US2502331A - Geiger-muller counter for radioactive emission - Google Patents

Description

L. MALTER March 28, 1950 GEIGER-MULLER COUNTER FOR RADIOACTIVE EMISSION Filed Dec. 21, 1948 MGM/777M -IIIIIIIIIIII,IIIIIIIIIIII1,111,111,111,

INVENTOR lazuli BY {m ATTORNEY Patented Mar. 28, 1950 GEIGER-M'ULLER COUNTER FOR RADIO- ACTIVE EMISSION Louis Malter, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 21, 1948, Serial No. 66,526 7 Claims. (01. 250-2'1.5)

This invention relates to Geiger-Muller counters and particularly to such counters in which spurious counts are reduced to a negligible quantity.

Geiger-Muller counters vary as to structural forms and arrangements, but consist essentially of an envelope or tube permeable to nuclear radiations in which tubes are mounted an anode and a cathode, with suitable leads passing through the envelope for connections to the external circuits. The envelope is exhausted in the conventional manner and certain gases or mixtures of gases are then drawn into the tube to provide a medium within the envelope susceptible to ionization by collision by nuclear radiation and primary and secondary cosmic rays.

A source of high electric potential is connected. between the anode and cathode and a high resistance, usually of the order of 10 ohms, is connected between the anode and cathode. The anode and cathode are also connected to a current indicating device.

In operation, the potential across the anode and cathode is adjusted to a value that in. the

absence of any nuclear radiation penetrating" the medium of the'tube, the potential is just'insufficient to break down the resistance of the medium. However, when nuclear radiation penetrates the medium, ionization of the medium occurs and large conduction currents flow in the external circuit. The electrons are drawn to the positive electrode, anode, and the positive ions are drawn to the cathode. Upon the flow of current, the drop, of potential along the high external resistance serves to restore the medium to its nonnumber of radiation units permeating the medium.

The spurious counts are believed to be due to electron emission from the cathode caused by the positive ions, produced by ionization, being deposited upon a thin insulating film or scattered particles of insulating material on the cathode. These ions set up a sufficiently strong field between themselves and the metal backing of the cathode to produce field emission of electrons from the metal through the insulating film.

These electrons pass into the medium and cause ionization as previously described, and the indicating means responds to the produced conduction currents.

Efforts have been made to eliminate the thin insulating film or particles on the cathode to avoid spurious counts, but without success. The, exact nature of the film or particles is not known, but it is believed to be a form of oxides of the. cathode material or it may be caused by or consist of impurities. The cathodes of these tubes have been cleaned in the past with meticulous care and spurious counts have been somewhat reduced, but the spurious counts have not been eliminated. The principal object of the invention is to provide improved methods of and means for eliminating spurious counts in Geiger-Muller counters.

Another object of the invention is to elimi ter tubes.

Other objects will be apparent from the disclosure of the invention as hereinafter set forth in detail and from the drawings made a part hereof in which Figure 1 is a diagrammatic sketch of a tube of the invention in which the cathode is heated by a coil of resistance wire; Figure 2 is a diagrammatic sketch of a tube of the invention in which the cathode is heated by a pancake resistance unit; and Figure 3 is a diagrammatic sketch of a tube of the invention, the cathode of which is heated by a current passing through the cathode.

Similar reference characters are applied to similar elements throughout the drawings,

Referring to Figure 1, numeral I represents the envelope or tube of a Geiger-Muller counter tube. The anode 2 is, in this embodiment, mounted in the longitudinal axis of the tube 1 and is held taut in position by spring 3 secured at the end of tube I in any conventional manner. The cathode 4, shown as a cylinder, is mounted coaxially the tube l. Leads 5 and 6 are the terminal connections of anode 2 and cathode t, respectively. Resistance 1 and a source of electric potential 8 are serially connected between leads 5 and 6. Surrounding cathode 4 is a resistance wire coil 9 connected to a second source of electric power l0.

Referring to Figure 2, the anode 2 and cathode 4 are in the form of plates secured in parallel position by any conventional means. The heating coil 9 is shown as a fiat or pancake type in contact with cathode 4 and connected to the second electric source HI. Resistance 1 is serially connected with the first electric source 8 between terminal leads .5 and- 6.

In Figure 3; the-:tube. construction isvsimilar'toz that shown in Figure 1, except that the cathode 4' is heated by passing electric current from the: second source I!) through the cathode 4 by connecting terminal lead 6 to source-.=l0,i.the other:

connection to the second source l0 beingiwire' I l which is connected to the farend of cathode l.

nitude' suflicient to produce field' emission' of electrons from the cathode.

Thecurrentsithroughthe heating; coils is so. adjusted that thermal electron emission from the 'heatingcoili'or the cathode: tdoes not occur and yet so adjusted that the resistivity of the thin film on the cathode is reduced; The condition of maximum currents in the'heatin'g coilsfof'this. invention may be: determined by observing; the, maximum current at which, the "counter tube in dicates' no ionization i whentthe mediumv of the.

tube' is not" subjected to" nuclear radiation;

Likewise, the minimum curr.ent is observed; as

that. at whichno spurious. counting occurs.

In each .of theembodiments of the invention. described heretofore, output" energy from leads;

5 and. B may be appliedto any, conventional Geiger-Muller."indicator device which. may include as much amplification; as iv desired.

I claim. as mytinvention;

1. The method of yeliminatingspurious"counts by. a'Geiger=Miiller:counterttubegwhich .tub'eriin. eludes" an. anode. and' a" cathode; comprising:

the applying of heat'to saidcath'ode inan. amount sufficient to reduce the internal tube impedance substantially and in :an'amount lessthan results in substantial cathode thermal electron emission.

2'; The -methodof f eliminating spurious counts by a. Geiger-Muller counter tube whichtubeine- 4. eludes an anode and a cathode on which cathode is a thin insulating film or scattered particles of insulating material, comprising: the applying of heat to said cathode in an amount sufiicient to substantially reduce the resistivity of the said insulating film or particles whereby the said film 01:. particles becomes conductive and in an amount less thaniresults; in substantial cathode thermal electron emission.

3. The method of eliminating spurious counts by a Geiger-Muller counter tube which tube includes an: anode. and a cathode, comprising: passing a current through said cathode, whereby the said cathode becomes heated, the value of v.said current.being sufficient to reduce the tube internal impedance and less than an amount which results in substantial cathode thermal electron emission.

4. A. Geiger-Muller counter tube comprising: an anode, a cathode, and means for heating said cathode in an amount/sufficient to reduce the internal tubeimp'edance substantially and in an.

amount'less'than resultsin substantial cathode thermal electron emission.

5. A Geiger-Muller counter tubev comprising: an anode; acathode; an electricresistance coil juxtaposed said cathode; and-means for passing an electric current through said coil, the value of said current being sufiicient to reduce the tube'internal impedance and less than an amount. wiriichresults in substantial-cathode thermal electron emission.

6; A Geiger-Muller countertube comprising: an anode; a cathode; and'means for passing an electric current-through said cathode electrode,

whereb'ysaid 'cathode is heated, the value of said current being suificie'nttoxreducethe tube in ternal impedance and less-than antamcunt which results inzsubstantial cathode thermal electronemission 7. A' Geiger-Muller counter tube comprising:

anranode; acathode, thesaicl cathode having an' insulating film or scattered particlesof insulat ing:materialrthereon; means 'forheating said cathode :inianxamountsufficient to substantially reduce :thelzresistivity of said film: or" particles whereby the'rsaid' 'film' or:parti'cles become conductive and iman amountless'than results'in substantial cathode thermal electron emission.

LQUIS. MALTER.

No references cited;

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