US2736816A - Ionization chamber - Google Patents

Description

Feb. 28, 1956 D. H. MORLEY IONIZATION CHAMBER Filed April 24, 195:5

Inventor: David H. Morley,

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United States Patent IONIZATION CHAMBER David H. Morley, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application April 24, 1953, Serial No. 350,9?7

3 Claims. (Cl. 250-831) My invention relates to constructional features of an ionization chamber.

In ionization chambers such as for neutron detection, the presence of neutrons is detected by the very small cur rents caused by ionizing particles produced when neutrons impinge upon a material such as boron, the particles ionizing a gaseous medium so that a small ion current proportional to the neutron density is produced. The ion current flows between a high voltage electrode and a collector electrode which must be very carefully and completely insulated from each other in order that leakage currents 'will not be large enough to afiect materially the ion current. Since in many applications the chamber must be able to operate at elevated temperatures, the insulation requirements are so stringent that problems are encountered in making the chamber mechanically strong and rugged. This difficulty is amplified where the apparatus must meet severe shock test requirements while at the same time incorporating the insulation and other design features necessary for sensitivity and efiiciency.

It is therefore a primary object of my invention to pro- 'vide an efiicient and accurate ion chamber which is mechanically strong.

It is a further object of my invention to provide an ionization chamber which will withstand severe shock accelerations without forfeiting high design standards of efliciency and sensitivity.

In accordance with my invention, an ionization chamber for neutron detection is provided with an inner hollow cylindrical collector electrode and an outer cylindrical high voltage electrode concentric therewith, the assembly being insulatingly supported within a closed guard cylinder. The high voltage electrode is supported by insulating rings between the high voltage cylinder and the guard, and a central support rod is rigidly fixed along the chamber axis between the ends of the guard cylinder. Insulating bushings having a high dielectric constant are mounted on the center support rod and the hollow collector inner electrode is supported by annular flanges or webs secured at their inner peripheries to the insulating bushings and fastened at their outer peripheries to the cylindrical collector electrode. The collector electrode itself has a relatively small mass, and the porcelain bushings, since they are substantially protected from axial compression due to the presence of the fixed center support rod between the end walls of the guard cylinder, are highly resistant to damage caused by axial shock which is usually the severest mechanical test requirement in conventional concentric electrode ionization chamber construction.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which the single figure is a sectional view of an ionization chamber embodying my invention.

Referring now to the drawing, an inner hollow cylindrical collector electrode 1 is shown therein surrounded by an outer cylindrical high voltage electrode 2 which is concentric with the electrode 1 and slightly longer so that it extends a short distance beyond the ends of the cylinder 1. Both cylinders are preferably made of electrolytic iron coated with boron containing the boron 1O (13 isotope which, when irradiated by neutrons, produces a nuclear reaction therewith as described in a following paragraph; An outer guard cylinder 3 surrounds the electrodes 1 and 2 and extends beyond the ends of both to also serve as an electrostatic shield and envelope for the apparatus. Metal end disks 4 and 5 are welded to the ends of the guard cylinder 3 to complete the envelope, both the cylinder 3 and the disks 4 being suitably made of steel.

To position the high voltage electrode within the guard envelope 3, annular insulators 6 and 7 are mounted on each end of the high voltage electrode with their outer peripheries engaging the inner surface of the guard cylinder 3. The insulators 6 and '7 are each pieferably made of a stack of annular mica laminations which are clamped together for mechanical rigidity. Inner guard rings or cylindrical sections 8 and 9 having the same diameter as the collector electrode 1 are respectively secured to the envelope end disks 4 and 5, the inner rings'being coaxial with the collector electrode and slightly spaced from the ends thereof. These rings, while not part of the active collector surface, are designed to operate at the collector potential and thus prevent fringing of the radial field between the collector and the high voltage electrode at the collector end surfaces, thus keeping the field uniform as desired for instrument linearity.

In accordance with my invention, a central support rod assembly it? extends along the chamber axis and is rigidly fixed at its ends to the disks 4 and 5 to serve as a support for the collector electrode. This rod suitably comprises a central portion made of a low mass hollow cylindrical steel member 11 having a steel stud 12 pinned or otherwise secured wi-thin each end and extending axially outwards. Each stud has a threaded portion 13 extending for a distance beyond the end of the cylindrical support 11 and a further rod-like extension 14 of smaller diameter extending to the end disk 4 or 5.

A tubular ceramic insulator 15 is positioned on each stud portion 14 with an end portion engaging each threaded portion 13 to prevent the insulator from rotating on the stud. The ceramic material is preferably alumina or aluminum oxide which, while it is subject to breakage due to its brittleness, is nevertheless well suited to the exacting insulation demands of very low leakage current at relatively high voltages and at elevated temperatures. Steel end cylinders 16 are fitted over the portions of each stud 1d projecting beyond the insulator tube 15 and are provided with a rectangular or other non-circular end key portion 17 to engage the respective end disks 4 and 5 in corresponding apertures 13 therein. The end of each stud 14 is brazed to each keep portion 17 to prevent the studs from rotating when the end keys 17 are inserted in position. While the studs might be welded directly to the end disks to prevent them from rotating the particular construction described here is designed for ease of assembly of the chamber. Soft metal pads 19 and 20, suitably made of copper washers, are also positioned respectively at each end of each ceramic tubing 15 to cushion the insulator from steel support tube iii and end cylinders 16. A metal annulus or web 21, suitably made of steel is positioned between each ceramic bushing 15 and the inner end surfaces of the collector electrode 1. This is suitably accomplished by providing each annulus 21 with an inner flange which is secured to the ceramic material by any known means such as the titanium hydride sealing process 3 and by providing each annulus with an outer flange which is welded to the cylinder 3..

To apply operating potentials to the chamber electrodes, the end disk 4 is apertured to accommodate insulating seals 22 and 23 for the respective electrode terminals or leads 24 and 25 of the collector 1 and high voltage electrode 2. The end disk 4 is also apertured to accommodate a tubulation 26 which is sealed after the chamber is evacuated and filled with an ionizable gas such as argon. A simplified circuit is schematically shown comprising a high voltage source 27 in the order of a hundred or hundreds of volts having its positive terminal connected to the high voltage terminal 25 and its negative terminal connected through ground to one terminal of a very high impedance voltmeter 28, which may suitably be an electrometer or a very high impedance vacuum tube volmeter, whose other terminal is connected to the collector electrode terminal 24. The guard cylinder 3 is also grounded so as to place the guard cylinder and its inner guard rings 8 and 9 at the same high voltage potential with respect to the high voltage electrode.

Referring briefly to the operation of the ionization chamber for a better understanding of the initial requirements met by the structural features embodying my invention, the chamber is intended for installation in a location where it is subject to the neutron radiation to be detected and measured. The neutrons are not in themselves ionizing particles and are not directly detectable but they pass through the chamber including the guard cylinder 3 and the electrodes 1 and 2 where they produce a nuclear reaction with the boron 10 isotope present between the facing surfaces of the collector electrode it and the high voltage electrode 2. As previously mentioned, metallic boron is placed on the chamber walls, or if desired the chamber may be filled with boron trifiuoride gas, the concentration of the boron 10 isotope depending upon the sensitivity required. The nuclear reaction can be expressed as B neutronalpha particle +Li +energy The energy appears as kinetic energy divided between the alpha particle and the lithium nucleus and hence each can cause ionization of the argon in the chamber. The ion current is collected by reason of the electric field between the electrodes and indicated by the instrument 2%. Since incident gamma rays will also cause ionization in proportion to the spacing between the electrodes 1 and 2 and thus effect the accuracy of the voltmeter indication of neutron density, the spacing is kept as small as practical. At the same time the spacing should not be indiscriminately smaller than the alpha particle range in which case the neutron sensitivity of the chamber would decrease. This alpha particle range is about .7 centimeter at atmospheric pressure. At a spacing equal to about one-half the alpha range (substantially Vs" spacing), the loss in neutron sensitivity is only about six per cent which is a fairly satisfactory compromise since further reduction in spacing would involve much more severe losses. Thus it may be seen that it is important that the chamber be mechanically designed to maintain small electrode spacings accurately.

At the same time the insulating means involved in providing the spacing must be of the highest quality without sacrifice of its insulating abilities for the purpose or" providing mechanical strength. It, for example, the current to be measured is of the order of 10- amperes, under normal conditions the insulation leakage current should have a much smaller value, say 1() amperes so that leakage error is less than 1 per cent or is not greater than 1 per cent. It is also desirable that the collector insulator be kept out of the collecting field so that ions will not be collected on it to cause local cancellation of that field.

Due to the particular construction of the rigid center support means 10 for the collector electrode, both the insulating and mechanical requirements are met. Not only is the use of a very high dielectric constant insulater 15 made feasible, but the insulator itself is also well protected from shock. For example, axial shock, which is often the severest test requirement, is absorbed in the metal portions of the central rod assembly 11, namely, the central support cylinder 19 and the studs 11 and 12. Transmission of this shock to the ceramic insulators is further decreased by the pads 19 and 20. The shock test requirements may be of the order of times gravity acceleration, and much greater shocks can be withstood by chambers constructed in accordance with my invention.

While I have shown and described a specific embodiment of my invention and certain modifications thereof, it will, of course, be understood in the art that modifications may be made without departing from the principles of the invention. I therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

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

1. An ionization chamber comprising a hollow cylinrical collector electrode extending along a given axis, a cylindrical high voltage electrode surrounding said collector electrode and coaxial therewith, a pair of metallic end disks insulatingly supported from said high voltage electrode end portions, and means for insulatingly supporting said collector electrode from said end disks comprising an axially disposed support rod positioned within said collector electrode along the axis thereof and fixed at each end to said end disks, an insulating bushing surrounding a portion of said rod near each end thereof and fixed thereto, and a metallic flange member secured between each end of said collector electrode and said bushing.

2. An ionization chamber comprising a pair of hollow concentric cylindrical electrodes extending along a given axis, a cylindrical envelope closed by metal end disks surrounding said pair of electrodes, means for supporting the outer electrode of said pair from said cylindrical envelope, and means for insulatingly supporting the inner electrode of said pair from said end disks comprising a rigid metal support rod positioned within said inner electrode along said axis and secured at each end to said disks, a ceramic bushing surrounding a portion of said rod near each end thereof and fixed thereto, and a metallic support flange member secured between each end of said inner electrode and said bushing.

3. An ionization chamber for neutron detection comprising a pair of hollow concentric cylindrical electrodes extending along a given axis, a boron coating formed on one of the said electrodes a cylindrical metallic envelope closed by metal end disks surrounding said pair of electrodes, means for insulatingly supporting the outer electrode of said pair from said cylindrical envelope, and means for insulatingly supporting the inner electrode of said pair from said end disks comprising a rigid metal support rod positioned within said inner electrode along said axis and secured at each end to said disks, a ceramic bushing in threaded engagement with a portion of said rod near each end thereof, and a metallic flange member secured between each end of said inner electrode and said bushing.

References Cited in the file of this patent UNITED STATES PATENTS 2,436,084 Weller Feb. 17, 1948 2,440,167 Broxon et al Apr. 20, 1948 2,605,435 Krasnow et a1. July 29, 1952 2,625,657 Kanne Jan. 13, 1953

Claims (1)

1. AN IONIZATION CHAMBER COMPRISING A HOLLOW CYLINDRICAL COLLECTOR ELECTRODE EXTENDING ALONG A GIVEN AXIS, A CYLINDRICAL HIGH VOLTAGE ELECTRODE SURROUNDING SAID COLLECTOR ELECTRODE AND COAXIAL THEREWITH, A PAIR OF METALLIC END DISKS INSULATINGLY, SUPPORTED FROM SAID HIGH VOLTAGE ELECTRODE AND PORTIONS, AND MEANS FOR INSULATINGLY SUPPORTING SAID COLLECTOR ELECTRODE FROM SAID END DISKS COMPRISING AN AXIALLY DISPOSED SUPPORT ROD POSITIONED WITHIN SAID COLLECTOR ELECTRODE ALONG THE AXIS THEREOF AND FIXED AT EACH END TO SAID END DISKS, AN INSULATING BUSHING SURROUNDING A PORTION OF SAID ROD NEAR EACH END THEREOF AND FIXED THEREOF, AND A METALLIC FLANGE MEMBER SECURED BETWEEN EACH END OF SAID COLLECTOR ELECTRODE AND SAID BUSHING.

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