US2856551A - Gaseous discharge tube - Google Patents

Description

Oct. 14, 1958 E LYNCH 2,856,551

GASEOUS DISCHARGE TUBE Filed Nov. 1, 1956 I2 I I INVENTOR.

ATTORNEY R bert E. Lynch GASEOUS DISCHARGE TUBE Appiication November 1, 1956, SerialNo. 619,846

1 Claim. or. sis-54y The invention pertains to gaseous discharge tubes and particularly to the gaseous filling of such tubes.

Two general types of gaseous discharge tubes are known in the art; the so-called hot-cathodegas tube and the glow discharge tube. They are similar in that almost complete ionization of a gaseous filling is required to attain the desired mode of operation. The ionization current in both types is substantially independent of the potential applied across the electrodes therein, so both may be considered to be constant current devices. The dissimilarities between the two types center mainly in the mechanism by which the required ionization is initiated.

In the hot-cathode gas tube, a thermionic cathode furnishes a source of electrons toxinitiate ionization of the gas. When the potential difference betweenthe cathode and the anode exceeds a certain critical value (about to volts in the ordinary case), sufficient kinetic energy is imparted to the emitted electrons to cause ionization of the gas by collision. Once the process of ionization has begun, successive collisions between the individual molecules of the gas multiply until saturation is substantially attained. The ionization current then reaches a stabilized and constant value, although a small increase in this current may be noted if the potential difference is increased considerably.

The glow discharge tube, however, has a cold or non thermionic cathode. Ionization is initiated by applying a potential difference (at least 100 volts in the ordinary case) between the anode and the cathode. Any ionized molecules present in the gas filling the tube are, therefore, accelerated toward the appropriate electrode. However, the mean free path of such ionized molecules is'so short in comparison to the distance between the electrodes that collisions between ionized and non-ionized molecules are certain to occur. The process is cumulative until a substantially constant ionization current is passed by the tube in the form of a glow type of discharge.

The exact potential required to create a glow discharge depends on many factors. Some, such as the geometry of the electrodes and type and pressure of the gas used, may determine constant parameters establishing the operation of the tube. Other factors, such as the degree of residual ionization of the gas when operation is initiated, must be treated as variables. While operationin accordance with desired constant parameters" may be obtained by proper design and by care in manufacture, the degree of residual ionization, depending as it does on the conditions under which the tube is used, cannot so be controlled in the common glow discharge device.

Independence of the degree of residual ionization of the gas in a glow discharge tube from environmental conditions may be attained by incorporating an ionizing source within the tube itself. More than one way of providing an ionizing source within a glow discharge tube is known. Thus, in some glow discharge tubes, selfluminous materials have been used to provide sufficient luminous flux to maintain a certain minimum degree of ionization of the gas and in other such tubes solid nited States Patent 0 radioactive materials have been disposed on various electrodes to accomplish the same purpose by radioactive emission. Of these two ways, the latter has proved the more efficient and offers the better promise of attaining the desired end.

Great difficulties are encountered in the use of a solid radioactive material for stabilizing the striking voltage of a glow discharge tube. Briefly, the difficulties encountered are: (1) The radioactive material may contaminate the electrodes in the tube to cause malfunctioning; (2) harmful radiations therefrom expose using or handling personnel to danger; and (3) safe disposal of a tube containing a solid radioactive material is not easy when the tube has reached the end of its life. Unfortunately, all known solid radioactive materials exhibit at least one of the enumerated deficiencies to a serious degree.

In addition to freedom from the aforementioned difficulties, any radioactive material used as an ionizing source in a tube should have a halflife of at least three years. In view of the time lag normally encountered between manufacturing and using a tube and the life expectancy of any tube in operation, the benefits and ad vantages of a radioactive ionizing source will not be obtained, in many cases, if the half-life of the radioactive material is less. While there are many radioactive materials having a half-life greater than the desired minimum, almost all such materials are either too expensive or too dangerous to use in glow discharge tubes. Further, the safety with which the radioactive material may be handled is of primary importance during manufacture of tubes using radioactive material. Obviously, although efiiciency demands that a relatively large supply of radioactive material be maintained at the manufacturing site, the hazards to health inherent in concentration of radioactive materials make it highly desirable that only small quantities of such materials be kept in any one place. As a result, a sufiiciently large quantity of almost all radioactive materials to attain a high degree of efliciency in manufacturing may not be safely assembled. Even though established safety practices allow 300 mr absorption of radiation per week as the maximum safe dose rate, it is recognized that such a limit should be considered for guidance only and not as a limit to justify unnecessary exposure. If exposure can be reduced by effort in selecting safer materials and developing better techniques, the expense of effecting such reductions is well worth while. This is especially true in view of the psychological effect of improving working conditions for persons who are not too familiar with the mechanism by which radiations affect the human body.

Another problem that affects the use of radioactive materials in tubes is that of contamination. If the material used can diffuse into the electrodes or the envelope, potential hazard is created. Once any part becomes irradiated, it is as dangerous as the irradiating source and must be treated accordingly. Therefore, it is highly important that contamination be avoided.

An object of the invention is to provide a safe, longlived and inexpensive ionizing source for glow discharge tubes;

Another object is to provide a radioactive ionizing source that does not affect electrical characteristics of glow discharge tubes other than to improve the stability of the striking voltage of the glow discharge therein under all operating conditions;

A further object of the invention is to provide a radioactive ionizing source that may be safely and easily used in the construction of glow discharge tubes;

A further object of the invention is to increase the ease with which glow discharge tubes containing radioactive materials may be manufactured and distributed;

A still further object of the invention is to minimize the possibility of contaminating the electrodes in glow discharge tubes with radioactive material used therein.

According to the invention, the desired results may be obtained by using a radioactive gas in filling a glow discharge tube. The gas should have a long life and emit only alpha or beta emanations whereby a desired minimum degree of ionization of the gas may be maintained in a glow discharge tube during the entire life expectancy of the tube. The radioactive gas may be mixed with any of the gases normally used to fill glow discharge tubes as a trace element, being added automatically from impermeable containers adapted to dispense the correct amount of gas into each tube envelope. The amount of radioactive gas in each tube is kept so low that, even if a large number of envelopes were to be broken simultaneously, there will be no ill effects to human beings. Further, in the event of accident, the radioactive material would be quickly dispersed into the atmosphere. However, the radioactivity of the gas is kept sufficiently high so that its alpha or beta emanations continuously ionize the gas in the tube to maintain a stable striking voltage. At the same time, after the life of the tube has expired, disposal of the radioactive gas is easily accomplished by breaking the envelope of the tube to allow the radioactive gas to be dissipated safely in the atmosphere.

In order that all of the practical advantages may readily be attained, an express embodiment, as example only, of the invention is described hereinafter with reference to the accompanying drawing in which the sole figure illustrates a glow discharge tube incorporating the invention.

There is shown in the drawing, a tube consisting of a gas tight envelope 11, preferably of glass, having a plurality of pins 12 and leads 13 sealed in the header portion of the envelope. Individual ones of the leads 13 are connected to a cathode electrode 14 and an anode electrode 15. A starting electrode 16 is integrally aflixed to the cathode electrode 14 as shown. While the particular shape illustrated is not essential to the present invention, it may be used to advantage as described in the co-pending U. S. patent application, Serial No. 463,846, of R. R. Law, entitled Voltage Regulator, filed October 22, 1954, now U. S. Patent No. 2,774,906 and assigned to the same assignee as the present application. The cathode electrode 14 and anode electrode are held in position with respect to each other and with the envelope 11 by means of electrically insulating spacing members 17 commonly made of mica. A gas 18 that may, for example, be a mixture of helium 99%; argon 0.99%; and krypton 85, 0.01%, by volume, at a pressure of 100 mm. Hg., fills the empty space within the envelope 11. The individual gases of the mixture may be changed in proportion or in kind, except for the krypton 85, or the pressure of the gas may be varied in order to change the operating characteristics of the tube. It is desirable, however, that the krypton 85 be the longer-lived isotope of krypton which is free of gamma rays and that its concentration be kept as low as possible consistent with obtaining the desired results in operation as will be explained in more "detail hereinafter. In this connection, it has been found that concentrations as low as 0.01 to 0.02 mc./cc. are satisfactory. Further, when precautions need be taken only against alpha and beta emanations and not against gamma rays, it is evident that manufacturing eificiency will increase because of the increased sense of security given workers.

The introduction of krypton 85 into the envelope 11 presents no particular problem to those skilled in the art. Thus, a mixture of gas containing the proper amount of krypton 85 may be stored in a protected tank placed adjacent the machine used to exhaust and seal the envelope 11 during manufacturing. By connecting appropriately shaped piping and using the proper kind of valve (both of which are well known), the exact amount of gas required to fill the envelope of each tube may be metered and injected into the envelope immediately after the envelope has been evacuated and the tube elements outgassed. Outgassing may be completed easily by a getter assembly 19 of any known type. This operation may be accomplished by automatic machinery so there is no danger to personnel even if the machinery breaks down.

In operation, the tube performs in exactly the same way as the conventional glow discharge tube, except for a greatly improved stability of the striking voltage characteristic brought about by the invention. That is, the potential difference between the starting electrode attached to the cathode electrode and the anode electrode required to initiate a glow discharge between the cathode and anode electrodes does not vary appreciably as operating conditions change. This stability is especially evident when the so-called dark starting voltage is considered. If a tube without an internal source of ionization is kept shielded from light for any appreciable time, it will be found that the striking voltage will vary erratically. However, a tube embodying the radioactive gas of the invention will be free of such failing. In addition, the materials used in glow discharge tubes adsorb only negligible amounts of krypton 85, so the possibility of contamination may be disregarded.

When a tube constructed according to the invention becomes inoperative for any reason, it is a simple matter to render it harmless. By merely breaking the envelope, the gas filling therein is freed to mix with the surrounding atmosphere. The very small amount of krypton 85 almost instantly is so diluted that it is no longer even slightly dangerous.

The invention claimed is:

A glow discharge tube containing a gaseous mixture having a pressure in the range of to 100 mm. of mercury, said gaseous mixture comprising, krypton in an amount of 1 part in 10,000 by volume thereof to provide a radioactive gas emanating only alpha and beta particles and auniform radioactive density within said glow discharge tube of from .01 to .02 microcurie per centimeter, whereby the striking potential of said glow discharge tube may be effectively stabilized and the remainder of said gaseous mixture consisting of an inert gas selected from the group of helium, argon and neon.

References Cited in the file of this patent UNITED STATES PATENTS 2,354,786 Wall Aug. 1, 1944 2,375,130 Perrin et al. May 1, 1945 2,576,100 Brown Nov. 27, 1951 2,616,986 Coleman Nov. 4, 1952 2,678,397 Herzog May 11, 1954 Flu-m

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