U.S. patent application number 12/045110 was filed with the patent office on 2009-09-10 for x-ray inspection and detection system and method.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Peter Michael Edic, Susanne Madeline Lee.
Application Number | 20090225944 12/045110 |
Document ID | / |
Family ID | 41053591 |
Filed Date | 2009-09-10 |
United States Patent
Application |
20090225944 |
Kind Code |
A1 |
Lee; Susanne Madeline ; et
al. |
September 10, 2009 |
X-RAY INSPECTION AND DETECTION SYSTEM AND METHOD
Abstract
An X-ray detection and inspection system is disclosed. The
system includes an X-ray source configured to generate an
interrogating X-ray beam, wherein the X-ray beam is directed
towards a probe volume in a sample, one or more two-dimensional
area detectors, wherein the one or more detectors are positioned at
angles other than 90 degrees with respect to the direction of the
interrogating beam and are configured to receive and detect
non-circular conic sections of diffracted X rays from the probe
volume, and an acquisition and analysis system configured to
generate position and intensity data of the non-circular conic
sections such that the corresponding mathematical equations of the
conic sections could be generated, to identify one of a
quasi-monochromatic or monochromatic XRD pattern from the
non-circular conic sections, and to determine a position of the
probe volume and at least two Bragg diffraction angles from said
XRD pattern.
Inventors: |
Lee; Susanne Madeline;
(Cohoes, NY) ; Edic; Peter Michael; (Albany,
NY) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY;GLOBAL RESEARCH
PATENT DOCKET RM. BLDG. K1-4A59
NISKAYUNA
NY
12309
US
|
Assignee: |
GENERAL ELECTRIC COMPANY
SCHENECTADY
NY
|
Family ID: |
41053591 |
Appl. No.: |
12/045110 |
Filed: |
March 10, 2008 |
Current U.S.
Class: |
378/71 |
Current CPC
Class: |
G01N 23/2076
20130101 |
Class at
Publication: |
378/71 |
International
Class: |
G01N 23/20 20060101
G01N023/20 |
Claims
1. An X-ray detection and inspection system comprising: an X-ray
source configured to generate an interrogating X-ray beam, wherein
the X-ray beam is directed towards a probe volume in a sample; one
or more two-dimensional (2D) area detectors, wherein the one or
more detectors are positioned at angles other than 90 degrees with
respect to the direction of the interrogating beam and are
configured to receive and detect non-circular conic sections of
diffracted X-rays from the probe volume; and an acquisition and
analysis system configured to generate position and intensity data
of the non-circular conic sections such that corresponding
mathematical equations of the non-circular conic sections could be
generated, to identify one of a quasi-monochromatic or
monochromatic XRD pattern from the non-circular conic sections, and
to determine a position of the probe volume and at least two Bragg
diffraction angles from said XRD pattern.
2. The system of claim 1, wherein the one or more detectors
comprise one or more detectors selected from the group consisting
of energy-integrating 2D area detectors, energy-discriminating 2D
area detectors, and combinations thereof.
3. The system of claim 1, wherein the acquisition and analysis
system is configured to determine at least two characteristic
spacings between planes of atoms or molecules in the probe volume
from the at least two Bragg diffraction angles.
4. The system of claim 3, wherein the acquisition and analysis
system is configured to determine a material composition from the
at least two characteristic planar spacings.
5. The system of claim 1, wherein a collimator is used to collimate
the interrogating X-ray beam.
6. The system of claim 5, wherein the collimator is an x-ray
optic.
7. The system of claim 1, wherein the X-ray beam is configured to
be one of a monochromatic, quasi-monochromatic, or polychromatic
X-ray beam.
8. The system of claim 1, further comprising at least two different
K-edge filters used sequentially, wherein the X-ray beam is a
polychromatic beam and wherein the acquisition and analysis system
is configured to generate a quasi-monochromatic X-ray diffraction
pattern from the diffraction patterns generated by each of the at
least two K-edge filters individually.
9. The system of claim 1, wherein the 2D area detector has a narrow
wavelength range sensitivity.
10. The system of claim 1, wherein the 2D area detector has a broad
wavelength range sensitivity.
11. The system of claim 1, wherein the energy or energies of the
interrogating X-ray beam are selected to be in a range from about 1
keV to about 5 MeV.
12. The system of claim 1, wherein an overlapping energy width of
the interrogating X-ray beam and the detector is in a range from
about 1 keV to 5 keV.
13. The system of claim 1, wherein an overlapping energy width of
the interrogating X-ray beam and the detector is in a range from
about 1 eV to 1 keV.
14. The system of claim 1, wherein the system is a screening
system.
15. The system of claim 14, wherein the screening system is a
baggage or cargo screening system.
16. The system of claim 15, wherein the system is configured to
detect and identify explosive or contraband material within the
probe volume.
17. The system of claim 14, comprising a frame wherein one or a
plurality of sources and detectors are distributed around the
frame, and wherein the frame is one of a stationary or rotary
frame.
18. The system of claim 1, wherein the system is a defect
inspection system for auto, rail, or aircraft parts.
19. The system of claim 1, wherein the system is combined with at
least one of a CT or radiographic system, which facilitates
identification of regions of interest.
20. A method of X-ray inspection and detection comprising:
generating an interrogating X-ray beam, wherein the X-ray beam is
directed towards a probe volume in a sample; interrogating the at
least one voxel within the probe volume by the X-ray beam to
generate diffracted X-rays; detecting non-circular conic sections
of diffracted X rays with one or more 2D area detectors positioned
at an angle other than 90 degrees with respect to the direction of
the interrogating X-ray beam; generating the position and intensity
data of the non-circular conic sections such that the corresponding
mathematical equations of the conic sections could be generated;
identifying at least one of a quasi-monochromatic or monochromatic
diffraction pattern from the non-circular conic sections;
determining the position of the at least one probed voxel by
determining at least one apex of at least one diffraction cone
corresponding to at least one of the non-circular conic sections of
diffracted X rays; and determining at least two Bragg diffraction
angles from the at least one of the quasi-monochromatic or
monochromatic diffraction pattern.
21. The method of claim 20, wherein the one or more detectors
comprise one or more detectors selected from the group consisting
of energy-integrating 2D area detectors, energy-discriminating 2D
area detectors, and combinations thereof.
22. The method of claim 20, comprising collimating the X-ray beam
using a collimator.
23. The method of claim 22, wherein the collimator is an x-ray
optic.
24. The method of claim 20, comprising determining at least two
characteristic spacings between planes of atoms or molecules in the
probe volume from the at least two Bragg diffraction angles.
25. The method of claim 24, comprising determining the material
composition from the at least two characteristic planar
spacings.
26. The method of claim 20, wherein the interrogating X-ray beam is
one of a monochromatic, quasi-monochromatic, or polychromatic X-ray
beam.
27. The method of claim 20, wherein generating and characterizing
position and intensity data of the quasi-monochromatic diffraction
pattern comprises subtracting diffraction patterns generated using
at least two different K-edge filters, wherein the interrogating
X-ray beam is a polychromatic beam.
28. The method of claim 20, wherein the 2D area detector has a
narrow wavelength range sensitivity.
29. The method of claim 20, wherein the 2D area detector has a
broad wavelength range sensitivity.
30. The method of claim 20, wherein the energy or energies of the
interrogating X-ray beam are selected to be in a range from about 1
keV to about 5 MeV.
31. The method of claim 20, wherein an overlapping energy width of
the interrogating X-ray beam and the detector is in a range from
about 1 keV to 5 keV.
32. The method of claim 20, wherein an overlapping energy width of
the interrogating X-ray beam and the detector is in a range from
about 1 eV to 1 keV.
33. The method of claim 20, wherein method is combined with a
method to identify a region of interest from data acquire from at
least one of a CT or radiographic system.
Description
BACKGROUND
[0001] The invention relates generally to an X-ray inspection
system and method, and more particularly to an X-ray inspection
system and method for screening.
[0002] Many applications use X-ray diffraction to identify the
crystal structure and composition of unknown objects spatially
distributed along an incident X-ray beam. For example, some airport
baggage screening systems use X-ray diffraction (XRD) to identify
explosive threats in scanned baggage. When such analyses need to be
performed quickly, the standard approach has been to employ energy
dispersive X-ray diffraction (EDXRD) with expensive,
energy-sensitive, liquid nitrogen (usually) cooled, single pixel,
line, or array of line detectors. Frequently these systems employ
X-ray sources that generate a divergent, polychromatic, X-ray beam
that needs to be collimated and combined with a detector collimator
in order to probe a localized volume of space. The size of the
localized volume depends on the degree of collimation provided by
the collimators. The major problem, therefore, becomes one of
measurement time versus spatial resolution. If two collimators are
used to provide a reasonable degree of collimation, i.e., good
spatial resolution, the photon flux drops dramatically, requiring
long measurement times to obtain good counting statistics.
[0003] In the case of EDXRD-based explosive detection systems
(EDS), for example, both good spatial resolution and fast
measurement times are needed. When both source and detector
collimators are used, the X-ray intensity is typically reduced by
more than 99.99%, leading to low-intensity signals that increase
the difficulty of correct explosive threat identification. To
compensate for these low signals and address the high false
positives and commensurate slow scan rate, EDS manufacturers use
single-photon counting, energy-sensitive, single pixel, line, or
array of line detectors, e.g., liquid nitrogen cooled, high purity
Ge (HPGe, high performance Ge) detectors with small X-ray sensitive
areas. An inherent problem with these single-photon counting
detectors is their inability to distinguish between two photons of
equal energy incident upon the detector simultaneously and a single
photon with twice the energy. Mischaracterization of the energy of
the incident photon will lead to reduced sensitivity and
specificity. Additionally, the detector dark currents (electrical
signals recorded with no photons impinging on the detector), which
are different for each detector element, are temperature-dependent.
At liquid nitrogen temperatures, the dark currents are negligible,
but increase non-linearly with increasing temperature. Thus, as the
liquid nitrogen boils off and the detector element temperature
changes, the dark current increase will not be uniform, degrading
the signal across the detector elements differently, which may
affect the accuracy of threat detection. The dark current also
changes with time as the detectors are exposed to high-energy X
rays, possibly affecting the accuracy of threat detection also.
Furthermore, the HPGe detectors typically have count rate
limitations; they saturate quickly when the diffracting volume
contains strongly diffracting crystals, which in turn increases the
difficulty of accurate composition identification. Moreover, the
operationally acceptable baggage scan rates are such that the
counting statistics in the diffracted signal are very low, leading
to classification errors.
[0004] One solution is to develop more powerful X-ray sources that
provide a higher photon flux density. However, most current X-ray
sources are already operating near the limit where the target
melts, which shortens their lifetime, making source maintenance a
concern in explosives detection systems.
[0005] Thus, it would be desirable to have an X-ray inspection and
detection system having higher accuracy, higher speed, and lower
maintenance costs.
BRIEF DESCRIPTION
[0006] One embodiment disclosed herein is an X-ray detection and
inspection system. The system includes an X-ray source configured
to generate an interrogating X-ray beam, wherein the X-ray beam is
directed towards a probe volume in a sample, one or more
two-dimensional area detectors, wherein the one or more detectors
are positioned at angles other than 90 degrees with respect to the
direction of the interrogating beam and are configured to receive
and detect non-circular conic sections of diffracted X rays from
the probe volume, and an acquisition and analysis system configured
to generate position and intensity data of the non-circular conic
sections such that the corresponding mathematical equations of the
conic sections could be generated, to identify one of a
quasi-monochromatic or monochromatic XRD pattern from the
non-circular conic sections, and to determine a position of the
probe volume and at least two Bragg diffraction angles from said
XRD pattern.
[0007] Another embodiment disclosed herein is a method of X-ray
inspection and detection. The method includes generating an
interrogating X-ray beam, wherein the X-ray beam is directed
towards a probe volume in a sample, interrogating with the X-ray
beam at least one voxel within the probe volume to generate
diffracted X-rays, detecting non-circular conic sections of
diffracted X rays with one or more two dimensional area detectors
positioned at an angle other than 90 degrees with respect to the
direction of the interrogating X-ray beam, generating the position
and intensity data of the non-circular conic sections such that the
corresponding mathematical equations of the conic sections could be
generated, identifying at least one of a quasi-monochromatic or
monochromatic diffraction pattern from the non-circular conic
sections, determining the position of the at least one probed voxel
by determining at least one apex of at least one diffraction cone
corresponding to at least one of the non-circular conic sections of
diffracted X rays, and determining at least two Bragg diffraction
angles from the at least one of the quasi-monochromatic or
monochromatic diffraction pattern.
[0008] These and other advantages and features will be more readily
understood from the following detailed description of preferred
embodiments of the invention that is provided in connection with
the accompanying drawings.
DRAWINGS
[0009] FIG. 1 is a schematic representation of X-ray diffraction
from planes of atoms or molecules in a crystal.
[0010] FIG. 2 is a schematic representation of an angular
dispersive quasi-monochromatic or monochromatic X-ray
interrogation-detection system in accordance with an embodiment of
the invention.
[0011] FIG. 3 is a schematic representation of conic sections
formed in an angular dispersive quasi-monochromatic or
monochromatic X-ray interrogation-detection technique in accordance
with an embodiment of the invention.
[0012] FIG. 4 is a flow chart representation of an angular
dispersive quasi-monochromatic or monochromatic X-ray
interrogation-detection method in accordance with an embodiment of
the invention.
[0013] FIG. 5 is an energy spectrum of an X-ray beam that has been
filtered with an Er K-edge filter in accordance with an embodiment
of the invention.
[0014] FIG. 6 is an energy spectrum of an X-ray beam that has been
filtered with a Yb K-edge filter in accordance with an embodiment
of the invention.
[0015] FIG. 7 is the difference between the Yb- and Er-filtered
spectra in accordance with an embodiment of the invention.
[0016] FIG. 8 is an energy spectrum of an unfiltered X-ray beam in
accordance with an embodiment of the invention.
[0017] FIG. 9 is a schematic representation of an angular
dispersive quasi-monochromatic or monochromatic X-ray
interrogation-detection baggage screening system in accordance with
an embodiment of the invention.
DETAILED DESCRIPTION
[0018] Embodiments of this invention disclose systems and methods
for angular dispersive, quasi-monochromatic or monochromatic X-ray
interrogation-detection.
[0019] In the following specification and the claims that follow,
the singular forms "a", "an", and "the" include plural referents
unless the context clearly dictates otherwise.
[0020] As used herein, the term "quasi-monochromatic or
monochromatic X-ray interrogation-detection technique" refers to
the combined configuration of an X-ray beam (monochromatic,
quasi-monochromatic, or polychromatic) and a two-dimensional (2D)
X-ray area detector (narrow or broad energy range) such that
monochromatic or quasi-monochromatic XRD patterns are the strongest
patterns detected. For example, the use of a monochromatic or
quasi-monochromatic X-ray beam with a narrow or broad wavelength
range detector, or the use of a polychromatic X-ray source with a
narrow wavelength range detector will result in the strongest
detected XRD patterns being quasi-monochromatic or monochromatic
ones. Alternatively, the combination of two different restricted
energy X-ray beams, a 2D area detector with either a narrow or
broad energy sensitivity, and post-processing of the detected XRD
patterns can result in the generation of a quasi-monochromatic XRD
pattern. For example, two different K-edge filters could be used to
create the two different restricted energy X-ray beams. In more
detail, to create a quasi-monochromatic XRD spectrum with 60
keV.+-.0.5 keV X rays, one pattern could be created with an Er
K-edge filter in the interrogating X-ray beam, while the other
pattern could be created with a Yb filter in the interrogating
X-ray beam. With appropriate filter thicknesses, when the two XRD
patterns are subtracted one from the other (Yb--Er), all but the
X-ray energies between the two K-edge filters will be suppressed
leaving a quasi-monochromatic XRD pattern created with
predominantly 60 keV X rays. This technique has the distinct
advantage of transmitting up to about 70% of the 60 keV source
photons, a significant improvement over conventional monochromator
crystals.
[0021] FIG. 1 is a schematic representation of X-ray diffraction
from crystal planes. X-ray diffraction from crystal planes is well
known in the art and the diffraction characteristics are governed
by Bragg's law, given in equation (1) for first order
diffraction.
.lamda.=2d sin .theta..sub.B (1)
where .lamda. is the wavelength of the incident X rays,
.theta..sub.B is the Bragg angle at which the diffracted intensity
is a maximum, and "d" is the spacing between planes of atoms or
molecules in the material. Rewriting equation (1), d is given
by
d=.lamda./(2 sin .theta..sub.B) (2)
d=hc/(2E sin .theta..sub.B) (3)
where E is the energy of the X rays, c is the velocity of light in
vacuum, and h is Planck's constant. Therefore, measuring
.theta..sub.B and knowing the energy of the interrogating X rays
will enable determination of the spacing between planes of atoms or
molecules in a material and the material's crystal structure,
uniquely identifying the material.
[0022] When X rays 10 are incident on planes of atoms or molecules
12 in the material, the X rays are diffracted by the various planes
as shown in FIG. 1. The diffracted X rays 13 interfere coherently
if the path difference 14 ("x"+"x"=2d sin .theta..sub.B) between
adjacent planes is an integral multiple of the wavelength of the
interrogating X-rays. The spacing "d" 15 between the planes of
atoms or molecules 12 can be determined by measuring the
diffraction angle 16 (.theta..sub.B) if the energy of the
interrogating X rays is accurately known.
[0023] FIG. 2 is a schematic representation of an angular
dispersive, quasi-monochromatic or monochromatic, X-ray
interrogation-detection system 18 in accordance with an embodiment
of the invention. In the illustrated embodiment, a collimated X-ray
beam 20 is incident on sample volumes 22 and 24 generating
diffraction cones, of which 26 (cone of solid lines pointed to by
element number 26) and 28 (cone of dashed lines pointed to by
element number 28) are representative, respectively. A detector 30
is placed along a plane such that diffracted conic sections other
than circles are captured on the detector 30. A conic section is
formed from the intersection of a plane and a cone.
[0024] The diffraction pattern in this scenario consists of
parabolas, hyperbolas, and/or ellipses resulting from the
intersection of the diffracted X-ray cones with the obliquely
angled plane of the detector. The diffraction cones can be
characterized by the equation
x.sup.2+z.sup.2-a.sup.2(y-y.sub.0).sup.2=0, (5)
where y.sub.0 is the position of the diffracting volume, a=tan
2.theta..sub.B, and .theta..sub.B is the Bragg diffraction
angle.
[0025] Intersecting the diffraction cones with a plane made by, for
example, a flat panel digital X-ray detector or a flat piece of
X-ray film will result in various conic sections being recorded by
the detector or on the film, depending on the angle between the
detection plane and the cone axis. If the detection plane is
perpendicular to the axis of the diffracted cones, the resulting
conic sections will be circles. It is possible to have two
spatially separated sample volumes of different composition that
will produce coincident x-ray diffraction circles. In other words,
the diffracted circles are not unique to the diffracting volume in
this scenario. This can be seen from the following equations. Let
the equation that represents the plane of the detector
perpendicular to the interrogating X-ray beam be:
y=H, (6)
where H is the distance between the X-ray source focal point and
the detector plane. This distance can be measured accurately.
Substituting Equation (6) into Equation (5) results in the equation
of the diffraction circles on the detection plane:
x.sup.2+z.sup.2=a.sup.2(H-y.sub.0) (7)
The quantity a(H-y.sub.0) is the radius of the diffraction circles.
To see that it is possible to have two spatially separated
diffracting volumes produce diffraction circles of the same radius,
consider the case of a diffracting volume closer to the source
(smaller y.sub.0) than a second diffracting volume. The first
diffracting volume's larger sample to detector distance will result
in a larger (H-y.sub.0) than for the second diffracting volume. If
the atomic planes in the first diffracting volume are selected
appropriately, a=tan 2.theta..sub.B may be smaller by just enough
to compensate precisely for the larger (H-y.sub.0), resulting in a
diffraction circle radius that is exactly the same as a diffraction
circle radius for the second diffracting volume. Thus, diffraction
circles cannot be used to uniquely identify the voxel from which
the X rays diffracted.
[0026] When the detection plane is not perpendicular to the axis of
the diffracted cones, the resulting conic sections are unique; no
other sample volume or composition can produce the same
(non-circular) conic section. For example, let the detection plane
be parallel to the X-axis and oriented such that the equation of
the detection plane is given by:
y=H-|m|z, (8)
where the slope of the plane with respect to the Z-axis is -|m| and
H is the intersection of the plane with the Y-axis. This plane
intercepts cones given by Equation (5) in non-circular conic
sections with equations:
x.sup.2+(1+a.sup.2m.sup.2)z.sup.2+2a.sup.2(H-y.sub.0)|m|z-a.sup.2(H-y.su-
b.0).sup.2=0 (9)
According to equation (9), it is not possible for "a" which is
related to the Bragg angle and hence the composition of the
material and "y.sub.0", which is the position of the diffracting
volume, to compensate for each other exactly, keeping the shape of
the conic section the same, because the z.sup.2 and z terms do not
contain combinations of a and (H-y.sub.0) only, as in the equation
of the circle. Thus, extracting the appropriate parameters from the
non-circular conic sections uniquely identifies both the position
(y.sub.0) of the small sample volume off which the X rays
diffracted and the composition (related to .theta..sub.B) of that
small sample volume. FIG. 3 schematically illustrates conic
sections 32 formed using this angular-dispersive,
quasi-monochromatic or monochromatic, X-ray interrogation-detection
technique in accordance with an embodiment of the invention. The
solid lines 34 represent the diffracted conic sections from a first
material and the dashed lines 36 represent those from a second
material. Depending on the location of the sample volumes and their
respective compositions, the conic sections 34 may be interleaved
with, but still distinguishable from, conic sections 36.
Alternatively, the conic sections 34 may even intersect the conic
sections 36.
[0027] Working from the equations for the conic sections and the
detector plane, the equation of the diffraction cones that gave
rise to the detected non-circular conic sections are uniquely
determined, i.e. the cone angles and spatial positions of each cone
vertex are uniquely determined from each non-circular conic
section. The spatial positions of the cone vertices determine the
diffracting sub-volume (hereinafter referred to as a volume pixel
or a voxel). Cones having a common apex spatially identify the
position of the voxel from which the X rays diffracted, producing
the detected diffraction pattern. Thus, no detector collimator is
required to establish the voxel's spatial position, as is required
in conventional energy-dispersive XRD.
[0028] Standard intensity versus two-theta plots for comparison to
material databases can be generated from the conic sections, by
selecting only those conic sections that correspond to cones with a
common apex, integrating the photon intensities contained in each
of these conic sections, determining relative integrated
intensities, and graphing those relative intensities as a function
of half the corresponding cone angle, i.e. 2.theta..sub.B, for that
conic section. Comparing such plots to standard XRD material
databases enables fast determination of voxel composition. In
summary, with the appropriate oblique angling of a flat area
detector and simple three-dimensional geometrical considerations,
both the spatial position of an object and its composition can be
rapidly determined. Objects dispersed along a wide range of
distances can be distinguished easily from each other and their
crystal structure and composition can be identified from the cone
angles and interrogating X-ray energy. Integrating the X-ray
intensities over the whole conic section increases the signal to
noise ratio for a given Bragg diffraction angle, which increases
the sensitivity and specificity of object identification.
Additionally, if the signal to noise ratio increases sufficiently,
the baggage scan rate can be increased without loss in sensitivity
and specificity.
[0029] Some embodiments of the invention described here address
issues of confounding diffraction patterns in conventional
monochromatic X-ray diffraction (XRD) when unknown objects are
distributed randomly in space along small (hundreds of micrometers)
or large distances (more than 1 centimeter) of the X-ray beam.
Embodiments of the invention alter the conventional monochromatic
XRD detector placement (perpendicular to the undiffracted X-ray
beam for an area detector) to positions that make angles other than
90.degree. with the undiffracted X-ray beam. In one embodiment, the
angle is chosen such that the smallest and largest potential
diffraction cones intersect the detector, ensuring that all
important lattice planes are imaged. With this angled detector, the
diffraction patterns are non-circular conic sections: hyperbolas,
parabolas, and ellipses.
[0030] FIG. 4 is a flow chart representation of an
angular-dispersive, quasi-monochromatic or monochromatic, X-ray
interrogation-detection method 37 in accordance with an embodiment
of the invention. The method includes generating a collimated beam
of X-rays 38.
[0031] In one example, a collimated beam may be formed by using a
physical collimator, which, for example, may create the collimated
beam by blocking divergent X rays from reaching the object under
interrogation. Alternatively, an X-ray optic may be used to
collimate the beam, wherein divergent X rays may be redirected to
form a collimated beam.
[0032] The collimated X-ray beam is used to interrogate at least
one voxel in a sample in step 39. The interrogation of the at least
one voxel results in the generation of at least one cone of X-rays.
In step 40, the at least one cone of diffracted X-rays from the at
least one voxel is detected by a 2D area detector placed at an
oblique angle to the incident X-ray beam direction. Many signal
processing techniques can be used to analyze the detector signals
and determine the equations of the non-circular conic sections
therein. For example, a collection of proposed conic sections or
paths may be generated and the measured detector signals correlated
along the length of the proposed sections. By varying the
parameters of the proposed non-circular conic sections and
considering maxima in the correlated signals, a best-fit equation
to the non-circular conic sections can be made. From the detected
and recorded conic sections, the apices of the conic sections are
determined in step 41. The coincident conic section apices are used
to locate the spatial position of the probed sample volume. In step
42, from the detected conic sections, the Bragg diffraction angles
are also determined. Once at least two Bragg diffraction angles are
determined, the characteristic spacings between planes of atoms or
molecules in the voxel can also be determined as discussed above,
thus identifying the composition of the material in the at least
one probed voxel.
[0033] In one embodiment, the detector is configured to receive and
detect non-circular conic sections of diffracted X-rays originating
from the probe volume. Position and intensity data of the detected
conic sections are generated from the detector signals. The
combination of interrogating X-ray energies and/or energy
sensitivity of the detector may be used to transform the detected
X-ray diffraction pattern into a quasi-monochromatic or
monochromatic diffraction pattern. The position and intensity data
of the conic sections are characterized and analyzed to determine
the position of the probed volume and at least two Bragg
diffraction angles.
[0034] In one example, the energy distribution of the interrogating
X-ray beam used in some embodiments of the invention may be about a
few eV wide (called monochromatic). In one alternate example, the
energy distribution in the interrogating X-ray beam may be about
one to about a few keV wide (called quasi-monochromatic). In one
embodiment, the interrogating X-ray beam may be polychromatic and
may have emissions in an energy range from about a few keV to about
200 keV or higher. In one example, a combination of filters may be
used to select a characteristic wavelength, such as
W--K.sub..alpha., from a polychromatic spectrum generated from a
tungsten (W) target. Conventionally, this type of a narrow energy
distribution X-ray beam is achieved by diffracting the
polychromatic X-ray output of a laboratory source off a strained
monochromator crystal. Such a monochromator usually reduces
significantly the interrogating beam intensity, typically between
one and several orders of magnitude less than the original source
intensity. With the filter technique, the polychromatic output of a
laboratory X-ray source is passed through a K-edge filter e.g., Er
with an absorption edge just below the desired characteristic X-ray
energy, e.g., 60 keV. This transmits X-ray energies below and above
the absorption edge, as shown in FIG. 5. Next this process is
repeated with a different K-edge filter, e.g., a Yb filter having
an absorption edge at a slightly higher energy than the desired
energy (60 keV, in our example), shown in FIG. 6. If the filter
thicknesses are chosen correctly, all but the X-ray energies
between the two K-edge filters will be suppressed. Thus, when the
two diffraction patterns recorded with the two different K-edge
filters are subtracted from each other (Yb filtered pattern-Er
filtered pattern), the resulting XRD pattern will be due to X-rays
within the narrow range between the K-edges of the two filters, as
shown in FIG. 7. The advantage of this technique is that the
intensity of the X rays in this narrow range may be up to about 70%
that of the unfiltered source spectrum, shown in FIG. 8, and at
least an order of magnitude more intense than that produced with
conventional monochromator crystals.
[0035] In another example, the spectral shaping ability of total
internal reflection (TIR) X-ray optics, such as polycapillary
optics or multilayer thin film TIR optics described in co-pending
U.S. patent application Ser. No. 11/619,009, can be used to
generate a collimated, quasi-monochromatic beam from a
polychromatic beam.
[0036] In another example, diffractive optics, such as curved
multilayer thin film diffractive optics or doubly curved crystal
optics or any number of other diffracting crystal optics, may be
used to generate a collimated, monochromatic or
quasi-monochromatic, interrogating, X-ray beam from a polychromatic
X-ray source.
[0037] Embodiments of the invention include systems with 2D area
detectors. Detector sensitivity to incident X-ray wavelengths may
be broad, such as standard energy-integrating 2D area detectors
(e.g. flat panel, multiwire, CCD, film), or narrow, such as
energy-sensitive CdTe or CZT-type 2D, multiple pixel, area
detectors described, for example, in US Patent publication
US20060071174. For example, an X-ray detector may have sensitivity
to a broad range of energies, from about a few keV to 200 keV or
higher, in which case the interrogating X-ray beam should contain
limited X-ray energies or the filter subtraction method described
earlier should be employed in order to produce a
quasi-monochromatic or monochromatic XRD pattern. In one alternate
example, the detector may be sensitive to a narrower energy range,
such as 60 keV plus or minus 2 keV, in which case a broad
polychromatic X-ray beam could be used to interrogate the sample
and create a quasi-monochromatic XRD pattern.
[0038] Although the following example application of an angular
dispersive quasi-monochromatic or monochromatic X-ray
interrogation-detection technique is described in detail in the
context of a baggage screening system, embodiments of the invention
are not limited to baggage screening systems. Other systems using
this technique to inspect or detect all animate or inanimate
objects fall within the scope of this invention. For example, the
system can also used be to screen, for example, passengers, to
probe for any hidden explosive or contraband being transported. In
another example, cargo containers can be screened using this
technique. In one embodiment, the system could be mounted on one or
more robotic arms to move the system to the position of an object
to be inspected. In a non-limiting embodiment, suitable X-ray
energies for inspecting cargo containers may be in the MeV
range.
[0039] Further, the detection and inspection systems described
herein have several applications including inspection or defect
analysis of rail or aircraft or automotive parts. In one example,
such parts may be composite parts. In a non-limiting embodiment,
suitable X-ray energies for such analysis may be in the hundreds of
keV range. The system can also be used to inspect parts such as but
not limited to railroad tracks, where a crystal structure in a
material has been altered due to environmental factors such as
stress or temperature.
[0040] FIG. 9 is an illustrated embodiment of an angular dispersive
quasi-monochromatic or monochromatic X-ray interrogation-detection
baggage screening system 44. The screening system 44 includes an
X-ray source 48 and detector 50 integrated or mounted onto a frame
or gantry 46. In the illustrated embodiment, a collimator 52 is
used to collimate the X-ray beam output of the source 48. In a
non-limiting example, the beam width may be about 2 mm.
Non-limiting examples of X-rays sources include Brehmsstrahlung
radiation sources.
[0041] The source and detector are positioned such that diffracted
X-rays from an interrogated volume are detected at an angle other
than 90 degrees from the collimated X-ray beam 58. In a
non-limiting example, one or more flat-panel digital X-ray
detectors may be used. In one example, the gantry 46 may be
circular as illustrated in FIG. 9. In some embodiments, the
circular frame may be rotary, allowing for the rotation of the
X-ray source and one or more detectors in combination or
independently of each other. In another non-limiting example, the
frame may be rectangular with multiple sources and one or more
detectors distributed around the stationary frame.
[0042] A conveyer system 54 moves a sample to be probed such as a
piece of baggage into an interrogation region located within the
gantry 46. The collimated X-ray beam 58 interrogates baggage voxels
60 and 62, which generate representative diffraction cones 61 and
63, respectively, with the cone apices located at the baggage
voxels 60 and 62. Detector 50, placed at an angle other than 90
degrees with respect to the direction of the interrogating X-ray
beam, detects the non-circular conic sections produced by the
intersection of the diffraction cones with the plane of the
detector. Thus, the detected conic sections are hyperbolas,
parabolas, and ellipses.
[0043] Data from the detector 50 is acquired by an acquisition and
analysis system 64, which curve fits the detected conic sections
and determines the voxel from which the diffracted X rays
originated. Once the voxel position is determined and combined with
the curve-fitted data, the spacings between the planes of atoms or
molecules in the voxel and the voxel's crystal structure can be
found, determining the voxel's composition.
[0044] The equations of the diffraction cones can be uniquely
determined from the non-circular conic sections recorded on the
angled detector. The equations of the diffraction cones are used to
determine the spatial position of the diffracting volume in the
baggage. Diffraction cones with common apices determine which conic
sections in the pattern correspond to a single diffracting object
and identify the position of the voxel that diffracted the X-rays
into those cones. From the equations of the conic sections
corresponding to a single diffracting voxel, the Bragg diffraction
angles (one quarter the cone angle) are determined, which in turn
determine the material crystal structure and atomic or molecular
plane spacings. The structure and plane spacings then determine the
voxel's composition according to standard crystallography theory.
Alternatively, once the Bragg diffraction angles have been
determined, the standard intensity versus two-theta plots can be
generated and compared to XRD material databases to identify
material composition. For example, explosive and contraband
material in a piece of baggage can be identified using embodiments
of the systems and methods described herein.
[0045] In one embodiment, the baggage screening system described
herein may be used in combination with a computed tomography (CT)
system. The CT system can be used as a primary scanning device to
identify regions of interest from a bigger volume. The angular
dispersive quasi-monochromatic or monochromatic X-ray
interrogation-detection part of the baggage screening system can
then be used in accordance with embodiments of the invention to
scan the regions of interest identified with CT and determine the
material compositions in those regions. In an alternate embodiment,
the baggage screening system described herein may be used in
combination with an X-ray radiography system, which identifies
regions of interest in the bigger volume.
[0046] Embodiments of the invention are expected to enable reduced
system and maintenance costs. On the system side,
energy-integrating 2D area detectors are typically less expensive
than the energy-sensitive, single pixel, line, or array of line
detectors currently used in XRD baggage scanning and, by
eliminating the detector collimator required by EDXRD-explosive
detection systems, the system design is simplified. Eliminating the
need for cryogenic detector cooling allows for simpler and more
flexible system designs, while reducing system size, as well as
system and maintenance costs. Additionally, the X-ray detection
area of an energy-integrating 2D area detector can be so much
larger than that of the energy-sensitive detector that, after
integrating the intensity along a path representing each conic
section, significantly better signal to noise in the detected
signal can be achieved in less time than with the energy-sensitive
detection technique. In baggage scanning applications, for example,
the improved signal to noise in detected signals would enable
higher baggage scanning rates with improved sensitivity and
specificity.
[0047] While the invention has been described in detail in
connection with only a limited number of embodiments, it should be
readily understood that the invention is not limited to such
disclosed embodiments. Rather, the invention can be modified to
incorporate any number of variations, alterations, substitutions or
equivalent arrangements not heretofore described, but which are
commensurate with the spirit and scope of the invention.
Additionally, while various embodiments of the invention have been
described, it is to be understood that aspects of the invention may
include only some of the described embodiments. Accordingly, the
invention is not to be seen as limited by the foregoing
description, but is only limited by the scope of the appended
claims. For example, although the method has been described in the
context of two diffracting volumes being probed by the collimated
X-ray beam, the system and method are applicable to one or more
diffracting volumes interrogated by the collimated X-ray beam.
* * * * *