Device for generating X-rays having a liquid metal anode

#The invention relates to a device for generating X-rays, which device comprises a source for emitting electrons accommodated in a vacuum space, a liquid metal for emitting X-rays as a result of the incidence of electrons, and a pumping means for causing a flow of the liquid metal through a constriction where the electrons emitted by the source impinge upon the liquid metal, said constriction being bounded by a window, which is transparent to electrons and X-rays and separates the constriction from the vacuum space.

#A device for generating X-rays of the kind mentioned in the opening paragraph is known from U.S. Pat. No. 6,185,277-B1. The window of the known device is relatively thin and is made from a material which is transparent to electrons and X-rays, e.g. diamond or molybdenum. The window prevents the vacuum space from being contaminated by the liquid metal. During operation of the known device, the liquid metal, e.g. mercury, flows through the constriction, which forms part of a closed channel system. The source generates an electron beam, which passes through the window and impinges upon the liquid metal in an impingement position in the constriction. The X-rays, emitted by the liquid metal as a result of the incidence of the electron beam, emanate through the window and through an X-ray exit window, which is provided in a housing enclosing the vacuum space. The velocity of the flow of the liquid metal in the constriction is relatively high, so that said flow is turbulent. As a result the heat, which is generated in the impingement position as a result of the incidence of the electron beam upon the liquid metal, is transported away from the impingement position by the flow of the liquid metal in an effective manner. As a result, an increase of the temperature of the liquid metal in the impingement position is limited, and a relatively high energy level of the electron beam is allowed without causing excessive heating of the liquid metal. The closed channel system of the known device further comprises a heat exchanger by means of which the liquid metal is cooled down.

#During operation of the known device, a relatively high pressure is generated by the pumping means in a portion of the channel system upstream from the constriction in order to achieve a sufficiently high velocity of the liquid metal in the constriction. As a result of said high velocity and the so-called Bernoulli effect, the liquid metal in the constriction has a pressure which is low relative to the pressure generated upstream from the constriction. A problem of the known device is that, although the pressure of the liquid metal in the constriction is relatively low, deformations and even breakage of the window occur as a result of said pressure because the window is relatively thin.

#In the following, embodiments of a device for generating X-rays according to the invention will be explained further in detail with reference to the Figures, in which

#FIG. 1 schematically shows a first embodiment of a device for generating X-rays according to the invention,

#FIG. 2 shows a constriction of the device of FIG. 1,

#In FIG. 1 only the main components of the first embodiment of a device for generating X-rays according to the invention are schematically shown. The device comprises a housing 1 which encloses a vacuum space 3 in which a source 5 or cathode for emitting electrons is accommodated. The device further comprises a closed channel system 7 comprising an inlet channel 9, a converging part 11, a constriction 13, a diverging part 15, an outlet channel 17, a heat exchanger 19, and a hydraulic pump 21. The channel system 7 is filled with a liquid metal which has the property of emitting X-rays as a result of the incidence of electrons. In the embodiment shown, the liquid metal is an alloy of Ga, In, and Sn, but also other kinds of metals or metal alloys which are liquid at room temperature, such as for example Hg, may be used. The constriction 13 is bounded by a window 23, which is transparent to electrons and X-rays, and by a wall 25 opposite to the window 23. In the embodiment shown, the window 23 comprises a relatively thin diamond plate, but also other kinds of materials which are sufficiently transparent to electrons and X-rays, such as for example Mo, may be used. The window 23 separates the constriction 13 from the vacuum space 3, thereby preventing the vacuum space 3 from being contaminated by particles of the liquid metal.

#During operation of the device, the liquid metal is caused to flow through the constriction 13 by means of the hydraulic pump 21. In the embodiment shown, the hydraulic pump 21 is of a conventional type, but also another suitable pumping means may be used instead, such as for example a magneto-hydrodynamic pump. The constriction 13 has a relatively small cross-sectional area, so that the flow of the liquid metal in the constriction 13 has a relatively high velocity and is turbulent. The source 5 generates an electron beam 27, which passes through the window 23 and impinges upon the liquid metal in an impingement position 29 in the constriction 13. As a result of the incidence of the electron beam 27 upon the liquid metal, X-rays 31 are generated in the impingement position 29. Thus, the liquid metal in the constriction 13 constitutes an anode of the device for generating X-rays. The X-rays 31 emanate through the window 23 and through an X-ray exit window 33, which is provided in the housing 1.

#A further result of the incidence of the electron beam 27 upon the liquid metal is the generation of a large amount of heat in the impingement position 29. This heat is transported away from the impingement position 29 in an effective manner by the flow of the liquid metal in the constriction 13, and the heated liquid metal is subsequently cooled down again in the heat exchanger 19. In this manner, excessive heating of the liquid metal in the impingement position 29 and of the surroundings of the constriction 13 is prevented. By means of the flow of the liquid metal in the constriction 13, a relatively high rate of heat transport away from the impingement position 29 is achieved, so that a relatively high energy level of the electron beam 27 and consequently a relatively high energy level of the X-rays 31 is allowed.