US3030538A - Ionisation chamber - Google Patents

Description

IONISATION CHAMBER Filed June 16, 1958 FIGJ INVENTOR NICOLAAS WARMOLTZ PAULUS PHILIPPUS MARIA SCHAMPERS United States Patent Fice i "3,030,538 IONISATION CHAMBER Nicolaas Warmo'ltz-and Paulus Philippus Maria', Schampers, Eindhoven, Netherlands,^assignors' to North American Philips Company, inc., New York, N.Y., a corporation ofDelaware Filed June 16, 1958, Ser. No. 742,241 Claims priority, application Netherlands .lune 28, 1957 5 Claims. (Cl. 313-93) This invention relates to ionisation chambers, especially for measuring gamma and X-rays.

For measuring 'y-rays and`X-rays, use is made ofthe ionisation vproduced by 'these rays in acertain volume of a gas `or gas mixture. "The gas volume is enclosed'in a chamber the walls of which are made entirely or in part of vinsulating material ari'd'inl which two electrodes are provided. Generally one electrode is applied to the wall while the .other is arrangedin the chamber. Each-electrode is connected to a conductor passing through the wall so that an electric potential is set up across them. The electrodes must 'be highly insulated from one another.

The wall material of ionisation chambers preferably consists of a substance which with respect to its permeability to the ionising radiation has about the same properties as air. Otherwise the sensitivity of the ionisation chamber will depend to a higher or lesser degree upon the hardness of the rays. Particularly in'small ionisation chambers of small volume the measuring results can be materially iniluenced by the composition of the wall material. As the wall material, use is frequently made of an organic substance which has substantially the same absorption and diffusion for the rays to be measured as air. In general, such materials are referred to as airwall equivalent materials.

ionisation chambers are increasingly used in radiation counters for detecting ionising radiation and for safeguarding persons whose work entails that they can be exposed to radiation. Consequently these apparatus must be permanently disposed in the vicinity of the person to be protected and therefore their size is made s0 that they can readily be worn on or in the clothing. Consequently the ionisation chambers used must be very small. Hence the electrodes are closely adjacent and the insulation distance is slight, so that the Wall material must have a very high insulation resistance.

It is known to use organic substances, such as polystyrene, as the wall material. The insulation resistance is suliciently high, but they have a limitation which restricts their use to ionisation chambers lled with air under atmospheric pressure. The material is not cornpletely airtight and consequently not completely reliable for chambers which are iilled with a gas or gas mixture other than air or in which the pressure of the gas lling differs from the atmospheric pressure. In course of time the composition of the gas or the pressure in the ionisation chamber is slightly changed. In addition, it may be desirable for chambers which are closed hermetically to be degassed in order to remove entirely detrimental constituents, more particularly hydrogen. The organic substances cannot withstand the temperature required in this process.

It is an object of the present invention to avoid these disadvantages. It relates to an ionisation chamber the wall of which is airtight, has very high insulation resistance, can be heated at a suflciently high temperature for the chamber to be completely degassed, while the radiation absorption is fairly similar to that of air, i.e., it is a substantially air-wall equivalent material. According to the invention, the wall of an ionisation chamber is made 3 ,030,538 Patented 'Apr'. I7, "1 962 of Aa vitreous substance, composed of pure bo'ric 'acid anhydride whichis .usually referred 'to Yas :boric oxide.

Glass `compositions areknown which contain 1a high percentage of'borieacid. *"Howeven'jthese -glasses do not possess the property required in manufacturing ionisation chambers, that the radiation absorption inthe material is similar to that of air. The absorption 'is greater, frequently even materiallyjgreaten l YOwing'to 'the'slight electrical conductivityof the vitreous B203 the insulation distances 'required'between the electrodes and the lead-in conductors 'canb'esmall'and the chamber can have the small size required'tforits use as the'envelope of a radiation 'counter tolbe worn onor in the clothing.

An advantage of the substance consists lin .that itcan be sealed lto aluminum. 'Conductors of this material can be sealed in the wall directly. Conductorsofcopper are locally coatedwith a soft enamel 'as an intermediate layer.

As is known, the vitreous substance consisting of boric oxide is highly hygroscopic. Consequently,\when it is used in an ionisation chamber in accordance with'the invention, the wall must .be protected against attack by moisture. lThe chamber canbe arranged inan'envelope filled with dry air or in a'vacuum. 'I'he space surrounding the ionisation'chamber Vwithin the envelope canibe used to accommodate the'electr'ostatic indicating system which'is employed in radiation counters for thedetection of the ionisingradiatio'n. -lf the ionisation chamber is to be used independently, a protective layer can be provided. This layer can consist of polyester or celluloid. After mounting, the ionisation chamber can be immersed in a solution of one of these substances and subsequently dried. If required, the immersion can be repeated until the layer has attained the required thickness.

In order that the invention may readily be carried out, reference is made to the accompanying drawing, in which- FIG. 1 is a cross-sectional view of an ionisation chamber in accordance with the invention, provided with a protective coating,

FIG. 2 shows the ionisation chamber arranged in a protective envelope.

Referring now to the gures, the gas volume to be ionised is enclosed in a tubular chamber 1, the Wall of which consists of a vitreous substance made of boric oxide. The gas volume may consist of air or a gas filling may be provided which absorbs X-rays to the same extent as air. A suitable pressure of the gas lies between l and 11/2 atm. At both ends the tube 1 is closed and provided with feed-through conductors 2 and 3 made, for example, of aluminum or copper. The copper wires are sealed in the wall 1 With the interposition of a vitreous enamel 4 and 5. An inner electrode 6, which may consist of a graphite rod, is connected to the lead-in 2. The outer electrode 7 comprises a conductive substance, preferably carbon, which -is applied to the wall 1. Prior to filling the chamber with the ionisable gas, to remove detrimental constituents, such as hydrogen, the chamber may be heated to a temperature of to 150 C.

On the Wall 1 a coating 8 of moisture-proof insulating material, made of for example polyester or celluloid, is applied In FIG. 2 the ionisation chamber 1 is situated Within a cylindrical or tubular housing 9 which is formed of an electrically conducting material that is pervious to radiation, such as for example aluminum. Within housing 9 is situated a lens 10 forming one end wall of a small compartment 11 the other end of which is closed by an insulating wall 12. One electrode 3 of the ionisation chamber 1 extends through the wall 12 and the second 3 electrode 2 supports an electroscope assembly 13, which is well known in the art so that it is not described in de@ tail. The lens and the wall 12 are spaced apart by a hollow cylindrical shell 14 having approximately the same Vouter diameter as the inner diameter of housing 9. The space 11 is preferably lledA with dry air. Y

The assembly as far as it is shown in FIG. 2 may form part of a radiation detection device as described in U.S. patent specification 2,613,327 to replace the electroscope assembly used therein.

What is claimed is:

1. An ionization chamber for 'measuring gamma rays, X-rays, and the like, comprising a sealed envelope having a wall portion of substantial surface area constituted of a substantial air-wall equivalent material and separating the envelope interior from the outside environment, an ionizable medium and electrode system Within the envelope, external terminals sealedpin the envelope and connected internally to the electrode system, said separating'wall portion Vconsisting of electrically-insulating, gas-tight, vitreous boric acid anhydride, and means surrounding the said ,Wall portion of boric acid anhydride and preventing moisture from coming into contact therewith.

2. An ionization chamber as set vforth in claim 1, wherein the last-named means comprises a moisture-impermeable coating provided on the said Wall portion.

3. A chamber as set forth in claim 2 wherein the moisture-impermeable coating is selected from the group consisting of a polyester and Celluloid.

4. An'ionization chamber for measuring gamma rays, X-rays, and the like, comprising a sealed envelope wholly constituted of a substantial air-wall equivalent material and separating the envelope interior from the outside environment, an ionizable medium and electrode system within the envelope, external terminals sealed in and through the envelope wall and connected internally to the electrode system, said envelope consisting of electricallyinsulating, gastight, pure vitreous boric oxide B203, and means surrounding the said envelope and preventing moisture from coming into contact therewith.

5. An ionization chamber as set forth in claim 4 wherein the last-named means comprises a hollow enclosure containing a moisture-free atmosphere.

References Cited in the le of this patent UNITED `STATES PATENTS 2,238,777 Lemmers et al. Apr. l5, 1941 2,367,595 Marden Ian. 16, 1945 2,500,941 Friedman Mar. 21, 1950 2,536,991 Wollan Ian. 2, 1951 2,568,459 Noel Sept. 18, 1951 2,573,999 Victoreen Nov. 6, 1951 2,596,080 Raper May 6, 1952 2,639,389 Landsverk May 19, 1953 2,683,234 Lynch July 6, 1954 2,837,678 Hendee et al June 3, 1958 FOREIGN PATENTS 606,013 Great Britain Aug. 4, 1948 OTHER REFERENCES Hanson et al.: Neutron Detector Uniform Sensitivity from 10 kev. to 3 mev., Physical Review, vol. 72, No. 8. October 15, 1947, pages 673-677.

Claims (1)

1. AN IONIZATION CHAMBER FOR MEASURING GAMMA RAYS, X-RAYS, AND THE LIKE, COMPRISING A SEALED ENVELOPE HAVING A WALL PORTION OF SUBSTANTIAL SURFACE AREA CONSTITUED OF A SUBSTATIAL AIR-WALL EQUIVALENT MATERIAL AND SEPARATING THE ENVELOPE INTERIOR FROM THE OUTSIDE ENVIRONMENT, AN IONIZABLE MEDIUM AND ELECTRODE SYSTEM WITHIN THE ENVELOPE, EXTERNAL TERMINALS SEALED IN THE ENVELOPE AND CONNECTED INTERNALLY TO THE ELECTRODE SYSTEM, SAID SEPARATING WALL PORTION CONSISTING OF ELECTRICALLY-INSULATING, GAS-TIGHT, VITREOUS BORIC ACID ANHYDRIDE, AND MEANS SURROUNDING THE SAID WALL PORTION OF BORIC ACID ANHYDRIDE AND PREVENTING MOISTURE FROM COMING INTO CONTACT THEREWITH.

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