US2436084A

US2436084A - Ionization chamber

Info

Publication number: US2436084A
Application number: US512070A
Authority: US
Inventors: Barton L Weller
Original assignee: National Aeronautics and Space Administration NASA
Current assignee: National Aeronautics and Space Administration NASA; US Atomic Energy Commission (AEC)
Priority date: 1943-11-27
Filing date: 1943-11-27
Publication date: 1948-02-17
Anticipated expiration: 1965-02-17

Description

Feb. 17, 1948.. B. L. WELLER IONIZATION CHAMBER Filed Nov. 27, 1943 M/ 8 9 t W Z d M .0

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WWI r ,M M U IJI JW lwb A24... a M z'iarnaw Patented Feb. 17, 1948 IONIZATION CHAMBER Barton L. Weller, Chicago, 111., aeaignor to the United States of Ameri Atomic Energy Comm ca, as represented by the lesion Application November 27, 1943, Serial N6. 512,070

8 Claims. (Cl. 25l-83.6)

My invention relates to apparatus for measuring the intensity of penetrating radiations such as gamma rays. More specifically, it relates to an ionization chamber having a collecting electrode and high potential electrode and an outer envelope serving as a guard, together with an asso-' ciated electrical metering circuit.

An object of my invention is to provide a device that is capable of creating measurable electrical currents in response to, and proportional to, penetrating radiations that may impinge on it, and that is of such simple construction that it is readily adaptable for large scale production.

Another object of my invention is to provide such a radiation measuring device that is safe to handle while in operation.

A further and more specific object of my invention is to provide an ion chamber having collecting and high potential electrodes in which the envelope, together with associated guard rings internally and externally thereof, serves as a guard system for preventing the intense electrostatic fields ot the high potential electrode and leads from disturbing the measurement of the ionization currents.

These and further objects will become more apparent from the study of the following description taken together with the drawing, in which:

Fig. 1 is a longitudinal cross-sectional view of an ion chamber and associated circuit showing the teachings of my invention; and Fig. 2 is a cross-sectional view taken on the line 2-2 of Fig. 1.

Referring more particularly to Fig. 1, numeral i denotes a metal chamber or envelope having welded thereto flanged

header plates

2 and 3. Fused to an eyelet portion 3a welded on plate 2 is an insulator 4 of glass, or any other similar material, which in turn is fused to a reentrant metallic support for a rod or lead-in 5 connected to a cylindrical collecting electrode 6, thus forming a glass to metal seal between the header plate 2 and lead-in 5. The header plate 2 is also pro-' vided with one or more flanged eyelet portions 1 having fused thereto glass beads 8 which in turn are fused to rod 9 which serves as a lead-in and support by being spot welded to a high potential cylindrical electrode l0. High potential electrode I 0 may be either positive or negative with respect to collecting electrode 6 depending upon whether it is desired to have electrode 6 collect positive ions or negative ions, respectively. A metal tubulation H, which is capable of being sealed oil, is provided for filling and exhausting the envelope. The envelope is filled with any desired gas such as argon or freon, preferably at high pressures such as between 1 and 5 atmospheres. A high voltage source such as a battery I! is connected between the high potential electrode lead-in II and envelope I. An electrical measuring instrument [3 such as an electrometer, galvanometer, or similar device, is connected between the collecting electrode lead-in 5 and metal chamber 1. A flexible lead-in conductor l4 connects lead-in I to meter I 3. A detachable connector II, of any suitable well-known design, may be used to make the conductor l4 readily detachable from lead-in I. A guard ring l8 having a cover plate I! forms an electrostatic shield for shielding insulator 4 from the high potential leadin. This shielding is continued along conductor I 4 by the coaxial connector l5 and coaxial cable 22 which incorporate sheaths that completely surround conductor l4 and are electrically connected to the header 2 and envelope I. An upper extension of chamber l, taken together with a cover plate I! and conduit 23, forms an external electrostatic shield for completely shielding the high potential and collecting electrode lead-ins and preventing their mechanical injury. Conduit 23 also prevents possible failure of the insulation of lead-in I! from harming the operator. An internal guard ring 21 is provided to protect collecting electrode insulator 4 from the electrostatic eilects of the high potential electrode II and lead-in Q.

In operation, when the device is subjected to penetrating radiation such as gamma rays, the high potential source applied to electrode III will cause ions formed in the chamber to move to-' wards the collecting electrode and will cause current to flow through the meter l3. Since header plate 2 is electrically grounded and interposed between bead 8 and insulator 4, the high potential gradient created by battery i2 appears only across head 8 and not across insulator 4, because only a small potential exists across meter l3 and insulator 4. It will be observed that leakage currents that tend to flow along the surface of glass beads I, because of the high potential gradientv placed thereon due to the high voltage, will not enter the collecting electrode circuit but instead will flow through the battery I! which is of sumcient capacity to supply this negligible loss. Guard ring I electrostatically shields the collecting electrode from disturbing efiects caused by any charge that may be present on the insulator" 4 moving over its surface as the result of the potential gradient from the strong electrostatic field created by the high potential electrode lead-in the envelope as well as internal and external guard rings, the collecting electrode metering circuit is substantially isolated electrically and electrostatically from disturbing variations due to the electrostatic effects resulting from leakage from the surface of the high potential lead-in insulator, thereby making it possible to measure very small ionization currents in the chamber even of the order of 10- an peres. Likewise, by

, virtue oi. the guard system, since a relatively low potential appears between the header plate 2 and lead-in 5 (perhaps of the order of 10 volts maximum), it is unnecessary to provide large and very high resistance insulating materials such as amber, for example-and instead, ordinary insulating materials can be used in the making of insulator 4.

It will be readily apparent that other modifications of my invention may be made without departing from the spirit of the invention disclosed.

Iclaim:

1. Apparatus for measuring pentrating radiations comprising, in combination, a vacuum tight sealed metal envelope, a collecting electrode and a high potential electrode contained therein, lead-ins connected thereto, insulators for in-- sulating said lead-ins from said envelope, a guard ring secured to said envelope and extending into the chamber formed thereby in a manner so as to electrostatically shield the interior surface of said collecting electrode insulator from said high potential electrode and lead-in, an electrical meter connected to said collecting electrode leadin, said collecting electrode being maintained at substantially the same potential as said envelope, a voltage source connected between said high voltage electrode and said envelope exclusive of said collecting electrode lead-in, thereby draining leakage currents from the surface of said high voltage insulator to the voltage source in preference to said collecting electrode or meter.

2. Apparatus recited in claim 1, together with metal guard means exterior of said envelope and between said lead-ins for electrostatically shielding ,the collecting electrode insulator from the .high potential lead-in.

3. An ionization chamber comprising, in combination, a metallic envelope having header plates sealed vacuum tight thereto by welds, a collecting electrode and lead-in, an insulator forming. part of the envelope walls and sealed to a, flanged portion of one of said header plates and to said lead-in, a high potential electrode andlead-in, a second insulator sealed to another flanged portion of said header plate and to said high potential lead-in, and an electrostatic shield exterior of said envelope and between said collecting electrode insulator and high potential lead-in.

4. Apparatus as recited in claim 3 together with electrical measuring means connected between said collecting electrodelead-in and said envelope. a source of high potential connected between said high voltage lead-in and said envelope exclusive of said collecting electrode leadin, said envelope being maintained at ground P tential.

5. Apparatus 'for measuring penetrating radiations such as gamma rays comprising a metallic, vacuum tight sealed envelope, a collecting electrode and a high potential electrode insulatingly supported therein by insulators supported by said envelope walls, lead-ins for each of said electrodes, electrical measuring means connected be,- tween said envelope and said collecting electrode lead-in, a high potential source connected to said high potential electrode lead-in, said envelope being maintained at substantially the same potential as said collecting electrode and an electrostatic shield inside said envelope for screening the interior surface of said collecting electrode insulator from the electrostatic efiects of said high potential electrode and lead-in.

6. Apparatus recited in claim 5 together with a metallic electrostatic shield exterior of said envelope and between said lead-ins.

7. Apparatus as recited in claim 3' together with a box-like metallic shield exterior of said envelope for surrounding said collecting electrode lead-in and a second box-like metallic shield integral with said envelope walls and surrounding the first-mentioned shield as well as said high potential lead-in.

8. An ion chamber measuring device comprising, in combination, a metallic vacuum tight casing maintained at ground potential, an ionizable gas therein, a collecting electrode,v a glass-tometalseal protruding exteriorly of said casing for insulatingly supporting said collecting electrode from said casing, a high potential electrode, a second glass-to-metal seal protruding exteriorly of said casing for insulatingly supporting said high potential electrode from said casing, leadins extending through said glass-to-metal seals, a source of high potential connected to-said high potential electrode, a cylindrical metallic guard surrounding said first-mentioned glass-to-metal seal, and an electrical meter interconnected between said collecting electrode and casing, exteriorly of said casing, for measuring the ion current flow to said collecting electrode as the result of ionization of said gas when the device is subjected to penetrating radiations such as gamma rays.

BARTON L. WELLER.

REFERENCES Cli'lllElD The following references are of record in the