X-ray or gamma ray systems or devices – Source – Target
Reexamination Certificate
1999-05-07
2001-02-06
Porta, David P. (Department: 2876)
X-ray or gamma ray systems or devices
Source
Target
C378S125000
Reexamination Certificate
active
06185277
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an X-ray source which includes an electron source for the emission of electrons and a target which emits X-rays in response to the incidence of the electrons and consists of a liquid metal which circulates in the operating condition of the X-ray source.
2. Description of Related Art
An X-ray source of this kind is known from U.S. Pat. No. 4,953,191. The liquid metal therein is contained in a pumping circuit which includes a distribution head wherefrom the liquid metal flows across a stainless steel plate and into a collecting basin wherefrom it is subsequently pumped to the distribution head again. The electron beam is incident on the liquid metal flowing across the stainless steel plate and generates X-rays therein.
The liquid metal thus flows through the vacuum space in which the electron source of the X-ray source is accommodated. Therefore, this type of tube is limited to liquid metals which have such a low vapor pressure that, even at the highest operating temperatures occurring, the vacuum in the X-ray source is not affected. Therefore, use must be made of gallium which has a comparatively low atomic number (30) and hence a comparatively low X-ray yield.
However, it is essential to prevent gallium particles from the circulating gallium flow from penetrating the vacuum space of the X-ray source, because the high-voltage strength of the X-ray source could suffer therefrom. This means that the flow of the gallium across the stainless steel plate should be purely laminar, because a turbulent flow could cause the escape of lubricant particles. The flow of the gallium from the distribution head to the stainless steel plate and notably the heating of the gallium by the electron beam favor the occurrence of turbulent flows. Therefore, the gallium may flow only in a thin layer of a thickness of substantially less than 1 mm and also at a speed which is significantly lower than indicated in the cited publication, so that the expected load carrying ability of the X-ray source is significantly reduced.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an X-ray source having an enhanced continuous load carrying ability. On the basis of an X-ray source of the kind set forth this object is achieved in that a window which can be traversed by the electrons and is cooled by the target is arranged between the electron source and the target.
It is an essential aspect of the invention that the electrons emitted by the electron source are not incident directly on the liquid lubricant, but pass through a window which separates the vacuum space of the X-ray source and the liquid lubricant from one another. It is to be noted that the window absorbs a part of the electrons. However, by choosing a suitable material and a suitably small thickness, the window can be conceived such that it absorbs only a small part of the electron energy (approximately 800 eV). Therefore, the electrons can penetrate the liquid metal and excite X-rays therein without being decelerated by the window to any significant extent. The liquid metal thus has three functions:
a) it converts high energy electrons into X-rays,
b) it effectively removes the heat from the region in which the electrons interact with the liquid metal, and
c) it cools the window.
The use of this window enables the coolant to be guided along the window as a turbulent flow. In the case of a turbulent flow, the mixing of the liquid metal is significantly better in comparison with a laminar flow, so that better cooling is achieved. Moreover, the liquid metal can be guided through the area of interaction with the electrons in a thicker layer and at a higher speed in comparison with a laminar flow. A significantly more effective cooling or a higher continuous load carrying ability is thus achieved.
Moreover, the separation of the vacuum space from the liquid metal allows for the choice of a metal having a vapor pressure higher than that of gallium, but also a higher atomic number so that it converts a larger part of the electron energy into X-rays.
It is to be noted that JP-A 08 036 978 already discloses an X-ray source in which the electrons emitted by an electron source are incident on a target through a window which seals the vacuum space of the X-ray source. The target, evidently being a solid state target, is arranged in a rotatable mount at some distance from the window. In the case of a defect it can be readily replaced by another target in said mount. Because a part of the energy of the electrons is converted into heat in the window, the load carrying ability of the X-ray source is only low, an additional problem being that the outer side of the window is subject to atmospheric conditions so that it must consist of a material which does not react with oxygen when heated.
The window of this invention must be constructed in such a manner that on the one hand it is as stable as possible so as to withstand the flow pressure of the circulating liquid metal, and on the other hand it should draw as little as possible energy from the electrons. A suitable material for the window is diamond, which preferably is arranged on a substrate that faces the electron source, the substrate having an opening at the area of incidence of the electrons.
Besides diamond, other window materials may also be used, for example beryllium or synthetic materials. Mercury, a mercury alloy, or an alloy containing lead and bismuth are suitable targets. Therefore, the term metal must be broadly interpreted in the context of the present invention. It should include not only metals defined by chemical elements, but also their alloys.
An embodiment which includes a pump for causing the liquid metal to circulate in a closed circuit with a predominately turbulent flow at the area of the window provides effective cooling which allows for an increased continuous power. A further embodiment wherein the cross-section of the circuit which is traversed by the liquid metal is substantially smaller at the area of the window than in an area situated farther from the window realizes a turbulent flow at the area of the window. Such a further embodiment can be realized in the simplest manner with a circuit including a duct whose circumference is provided with the window and with a constriction at the area of the window.
An embodiment wherein the electron source is accommodated in an evacuated envelope which is sealed by the window ensures that the vacuum space enclosed by the envelope and the space in which the liquid metal flows are hermetically sealed from one another. Therefore, the liquid metal need not have a low vapor pressure as in the known X-ray source. In the further embodiment wherein the envelope is provided with an exit for the x-rays generated in the target, the X-rays produced in the liquid metal first pass through the window for the electrons before emanating as useful radiation from the X-ray exit window. When the electron beam emitted by the electron source has an elongate cross-section (“strip focus principle”), the plane defined by the electron beam and the emergence of the useful radiation beam should extend perpendicularly to the direction in which the liquid metal flows past the window.
REFERENCES:
patent: 4723262 (1988-02-01), Noda et al.
patent: 4953191 (1990-08-01), Smither et al.
patent: 5052034 (1991-09-01), Schuster
patent: 08036978A (1996-02-01), None
Dunn Drew A.
Porta David P.
U.S. Philips Corporation
Vodopia John F.
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