IL116600A - Energy dispersive system for detecting specific atomic elements - Google Patents

Description

α»»ΐϋΝ jiHif > \y *ib>air x - i> notnattn n>*nn» naive A novel energy dispersive system for detecting specific atomic elements The State of Israel, Atomic Energy n»¾*ijNt» rrrjnn , >n\y> nj»Ttt Commission Soreq Nuclear Research Center p-n\y ru j>m> *ιρη»ί> τ:ηβπ The inventors: Yosef EISEN Gideon ENGLER C. 98437 FIELD OF THE INVENTION The present invention is in the field of x-ray detection of heavy metals and relates in particular to a fast inspection system and method for the detection of small amounts of elemental and compounded heavy metals such as gold, lead, bismuth, uranium and the like. One particular application of the invention concerns the inspection of luggage, parcels, letters and the like for the detection of smuggled or compounded heavy metals of the kind specified.

BACKGROUND OF THE INVENTION US 5,060,249 and the corresponding EP 0 358965 Bl disclose a method and apparatus for the detection of small amounts of heavy elements, i.e. elements with a high atomic number Z. By that method an inspected object such as a piece of luggage, is placed between a source of collimated x-ray radiation with energy bins centered around the K-edge energy of the target metal and an array of the solid state spectroscopy grade detectors, and the electric signals produced by the detectors are suitably processed. For instance, to detect gold, two energy bins are centered above and below 80.7 keV and in the case of uranium around 115 keV. A suitable photon filter system is provided in conjunction v/ith the x-ray source to ensure that the radiation incident on the inspected object is within the desired energy range.

An alternative method is described by G.T. Barnes et al. in Radiology, 156 (1985) p. 537. In accordance with that disclosure, the use of spectroscopy grade detectors in combination with photon filters is replaced by aggregated scintillation type detectors each coupled to a photodiode, each aggregated detector having a relatively thin upper layer facing the x-ray source and capable of absorbing mainly the low energy components of the incident radiation while transmitting the high energy components, and a relatively thick bottom detector layer distal from the x- ray source capable of absorbing the high energy components of the incident radiation. In these aggregated detectors, the upper detector layer is meant to probe the low energy region and the bottom detector layer is meant to probe the transmitted high energy region. Both of these regions are not narrow and not centered below and above the K-edge of a specific high Z element material. Therefore, this aggregate is capable of discriminating low Z from medium Z materials but is not sensitive to a specific high Z element material. As a result, when using this configuration for detecting a specific element, a high degree of false alarms is expected.

GLOSSARY In the following specification and claims, the following terms will be used: Element and Metal - these terms refer to both the elemental and compounded forms.

Detector - an x-ray scintillator coupled to a photodiode.

Aggregated detector - a multi-layer detector body in which at least two layers have different x-ray absorption capabilities.

Bottom detector layer - in a system that comprises an x-ray source and an array of aggregated detectors at a distance therefrom, a detector layer distal from the x-ray source.

Upper detector layer - in a system that comprises an x-ray source and an array of aggregated detectors at a distance therefrom, any detector layer that is closer to the x-ray source than the bottom detector layer.

Target element - the element to be detected in an inspected object.

Inspection specific detector - an x-ray radiation detector that is specific to the target element or to an element close in atomic number to the target element.

Inspection non-specific detector - an x-ray radiation detector that is not specific to the target element.

Low Z element: Z < 10.

Medium Z element: 10 < Z < 53.

High Z element: Z > 53.

SUMMARY OF THE INVENTION The invention makes use of aggregated detectors in which the upper detector layer holds a heavy element, e.g. a heavy metal which either is the target element or an element that has a very close atomic number while the bottom detector layer holds an element that is either the same as that of the upper detector layer or of a lower atomic number.

In accordance with the present invention, there is provided a method of detecting in an inspected object an element of high atomic number; providing a source of x-ray radiation and at a distance therefrom an array of aggregated detectors having at least one upper detector layer holding inspection specific detectors for detecting high energy components of the x-ray radiation and a bottom detector layer holding x-ray radiation detectors for detecting low energy components of the x-ray radiation and capable of absorbing all x-ray radiation transmitted by the upper detector layer; associating said array of aggregated detectors with processor means; projecting on said array of aggregated detectors an x-ray flux having a characteristic energy spectrum that includes components centered around the K-binding energy of a target element, whereby a first set of electric reference signals is obtained from each of the upper and bottom detector layers; recording and storing said reference signals; placing an inspected object between said source of x-ray radiation and array of aggregated detectors whereby a second set of electric signals is obtained from each of the upper and bottom detector layers; and processing said second set of electric signals and comparing them with the reference signals.

If desired, imaging means may be combined with said processor means for automatically detecting a target of high atomic number.

In order to ensure that the bottom detector layer absorbs essentially all the components of the x-ray spectrum that are transmitted by the upper detector layer, the bottom detector layer is as a rule thicker than the upper one.

Due to the fact that as distinct from the prior art, in accordance with the invention, the inspection specific detectors are located in the upper detector layer, the bulk of the high energy components of the incident radiation that are centered around the K-edge energy of the target element reaches the upper detector layer which absorbs predominantly the flux above the K-edge of the high Z element comprising the detector. The flux transmitted is located mainly below and close to the K-edge. This provides a unique signature of the high Z target element and so enables to determine its specificity. Furthermore, by using inspection specific detectors it becomes possible to employ simple x-ray scintillators or low grade solid state counters such as Hgl2 or Pbl2 operating in either pulse counting mode or current mode, rather than the more refined and expensive spectroscopy grade solid state detectors which are required in accordance with US 5,060,249 for internal resolution of the incident x-ray radiation spectrum.

In accordance with the invention, it is possible to have available units of aggregated detector arrays, e.g. in cartridge form, differing from each other by the nature of the upper detector layer so as to be specific for different elements with relatively high atomic number, e.g. heavy metals such as gold, lead, bismuth, uranium, etc. and to select for each specific operation the appropriate detector array.

In accordance with one embodiment of the invention, each aggregated detector of the array comprises two upper detector layers one on top of the other, each being specific for a different element, whereby the presence or absence of two different elements may be determined simultaneously.

By one specific application the bottom detector layer comprises inspection specific detectors.

By another specific application the bottom detector layer comprises inspection non-specific detectors.

By a preferred embodiment of the invention the upper detector layers are separated from the bottom detector layer by a gap. The purpose of the gap between the layers is to reduce internal scattering.

In another embodiment the upper detector layer is covered by a thin non-specific detector where lower energy components of the incident x-ray radiation are mainly absorbed.

In accordance with yet another embodiment of the invention, the significance of the results may be improved by sandwiching between the upper and bottom detector layers a foil having the same atomic number of the target element, whereby the energy groups above and below the K-edge energy of a target element with a high atomic number are better defined.

By one specific application the bottom detector layer comprises inspection specific detectors.

By another specific application the bottom detector layer comprises inspection non-specific detectors.

By another preferred embodiment of the invention the foil is separated from the bottom detector layer by a gap.

The invention also provides for use in the performance of the above method an aggregated detector comprising at least two detector layers differing from each other by the atomic number of the elements which they hold, the layer with the element of the higher atomic number being thinner than that of the lower atomic number.

The invention further provides a detector unit holding an array of aggregated detectors of the kind specified. In accordance with one embodiment such unit is one-dimensional in that it comprises only a single detector row, while in accordance with another embodiment the unit is two-dimensional and comprises several juxtaposed detector rows.

Preferably the above detector array units are shaped in form of a circular arc such that in use all detectors in each layer of the array are equidistant from the x-ray source.

The invention further provides an apparatus for detecting in an object the presence of at least one target element comprising: an x-ray source capable of supplying an x-ray flux having energy components around the K-edge energy of at least one target element; an array of aggregated detectors spaced from said x-ray source and comprising at least one upper detector layer holding each an inspection specific detector and a bottom detector layer holding an inspection nonspecific detector; processor means coupled to said array of aggregated detectors; and means for placing an inspected object between said x-ray source and array of aggregated detectors so as to intercept a flux of x-rays emanating from said x-ray source.

If desired, imaging means are combined with said processor means.

By one embodiment said apparatus comprises a one-dimensional detector array of aggregate detectors and is designed for a continuous transport of the inspected object across the x-ray flux.

By another embodiment said apparatus comprises a two-dimensional array of aggregate detectors and is designed either to hold an inspected object stationary or to advance it across the x-ray flux in discrete steps.

BRIEF DESCRIPTION OF THE DRAWINGS For better understanding the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which: Fig. 1 shows a side elevation of a two-layer aggregated detector array of the invention; Fig. 2 shows a side elevation of a modification of the embodiment of Fig- 1; Fig. 3 shows a side elevation of a three-layer aggregated detector array of the invention; Fig. 4 shows a side elevation of another embodiment of a three-layer aggregated detector array of the invention for detecting simultaneously two high Z elements; Fig. 5 shows a three-layer aggregated detector array of the invention in association with an x-ray source and an inspected object present between the detector array and the x-ray source; Fig. 6 shows the calculated spectrum of the x-ray flux, in arbitrary units, emanating from an x-ray source used in accordance with the invention; Figs. 7, 8 and 9 show, respectively, the calculated currents generated in the top, middle and bottom detector layers of the three-layer aggregated detector array of Fig. 3, without the presence of an inspected object; Figs. 10, 11 and 12 show, respectively, the calculated currents generated in the top, middle and bottom detector layers of the three-layer aggregated detector array of Fig. 3, in the presence of an inspected object containing bismuth; Fig. 13 shows an inspection assembly of the invention with a one-dimensional array of aggregated detectors; Fig. 14 shows an inspection assembly of the invention with a two-dimensional array of aggregated detectors; Fig. 15 shows a block diagram of a detection assembly according to the invention; and Fig. 16 shows a side elevation of a three-layer aggregated detector array of the invention with gaps between adjacent detector layers.

DETAILED DESCRIPTION OF THE INVENTION Attention is first directed to Fig. 1 showing a two-layer aggregated detector array 1 of the invention wherein the upper detector layer 2, typically 0.5 mm thick, comprises an array of bismuth germanate arbitrarily designated [BGO, ZBi = 83] x-ray scintillators 3 and the bottom detector layer 4, typically 3 mm thick, comprises an array of cesium iodide x-ray scintillators 5 which are doped with thallium [CsI(Tl), = 55]. In order to improve the specificity of the system for detecting the target material (Bi), the spectrum of the incident x-ray flux should be centered around the K-edge energy of this material. This is achieved with the aid of an external metal filter (commonly made of copper and aluminum) at the exit of the x-ray source (not shown in Fig. 1). Upon the incidence of x-rays on the upper detector layer 2 of the detector array 1, the high energy components of the incident x-ray spectrum above 90.5 keV are highly absorbed, whereas primarily medium energy components below the K-edge energy of Bi are transmitted across layer 2 and are absorbed by the bottom detector layer 4. The absorbed energies are converted into electric currents or pulses by photodiodes (not shown) that are coupled to the scintillators 3 and 5. If an object that contains bismuth is placed between the upper detector layer 2 and an x-ray source (not shown in Fig. 1), some of the high energy components of the x-ray radiation will be absorbed by such object before the radiation reaches the upper detector layer 2 of the detector array and this absorption will be manifested by a lower intensity of the electric current or pulses produced by the x-ray scintillators in the upper detector layer 2. This layer then is bismuth specific.

Fig. 2 shows a modified two-layer aggregated detector array 6 of a composition and structure similar to that of Fig. 1, in which a foil 7, having the atomic number equal to the target element is sandwiched between the upper and lower layers 2 and 4 in order to better define the energy groups above and below the K-edge energy of the target metal which in this case is bismuth.

Fig. 3 shows a three-layer aggregated detector array 9 having a top detector layer 10, a middle detector layer 11 and a bottom detector layer 12. The top layer 10 which is typically 0.5 mm thick is of CsI(Tl) and preferably absorbs the low energy component. The middle layer 11 is of BGO and is thus Bi specific while the thick bottom detector layer, which is again of CsI(Tl), is designed by thickness to absorb the residual radiation transmitted by layers 10 and 11. Accordingly the aggregated detector of Fig. 3 is capable of detecting simultaneously the presence of bismuth and nonspecific low and medium atomic number materials and within the terminology used herein it has two upper detector layers, namely layer 10 and 11.

Fig. 4 shows another embodiment of a three-layer aggregated detector array 14 having top, middle and bottom detector layers 15, 16 and 17. The top layer 15 is of BGO and accordingly bismuth specific and the middle layer 16 which in accordance with the terminology herein is a second upper detector layer is of CdWO4 and is accordingly wolfram (tungsten) specific. The bottom detector layer 17 is of CsI(Tl) and fulfills the same function as in the embodiments of Figs. 1 to 3.

Attention is now directed to Fig. 5, which shows a layout of a detection assembly comprising a detector array 9 of the kind shown in Fig. 3, in association with an x-ray source 18 spaced therefrom, and an inspected object 19 located between the x-ray radiation source 18 and the detector array 9. In operation, the object 19 may be moved in a direction normal to that of the propagation of the x-ray radiation either continuously or in discrete steps, whereby it is fully scanned. Alternatively, if a sufficiently large two-dimensional detector array is used, the object 19 may be stationary.

The spectrum of the x-rays incident on the three-layer detector aggregate in the arrangement of Fig. 5 is shown in Fig. 6. in arbitrary units. The resulting currents generated in the three detector layers of the array 9 are shown in Figs. 7 to 9, again in arbitrary units. Fig. 7 shows the current generated in the top detector layer 10, Fig. 8 the current generated in the middle detector layer 11 and Fig. 9 the current generated in the bottom detector layer 12. Referring first to Fig. 7, it is seen that the current generated by the top detector layer 10 that holds elements of comparatively , low atomic number represents an absorption around a mean lower energy of about 48 keV and this absorption and the resulting currents may assist in discriminating low atomic number from medium atomic number materials, for example, plastics from steel.

Referring now to Fig. 8, it is seen that the middle detector layer 11 holds bismuth in the form of BGO which has the relatively high atomic number 83, shows less absorption of low energies in the lower part of the spectrum and, gives rise to a discontinuity at 90.5 keV which is the K-edge energy of bismuth.

Turning now to Fig. 9, it can be seen that the bottom detector layer 12 which is 3 mm thick generates an electric current which represents a full absorption of the remaining radiation. In that layer, the lower energies are naturally absorbed by the low atomic number of the scintillator constituents while the higher energies are absorbed .due to the sheer thickness of that layer. The main component of the current is centered close to the K-edge of bismuth.

Figs. 10 to 12 show the attenuated current intensities as a function of energy, again in arbitrary units, in the three-layers 10, 11 and 12 of the detector array 9 in a testing experiment in accordance with the layout shown in Fig. 5 in which a bismuth sample 19 is placed between the x-ray source 18 and the detector array 9. By comparing each of the graphs in Figs. 10, 11 and 12 with, respectively, those of Figs. 7, 8 and 9, strong absorption of energies is seen at and above 90.5 keV which is the K-edge energy of bismuth.

Attention is now directed to Figs. 13 and 14 which are perspective illustrations of the experimental setup of Fig. 5. Fig. 13 shows the use of a one-dimensional detector array and Fig. 14 the use of a two-dimensional detector array. In both these embodiments, the detector arrays 9' and 9" are shaped in the form of a circular arc whose radial center is the radiation source 18 and in this way all individual scintillators of each array layer are equidistant from source 18. As the object of inspection 19 passes between the x-ray source 18 and the detector aggregate 9' or 9", each individual three-layer aggregated detector responds to a different region of the object of inspection 19. Hence the array of detectors forms an image of the distribution of target elements in the inspected object. Each individual three-layer aggregated detector defines a pixel of the formed image. In the embodiment of Fig. 13, the object of investigation 19 has to be moved continuously across the x-ray beam produced by source 18. Against this in the two-dimensional array embodiment of Fig. 14, a field of pixels is produced simultaneously and depending on circumstances, the object of investigation may not have to be moved at all in the course of inspection or alternatively, may have to be moved in discrete steps.

By replacing the bismuth object by samples of various elements of high, medium and low atomic numbers and repeating the above described measurements, general conclusions can be reached as to the behavior of the currents generated in each of the three layers of the aggregated detector array in the presence of these elements. The attenuation of the x-rays in each detector in each of the three-layers of the array can be derived by taking the ratio of the currents generated in the presence of the object under investigation to those generated in the absence of any intervening object.

In order to demonstrate the performance of the proposed invention, an example of a possible algorithm for detecting bismuth and discriminating between low Z and medium Z materials is given. For this purpose let Aj be the attenuation of the induced current in detector i of the j triple layer detector aggregate of the array (i = 1,2,3; j=l,...,N, where N is the number of the triple layer detector aggregate of the array). Then fulfillment of the following relations: indicates that the qth detector of the array of the triple layer detectors has detected the presence of bismuth. No definitive conclusion can be reached from these relations as to the elements detected by the pth detector of the array of triple layers detectors, except that they do not indicate the existence of bismuth.

The presence of low atomic number materials is indicated triple layer detectors if the following conditions are satisfied for a given detector: bismuth are nearly equal).

The presence of medium atomic number materials is indicated if the following condition is satisfied: A p < 1 (3) (In all relations: "< " means smaller than about 30%, and means equal within the range of 5%).

Attention is now directed to Fig. 15 which is a block diagram of a detection assembly according to the invention. In this figure, each block includes a functional description and there is no need for any further explanation. It is, however, pointed out that the filter at the exit of the x-ray generator is for achieving an x-ray flux centered around the K-edge energy of the inspected object (placed in the screened box) as described in the foregoing description.

Although the various embodiments of the aggregated detectors of the invention have been shown in Figs. 1 to 5, with adjacent layers in contact, they can also be configured with a gap between adjacent layers. Accordingly, the embodiment shown in Fig. 3 could just as well be arranged as shown in Fig. 16, with a gap 20 separating the top layer 10 and the middle layer 11 and a further gap 21 separating the middle layer 11 from the bottom detector layer 12. This property has been demonstrated for the triple layer detector shown in Fig. 3, but it clearly applies to the embodiments illustrated in Figs. 1 and 4 and also to the embodiment shown in Fig. 2 where the gap would be between the bottom detector layer and the thin foil.

Claims (30)

CLAIMS:

1. A method of detecting in an inspected object an element having a high atomic number; providing a source of x-ray radiation and at a distance therefrom an array of aggregated detectors having at least one upper detector layer holding inspection specific detectors for detecting high energy components of the x-ray radiation and a bottom detector layer holding x-ray radiation detectors for detecting low energy components of the x-ray radiation and capable of absorbing all x-ray radiation transmitted by the upper detector layer; associating said array of aggregated detectors with processor means; projecting on said array of aggregated detectors an x-ray flux having a characteristic energy spectrum that includes components centered around the K-binding energy of a target element, whereby a first set of electric reference signals is obtained from each of the upper and bottom detector layers; recording and storing said reference signals; placing an inspected object between said source of x-ray radiation and array of aggregated detectors whereby a second set of electric signals is obtained from each of the upper and bottom detector layers; and processing said second set of electric signals and comparing them with the reference signals.

2. The method of Claim 1, wherein imaging means are combined with said processor means for automatically detecting a target of high atomic number.

3. The method of Claim 1 or 2, wherein each aggregated detector of the array comprises two upper detector layers one on top of the other, each being specific for two high atomic number elements, whereby the presence of two different elements may be determined simultaneously.

4. A method according to any one of Claims 1 to 3, wherein the upper detector layer is covered by a low or medium atomic number detector layer, whereby the presence of non-specific low and medium atomic number materials can be detected.

5. The method of any one of Claims 1 to 3, wherein the bottom detector layer is thicker than said at least one upper detector layer.

6. The method according to any one of Claims 1 to 5, wherein the bottom detector layer comprises inspection specific detectors.

7. The method according to any one of Claims 1 to 5, wherein the bottom detector layer comprises inspection non-specific detectors.

8. The method according to any one of Claims 1 to 7, wherein the bottom detector layer is separated from the detector layer above it by a gap.

9. The method of Claim 1 or 2, wherein a foil having the same atomic number as the inspected object is sandwiched between upper and lower detector layers.

10. The method according to Claim 9, wherein the bottom detector layer comprises inspection specific detectors.

11. The method according to Claim 9, wherein the bottom detector layer comprises inspection non-specific detectors.

12. The method according to any one of Claims 9 to 11, wherein the bottom detector layer is separated from the foil by a gap.

13. An aggregated detector comprising at least two detector layers differing from each other by the atomic number of the elements which they hold, the top layer with the element of the higher atomic number being thinner than that of the bottom lower atomic number layer.

14. The aggregated detector according to Claim 13, wherein the bottom detector layer comprises inspection specific detectors.

15. The aggregated detector according to Claim 13, wherein the bottom detector layer comprises inspection non-specific detectors.

16. The aggregated detector according to any one of Claims 13 to 15, wherein the bottom detector layer is separated from the top detector layer by a gap.

17. An aggregated detector according to any one of Claims 13 to 16, wherein said detectors comprise scintillators coupled to photodiodes.

18. An aggregated detector according to any one of Claims 13 to 16, wherein said detectors comprise room temperature solid state detectors.

19. An aggregated detector according to Claim 18, wherein said detectors operate in a pulse counting mode.

20. An aggregated detector according to Claim 18, wherein said detectors operate in a current mode.

21. An aggregated detector according to any one of Claims 13 to 16, wherein said detectors comprise storage phosphors.

22. A detector unit comprising an array of aggregated detectors according to any of Claims 13 to 21.

23. The unit of Claim 22, comprising a single row of aggregated detectors.

24. The unit of Claim 22, comprising several juxtaposed rows of aggregated detectors.

25. A detector unit according to any one of Claims 22 to 24 being shaped in form of a circular arc.

26. An apparatus for detecting in an object the presence of at least one target element comprising: an x-ray source capable of generating an x-ray flux having energy components around the K-edge energy of at least one target element; an array of aggregated detectors spaced from said x-ray source and comprising a bottom detector layer holding inspection non-specific detectors and at least one upper detector layer holding each an inspection specific detector; - ? - 18 - processor means coupled to said array of aggregated detectors; and means for placing an inspected object between said x-ray source and array of aggregated detectors so as to interact a flux of x-rays emanating from said x-ray source.

27. The apparatus of Claim 26, wherein imaging means are combined with said processor means.

28. An apparatus of Claim 26 or 27, comprising a detector array with a single row of aggregated detectors and designed for continuous transport of an inspected object across the x-ray flux.

29. An apparatus according to Claim 26 or 27, comprising a detector array with several juxtaposed detector rows and designed to hold a stationary inspected object.

30. An apparatus according to Claim 26 or 27, comprising a detector array with several juxtaposed detector rows and designed to transport an inspected object stepwise across the x-ray flux. 9S437-7-SPC-MC,MICbe/24.12.1995