Note: Descriptions are shown in the official language in which they were submitted.
CA 02394360 2002-06-14
WO 01/44792 PCT/GB00/04361
~1~
APPARATUS FOR FAST DETECTION OF X-RAYS
This invention relates to the field of X-ray inspection systems and more
particularly
those that use X-ray diffraction to analyse an object under inspection.
X-ray diffraction has long been used as an aid to structural analysis. The
technique
derives from a well-known property of crystalline materials: that they
diffract incident
X-rays in accordance with the Bragg equation:
n7~ = 2d sin6
where 28 is the angle, measured relative to an axis through an X-ray source
and
scattering centre within the crystalline material, through which incident X-
rays of
wavelength ~, are coherently scattered; d is the crystal lattice spacing (d-
spacing) and
n is an integer. It is also known from, for example, Harding and Kosanetzky J.
Opt.
Soc. Am. A 4(5) 933 - 944, 1987, that information can also be extracted from X-
ray
diffraction patterns derived from non-crystalline or poorly-ordered materials.
A crystal lattice will possess a large characteristic set of d-spacings. The
range of d-
spacings present in a material can be extracted from measurements taken while
either ~, or 8 is varied. The position of each spot (or peak) in a diffraction
pattern,
formed when the diffracted beam hits some form of X-ray detector, arises from
a
characteristic set of values {d, 7~, 8}. The spot intensities contain
information
regarding the molecular content of the material. Information about a
diffracting
material is commonly derived from the position and intensity of peaks in the
diffraction
pattern by either of two methods: detecting the range of wavelengths
diffracted
through a constant scattering angle (energy dispersion) or looking at
monochromatic
X-rays scattered through a range of angles (angular dispersion).
Essential to prior art diffraction systems employing angular dispersion is the
provision
of a monochromatic incident X-ray beam. The terms monochromatic and
polychromatic are used herein to refer to narrow b7~ and broad 4~, finite
spectral
ranges respectively, 8~, « ~~,. The midpoint ~,o of the narrow spectral range
S7~ is
taken to be the wavelength of monochromatic X-rays.
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In a conventional X-ray source, polychromatic X-rays are generated by
bombardment
of a target anode material such as copper or tungsten with high-energy
electrons.
The X-ray spectrum so-produced comprises a continuous spectrum ("continuum")
superimposed with peaks at characteristic (of the anode material) energies.
The
continuum emission in a conventional X-ray source tends to dominate the
emission.
Monochromation is achieved by filtering out the spectral region around a peak.
Quasi-monochromation refers to any source that will provide a narrow spectral
output
in comparison with a conventional source. Whereas in a conventional source the
continuum emission dominates the line emissions (peaks), in a quasi-
monochromatic
source the continuum will be background and the line emission will dominate.
An
example of a quasi-monochromatic source would be a Fluorescence X-ray source
in
which a high energy X-ray source is used to excite X-ray fluorescence in a
material.
Although a quasi-monochromatic source produces a much narrower spectral range
than a conventional polychromatic source it is not truly monochromatic and so
full
monochromation will still be achieved by filtering out the spectral region
around a
peak.
To achieve a degree of monochromation suitable for applications in which the X-
ray
scattering power of a substance under investigation is weak, it is necessary
to avoid
contamination of the diffraction pattern by diffracted X-rays from other
spectral
regions. Thus, while for some applications, a single filter is sufficient for
the peak
wavelength to dominate in the diffraction pattern, more discriminating
applications
require a balanced filter technique to be used.
The balanced filter technique involves taking two diffraction images of the
same
spectral region through two high-pass filters whose cut-off edges are either
side of the
desired spectral peak. One image is subtracted from the other, and the result
is a
filtered monochromatic (occupying the spectral region between the two filters'
absorption edges) diffraction pattern. There are numerous disadvantages of
this
technique. In practice any filter will inevitably attenuate the beam to some
extent
within the spectral range of interest; this is compounded by the need to
provide two
filters. It is very difficult to arrange for close matching of the attenuation
properties of
the filters, which is necessary to ensure that a narrow energy band is
sampled. Two
25-~~-2002 CA 02394360 2002-06-14 GB0004301
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_3'
images are required of the same scattering target (one per filter) with the
result that
either the object under investigation must be stationary during data
acquisition to
allow successive images to be taken or two detector systems are required.
Furthermore the subtraction of the two images results in data of poor
statistical
quaiit~r.
There is therefore a perceived need to provide an alternative technique to
improve
the extraction of structural infomyation using angular dispersive X-ray
diffraction from
weakly diffracting or otherwise noisy materials.
(t is an abject of this invention to provide apparatus suitable for structural
investigation
by angular dispersive X ray ditfradivn without the intensity loss inherent to
the frtter
techniques of the prior art
Accordingly this invention provides an X-ray inspection system comprising an X-
ray
source, a target material ~ be investigated and a detection system arranged
such
that a quasi-monochromatic yr polychromatic X ray beam generated by the source
is
scattered by the target material across an angular range rba - ~d wherein the
detection
. system comprises an array of semiconduc~ing detector elements subtending the
angular range ~o - ~a respectively connected to a corresponding array of
readout
channels characterised in that the detector elements are fabricated from a
semiconducting material with band gap transport responsive to irradiation by X-
rays
whereby each detector element generates art electrical response whose
magnitude is
dependent on inadent X-ray energy and each rnsdout channel comprises front-end
electronics ananged to transform the semiconductor electrical response to an
electrical pulse with ~ a parameter representative of response magnitude;
discriminating electronics arranged to output a digital signal if the pulse
parameter lies
between first and second pre-selected discriminator values and a counter
arranged to
count the number of digital signals output from the discriminating
electronics.
This invention provides the advantage that information about the target
material can
be extracted with shorter interrogation time of the target material in
comparison with
the prior art. Scattering is detected across the spectmm of the quasi-
monochromatic
yr polychromatic X-rays within all angles subtended by the detector array.
Each
parameter is separable. The scattering angle is determinable from the position
of the
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2~-0'1-2~17~ CA 02394360 2002-06-14 GB~~~~r317~i
-4-
detector element at which an X-ray beam is intercepted. To provide a rapid
asSesSment Of X-ray energy a relatively simple electronic circuit is used in
which an
analogue detector element response is converted to a digital output - the
vu~ut
having vne of two logic states to indicate merely the presence or absence of X-
ray
incidence within a particular narrow energy range.
In such systems it Is desirable to know both energy (or equivalently
wavelength] and
angular deflection of an X-ray beam in order to deduce the lattice spacings d
characteristic of the material under investigation. Detector arrays have be$n
used in
the prior art to determine scattering angle, but detector elements have
generally been
unsuitable for providing energy resoluflon, certainty within a reasonable
exposure
timescale. This has necessitated the use of X-tey monochromation techniques
prior
to diffraction pattern detection. The monochromation process itself inevitably
leads tv
a reduction in inten$rty of the detected probe beam, leading to a c~sequent
incxease
in target interrogation time. By way of contrast, this invention provides an
inspection
system which is capable of measuring both scattering angle and energy of
difTraded
X-rays. and so reduces the need for beam monoctrromabon prior to detection.
Ail X-
rays scattered within the accepted angular range will be detected by the
semiconducting detector elements. It is only after detecfion that the
eledronic~
provide a fast means of recording an essentially monochromatic diffraction
pattern,
with no significant discarding of detection events within the required energy
band. In
this way two of the characteristic set of values ~d, s(~), 8} Inked by Bragg's
equation
are measurable and the characteristic d spaangs can be calculated in orxter to
extract
inforrnarlion about the target material.
The system may include at least two arrays of readout channels, each
semiconducting detector element being connected to respac~ive readout
channels,
vne from each array, wherein readout channels in the same array have
discriminating
electronics set to substantially the same first and second discxirninator
values, and
which in turn are different from discriminator values appropriate to n3adout
channels
in other arrays.
This embodiment of the inspection system of the invention provides a means by
which further information can be extracted simultaneously with the
monochromatic
diffraction pattern described above. ~ifierent arrays of readout channels are
Set to
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25'1-2002 CA 02394360 2002-06-14 GB0004301
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measure monochromatic diffraction patterns occupying different parts of the
quasi-
monochromatidpolychromatic spectral range. Thus mere- informatron can be
extracted in the same interrogation time, further increasing the advantage in
speed
that this detection system has over the prior art.
The system may also include display means arranged to display for each readout
array an X-ray scattering pattern derived from position of each detector
element in the
array plotted against number. of courts ragistared in respectively connected
readout
channels. This provides the advantage of simplicity, the pattern is displayed
in a
fomtat commonly used for powder diffraction patterns. This faalitates a quick
comparison with known diffraction patterns to assist in identification of an
unknown
substance.
The semiconducbng elements are preferably fabricated from cadmium zinc
telluride,
gallium arsenide, lead iodide or mercury iodide. These high atomic number
semicvnducting materials are currently fabricated into commercially available
arrays
and meet the preferred criteria of detector element materials for use in this
aspect of
the invention. These materials can be used to provide detectors sensitive to
the
fOkeV region of enhancxd emission from a tungsten source. Such materials, with
their high attenuation coefficients and high photoelectric absorption to
Cornpton
scatter raatios, enable the deteccting elements to record practically all
incident photons.
Additionally, they can be used to consbuct detector elements with sufficient
en~rgy
resolution (less that 10°/° at full-width-half maximum) in the
60keV energy region to
be able to record the true energy of all relevant inadent X try photons.
Preferably, the angular range ~, to ~~ is 2° to 8°. This
corresponds to the angular
range of interest in measuring diffraction patterns from powdered materials
using the
80keV tungsten enhanced emission band. Moreover the materials referred to in
the
previous paragraph provide sufficient spatial resolution over this angular
range.
The quasi-rnonotd~romabdpotychromatic X-ray beam is preferably collimated into
a
fan beam in order to illuminate a coplanar two-dimensional array of voxels
within the
target material and the systerrt includes focusing coltimation means arranged
to pass
only X-rays scattered from a single voxel at one depth and height within the
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25'01-2002 CA 02394360 2002-06-14 GB0004301
_6_
illuminated array to the detection system. The system may also indude an array
of
focusing collimation- means - and a respective array of detection 5ystems;--
the - -
col~mation array members being stacked so as to pass simuttan~ously to
respective
detection systems X-rays scattered from respective voxels at different heights
within
the illuminated voxei array. Further, the array of collimation means is
preferably
moveable relative to ttte target material in the direction of unscattered X
rays In order
to enable detection of X-rays scattered from voxels at different depths within
the
target material.
Thes~ features lend themseiv~ to implementation in a scanning X-ray inspection
system which can be used to inspect the entire wlurne ef a bulk target in
reaf~stic
time periods. This reduces the total time taken to complete a thorough
inspection in
comparison with prior art inspection systems. These embodiments are thensfore
particularly applicable to scanning airport baggage in which a high throughput
is
desired within a timescal~ which falls within passenger tolerance and yet with
a very
high prvbabitity of detection of any explosives, drugs or other contraband
material.
The inspection system may indude multipl~ detedion systems symmetrically
oriented
to intercept a conical distribution of diffracted X-rays at symmetrically
equivalent
regions. This provides the advantage of improved acxuracy. The diffraction
patterns
detected by different detection systems can be averaced over the carne
monochromation range to provide more accurate counting statistics and hence
incxease the certainty with which a target material may be identified.
in order that the invention may be more fully understood ernbodimertts thereof
will
now be described with reference to the accompanying drawings in which
Figure 1 illuschematically an embodiment of the X-ray inspection system of
the invention.
Figures Za and 26 are sch~matic illustrations of the readout electronics of
the
inspection system of Figure f .
Figure 3 is a schematic flfustration of the pulse processing performed by the
readout
electronics of Figure 2.
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Fgure 4 is an illustration of a mufti-channel readout drcuit for use with the
inspection
system of Figure 1.
F'~rre 5 is an illustration of X-ray diffradivn in a prtor art system for
baggage
scanning.
Figure 9 illustrates an X-ray inspection system indicated generally by 10. The
system
comprises a collimator (not shown} arranged to pass X-rays 12 scattered from a
volume element (voxel) 14 of a substance under investigation towards an
arrangement of four Gnear d~tectvr arrays 18, 18, 20, 22. Each array, for
example 1 s,
comprises a series of cadmium zinc telluride (CZ~y detector elements 16a,
16b,16c,
........ . tack detector element lt3a, lfib, 16c, ....... is connected to a
respective
readout circuit, respective arrays 24, 26 of readout drcults for two detector
arrays 16,
2a being shown in the Figure. Each array of readout rdrarits 24, 2fi processes
signals
received from one of the detector arrays 16, 18, 20, 22 and outputs a
diffract(on
pattern 28, 30 representative of the intensit5r of X-ray radiation 12 incident
at the
positions of each detector element 16a, 16b, 16c, :...... in the detector
artery 16, 18,
20, 22.
Ffgrrre 2a illustrates a sings detection channel 44 of the X-ray system
illustrated in
Figure 1. Each such channel comprises one CZT detector element 16a connected
to
an associated readout drcuit 24a. The detector element 16a develops an
ei~trical
response when struck by an incident X-ray photon 42a. The magnitude of this
response is proportional to the energy of the striking phcton(s) and it is
this which is
processed by the readout circuit 24a. The drcuit 24a comprises a preamplifier
44, a
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_g_
shaping amplifier 46, a lower-level discriminator 48, an upper-level
discriminator 50
and a counter / buffer 52.
Figure 2b illustrates an array 60 of the detection channels 40 illustrated in
Figure 2a.
The array 60 comprises an array 24 of the readout circuits respectively
connected to
an array 16 of CZT detector elements.
Figure 3 illustrates the principle of operation of the readout circuit in
producing a pulse
signal from the detector response. The Figure shows an array 16 of CZT
detector
elements 16a-h. Each detector element 16a-h develops an electrical response to
an
incident X-ray photon 42a-h, the magnitude of this response being proportional
to the
energy of the striking photon(s). This response is amplified and shaped by
readout
array 24 pre- 44 and shaping- 46 amplifiers to produce an electrical pulse 72a-
h. The
discriminators 48, 50 within the readout array 24 are arranged to produce a
digital
output response (or count) 74a, c, e, g, h if the magnitude of the electrical
pulse 72a-h
falls between lower s, and higher s~ discriminator levels. The counter /
buffers 52 are
arranged to measure the number of counts in each channel 40 over a given
period of
time.
With reference to Figure 1, the detection of an X-ray diffraction pattern in
accordance
with this invention will now be explained. The target voxel 14 is an elemental
volume
that is to be investigated by X-ray diffraction. For the moment, it is assumed
that the
voxel 14 and its associated diffraction pattern 12 are isolated from
neighbouring
voxels and their associated diffraction patterns i.e. either the voxel 14
comprises the
entire object under investigation or some arrangement is used whereby such
isolation
can be effectively achieved. An arrangement of apparatus known to be able to
effectively isolate a voxel in this way is described in a PCT patent
application,
publication number WO 96/24863. However, this aspect is not central to the
invention, although the two systems can be very advantageously combined.
Accordingly, a description of how isolation is achieved will be given later.
For the
present however, X-rays diffracted from the voxel 14 are uncontaminated by
neighbouring voxel diffraction patterns.
A collimated beam (not shown) of polychromatic X-rays is incident on the voxel
14.
(Note: the skilled man will appreciate that a quasi-monochromatic beam can
also be
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used). The voxel 14 comprises a number of crystallites with a range of d-
spacings
which diffract X-rays in accordance with the Bragg equation. In a powdered
material,
the crystallites are randomly oriented and each d-spacing is present in a
number of
orientations. For a given d-spacing and incident wavelength ~,, the diffracted
beams
therefore lie along the surface of a cone with semi-angle 8. With a
polychromatic
incident beam, the diffracted X-rays 12 form a continuous series of cones with
a
range of semi-angles 0 to 6max. However, for each value of 8, there is a range
of
combinations of d and 7~ which satisfy the Bragg equation and hence possible
wavelengths of X-rays forming the diffracted beams 12. The set 12 of
diffracted
beams can be envisaged as forming a spatial superposition of a number of
monochromatic (covering wavelength region ~~,) angular-dispersive diffraction
patterns.
The diffracted beams 12 are incident on four linear arrays 16, 18, 20, 22 of
CZT
detector elements. Since the diffraction pattern 12 is conically symmetric, a
single
linear array suffices for its detection. However four detector arrays 16, 18,
20, 22,
symmetrically oriented, provide better counting statistics if detector results
are
averaged. The operation of one such array 16 will be explained, it being
understood
that the remaining three 18, 20, 22 operate in the same way. This array 16
comprises
detector elements 16a, 16b, 16c, .... arranged to extend over an angular range
0 to
~d. The range ~d is selected to encompass a higher intensity region of the
range of
the diffraction beam 0 to 6max. For powdered compositions of interest at
airports, this
range will correspond to small angle scattering. Each detector 16a, 16b, 16c,
.... in
the array therefore intercepts one angular region 80 of the diffraction
pattern.
Referring now to Figures 2a and 3, the function of the readout channels 24 in
providing energy discrimination (equivalently resolving the superimposed
monochromatic diffraction patterns) will be described. The CZT detector
element 16a
develops a charge pulse in response to X-ray 42a illumination. The magnitude
of this
response is proportional to the energy of X-rays incident on the element 16a
during a
response time interval. The preamplifier 44 and shaping amplifier 46 transfer
the CZT
detector response to an electrical pulse 72a whose height is representative of
the
energy of the incident X-ray 42a. If this pulse height then falls between a
lower
threshold E, set by the lower-level discriminator 48 and an upper threshold Ez
set by
the upper-level discriminator 50 a signal 74a is sent to the counter 52 which
is
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registered as a "hit". If the peak pulse height is outside the range 8s = EZ -
E, set by
the discriminators 48, 50 then no hit is registered. The counter 50 then
buffers the
number of hits registered over an observation period and these are displayed
as an
intensity reading at the position of the detector element 16a in the
diffraction pattern
28.
The diffraction patterns 28, 30 displayed by the apparatus 10 and shown in
Figure 1
therefore comprise plots of scattered intensity (number of counts) against
scattering
angle (detector element position) of monochromatic X-rays (energy, or
equivalently
wavelength, within the range s, - s2). These patterns are equivalent to those
detected
in prior art angular dispersion diffraction systems, but without the
disadvantages
inherent to the use of filters in obtaining monochromaticity. Any apparatus
based on
the prior art technique of using filters to achieve monochromation inherently
introduces a loss in intensity from either the incident or diffracted beam
(depending on
filter positioning), with the consequent reduction in signal to noise ratio.
In contrast,
although in this embodiment the present invention also discards potential
information-
carrying X-rays from outside the monochromation range, it does not
consequently
reduce the intensity of X-rays in the detected monochromatic spectral region.
A further advantage of this invention over the prior art is achieved if a
multi-channel
readout circuit, as illustrated in Figure 4, is connected to each detector
element 16a.
In this Figure, the single detector element 16a is shown connected to first
24a,
second 80a and third 82a readout circuits. The multi-channel circuit comprises
common pre- 44' and shaping- 46' amplifiers and multiple discriminators 48, 50
and
counter / buffers 52. Each readout circuit 24a, 80a, 82a is identical to that
24a
described previously except that the discriminators 48, 50 in each are set to
provide
different threshold levels s,, s2. Thus while the first readout circuit 24a
extracts
information relating to the monochromation range s, - E2, the second 80a
extracts
information from a different monochromation range E3 - s4 and the third 82a
extracts
information from a third range s5 - E6. Counts from the first 24a, second 80a
and third
82a readout circuits are then combined with counts from readout circuits set
to the
same respective thresholds from different detector elements to derive three
diffraction
patterns of the form of 28, each plotting intensity against scattering angle
for a
different part of the X-ray spectrum. Clearly this set up is not limited to
only three
readout circuits. A series of circuits can be provided which readout
information from a
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series of different monochromation ranges. In this way information from other
parts of
the diffraction spectrum can be extracted in segments from the detector
elements
during a single period of voxel irradiation.
The parallel processing capability of the readout circuits enables a number of
monochromatic diffraction patterns to be simultaneously extracted from a
general
polychromatic diffraction pattern. This reduces the information content to a
number of
readily interpretable conventional monochromatic presentations, which can
ultimately
be used to increase the accuracy of extracted structural detail.
This invention is particularly applicable to situations in which data
collection needs to
be completed as quickly as possible. For example baggage scanners at airports
have a high throughput of passenger baggage and it is necessary to have a
trusted
and reliable system with which to detect possibly small amounts of illegal
substances
such as drugs or explosives concealed in larger containers. Similarly this
invention
can be used to scan rapidly foodstuffs such as meat on a conveyor belt in
order to
detect bone, cartilage or other inedible contaminant.
In order to facilitate this rapid scanning of bulk objects it is necessary to
be able to
avoid interference between diffraction patterns generated from neighbouring
scattering centres. Thus each voxel must be independently addressable. As
mentioned above a method of achieving this is described WO 96/24863, which
explicitly cites its applicability to airport baggage scanning. The advantages
of
combining the present invention with that one are thus readily seen.
WO 96/24863 describes how illumination of a large object by an incident fan
beam in
combination with a particular form of collimation of the diffracted beam
enables
enhanced discrimination between scattering from the target voxel and that from
neighbouring material. The advantage provided by that invention, namely a more
rapid three-dimensional scanning of bulk objects, is further achieved by
application of
the present invention.
Consider a three-dimensional array of voxels illuminated by an X-ray fan beam.
In a
depth dimension, defined by stacking in the direction of propagation of the
incident X-
ray beam, conic diffraction patterns, similar to 12, will be generated from
successive
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voxels. A focusing collimator, which reflects the conic symmetry of the
diffraction
pattern, can be used to focus in on one particular depth.
Figure 5 illustrates X-ray diffraction from a plane of voxel elements at one
particular
depth in a bulk three-dimensional object. This illustration is of a plane
parallel to the
voxel plane, displaced in the direction of propagation of the incident X-ray
beam.
Diffracted beams from neighbouring voxels intersect this plane in circular
profiles 90a,
90b, 90c, ....., centred on projected voxel positions in this plane. The
incident X-ray
fan beam is collimated to intersect one line only of voxels at each depth, and
so
defines a linear intersection 92 with the plane of Figure 5. A section 94 of
the
focusing collimator also intersects this plane. The section 94 comprises
horizontal
94a and vertical 94b collimation sheets which respectively provide vertical
and depth
specificity.
At any one time, interference arising from scattering from neighbouring
horizontal, i.e.
perpendicular to the plane of the fan beam, voxels 90a, 90b, 90c is thus
readily
avoided by collimating the beam such that only one "line" of voxels 90e, 90a,
90d is
illuminated by what is effectively an X-ray line 92 formed by the fan beam.
The
voxels 90a, 90b, 90c can then be moved relative to the fan beam such that
diffraction
patterns from neighbouring voxels are separated in time. In order to avoid
interference from voxels stacked in the vertical, i.e. within the plane of the
fan beam,
direction, only a section of each diffraction pattern, of finite height, is
accepted by the
focusing collimator. This is facilitated by the horizontal collimation sheets
94a.
The invention of WO 96/24863 can be very advantageously combined with the
present invention to provide a fast X-ray scanner for detection of explosives
and / or
drugs carried in airport baggage. Each elemental portion of a piece of baggage
acts
as a scattering centre (voxel) when irradiated by X-rays. The object of an
airport
scanner is to scan rapidly the volume of the baggage in order first to detect
whether
or not any prohibited substance is present and secondly, if something is
detected, to
identify what it is and where within the baggage it is stowed. Each elemental
diffraction pattern is therefore analysed for evidence of certain diffraction
peaks,
characteristic of any anticipated prohibited substance. Identification can be
most
rapidly achieved by comparison with a look-up table of diffraction patterns of
known
prohibited materials.
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As described in WO 96/24863, the focusing collimators can be stacked in the
vertical
direction of Figure 5, to provide simultaneous collimation of diffraction
patterns from
all voxels at a particular depth illuminated by the fan beam. One 16 of the
linear
detection arrays of Figure 1 and its associated array of readout circuits 24
is used in
this combined system, also stacked in the same direction. Thus each detector
system (comprising detector array 16 and readout circuit 24) can be used to
detect, at
rapid speed, one, or a number of, the spectral series of diffraction patterns
generated
by each voxel element isolated by the combination of fan-beam illumination 92
and
each collimation system 94 of WO 96/24863.
To scan the entire baggage volume, the fan beam is arranged to illuminate one
dimension (say height). The baggage is commonly moved by conveyor belt to
provide for illumination by the fan beam along its length. The collimation 94
and
detection 16, 24 systems are moved relative to the baggage to scan its depth.
At
each scan point, sufficient information must be collected to identify (in so
far as
whether or not it is contained in the prohibited substances look-up table) the
composition of the scattering voxel. The position of this voxel is identified
from the
mechanics of the conveyor belt / depth scanning system and which detection
system
registers the pattern. The ability to perform this is well known in the prior
art. In order
to be able to reduce the scan time of each voxel to a sufficiently short value
that the
entire baggage throughput of an airport can be scanned with acceptable
passenger
delay, it is essential to have detectors which are capable of rapidly
collecting enough
information for composition identification. This is achieved in the present
invention by
the use of energy-sensitive detectors which obviate the need for
monochromation
filters and their inherent reduction in X-ray intensity present in the
diffraction pattern.
The time needed to collect data from each diffracting element is consequently
reduced.
