Rotating Anode X-ray Tube

Abstract

A rotating anode x-ray tube employing a generally cylindrical anode which is axially moveable relative to the cathode so that the focal spot traverses a spiral path as the anode rotates and moves axially. The axial displacement of the anode can be controlled by the temperature of the focal spot, i.e. by measuring the light emitted from the focal spot area.

Description

The invention relates to a rotating anode x-ray tube in which the anode is axially displaceable to thereby increase the loadability of the focal spot by increasing the cooling surface.

It has been proposed to increase the unit area loading of the anode of an x-ray tube, and thereby increase the x-ray output, by forced cooling of the anode, by rotating the anode, and by a combination of such expedients. It has also been proposed to oscillate a rotating target effectively to increase the amount of fresh metal presented to the electron beam per target revolution as described in U.S. Pat. No. 2,926,270.

It is an object of the present invention to substantially increase the unit area loading of a rotary anode x-ray tube without resort to forced cooling.

It is another object of the invention to increase the cooling surface of a rotary anode x-ray tube while maintaining a small focal spot.

These and further objects of the invention will appear as the specification progresses.

Broadly stated, the invention relates to a rotary anode x-ray tube employing a generally cylindrical, preferrably cup-shaped anode which is axially displaceable during exposure which increases the cooling surface because the focal track becomes a spiral on a cylindrical drum. The axial displacement can be controlled by the temperature of the focal spot, for instance by measuring the light emitted from the focal spot area on the inside of the cup. The light emitted from the inside of the cup may be detected and converted into an electrical signal which may be amplified and used to drive the motor which controls the axial movement of the anode.

The invention will be described with reference to the accompanying drawing which shows a single, exemplary embodiment of a rotary anode tube according to the invention.

The x-ray tube as shown in drawing includes a generally cylindrical evacuated envelope 1 having an end portion with a metal wall 2 sealed to the remainder of the envelope by a metal-to-glass seal 3 and provided with an x-ray permeable window 4 through which x-rays generated within the tube are transmitted. Mounted for rotation within the envelope is a cup-shaped anode 5 having a generally cylindrical outer surface. A cathode cup 6 is positioned opposite the outer cylindrical surface of the cup-shaped anode and is energized by a filament 7 to produce a beam of electrons which impinges on the anode surface and forms a focal spot 7 visible through the window 4.

Anode 5 is secured to a shaft 8 which is connected to a rotor 9 driven by a stator 10 mounted externally of the envelope. Rotor 9 is mounted for rotation on an axial slide 11 by radial bearings 12.

Axial slide 11 in turn is mounted on a stationary shaft 13 for axial movement by axial bearings 14 and is moved by a rack 15 which forms part of the inner surface of the slide and a pinion 16 driven by an axial drive rotor 17. The stationary shaft 13 is sealed in the end of the envelope so that the envelope may be evacuated.

In operation the anode 5 is driven by rotor 9 and presents a continually fresh surface to the impinging beam thus minimizing local heating where the electron beam impinges and forms the focal spot. However, due to thermal lag, the temperature of path described by the focal spot increases with each revolution thus limiting the loading of the anode.

In order to increase the loading, the anode is moved axially by driving the axial slide 9. This causes the focal spot to describe a spiral on the outer surface of the anode which increases the path length and thus reduces the anode temperature rise due to thermal lag.

To further control the rate at which the anode temperature uses, and thus increase the loadability, the metal end cap 2 is provided with a small window 20 through which the inner surface of the cup-shaped anode can be viewed by a light detector 18 which produces an electrical signal 19 proportional to the intensity of the light emitted by the focal spot 7. This signal is amplified by amplifier 19 and is used to control the axial drive motor 17. Thus, as the anode temperature rises, the light emitted by the focal spot will increase which will increase the signal applied to the axial drive motor causing the anode to be moved axially in response to the increased loading.

Sometimes anodes crack and portions hit the tube window and may leave the shield. The proposed construction provides a metal shield around the anode and the x-ray window is not in the path of an anode part driven by a centrifugal force outside the shield.

Claims

What I claim is:

1. An x-ray tube comprising an evacuated envelope, a generally cylindrical anode rotatable about a given axis within said envelope and moveable longitudinally therealong, a cathode for producing an electron beam which impinges on the surface of said anode forming a focal spot thereon where x-ray are generated which emerge from said envelope through an x-ray permeable window therein, means for rotating said anode and means for moving said anode longitudinally along said axis in response to the temperature at the focal spot to thereby present a fresh surface of the anode to the electron beam for producing the focal spot.

2. An x-ray tube as claimed in claim 1 in which the focal spot moves in a spiral path in response to movement of the anode.

3. An x-ray tube as claimed in claim 1 in which the anode is cup-shaped and the cathode faces the outer surface of the cup.

4. An x-ray tube as claimed in claim 3 in which the anode is connected to a rotor which rotates about an axially moveable slide.

5. An x-ray tube as claimed in claim 4 in which the slide is moved by an axial drive mechanism which is actuated by means responsive to the focal spot temperature.

6. An x-ray tube as claimed in claim 5 in which the means responsive to the focal spot temperature is a member responsive to light emitted from the focal spot area inside the cup-shaped anode.

7. An x-ray tube as claimed in claim 5 in which the slide is coupled to the axial drive mechanism through a rack and pinion, the pinion being fixed by positioned and the rack being mounted on said slide.

8. An x-ray tube as claimed in claim 7 in which the rotor is journalled for rotation about the slide.

9. An x-ray tube as claimed in claim 8 in which the slide and rotor are mounted on said shaft by bearing means.

10. An x-ray tube as claimed in claim 9, in which said envelope includes an uninterrupted metal shield around the anode providing better safety from anode explosions.