Method and arrangement for identifying crystalline and polycrystalline materials

A method and apparatus is provided for identifying crystalline and polycrystalline material in an object placed in an examination region. X-rays having a polychromatic energy distribution are passed through a diaphragm to create a central x-ray beam in a fan plane that is projected into the examination region for irradiating a cross section of the object. The x-rays are diffracted by individual subregions of the object along the cross section in dependence of the presence of crystalline and/or polycrystalline material in the individual subregions. Collimators with collimating windows are arranged beyond the examination region with respect to the diaphragm, each collimating window covering a fixed, predetermined subregion of the examination region and extracting at least one diffracted plane fan beam from the respective individual subregion of the object. Energy spectra of the diffracted x-ray plane fan beams exiting the respective one of the collimating windows are captured with a detector located behind each of the collimating windows for converting the captured energy spectra into signals usable in a data processing arrangement.

CROSS-REFERENCE TO RELATED APPLICATIONS 
This application claims priority of German application No. 195 10 168.5, 
filed in Germany on Mar. 21, 1995, the disclosure of which is incorporated 
herein by reference. 
BACKGROUND OF THE INVENTION 
the invention relates to a method and arrangement for identifying 
crystalline and polycrystalline material in an object, and in particular 
to such a method and arrangement wherein the object is irradiated in an 
examination region with x-rays having a polychromatic energy distribution, 
which x-rays are diffracted along a material volume of the object in 
dependence of the presence of crystalline and polycrystalline material and 
the energy spectra of the diffracted x-rays are captured in detectors and 
converted to usable signals in a data processing arrangement. 
In order to ensure aviation safety, it is necessary to check passenger 
baggage by employing the most modern technical resources. In this context, 
the detection of bombs or plastic explosives contained in baggage is of 
particular importance since the hazard potential increases from year to 
year. Inspection or detection arrangements are required to have a low rate 
of false alarms and a high likelihood of detection while working at high 
throughput rates. At the same time, such systems are required to portray a 
high degree of sturdiness and availability. Analysis methods as they are 
employed in the laboratory can only be applied to a limited extent. In 
order to meet these demands, multi-stage systems, for example, are used. 
It is possible, in principle, to employ the physical effect of x-ray 
diffraction at lattice planes or crystalline and polycrystalline materials 
by an appropriate selection of a beam geometry. X-ray diffractometry has 
been known for many years for identifying and classifying materials, not 
only on surfaces but also in a transmission process to obtain material 
information within spatially expanded examination objects, for example, 
airplane baggage. 
In the simplest case, an object is penetrated by an x-ray having a small 
cross section. If the object is comprised of materials with a crystalline 
or polycrystalline lattice structure, individual quanta are diffracted at 
this structure. In general, the diffracted radiation will disappear 
through destructive interference. But intensifications of these energy 
emissions occur as well. 
This is always the case if an allocated wavelength 1 appears at a specific 
angle .theta. to the crystal plane and a structural interference appears 
at a material-specific spacing d between the crystal planes. The known 
Bragg interference condition summarizes the relationships between 
.lambda., .theta. and the lattice spacing d as follows: 
EQU 2d.times.sin.theta.=n.times..lambda. 
As can be seen from tis relationship, the effect may be applied in 
different ways. If a polycrystalline material is exposed to a 
polychromatic x-ray source, different energy maxima appear at an angle 
.theta. which, for example, is to be considered as being fixed. These 
maxima are characteristic for the lattice spacings in the examined 
material. Studies have shown that, due to their polycrystalline structure, 
explosive substances generate such energy spectra. Thus, this method is 
suitable, in principle, for detecting explosive substances. A sought after 
material is identified by comparing the measured spectrum with a catalog 
of relevant spectra deposited in a data memory. Such methods are 
customary, for example, in x-ray diffractometry IR spectroscopy and gas 
chromatography and they are not explained here in detail. 
European patent publication EP-0 209 952 A2 discloses that products which 
are comprised of different scattering angles and the energies associated 
therewith are combined in groups and analyzed. In the arrangement 
disclosed in this publication, cylinder collimators upstream and 
downstream of the object to be examined result in circular or annular 
images and corresponding detector geometries. For the examination of the 
volume regions along the irradiated x-ray bundle, that is in the depth of 
the object, it is necessary that the object be displaced longitudinally 
with respect to the incident x-ray and/or that the entire arrangement be 
subjected to a lateral relative movement with respect to the object to be 
examined. For this method, a plurality of consecutive measurements is 
required because only a single point of the cross-sectional plane along 
the irradiated x-ray bundle can be examined during each measurement. 
During such point-by-point detection it is necessary that each detector 
captures the scattered radiation in a plurality of angle positions and 
displacement positions, which means that a large time and computational 
expenditure is necessary. 
SUMMARY OF THE INVENTION 
It is an object of the invention to improve upon the known method and 
arrangement described above so that a substantial simplification and a 
more cost-advantageous solution are created which solution allows a 
time-parallel examination of all volume elements of an object along an 
incident x-ray. 
The above and other objects are accomplished according to the invention by 
the provision of a method for identifying crystalline and polycrystalline 
material in an object, comprising: placing the object in an examination 
region; passing x-rays having a polychromatic energy distribution through 
a diaphragm to create a central x-ray beam in a fan plane that is 
projected into the examination region for irradiating a cross section of 
the object, the x-rays being diffracted by individual subregions of the 
object along the cross section in dependence of the presence of at least 
one of crystalline and polycrystalline material in a respective one of the 
individual subregions; arranging collimators with collimating windows 
beyond the examination region with respect to the diaphragm, each 
collimating window covering a fixed, predetermined subregion of the 
examination region and extracting at least one diffracted plane fan beam 
from the respective individual subregion of the object, and capturing 
energy spectra of the diffracted x-ray plane fan beam exiting a respective 
one of the collimating windows with detectors each located behind a 
respective one of the collimating widows for converting the captured 
energy spectra into signals usable in a data processing arrangement. 
According to another aspect of the invention there is provided an 
arrangement for identifying crystalline and polycrystalline material in an 
object, comprising: an x-ray source including a diaphragm for projecting a 
central x-ray beam having a polychromatic energy distribution in a fan 
plane into an examination region containing the object for irradiating a 
cross section of the object, the x-rays being diffracted by individual 
subregions of the object along the cross section in dependence of the 
presence of at least one of crystalline and polycrystalline material in a 
respective one of the individual subregions; collimators arranged beyond 
the examination region relative to the x-ray source, the collimators being 
arranged in at least one row symmetrically around the axis of the central 
x-ray beam in a plane extending perpendicularly to the fan plane of the 
central x-ray beam and including collimating windows extending in parallel 
with respect to one another and respectively at a fixed angle .alpha. with 
respect to the axis of the central x-ray beam, each collimating window 
covering a fixed, predetermined subregion of the examination region and 
extracting at least one diffracted plane fan beam from the respective 
individual subregion of the object; and detectors each arranged at a 
respective one of the collimating windows of the collimators in the plane 
of the respectively collimated fan beam for capturing energy spectra of 
the diffracted x-ray plane fan beam exiting a respective one of the 
collimating windows and converting the energy spectra into signals for 
subsequent use in a data processing arrangement. 
According to the invention the circular collimation devices used in the 
known arrangement are replaced by linear collimating devices and 
substantial improvements are accomplished by conducting a time-parallel 
examination of all volume elements or subregions of the object along the 
incident x-ray beam. A simplification is essentially accomplished in that 
the collimators arranged beyond the examination region extract at least 
one diffracted, fan beam from the respective subregion and each 
collimating window of the collimators covers a fixed, predetermined 
subregion of the cross-sectional plane of the examined object. 
The method and the arrangement of the invention allow a simultaneous 
detection of all of the subregions that are fixedly predetermined in the 
cross-sectional plane of the object so that a complete cross section of 
the object can be inspected successively at short time intervals. 
According to another aspect of the method and arrangement of the 
invention, a high detection accuracy is achieved because the collimation 
and detection unit can be oriented automatically toward the focus of the 
x-ray source and can be adjusted. The collimating and detection unit may 
be arranged in different planes along the incident x-ray, preferably in a 
horizontal and vertical plane, so that a faster detection of the material 
of interest in the examined object is possible by way of multiple 
measurements of an examination region. 
In a further advantageous manner, the method and the arrangement of the 
invention make possible the use of silicon photodiodes as semiconductor 
detectors and thus offer a simplification and a more cost-advantageous 
solution compared to the use of, for example, cooled germanium detectors 
which are normally required for this technique.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 1 and 2, there is shown an x-ray source 1 (x-ray 
generator) which collimates x-ray radiation smaller than 100 KeV, for 
example, by means of a diaphragm 2, to create an incident x-ray fan beam 3 
having a small cross section of less than 1 mm thickness and approximately 
10 mm in height. X-ray fan beam 3 exiting diaphragm 2 has a polychromatic 
energy distribution and penetrates an object 4 in an examination region A 
for identifying crystalline and polycrystalline materials at predetermined 
locations, or subregions, at which diffraction centers 5 are generated 
along the x-ray. The portion of the incident x-ray fan beam 3 transmitted 
straight through object 4 is designated with reference numeral 15. On the 
opposite side of examination region A from x-ray source 1, collimators 8 
are disposed having slot-shaped collimating windows 18 arranged 
symmetrically around the axis of central x-ray 3, 15 in a plane 20 
extending perpendicularly to the fan plane 19 of x-ray fan beam 3, 15. 
Collimators 8 may be arranged in a single row or behind one another in a 
multi-row construction, with the collimating windows 18 extending parallel 
to one another on the respective sides of the axis of central x-ray fan 
beam 3, 15, which windows are respectively arranged at a fixed angle 
.alpha. to the axis of central x-ray fan beam 3, 15. At the respective 
collimating windows 18 of the rear collimators 8, detectors 9 are arranged 
in the respective planes 19 of the fan beams 6 collimated by collimating 
windows 18, which detectors capture the radiation of x-ray fan beams 6 
diffracted by diffraction centers 5. 
Detectors 9 capture the energy spectra of the diffracted radiation and 
forward them to a data processing arrangement 16 in which the data are 
converted into usable signals and can be displayed in an adjoining output 
unit 17. Furthermore, an automatic evaluation of the examination is 
possible by comparing the measured spectra to known spectra of explosive 
substances that are stored in the system. 
Deviating from the representations of FIGS. 1 and 2, collimating windows 18 
may be arranged in a plurality in parallel next to one another on the 
collimators 8 at a respectively constant angle .alpha. within an angular 
region between 2.4.degree. and 3.degree. with respect to the axis of 
central x-ray fan beam 15. 
Collimating windows 18 of collimators 8 have a width of.ltoreq.1 mm, 
preferably in the range between about 0.3.degree.-0.5.degree. mm, and a 
height of approximately 10 mm. Silicon photodetectors 9, for example, are 
arranged at collimating windows 18, with the photodetectors having an end 
face having an area of about 1 mm.sup.2 to 5 mm.sup.2 to receive the 
diffracted, collimated fan beam 6. Since the thickness of the collimated 
beam bundle, preferably at the detector, is in the range between about 0.3 
to 0.5 mm for reasons of attaining a high energy resolution, it becomes 
possible to use a silicon photodiode 9 as a semiconductor detector. In 
x-ray technology, silicon photodiodes are generally used as detectors for 
.alpha. and .beta. radiation. This is done, in particular, because the 
radiation-sensitive charging zone within the semiconductor material has a 
thickness of approximately 0.3 mm. According to its ordinal number, 
silicon is in a position to completely absorb .alpha. and .beta. radiation 
within this material thickness. This is the case to a much lesser extent 
with high energetic .gamma. radiation between 10 and 100 KeV. Therefore, 
the silicon photodiode 9 is employed such that the collimated fan beam 6 
is incident parallel or longitudinal to the semiconductor-sensitive zone 
or plane. Since the thickness of the fan beam and the sensitive 
semiconductor zone are within the same magnitude, a relevant loss of 
information does not here occur. Furthermore, it is advantageous that the 
thin detector zone offers an additional collimation because radiation 
which passes the detection plane laterally must, by its nature, come from 
a solid angle which is not to be considered. 
The known drawbacks of silicon material which are due to its small 
photoabsorption in the energy range of larger than 50 KeV can be mitigated 
by connecting several detectors in parallel. Sine the energy range of 
smaller than 20 KeV is not of interest for obtaining the diffraction 
spectra, the Compton edge does not have any negative effects on the 
evaluation. 
The subregions having diffraction centers 5 formed in the depth of the 
examination object along the incident x-ray fan beam 3 may also be 
examined in the manner described above in further planes, for example, in 
an inclined or perpendicular plane. In such a case, diaphragm 2 at x-ray 
source 1 would be supplemented by a further perpendicular or inclined 
diaphragm and the collimation and detection arrangement would be 
supplemented by further collimators and detectors in these planes in a 
manner which is not shown. 
The collimation and detection arrangement offers the further advantage of a 
compact construction, which makes it possible to adjust the entire system 
in a simple manner. It is obvious that the adjustment has a decisive 
influence on the selectivity and thus on the recognizability of the 
material and the detection probability. A one-time adjustment of such a 
system, for example, during its assembly, is not sufficient. On the 
contrary, it must be assumed that an automatic readjustment must take 
place at regular intervals, for example, prior to every measurement of an 
examination object. 
For these reasons, the above-described arrangement is additionally provided 
with a corresponding adjustment arrangement and therefore the collimator 
and detector arrangement 8, 9 is arranged on a joint support unit 10 and 
comprises a central collimator 11 which can be oriented toward the focus 
of x-ray source 1 via a front bearing point 7 for automatic orientation 
and adjustment of collimators 8 and detectors 9. Referring additionally to 
FIG. 3, central collimator 11 is comprised of individual detectors 12, 13 
which are arranged in a pair opposite of one another at their contact 
surfaces 14 and which are decoupled in terms of their signals, which is 
carried out by a thin light barrier (not shown) between the two detectors. 
The two detectors 12, 13 are hit in their adjusted state by central ray 15 
with equal intensity. This results in output signals of identical 
amplitude which are amplified by amplifiers 23 and 24 and supplied to a 
subtractor 25, which may receive an offset voltage from an offset 
potentiometer 26 for initially adjusting subtractor 25 when the system is 
put into service. If the amplified signals input to subtractor 25 are 
identical in amplitude, the output is "0", i.e., an error signal is not 
forwarded to the servodriver 27, and servomotor 28 remains at rest. 
If the system is maladjusted, however, one of the two detectors 12, 13 is 
irradiated more intensively than the other detector. Corresponding to the 
direction of the maladjustment, a positive or negative output signal is 
generated at the output of the subtractor 25 which is forwarded to 
servodriver 27 for causing servomotor 28 to shift the entire collimating 
unit until the two detectors are again irradiated with the same intensity. 
Automatic adjustment takes place such that, if the detection unit is 
oriented precisely, central beam 15 emitted by x-ray source 1 penetrates 
central collimator 11 and preferably identical signal components are 
generated in each individual detector 12, 13. In the event of a faulty 
adjustment, automatic readjustment can take place by an adjustment device, 
(see adjustment control), via the evaluation of the detection signals. The 
adjustment of a second plane takes place in the same manner, for example, 
with a second detector pair which is offset by 90.degree.. 
The individual detectors 12, 13, also referred to as a split detector, may 
be comprised of 2 scintillation detectors if the adjustment of the 
arrangement takes place in one plane or of four detectors if the 
adjustment of the arrangement takes place, for example, in both horizontal 
and vertical planes. 
FIG. 4 shows the principle of the physical effect of diffraction, for 
example, of a material with a crystalline or polycrystalline lattice 
structure which is penetrated by an x-ray fan beam having a small cross 
section. In the example shown here, x-ray fan beam 3 penetrates the 
crystal planes separated from one another at a material-specific spacing 
d, with the individual quanta being diffracted at this structure. 
FIG. 5 shows a graph of characteristic energy maxima obtained in accordance 
with the above-described method, as they are generated, for example, by 
explosive substances due to their polycrystalline structure. 
The invention has been described in detail with respect to preferred 
embodiments, and it will now be apparent from the foregoing to those 
skilled in the art that changes and modifications may be made without 
departing from the invention in its broader aspects, and the invention, 
therefore, as defined in the appended claims is intended to cover all such 
changes and modifications as fall within the true spirit of the invention.