US2715195A - Photon-counter with adjustable threshold - Google Patents

Description

9, 1955 H. FRIEDMAN PHOTON-COUNTER WITH ADJUSTABLE THRESHOLD Filed July 19, 1946 gwumvtom x aw III I HERBERT FRIEDMAN Unite States Patent Ofiice 2,715,135 Fatented Aug. 9, 1955 PiifiTUN-CGUNTER WITH ADJUSTABLE THRESHOLD Herbert Friedman, Arlington, Va.

Application July 19, 1946, Serial No. 684,907

1 Claim. (Cl. 313-93) (Granted under Title 35, U. S. Code (1952), sec. 266) My invention relates to Geiger-Mueller counters and has particular reference to a form of Geiger-Mueller counter which may be made selectively sensitive to a particular portion of the electromagnetic radiation spectrum.

An object of my invention is to provide a Geiger-Mueller counter which is sensitive to radiations in the ultraviolet spectrum.

An object of my invention is to provide a Geiger-Mueller counter having an adjustable lower frequency threshold, so that it can be made sensitive only to radiations above a particular frequency.

Still another object of my invention is to provide a Geiger-Mueller counter which will be sensitive to ultraviolet radiation but not to visible radiation.

A further object of my invention is to provide a Geiger- Mueller counter tube which is sensitive to a relatively narrow band of frequencies of electromagnetic radiation, which frequency band may be predetermined by the construction and operation of the counter tube itself.

Still further objects of my invention reside in the de tails of construction, combinations of elements, arrange ment of parts and operation of the device as described hereinafter.

The Geiger-Mueller counter is a device with a wide range of applications for the measurement of electromagnetic radiation. In its basic form it comprises an anode wire surrounded by a cylindrical cathode, both elements being enclosed in a radiation permeable envelope containing a gaseous filling. The gas filling is preferably hydrogen or a mixture of inert gas or gases, such as argon, neon, krypton, or xenon with oxygen, in the ratio of about ten parts of inert gas to one of oxygen. The gas is admixed with a small amount of organic vapor such as alcohol, xylene, or petroleum ether as a quenching agent.

In operation the Geiger-Mueller counter tube is connected to a source of potential of the order of a thousand or more volts. Radiation entering the tube produces an ionization of the inert gas filling either directly in the gas or by causing the emission of electrons from the cathode surface. When the potential applied across the tube elements is of the magnitude of about a thousand volts, a chain of ionizations occurs within the inert gas such that in a very short period of time a gaseous discharge occurs between the cathode and anode resulting in a decreased potential between these two tube elements. Were it not for the deionizing effect of the quenching vapor or of an external electronic circuit, such as the Neher-Harper or Neher-Pickering circuits, the gaseous discharge might continue unabated for an indefinite period of time. These quenching systems act so rapidly, however, that within a few microseconds the gaseous discharge has stopped, the inert gas is deionized, and the Geiger-Mueller counter tube is again ready to repeat the counting cycle.

The initial ionization may be produced by the effect of penetrating radiation such as hard X-rays directly upon the inert gas, or by the elfect of lower frequency radiation upon the cathode surface of the tube, which surface will produce photoelectrons which will in turn initiate ionization in the gas. It is this latter type of action with which this invention is particularly concerned.

The Geiger-Mueller counter may be applied very advantageously as a means for measuring the intensity of radiation in the ultraviolet region of the spectrum. In such a counter the ionizing action depends upon the emission of photoelectrons from the cathode surface when it is activated by radiation in the ultraviolet region. A Geiger-Mueller counter for this purpose effectively counts the photons of radiation striking the cathode surface and it is necessary, therefore, to construct the cathode of such a material that it will be effectively photosensitive to the particular radiation concerned. It is further necessary that the envelope be of a radiation permeable material or have a Window which will transmit the radiation to be measured without significant loss of intensity.

A particular difficulty which is encountered in using the Geiger-Mueller counter as a means for detecting ultraviolet radiation, however, is that heretofore no means have been available for controlling the bandwidth sensitivity of such a device. Consequently visible radiation, as well as the invisible ultraviolet radiations from a particular source, would be detected, with a resultant confusion in the interpretation of the counter output. My invention, by providing a means for electrically excluding the visible spectrum from the counter, when required, permits its use in a series of embodiments which were heretofore impracticable.

Accordingly my invention comprises a Geiger-Mueller counter having an anode and a coaxially mounted cathode having a surface capable of emitting photoelectrons when subjected to radiations of a particular frequency range. In combination with the cathode I use a fine mesh which is supplied with an electron retarding voltage which permits only photoelectrons having an energy or electron voltage equal to or greater than the mesh potential to pass through the mesh and be counted. Since the retarding potential may be adjusted, and since the energy of a photoelectron depends upon the frequency of the photon producing this effect, the device, in consequence, controls the threshold frequency sensitivity for the Geiger-Mueller tube and permits selective measurement of radiation in a narrow frequency band.

For a better understanding of my invention, reference may be had to the accompanying drawings wherein:

Figure l is a rear view of an embodiment of a Geiger- Mueller counter tube made according to my invention;

Figure 2 is a side view of the tube rotated 90 from the position of Figure 1;

Figure 3 is a cross-sectional representation taken along the line 3-3 of Figures 1 and 2.

The Geiger-Mueller counter tube shown in Figures 1, 2, and 3 comprises a wire anode 10 which is partially enclosed in glass capillary tubes 11 and 12 and exposed only throughout that portion of its total length contained within the cathode cylinder 13.

The cathode is constructed in the form of an opensided cylinder so that radiations may enter and strike the photosensitive inner surface which is so prepared that it is photo-sensitive to the band of radiation which is to be detected. in close proximity to the active surface of the cathode, a fine wire mesh 14 is mounted and insulated therefrom by insulating supports l5, 16, 17 etc.

The cathode cylinder is terminated on each end by insulating discs 18 and 19, preferably mica, which serve to support the cathode structure in place by means of metal supports 20, 21, 22, 23. The anode wire 10 is connected to an external cap 24 through a metal-glass seal, while the cathode structure is connected to wire 25 and thence to external cap 26. The wire mesh grid 14 is connected to external cap 27. The mesh may be supported either by a series of thin insulating strips cemented longitudinally, i. e., parallel to the axis of the cathode cylinder as shown in Figure 3, or by two or more thin insulating strips affixed to the cathode cylinder perpendicular. to the axis of the cylinder and cemented to the inner surface of the cathode. In both cases the mesh spacing is approximately determined by the thickness of the insulating spaces which is preferably about 0.5 millimeter.

In order for the radiation to enter the counter without significant loss of intensity, a quartz or other radiation permeable window 28 is cemented to the envelope and supported with a retaining ring 29. In a variation of this design the side of the glass envelope through which the radiation is to be admitted maybe blown into a thin bubble window through which substantially all of the radiation may pass. If the tube is to be used for the detection of ultra violet radiation, a preferred type of gaseous filling consists of a mixture of alcohol vapor at 4 millimeters, and argon at 4 centimeters of mercury pressure. Other gases such as hydrogen may also be used, however.

The photoelectric effect consists of the liberation of photoelectrons by a material when subjected to electromagnetic radiations. In effect the energy of the impinging radiation is transferred to certain electrons in the photosensitive material, and these electrons are thereby enabled to escape past the surface forces which hold them in the material. It is an established physical principle that electromagnetic radiations have associated with them an amount of energy which may be considered as existing in discrete units or photons. The amount of energy of these photons of radiation is proportional to the frequency. Thus, a higher frequency radiation will have associated with it a large amount of energy, and consequently its photoelectric effect upon the material upon which it falls will be great.

In this regard, the materials exhibiting photosensitive properties vary in their behavior with regard to the energy required for photoelectrons to be liberated. Each metal may be said, therefore, to have a photoelectric threshold which can be expressed in electron-volts or in units of consequently the photoelectric threshold of a surface determines the minimum frequency which may produce electrons.

The mesh material is selected for having a low photoelectric threshold, and such metals as platinum and nickel are satisfactory in this respect, although an alternative means for obtaining a low threshold mesh is to use a screen of a metal such as copper, and to prepare the surface by applying a thin layer of a clear lacquer or 'glyptal diluted with amyl acetate.

For a cathode, such metals as copper, zinc, tungsten or silver have been found satisfactory for use in the ultraviolet regions of the spectrum. In the case of copper,

however, it is sometimes desirable to lower the threshold for photoelectric emission by a treatment comprising slightly oxidizing the surface by applying a coating of an iodine film, which may be accomplished by painting the cathode with a solution of iodine in alcohol, or by applying a thin lacquer film which may be accomplished by painting the surface with a solution of one drop of pure lacquer in 25 cubic centimeters of amyl acetate.

In the operation of the preferred form of this invention the tube is oriented in such a manner that the radiation permeable window receives the radiations to be measured. A potential of the order of a thousand volts is applied between the anode and cathode and a potentiometer and voltage dropping resistor or installed in the circuit so that the electron retarding mesh may be supplied with a voltage which varies from zero to about 10 volts negative with respect to the cathode. When the voltage of the mesh is made increasingly negative the threshold for counting of ultraviolet radiation moves from the visible spectrum to the direction of higher frequencies. With the proper adjustment, which must be empirically determined for a particular embodiment of this invention, the spectrum of radiation which will be counted by the tube will depend, for its lower frequency limits, upon the voltage setting of the mesh, and for its upper frequency limits upon the photochemical sensitivity properties of the cathode surface itself.

It is to be noted that the band selecting eflFect described herein is obtained by coupling the retarding effect of the negative mesh voltage upon the electrons produced at the cathode with the chemical nature of the cathode. It is known that photons of radiation have as an inherent property a particular energy which is related to the frequency of the radiation. In consequence, the frequency of radiation striking the cathode surface will determine the energy of the liberated photoelectrons. Higher frequency radiations will produce photoelectrons of higher energy content, consequently a more negative retarding voltage on the mesh will permit only those electrons to pass which were liberated by higher frequencies of incoming radiation.

A Geiger-Mueller counter tube constructed according to this invention may be so designed and operated as to be sensitive to a rather narrow band of frequencies. The upper frequency limit will to some degree be determined for a particular counter by the composition and preparation of the cathode surface and of the gaseous content of the tube.

Another effective method for controlling the upper frequency range is by'the use of radiation filters constructed of special types of glass and mounted in front of the window of the tube. Such metals as iron, calcium, zinc, and aluminum, when used as cathodes, have a sensitivity to wavelengths of 4000 Angstroms or lower. It is likewise possible to produce a cathode structure which is sensitive in the visible spectrum by using a cathode of an evaporated and deposited film of a metal such as calcium, magnesium, or sodium. The mesh structure provides a control of the lower extreme of the frequency spectrum by, in efiect, preventing photoelectrons which have energies below a particular threshold from entering the ionizable gas counting portion or active volume of the tube.

An important consideration in selecting the mesh composition is that during operation of the counter tube the mesh itself must either be constructed of a metal which because of a high threshold is incapable of emitting photoelectrons, such as platinum or nickel, or the surface of the mesh must be covered with a material which prevents the emission of photoelectrons. A mesh of copper, brass, or tin may be used, provided its surface iscoated with a thin layer of an insulating material such as glyptal or diluted clear lacquer.

A mesh so coated, however, presents an operational problem, since during normal operation of the tube the positive ions produced by the gaseous discharge tend to migrate to and collect about the mesh, until such an accumulation of positive charge is present that no electrons emitted at the cathode are allowed to pass the mesh surface. When the mesh consists of a conductive material such as untreated platinum or other metal having a high photoelectric threshold the ions are conducted off from the mesh and no charge accumulates. When, however, a metal is used for the mesh which has a low photoelectric threshold and which must consequently be treated by coating with an insulating material the ions cannot be removed and a strong positive charge accumulates, effectively blocking the tube.

To overcome this condition special arrangements must be made to discharge the mesh. Probably the most effective means is to apply a square wave to the mesh which periodically makes its potential positive with respect to the cathode. By this method the positive ions are attracted away from the mesh and toward the cathode, Where their charge is removed. If electronic quenching means are used in the circuit this mesh discharge should be synchronized with the quenching action in the tube whereby the potential on the anode is momentarily lowered to deionize the gas in the tube and stop the gaseous discharge in the counter tube.

In the foregoing specification it is seen that my invention comprises a Geiger-Mueller counter capable of selectively detecting a band of electromagnetic radiation, thereby making possible the exclusion of unwanted detection of other frequencies.

It will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claim.

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:

A Geiger counter of adjustable frequency threshold comprising an envelope having at least a portion thereof transparent over a range of radiation frequencies, a cathode within the envelope having a photo emissive surface positioned to receive radiation through said envelope portion and responsive over a range of received radiation frequencies to emit photo electrons at a velocity determined by the incident radiation frequency, an anode facing and parallel the photo emissive surface of the cathode, a Geiger counting gaseous filling in the envelope comprising an ionizable gas with a minor amount of quenching gas, and a photo electrically inactive grid electrode structure interposed between the cathode and anode and adjacent the former operative under negative bias selectively to prevent entry of low velocity electrons from the cathode in the grid-anode space.

References Cited in the file of this patent UNITED STATES PATENTS 1,668,383 Smith et a1 May 1, 1928 1,809,676 Culver June 9, 1931 2,103,498 Schroter Dec. 28, 1937 2,316,772 Dench Apr. 20, 1943 OTHER REFERENCES Photoelectric Quantum Counters for Visible and Ultraviolet Light, Locher, Physical Review, November 15, 1932, vol. 42, pp. 525-528.

Korfi and Ramsey, Article in Review of Scientific Instruments, August 1940, vol. 11, pages 267-269.

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