GB2359717A

GB2359717A - Collimator adjustment apparatus

Info

Publication number: GB2359717A
Application number: GB0027193A
Authority: GB
Inventors: Martin Hartick; Frank Cordes
Original assignee: Heimann Systems GmbH; Heimann Systems GmbH and Co KG
Current assignee: Heimann Systems GmbH; Heimann Systems GmbH and Co KG
Priority date: 1999-11-13
Filing date: 2000-11-07
Publication date: 2001-08-29
Anticipated expiration: 2020-11-07
Also published as: GB0027193D0; DE19954661A1; NL1016473C2; FR2801134A1; NL1016473A1; GB2359717B; FR2801134B1; DE19954661C2

Abstract

Apparatus for adjusting a collimator in an X-ray diffraction testing facility. In order to provide exact measurement and determination of the material it is necessary to adjust precisely the angular position of the collimating and detecting apparatus in relation to the incident X-radiation. The adjustment is made with the aid of a detection system arranged in the collimator 6 consisting of at least two local and separated detection instruments 8, 9 arranged in series and spaced from one another.

Description

Title:

2359717 Apparatus and method for adjusting a collimator The invention relates to an apparatus and a method for adjusting a collimator in an X-ray installation.

The adjustment of a collimating and detecting apparatus in an X-ray facility has a decisive influence on the selectivity and hence on the capacity to recognise materials and the probability of detection of an object to be transilluminated, In particular in the case of X-ray facilities that make use of the physical effect of the lo diffraction of X-radiation, an exact adjustment is necessary in order that the exact angular position of the collimating and detecting apparatus in relation to the X-ray enables an exact measurement and determination of the detected material.

In DE 195 10 168 A1 the adjustment of a collimating and detecting apparatus in an X-ray testing facility is disclosed in an embodiment as example. For this purpose an automatic readjustment is undertaken prior to each measurement. The collimating and detecting apparatus, which consists of several collimators with detectors behind them in each instance, is arranged in a common carrier unit, said carrier unit containing an additional central collimator which is aligned with the focus of an X-ray source generating the X-radiation or which is aligned in the course of a readjustment. In the course of an adjustment, detection signals are generated by the central ray (primary ray) emitted by the X-ray source, which passes through the central collimator in the case of an exact alignment, on individual detectors arranged alongside one another and located behind said central collimator, said signals being of equal magnitude if an exact adjustment obtains.

2 An object of this invention is to provide for a further adjustment of a collimator in an X-ray facility, which is of simpler structural design and which operates in fully automated manner.

According to this invention there is provided According to this invention there is also provided The feature of this invention is to perform the adjustment with the aid of a detection system which is arranged in the collimator, consisting of at least two locally separated detection instruments arranged in series and spaced from one another. in the case of a homogeneous attenuation of a primary ray emitted from an X-ray lo source by the first detection instrument of the detection system, the residual intensity of the ray is used once more for the final alignment of the collimator with the aid of the second detection instrument. In this way an exact, coaxial alignment of the collimator, together with the detection system located in the collimator, with or along the primary ray is possible, as a result of which a higher accuracy of measurement is within an X-ray facility is guaranteed. The greater the distance between the detection instruments then the more precise does the alignment of the collimator within an X-ray facility become.

For the spatially local alignment of the collimator at a first point, the first detection instrument is displaced in the primary ray until such time as a signal that is generated in the method is maximal, that is strongest. With a view to optimal alignment of the collimator along the primary ray, alignment with a further point in the collimator is then undertaken. This is obtained with the aid of the second detection instrument, the collimator being rotated in an (upper) plane about an imaginary point located as close as possible to the centre of the first detection instrument In two 3 independent planes until such time as the signal is maximal also in these cases, without the signal at the first detection instrument thereby being minimised. Therefore a further rotation in the second plane of rotation is undertaken after the maximum in the first plane of rotation has been set, the centre of rotation being situated also in this case as close as possible to the centre of the first detection instrument. The optimal alignment is guaranteed when the intensity maximum at both detection instruments has been set in all three planes of rotation.

Thus in a simple embodiment a diaphragm arrangement, consisting for example of a pinhole diaphragm in each instance, may be placed in front of each of lo the two detection instruments in the collimator, which serves to adapt the detection area to the diameter of the primary ray.

In the case where use is made of detection instruments having a sensitive area that coincides sufficiently well with the diameter of the ray, the diaphragm arrangement can be dispensed with.

is By way of first detection instrument, use may be made, for example, of a semiconductor counter, gas counter or scintillation counter that is sufficiently thin and homogeneous, as a result of which the primary ray is only slightly attenuated and an intensity distribution is substantially maintained. By way of second detection instrument, a semiconductor counter, a gas counter or a scintillation counter may likewise be employed. However, the absorption properties thereof must be matched to the, on average, higher energy of the quanta which results by reason of the transmission by the first detection instrument.

The two locally separated detection instruments may each consist of a four quadrant detector, at least two individual detectors, a detector array or location ......

---51 4 sensitive detectors with multi-segmented diodes.

The method is preferably used for adjusting round-slot collimators with a crystal detector behind them, the entire system being set to a predetermined angle in relation to the primary ray for an exact measurement with the aid of X-ray diffraction.

It is also possible to adjust a simple col[ imati ng/detecting instrument which, for example, is used for the conventional detection of material, the first detection instrument being designed as a detector for the lowenergy portion of the radiation and the second detection instrument being designed as a detector for the higherenergy portion.

is The invention will be described in more detail by reference to an embodiment shown by way of an example and illustrated in the drawings, wherein- Fig. 1 shows a schematic diagram of a measuring section according to the state of the art, Fig. 2 shows a round-slot collimator with the apparatus according to this invention, Fig. 3 shows the apparatus according to this invention in a simple collimator, Fig. 4 shows a first embodiment of the apparatus according to this invention, Fig. 5 shows another embodiment of the apparatus, and Fig. 6 shows a schematic representation of the alignment of the collimator with a primary ray.

Referring to the drawings, a measuring section in an X-ray (testing) facility which is not represented in any detail is shown in Fig. 1. As is generally known, an X-radiation FX is generated by an X-ray source 1 and radiated through an object 2 to be transilluminated which is located on a transport apparatus 3. By means of a diaphragm apparatus 4a, 4b a primary ray FX, is generated, preferably in the form of a pencil beam.

Owing to the crystal-lattice structure of the material of the object 2 to be transilluminated, the primary ray FX1 is diffracted in known manner at several lattice points G (here represented once only), whereby at a certain radiant energy the primary ray FX, is deflected as radiation FX,' at an angle 0m which is dependent upon the material. By utilising this fact, the material that has been detected is determined in known manner on the basis of the physical effect of X-ray diffraction (formation of Bragg's interference pattern), whereby by pre-setting a certain angle 0m different energies are measured (according to Bragg) and compared with known values.

In Fig. 2 a collimator 6 is represented, with the aid of which the material or the material-type of the object 2 (from Fig. 1) which is transilluminated with the primary ray FX1 can be determined. The collimator 6 in this case is a circular- slot collimator with a crystal detector 11 located behind it, which are both used for a measurement by utilising X-ray diffraction. A blind-hole- type opening 7 acting as a central collimator is centrally integrated within the collimator 6. At a spacing from the centre opening 7 the collimator 6 exhibits a conically divergent circular slot 10 which simulates an angular path at the predetermined angle 0m. A first detection instrument 8 and, behind it at a defined spacing therefrom, a second detection instrument 9 are arranged within the opening 7. The crystal detector 11 is so large in terms of surface area that the scattering-cone rays (radiation FX,') emerging through the round slot 10 can be received, and it exhibits a preferably circular, X-ray-sensitive 6 surface 12 pointing towards the collimator 6. The X-ray source from Fig. 1 is denoted by 1.

In Fig. 3 another collimator 26 is represented which exhibits a first detection instrument 28 in a centre bore 27 and, at a defined spacing, preferably at the rear s end of the bushing 27, another detection instrument 29. In this case the first detection instrument 28 is designed as a detector for lower X-ray energies and the second detection instrument 29 is designed as a detector for higher X-ray energies. This col J imating/detecting instrument is used, for example, for the conventional detection of material.

The internal structure of the collimator 6, 26 with the electrical connections for signal evaluation that are necessary for the adjustment is represented schematically in Fig. 4. In this case, pinhole-diaphragm arrangements 13a, 13b and 14a, 14b are arranged in front of the detection instrument 8, 28 and in front of the detection instrument 9, 29, respectively, which serve to adapt the detection area of the detection instrument 8, 28 and of the detection instrument 9, 29 to the diameter of the primary ray. Denoted by 15 and 17 are signal-amplifier stages which are necessary for the purpose of amplifying the signals that are picked up at the respective detection instruments 8, 28 and 9, 29. These amplifier stages 15 and 17 are each routed to an indicating unit 16 and 18, respectively, and are connected in parallel with these to a microprocessor 23. The crystal detector 11 which is represented additionally in this case is dispensed with if, instead of an annular-slot collimator 6, a simple collimator 26 is to be adjusted.

in another embodiment, according to Fig. 5, detection instruments 8, 28, 9, 29 are used, with the aid of which it is possible to determine the centre of the intensity 7 distribution of the Xray or primary ray FX, impinging on them. These detectors may be several individual detectors, a detector array, four quadrant detectors or locationsensitive detectors with multi-segmented diodes. Connected in series downstream of these detection instruments 8, 28 and 9, 29 are amplifier stages operating in parallel which, for the sake of clarity, are each denoted by only one reference symbol 19 and 21, respectively, with an amplifier of the respective amplifier stage 19 or 21 being assigned, in each instance, to each individual detector, to each detector in the array and also to each diode in the detection instrument 8, 28, 9, 29. Connected in series downstream of these amplifier stages 19, 21 are, preferably, indicating units lo 20 and 22, respectively. In parallel with these, the amplifier stages 19, 21 are electrically connected to the microprocessor 23. In this case the first diaphragm arrangement 13a, 13b and the second diaphragm arrangement 14a, 14b, or only the second diaphragm arrangement 14a, 14b, from Fig. 4 may be dispensed with if the alignment is undertaken in a pencil beam (point ray) FXl.

The adjustment method proceeds, independently of the embodiment, as follows:

As represented in Fig. 6, the X-ray considered within the bushing 7 which is emitted by way of primary ray FX, is situated prior to the adjustment outside the centre of the first and second detection instruments 8, 28 and 9, 29. A first point P1 which is predetermined for the adjustment and a second point P2 on the primary ray do not in this case coincide with the respective midpoint (centre) of the detection instrument 8, 28 and 9, 29. This has the effect that the collimator 6, 26 has to be aligned spatially, i.e. in three planes, in order to attain its optimal local position without canting relative to the primary ray FXi.

8 In a first step, in the course of the adjustment the collimator 6, 26 is now displaced until such time as the signal that is generated in the first detection instrument 8, 28 is maximal, there still being no full coincidence between the midpoint of the detection instrument 8, 28 and the point P1 of the primary ray FXl. In this case the primary ray FX1 at point P2 continues to be situated outside the centre of the second detection instrument 9, 29. The signals that are generated in the detection instruments 8, 28 and 9, 29 are indicated on the indicating unit 16 and 20, respectively.

With a view to optimal alignment of the collimator 6, 26, in a second step the alignment with the second point P2 is required. With a view to alignment with this second point P2, the collimator 6, 26 is rotated or adjusted about an imaginary point P3, which is preferably situated in the vicinity of the centre of the detection instrument 8, 28, in two independent planes until the signal from the second detection instrument 9, 29 also becomes maximal. This plane of rotation comprises a pointed cone shaped region (see arrows) emanating from the point P3. After this second step the maximum of the first plane (of rotation) is set, as a result of which a first local pre-alignment of the collimator 6, 26 has been undertaken.

After the intensity maximum in the first plane has been ascertained, the rotation in the second plane (of rotation) is begun with a view to further spatial alignment. A 2 0 point close to the centre of the first detection instrument 8, 28 also has to be selected for this alignment, it being possible for point P3 to be called upon. Also in this case the collimator 6, 26 with the pointed cone shaped region is rotated in such a way that the intensity maximum Is set at the respective detection instruments 8, 28 and 9, 29. This then also has to be performed in the third plane (of rotation), so that the 9 primary ray FX1 is then perpendicularly and centrally incident on the first and on the second detection instruments 8, 28, 9, 29, i.e. in the centres thereof.

In the case where use is made of several location-sensitive detectors, the individual signals of the detection instruments 8, 28 and 9, 29 are firstly analysed stepwise in the microprocessor 23 with regard to their amplitude and are subjected to further processing, whereby the current position of the centre of the primary ray FX1 is ascertained from the individual signals and compared with the centre of the respective detection instrument 8, 28 and 9, 29, respectively. Readjustment of the detection centres to the centre of the primary ray FX1 is undertaken until such time lo as the signal values generated are maximal. By this means, with the aid of a single calibration a misalignment between the respective detection centre and the centre of the primary ray can be ascertained more clearly and corrected in one step.

In the case where use is made of a 4-quadrant detector in the detection instrument 8, 28 or 9, 29, evaluation of the signals generated by each quadrant is undertaken, the detection centre being set when the primary ray FX, is incident in the common cross of the quadrants and generates signals of equal magnitude.

Consequently an optimal alignment of the collimator 6, 26 along a primary ray FX, is realised by simple means. The position of the collimator in an X-ray testing facility which is not represented here in any detail is varied sequentially with the aid of a control instrument, which is likewise not represented in any detail, until both detection instruments 8, 28 and 9, 29 are impinged maximally by the primary ray FX1.

The respective rotation of the collimator 6, 26 in the individual planes is undertaken in stepwise manner via mechanical means which are installed in the X- ray facility and which are controlled and regulated automatically via a program which is controlled by the microprocessor 23. The intensity maxima that are ascertained in the course of the first adjustment can be stored with a view to further utilisation.

In the case where a calibrated collimator 6, 26 having several locationsensitive detectors is employed, the misalignment between the primary-ray maximum and the detection centre can be determined immediately with a single measurement and corrected by this method.

Use of several detection instruments arranged in series within the bushing 7 is possible but is not necessary for the adjustment method itself, since the latter then 10 becomes more elaborate.

Claims

1. An apparatus for adjusting a collimator in an X-ray testing facility, including an Xray source for generating an X-radiation and means for generating a primary ray, wherein at least two locally separated detection instruments which are situated in series and spaced from one another are arranged within a central bore in the collimator with a view to alignment of the same with the primary ray.

2. Apparatus according to Claim 1, wherein a diaphragm arrangement is placed in front of the first detection instrument.

3. Apparatus according to Claim 2, wherein a diaphragm arrangement is placed in front of the second detection instrument.

4. Apparatus according to any one of Claims 1 to 3, wherein the collimator is an annular-slot collimator which exhibits, at a spacing from the central bore, a conically divergent round slot which simulates an angular path at a predetermined angle.

5. Apparatus according to Claim 4, wherein a crystal detector is permanently assigned to the collimator, said crystal detector possessing an X-ray sensitive surface which points towards the collimator and which is aligned with the predetermined angle of the round slot in such a way that the scattering cone rays 1 12 emerging from the round slot are received.

6. Apparatus according to any one of Claims 1 to 5, wherein the detection instruments each consist of a four quadrant detector.

7. Apparatus according to any one of Claims 1 to 5, wherein the detection instruments each consist of at least two individual detectors.

8. Apparatus according to any one of Claims 1 to 5, wherein the detection lo instruments are constructed from position sensitive detectors with multi-segmented diodes.

9. Apparatus according to any one of Claims 1 to 6, wherein the detection instruments each consist of a detector array.

10. Apparatus according to any one of Claims 1 to 8, wherein one detection instrument is designed as a detector for low X-ray energies and the other detection instrument is designed as a detector for higher X-ray energies.

2 0

11. Apparatus according to any one of the preceding Claims 1 to 10, wherein the first detection instrument is electrically connected to an amplifier stage and the second detection instrument is electrically connected to an amplifier stage, downstream of 13 which a central microprocessor is connected in series.

12. A method for adjusting a collimator in an X-ray testing facility with the aid of emitted X-radiation and a generated primary ray, wherein.

a spatial alignment of the collimator is undertaken with the aid of two locally separated detection instruments arranged in series and spaced from one another along the primary ray, whereby in a first step the collimator is displaced until such time as the signal that is generated at the first detection instrument is maximal, in a second step the collimator is rotated about a point in a first plane of rotation in two independent planes until the signal from the second detection instrument is also maximal, the point preferably being situated in the vicinity of the centre of the first detection instrument, after the intensity maximum in the first plane of rotation has been ascertained, the rotation of the collimator in the second and then in the third plane of rotation is undertaken until the intensity maximum has been set at the respective detection instruments also in these cases, so that the primary ray is perpendicularly and centrally incident on the first and on the second detection instruments.

13. Method according to Claim 12, wherein with the aid of a microprocessor the signals are evaluated and the intensity maximum of the plane of rotation to be set is ascertained, whereby a direct determination of the misalignment to be readjusted

14 between a primary-ray centre and the detection centres of the detection determined with the aid of the offset of the individual signals.

is 14. An apparatus for adjusting a collimator in an X-ray testing facility, constructed and arranged to function as described herein and exemplified with reference to the 5 drawings.

15. A method for adjusting a collimator in an X-ray testing facility, substantially as herein described and illustrated with reference to the drawings.