JPS5935502B2 - acceleration tube - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in accelerating tubes used in high-voltage electron microscopes with accelerating voltages of, for example, 200 KV or higher.

As shown in the schematic diagram of FIG. 1, this type of accelerating tube has a plurality of accelerating electrodes 1. It is constructed by stacking elements a, 1b, . . . with insulators 2a, 2b, 2c, . . . interposed therebetween.

The acceleration tube and electron gun 3 are housed in a housing 4 sealed with an insulating gas of about 3 atmospheres, such as Freon (CCl2F2), and the inside of the acceleration tube is connected to the electron microscope body and is kept in a high vacuum. Exhausted.

5a and 5b in the figure. 5c, 5d, . . . are OIJ packings placed at the connection portions between each accelerating electrode and the insulator, and are for creating a vacuum seal between the inside A of the accelerating tube and the inside B of the casing.

However, if such an accelerating tube is operated for a long period of time (for example, 1 to 3 years), the surface of the accelerating electrode and insulator in the vacuum will become contaminated with hydrocarbon-based or fluorine-based polymers, making it difficult to apply high voltage. occurs.

Conventionally, it has been generally thought that the cause of contamination on the surfaces of the accelerating electrodes and insulators is that oil vapor or vacuum grease vapor from the oil diffusion pump that exhausts the inside of the accelerator tube decomposes and adheres to the surface due to the impact of the electron beam. Ta.

However, in view of this point, the inventors of the present invention have conducted experiments over a long period of time and have found that most of the contaminants on the surface of the accelerating electrode are Freon gas placed outside the accelerating tube.

The reason for this will be explained below.

Generally, there is a vacuum seal between the inside of the accelerator tube and the inside of the casing.
Fluororubber, which releases less gas, is used for ring backing, but this fluororubber O-ring backing is easily affected by freon gas, and when used for a long period of time, the fluororubber O-ring backing swells significantly and cannot be used under vacuum. It loses its function as a seal.

Therefore, the Freon gas inside the housing leaks into the accelerating tube, contaminating the accelerating electrode surface and the like.

The present invention is intended to solve such inconveniences, and an embodiment of the present invention will be described below in detail based on FIG. 2.

FIG. 2 is an enlarged sectional view corresponding to the circle a shown by the dashed line in FIG. 1, and the same numbers as in FIG. 1 indicate the same components.

That is, in this embodiment, the insulators 2a and 2b of the accelerating electrode 1a
Two annular 01J grooves 6a, 6b and 6a, 6b are formed approximately concentrically at the respective connecting portions with the accelerator tube, and the grooves 6a and 6a on the side closer to the inside A of the acceleration tube are filled with gas. Fluorine O-ring backings 5a and sb, which emit less, are fitted, and grooves 6b and 6b' on the side closer to the inside B of the other housing are filled with, for example, nitrile rubber or chloroprene rubber 01J, which is less likely to swell with Freon gas. The ring packing 7a and 7b are fitted together.

The painter's accelerating electrode 1b... has a similar configuration.

And the above fluororubber O-ring backing 5a, sb
The spaces c and c' between the and the nitrile rubber O-ring backings 7a and 7b are air spaces at approximately atmospheric pressure.

With such a configuration, the fluororubber O-ring backing does not come into contact with Freon gas unlike in the past, so it can be prevented from swelling due to the gas, and the connection between space C and the inside of the housing B can be prevented. Since a nitrile rubber backing or the like which hardly swells with Freon gas is used between them, leakage of Freon gas into the space C can be prevented.

Therefore, even if the accelerating tube is operated for a long time, Freon gas will not leak into the accelerating tube, so there will be no contamination of the accelerating electrode surface, etc., and disassembly and cleaning will not be necessary, which has great practical effects.

In the above-mentioned embodiment, the two concentric 01J packings placed at the connection part between the accelerating electrode and the insulator are installed in the same plane, but they may be installed in different planes. .

Further, in the above explanation, the case where the insulator and the accelerating electrode can be separated from each other was described, but if the accelerating voltage is, for example, 200K,
In an accelerating tube of about V, the insulator and accelerating electrode may be integrated by welding or the like.

In this case, the sealing location between the inside and outside of the acceleration tube (that is, the location where the O-ring backing is doubled) is only the flange portion at the end of the acceleration tube.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing a conventional example, and FIG. 2 is an enlarged sectional view showing an embodiment of the present invention. In the figure, 1a and 1b are accelerating electrodes, 2a to 2c are insulators, 3 is an electron gun, 4 is a housing, 5a, 5b, 5c
, 7a and 7b are 0 ring bucking, 6ay6a'y
6b and 6b' are grooves.

Claims (1)

[Claims] 1 In a device that maintains high voltage insulation by using an insulating gas in the outside atmosphere of an acceleration tube whose inside is kept in a vacuum, two O-ring backings are installed approximately concentrically at the sealing portion between the inside and outside of the acceleration tube. Of these OIJ song socks, a material that releases less gas is used on the vacuum side, and a material that does not swell with the insulating gas is used on the insulating gas side. An accelerator tube.