US2657316A - Method of suppressing photoelectric threshold - Google Patents

Description

Oct. 27, 1953 H. FRIEDMAN 2,657,313

METHOD OF SUPPRESSINGPHOTOELECTRIC THRESHOLD Fixed- Nov. 7, 1950 INVENTOR HERBERT FRIEDMAN BY nQ ATTORNEYS Patented ct. 27, 1953 METHOD OF SUPPRESSING PHOTO- ELECTRIC THRESHOLD Herbert Friedman, Arlington, Va. Application November 7, 1950, Serial No. 194,543

(Granted under Title 35, U. S. Code (1952),

sec. 266) Claims.

This invention relates to Geiger-Mueller tubes.

More specifically this invention relates to a novel structure for a Geiger-Mueller tube, which provides a means for increasing the effective photoelectric threshold value of the cathode electrode.

It is generally known that metals give off electrons when electromagnetic energy exceeding certain energy value impinges thereon. This energy level above :which electrons are liberated is called the work function of the metal and is usually given in volts. This effect is called the photoelectric effect and electrons emitted thereby are called photoelectrons.

When the cathode of the Geiger tube is struck by energy exceeding the work function of the metal of which it is made, the electrons emitted thereby cause erratic and undesirable operation of the Geiger tube. The electrons which it is usually desired to free in a Geiger tube are those due to the ionization of the gas molecules and not those due to photo-emission of the cathode.

Some prior art Geiger tubes eliminated this undesirable photoemission by placing a grid structure adjacent the cathode in the space between the cathode and anode electrodes. By applying a negative voltage to this grid, any electrons which were emitted by the cathode due to this so called photoelectric effect were turned back to the cathode by the negative grid voltage thus preventing photoelectrons originating from the cathode from reaching the anode or the space between the control grid and anode. This control grid, however, also has been found to emit electrons by photoelectric efiect though perhaps to a smaller extent than the solid cylindrical cathode.

The present invention also utilizes a control electrode to prevent photoelectron emission from the cathode but its relationship to the cathode is entirely difierent from grid structure of the prior art.

The present invention comprises the combination of a cathode made in the form of a mesh structure, a control electrode located on the side of the cathode remote from the anode. and an insulating structure placed between the control electrode and cathode.

One object of the present invention is therefore to provide a novel Geiger-tube structure wherein photoelectric emission of electrons by the tube electrodes is substantially reduced.

Another object of the present invention is to provide a novel Geiger tube structure wherein the photoelectric emission of electrons from the cathode and control electrode is reduced to a minimum.

These and other objects will become apparent to those skilled in the art upon reading the specification to follow and the attached drawings wherein:

Figure 1 is an elevational view of the elements of a Geiger tube forming the present invention.

Figure 2 is a cross sectional view of figure along section line 2-2.

Referring now to the drawings where like reference numbers refer to the same elements, the tube of the present invention comprises a cylindrical cathode I and an anode 2 preferably coaxially disposed within the cathode cylinder. The cathode cylinder is made in a mesh-like structure for reasons which will be hereinafter explained.

Surrounding the cathode I is a second cylindrical electrode 3 which is made positive with respect to cathode I so that electrons which tend to be emitted by photoelectric effect from cathode l are drawn back either to cathode I or possibly to electrode 3 due to the force of the positive electric field emanating from the latter electrode. As noted above the cathode is made in the form of a mesh so that the electric field from electrode 3 can effect to a greater degree the photoelectrons which are liberated from the surface of the cathode l facing anode 2. Interposed between the cathode I and the outer electrode 3 is a cylindrical member 4 made of an insulating material such as mica. This element prevents any photoelectrons which may be emitted by electrode 3 from reaching the main portions of the Geiger tube, and likewise prevents any electrons emitted by cathode I from striking control electrode 3.

The numeral 5 represents the enclosing envelope of the Geiger tube which may be made of any well known suitable material such as glass. As is well known in the art, portion 3 of envelope 5 must be transparent to the type of energy to be detected by the Geiger tube.

One specific embodiment found satisfactory comprises a fine cathode mesh I wires to the inch), spaced about .4 of an inch from the anode. The cathode mesh was located adjacent the cylindrical mica member 4 which was about .00015 inch thick.

The cathode electrode mesh I should have a large ratio of open area to wire area to ensure that electrode 3 has an appreciable effect in preventing the photoelectrons emitted from cathode 3 from being drawn to the anode electrode 2.

The thickness of the insulation member 4 should be as thin as possible to increase the effectiveness of electrode 3.

The cathode electrode should be made of a material which will not readily emit photoelectrons at the wavelength at which it is desired to have a negligible photoelectric efl'ect.

Many modifications may be made of the specific embodiment just disclosed without deviating from the scope of the present invention.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. Means for reducing photoelectron emission from a Geiger tube comprising a control electrode surrounding the Geiger tube cathode electrode, means coupled between said control electrode and said cathode electrode for rendering said former electrode positive with respect to said latter electrode, a solid insulating member through which electric fields may freely pass located between and substantially coextensive with the opposed areas of said two electrodes for preventing passage of electrons therebetween.

2. A Geiger tube comprising the combination of an anode member a cathode member surrounding said anode member, said cathode memher having a mesh-like structure, a control electrode surrounding said cathode, and a solid insulating member through which electric fields may freely pass located between and substantially coextensive with the opposed areas of said cathode and control electrodes for preventing the passage of electrons therebetween.

3. A Geiger tube comprising the combination of an anode electrode, a cathode electrode surrounding said anode electrode and spaced a predetermined distance therefrom, said cathode electrode having a mesh-like structure, a control electrode surrounding said cathode at a distance substantially less than said predetermined distance, and insulating means located between said cathode and control electrodes for preventing passage of electrons therebetween.

4. Means for reducing photoelectric emission from a Geiger tube comprising a control electrode surrounding the Geiger tube cathode electrode at a distance substantially less that the distance between said cathode electrode and the Geiger tube anode, said cathode electrode having a mesh-like structure, means coupled between said control and cathode electrodes for rendering said control electrode positive with respect to said cathode electrode, and insulating means located between said control and cathode electrodes for preventing passage of electrons therebetween.

5. In a Geiger tube the combination of an anode electrode, a cathode electrode having a mesh-like structure surrounding said anode electrode, and means for reducing the photoelectric emission from said cathode comprising a control electrode surrounding said cathode electrode and having a positive potential with respect thereto, said control electrode being closely adjacent to said cathode relative to said anode-cathode spacing, and the potential difference between said control and cathode electrodes being such that the electric field due to said control electrode is at least as great at said cathode as that due to said anode.

HERBERT FRIEDMAN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,092,804 Jobst Sept. 14, 1937 2,107,518 Bull et a1 Feb. 8, 1938 2,197,453 Hassler Apr. 16, 1940 2,524,100 Dauvillier et a1. Oct. 3, 1950 OTHER REFERENCES Use of a Grid etc., Korff et al., Rev. of Sci. Inst, August 1940, vol. 11, #8, pages 267-269.

Increased Gamma-Ray Sensitivity, etc, Evans et al., Rev. of Sci. Inst, December 1936, vol. '7, pages 441-449.

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