US 3296476 A
Description (OCR text may contain errors)
. 1967 ERNST-GUNTER HOFMANN 3,295,476
X-RAY TUBE Filed Oct. 30, 1962 4 Sheets-Sheet 1 F'I' a 9 A PRIOR ART i PRIOR ART Fig.3
PRIOR ART Jnrenzor ERNST-GCJNTER HOMANN sv um 74 1/ ATTORNEYS Jan. 3, 1967 Filed 001;. 50 1962 ERNST-GUNTER HOFMANN X-RAY TUBE 4 SheetsSheet 2 jfc a HHH41HH PRIOR ART Fig.5
Jnrentar- ERNST-GCINTER HOFMANN ATTORNEYS Jan. 3, 1967 ERNST-GUNTER HOFMANNQ 3,296,476
X-HAY TUBE 4 Sheets-Sheet 4. Fig. 7::
Filed Oct. 30, 1962 mumxmm,
Jnrentar- ERNsT-ebNTER HOT-MANN Y: W 1 B J ATTORNEYS United States Patent 40, 1 13 Claims. ((1313-55) The present invention relates to X-ray tubes, particularly to high power X-ray tubes.
One of the requirements made of X-ray equipment is that it deliver, in continuous operation, as much useful radiation as possible. Furthermore, the apparatus should deliver as high a dosage as possible and should provide suitable spatial distribution. For economic reasons, the apparatus should operate at the highest possible efi'iciency, i.e., the conversion from electron to X-ray energy should be as efficient as possible. This means that the apparatus should make use of as large an optimal angle as possible, and also, that the conversion of electron energy to X- ray energy be effected as efiiciently as possible by appro priately selecting the electron retarding material of which the anode is made, the tube voltage, and the type of voltage.
The Roentgen or X-rays which are produced are emitted over a solid angle of 411-; if the anode is considered as being infinitely thin, the spatial intensity distribution is dependent on the acceleration voltage of the electrons. If the finite thickness of the anode and the tube wall is. considered, the spatial distribution is additionally dependent on the absorption by the anode. The solid angle is particularly well utilized when the anode is a membrane through which the radiation penetrates, in which case a solid angle of almost 2.1 is utilized in a direction in which maximum intensity is desired. The membrane anode serves simultaneously as a tube wall. The self absorption must therefore be kept small, while giving due consideration to maintaining sufficient mechanical stability. The membrane-type anode can therefore be made only of a metal of low or medium atomic number. In order to increase the efiiciency, a layer of heavy metal is applied on the vacuum side, the thickness of which layer will be approximately equal to the maximum reach of the electrons. The radiation penetrating through the anode will be utilized directly on the outside of the anode, or, if a cooling system is provided, directly on the outside of such cooling system through which the radiation will also pass. In either case, the radiation is used relatively near the place at which the radiation is generated. In the case of point-shaped radiation sources, and neglecting the absorption in the radiation chamber, the dosage would drop as the square of the distance from the source of radiation. If, however, the focal point is large with respect to the effective distance, the radiation field exteriorly of the anode will not follow the inverse square law. (The term focal point, as hereinafter used in the specification and claims, is intended to refer to the surface area upon which the electrons impinge, i.e., the word point is not meant in its strict geometric sense but is intended to include an area having larger than negligible dimensions.) Instead, the dosage power will, up to distances which correspond approximately to the linear dimensions of the focal point, decrease less than quadratically. In contradistinction to conventional X-ray tubes, in which the effective distance is always large with respect to the dimensions of the focal point, the dosage power, in the case of focal points of large areas, which are generally used in tubes of this type, will be dependent upon the specific loading of the focal 3,296,476 Patented Jan. 3, 1967 ICC point. (The term specific loading, as hereinafter used, represents the power per unit area, e.g., watts per square centimeter.) Consequently, the specific loading of the focal point will be sought to be made as high as possible.
In order to facilitate an explanation of the present invention, and its significance in view of the prior art, reference will now be made to the accompanying drawings in which:
FIGURE 1 is a cross section of a prior art X-ray tube.
FIGURE 2 is a cross section of another prior are X- ray tube.
FIGURE 3 i a cros section of yet another prior art X-ray tube.
FIGURE 4 shows the dosage field produced by an X-ray tube according to the prior art.
FIGURE 5 shows the dosage field produced by an X-ray tube according to the present invention.
FIGURE 6a is a cross section of an X-ray tube according to the present invention.
FIGURE 6b shows the configuration of the focal point produced on the anode of the X-ray tube illustrated in FIGURE 6a.
FIGURE 7a is a cross section of another X-ray tube according to the present invention.
FIGURE 7b shows the configuration of the focal point produced on the anode of the X-ray tube illustrated in FIGURE 7a.
FIGURE 8a is a cross section of yet another X-ray tube according to the present invention.
FIGURE 8b shows the configuration of the focal point produced on the anode of the X-ray tube illustrated in FIGURE 8a.
There exist various high power X-ray tubes which are based on the above-explained principle and in which the heat losses are carried away by a suitable cooling system associated with the membrane anode. In such tubes, the anode as well as the cooling system will be penetrated by the radiation. Different types of anode forms have been used or suggested. Thus, it is possible, for example, to provide a water-cooled large area membrane anode A, which is irradiated homogeneously by radiation a producing a large area focal point, with a single cooling channel C, as shown in FIGURE 1, to produce a usable cone of radiation indicated at b. Alternatively, a large area membrane anode can be irradiated with rays a to produce sub-divided focal points A and be water-cooled with individual channels C. This will result in individual cones b, as shown in FIGURE 2. In this case, the individual focal points will be loaded evenly and be spaced equidistantly apart. Each individually cooling channel is associated with one band-shaped focal point The anodes according to the embodiments of FIG- URES l and 2 are preferably made of light metal. If the anode is made of a material such as a metal of middle atomic number, as, for example, stainless steel or nickel, it is possible to use water-cooled large area membrane anodes with sub-divided focal points and lenticular cooling channel C", as shown in FIGURE 3. Each'lenticular anode portion A" is bombarded with electrons a" emanating from a respective linear heating filament (not shown in FIGURE 3), these electrons being focussed, by means of a suitable focussing system, such that here, too, a homogeneous band-shaped focal point will be produced in the middle of each anode portion. The individual anode portions A are spaced equidistantly from each other. The usable cones of rays are indicated at b".
All of the above-described embodiments have in common that there is either a focal point having a uniform specific loading or that there are a plurality of focal points, of the same size, having uniform specific loadings, the individual focal points being spaced equidistantly from each other. In the embodiments of FIG- URES 2 and 3, a focal point of uniform Specific loading is reproduced and a dosage field is built up on the outside of the anode which is similar to the field obtained with the embodiment of FIGURE 1 operating with a large area homogeneous focal point. This is shown in FIG- URE 4 wherein the isodosage lines are shown at c.
The drawback of this dosage distribution is that, with respect to planes that are parallel to the anode surfaces, the dosage power decreases sharply on the outside. As a result, articles which are transported past the anode will receive less radiation on their sides than in their midortions.
It is known to correct the radiation field of X-ray tubes by means of suitably shaped absorption filters, so that there will be formed, in planes vertical to the center beam a limited homogeneous fields of constant dosage power. The use of absorption filters, however, involves the loss of usable radiation, which, of course, results in reduced efficiency.
It is, therefore, an object of the present invention to overcome the above disadvantages, and this object is accomplished in the following manner: without using any means which reduce the efficiency of the tube and by suitably shaping and distributing the focal point and/or by suitably selecting the electron beam density, there is produced, in the working space or at least in parts thereof, from the X-rays coming from the anode, a radiation field extending in one or two dimensions and having approximately rectilinear and mutually parallel isodosages, which radiation field has dimensions that correspond approximately to the dimensions, in the respective directions, of the focal point. This result can be achieved in the case of anodes which are penetrated by radiation regardless of the shape of such anodes, the present invention, however, being particularly applicable to planar anodes having large surface areas. In such tubes the focal point is flat and it is the primary object of the present invention to produce a dosage field whose lines of isodosages below the region of the focal area, are parallel to the surface of the anode. According to the present invention, the radiation field correction is effected primarily in the case of square, rectangular and circular focal areas. In principle, however, it is also possible with differently shaped focal areas. In general, it will be sought to correct the radiation field in two dimensions, although in some cases a unidimensional correction will suffice.
In the case of a coherent, i.e., undivided, large area focal point, the desired effect can be achieved as follows: in contradistinction to hitherto conventional techniques, by which it is sought to obtain a constant current density of the electrons over the entire focal point, the current density of the electrons is such that it increases at the edges of the focal point. This can be done, for example, by using electrostatic or magnetic focussing means which cause the electrons emitted by the cathode to be radiated toward the anode with a current density which increases from the inside to the outside. This is shown in FIG- URE 5, in which the current density is represented by the spacing between the arrows symbolizing the electrons. As is apparent from FIGURE 5, there will be a region below the focal points in which there is an isodosage field parallel to the anode.
Alternatively, the desired effect can be obtained by suitably shaping and sub-dividing the large focal area into individual focal points and/or by suitably selecting the electron beam density of the individual focal points. In this way, a coherent large area focal point is derived from the individual focal points having the aforementioned characteristics. This is frequently of advantage because the cathode and/or anode systems of the tube can then be made as simple as possible.
It is possible, in the above-described manner, to obtain large circular focal points by providing a spiral focal band of constant width and surface load, the distance between turns of the spiral decreasing the further the band is from the center of the spiral. Such a focal point can be produced by a cathode system having a spiral heating filament which, throughout its length, is equidistant from the anode surface and whose turns are spaced increasingly closer together, which filament is incorporated in a suitably fashioned focussing system.
In order to form square or rectangular focal points, linear focal bands, produced by cathode systems having linear heating filaments, can be used.
A unidimensional radiation field correction can be obtained in various ways. If the system uses parallel bandshaped focal points of equal lengths, the desired result can be obtained under the following conditions:
(1) If the individual focal points are of the same width and are spaced equidistantly apart, the surface loading of the individual focal points is kept constant for each such focal point, while the absolute value of the area load increases toward the outside.
(2) If the individual focal points are of the same width and have the same surface loading, the distance between individual focal points is made smaller at the outside.
(3) If the individual focal points have the same surf-ace loading and are spaced equidistantly apart, the width of the individual focal points is made to increase toward the outside.
In practice, the arrangement defined in (2) is considered to be the most preferable because it makes use of heating filaments which have the same cross section and which are heated to the same extent, as a result of which all filaments will have approximately the same useful life. Moreover, the focussing system can be kept quite simple.
If a unidimensional radiation field correction is carried out, a field is produced which, in a median plane at right angles to the anode and at right angles to the longitudinal axis of the focal point strips, will have the configuration shown in FIGURE 5, and, in a median plane at right angles to the anode and parallel to the longitudinal axis of the focal point strips, will have the configuration shown in FIGURE 4. In order to put to use the advantages of these features, it is generally necessary that the material subjected to the radiation be transported, such as by a conveyor belt, through the radiation field in a direction which is parallel to the anode surface and which extends in a direction coextensive with the longitudinal axis of the focal point strips. In this way, it is possible to make optimal use of the radiation field and to subject the material being irradiated, if such material is of relatively large width, with uniform dosages in parallel layers. If the material is irradiated on two sides, it is possible, if the material is of appropriate thickness, to obtain a substantially homogeneous radiation effect. The material is turned over after one pass through the radiation field and is thereafter again fed through the field. Alternatively, two tubes according to the present invention may be used, which tubes are arranged opposite each other and be tween which the material passes but once. Such an arrangement is referred to as the tandem system.
The present invention can be refined further by providing an arrangement which eliminates those disadvantages which still remain if the radiation field is corrected in but one dimension. In the case of rectangular or square focal points, this can be done by forming the overall focal point by using two identical cathode means each comprising the spaced linear filaments which produce the strip-shaped individual focal points and meet the aboveenumerated criteria (1), (2), and/or (3). These cathode means are arranged mutually perpendicular to each other so as to form a raster-like over-all focal point. Such a cathode arrangement may comprise two sets of parallel heating filaments which are built into a focussing system and are spaced closely together. Alternatively, the cathode arrangement may be a mesh-type cathode member, in which case the raster need not necessarily be a right-angle raster in that other suitably configured raster-like focal points will produce the desired result.
In general, it will be found preferable to make use of the two-dimensional radiation field correction-feature, in which case it is not absolutely essential that the material being irradiated be passed through the radiation field in any particular direction in order to produce, in layers parallel to the anode surface, an approximately uniform radiation effect.
Nor is it essential that the two-dimensionally compensated radiation field be rectangular, instead, the focal point can be circular.
As in all high-power X-ray tubes having large anode areas which are penetrated by the radiation, it will be sought to make the specific loading as high as possible, as, for example, up to several 1000 watt per square centimeter. Therefore, the anodes will have to be suitably cooled, in which case a coolant such as distilled or demineralized water will be circulated through cooling means associated with the anode, which coolant after being heated up by the anode, will be cooled down by a suitable heat exchanger which itself is fed from an appropriate external water supply. The cooling means may be in theforrn of appropriately positioned cooling channels which may include one or more walls spaced from the anode and form therewith one or more spaces or channels through which the coolant is circulated.
In order to increase the efficiency of the energy conversion with which the electrons are changed into X-ray energy, as well as the yield of the tube, the anode may comprise a carrier which itself is of adequate mechanical strength and which absorbs little energy, and a layer of heavy metal on that side of the carrier which faces the anode. The carrier will be suitably thin and/ or be made of a light metal.
X-ray tubes of the type contemplated by the present invention will, in practice, be operated with anode volt ages up to 400,000 volts and have anode powers of between 50,000 and 500,000 watts. If desired, however, the tubes can be operated at even higher values, particularly at higher power ratings.
Referring once again to the drawing, FIGURE 6a shows a tube according to the present invention, while FIGURE 6b shows the configuration of the focal point produced thereby, the latter being generally circular and made up of a spiral whose turns are progressively closer together from the inside out, the width of the strip making up the spiral and the specific loading of the spiral being constant, thereby producing a two-dimensionally compensated radiation field whose lines of isodosage lie in planes parallel to the focal point on the anode. The X-ray tube itself comprises an envelope 1 on which is mounted an insulating column 2 having a head 3 through which pass the current and voltage leads 4. The cathode 5 is connected to a mounting 6 which is concentric with the column 2 and which also serves as a current lead. The cathode is located opposite an anode 7 which is bombarded with electrons and through which passes the radiation. The cathode comprises a spirally configured heating filament 9 which lies in a plane that is in parallel spaced relationship with the surface of the anode and which has the configuration of the spiral shown in FIGURE 6b, i.e., the turns of the spiral filament 9 are progressively closer together from the inside out.
The anode 7 has two spaced walls forming between themselves an interspace 8 through which the coolant, such as water, is circulated.
The X-ray tube shown in FIGURE 7a includes a large area liquid-cooled anode and a cathode which produces a series of strip-shaped and mutually parallel individual focal points of the same width and specific loading, as shown in FIGURE 7b. The individual strips are spaced progressively closer together from the inside out, so as to produce, in planes at right angles to the surfaces of the anode and to the length of the individual strips (i.e.,
in the plane in which FIGURE 7a is drawn), an approximately rectilinear isodosage field parallel to the surface of the anode. The cathode 10 is attached to the head 3 in the manner shown in FIGURE 6a. The individual strip-shaped focal points are produced by means of parallel heating filaments 11 which are carried by a suitable focussing member and lie in a plane which is in parallel spaced relationship with respect to the anode surface. The anode 12 is provided with a plurality of individual cooling channels 13 arranged in alignment with the respective strip-shaped focal points. In tubes of this type, the material being irradiated has to be passed through the radiation field parallel to the outside of the anode and in the direction of the coolant channels 13, if the material is to receive approximately homogeneous radiation in layers parallel to the surface of the anode.
FIGURE 8a shows an X-ray tube having a cathode 14 provided with a mesh-type heating grid 15 to produce the raster-like focal point shown in FIGURE 8b. Such a cathode produces a two-dimensionally compensated coherent focal point made up, in effect, of a superposition of two focal points as shown in FIGURE 7b, which focal points, however, are positioned at right angles to each other. Thus, there will be in each direction a series of strips of the same width and of the same specific loading, the distance between the strips progressively decreasing from the inside out. The focal points shown in FIGURE 812 will thus produce a radiation field whose lines of isodosage lie in planes that are parallel to the anode surface.
It will be seen from the above that an X-ray tube according to the present invention comprises, in essence, an anode having a large surface area, and a cathode for producing electrons which impinge upon a focal point on the area of the anode to produce X-rays which penetrate the anode, the anode and cathode being arranged to produce a radiation field which has substantially parallel lines of isodosage and which, in the direction of the lines, is of approximately the same size as the focal point. More particularly, the anode and cathode are configured to produce an over-all focal point which is more concentrated on the outside than on the inside, this being achieved by considering the over-all focal point as being composed of a plurality of individual focal points with those at the peripheral region being stronger, in the sense of having greater specific loadings, or being spaced more closely together, or being larger, or any combination of these factors, than those at the central region of the over-all focal point.
It will be understood that the above description of the present invention is susceptible to various modifications, changes, and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.
What is claimed is:
1. A high power X-ray tube including means for producing an X-ray radiation field which has lines of isodosage that are substantially parallel to each other, said means comprising, in combination: an anode having a large surface area defining a focal point; and a cathode including a plurality of electron-emitting elements which are spaced more closely together at the edges of said cathode than at the center thereof for producing an electron beam which impinges on said focal point and which has a current density which increases progressively from its central region to its periphery.
2. An X-ray tube as defined in claim 1 wherein said surface area of said anode is flat, and wherein said lines of isodosage extend parallel to said focal point.
3. An X-ray tube as defined in claim 2 wherein said focal point is rectangular, and wherein said lines of isodosage are approximately rectilinear and extend parallel to each other in but one dimension.
4. An X-ray tube as defined in claim 1 wherein said electron-emitting elements are arranged for producing a coherent beam at said focal point, which beam has a greater current density in the region near its edges than in its central region.
5. An X-ray tube as defined in claim 1 wherein said surface area of said anode is planar and said electronemitting elements produce a beam which has a greater current density, in at least one direction parallel to the surface of said anode, at the edges of said focal point than at the central region thereof, thereby to produce an X-ray radiation field which has lines of isodosage that are parallel to said focal point in at least one direction.
6. An X-ray tube as defined in claim 5 wherein said emitting elements produce a coherent electron beam composed of a plurality of individual parallel and equally long strip-shaped beams at said focal point.
7. An X-ray tube as defined in claim 1 wherein said surface area of said anode is planar and said electronemitting elements produce a coherent electron beam which, at said focal point, has a greater current density at its periphery than in the central region thereof in at least two directions parallel to said surface area, thereby to produce an X-ray radiation field having lines of isodosage that lie in planes parallel to said focal point.
8. An X-ray tube as defined in claim 7 wherein said focal point is rectangular.
9. An X-ray tube as defined in claim 7 wherein said cathode includes two intersecting cathode means each forming parallel strip-shaped focal points.
10. An X-ray tube as defined in claim 9 wherein said cathode means are oriented mutually perpendicular to each other.
11. An X-ray tube as defined in claim 10 wherein said cathode means are arranged on a focussing member and are spaced close to each other.
12. An X-ray tube as defined in claim 10 wherein said cathode means are constituted by a mesh-type cathode member.
13. A high-power X-ray tube including means for producing an X-ray radiation field having lines of isodosage which are substantially parallel to each other, said means comprising, in combination: an anode having a large surface area defining a focal point; and a substantially flat cathode including a plurality of spaced electron-emitting elements, some of which elements are disposed adjacent the edges of said cathode and others of which elements are disposed near the center thereof, for producing an electron beam which, in the region adjacent said cathode, has a greater current density in the vicinity of the edges of said cathode than in the vicinity of the center thereof.
References Cited by the Examiner UNITED STATES PATENTS 1,289,672 12/1918 Coolidge 313-57 X 1,510,780 10/1924 Hollnagel 3l357 X 1,622,149 3/1927 John 313-57 X 1,645,304 10/ 1927 Slephian 313 X 1,977,541 10/1934 Bouwers 3l357 2,019,600 11/1935 Ehrke 31357 2,108,573 2/1938 Alfter 31357 2,482,275 9/ 1949 Horsley 313344 X 2,896,105 7/1959 Hosemann 31355 2,905,841 9/1959 Meyer et al. 31355 2,921,214 1/1960 Broad 31355 X 2,922,060 1/1960 Rajewsky 31355 X JAMES W. LAWRENCE, Primary Examiner.
R. SEGAL, P. C. DEMEO, Assistant Examiners.