US2532956A - Air proportional counter - Google Patents

Description

-2 Sheets-Sheet l1 INVENTOR. .l/1v1 ;Zapasazar /w a, M'

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J A SIMPSON, JR

AIR PROPORTIONAL COUNTER Dec. 5, 1950 Filed Deo. 31, 1946 Dec. 5, 1950 J. A. SIMPSON, JR 2.532,955l

AIR PRoPoRTIoNAL COUNTER Filed Dec. :51, 194e 2 sheets-sheet 2 f8 I @4Z ff .f y... y A f77-HHH v 5228 Wma @WJ an improved center-wire suspension for Lproportional counters.

It is a further object of the invention to provide an air proportional counter which is not subject to spurious counts and discharges.

It is a still further object to provide an electrically conducting window in the cathode of a proportional counter. y

It is a still further object of the invention to provide an improved proportional counter having parallel plate cathodes and a lplurality of equally spaced anode center-wires.

The manner in which the foregoing and other objects of the invention are accomplished is i1- lustrated inthe embodiment of the attached drawing, in which:

Fig. 1 is a plan view of a proportional counter with covering screens to be described below, partially broken away to show the internal construction.

Fig. 2 is a longitudinal cross sectional view taken along the line 2-2 of Fig. 1 in the direction indicated by arrows;

Fig. 3 is a transverse cross sectional view of the proportional counter taken along the line 3--3 of Fig. 2 inthe direction indicated by the arrows;-

Fig. 4 is an enlarged fragmentary cross sectional view of a portion of the counter illustrated in Fig. l, showing certain details of the mounting of the center wires thereof; V

Fig. 5 is a fragmentary cross sectional view illustrating the mounting of an insulator shown in Fig. 4 taken along the line 5-5 of Fig. 4 in the direction illustrated by arrows; and

Fig. 6 is an enlarged perspective View illustrating the manner of suspension of the centerwires of the counter of Fig. 1.

Referring rst to Figs. 1 and 2, a rectangular back plate lll has mounted thereon as by screws at the outer perimeter thereof two Vside walls I2 and two end walls I4, for example of aluminum, defining the body of the proportional counter. The interior dimensions may be, say, 5 inches by l2 inches by inch. On the interior surface of each of the side walls I2 are two grooves I6 adapted to receive insulators I8. As appears more clearly in the transverse View of Fig. 3, each of the insulators I8 consists of a substantially rectangular bar of an insulating material, preferably polystyrene, extending across the counter from one side wall I2 to the other. Spaced along the edge of each of the insulators I8 are transverse notches 20 extending part way through the insulator. As illustrated in Fig. 3, the insulators I8 are wider at the extreme ends thereof than at the central portion in which the notches 20 appear, thus leaving air gaps 22 between the insulator I8 and the back plate Ill and between the insulator I8 and the upper conducting surface hereinafter to be described.

- Centrally of the back plate ID adjacent one end thereof is an aperture 24 (Figs. 1 and 2) through which'enters the center conductor 26 of a conventional coaxial cable connector 21 which is fastened to the back plate II) by conventional means. Two insulating posts 23, preferably of polystyrene,'are equally spaced from the coaxial cable collector 21 along a transverse line slightly forward of the connector 21. The insulating posts 28, as appears more clearly in Fig. 5, are fastened to the back plate I0 by screws 30 extending therethrough. the posts 28 being internally threaded for this purpose. The

edge thereof.

therein adapted to receive in tight-fitting fashion a rod 32, for example of brass. The rod 32, for example 1/8 inch in diameter, is supported transversely of the counter by the insulating posts 28. A connecting wire 34 is soldered at one end to the center conductor 26 of the connector 2 and at the other end to the central portion of the rod 32. Extending transversely of the rod 32, and thus longitudinally Vof the counter, are small apertures 3e in the rod 32, as clearly appears in Fig. 4.

Threaded through each of the apertures 3% is the end of a coiled spring 38. TheV end of each of the coiled springs 38 is prevented from coming out of the aperture 36 by a right-angle bend at the end of the spring. The other ends of the springs 38, as more clearly shown in Flg. 6, are bent into the lorm of small hooks fill which support the center-wires 42 of the counter. The center-wires 42 are preferably of'l mil tungsten wire. At each end of the wire i2 is a small conducting ball 44, preferably spherical, for example of silver solder, substantially integral with the wire 42. rlhe ball 44' at one end of the wire 42 is retained in place by the hook 40 which is smaller in diameter than the ball 44 but greater in diameter than the wire 42. The spring t exerts tension on the wire 42. Each wire l2 traverses the corresponding notch 20 in the nrst insulator I3 and extends to the other end of the counter where the other end of the wire l2 is held in place by the other spherical ball lili which is larger in diameter than the width of the notch 2u in the second insulator I3, so that the tension in the spring 38 holds the ball tightly against the notch 2li, and thus secures the center-wire in place. It will be seen that the center-wires 42 may be readily replaced without the necessity of soldering.

Over the lace or the counter, supported by the walls I2 and I of the counter is a rectangular frame iii constituting the support for a window 48, which is secured, as by cement, to the The window 48 may be of nylon of less than 0.5 mil thicliness, and preferably of 0.2 mil thickness. The window 48 extends ovel` approximately all the face of the counter corresponding to the region between insulators I8 near either end thereof. The window 48 has deposlted upon the inner surface thereof a thin coating of a conducting substance such as graphite (not visible in the drawing). superimposed upon the frame 46 for the window 48 is an addltional frame 5t, preferably of a plastic such as polymerized methyl methacrylate, commonly called lucite. The frame 5f) constitutes the support for a wire mesh screen 52 preferably of stainless steel which extends thereacross and the edges of which are preferably molded into the frame 5t. The frames 4b and 56 are secured to the walls l2 and lll by screws 4l and 5I, respectively. The window 4S constitutes, with the back plate I0, a parallel-plate cathode, the anode center wires 2 preferably being approximately midway therebetween. The window 43 permits alpha particles from the exterior to enter the counter. The screen 52 protects the window 48 against both protrusions and dirt accumulations on surfaces, and may readily be removed for cleaning, for example in an acid bath if made of acid-resistant materials such a suggested above.

In the construction of the counter illustrated in the drawing the joints between the Various' parts defining the counter body are preferably air-permeable for it'wiu te readily seen that otherwise apressure differential could be created acossfthe delicate window 48. t `is not neces-A sary, hwever, to provide any merture to yallow thefinlux of air Since in the absenceof affirmative sealing means such as greases,there will be siicient leakage `between the interior` of the counter and the ambient atmosphere to insure that no substantial pressure diiTerential between theinside and the outside of the counter may exist. This will be true even with the greatest care in `machining and lassembling of the conn-l ter. `Since it isnecessary for proper operation thatthe counter be absolutely free of dust, it is highly desirable that the parts described abve, particularly those between which air leakage will occur, be'machined to the finest possible tolerances soas to allow a minimum of dust to enter the counter. l

The structure ofthe embodiment of the presi entinvention illustrated in the drawing having thus 'been described, the theory of operation and the basic principles of the design may now be disciissed.

Air is admittedly not an optimum choice from the theoretical standpoint of a filling for aproportional counter. However, the use of air at atmospheric pressure as such ionization medium, as illustrated in the exemplary enibodilnet of the drawing, makes possible the performance of counting operations which heretofore could not adequately be performed. For example, a `prportional counter such as that illustrated in the drawing may be used in making routine surveys of areas suspected of contamination by alpha particle emitting substances. l

The general theory of proportional counters is wiell kncwn in the art and will vnot be set forth here. The theory and observations below will, therefore, be confined principally to phenomena in respect to which air substantially differs 'from other ionizing media. y y

The oxygen and carbon dioxide content of air introduces the problem of negative ion formation. "The electron attachment for oxygen is' 1.73 jelectronvolts. This results in serious lss oielectrons formed in Vthe primary ionization of air by an alpha particle since some oi 'the elec; trons Yprimarily induced will becerne attached to oxygen atoins beforecausing additional ionization. This is especially true in the outer, or low intensity, field region of the proportional counter. Such an eiect is observed ras an apparent decrease in counter efliciency. At high voltages,

furthermore, this results in spurious counts since' the electron which has become attached tothe oxygen atom may again become detached long after the original ionizing event, and thus cause a spurious count. Although these eifects are irnportant in using such a counter as herein de#I scribed for particle energy measurements, they are practically negligible for determining the nurnberof ionizing events, as the operating Volt-4 age of the counter is selected so as to be con siderably below the semi-proportional region.

"Thelpresenee of nitrogen in the counter re'- sults a relatively high operating potential. This requires that very small center-wire diar`n et'er's be used in order to obtain adequate gaslainpliication at reasonably moderate potentials. The center-'wires should therefore be less than 3 milsin diameterand preferably l ALrriil or less.

In order to design air proportional conters, it 'isfnecessary to understand the principal diffeifefces between theele'ctrical' discharge niecliaera few of the el l In addition, any photons the gas.` Under these ni'sil iii "a "eas the *disch mechanism in aumpixture f The properties" of the passe the" "h, po are f Qi" f portional region f l plied betweii'the required to give' .angels ron enough produce iiiiatio'ii last before being 0j A a at nie in ythe potencial difference ionization and 'mitiiineatid Many of theieiecuis .1p y Charge become exert ind,

erranti". fi he effect .of the .d l stitutes one ofthe 'pr' the Velischalr ge reces` thatlnre ro the polyatoginic' .gas-are` Messa" ily becanse of the wide photojfabso" ti such typeso-f nioleciiles. f air discharge .produces A range since vthe photon Aabs air is ra small, Under such conditions,"photo-electrcns: may be produced yat `thY eat such photo-electrons anode may pro'du'ce addi 1o electrons produeedfi-n t bytheanode willnA Y 4 p n mary lions created by @been v ticle whose impingement-is Sought tope recorded'.- Since isnch' vproportic'mality *is requisitel to proper operation *of the proportional counter; in cointers. tlreproportional reeion isflirnited to the regionlying above the popoftinalf threshold 'and below `the pointI iwhre'ipliotons play a :signifi l-cant role.V fExperimentally, at' has "been-n found that this mnltiplication for l'gas :amplification in the' cdiintrLcontaiv-ing air atmospheric 1pm-:isa sure extends up to lappr-c atel'y 5103, whereas higher* orders of multiplication .of gas ampli: "be Thev airpla critically `del pendent upo counters vinthe and, therefere, air Vcou dependent foiie potential.

esaminano-n factor or 'i dependan aA assente field distribution, may be considered unimportant.

Air counters possess relatively high resolution, i. e., better than "5 seconds.

In air counters heretofore employed, it has been thought that since the ionization medium is air at atmospheric pressure no window should be required, and a metallic screen has been used as all or a portionof the cathode. But, it has been found thatsuch designs have the disadvantage of the entry of clust into the high field regions and consequentv spurious counting. It is thus imperative that there be introduced a barrier for dust which is nevertheless adapted to transmit alpha particles without substantial absorption. The best material for the dust barrier window has been found to be sheets of nylon nlm approximately 0.2 mil thick. The window of thin Amaterial suchas nylon has been found to require a conducting coating of, .for example, graphite in order tov prevent the collection of electrostatic charge and consequent spurious counting. Of course, a compromise must be made between maximizingf theconductivity of the coating and minimizing the particle energy absorption of the coating. It has been found that a satisfactory coating may have a resistance from the center of the window 48 to the frame 46 of the order of 1,000 to- 10,000 ohms, the measurement being made by measuring the resistance between the framed and a coin such as a dime placed. at the'center of the window 48. The coating shouldhof course, be as uniform as possible, and is preferably of evaporated colloidal graphite. .l

In operating a proportional counter filled with air at atmospheric pressure and being sufiiciently open to the atmosphere so that pressure inside Y and outside the counter is substantially the same, itis obvious that the characteristics of the counter, which are dependent upon the pressure of the gas, will vary from day to day in accordance with the variations in ambient atmospheric pressure. Experimentally, it has been found that with counters using 1 mil diameter centerwires the shift in threshold potential in response to an atmospheric pressure change of 1 cm. Hg (the maximum daily fluctuation of atmospheric pressure in most locations). is approximately 15 to 2G Volts, the normal operating potential being in the neighborhood of 21200 volts. This shift is linear' over a range of at least 20 cm. Hg in the vicinity of sea level at atmospheric pressure. It has been found 'that in any given location the shift in operating'characteristics due to atmospheric pressure changes can be virtually disregarded when the counter is incorporated inV instruments Afor alpha' particle and proton.

surveying and detection, which do not require extreme accuracy. At room temperatures, it has been found that there is vapproximately a 1 per cent change in the counting rate of the counter with a 1 F. change in temperature.

A thin window of, for example, nylonhas afairly high permeability to Vwater vapor. The

water vvapor content'of air changes from'time ing'in a counter with a high background, ex-f'v cept that the counts are somewhat periodic rather than random in occurrence. Counters with fairly clean insulators may show the effect above per cent relative humidity. It has been found that the best performance in this regard is obtained when the insulators are molded with proper precautions as to cleanliness rather than being machined. It is, of course, desirable` that the surface leakage paths on all insulators be tween the anodes and cathodes be maximized in length. In the illustration of the drawing, the gaps 22 and the illustrated tapering of the insulating posts 28 serve this purpose.

The counter illustrated in the drawing is of the flat type which has the maximum effective area in making contamination surveys on fiat surfaces. A general criterion for the proper design of the counter is that the distance between the center-wires be approximately twice the distance between the parallel plates which constitute the cathode. It has been found that such dimensions produce a satisfactory field distribution in such a counter without vrequiring the use of a large number of center-wires.

The counter illustrated in the drawing is only one embodiment of the present invention. Persons skilled in the art will readily devise other proportional counters embodying the teachings of the invention and will readily adapt some of the features above disclosed to devices otherV than proportional counters.

What is claimedis:

1 In an electron discharge device, in combination: an electrode comprising aconductingwire of uniform diameter having two ends, and end portions having transverse dimensions greater than the diameter of said wire, the respective,

wire extending through the apertures and the end portions abutting against the outer surfaceY of the respective supporting members, so that the electrode is firmly supported but readily replaceable.

2. In an electron discharge device, in combination, an'electrode comprising a wire having a central portion of uniform diameter and two end portions of larger diameter than said central portion, two spaced support means having apertures therein larger than said central portion but smaller than said end portion, said apertures being open at the periphery thereof to permit peripheral introduction of the wire into the apertures, said wire extending through said apertures, and means for springily mounting at least one of said support means so as to exert tension on said wire. 3. A proportional counter comprising, in combination, a plurality of electrodes, an ionizing medium consisting of air at atmospheric pressure, and an enclosure surrounding said electrodes and said ionizing medium, said enclosure being impermeable to dust of the ambient air but having at least a portion permeable to alpha particles.

4. A proportional counter comprising in combination, a plurality of electrodes, an ionizing medium consisting of air at atmospheric pressure, an enclosure surrounding said electrodes and said ionizing medium, and a window in said enclosure comprising a sheet of nylon less than 0.5 mil in thickness, said window being impermeable to dust of the ambient air.

5. A proportional counter comprising, in combination, a conducting center-wire electrode, a conducting wall electrode impermeable to dust and an ionizing medium between said electrodes, said ionizing medium consisting of air at atmospheric pressure and said conducting Wall electrode consisting at least in part of a thin membrane of non-conducting material having on at least one side thereof a thin coating of a conducting material, said membrane and coating being permeable to alpha particles but impermeable to dust.

6. A proportional counter in accordance with claim wherein said membrane consists of sheet nylon less than 0.5 mil thick and said coating comprises evaporated colloidal graphite.

7. The apparatus of claim 1 wherein at least one of the supporting members is springily mounted to exert tension on the Wire.

8. The apparatus of claim 1 wherein at least one of the supporting members comprises a helical spring having at the end thereof a substantially circular hook constituting said peripherally open aperture.

9. The apparatus of claim 8 wherein the end portion of the electrode is substantially in the form of a sphere.

10. A proportional counter comprising in combination, a conducting center wire electrode, a conducting wall electrode impermeable to dust, and an ionizing medium between said electrodes, said ionizing medium consisting of a gas at substantially atmospheric pressure, and said conducting Wall electrode consisting at least in part of a sheet of nylon less than 0.5 mil thick having on at least one side thereof a thin coating of conducting material, said nylon sheet and coating being permeable to alpha particles but impermeable to dust.

11. A proportional counter comprising in combination, a conducting center wire electrode, a conducting Wall electrode impermeable to dust, and an ionizing medium between said electrodes, said ionizing medium consisting of a g-as at substantially atmospheric pressure and said conducting electrode consisting at least in part of a thin membrane of non-conducting material having on at least one side thereof a thin coating of colloidal graphite, said membrane and coating being permeable to alpha particles but impermeable to dust.

12. A counter having a plurality of electrodes, an ionizing medium, an envelope surrounding at least one of the electrodes and the ionizing medium, and a window in the envelope permeable to radiation, characterized by the improvement wherein the Window consists of a sheet of nylon less than 0.5 mil thick coated with colloidal graphite.

13. A proportional counter comprising, in combination, a plurality of electrodes, an enclosure surrounding said electrodes, a window in said enclosure, said window consisting of a thin membrane permeable to alpha particles and impermeable to clust, an ionizable gas within the enclosure, and means for maintaining the gas at a pressure approximately the same as the pressure exterior of the enclosure.

JOHN A. SIMPSON, JR.

REFERENCES CITED The following references are of record. in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,562,403 Wilson Nov. 17, 1925 1,859,678 Nachumsohn May 24, 1932 2,130,510 Samuel Sept. 20, 1938 2,368,486 Mullane Jan. 30, 1945 2,397,075 Hare et al. Mar. 19, 1946 FOREIGN PATENTS Number Country Date 225,293 Great Britain Dec. 1, 1924 THER, REFERENCES Korn-Electron and Nuclear Counters, D. Van Nostrand Co., Inc., Apr. 1946, pp. -77, 80-83 and 122.

McCusker: Journal of Scientic Instruments, Vol. 2l, July 1944, pp. 12S-130.

Kori Electron and Nuclear Counters, 2D. Van Nostrand Co., Apr. 1946, pp. 128 and 129.

Geiger and Muller: Article in Physikalische Zeitschrift, vol. 30, Aug. 15, 1929, pp. 489-493.

Copp and Greenberg: Review o Scientic Instruments, July 1943.

Brown et al.: Review of Scientic Instruments, vol. 16, No. 5, May 1945, pp. -129.

Dunning et al.: Review of Scientific Instruments, vol. 6, Aug. 1935, pp. 243-246.

Brubaker et al.: Review of Scientic Instruments, vol. 8, July 1937, pp. 254258.

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