US 3344298 A
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Sept-26,1967 1c. MARTIN 3,344,298
FLASH X-RAY TUBE WITH GAS FOCUSING OF BEA! Filed Mai 24, 1965 United States Patent 3,344,298 FLASH X-RAY TUBE WITH GAS FOCUSING 0F BEAM John Christopher Martin, Tadley, England, assignor to United Kingdom Atomic Energy Authority, London, England Filed May 24, 1965, Ser. No. 458,327 Claims priority, application Great Britain, May 29, 1964, 22,425/ 64 6 Claims. (Cl. 313-57) ABSTRACT OF THE DISCLOSURE A flashX-ray tube having a field-emission cathode, an anode made of thin electrically conducting material through which-electrons striking the anode are transmitted, and a target located remotely from the accelerating gap. The space between anode and target contains gas under pressure to provide magnetic self-focussing of the transmitted electrons.
This invention relates to flash X-ray tubes, i.e. tubes to which a high-voltage pulse is applied between anode and cathode for a very short period, e.g. of the order of a few tens of nanoseconds. Such tubes may use field-emission cathodes of various forms.
In such tubes, the accelerating gap between the anode and cathode proper may be very small, of the order of 1 cm. Since the use of short voltage pulses necessitates the tube being in close proximity to the generator in order to keep the inductance of the connections low, this means that the location of the anode, hitherto the source of the X-rays, is fixed, and is difiicult to adjust in relation to the object to be X-rayed. This is particularly inconvenient where the object itself is large and relatively immobile.
Furthermore, in order to obtain sharp X-ray photographs, the anode should be, as nearly as possible, a point source of X-rays. In practice the electron beam is of finite diameter and the X-rays are emitted from a correspondingly finite area or spot on the anode, which spot should be as small as possible. The electrons leave the field-emission cathode as a divergent beam. It is possible to focus the electrons into a parallel beam by providing a magnetic focussing coil which encircles the beam between anode and cathode, but the arrangement of a satisfactory focusing coil in the high-field region of the accelerating gap is diflicult to achieve.
The present invention provides a tube in which the abovedescribed problems are alleviated by allowing the source of the X-rays, or target, to be located remotely from the accelerating gap.
According to the present invention a flash X-ray tube comprises an anode made of thin electrically conducting material capable of transmitting a substantial proportion of the electrons striking the material, and a target located a distance behind the anode, the space between anode and target being adapted to contain gas at a pressure sufliciently above that in the accelerating gap to provide magnetic self-focussing of the transmitted electrons traversing the space. The tube may also include a magnetic focussing coil arranged to encircle the selffocussed beam adjacent the target.
To enable the nature of the present invention to be more readily understood, attention is directed, by way of example, to the accompanying FIGURE 1, which is a sectional elevation of a flash X-ray tube embodying the present invention, and to FIGURE 2 which shows a modification thereof.
In FIGURE 1 an anode plate 1 and a cathode plate 2 are separated by four series-arranged annular insulators 3 separated by three annular conducting rings 4. The inner surfaces of the insulators 3 are angled in the manner described in our copending application No. 22424/ 64 filed in Great Britain. The high-voltage DC pulse is applied between plates 1 and 2. Mounted on plate 2 is a cathode assembly comprising a stem 5, a smooth ball 6, and a needle 7 having a rounded tip and constituting the field-emission cathode proper.
At the other side of the accelerating gap is the anode proper 8, consisting of a disc of thin aluminium foil (typically 0.001 or 0.002 inch thick) which seals the end of an anode tube 9 mounted on the anode plate 1. At the other end of the tube 9, mounted on the plate 1, is a target 10, typically of lead or some other dense metal. The air pressure in the accelerating gap is reduced to the working pressure, typically 10- or 10" mm. Hg, by connecting a vacuum pump to duct 12, and the pressure Within the tube 9 to a higher pressure, typically in the range 0.2 mm.10 mm. Hg, via duct 13.
It is found that, provided the gap voltage is sui'ficient to raise the electrons to relativistic velocities, and provided the electron current transmitted by the foil 8 exceeds about 12 kA, the initially divergent beam 14 is self-focussing within the tube 9, where it becomes substantially non-divergent (parallel-sided) as shown.
Also shown in the present embodiment is a magnetic focussing coil 15 encircling the tube 9 adjacent the target end thereof i.e. remote from the gap region, to reduce the beam diameter before it strikes the target and hence reduce the spot size. However, the provision of such a coil is not essential in all applications of the invention.
The explanation of the self-focussing effect is thought to be as follows. Under relatively high-vacuum conditions, the number of positive gas ions produced by the beam of relativistic electrons is small, the mutual repulsion of the electrons in the beam is balanced by the constricting eifect of the self-magnetic field of the beam, and the beam therefore continues with its initial divergence. When the gas pressure is increased, however, as in the tube 9, the number of positive ions is increased to an extent which tends to neutralise the mutual repulsion of the electrons, and the self-field of the beam becomes unbalanced, thereby constricting the beam and reducing the divergence.
In one experiment a beam of electrons originating from an approximately 1 mm. diameter cathode diverged into a cone of semi-angle 30 to produce a spot about 8 mm. in diameter on a conventional anode spaced 7 mm. from the cathode. When this anode was replaced by the described arrangement (without the focussing coil 15) the target 10 being a distance of 20 cm. from the foil 8, the beam diameter as measured by the spot size had only increased to about 10 mm. At least a third of the electrons leaving the cathode was calculated to have reached the target, the current initially leaving the cathode being about 14 kA. The air pressure in the tube 9 was not critical, the effect on the beam being broadly independent of the pressure in the range 0.2 mm. to 10 mm. Hg. Using as the cathode a ball-bearing having a roughened surface, which gives an initially less divergent beam than the needle, it was estimated that two-thirds of the initial electrons reached the target, most of the remaining third being lost in the ionisation process.
As FIGURE 2 shows, the target 10 can be located behind the anode plate 1 by providing a backwardly extending tube 16 sealed to the plate. In the present embodiment tube 16 is effectively a backward extension of tube 9 through plate 1, allowing the target to be located still further from the anode 8'. A focussing coil, corre- 3 sponding to coil 15 in FIGURE 1, can be arranged to encircle tube 16 if desired.
1. A flash X-ray tube comprising a fieldemission cathode, an anode made of thin electrically conducting material capable of transmitting a substantial proportion of electrons striking the material, and a target located a distance behind the anode which distance is very much greater than the anode to cathode distance, the space between anode and target being adapted to contain gas at a pressure sufliciently above that in the accelerating gap but below atomspheric pressure to provide magnetic self-focussing of the transmitted electrons traversing the space.
2. A flash X-ray tube as claimed in claim 1 wherein the target is mounted on an anode plate, an anode tube surrounding said target is sealed to and extends from said anode plate towards the cathode end of the X-ray tube, with the end of the anode tube remote from the anode plate being closed by a foil constituting said anode of thin electrically conducting material.
3. A flash X-ray tube as claimed in claim 1 wherein the target is mounted on an anode plate and an extension tube is sealed to and extendsbackwardly from said anode plate away from the cathode end of the X-ray tube, said target being located Within said extension tube.
4. A flash X-ray tube as claimed in claim 1 wherein a magnetic focusing coil is arranged to encircle the selffocussed beam adjacent to the target.
5. A flash X-ray tube as claimed in claim 1 wherein the gas in the space between anode and target is air at a pressure in the range 0.2 mm. to 10 mm. Hg.
6. A flash X-ray tube as claimed in claim 1 wherein the electron current transmitted by said thin electrically conducting material exceeds 12 kA.
References Cited UNITED STATES PATENTS 2,362,816 11/1944 Harker 313-32 2,755,413 7/1956 Wagner 3l384 3,239,706 3/1966 Farrell et a1 313-55 DAVID J. GALVIN, Primary Examiner.