assembly 270 collects the
diffracted x-rays 265. A linear detector 290 may be positioned
underneath the conveyor at the primary axis of the x-ray sheet beam
220 to detected undiffracted components of the x-ray sheet beam
220. The linear detector (e.g., composed of a line of
photoconducting diodes) measures the undiffracted x-ray beam and
provides a reference signal and projection line scan image of
container 240.
[0039] In one embodiment, the detector assembly 270 may include
first and second collimators 272 and 274 and a flat panel detector
276. The flat panel detector 276 may have a conventional TFT
structure with a scintillator or photoconductor x-ray direct
conversion layer, as discussed below in relation to FIGS. 5A and
5B. In one embodiment, the conversion layer is amorphous and, in
particular, may have a polycrystalline structure. Alternatively,
the conversion layer may have other crystalline structures. The
detector assembly 270 may be configured to have a narrow acceptance
angle of approximately 0.2 degrees full width at half maximum
(FWHM) by using one or more collimators placed in front of the flat
panel detector 276. The collimation planes of the first collimators
272 plates (e.g., plate 283) and the second collimator 274 plates
(e.g., plate 293) may be substantially orthogonal to each other, as
illustrated in FIG. 2C. The collimators 275 ensure that each pixel
279 of the flat panel detector 276 views a separate area of the
beam, thus dividing the diffracted x-rays 265 into volume elements.
In particular, first collimator 272 divides the x-ray sheet beam
into individual vertical (e.g., beam direction 241 mapping to flat
panel detector length 271) segments and second collimator 274
provides the angular resolution for accepting diffracted x-rays of
a particular angle. Photons having a particular diffraction angle
are selected by the angle of the collimators 275 and flat panel
detector 276 with respect to the primary x-ray sheet beam 220. By
tilting the second collimator 274 and flat panel detector 276
together, it is possible to scan through the diffraction spectrum
in terms of diffraction angle. The first collimator 272 need not
reside within the detector assembly 270, such that only the second
collimator 274 and the flat panel detector 276 are movable. In an
alternative embodiment, the plates of the first and second
collimators may be integrated together to form a single collimator
having orthogonal plates. Alternatively, other collimator
arrangements known in the art may be used, for examples, hexagonal
collimators.
[0040] In FIGS. 2A and 2B, only a single detector assembly 270
having a large area flat panel detector 276 is shown for clarity.
Diffraction is symmetrical about the primary beam and multiple
detector assemblies using smaller area flat panel detectors can be
located on the radial periphery of cone 260 such as, for example,
on diametrically opposed sides of the primary sheet beam (as
discussed below in relation to FIG. 4B), for improved sensitivity
and signal to noise. In one embodiment, a pair of Soller
collimators may be used. Soller collimators are compact collimators
obtainable with a FWHM acceptance angle of 0.16.degree. with
greater than 70% transmission and are commercially available from
JJ X-Ray of Denmark.
[0041] By collecting the diffracted x-rays from each volume element
(voxel), it is possible to detect, identify and physically locate
substances (e.g., explosives) within container 240 as it is moved
through the x-ray sheet beam 220. The method is substance-specific
and sensitive, with sub-voxel detection capability because the
substance does not need to fill an entire voxel to be identified.
The requirement is merely that sufficient material of the substance
is intersected by the x-ray sheet beam 220 to give a diffracted
photon signal that is above the detector noise level.
[0042] The diffracted x-rays may be characterized in different
ways, for examples, by wavelength dispersive (WD) diffraction and
energy dispersive (ED) diffraction. In wavelength dispersive
diffraction, an incident x-ray beam may be composed of
monochromatized x-rays containing a narrow range of wavelengths
typically 1% or less, centered upon an x-ray emission line
characteristic of the x-ray target material, for example, a K-alpha
line, to increase the photon flux in the monochromatized beam. The
incident beam may be monochromatized, for examples, by diffraction
off a crystal, by absorption edge filtering, via a graded
multilayer mirror, or by other means known in the art. The latter
has the advantage of improved x-ray collection efficiency.
[0043] In one embodiment, a copper (Cu) target generating filtered
Cu K-alpha radiation is used for the x-ray source. The radiation's
1.54 .ANG. wavelength results in large diffraction angles that can
be measured with high precision. However, Cu K-alpha radiation is
rapidly attenuated even by atmospheric air and more energetic x-ray
photons may be necessary for container scanning where the radiation
must penetrate the full thickness of a container which may contain
strongly absorbing objects.
[0044] As such, in alternative embodiments, other types of targets
211 generating other types of radiation may be used for the x-ray
source. In one embodiment, radiation having energy in the
approximate range of 30 to 120 KeV is used. Even higher energies
may be used for still greater penetrating power but at the cost of
decreased diffraction angles. In one particular embodiment, for
example, a tungsten (W) target 211 generating W K-alpha
characteristic radiation, with a photon energy of 59.3 KeV and
wavelength of 0.21 .ANG., may be more suitable for checked bag
scanning. There is the added advantage that a tungsten target can
be operated with high beam voltage and at high beam power. The
x-ray output rises with both beam voltage and the atomic number of
the target material. Thus a tungsten target is a more efficient
producer of x-rays than a copper target. The tradeoff is reduced
diffraction angles due to the shorter x-ray wavelength.
[0045] FIG. 3 shows a plot of diffraction angle against x-ray
photon energy for the six strongest diffraction lines of
trinitrotoluene (TNT), which has d-spacings typical of organic
explosive compounds that might be encountered in a terrorist
device. The range of angles at the 59.3 keV energy of W k-alpha is
of the order of 2.5.degree.. With W K-alpha radiation in WD mode
and measurement of crystal d-spacings down to 1.5 .ANG., the
minimum value required for explosives screening, .theta. reaches a
maximum of just over 4.degree.. This parameter, together with the
vertical height of the container 240 to be scanned, may be used, in
one embodiment, to determine the required detector size in the
direction 243 parallel to the conveyer motion.
[0046] In x-ray crystallography, highly accurate measurement of
diffraction angles is necessary for structure determination. The
requirements are less stringent for matching a diffraction spectrum
to a database of spectra of threat compounds. The angular
discrimination provided by a simple collimator system placed in
front of the detector is sufficient to resolve "fingerprint"
spectra adequate for matching purposes. For example, if the W
k-alpha diffraction spectrum of TNT is convolved with the
0.16.degree. FWHM of a commercial Soller collimator, the
diffraction lines show significant broadening but the line
definition is still adequate for identifying proscribed material by
comparison with reference spectra.
[0047] FIG. 4A illustrates one embodiment of x-ray diffraction
system 200 having a movable detector assembly 470 that may be used
for operation with wavelength dispersive diffraction. As previously
discussed, in the WD mode the x-ray source of generator 210 is
filtered so that the x-ray sheet beam 220 contains only a narrow
range of x-ray photon wavelengths that are centered upon, for
example, the W K-alpha line. In WD mode, the x-ray diffraction
lines 265 (only two exemplary diffraction lines are illustrated)
are scanned by moving (e.g., by tilting, pivoting, rotating,
sliding, etc.) 471 the detector assembly 470 from an angle
.theta..sub.1 to an angle .theta..sub.2 on the order of a few
degrees. For example, for W K-alpha radiation, the angular extent
of the scan may be of the order of 2.5 degrees to cover the range
of d-spacings that is of interest for the detection and
identification of explosives. With W K-alpha radiation in the WD
mode and measurement of crystal d-spacings down to 1.5 .ANG., the
maximum value of .theta. may be approximately 4 degrees.
Alternatively, another angular range .theta..sub.2-.theta..sub.1
may be used based on the particular substance to be detected. The
conveyor 250 stops the container 240 at each scan location and
container 240 is scanned serially through the angles .theta..sub.1
to .theta..sub.2 that the x-ray sheet beam 220 is diffracted
through. The vertical position in the incident x-ray sheet beam
direction 241 maps onto the flat panel detector 276. This mapping
changes with the angle .theta. at which the detector assembly 470
is positioned with respect to the primary axis of the x-ray sheet
beam 220. A software routine may be used to perform such a
mapping.
[0048] Referring again to FIG. 2B, the vertical dimension 441 of
the scanned area is compressed by a factor of sine .theta. in the
diffracted image on the surface of flat panel detector 270 (and 470
of FIG. 4A). Thus, for example, a 100 cm wide 242.times.75 cm high
241 scanned area in container 240 maps onto a 100 cm wide
272.times.5.25 cm long 271 flat panel detector area. In practice,
the flat panel detector 276 need not be in the form of a single
elongated panel (e.g., 100 cm.times.5.25 cm). Since x-rays are
diffracted symmetrically outwards, the flat panel detector 276 can
be divided into several smaller panels, as illustrated in FIG.
4B.
[0049] FIG. 4B is a top view along the primary axis of the x-ray
sheet beam of system 200 illustrating one embodiment of a detector
assembly. Continuing the above example, the flat panel detector 475
may be divided into four panels 476-479, each panel with active
area of 25 cm.times.5.25 cm, located alternately on either side of
the x-ray sheet beam primary axis 221. In yet another embodiment,
the panels residing on a side of primary axis 221 may be disposed
within a separate assembly (e.g., panels 476, 478 in one assembly
and panels 477 and 479 in another assembly) or each panel may be
disposed within its own assembly.
[0050] FIG. 5A illustrates one embodiment of components of a flat
panel detector. The flat panel detector 276 may be constructed as a
panel with a matrix of photosensitive devices with readout
electronics to transfer the light intensity of a pixel to a digital
video signal for further processing or viewing. Flat panel detector
276 includes a conversion layer formed by a scintillator layer 521
and a photodetector layer 526. Flat panel detector 276 also
includes a substrate 527, a supply voltage 523, capacitor 528, and
switch 532. In one embodiment, for example, photodetector layer 526
may include photoconductors or photovoltaic components (e.g., a
photodiodes) that receive light photons from a scintillator 521.
The scintillator 521 is a conversion layer that receives x-rays and
generates visible light that strikes photodetector layer 526.
Photodetector layer 526 captures the visible light produced in the
scintillator and generates an electric current (I) 529. The
electric current 529 charges capacitor 528 and leaves a charge
value on capacitor 528, where the integrated charge on capacitor
258 is proportional to the integrated light intensity striking
photodetectors 526 for a given integration time. Capacitor 528 is
coupled to switch 532 such as a thin-film-transistor (TFT). The
operation of switch 532 may be discussed herein in relation to a
TFT for ease of discussion purposes only. Other types of switch
devices, for example, switching diodes may also be used.
[0051] At an appropriate time, the control input 530 (e.g., gate of
a TFT) activates switch 532 and reads out the charge on capacitor
528 at node 534. The charge at node 534 is further amplified and
processed for a corresponding pixel of flat panel detector 276, as
discussed below in relation to FIG. 7.
[0052] In an alternative embodiment, flat panel detector 276 may
have other configurations. For example, flat panel detector 276 may
utilize a semiconductor material as a direct conversion layer to
convert x-rays to electric charges directly, without an
intermediate step of converting x-rays to visible light. FIG. 5B
illustrates one embodiment of flat panel detector 276 having a
direct conversion layer 535. The flat panel detector 276 has
conversion layer 535 composed of a semiconductor material disposed
between a top electrode layer 531 and charge-collection electrode
layer 533. A bias voltage 536 is applied across semiconductor layer
535 incident to the top electrode 531. As x-rays 524 propagate
through the semiconductor layer 535 through the top electrode 531,
it creates electric charges within the semiconductor layer 535 that
are drawn to the charge-collection layer 533. The charge is
collected, amplified and processed for a corresponding pixel of
flat panel detector 276.
[0053] A significant increase in detector sensitivity can be gained
by utilizing a flat panel detector 276 coated with a wide bandgap
(e.g., in the approximate range of 0.5 to 3 eV) semiconductor as a
conversion layer 535. Semiconductors act as direct conversion
materials. The passage of x-ray photons generates electrons and
holes that are swept out of the photoconducting conversion layer
535 by an applied bias voltage and collected on a switch (e.g.,
TFT) array. In one embodiment, a polycrystalline mercury-iodide
(HgI.sub.2) may be deposited on TFT panels to form direct detection
arrays with very high sensitivity, approaching the theoretical
maximum. This sensitivity may be more than five times better than
that of flat panel detectors with scintillator layers that employ
indirect conversion.
[0054] It should be noted that alternative configurations and
components known in the art may be used with flat panel detector
276. For example, the flat panel detector 276 may be integrated
with CCD-based or CMOS-based photodetectors. Flat panel detectors
are available from manufacturers such as Varian Medical Systems,
Inc. of California.
[0055] FIG. 6 shows the x-ray sensitivity of an exemplary
polycrystalline HgI.sub.2 detector layer operating at room
temperature with 80 KV(p) photons. The material has sensitivity in
the range of 15 .mu.Coulombs/R/cm.sup.2 that is approximately five
times higher than some of the better performing CsI scintillator
materials. The curves of sensitivity versus bias voltage show
saturation at a bias field below 1 volt/micron that is important to
avoid voltage dependent gain variations during operation at high
signal levels.
[0056] Although detector 276 may be operated at room temperature,
detector 276 may also be cooled below room temperature (e.g., down
to approximately minus 100 degrees C.) in order to reduce dark
current contributions to the collected charge on the capacitors 528
of the detector 276. Cooling of a semiconductor conversion layer
detector results in lower noise and permits a higher bias voltage.
The latter improves the rate at which charge carriers are swept out
of the semiconductor, which increases the maximum attainable count
rate for photons and improves the energy resolution. Wide band-gap
semiconductors have relatively low dark currents even at room
temperature and the dark current has a steep dependence upon
temperature. Although operation at liquid nitrogen temperature is
not required, cooling semiconductors such as HgI.sub.2 to
temperatures of the order of minus 30.degree. C. is beneficial,
with a somewhat lower optimum operating temperature for CZT. These
temperatures can be reached, for examples, with a solid state
Peltier cooler (down to approximately minus 70.degree. C.). Peltier
coolers, also known as thermoelectric coolers, are solid state heat
pumps that take advantage of the Peltier Effect. The Peltier effect
takes place when an electric current is sent through two dissimilar
materials that have been connected to one another at two junctions.
One junction between the two materials is made to become warm while
the other becomes cool, in what amounts to an electrically driven
transfer of heat from one side of the device to the other. Peltier
coolers are available from different manufacturers such as Swiftech
of California, USA. In alternative embodiments, other temperatures
and other types of cooling systems may be used, for example, a
closed circuit cascade cooling system (e.g., available from IGC
Polycold Systems of California, USA) that can cool as low as
approximately minus 100.degree. C.
[0057] FIG. 7 illustrates one embodiment of, in particular,
electronic components of the scanning system 200. Scanning system
200 includes a computing device 704 coupled to a flat panel
detector 276. As previously discussed in relation to FIGS. 5A and
5B, flat panel detector 276 operates by accumulating charge on
capacitors (e.g., capacitor 528) generated by pixels (e.g., pixel
279 of FIG. 2C) of photodetectors with a scintillator layer 521 or
by pixels of a direct conversion layer 535. Typically, many pixels
are arranged over a surface of flat panel detector 276 where, for
example, TFTs (or e.g., single and/or double diodes) at each pixel
connect a charged capacitor 528 to charge sensitive amplifier 719
at the appropriate time. Charge sensitive amplifier 719 drives
analog to digital (A/D) converter 717 that, in turn, converts the
analog signals received from amplifier 719 into digital signals for
processing by computer device 704. A/D converter 717 may be coupled
to computing device 704 using, for example, I/O device 710 or
interconnect 714. A/D converter 717 and charge sensitive amplifiers
719 may reside within computing device 704 or flat panel detector
276 or external to either device. Amplifiers 719 count the photons
received by flat panel detector 276 and provide a pulse
proportional to the received energy. Amplifiers 719 transmit the
pulse to A/D converter 717. A/D converter 717 converts the pulse
heights to a digital value that is provided to computing device
704.
[0058] The methods, steps, instructions, etc. that are performed by
computer device 704, as discussed below, may be performed by
hardware components or may be embodied in machine-executable
instructions (or a combination thereof), which may be used to cause
a general-purpose or special-purpose processor programmed with the
instructions to perform the steps. Machine-executable instructions
may be contained in machine readable medium that includes any
mechanism for storing or transmitting information in a form (e.g.,
software, processing application) readable by a machine (e.g., a
computer). The machine-readable medium may includes, but is not
limited to, magnetic storage medium (e.g., floppy diskette);
optical storage medium (e.g., CD-ROM); magneto-optical storage
medium; read only memory (ROM); random access memory (RAM);
erasable programmable memory (e.g., EPROM and EEPROM); flash
memory; electrical, optical, acoustical or other form of propagated
signal (e.g., carrier waves, infrared signals, digital signals,
etc.); or other type of medium suitable for storing electronic
instructions. Particular reference to hardware or software herein
is made only for ease of discussion.
[0059] Computing device 704 is coupled to conveyor 250 to control
the position of container 240 through, for example, position
control circuitry 721. Computing device 704 is coupled to
projection image acquisition circuitry to 722 to receive signals
indicative of the undiffracted x-ray beam from linear detector 290
and provide a projection line scan image of container 240.
Computing device 704 implements the methods for processing of the
digital signals provided by A/D converter 717 to provide an output
for identification of the substance being scanned as is known in
the art. In one embodiment, computing device 704 includes a
processor 706, storage device 708, input/output (IO) device(s) 710,
and memory 712 that are all coupled together with interconnect 714,
such as a bus or other data path.
[0060] Processor 706 represents a central processing unit of any
type of architecture (e.g., Intel architecture or Sun Microsystems
architecture), or hybrid architecture. In addition, processor 706
could be implemented in one or more semiconductor chips. Storage
device 708 represents one or more mechanisms for storing data
and/or instructions such as the method steps of the invention.
Storage device 708 represents read-only memory (ROM), random access
memory (RAM), magnetic disk storage media, optical storage media,
flash memory devices, and/or other machine-readable media.
Interconnect 714 represents one or more buses (e.g., accelerated
graphics port bus, peripheral component interconnect bus, industry
standard architecture bus, X-Bus, video electronics standards
association related buses, etc.) and bridges (also termed bus
controllers). I/O device(s) 710 represents any of a set of
conventional computer input and/or output devices including, for
example, a keyboard, mouse, trackball or other pointing device,
serial or parallel input device, display monitor, plasma screen, or
similar conventional computer I/O devices. Memory 712 represents a
memory device for retaining data and processor instructions for
processor 6 according to the method steps of the invention. Memory
712 can be implemented using any of the memory devices described
above for storage device 8. In addition, memory 712 can be used as
a data cache for processor 706. It should be noted that the
architecture illustrated in FIG. 7 is only exemplary. In
alternative embodiments, other architectures may be used for
computing device 704. For examples, computing device 704 may
utilize memory controller(s) and/or I/O controller(s) that are
dedicated or integrated into one or more components. For other
examples, while computing device 704 is described in relation to a
single processor computing system, a multi-processor computing
device may be used and/or a distributed computing environment may
be used where the machine readable medium is stored on and/or
executed by more than one computing devices.
[0061] Referring again to FIGS. 2A and 2B, an alternative method of
obtaining diffraction data from the contents of the container is to
position the collimator and detector assembly at a fixed angle to
the incident beam. If the acceptance angle .theta. for photons is
fixed then, from the Bragg equation, specific x-ray photon
wavelengths will be diffracted from the various crystal lattice
d-spacings at a given diffraction angle. The photon wavelengths,
and hence energies, are defined by the lattice spacings and the
selected photon acceptance angle. Energy dispersive diffraction
scanning may be capable of much more efficient use of the x-rays
from the line-source x-ray tube, coupled with the ability to scan
through the entire diffraction spectrum in parallel rather than
serially.
[0062] For energy dispersive diffraction the x-ray sheet beam does
not need to be monochromatic but may be merely collimated
directionally by techniques known in the art. As such, much greater
total power in the continuum output of source can be used. With
energy dispersive diffraction scanning, the incident x-ray sheet
beam 220 contains a broad range of photon energies. Each wavelength
has an associated photon energy. The smaller the wavelength, the
higher the photon energy. The unfiltered bremsstrahlung radiation
from, for example, a tungsten (W) target contains a suitable spread
of photon energies. Alternatively, other materials for target 211
may be used. The incident electron beam energy is chosen to furnish
a sufficiently high cut-off in the x-ray photon energy.
[0063] The appropriate photon takeoff angle may be derived by
simulation or empirical data. For example, FIG. 3 that shows the
diffraction lines of TNT. As the diffraction angle .theta. is
increased, the diffracted photon energies shift downwards. Lower
photon energies will be preferentially absorbed during passage
through the container. A photon takeoff angle of 2-2.5.degree.
results in an appropriate range of photon energies for detection of
TNT.
[0064] The flat panel detector 276 is provided with a
photoconductor layer 535 capable of providing a pulse height output
that is proportional to the energy of the photons incident on the
detector. Photoconductors that may be used for conversion layer 535
include, for examples, CZT, HgI.sub.2, PbI.sub.2, Se or other wide
bandgap semiconductor materials. The acceptance angle of the panel
for incident photons may be defined by the collimator(s) in a
similar manner to the wavelength dispersive arrangement. However,
the acceptance angle may be held constant (e.g., typically 2-3
degrees away from the sheet beam axis). In this case, specific
x-ray photon energies are diffracted by the d-spacings of the
material at this chosen angle. Only photons with these specific
energies can pass through the collimator 274 at the selected angle.
The flat panel detector 276 measures the energy of the diffracted
photons, thus characterizing the material. This method has the
advantage that the energy data can be measured in parallel, hence
greatly increasing the scan speed. Additionally, a mono-energetic
primary x-ray sheet beam 220 is not required and the entire
continuum output of the x-ray generator 210 can be used that
increases the efficiency of x-ray generation and utilization in the
incident x-ray sheet beam 220.
[0065] In an alternative embodiment, the electronic components of
flat panel detector 276 and scanning system 200 may be modified
from that shown in FIGS. 5A, 5B, and 7 for use with the energy
dispersive mode. FIG. 9 illustrates an alternative embodiment of a
flat panel detector that may be used with an energy dispersive mode
of scan operation. In this embodiment, with reference to FIGS. 5B,
7 and 9, flat panel detector 276 includes an array of pixels having
two sets of parallel conductor lines on the top and bottom of a
semiconductor conversion layer (e.g., conversion layer 535 of FIG.
5B). A set of conducting read-out lines (X.sub.1 to X.sub.N) 971
runs in the x-direction on the top face and a set of conductors
(Y.sub.1 to Y.sub.N) 972 runs in the y-direction on the bottom face
of the semiconductor layer 535. A bias voltage 536 is applied
through the thickness of the semiconductor layer 535 between the
set of x lines 971 and the set of y lines 972. Pulse measurement
circuits may be disposed around the periphery of the detector array
276 to measure pulses, as the pulses appear on the x and y lines
971 and 972, respectively.
[0066] The pulse measurement circuits may include pulse amplifiers
(PA) 910 and 915, timing correlator 920, pulse shaping amplifier
925, and peak detector 930. The output of amplifiers 910 and 915
may be coupled to timing correlator 920. Timing correlator 920 uses
coincidence timing to assign an x-y line pair to a particular pulse
event, thereby localizing the position (pixel) at which the charge
originated in the semiconductor layer 535. The pulses of charge are
passed though a pulse shaping amplifier 925. In one embodiment, for
example, the pulse shaping amplifier 925 may have a time resolution
of the order of 2-20 microseconds that gives an output pulse
proportional to the charge delivered in the initial pulse. The
output pulse is fed to a peak detector 930 and an A/D converter 917
where the pulse amplitudes are converted to a digital value. The
resulting A/D output is used as the address of a memory location in
an array (e.g., array 941) of a digital memory 712. In effect, the
memory arrays act as a series of individual counters each covering
a narrow range in energy. Counts are accumulated in these "bins" or
"channels" building up a digital representation of the energy
distribution of the diffracted photons, which is the diffraction
spectrum in the energy domain. Each pixel in the detector array 976
has an associated array of digital memory addresses and a spectrum
in the energy domain energy is accumulated for each 2-D pixel
position in the detector. The pixels map back to the equivalent
positions in the plane of the x-ray sheet beam 220 as it passes
transversely through container 240.
[0067] Various alternatives may also be used. It is desirable to
keep the pulse of charge from each incident photon separate from
the pulse from subsequent photons. At high count rates, a second
pulse may reach the amplifier before the pulse from the first
photon is fully processed. This results in a phenomenon known as
pulse pile-up of a total pulse that is a combination of two pulses.
In one embodiment, pulse pile-up rejection circuits may be
incorporated into the electronics.
[0068] Considerable improvement in signal-to-noise can be achieved
by locating a front-end amplifier at each pixel rather than having
simple x-y read-out lines and peripheral read-out pulse measurement
circuits. In one embodiment, for example, the front-end amplifier
may be implemented as a layer 538 between the substrate 527 and the
conversion layer 535 as illustrated in FIG. 5C.
[0069] In yet another embodiment, the x and y read-out lines 971
and 972 may also be incorporated into substrate 527. The top
surface of semiconductor conversion layer 535 has a top electrode
531 in the form of a continuous conducting layer to which the bias
voltage 536 is applied. Such a configuration may simplify
fabrication of the flat panel detector 276 when using certain
semiconductor materials for conversion layer 535 such as HgI.sub.2
that are difficult to lay down contacts upon. A continuous contact
sheet of, for example, evaporated palladium, may be easier to
arrange than many narrow contact lines. Alternatively, other metal
and metal alloys may be used for the contact layer 531. The x and y
read-out lines 971 and 972 may be fabricated into substrate 527 at
the same time as the pixel amplifier layer 538 before deposition of
conversion layer 538. Each pixel would have a contact pad (defining
the area of the pixel) above the amplifier in layer 533. The
contact pads collect the pulse of the charge from semiconductor
conversion layer 535, feed the pulse to the amplifier 538 that
amplifies the pulse and provides it into the x-y lines in substrate
527.
[0070] In an embodiment where CZT is used as the conversion layer
535, the conversion layer 535 may have the form of a mosaic of
tiles or strips of CZT supported upon a substrate. High performance
CZT is not a polycrystalline material that can be deposited by
physical vapor deposition (PVD) but is composed of monolithic
single crystal pieces sliced from boules grown by high pressure
Bridgeman crystallization. The same x-y read-out line scheme may be
used and if individual pixel amplifiers are used they can be
implemented, for example, as an amplifier layer 538 above the
substrate 527 as illustrated in FIG. 5C. The amplifier layer 538
may be coupled to the CZT conversion layer 535, for example, by
bump-bonding amplifier layer 538 to the underside of the CZT layer.
Alternatively, other coupling methods may be used.
[0071] In one embodiment, the use of very large pixels (by display
standards) in the diffraction detector makes it possible to
subdivide each pixel into subpixels to reduce the performance
requirement of the front end amplifier and read-out electronics.
Each sub-pixel receives only a fraction of the photon flux incident
on the pixel that reduces pulse pile-up and increases the maximum
photon count rate under large signal conditions.
[0072] As previously discussed, a scintillator based detector may
be used in pulse counting mode but is not commonly employed because
of its low x-ray conversion efficiency compared to semiconductor
direct conversion detectors. For the same x-ray photon energy
deposited in the detector a factor of five or more electrons are
obtained from a semiconductor detector as compared to a
scintillator detector. This means that scintillator detectors have
a lower signal to noise ratio and the lower absolute signal means
less precision of energy measurement.
[0073] The 2D area flat panel detector discussed herein permits the
spatial location of the source of diffracted x-rays within
container 240 where the searched-for substance is concealed. The
cross section of the container 240 defined by the x-ray sheet beam
220 is mapped onto the flat panel detector 276 array surface with
compression of the vertical direction 241. The use of a low cost,
large-area, wide bandgap semiconductor detectors capable of single
photon counting with adequate energy resolution would eliminate the
constraints of existing scanning systems and permit single pass
diffraction scanning at much higher speed. It should be noted again
that the diffraction-based detection system discussed herein can
detect almost any crystalline and, in particular, microcrystalline
substance, including many illegal drugs and other contraband
materials in addition to explosive materials.
[0074] In the foregoing specification, the invention has been
described with reference to specific exemplary embodiments thereof.
It will, however, be evident that various modifications and changes
may be made thereto without departing from the broader spirit and
scope of the invention as set forth in the appended claims. The
specification and drawings are, accordingly, to be regarded in an
illustrative sense rather than a restrictive sense.
* * * * *