X-RAY BEAM POSITION MONITORS

Abstract: An X-ray beam position monitor has a first series of three collection electrodes (1a, 1b, 1c) and a first biasing electrode (3) for detecting the rotational position of the X-ray beam about a vertical axis (26). The monitor also has a second series of three collection electrodes (5a, 5b, 5c) and a second biasing electrode (7) for detecting the rotational position of the X-ray beam about a horizontal axis (27). Signals generated at the collection electrodes are processed and used to derive the rotational position of the beam about the two axes (26, 27). These signals are also used to derive the translational position of the X-ray beam in horizontal and vertical planes. The beam is centered so that the monitor acts as a null-seeking monitor.

Description: TITLE: X-RAY BEAM POSITION MONITORS This invention relates to X-ray beam position monitors and represents a development of the invention disclosed in the applicants' co-pending UK Patent Application No 9913615.2. For carrying out X-ray diffraction experiments it is necessary to align the X-ray generator with respect to an X-ray diffractometer or an X-ray camera. Complete alignment of the X-ray beam requires both translational and rotational positioning. In some applications it is more convenient to keep the X-ray generator fixed and to move the diffractometer into alignment. In other applications it is easier to keep the diffractometer fixed and to move the generator with respect to it. In either case alignment is greatly helped by an X- ray beam position monitor, preferably one which has electrical outputs which can be used for controlling the translations and rotations to make the alignment automatic. The aim of the invention is to provide monitoring of at least the rotational positions of the X-ray beam, and preferably both translational and rotational positions. According to the invention an X-ray beam position monitor comprises a first electrode assembly for detecting the rotational position of the X-ray beam about an axis orthogonal to one plane, the first electrode assembly comprising a first series of three collection electrodes and a first biasing electrode, a second electrode assembly for detecting the rotational position of the X-ray beam about another axis orthogonal to another plane transverse to said one plane, the second electrode assembly comprising a second series of three collection electrodes and a second biasing electrode, means for applying a bias voltage to the biasing electrodes and signal processing means for processing electrical signals which are generated at the collection electrodes and deriving therefrom signals which are representative of the rotational position of the X-ray beam about said axes. The three collection electrodes of the first series are preferably constituted by an intermediate electrode in the shape of a parallelogram adjacent to which are two end electrodes each triangular in shape, the series of the three collection electrodes preferably having in overall outline a generally rectangular shape. The processing means are preferably operative to sum the signals from the end electrodes and subtract therefrom the signal from the intermediate electrode, in order to derive a signal representative of the rotational position of the X-ray beam in said one plane. This latter signal may be normalised by dividing it by the sum of the signals from the three electrodes. In addition to detecting the rotational position of the X-ray beam, the monitor may also detect the translational position of the X-ray beam in said one plane, in which case the signal processing means are additionally operative to obtain the difference between the signals from the two end electrodes, in order to derive a signal representative of the translational position of the X-ray beam in said one plane. This latter may be normalised by dividing it by the signal from the intermediate electrode. Similarly, the three collection electrodes of the second series are preferably constituted by a second intermediate electrode in the shape of a parallelogram adjacent to which are two second end electrodes each triangular in shape, the series of the three collection electrodes of the second series preferably having in overall outline a generally rectangular shape. The processing means are preferably operative to sum the signals from the second end electrodes and subtract therefrom the signal from the second intermediate electrode, in order to derive a signal representative of the rotational position of the X-ray beam in said another plane. In addition to detecting the rotational position of the X-ray beam in said another plane, the monitor may also detect the translational position of the X-ray beam in said another plane, in which case the signal processing means are additionally operative to obtain the difference between the signals from the two second end electrodes, in order to derive a signal representative of the translational position of the X-ray beam in said another plane. The first electrode assembly and the second electrode assembly may be positioned at substantially the same axial position along the direction of propagation of the X-ray beam, in which case the means for applying the bias voltage include switching means for applying the bias voltage to the first biasing electrode or the second biasing electrode. The placement of the first electrode assembly and the second electrode assembly at substantially the same position along the direction of propagation of the X-ray beam provides a compact arrangement which renders the monitor particularly useful for use with X-ray diffractometers and for laboratory use generally. In an alternative embodiment, the first electrode assembly and the second electrode assembly are not at the same axial position along the direction of propagation of the X-ray beam. The electrodes of the first assembly are preferably orthogonal to the electrodes of the second assembly, so that said one plane and said other plane are mutually orthogonal. When the first and second electrode assemblies are placed at substantially the same position along the direction of propagation of the X-ray beam, the first and second electrode assemblies preferably constitute the four walls of a square-section tunnel-like structure through which the X-ray beam is propagated. The X-ray beam position sensor preferably acts as a null-seeking device, the beam being centred (in both translational and rotational senses) by means of adjustments in the two planes of positioning, until the electrical signals are representative of a centred position of the X-ray beam. This adjustment can be simultaneous when the first and second electrode assemblies are axially spaced, but is sequential in the two planes of positioning when the first and second electrode assemblies are at the same axial position. Adjustment may be achieved by applying a centring movement to the beam, to the assembly or to a combination of both beam and assembly. Two embodiments of X-ray beam position sensor according to the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a view showing the structure of collection and bias electrodes in the first embodiment. Figure 2 is a view showing three collection electrodes of the first embodiment. Figure 3 shows the electrical circuitry of the embodiment of Figure 1 , and Figure 4 is a view showing the structure of collection and bias electrodes of the second embodiment. Referring to Figure 1 , the sensor comprises a first electrode assembly comprising a first series of three collection electrodes la, lb, lc printed on a first anode board 2, and a first biasing electrode 3 printed on a first cathode board 4. The anode and cathode boards 2 and 4 occupy vertically spaced horizontal planes with the first series of collection electrodes la, lb, lc facing the first biasing electrode 3. The first biasing electrode 3 is rectangular in shape, the first series of three collection electrodes la, lb, lc, having, in overall outline, a similar rectangular shape which is divided by two angled but mutually parallel lines of separation so that the electrode lb defines an intermediate electrode in the shape of a parallelogram and the two electrodes la and lc are end electrodes each in the shape of a right-angled triangle. As shown in Figure 1 , the pair of boards 2 and 8 are separated by a short distance from the pair of boards 3 and 6. For clarity, the first anode board 2 and its three collection electrodes la, lb and lc are shown in Figure 2. Also shown diagrammatically in Figure 2 are the respective electrical connections to the three electrodes la, lb and lc. Similarly, the second electrode assembly comprises a second series of three collection electrodes 5a, 5b, 5c printed on a second anode board 6, and a second biasing electrode 7 printed on a second cathode board 8. The second anode and cathode boards occupy horizontally spaced vertical planes, with the second series of collection electrodes 5a, 5b, 5c facing the second biasing electrode 7. The second biasing electrode 7 is rectangular in shape, the second series of three collection electrodes 5a, 5b, 5c having, in overall outline, a similar rectangular shape which is divided by two angled but mutually parallel lines of separation so that the electrode 5b defines an intermediate electrode in the shape of a parallelogram and the two end electrodes 5a, 5b are in the shape of right-angled triangles. Each electrode la, lb, lc, 5a, 5b, 5c, 3 and 7 is formed by an area of copper deposited on the appropriate board. The first and second electrode assemblies are positioned at the same axial position along the direction of propagation of the X-ray beam, the centred direction of which is indicated at 9 in Figure 1. The first and second electrode assemblies thus form a tunnel like structure of square cross-sectional shape, through which the X-ray beam is propagated. The square section tunnel structure is housed within a tube. In a preferred embodiment, each board is a rectangle 8mm wide by 36mm long, with a spacing of 10mm between anode and cathode. The air-filled tunnel-like structure is 36mm long and has a square cross-sectional shape with an edge dimension of 10mm. This structure fits within a 25mm diameter tube, thus providing a compact arrangement. Referring to Figure 3, the two cathode or biasing electrodes 3, 7 are connected to a double pole switch 10, in one position of which (illustrated in Figure 3) the electrode 7 is grounded and the electrode 3 is connected to a -300 volt source 12, and in the other position of which the electrode 7 is connected to the -300 volt source 12 and the electrode 3 is grounded. In showing all six electrodes la, lb, lc, 5a, 5b, 5c, Figure 3 is diagrammatic because there is no transverse plane which would show all six of these electrodes. The three collection electrodes la, lb, lc are respectively electrically connected to three current to voltage amplifiers 13, 14, 15 each having a respective feedback resistor 16, 17, 18 typically of 20 GΩ. The amplifiers 13, 14, 15 have respective voltage outputs Va,, Vb, and Vc, respectively proportional to the charges collected on the electrodes la, lb and lc as a result of the ionisation produced by the X-ray beam. In a corresponding manner, the three collection electrodes 5a, 5b, 5c are respectively connected to three current to voltage amplifiers 19, 20, 21 each having a respective feedback resistor 23, 24, 25 of 20 GΩ. The three amplifiers 19, 20, 21 have respective voltage outputs Va 2 , Vb 2 and Vc 2 respectively proportional to the charges collected on the electrodes 5a, 5b and 5c as a result of the ionisation produced by the X-ray beam. With the switch 10 in the position illustrated in Figure 3, the voltages Va, Vb, Vc, are used to derive a first signal R, representative of the rotational displacement (about the central vertical axis 26) of the X-ray beam from the central axis. R = Va, + Vc, -Vb, Va, + Vb, + Vc, R, is indicative of the rotation (about the vertical axis 26) required to make the X-ray beam parallel to the central axis. The denominator in the above expression for R, normalises the signal. It can be shown that: R, = -L tan θ W - L tan θ where 2L is the length of the electrodes, W is their width and θ is the tilt angle in the horizontal plane, ie about the axis 26. Also, the voltages Va,, Vb,, and Vc, are used to derive a first signal T, representative of the translational displacement of the X-ray beam, in a horizontal plane, from the centre of the chamber. T, = Va, - Vc, Vb, where the denominator normalises the signal. It can be shown that T, changes from + 1 through 0 to -1 as the position of the beam moves from one long edge of the electrode through the centre to the other long edge. As a result of normalisation, the values of R, and T, are independent of the X-ray beam intensity and depend solely on the beam position. With the switch in the alternative position (i.e. the non-illustrated position in Figure 3) the voltages Va 2 Vb 2 and Vc 2 are used to derive a second signal R 2 representative of the rotational displacement (about the transverse horizontal axis 27) of the X-ray beam from the central axis. R 2 = Va, + Vc, - Vb, Va 2 + Vb 2 + Vc 2 R 2 is indicative of the rotation (about the horizontal axis 27) required to make the beam parallel to the central axis. It can be shown that R 2 = - L tan Φ W - L tan Φ where 2L is the length of the electrodes, W is their width and Φ is the tilt angle in the vertical plane, ie about the axis 27. Also, the voltages Va 2 Vb 2 and Vc 2 are used to derive a second signal T 2 representative of the translational displacement of the X-ray beam (in a vertical plane, from the centre of the chamber) . T 2 = Va, - Vc Vb 2 It can be shown that T 2 changes from + 1 through 0 to -1 as the position of the beam moves from one long edge of the electrode through the centre to the other long edge. As before, the values of R 2 and T, are independent of the X-ray beam intensity and depend solely on the beam position. The beam is positioned so as to be maintained in its aligned central position, the alignment of the beam being carried out by sequential adjustment in the horizontal and vertical planes until R, T,, R, and T, are all 0. This centring process can be carried out automatically by a central processing unit. The embodiment of Figure 1 is a compact arrangement in which the first and second series of electrodes are located at the same axial position along the direction of propagation of the X-ray beam. In the second embodiment of Figure 4, the first series of electrodes la, lb, lc, 3 are located at a different position along the direction of propagation of the X-ray beam from the second series of electrodes 5a, 5b, 5c, 7. In Figure 4, the X-ray beam passes first through the first series of electrodes and then through the second series of electrodes. The signal processing for the embodiment of Figure 4 is the same as for Figure 2, but it is not necessary to use a change-over switch 10 because the bias voltage 12 can be applied to both collection electrodes simultaneously and the signals R, T, R 2 and T 2 are thus obtainable simultaneously and continuously over a chosen period of time.

Claims: CLAIMS 1. An X-ray beam position monitor comprising a first electrode assembly for detecting the rotational position of the X-ray beam about an axis orthogonal to one plane, the first electrode assembly comprising a first series of three collection electrodes and a first biasing electrode, a second electrode assembly for detecting the rotational position of the X-ray beam about another axis orthogonal to another plane transverse to said one plane, the second electrode assembly comprising a second series of three collection electrodes and a second biasing electrode, means for applying a bias voltage to the biasing electrodes and signal processing means for processing electrical signals which are generated at the collection electrodes and deriving therefrom signals which are representative of the rotational position of the X-ray beam about said axes. 2. An X-ray beam position monitor according to claim 1, wherein the three collection electrodes of each series are constituted by an intermediate electrode in the shape of a parallelogram and two end electrodes each triangular in shape. 3. An X-ray beam position monitor according to claim 2, wherein the three collection electrodes of each series have in overall outline a substantially rectangular shape. 4. An X-ray beam position monitor according to claim 2 or 3, wherein for each series of electrodes the processing means are operative to sum the signals from the end electrodes and subtract therefrom the signal from the intermediate electrode, in order to derive a signal representative of the rotational position of the X-ray beam in said one or other plane. 5. An X-ray beam position monitor according to claim 4, wherein the signal representative of the rotational position of the X-ray beam in said one or other plane is normalised by dividing it by the sum of the signals from the three electrodes of the corresponding series. 6. An X-ray beam position monitor according to any of claims 2 to 5, wherein in addition to detecting the rotational position of the X-ray beam, the monitor also detects the translational position of the X-ray beam in said one plane, the signal processing means being additionally operative to obtain the difference between the signals from the two end electrodes of the first series, in order to derive a signal representative of the translational position of the X-ray beam in said one plane. 7. An X-ray beam position monitor according to claim 6, wherein the signal representative of the translational position of the X-ray beam in said one plane is normalised by dividing it by the signal from the intermediate electrode of the first series. 8. An X-ray beam position monitor according to any of claims 2 to 7, wherein in addition to detecting the rotational position of the X-ray beam, the monitor also detects the translational position of the X-ray beam in said another plane, the signal processing means being additionally operative to obtain the difference between the signals from the two end electrodes of the second series, in order to derive a signal representative of the translational position of the X-ray beam in said another plane. 9. An X-ray beam position monitor according to claim 8, wherein the signal representative of the translational position of the X-ray beam in said another plane is normalised by dividing it by the signal from the intermediate electrode of the second series. 10. An X-ray beam position monitor according to any of the preceding claims, wherein the first electrode assembly and the second electrode assembly are positioned at substantially the same axial position along the direction of propagation of the X-ray beam. 11. An X-ray beam position monitor according to claim 10, wherein the means for applying the bias voltage include switching means for applying the bias voltage to the first biasing electrode or the second biasing electrode. 12. An X-ray beam position monitor according to any of claims 1 to 9, wherein the first electrode assembly and the second electrode assembly are not at the same axial position along the direction of propagation of the X-ray beam. 13. An X-ray beam position monitor according to any of the preceding claims, wherein the electrodes of the first assembly are orthogonal to the electrodes of the second assembly, so that said one plane and said other plane are mutually orthogonal. 14. An X-ray beam monitor according to claims 10 and 13, wherein the first and second electrode assemblies constitute the four walls of a square-section tunnel-like structure through which the X-ray beam is propagated. 15. An X-ray beam position monitor according to any of the preceding claims, wherein the X-ray beam position sensor acts as a null-seeking device, the beam being centred by means of adjustments in the two planes of positioning, until the electrical signals are representative of a centred position of the X-ray beam.

Inventors: ARNDT ULRICH WOLFGANG, KYTE MARTIN PAUL, ARNDT, ULRICH, WOLFGANG, KYTE, MARTIN, PAUL
Applicants: ARNDT ULRICH WOLFGANG, KYTE MARTIN PAUL, MEDICAL RES COUNCIL, ARNDT, ULRICH, WOLFGANG, KYTE, MARTIN, PAUL, MEDICAL RESEARCH COUNCIL
Application year: 2000
Main classification: G01T1
IPC codes: G01T1