US2499489A - Exploring for radioactive bodies - Google Patents

Description

March 7, 1950 L. sows-rem ETAL EXPLORING FOR RADIOACTIVE BODIES Filed Jan. 2-1, 1944 a 6W INVENTORS W WHY M H T ORNE Y5 Patented Mar. 7, 1950 Ladislas Goldstein N. Y., assignors and Boris Pregel, New York. to Canadian Radium & Uranium Corporation, New York, N. Y., a corporation of New York Application January 24, 1944, Serial No.519,5oe

1 Our invention relates to a new and improved method of locating bodies of radio-active material, and an improved apparatus for this purpose.

The main object of our invention is to facilitate the discovery of deposits of uranium ores.

The uranium compound or compounds usually form only a small percentage of such deposits, usually about 1% by weight.

The present method of exploration is very slow and expensive.

According to our method, a suitable ionization chamber is transported over the territory to be explored, preferably by means of an aircraft. This is of great advantage in exploring large areas.

The annexed drawing illustrates the ionization chamber which we utilize. The invention is not limited to the details stated herein.

Such ionization chamber A is of general cylindrical shape. Its axial length is 100 centimeters, or about 40 inches. Its diameter is 20 centimeters, or about eight inches. chamber A is made of aluminum, duralumin, or other suitable metal or alloy. As an example, this wall I is maintained at a constant negative voltage of 600 volts. The base 2 of said chamber has a bore, which is closed gas-tight by a plug 3, which is made of insulating material. Said plug The wall I of the 3 and said bore of base 2 are of frusto-conical shape.

A conductive screen 4 is fixed to plug 3, or is otherwise held fixed in spaced relation from the interior surface of wall I and base 2. This screen 4 defines an inner ionization chamber 5a. Said screen 4 is insulated from wall I and base 2. The wall I usually contains a small percentage of radio-active material as an impurity, and such radio-active material emits alpha rays. It is desired to prevent such alpha rays from penetrating the inner ionization chamber 5a. The fixed spacing between screen 4 and the interior face of wall I, therefore depends upon the pressure and composition of the gaseous filling. As an example, if the gaseous filling is argon, under a pressure above ten atmospheres, the spacing between screen 4 and the inner face of wall I, including its base 2, is about five millimeters. Hence the volume of the ionization chamber 5a which is enclosed by screen 4, is substantially equal to the volume of ionization chamber A. In this example, said volume V is substantially 2,000 cubic centimeters. If the pressure P is 150 pounds per square inch, VP equals 300,000.

The wall I may be made originally in two or more transverse sections, so that the screen 4 is filled with a suitable gas,

c t o e. If screen 5 Claims. (01. 250-8345) and plug 3 may be located relative to base 2, in final relation. The other section of-wall I can then be fixed to the section which is integral with base 2. A gas-tight joint, such as a threaded joint, is formed between sections of wall I.

An axial cylindrical electrode 5, whose diameter is about two millimeters, fits gas-tight in an axial bore of insulating plug 3. The chamber A such as argon, under suitable pressure. This pressure may be thirty atmospheres, or about 440 lbs. per square inch. Said pressure may be as high as atmospheres or even more. A frusto-conical conductive member 6 is located gas-tight in a bore of plug 3.

Said member 0 is a guard ring, which is maintained at zero potential by a wire I, which is connected to a metal frame of the aircraft, or is maintained at zero potential in any manner. This guard ring 6 acts as a ground electrostatic shield for the respective adjacent part of the electrode 5. It also prevents any leakage ofcharge or current between the electrode 5 and the screen 4, or between the wall I and electrode 5. The guard ring 6 may extend longitudinally inwardly and outwardly from the insulating plug 3.

The screen 4 is maintained at a fixed high Volt age, which may be either positive or negative. We prefer a negative voltage. The screen 4 is connected to the negative terminal of the source of biasing potential by wire 8. The positive terminal of the source of grounded in the usual manner. As an example, the fixed or constant negative voltage of screen 4 may be 800-1600 volts. -When alpha rays are radiated from wall I, such alpha rays produce gaseous ions in the space between screen 4 and wall I. The function of the charged screen 4 is to extract such ions, in order to prevent such ions from entering the inner ionization chamber in. It is well known that the ionization of a gas produces positive and negative particles or ions. If a gas is thus ionized between two electrodes which are at different respective potentials, the positive ions will move to the electrode of lower potential to be extracted, and the negative ions will move to the electrode of lower potential to be extracted. In this example, the wall I is maintained at a fixed negative potential of minus 600 volts. This can be done in the usual manner,- by connecting wall I to the negative terminal of a source of unidirectional and constant voltage. The positive terminal of said source may be grounded, as is customary in providing a biasing potential. In this example, wall I is the anode and screen 4 is the 4 and wall I were connected biasing, potential may by a wire, current would flow through said wire from wall I to screen 4. Hence the negative ions which are formed by the alpha. rays in the gas between screen 4 and wall I, will be extracted at wall I in this example, and the corresponding positive ions will be extracted at screen 4. Such ions will be extracted, as long as there is a suitable difierence of potential between screen 4 and wall I. Thus, if screen 4 is at a positive potential and wall I is at a higher suitable positive potential, the ions which are formed by the alpha rays between screen 4 and wall I, will be extracted by said screen 4 and wall I.

Externally to chamber A, electrode 5 is connected to wire 1, through a resistor 9, whose resistance is 4.10 ohms, namely, 40,000 megohms. This resistancemay, for example, be within a range of 10 ohms and 10 ohms. The free end I of electrode is connected to any suitable direct current amplifier.

The wall I and the electrode 5 form a small capacitance, whose capacity is about 25 micromicrofarads, namely, 25.10- farads. If C designates the capacity of said capacitance, and R designates the resistance of resistor 9, then CR is the time constant of the circuit. In this example CR equals one second. The time constant can be varied, but it preferably should not greatly exceed one second. In a capacitance-resistor circuit, the time constant is defined as being the number of seconds required to impress 63.2% of the full charge upon the capacitance, after a unidirectional and constant voltage is applied to the capacitance-resistor circuit. In this example, the constant charging voltage is 600 volts, because electrode 5 is maintained at zero potential, and wall I is maintained at a negative potential of 600 volts. The charging current must flow through resistor 9, which is connected in series to the capacitance which is provided by electrode 5 and wall I. The electrode 5 and screen 4 provide another capacitance, which is connected in series with resistor 9.

This chamber A can be transported over the territory which is to be explored, in an aircraft, at a low altitude, which may be about 100 meters, or about 330 feet, at a low horizontal or ground speed, which may be about 100 feet per second.

When the chamber A is at this altitude, it will receive any gamma radiations from an area which has a radius of about 100 meters, namely, from an area of about 30,000 square meters, above whose center said chamber A is located.

If said area has a deposit of uranium ore, which is equivalent to one per cent of uranium oxide by weight, and if the area of said deposit is at least 100 square meters, such deposit will be detected by chamber A. The detecting apparatus will indicate a maximum ionization current, when the chamber A is substantially above this deposit, making allowance for the time delay of response to ionization. As the aircraft approaches said deposit, the ionization current will increase. As the aircraft moves away from said deposit, the ionization current will decrease.

In order to detect such a deposit, it need not be a contmuous deposit. Such deposit may consist of separate adjacent deposits.

The gamma rays of such a deposit will penetrate the atmosphere through said height of about 100 meters, and such gamma rays will also penetrate wall I and pass into the inner ionization chamber 5a which is defined by screen 4.

There is always some ionization in the ioniza- 4 tion chamber which is defined by screen 4, because of cosmic rays and other penetrating radiations.

In this example, a potential difference of 800 volts is maintained between screen 4 and electrode 5, and screen 4 is negative relative-to electrode 5. When the gamma rays ionize the gas between electrode 5 and screen 4, an ionization current will flow through the gas between then so that the positive ions are extracted at screen 4 and the negative ions are extracted at electrode 5. This provides a detecting ionization current between the grounded point of electrode 5, and the source of biasing voltage whose negative ter minal is connected through wire 8 to screen 4. This detecting current will increase or decrease as the ionizing effect of the received gamma-rays increases or decreases. In efiect, the gamma rays vary the resistance of the circuit of the detecting ion zation current between electrode 5 and screen 4. When the aircraft approaches a source of gamma rays, the detecting current will increase, and such detecting current w'll decrease as the aircraft moves away from the source of gamma rays. Due to the corresponding change in efl'ective resistance of the gas between electrode 5 and screen 4, there will be a variation in the potential of the point of electrode 5 which iS connected to resistor 9, with a corresponding variation in potential at point I 0 of electrode 5.

Hence, the ionization current, in this example, will detect the presence of a commercial uranium ore deposit, in a belt which extends laterally meters at each side of the course 01' the aircraft. The period of each indicating pulse of the ionization current, in which it rises to a maximum from a minimum value and in which it is reduced to said minimum value, depends up n the ground speed of the aircraft, allowance being also made for said time constant, CR.

If the aircraft is flown over the territory to be explored at the low ground speed of a out 100 feet per second, the voltage of electrode 5 at point I0 will thus be varied by an average uranium deposit of commercial value, during a respective detection period of about three seconds. The detecting variation in voltage of electrode 5 at point I0 can be amplified by a conventional direct current amplifier, whose current variaton can be continuously recorded by conventional means, or transformed to an A. C. voltage and thus amplified by a conventional A. C. amplifier. Each detecting voltage pulse of electrode 5 at point I0 thus produces a detecting current increase pulse, during a few seconds, in the output of the amplifier.

During each detecting pulse, the voltage of ectrode 5 at point III passes through a maximum. This variation in voltage can be used to produce an alternating current by conventional means, and such alternating current can be amplified and detected. Such voltage pulse can be separated into shorter pulses, by using a suitable interrupter or chopper.

The altitude of the aircraft is suitably determined and recorded by using an altimeter, because the intensity of the gamma radiation which is received from the ground, depends on the altitude of the aircraft.

Hence, at the end of an exploration fli h ch detecting current pulse or voltage pulse which is delivered by'the amplifier will have been detected and recorded. A suitable record is simultaneously and continuously made of the explored territory, as by a continuous series of pictures. so that each detecting current pulse can be identified with the respective-area or the explored territory.

As previously stated, the cosmic rays and stray radio-active rays will always produce some ionization in the chamber to, so that current will always flow from the point of zero potential to electrode 5, in this example.

If the gaseous filling is argon, and VP equals substantially 300,000 in chamber to, as previously stated, the normal stray-ionization current is about 2.10- ampere, namely, about 0.2 micromicroampere. In such case. if the potential difference between electrode I and screen 4 is 800 volts, the potential of electrode I is changed from zero potential by 8.10 volts, namely, 0.008 volt, because substantially the entire voltage drop of the stray ionization current is through the gas between electrode and screen 4. Since the screen 4 is substantially of the same size as wall i, there will be little difference between the time constant of the circuit of the capacitance 0-4, and the circuit of the capacitance 5l, since the resistor l is 'common to the circuits of H and 5-4.

By providing the screen 4 with a higher negative potential than wall I, we suppress or minimize the effect of alpha rays which are emitted by wall I.

There is no discharge between wall I and screen 4, because the compressed gas can easily withstand a voltage diflerence of 200 volts.

Even if the surface of the ground is irregular, or if the altitude of the aircraft is varied within reasonable limits, the presence of a commercially workable uranium deposit will produce a substantial and easil detectable increase of at least 20% in the current which is delivered by the The invention is not limited to the details stated herein. By diminishing the CR factor, we can use an aircraft which operates at higher speed. The axis of the chamber A is held horizontal.

The invention applies to the detection of uranium ore deposits which are at or close to the surface of the ground, so enter the atmosphere.

Our invention is useful for detecting any source of gamma rays, such as a plant which has large quantities of radium compounds, uranium compounds, etc.

The correlation of the various factors is one of the features of our invention. The CR. factor must be in proportion to the speed of the aircraft. The time factor, CR, must be regulated so that a perceptible pulse of ionization current is secured during the scanning period. Instead of making a photographic record of the explored territory, maps and other means can be'used for determining the regionof the deposits. The current of the amplifier can be designated by a suitable continuous graph, on a conventional recording device. By plotting the course of the aircraft with reference to a map, and thus recording the direction of movement of the aircraft and its speed, it is possible to ascertain the position of the aircraft during each instant of time. The altitude of the aircraft should also be continuously determined and recorded.

Upon comparing this with the ionization current graph, which is also plotted with reference to time, the desired data can be determined.

The screen 4 may have a frusto-conical extension 40, which is made of insulatinz material. and which is clamped between base I and plug 3. Any

that the gamma rays can rays and other means may be used for supporting screen 4 in ilxed spaced relation from wall I, and insulated from wall I.

The undesired radiations, such as cosmic rays, internal alpha rays, gamma radiations from the ground which are found even in the absence of any substantial mass of radio-active material, etc.,- are conveniently designated as stray radiations. Electric charge is withdrawn from the electrode I at a sufficient rate, either wholly to compensate for such stray radiations, or sumciently to compensate for such stray radiations, to detect a voltage change on electrode 5 which corresponds to a predetermined intensity of gamma ray radiation.

The gaseous filling of the chamber is preferably free from water vapor.

We have described a preferred embodiment of our invention, changes and omissions and additions can be made without departing from its scope.

We claim:

1. A method of detecting the presence of a mass of radio-active material which emits gamma which is located sufiiciently close to the surface of the earth to emit said gamma rays upwardly into the atmosphere, which consists in moving an aircraft over the region to be explored, said aircraft being thus moved at an altitude within the upward range of said gamma rays, supporting a detecting ionization chamber on said aircraft to receive said gamma rays within said ionization chamber, maintaining a gaseous filling in said detecting ionization chamber in excess of normal atmospheric pressure, maintaining a difference of potential between the wall of said detecting ionization chamber and an internal electrode thereof to extract the ions which result from said gamma rav ionization so that said ions are extracted at said wall and at said internal electrode to produce a flow of detecting ionization current between said wall and said internal electrode in a predetermined circuit relative to said internal electrode, substantiallv shielding said detecting ionization chamber from alpha rays, determining the stray ionization in said detecting ionization chamber which results from cosmic rays and other stray rays by determining the tected, said aircraft being respective stray ionization current in said detecting ionization chamber which corresponds to the stray ionization which is produced by said stray rays, observing the increase in the flow of said detecting ionization current which is produced by the gamma ray radiation which is to be demoved at a speed which is sufllciently low in proportion to CR to produce said increase in said flow of current during a period which can be readily observed, C being the ca acity between said wall of said detecting ionization chamber and said electrode. B being the resistance of the circuit in which said detecting ionization current flows.

2. An ionization device for detecting gamma rays, said device comprising an enclosed chamber which has a metal wallwhich is permeable to gamma rays and which emits alpha rays, said metal wall having an opening which is closed by a plug which is made of insulating material, said chamber having an ionizable gaseous filling, a perforate metal screen which is permeable to gamma rays, said screen being located in said chamber and being spaced from and being insulated from said metal wall, said screen substantially enclosing a part of the inner space of said chamber to provide an inner but it is clear that numerous trode extending outwardly of ionization chamber, means maintaining said metal wall and said metal screen at a diiference of potential which is sufiicient to produce an ionization current through said gaseous filling between said metal wall and said metal screen by extracting ions which are produced by said alpha rays in said gaseous filling in the space between said wall and said screen, said gaseous filling being under suflicient pressure to limit said ionization current between said wall and said screen to the ionization current which is produced by the extraction of said ions, an elecsaid enclosed chamber and said ionization chamber through said plug, said electrode being connected through a resistor to a point whose potential is different from the potential of said screen, the CR factor of said ionization device being at least substantially one second, being the capacity between said wall and said screen, R being the resistance of said resistor.

3. A device according to claim 2, in which said screen is negative relative to said wall and said electrode.

4. A device according to claim 2, in which said metal wall is maintained at a negative potential, said screen is maintained at a greater negative potential than said wall, and said electrode is connected through said resistor to a .point of zero potential.

8 5. A device according to claim 2, in which said screen has an insulating material said plug and said being located in said and said extension,

extension which is made of and which is fixed between metal wall, a metal shield plug between said electrode to said electrode through said resistor.

LADISLAS GOLDSTEIN. BORIS PREGEL.

REFERENCES CITED Number Number 340,231

UNITED STATES PATENTS Name Date 'Hassler Apr. 6, 1940 Neufeld Mar. 10, 1942 Fearon Apr. 13, 1943 Cravath Oct. 24, 1944 FOREIGN PATENTS Country Date Great Britain Dec. 12, 1930 OTHER REFERENCES Review of Scientific Instruments," vol. 11; No. 8, Aug, 1940, pp. 267-269.

said shield being connected

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