Thin Target X-ray Tube With Means For Protecting The Target

Abstract

In X-ray tubes having thin targets such as foils, a problem is encountered in the deterioration of the foils, due to heat caused by bombardment by an electron beam. The invention avoids this deterioration in accordance with the one embodiment by providing water-cooled blocks in contact with the target and provided with openings through which the electron beam and/or emitted X-rays can pass and in accordance with another embodiment by providing an antioxidizing coating on the target.

Description

DRAWING

FIG. 1 diagrammatically illustrates in cross section a part of a preferred embodiment of the present invention;

FIG. 2 diagrammatically illustrates in cross section the target section of the embodiment of FIG. 1;

FIG. 3 is a cross section taken along line III--III in FIG. 2;

FIG. 4 diagrammatically illustrates in cross section a second embodiment of the present invention;

FIG. 5 is a cross section taken along line V--V in FIG. 4;

FIG. 6 diagrammatically illustrates in cross section another modified form of the X-ray tube of the present invention;

FIG. 7 diagrammatically illustrates in cross section, on enlarged scale, the target portion of FIG. 6;

FIG. 8 is a cross section taken along line VIII--VIII in FIG. 7;

FIG. 9 is a schematic illustration of the principle of the present invention;

FIG. 10 is a diagram showing a heat radiation curve used to describe the present invention; and

FIG. 11 shows an enlarged section of the target portion of another embodiment of the present invention.

DETAILED DESCRIPTION

This invention generally relates to X-ray tube devices and more particularly to cooling systems for thin target X-ray tube devices.

In various types of X-ray measurement or observation equipment including X-ray microscopes and X-ray diffraction devices, and particularly for research on X-ray diffraction figures by the Kossel-pattern technique or in the microscopic photographing of X-ray figures, an X-ray tube having a focal point of sufficiently small size and capable of generating a strong X-ray beam is required. In such an X-ray tube, a minute focal point of the ray having a size of, for example, several tens of microns is formed on a thin metal target having a thickness of, for example, several tens of microns.

In such an X-ray tube, the focal point area on the thin target is heated locally to a high temperature and consequently is fused. Therefore, an electron current higher than about 100 .mu.A. cannot pass. Even when the back surface of a thin target is cooled directly by passing cooling water thereover, the cooling water boils at the back side of the focal point on the back surface of the target, undesirably resulting in the formation and building up of "fur."

A primary object of the present invention is to provide means for effectively cooling a thin target of an X-ray tube and avoiding the above problems.

Another object of the present invention is to provide a water cooling system for cooling the thin target of an X-ray tube, which system is simple in construction but excellent in effect.

Still another object of the present invention is to provide an X-ray tube which is capable of generating strong X-rays to be passed through a minute focal point formed on a thin foillike target.

It is a further object of the present invention to provide an X-ray tube which is arranged to prevent a minute focal point on a thin foillike target from oxidizing due to heating.

The present invention will next be described in detail with reference to the preferred embodiments illustrated in the accompanying drawings.

In FIG. 1, an elongated metal tube 1 provided at one end thereof with a Wehnelt electrode 2 and a filament 3 is grounded. A negative high voltage is applied to the electrode 2 and the filament 3, and an electron-beam-focusing coil 4 is fitted over and encircles the tube 1. The other end of the tube 1 is provided with a pair of cooling water supplying members and a target foil as will be explained with reference to FIG. 2.

More particularly, the other end of the tube 1 is provided with a suitable target foil 7 of a metal such as copper, aluminum or gold interposed between a pair of cooling-water-supplying discs or blocks 5 and 6 fitted into the tube 1. The target foil 7 has a thickness within the range from several tens of microns to several hundreds of microns.

The discs 5 and 6 are formed with central conical openings 8 and 9, respectively, the diameter 2R of the openings at their apices being formed to provide the desired diameter for the focal point. A pair of cooling water passages 10 and 11 are formed in the discs 5 and 6, respectively, through the portions of discs 5 and 6 nearest to the apices of the conical openings 8 and 9. The passages 10 and 11 are shaped in discs 5 and 6 to form halves of a complete circle and the target foil 7 is centrally disposed to pass through the center of the circle. In other words, the foil 7 divides the passage into two halves. The passages 10 and 11 are respectively extended toward the edges of the discs 5 and 6 and are connected to inlet and outlet pipes P1 and P2 respectively connected to the elongated tube 1 (see FIG. 3).

In an X-ray tube arranged as described above, the electron current discharged from the filament 3 is intensified by the elongated tube 1 to a voltage of several tens of thousands. The resultant electron current e is focused at the apex of the conical opening 9 by the coil 4. A portion of the focused current hits the portion of the target foil 7 exposed in the opening at the apex. Since the foil is very thin, a portion of the X-rays generated by the electron beam bombardment passes through the foil and is emitted through the opposite opening 8 to the outside as indicated by arrows x. The electron beam e thus hits a relatively large area of the inner surface of the conical opening 9, but the portion permitted to pass through the opening 9, foil 7 and opening 8, to become the source of X-rays, is defined by the diameter of the openings in the apices 2R which is, for example, in the order of several tens of microns.

Although electron beam e hits a relatively large inner surface area of the conical opening 9 and the portion of the foil 7 exposed at the opening by the apex, there is no fear of local elevation of the temperature at the portions bombarded by the electron beam since the opening 9 is formed in the disc 6 which has a sufficient thickness and is formed of a metal having a good thermal conductivity. The foil 7 is bombarded and heated by the electron beam only at its exposed portion which, as noted, has a diameter of several tens of microns. Since the exposed portion is in contact directly with the massive discs 5 and 6 formed of a good heat conductive material and surrounding the exposed portion of the foil and having cooling water passages 10 and 11 contiguous with the exposed portion, the vicinity of the exposed portion of the foil 7 is maintained at a temperature substantially equal to that of the cooling water. Assuming that the thickness of the foil 7 is s and that the calorific value Q due to the electronic impulse is generated uniformly in the foil and the density of the electron beam is uniform for simplifying the analysis, calories Q.sub.1 generated within a circle with a radius r from the center of the exposed portion of the foil 7 may be expressed as follows:

Where the temperature gradient at radius r is dt/dr and the thermal conductivity of foil 7 is h, the calories Q.sub.2 in a circular cross section of the foil 7 with radius r is:

If the scattering of heat due to radiation and convection is ignored, since calorific value Q.sub.1 and the propagating calories Q.sub.2 are equal under normal conditions,

thus,

Therefore, the temperature difference T between the center of the exposed foil portion and its periphery is: ##SPC1##

Hence, if the temperature of cooling water is T.sub.1 , the temperature at the center of the exposed portion of the foil 7 may be obtained in the following manner:

Thus the temperature at the center of the exposed foil portion where it is heated most extremely has no relationship to the radius R of the exposed portion.

If the acceleration potential of the electron beam is constant, the total calorific value at the exposed portion of the foil will depend upon the value of the electron current hitting the exposed portion. Therefore, if the highest permissible temperature (i.e., without melting or damaging the foil 7) is constant, the exposed foil portion regardless of its radius R may be bombarded with a constant electron beam whereby X-rays are generated. In other words, with the smaller X-ray focal point provided by forming smaller apices of the conical openings 8 and 9, the electron beam density is made greater, whereby the magnitude of the electron current hitting the exposed portion of the foil 7 may be always maintained at a constant value. In this manner, X-rays of a substantially constant intensity may be provided regardless of the size of the X-ray focal point. Thus according to the present invention, an X-ray tube having an extremely small focal point and capable of generating strong X-rays may be provided.

According to the present invention, since the X-ray generating portion of the X-ray tube is directly cooled by water, the cooling at this portion may be carried out very effectively and efficiently. Furthermore, X-rays of a constant intensity may always be provided regardless of the size of the focal point and since the cooling efficiency is high, the electron-beam-accelerating potential may be increased. In such a case, the electron beam permeates deeper into the foil to generate X-rays at deeper portions of the foil, thus to emit stronger X-rays from the back surface of the foil.

It is a further advantage of the present invention that with the smaller focal point, a lower atmospheric pressure is applied to the foil. Therefore, a thinner foil may be used whereby even stronger X-rays may be obtained.

In the second embodiment of the present invention shown in FIGS. 4 and 5, the cooling water introducing member 6 as shown in FIGS. 2 and 3 is omitted while an X-ray window 12 is formed through the wall of the tube 1 and the window 12 is closed and hermetically sealed with a beryllium cover plate 13. In this embodiment, laminar electron beams e bombard the apex of the opening 8. The manner of emitting X-rays X.sub.1 from the back surface of the foil 7 through the opening 8 is similar to that of the embodiment shown and described with reference to FIGS. 2 and 3. In this embodiment, however, when the window 12 is so positioned as to face the side of a wide electron flux e, X-rays X.sub.2 emitted from the surface of the foil 7 through the window 12 and the beryllium cover plate 13 form a linear focal point as shown, when the window is positioned to face the side of a narrow electron flux, X-rays X.sub.2 form a spot focal point effectively. The X-ray tube of this form is of a multipurpose type; i.e. X-ray microscopic photographs may be taken by utilizing X-rays emitted from the opening 8, while, for example, the observation of X-ray diffraction may be carried out using X-rays emitted through the window 12.

The third embodiment of the present invention is next described with reference to FIGS. 6 through 10. In this embodiment, the elongated metal tube 1, which is similar to that shown in FIG. 1, is fitted at its one end with a thick disc-shaped target holder 105 formed of a good heat-conductive material such as steel to form a part of the wall of an airtight casing. This portion is shown on enlarged scale in FIGS. 7 and 8 and, as clearly illustrated, the target holder 105 is formed with an axial hole 106 along its axis and a radially extended hole 107 at the outer end. A metal target foil 108 is secured to the outer end face of the holder 105 and the open ends of the radial hole 107 are respectively closed in airtight condition by cover plates 109 made of a highly X-ray permeable material such as beryllium. The cooling water pipe 110 is provided with a hole into which the end of the tube 1 is fitted, thus to form a passage 111 for cooling water to cool the outer surface of the foil 108.

Cooling water is passed through the passage 111 as shown by arrow a in FIG. 7, the electron current discharge from the filament 3 being accelerated through the elongated tube 1 to a voltage of several tens of thousands to form X-ray bean e as shown by dotted lines in FIG. 8. The X-ray beam e is fluxed by coil 4 and the flux is passed through the axial hole 106 to form focal point P on the surface of the target foil 108 whereby to emit X-rays from the foil 108. A portion of the X-rays is externally passed through the radial hole 107, which may be termed an X-ray outlet hole, to utilize it for any desired purpose such as for the observation of X-ray diffraction.

FIG. 9 shows a section of the focal point P, on enlarged scale, in which r designates the radius of the focal point P of the electron beam e, and B is a hemisphere with radius R; the heat radiation E may be obtained as follows:

wherein, T.sub.1 represents the temperature at the focal point P, TT.sub.2 represents the temperature of the hemisphere B and k represents the thermal conductivity of the target foil 108. If the electric current value of the electron current is I and the velocity thereof is V, the power applied to the focal point P will be VI, therefore, the value VI should be maintained below the value E obtainable by expression (7) given hereinabove. Assuming that all values T.sub.1, T.sub.2, k and r are constant, it can be seen that there is a relationship between the radius R and the heat radiation E as shown in FIG. 5. Accordingly, it is clear that the radius R of the hemisphere B becomes smaller at a constant temperature T.sub.2 , the permissible power VI of electron beam may be increased. In the above-described X-ray tube, a thin foil 108 is used as the target provided with a cooling water passage 111 on its back surface to be cooled directly, and a member 105 for holding the target foil 108 is formed with a good heat conductive material and provided with an axial hole 106 over which the foil 108 is directly secured. When the temperature of the cooling water is T.sub.2, the back side of the foil 108 and the hole 105 is naturally maintained at the temperature T.sub.2 . Therefore, the heat radiation E may be obtained by substituting for the radius R in expression (7) the thickness d of the foil 108. As shown in FIG. 10, the smaller becomes the value R substituted by thickness d, the more heat radiation E increases. Thus, the power VI to be applied to the target becomes higher due to the stronger bombardment of electron beam, consequently emitting stronger X-rays. Since the target foil 108 is subjected to atmospheric pressure at only the portion exposed through the axial hole 106 and the radial hole 107, a very thin foil may be used as the target. Since the foil is simply secured to and held by the holder 105, the manufacture of the X-ray tube may be greatly facilitated.

FIG. 11 shows the fourth embodiment of the present invention. In this embodiment, the end of the tube 1 opposite its electrode side is fitted with a cover plate 215 formed of a metal having a good heat conductivity. The cover plate 215 is formed with a central opening 216 the open end of which is covered with a thin-target foil 217 of a suitable metal such as steel, molybdenum, iron or tungsten such that the target foil 217 forms of a portion of the wall of the hermetically sealed casing. The elongated tube 1 is surrounded by an electron beam fluxing coil and electron beam e emitted from a filament by grounding the tube 1 and applying a high negative voltage to the filament and a Waynelt electrode is accelerated through the tube 1 to a voltage of several tens of thousands. The beam is fluxed by the coil so that a focal point is formed on the surface of the foil 217 through the axial hole 216. The size of the focal point thus formed on the target foil is very small such as of several microns to several tens of microns. The thickness of the foil 217 is also within the same range. Since the penetration of electron beam e is shallow relative to the thickness of the foil, X-rays are generated at the inner surface of the foil, and a portion of X-rays passes through the foil and is emitted to the outside as shown by arrows x in FIG. 11. In the embodiment illustrated in FIG. 11, the outer surface of the target foil 217 is coated with a heat-resistive antioxidizing layer 218. In case a steel foil is used as the target foil 217, one surface of the steel foil is preplated with platinum so that the plated platinum layer will serve as the antioxidizing layer. When an aluminum foil is used as the target foil, one surface of the foil may be covered with an alumite layer for the same purpose.

In known X-ray tubes, when it is attempted to obtain strong X-rays by increasing the intensity of the bombardment of the electron beam against the target, the temperature of the target surface rises. In an X-ray tube having the target contained in a hermetically sealed casing, the bombardment intensity is limited by the temperature which should be lower than that which melts the tube. It has been found that in such a known type of thin-target X-ray tube, the target foil 217 is gradually oxidized and deteriorated due to the temperature rise. It usually cracks and causes leaks. Therefore, such X-ray tubes of the prior art have been generally found to be short lived due to oxidation of their target foils. According to the present invention, however, the target foil 217 is covered with a heat-resistive antioxidizing layer, whereby to avoid the formation of cracks in the foil thus to increase the life of the X-ray tube. Since the target foil of the present invention is prevented from oxidizing, the level of permissible temperature may be elevated to allow increase of the density of the electron beam to thereby increase the intensity of X-rays emitted. The antioxidizing layer can be made very thin. Therefore, its absorption of X-rays may be minimized. However, in some applications, the tube may be arranged to emit secondary X-rays of suitable wavelengths from the antioxidizing layer for any particular purpose.

The discs 5 and 6 and the element 215 noted hereinabove can be made of metals having good thermal conducting qualities, such as, for example, copper.

Claims

I claim:

1. An X-ray tube comprising a source is an electron beam, first means to accelerate said beam, a target arranged to be bombarded by the accelerated beam, and second means associated with the target to enhance the ability thereof to withstand deterioration which would result from bombardment by said beam, said second means including cooling means for cooling said target, said first means including an elongated tube extending between said source and target and wherein said cooling means includes at least one cooling block having an opening aligned with said beam and being provided with a passage for the passing of a cooling fluid, said block being supported by said tube and being in contact with the target over a major portion of the latter, said target being exposed to said beam in correspondence with said opening.

2. A tube as claimed in claim 1 wherein said second means includes antioxidizing means to prevent the target from oxidizing.

3. A tube as claimed in claim 1 wherein the block is arranged between the target and source and the beam passes through said hole.

4. A tube as claimed in claim 1 wherein the target is arranged between the block and source and at least part of the X-rays emitted by the target pass through the hole.

5. A tube as claimed in claim 1 comprising a second block corresponding to the first, said blocks sandwiching the target therebetween, said blocks having said holes in coaxial conical form tapering towards each other to corresponding apex end openings, said passageway being constituted by corresponding annular semicircular grooves in the blocks, said grooves being juxtaposed to form a passage of toroidal shape, said tube further comprising means to supply and remove a cooling fluid to and from said passage.

6. A tube as claimed in claim 4 wherein said tube includes window means between the target and source for the passing of another part of the X-rays emitted by the target.

7. A tube as claimed in claim 1 wherein the passage is positioned internally of said tube.

8. A tube as claimed in claim 1 wherein the passage is positioned externally of said tube.

9. A tube as claimed in claim 1 wherein the target is a thin foil.

10. A tube as claimed in claim 2 wherein the antioxidizing means is a layer adhering to the target.

11. A tube as claimed in claim 1 wherein the block is of a metal having good thermal conductivity.