US2683234A - Ionization chamber - Google Patents

Description

July 6, 1954 E. E. LYNCH IONIZATION CHAMBER.

2 Sheets-Sheet 1 Filed Aug. 27, 1952 N .wmf

Inventor? K Eidvvahd E .Lamer-w., H l-Iis Attohneg Patented July 6, 1.954"

'l' OFFICE IONIZATION CHAMBER Edward E. Lynch, Wakeiield,` Mass., assigner to General Electric Company, a corporation of New York Applicationy August 2237,1952,l Serial No. 306,621

(cutis- 93) 6 Claims. 1

My invention relates to improvements in ionization chambers, and its principal object is to provide an improved air-equivalent ionization chamber having high sensitivity, low spectral dependence, low voltage dependence, a great degree of freedom from directional eiects, and an economical, relatively trouble-free construction. Other objects and advantages will appear as the description proceeds.

My invention Will be better understood from the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

ln the drawings, Fig. 1 is a front elevation of my improved ionization chamber, showing the sliding shutters, hereinafter described, in the closed position;

Fig. 2 is a front elevation showing the sliding shutters partially opened;

Fig. 3 is a section taken generally along the line 3-3 of Fig. 2;

Fig. 4 is a section taken generally along the line 1 -i of Fig. 3; and

Fig. 5 is an exploded detail of a Window construction used in my improved ionization chamber.

Referring now to the drawings, my improved ionization chamber comprises. a generally cylindrical casing l having dome-shaped ends 2 and 3. Preferably, to provide air equivalence, theY parts of the casing are molded of an acrylic plastic material having approximately the same equivalent atomic number as air, and are cemented together. Suitable materials are Lucite and Textolite.

The term air equivalence means substantial equivalence to an infinite Wall` oi air insofar as the ionization equilibrium within the chamber is concerned. This' is important since the common unit of radiation exposure, the lftoentgen, is based upon the amount of ionization produced in a speciiled volume o1" air. The volume considered should be in equilibrium with the surrounding medium, preferably air or its equivalent, suchl that -an equilibrium exists between the number of ions entering and leaving the volume under consideration by ion .migration across its boundaries. Substances, such as Lucite and Textolte, which have approximately the same equivalent atomic Referring particularly to Figs. 2 and 3, the casing may be provided with several relatively thin Windows 5 along one of its sides, as shown. Windows 5 are preferably thin sheets of cellulose acetate in the order of .015" thick. Such windoWs are substantially permeable to beta rays, while the remainder of the casing, which may be about 1A, thick, is substantially impermeable to such rays havingv energy values less than about 0.95 m. e. v. The windows may be cemented into recessed slots in the casing as shown in Fig. 3.

A. pair of sliding shutters 6 are provided as shown in Figs. 1, 2, 3 and 4. Shutters 6 are shaped generally as. cylindrical segments, as shown, and are slidable circumferentially about the outer surface of casing l selectively to cover the Windows 5, as shown in Fig. l, or t uncover the windows 5 as shown in Figs. 2 and 3. The shutters may be of substantially the same material and thickness as the casing I; and hence, they are substantially impermeable to beta rays.

When the shutters are closed, as shown in Fig. l, only gamma rays and X-rays may enter 'the chamber, so that these rays may be measured independently of any beta or alpha radiation present. When the shutters are opened, thus uncovering the windows 5, beta rays may enter the chamber, and the sum ofthe beta, gamma and XL-radiation present can be measured. To determine the amount of beta radiation present, the value measured with the' shutters close is subtracted trom the value measured with the shutters open.

Even the thin windows provided exclude alpha rays. To measure alpha emission, it is necessary that the sample to be measured be placed inside the ionization chamber. This may be done by making one or more of the windows 5 removable. However, when the Window is replaced, it is essen tial that the conductive inner surface oi the Window bein good electrical contact with the conductive inner surface oi' the remainder or" the casing. Such electrical contact is insured by the arrangement illustrated in Fig. 5. The conductive coating inside casing I is carried up overV the shoulder 1 of the recessed slot into which Window ts. The entire inner surface of the window is covered with a similar conductive layer, so that when the Window is placed in its slot the conductive layer on the Window is in electrical contact with the conductive layer on shoulder l.

Referring now to Figs. 3 and 4, a rod S is centrally located Within the casing l and is coaxial therewith. Preferably, this rod is also of acrylic plastic material, and is coated on its outer surface with an electrically conductive graphite layer toform an inner electrode of the ionization chamber. -Aluminumplugs 9 areheldy inl place at respective ends of rod E by rivets I6. Attached to plugs 9 are bifurcated metal pins II, as shown. The metal pins I I slidably engage the respective inner surfaces of cylindrical metallic linings I2 within two cylindrical insulating bushings I3, which are coaxial with rod 8 and are attached to respective ends of casing I by caps I4, as shown.

Each bushing is surrounded by a metal sleeve I5, and is held in place by a lock nut I6. The bushings I3 are of material having high insulation value, such as polystyrene or Teflon, since they should provide an electrical insulation oi' 1013 ohms or better. Extreme care must be exercised to avoid handling these bushings, or their insulating properties may be impaired. The bushings are greatly protected in this respect by their cylindrical shape, and by the metal sleeves I5. An important advantage of the construction shown is that the insulating bushings may be replaced easily without undue handling. To replace the bushings, it is necessary merely to loosen the lock nuts It, remove sleeves I5, and insert other sleeves containing new insulating bushings. The slidable engagement of pins II with the bushing linings I2 is also advantageous in that diiferences expansion between casing I and rod 8, due to temperature changes or other causes, place no strain upon the insulators or other parts, yet the rod S is firmly held in place within casing thereby minimizing relative vibration of the parts which could cause microphonic disturbances.

A pair ,of cylindrical guard rings I'I are coaxial with respective ends of rod t, as shown. Preferably, guard rings Il are made of acrylic plastic, and both the inside and the outside surface of the guard rings are coated with a layer of conducting graphite. However, the ends of the guard rings are not coated, so that the two graphite layers are electrically insulated from each other. The inner graphite layer extends across the inside surface of cap I4, as shown, and thus is electrically connected to sleeve I5. The guard rings are arranged so that no part of insulating bushings I3 has a direct line-of-sight pat-h to any part of outer electrode 4. Since, as hereinafter explained, there is little if any potential diiference between the inner surface of the guard rings and the inner electrode 8, there is no substantial potential gradient in the vicinity of the insulating bushings to produce undesirable movement of ions trapped on the insulators, which could otherwise produce spurious signals.

The metal sleeves I5 are connected together by Y a wire, not shown, embedded in a groove along the backside of casing i, so that the inner surfaces of the two guard rings are maintained at the same potential.

A terminal It is provided, as shown, in electrical contact with the conductive surface on the inside of casing l. In operation, a negative p0- tential of 45 to 250 volts is applied to terminal I, by a battery or other suitable means, and, hence, is applied to the outer electrode of the ionization chamber. The positive terminal of the battery is connected to sleeves I5, through a suit able connector attached to one of the sleeves. The external connector also has a pin which contacts the metal lining i2 of insulator I3, and thus is in electrical contact with the inner electrode of the ionization chamber. An external resistor is connected lbetween the inner electrode and sleeves I5, and measurements are obtained in the usual manner by measuring the voltage 4 drop across this external resistor produced by ionization current flowing within the chamber.

The connector for the external circuit may be attached to either end of the ionization chamber, whichever is more convenient, and the sleeve at the other end preferably is covered by a protective metal cap I9. Since the voltage drop, due to the ionization current, across the external resistor is seldom more than one or two volts, the inner electrode and the guard rings are always at substantially the same potential, whereas the outer electrode is at a high negative potential relative to the inner electrode.

Finger holes 20, as shown in Figs. l and 4, may be provided to facilitate movement of the sliding shutters. The construction described is particularly advantageous in that no hinges or other metal parts are required by the shutters. In fact, the only metal parts in the entire ionization chamber are the metal plug and pin at each end of rod 3, the metal in the terminal IS, and the metal parts around the two insulating bushings. Metal in the vicinity of the ionization chamber is disadvantageous in that it may affect the energy dependence of the chamber.

Preferably, the inner electrode 8 is covered with a grease which will trap any lint or dust particles which may get into the chamber. All such particles are generally trapped within 15 minutes operation of the chamber, and thereafter can produce no adverse effects. This grease may be made conductive by mixing therewith finely-divided particles of graphite.

The domed shape of casing ends 2 and 3 is advantageous in that it eliminates long paths from ionization points to the inner electrode which would exist, for example, from the corners of the casing to the inner electrode if the ends were flat. This elimination of long paths substantially lowers the voltage dependence of the chamber. ln addition, the guard ring arrangement used makes it possible to construct the casing so that the end portions are no thicker than the remainder of the casing. This reduces directional effects of the chamber.

A desiccant `holder 2| may be provided at one end of the ionization chamber, as shown in Figs. 1, 2 and 4. Holder 2I preferably is a plastic bottle which may be filled with any suitable desiccant material, such as activated alumina. The mouth of bottle 2| forms an air passage which communicates with the interior of the ionization chamber through a hole 22 in the casing. Within this air passage, held in place between the casing and the top of the desiccant holder, there may be placed an air lter 23 to keep dust, lint and other particles from entering the chamber. The holder 2l also has a vent 24 to the outside atmosphere. This arrangement provides very efficient drying of the air within the ionization chamber. Whenever the pressure relation of air inside the chamber and the outside air tends to change, air passes through vent 24, the desiccant holder, and air passage 22 to equalize the pressure. Thus, under changing atmospheric conditions, the ionization chamber breathes through the desiccant holder. As the air passes through the desiccant holder, it is thoroughly dried by the desiccant.

It will be understood that my invention is not limited to the specific embodiment herein illustrated and described, and that the following claims are intended to cover all changes and modifications which do not depart from the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An ionization chamber comprising a generally cylindrical casing having dome-shaped ends, the inner surface of said casing being coated With electrically conductive material to form an outer electrode, a rod centrally located within said casing and coaxial therewith, the outer surface of said rod being electrically conductive to form an inner electrode, a pair of cylindrical insulating bushings having cylindrical metal linings located at respective ends of said casing in axial alignment with said rod, and a pair of metal pins attached to opposite ends of said rod and slidably engaging the respective inner surfaces of said linings.

2. An ionization chamber comprising a generally cylindrical casing having dome-shaped ends, said casing being of a substantially airequivalent acrylic plastic material coated on its inner surface with an electrically conductive graphite layer to form an outer electrode, a rod centrally located within said casing and coaxial therewith, said rod being of a plastic material coated on its outer surface with an electrically conductive graphite layer to form an inner electrode, a pair lof cylindrical insulating bushings having conductive inner surfaces located at respective ends of said casing in axial alignment with said rod, and a pair of metal pins attached to opposite ends of said rod and engaging the respective inner surfaces of said bushings.

3. An ionization chamber comprising a generally cylindrical casing having dome-shaped ends, the inner surface of said casing being coated with electrically conductive material to form an outer electrode, a rod centrally located within said casing and coaxial therewith, the outer surface of said rod being coated with electrically conductive material to form an inner electrode, a pair of cylindrical insulating bushings having conductive inner surfaces located at respective ends of said casing in axial alignment with said rod, a pair of metal pins attached to opposite ends of said rod engaging an electrical conductor on the respective inner surfaces of said bushings, and a pair of cylindrical guard rings coaxial with respective ends of said rod arranged to intercept all line-of-sight paths between said bushings and said outer electrode.

4. An ionization chamber comprising a generally cylindrical hollow casing having an opening through the side thereof, dome-shaped end members substantially closing the open ends of said casing, said casing and end members being of material and Ithickness such that they are impermeable to beta rays, a window member shaped to mate with said casing about said opening and movable to and from mating engagement with said casing to open and close said casing, said window member being of material and thickness such that it is permeable to beta rays and substantially impermeable to alpha rays, at least one shutter having the general shape of a cylindrical segment and mounted for sliding circumferential movement about the outer surface of said casing between positions wherein it overlays and uncovers said window member, said shutter being of material and thickness such that it is impermeable to beta rays, a rod-shaped inner electrode within and coaxial with said casing and having a conductive surface, a layer of conductive material coating the inner surfaces of said casing, and a coating of conductive material coating the inner surface of said Window member, sai-d conductive layers extending to mating surfaces of said window member and casing to provide conductive interconnection therebetween when said Window member closes said casing.

5. An. ionization chamber comprising a generally cylindrical casing having dome-shaped ends, the inner surface of said casing being coated with electrically conductive material to form an outer electrode, a rod-shaped member within and coaxial with said casing, the outer surface of said rod being coated With electrically conductive material to form an inner electrode, a pair of conductive metal contacts attached one at each end of said rod-shaped member and connected with the conductive coating on said rodshaped member, a pair of insulatingl bushings having conductive inner surfaces and mounted one on each of said dome-shaped ends of said casing to receive said contacts therein, said conductive contacts being slidably engaged with said inner surfaces of said bushings to mount said rod-shaped member coaxially with said casing and to interconnect said rod-shaped member coating and said bushing surfaces electrically, and a pair of guard rings coaxial with respective ends of said rod-shaped member arranged to intercept all line-of-sight paths between said bushings and said outer electrode.

6. An ionization chamber comprising a generally cylindrical casing having dome-shaped ends, said casing being of a substantially airequivalent acrylic plastic material coated on its inner surface with an electrically conductive graphite layer to form an outer electrode, said casing being provided with a plurality of relatively thin windows along one of its sides, at least one of said Windows being removable, said Windows being substantially permeable to beta rays and the remainder of said casingI being substantially impermeable to such rays, a pair of shutters having the general shape of cylindrical segments and being slidable circumferentially about the outer surface of said casing selectively to cover or to uncover said windows, said shutters vbeing substantially impermeable to beta rays, a rod centrally located within said casing and coaxial therewith, said rod being of a plastic material coated on its outer surface with an electrically conductive graphite layer to form an inner electrode, a pair of cylindrical insulating bushings located at respective ends of said casing in axial alignment with said rod, cylindrical metal liners for said bushings, a pair of bifurcated metal pins attached to opposite ends of said rod and slidably engaging the respective inner surfaces of said liners, a pair of cylindrical guard rings coaxial With respective ends of said rod arranged to intercept all line-of-sight paths between said bushings and said outer electrode, a desiccant holder attached to said casing, said desiccant holder having an air passage to the casing interior and also having a vent to the outside atmosphere, and an air filter Within said air passage.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,496,218 Kieffer Jan. 31, 1950` 2,536,991 Wollan et al. Jan. 2, 1951 2,573,999 Victoreen Nov. 6, 1951 2,574,000 Victoreen Nov. 6, 1951 2,587,254 Victoreen Feb. 26, 1952 2,596,080 Raper et a1. May 6, 1952

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