JPS5923063B2 - x-ray tube - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an X-ray tube with a fixed hooded anode and in which a relatively small focus can be easily obtained.

As is well known, an X-ray tube includes a cathode and an anode, and electrons emitted from the cathode are made to collide with a so-called target of the anode to which a high voltage is applied, so that X-rays are emitted from the target.

When electrons collide with a target, in addition to emitting X-rays, many secondary electrons and backscattered electrons are also emitted.

Some of these electrons return to the positive anode, but many of them attach to the inner wall of the vacuum envelope, usually a glass bulb, and make that part negatively charged.

In general, the inner wall surface of the glass bulb is very clean during the process of evacuating the inside of the X-ray tube to a high vacuum, so it has good insulation.

Therefore, the charge generated on the inner wall surface of the glass bulb by secondary electrons, etc. as described above is difficult to neutralize, and after reaching a considerably high voltage, intense creeping discharge occurs and is neutralized. many.

It even occurs that the glass bulb is damaged during this discharge.

In order to prevent such accidents, conventionally, a metal hood, which has a cylindrical shape and has approximately the same outer diameter as the anode and projects toward the cathode, is attached to the anode.

Fig. 1 shows an example of a conventional fixed anode X-ray tube with a hood, where 1 is an anode, 2 is a hood, 3 is a filament, 4 is a cathode, 5 is a valve, 6 is a beryllium plate, and 7 is a brazed Department,
8 is the target.

The electrons emitted from the filament 3 heated by electricity are
It is attracted to the anode 1, which is at a high positive potential with respect to the cathode, and collides with the target 8 on the anode surface.

Target 8, where many high-energy electrons usually collide
Since the anode 1 reaches a high temperature, it is made of tungsten, which has a high melting point, and the anode 1 (other than the target) is made of copper so that the heat is well dissipated.

A window is provided in the hood 2 where the X-rays emitted from the target 8 pass, and a metal plate 6, which transmits X-rays best among practically available metals, is fitted in this window, so that the X-rays can pass through. However, secondary electrons and the like cannot pass through.

Conventional hood 2kL Because it is a conductive metal cylinder, even if secondary electrons adhere to it, it will not be charged and the potential will fluctuate, and the potential will always be the same as the target 8 and the anode 1. Equipotential.

However, the fact that the hood 2 is at the same potential as the entire anode 1 means that
Since the diverging electron lens is provided near the tip of the hood 2 (near the cathode), it becomes difficult to obtain a minute focus.

In addition, the tip of the hood 2 and the cathode 4, which are at the same potential as the anode 1, must stand above a predetermined value due to the withstand voltage characteristics between the cathode and anode. Therefore, it becomes necessary to lengthen the pipe (in the axial direction).

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the conventional fixed anode X-ray tube with a hood, and to provide an X-ray tube that can obtain a small focus and is not susceptible to creeping discharge on the inner surface of the bulb.

In order to achieve the above object, in the present invention, a portion of the hood near the cathode is formed of an insulating material.

FIG. 2 is a diagram showing an embodiment of the present invention.

In the figure, 9 is a hood according to the present invention, and the other symbols (salt) are the same as in the case of Figure 1.

The hood 9 according to the present invention (as shown in the figure) is made of metal up to the window where the beryllium plate 6 of the X-ray passage is fitted, but the part near the cathode is made of inorganic insulation such as highly heat-resistant ceramic. made of material.

In this way, unlike in the case of the conventional all-metal hood 2, a diverging electrostatic lens is generated at a distance from the target 8 to the phase foil cathode 4 side, and the focal point is broadened. There isn't.

Even if creeping discharge due to secondary electron charging occurs on the inner wall surface of the insulating material portion of the hood 9, there is very little possibility that the glass bulb will be damaged, that is, the vacuum envelope will be damaged, or that it will become unusable.

The insulating material used for the hood 9 is the inner material of the vacuum electron tube, so it must withstand high temperatures for exhaust work and not evaporate, but it must also be vacuum-tight (including where it connects to metal). Not done.

In addition, due to the requirements for pressure resistance between the conventional X-ray tube using a hooded anode and the '4 cathode shown in Figure 1, the dimension in the tube axis direction of the X-ray tube is longer than that without a hood. (However, in the embodiment of the present invention, the dimension &L in the tube axis direction may be almost the same as the case without the hood.

As explained above (according to the present invention, a relatively small focal point can be easily formed, and accidents such as creeping discharge on the inner surface of the vacuum envelope caused by secondary electrons and vacuum breakdown) can be easily formed. It has a preventive effect.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of a conventional fixed anode X-ray tube with a hood, and FIG. 2 is a diagram showing an embodiment of the present invention. 1... Anode, 4... Cathode, 5... Bulb, 8...
- Target, 9... Food according to the present invention.

Claims (1)

[Claims] 1. In a fixed anode X-ray tube, in which a cylindrical hood with approximately the same outer diameter as the anode and protruding toward the cathode is attached to the anode,
An X-ray tube characterized in that a portion of the hood near the cathode is formed of an insulating material.