GB2295266A - X-ray generator - Google Patents

Abstract

An x-ray generator, e.g. for testing circuit boards includes an electron generator (13), means (21) for rastering an electron beam (30) produced by the electron generator, a target plate (24), and an end plate (15) opaque to x-rays, the end plate having, on a centre-line of the generator, an x-ray transparency (16) through which x-rays (31), generated by impingement of the electron beam on the target plate, can pass. The transparency may be an effective pinhole, an x-ray lens or a Fresnel Zone lens. An x-ray detector (27) inputs signals to a computer display (28) rastered in synchronism with the electron beam (30). An anode HT connector is described (Fig. 3) comprising two integral PTFE portions at right angles, each having a central bore containing conductors for connecting the inner core of a coaxial cable to the anode. The outer braid of the cable is connected to earth and sandwiched between two heat shrunk tubes. <IMAGE>

Description

X-RAY APPARATUS The present invention relates to x-ray apparatus.

X-rays are best known for their ability to penetrate certain materials, and are commonly used for medical and similar purposes where an x-ray generator on one side of a body transmits rays through the body to a detector on the other side. Differential transmission of the x-rays through various materials (for example flesh, bones) within the body results in a pattern which can be filmed, or otherwise prepared for viewing, to give information about the contents of the body.

X-ray systems are in general bulky and complicated, and x-rays are inherently damaging to the human body. In order to reduce the effect of these problems it has been suggested that a rastered beam of x-rays be used rather than a single overall exposure. Early rastered beam equipment uses mechanical rastering systems and are themselves bulky such that they are suitable only for non-mobile installations.

It has been suggested that the use of an electrically controlled rastering system, similar to that used in television sets, reduces both the unwieldiness of equipment and the danger of over-exposure.

Early rastering systems used detector systems similar to those used by the overall exposure systems. However suitable detector systems of the required size and capable of developing a suitable image from a rastered rather than from an overall exposure are not readily available. The use has therefore been suggested, for example in US Patents 3,949,229 and 4,057,745 of a small area detector which supplies a signal to an image developer having a rastered display slaved to the rastered source. In these Patents the x-rays are developed by rastering an electron beam over a target plate. X-rays are developed at the point where the beam meets the plate and are radiated in all directions. Only rays which pass in a straight line from the development point, through a specimen, to the detector, are recorded.

US Patent 4,519,092 describes a development of the equipment used in US Patents 3,949,229, and 4,057,745 where a collimator, having alternating transmissive zones and absorbent zones, is interposed between the target plate and the specimen. The described use of this arrangement is particularly concerned with a method whereby secondary x-rays caused by primary x-rays falling on a specimen specimen are be used to determine properties of that specimen. In this method the x-ray generator and the x-ray detector are both on the same side of the specimen. This form of x-ray dissemination is referred to herein as back-scatter. Depending on the nature and size of the specimen the transmissive zones in the collimator are aligned to form a radiation pattern which may be convergent or divergent. Thus different collimators have to be used for different specimens.Also the collimators are heavy and expensive and rely on short channels to define the annular dispersion of the beam, this dispersion decreasing with the length of the channels. A further disadvantage of collimators is that they leave blank patches, corresponding to the opaque parts of the collimator, in the image, resulting in a visible pattern on the image or a loss in image brightness.

The present invention provides an x-ray generator, suitable for back-scatter operation, which is sturdy, portable, and suitable for use in harsh environments, and which does not require different collimators for different specimens.

According to the present invention an x-ray generator includes an electron generator, means for rastering an electron beam produced by the electron generator, a target plate, and an end plate opaque to x-rays. the end plate having, on a centre-line of the generator. an x-ray transparency through which x-rays, generated by impingement of the electron beam on the target plate, can pass.

The annular dispersion of the x-ray beam with this arrangement decreases with the distance between the target plate and the x-ray transparency, which can easily be made much greater than the channel length of a practical collimator as used in the prior art.

The x-ray transparency might be, for example. an effective pinhole, (by which is meant a pinhole in x-ray opaque material such as tungsten which might be mechanically closed by an x-ray translucent material such as aluminium), an x-ray lens or a Fresnel Zone Plate.

The effective pinhole will, for convenience, be referred to simply as a pinhole.

The target plate is preferably positioned downstream of a generally cylindrical anode to which can be applied a high voltage in order to accelerate electrons in the electron beam.

The generator can conveniently be of cylindrical shape and may have a body shell of glass, of ceramic, or of metallic, for example aluminium, construction, the anode being insulated therefrom by an insulator constructed of an insulating material such as, preferably, polytetrafluoroethane (PTFE).

An electrical (HT) connection to the anode might be by means of a centre pin projecting at right angles thereto and carried by a connector body.

A preferred form of HT connector, constructed of insulating material such as PTFE is of unitary construction and has; a first portion of generally cylindrical shape with an external flange whereby it can be secured to the body shell, there being a first central bore in which the terminal rests; a second portion at right angles to the first portion, the second portion having a second central bore which meets the first central bore; the second central bore having a first length of a lesser diameter which carries a closed-headed nut having in its head a diametrical passage in which the terminal rides; the nut being secured in the second bore by a screw which fits a larger diameter second length of the second bore, the screw having a head with an internally chamfered edge which penetrates the material of the connector at the juncture of the first and second lengths;; the second portion being of cylindrical section at its juncture with the first portion and of low-angle frusto-conical section thereafter.

The construction of the body shell, insulator and HT connector is preferably such that, when assembled together, part of the HT connector passes through the body and lies within the insulator in plug and socket fashion, but with a small gap between insulator and connector.

The target plate may be attached to the anode by means of screws.

Some embodiments of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, of which: Figure 1 is a side elevation, partially in section and partly in false perspective, an x-ray generator according to the invention in use in a typical back-scatter mode, Figure 2 is a side elevation, in section and in detail, of a preferred form of x-ray generator according to the invention, Figure 3 is a side elevation, in section, of an HT connector as used in the x-ray generator of Figure 1, Figure 4 is a side elevation, in section, of a detail of the connection between the HT connector and a main body of the x-ray generator, Figure 5 is a side elevation, in section along line I-I of Figure 7, of first and second portions of the HT connector body.

Figure 6 is an end elevation in the direction of arrow A in Figure 5, Figure 7 is a plan view in the direction of arrow B in Figure 5.

Figure 8 is a plan view of a split washer used in the connection of the HT connector to the x-ray generator, Figure 9 is an end view in the direction of arrow C in Figure 8, Figure 10 is a side elevation of a centre pin, Figure 11 is a plan view in the direction of arrow D in Figure 10, Figure 12 is a side elevation of a nut, Figure 13 is an end view in the direction of arrow E in Figure 12, Figure 14 is a plan view in the direction of arrow F in Figure 12, Figure 15 is a plan view of sealing screw, Figure 16 is a side elevation, in section along line II-II of Figure 15, Figure 17 is an end view of a terminal, Figure 18 is a side view, in section along line III-III of Figure 17.

Figure 19 is an exploded view, in perspective, of an end plate assembly, and Figure 20 is an exploded elevation, in section, of the end plate assembly of Figure 20.

An x-ray generator according to the invention (Figure 1) has a body 10 consisting of a casing 11 closed at both ends. At a first end 12 is an electron generator 13 and at a second end 14 is an end plate assembly 15 opaque to x-rays apart from an effective pinhole 16.

Within the casing 11 is an anode 17 consisting of two co-axial cylinders 18, 19 joined together in series and mounted in an an insulator 20. Positioned adjacent the smaller diameter cylinder 18, which is closest the electron generator 13, are firstly a focusing coil 120 and secondly deflection coils 21. The deflection coils might most conveniently consist of two pairs of coils mutually at right angles of the type commonly used in, for example, television sets and will not be described further here.

Positioned at an end 22 of the larger diameter cylinder 19 remote from the electron generator 13 is a target screen 23 held in place by a target retaining plate 24 (items 23 and 24 are illustrated in perspective).

A vacuum pump connection 130 is positioned in the casing 11.

In use a vacuum pump is connected to the connection 130 and the interior of the body evacuated. Low tension power 122 is supplied to the electron generator 13, focusing coil 120 and deflection coils 21.

A high tension power supply 25 is connected to the anode 17. The x-ray generator 10 is positioned adjacent a specimen 26 be x-rayed and an x-ray detector 27 is positioned to detect x-rays back scattered from the specimen 26. The distance between the specimen 26 and generator 10 will depend on the dimensions of the specimen and of the generator- for example the distance between the target plate 23 and the effective pinhole 16. A computer 28 having a screen 29 is operatively connected as required to the various components of the x-ray generator 10 and receives signals from the detector 27.

Under control of the computer 28 an electron beam 30 is ejected, in either continuous or pulsed mode, by the electron generator 13 and the component electrons are accelerated towards the anode 17, being focussed by the focusing coil 120. The deflection coils 21 are operated in well known fashion to raster the focussed electron beam 30 over the target plate 23. On the opposite side of the target plate 23 to the beam 30 x-rays are generated at the point where the beam impinges on the plate. A beam of x-rays 31 passes, in rastered fashion, through the pinhole 16 and falls on the specimen 26. X-rays 123, which might be primary or secondary, are back-scattered from the specimen 26 and detected by the detector 27 which sends a signal to the computer 28.The computer 28 processes the signal and activates the screen 29 in rastered fashion corresponding to the rastering action of the deflection coils 21. Analysis of the picture on the screen 29 is conducted in the usual fashion.

In practice some design details of the x-ray generator have given problems. An embodiment of the generator 10 will therefore now be described in more detail with reference to Figures 2 to 20.

The main body of the generator 10 is shown in Figure 2 where like numbering is used, where appropriate, to that in Figure 1.

The construction of an HT connector 39 in an apparatus designed to be portable and rugged has proved troublesome. A rugged connection offering considerable resistance to arcing, illustrated in Figures 5, 6 and 7 is constructed of insulating material such as PTFE, is of unitary construction and has a first portion 40 of generally cylindrical shape with an external flange 41. A second portion 42 integral with and at right angles to the first portion 40 has a cylindrical length 43 extending from the junction and a frusto-conical length 44 extending to a termination 45. A first two stage central bore 46 in the first portion 40 meets a second central bore 47 in the second portion 42. The second central bore has a first length 48 of a lesser diameter and a second length 49 of a larger diameter.Three holes 124 connecting the bore 49 to the exterior surface of the frusto-conical portion 42 are positioned adjacent the termination 45.

The HT connector is prepared for attachment to the anode 17 and casing 11 as follows (see Figure 3). A nut 50 (Figures 12, 13 and 14) has an axial threaded bore 51 and a head 52 in which is a diametrical passage 53. The nut 50 is inserted into the first length 48 of the second central bore 47 with the diametrical passage 53 aligned with the first central bore 46.

A centre pin 54 (Figures 10 and 11) is of two stage cylindrical construction having at a larger diameter end 55 a central hollow 56 with six radial slots 57. The centre pin 54 is inserted into the first central bore 46 so that their two stages coincide and with a smaller diameter end 58 protruding through the diametrical passage 53 in the nut 50.

A screw 59 (Figures 15, 16) has a head 60 which is a sliding fit in the second length 49 of the second central bore 47. The head 60 has an internally chamfered edge 61. The screw 59 is inserted into the second length 49 and screwed into the nut 54 so that the chamfered edge 61 cuts into the material of the connector 39 at the junction of the two lengths 48. 49 of the second central bore 47.

A spring 62 is then inserted into the second length 49 followed by a terminal 63 (Figures 17, 18) which is a sliding fit.

A HT cable 70, of the usual form having a conductor 64, an inner core 65, braid 66 and outer core 67 has the outer core pared off to a length slightly greater than the length required for the inner core and conductor to reach the terminal 63. The braid 66 is laid back and the inner core 65 and conductor 64 introduced into the second central bore 47 until the conductor is brought into firm contact with the terminal 63 forcing the terminal up the bore against the resistance of the spring 62. A first layer of heat shrink tubing 68 is then laid over the outer surface of the second portion 42, extending over the length of the frusto-conical length 44 and a substantial length of the cylindrical length 43 and over a length of the inner core 65 beyond the termination 45. The braid 66 is then laid over the layer 68, with an earth connection 131 extending therefrom, and a second layer of heat shrink tubing 69 laid over the first.

The casing 1; of the x-ray generator body 10 has an aperture 71 (see particularly Figure 4) adapted to allow passage of the first portion 40 of the HT connector 39. Co-axial with this aperture is a recess 72 in the insulator 20, the diameter and depth of the recess being slightly greater than required for entry of the end of the first portion 40. Holes 73 in the insulator 20 and 74 in the anode 17 are co-axial with the aperture 71 and recess 72.

The HT connector 39 is secured to the casing 11 by studs and nuts (not shown) passing through a pair of split washers 75 (of which one is shown in Figures 8, 9), through the flange 41 and into threaded bores (not shown) in the casing 11 until the flange 41 is in hard contact with the casing 11. The length of the centre pin 54 is such that it passes through the hole 73 into the hole 74, which has dimensions such that, with the springiness induced in the end of the centre pin 54 by the hole 56 and radial slots 57, ensures tight contact. In this position there is a slight gap 175 between the insulating materials of the insulator 17 and the HT connector 39. It has been found that this gap, when evacuated, greatly reduces electrical tracking which has on occasion been found to occur in use when the surfaces of these items are in contact.During the securing process the earthing extension 131 of the braid 66 is connected to one of the studs.

The target plate 23 is secured to the anode 20 by a target retaining plate 24 (Figure 2).

In one form of end plate 15 (Figures 19, 20) two tungsten plates 81, 82, which are opaque to x-rays, are sandwiched between three aluminium plates 83, 84 and 85, lying in recesses 86, 87. The innermost aluminium plate 83 has a central recess 88 sealed by a thin layer of aluminium 89 which is substantially transparent to x-rays.

The tungsten plate 81 which fits in the recess 86 in plate 83 has, at its centre, the pinhole 16. The aluminium plates 84 and 85, and the second tungsten plate 82 have, respecively, holes 90, 91 and 92 so dimensioned as not to interfere with x-rays generated by the target plate 23 which have passed through the pinhole 16.

Means are provided, for example by a hole (not shown) in the casing 11, for evacuating the inside of the body 10. It is important in the design that the gap 75 should be connected by an air passage to the inside of the body 10.

In general, where not mentioned, suitable materials in the design of x-ray generators according to the invention are aluminium or the like for body items such as cylinder 11 and plates 81, 82,83; brass or the like for electrical conducting items (copper for the cable conductor 64) and PTFE or the like for insulating components.

A suitable target plate 23 might consist of a light metal plate of aluminium or titanium, about lmm thick, coated with a thin (microns) layer of tungsten on which the electron beam 30 is rastered.

The tungsten is a good x-ray generator but a bad transmitter, whilst the light metals suggested provide mechanical strength and are good x-ray transmitters.

In the back-scatter mode the x-ray generator can be used, for example, to test electronic circuit boards or nominally constant thickness layers of material, or for testing specimens which are x-ray translucent only to a certain depth.

X-ray generators according to the invention can also, of course, be used in the normal way with a generator and detector on opposite sides of a specimen.

It will be realised, of course, that x-ray transparencies other than pinholes might be used in the invention, such as the various forms of x-ray lenses and Fresnel Zone lenses which have been suggested for use with x-rays.

It will also be realised that many variations in the above described embodiment are possible within the scope of the invention.

For example many constructional details, such as the means of attaching items together, may be varied according to normal engineering practice.

Claims (12)

1. What is claimed is: 1. An x-ray generator including an electron generator, means for rastering an electron beam produced by the electron generator, a target plate, and an end plate opaque to x-rays, the end plate having, on a centre-line of the generator, an x-ray transparency through which x-rays, generated by impingement of the electron beam on the target plate, can pass.
2. 2. An x-ray generator as claimed in Claim 1 wherein the x-ray transparency is an effective pinhole.
3. 3. An x-ray generator as claimed in Claim 1 wherein the x-ray transparency is an x-ray lens.
4. 4. An x-ray generator as claimed in Claim 1 wherein the x-ray transparency is a Fresnel Zone lens.
5. 5. An x-ray generator as claimed in any one of Claims 1 to 4 wherein the target plate is positioned downstream of a generally cylindrical anode to which can be applied a high voltage in order to accelerate electrons in the electron beam.
6. 6. An x-ray generator as claimed in in Claim 5 wherein the generator is of cylindrical shape and has a body shell of metallic, for example aluminium, construction.
7. 7. An x-ray generator as claimed in Claim 6 wherein the anode is insulated from the body shell by an insulator constructed of an insulating material such as polytetrafluoroethane (PTFE).
8. 8. An x-ray detector as claimed in any one of Claims 1 to 7 wherein an electrical (HT) connection to the anode is made by means of a centre pin projecting at right angles thereto and carried by a connector body.
9. 9. An x-ray detector as claimed in any one of Claims 1 to 8 including a HT connector constructed of insulating material such as PTFE of unitary construction and having; a first portion of generally cylindrical shape with an external flange whereby it can be secured to the body shell, there being a first central bore in which the terminal rests; a second portion at right angles to the first portion, the second portion having a second central bore which meets the first central bore; the second central bore having a first length of a lesser diameter which carries a closed-headed nut having in its head a diametrical passage in which the terminal rides;; the nut being secured in the second bore by a screw which fits a larger diameter second length of the second bore, the screw having a head with an internally chamfered edge which penetrates the material of the connector at the juncture of the first and second lengths; the second portion being of cylindrical section at its juncture with the first portion and of low-angle frusto-conical section thereafter.
10. 10. An x-ray generator as claimed in Claim 9 wherein the construction of the body shell, insulator and HT connector is such that, when assembled together, part of the HT connector passes through the body and lies within the insulator in plug and socket fashion, but with a small gap between insulator and connector.
11. 11. An x-ray generator substantially as herein described with reference to Figures 1 to 20 of the accompanying drawings.
12. 12. An x-ray generator substantially as herein described.