Abstract:
An improved cylindrical Geiger-Mueller tube with a life at least ten timesreater than that heretofore obtainable includes a re-entrant insulator at each end of the tube to support the coaxial anode and to shield the tube ends from the Geiger discharge.

Description:
GOVERNMENT LICENSE 
     The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     Broadly speaking, this invention relates to radiation detection. More particularly, in a preferred embodiment, this invention relates to an improved radiation detection tube having a useful life which is at least one order of magnitude greater than that therefore obtained. 
     (b) Discussion of the Prior Art 
     Radiation detectors find widespread application in both civil and military environments. By far the most common detector is the Geiger-Mueller tube which is less expensive and more rugged than the scintillation counter. 
     Unfortunately, the Geiger-Mueller tube has been found to have a limited life, which is believed to be caused by cathode degradation in the region of the insulator-cathode seal. For example, prior to the instant invention, commercial G-M tubes, especially small-sized units, had useful lives of from 10 9  to 10 10  counts when operated at moderate loadings and moderate over-voltages. The end of useful tube life occurs when the counter no longer yields an accurate measure of gamma-radiation. The end of useful life could result, for example, from the end of the tube&#39;s plateau voltage falling below the operating point. This reduction in plateau length results from increased secondary electron emission from the tube&#39;s cathode and manifests itself as multiple tube counts. 
     Investigation has revealed that the undesirable secondary emission is caused by damage to the cathode&#39;s passivation layer. More particularly, within the G-M tube, the abrupt termination of the glass frit seal and the metal electrodes distorts the applied electric field creating field intensities of such magnitude that the aforementioned damage to the passivation layer can occur. The above observations have been confirmed by the use of a scanning electron microscope which clearly revealed the existence of an abrupt insulator-metal interface in the cathode seal region of several commercial G-M tubes. Of course, in such tubes the secondary emission would increase with rising voltage to the point where multiple tube counts would cause the G-M tube to go into continuous discharge. 
     SUMMARY OF THE INVENTION 
     The problem, then, is to design a Geiger-Mueller tube which has an insulator design such that not only are the anode and cathode electrically isolated, one from the other, but the dielectric-metal interface is shielded from the Geiger discharge. 
     As will be disclosed below, this problem has been solved by a re-entrant insulator design which can be applied to an all cylindrical metal-walled G-M counter. In this novel design, the shielding of the discharge from the field-distorted region near the dielectric-metal interface means that the ions that reach the cathode will impinge upon its passivated surface far more uniformly and with less destructive energy than heretofore. This results in a more stable Geiger-Mueller plateau and hence a longer useful life. 
    
    
     The invention and its mode of operation will be more fully understood from the following detailed description when taken with the appended drawing in which: 
     DESCRIPTION OF THE DRAWING 
     FIG. 1 is a cross-sectional view of an illustrative Geiger-Mueller tube with a re-entrant insulator according to the invention; 
     FIG. 2 is a cross-sectional view of an alternative insulator of somewhat simpler design; and 
     FIG. 3 is a cross-sectional view of yet another alternative insulator which is self-aligning. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a cross-sectional view of an illustrative Geiger-Mueller tube according to the invention. It will be understood that the particular tube shown is not intended to be limiting and that the invention has application to Geiger-Mueller tubes of other configurations. Indeed, in its broadest sense, the invention is not limited to Geiger-Mueller tubes at all, but has application to any electronic device where a distorted electric field contributes to destruction of a passivation layer and hence secondary electron emission. 
     As shown, Geiger-Mueller tube 10 comprises a cylindrical metal cathode 11 with a metal anode 12 coaxially positioned therein. At one end, anode 12 is surrounded by a glass tubulation 13 which is used to evacuate the tube and then to introduce the desired gas mixture. Glass tubulation 13, in turn, is supported by a re-entrant insulator 14 which has an inwardly extending portion 16 and an outwardly extending portion 17 at the far end thereof. 
     A glass-frit seal 18 seals the inwardly extending portion 16 to the tubulation 13 while a second glass-frit seal 19, through which the tubulation 13 extends, seals the ends of insulator 14 and cathode 11. 
     In like fashion, at the other end of the tube, anode 12 is supported by a glass bead spacer 21 which, in turn is supported by a second re-entrant insulator 14&#39; essentially identical to insulator 14 at the other end of the tube. A third glass-frit seal 22 seals the anode 12 to the glass bead spacer 21 and the inwardly extending portion 16&#39;. A fourth glass-frit seal 23 seals insulator 14&#39; to the cathode 11. 
     As previously mentioned, the tube 10 is first evacuated through tubulation 13, then filled with a suitable gas mixture, for example, a halogen-rare gas mixture comprising 175 Torr of 6.2% Neon, 0.10% Argon and 93.7% Helium plus 2.5 Torr of Bromine. The electrodes may comprise any of the metals heretofore employed, for example, copper which has been chromium plated. At least the surface of the chromium plating should be oxidized to form chromium sesqui-oxide to form a protective layer which keeps the gases, particularly the Bromine, from direct contact with the electrodes. Alternatively, the anode may comprise pure tungsten and the cathode a Chromium Manganese, Silicon, Carbon Iron alloy. Electrical connection to the tube is made by way of a wire ring positioned about the outside of the cathode and a crimped lug positioned around the anode wire extension (not shown). 
     FIGS. 2 and 3 respectively depict alternate embodiments for insulators 14 and 14&#39;. The embodiment shown in FIG. 2, for example, is a somewhat simpler design wherein the inwardly extending portion 31 and the outwardly extending portion 32 are aligned. 
     The embodiment shown in FIG. 3 is more complex. However, it has the advantage that it is self-aligning within the cathode shell while still keeping the inner frit-seal protected from the outside, the tube thus being less susceptible to damage. The self-aligning feature is obtained by splitting the outwardly extending portion 32 into two separate portions 32a and 32b, as shown. 
     Insulators 14, 14&#39; are advantageously made from glass or ceramic, although other non-conducting materials of similar characteristics may also be used. 
     In summary, the novel re-entrant insulator disclosed and claimed herein yields a Geiger-Mueller tube which: 
     (1) has a longer tube life--at least a tenfold increase in counts before failure; 
     (2) a more stable and longer Geiger plateau; and 
     (3) the ability to alter the length of insulator protruding into the tube, thus permitting the tailoring of the active volume and, hence, controlling the radiation response of the tube without altering its overall physical dimensions. 
     One skilled in the art can make various changes and substitutions to the layout of parts shown without departing from the spirit and scope of the invention.