US2502617A

US2502617A - Alternating current ionization chamber

Info

Publication number: US2502617A
Application number: US34488A
Authority: US
Inventors: Robert E Fearon
Original assignee: Well Surveys Inc
Current assignee: Well Surveys Inc
Priority date: 1948-06-22
Filing date: 1948-06-22
Publication date: 1950-04-04
Anticipated expiration: 1967-04-04

Description

April 4, 1950 R. E. FEARON 2,502,617

ALTERNATING CURRENT IONIZATION CHAMBER Filed June 22, 1948 I Patented Apr. 4, 1950 ALTERNATING CURRENT IONIZATION CHAMBER Robert E. Fearon, Tulsa, Okla., assignor to Well Surveys, Incorporated, Tulsa, Okla., a corporation of Delaware Application June 22, 1948, Serial No. 34,488

9 Claims.

This invention relates generally to the art of geophysical prospecting and more particularly to radioactivity well logging.

Specifically this invention is directed to a method and apparatus for making an ionization chamber generate alternating current by controlling the circulation of the ionizable medium in a convection type ionization chamber, such as that disclosed in my copending application Serial Number 572,666 now Patent No. 2,472,153.

The convection type ionization chamber employs an ionizable medium such as argon or helium which contains a trace of a substance, such as xenon, which reduces the electrical mobility of the positive ions without affecting the electrical mobility of the negative ions. This ionizable medium is circulated continuously in the system by a pump. At one point in the system the medium is completely de-ionized, then the ionizable medium is subjected to the gamma radiation that it is desired to measure. The gamma radiation entering the ionizable medium produced ionization thereof. The area in which the ionization occurs has disposed therein collector plates to which the negative ions diffuse immediately while the more cumbersome positive ions, by the circulation of the ionizable medium, are swept out of this area and into a third area where de-ionization is afiected. A measurement of the current produced by collecting either the negative or positive ions is a measurement of the gamma radiation which produced the ionization.

It is obvious, that since the operation of the detector described above depends on the propulsion of the ionizable medium, the ionization current would fall to a much smaller value if the ionizable medium was at rest. Consequently, if the circulating pump is so arranged that it delivers a current of the ionizable medium intermittently, the output of the electrical system would vary in a corresponding manner. For example, assuming that the operation of the pump occurs in cycles of one second of pumping and one second of not pumping, it is apparent that the output of the electrical system will contain a component of alternating current at one-half cycle per second, inasmuch as two seconds are needed for the cycle which has been ascribed to the pump.

' There are many ways of causing the current of the ionizable medium to flow intermittently. As examples: an ordinary reciprocating pistontype pump, or centrifugal pump of the type employed in sirens, using as the output any one of the peripheral ports. Another way of producing intermittent flow of the ionizable medium through the area in the system in which the gamma rays are detected is by opening and closing a valve in a pipe which acts as a shunt across the circulating pump. If such a valve is closed, the current of the ionizable medium will pass through the usual delivery channel of the pump, and if the valve in the shunt line is open, the ionizable medium will pass through the shunt line thereby reducing the delivery, as would be observed in the normal delivery channel of the pump, to a very small value. Accordingly cyclical openings and closings of the value in the shunt line will produce cyclical variation in the electrical output which appears from the ionization chamber. It is obvious that similar results could be obtained by placing a valve directly in the delivery line of the pump and opening and closing it cyclically.

The choice of frequency to be employed in interrupting the fiow of the ionizable medium would depend upon the spacing of the plates in the zone of the instrument where the electrons are collected by diffusion. The frequency will also depend on the length of this group of plates parallel to the direction of the flow of the ionizable medium. The choice of frequency will also be influenced by the diffusibility of both the positive and negative ions. Generally speaking, the upper limit of the required frequency will be fixed by the difiusibility or electrical mobility of the electrons in the working medium, while the lower limit of the operating frequency will be fixed by the difiusibility or electrical mobility of the positive ions in the ionizable medium. The amplitude of movement of the medium corresponding with an interval of pumping should be enough to sweep out the medium from between the working plates which collects the electrons.

The working plates which collect the electrons will be replaced by a system of small tubes. An equivalent will also be a mass of steel Wool or shavings, or, in fact, any shaping or conformation of electrically conducting matter having fine openings between metal surfaces, and disposing an extensive area of metal surface in a small amount of volume.

The way in which the desirable operating frequencies can be fixed, can easily be understood if the fundamental principle, that every photo-electron which occurs in the zone of the electron collecting plates must contribute to the output current, is remembered. Assume for the of the positive ions have thus been lost most of.

these ions will be swept out of the plates. If the flow had recommenced too quickly to permit the electrons to be lost by diffusion the electrons along with the positive ions would have to be swept out, which would result in no net chargebeing carried to the collecting plates and hence no electrical output would result. It is accordingly necessary to permit the ionizable medium to remain at rest sufiiciently long to lose a. very large fraction of the electrons which occurred during the rest, period. but not sufiiciently long to. bring about: an appreciable loss of positive ions. The interval of the flow must be approximately. equaLt the interval of the rest for the greatest; electrical output at the fundamental frequency of. the cyclic process.

There are moreover. other limitations on the frequency choice which depend on the intensity of. the. radiation being detected. Intense radiation willnot be satisfactorilyregistered by low frequency. circulation for the reason that the electronswill not. be lost by. diffusion if too many positiveionsremain during an interval, because of. space) charge efiect of the residual positive charge- Therefore the principalobject of this invention.resides. in the provision of a method and apparatusfon detecting ionizing radiation, such asgamma rays, byproducing an alternating current whose amplitude is a measure of the ionizing'. radiation.

Another object of .this invention resides in the provision of an ionization chamber. adapted, on exposure of; radiation, to deliver analternating currentoutput.

Still. another object= of thisinvention, resides in-theprovisioniof a convection current type ionization. chambe which, when exposed to ionizing radiation, will. deliver alternating current.

This invention-alsocontemplates a methodof producing alternating current in an ionization chamber by, causing the ionizable; medium therein to. circulate. in pulses cyclically.

Another objectof this invention resides in the provision ofnovel means, for detecting extremely weak penetrating radiations, such as gamma rays, by generating an alternating current whose amplitude is clirectlyproportional to the penetrating radiation.

Other objects and advantages of the present invention willbecome. apparent fromthe following detailed; description when considered with eu r wine which.

Figure Us a. diagrammaticillustration of one formof thepresentinventionillustrating the use of a. pump for circulating theionizable medium by pulses;

Figure 2 is a diagrammaticillustrationof the second form of the. present invention showing the use. of a valve in the. supply line from the pump for controlling the pulses in the ionizable medium;

Figure 3 is a diagrammatic illustration of a third form of the instant invention illustrating the use of a valve in the shunt line of a pump; and

Figure 4 diagrammatically illustrates a still further modification of the present invention in that the multiple detectors are employed in producing the alternating current.

Referring to the drawings in detail particularlyv Figure 1, there is illustrated an ionizable medium circulating system I0, having a pump ii, that is adapted on operation to circulate the ionizable medium in the direction indicated by the arrows. The ionizable medium may consist of a gas, such as argon, plus a trace of propylamine; The trace of Xenon is added to the gas to reduce the electrical mobility of the positive ions. The gas being circulated in the form of pulses by thepump. II, passes through a set of de-ionizing plates I2, of which every other plate is made positive with respect to ground by means of a battery I3. Those plates which are not positive with respect to. ground, are connected to the ground and'to -the negative terminal of the battery I3. The electricalficld:between the plates I 2 is made sufliciently strong to sweep all ions from the circulating gas. The gas flowing through the plates I2; is thus completely deionized, and on reaching the zone occupied by a second set of plates I4, becomes subjected to the gammaradiation that. it is desired to measure. The gamma radiation. causes the gasto become ionized. Since the electrons have considerably more difiusibility than the positive ionsthey will be. immediately absorbed by the plates I4. The reduced diffusibility. or electrical mobility. of 1 the positive-ions, due'to-the propylamine, will cause them to be carriedvout of. the zone occupied by the plates. I4", intothezone occupied by a third set: ofplates I51 Every other one. of the plates I5; are connected to-thepositive side of the battery I6, and the remaining-platesare connected to the negative:side-of the-battery I6, andto ground. The plates I5, in the manner described inconnection with theplates I2; also function as means=for de ionizing-thegas as itpassesover them.

The electronsabsorbed by plates III-produce a flow of current inthe conductor l'l; through the input of amplifier I8-to-ground-at I9. Due to the fact: that: no surge-- chamber is provided the gas-willflowin pulses and as a result the current produced will be a pulsating current having a frequency that is equal tothe number of strokes of the pump II per second-.- The alternating component ofthe: signal-thusproduced in the input-of amplifier-I 8 is amplified at the frequency ofalternation of the pump and indicated or recorded by the meter-or recorder-2 0' as a measure of. the gammaradiation entering-"the region of the plates- I4:

In-Fig u-re-Z there-is illustrated a modification of the-present invention. This-form of the'invention differs from that shown in Figure 1 inthat a'valve 2! is disposedin the ionizable medium circulatory system' I Il-onthe exhaust sideof the pump lI. ValveZI is-caused-to openand close ata predeterminedrate by the synchronous m0- to 22 which drives it through the-gearbox 21. Gear-box 23 is equipped with a plurality; of gears whereby the speed at which valve 2I is opened and closed can be varied at will to control the frequency of pulsationof the gas flow-and thereby control the frequencyof the signal amplified and recorded-by the amplifier I8 and the recorder 20.

Still another embodiment-of'this invention is illustrated in Figure-3. In'thls form of the invention the circulatorysystem I is provided with a by-pass 24 which permits the gas being circulated by the pump II to by-pass the plates l2, I4 and i5. A valve 25 is disposed in the by-pass 24. Valve 25 may be driven by a synchronous motor 26 through a gear box 21 in the same manner as described in connection with Figure 2 to produce pulsations in the flow of gas over the plates l2, l4 and [5. In this form of the invention, as well as'those illustrated by Figures 1 and 2, the useful signal is produced by diffusion of electrons to the plates l4.

Another embodiment of the present invention is illustrated in Figure 4. In this form of the invention the circulatory system I0 is arranged to'provide two like paths 26 and 21 for the flow of gas. Each path is provided with a similar set of plates which function in the same manner as those described above inconnection with Figures 1, 2 and 3. The plates in path 26 have the same reference characters as used in the preceding figures and the plates in path 21 have the same characters with primes added. The battery I3 is used for both sets of plates [2 and i2 and the battery I6 is used for both sets of plates I5 and The plates 14 and I4 are connected to opposite sides of the input of a push-pull amplifier 28 which in turn is connected to the recorder or indicator 29.

In the operation of this form of the invention r the pump ll continually forces the gas toward a valve 30 which is disposed in a Y formed by the circulating paths 26 and 21 and the path leading from the pump. Valve 30 may be of the rotary type and be driven by a synchronous motor 3| through a speed control gear box 32. The ports in the valve are so arranged that the valve intermittently passes the current of gas supplied by pump II into path 26 and path 21. The currents produced by diffusion of electrons on the separate groups of plates l4 and M are brought by separate leads to the opposite sides of the push-pull amplifier 28 which impresses an alternating current signal at the frequency of operation of the valve 30 on the indicator or recorder 29 which indicates or records the intensity of the alternating current signal as an indication of the gamma rays falling on the two sets of plates 14 and I4.

Although the present invention finds broad application in industry it is to be understood that it is particularly applicable to well surveying systems. The manner of incorporating the present invention in a well surveying system, except for small changes, is the same as that disclosed in my copending application Serial Number 572,666.

I claim:

1. A detector of ionizing radiation comprising in combination means defining a closed circulatory system, at least a portion of which is adapted to admit ionizing radiation, an ionizable medium in said system, means for circulating said medium in the system to expose it to the admitted radioactive ionizing emanations, said means being adapted to impart surges to said ionizable medium that are separated by uniform time intervals, means for de-ionizing said medium before it enters the portion adapted to admit ionizing radiation, means for separating the ions produced by said radiation when the medium enters that portion, means for collecting one group of the separated ions, and means for measuring the alternating component of the electrical current produced by the collection of said ions as a measure of radiation.

2. An apparatus for detecting penetrating radiation that comprises in combination a sealed system, an ionizable medium in, said system, means for cyclically circulating said medium in said system, said means being adapted to impart surges to said medium which are separated by uniform time intervals, means in the path of said medium for de-ionizing said medium, said system having a zone in which the penetrating radiation enters the ionizable medium to produce ionization of the de-ionized medium, means adjacent said zone for collecting the negative ions, means at another point in the path of circulation of said surging medium for subsequently collecting the positive ions, and means for amplifying and recording the alternating component of the current produced by the collection of the negative ions.

3. An apparatus for detectingpenetratingv radiation that comprises in combination a sealed system, an ionizable medium in said system, means for circulating said medium, said circulating means being adapted to impart surges to said medium separated by uniform time intervals, means in the path of said medium for deionizing said medium, means for separating positive and negative ions formed by subjecting the ole-ionized medium to penetrating radiation, and means for recording the alternating component of the current produced by collecting the separated negative ions as an indication of the intensity of the penetrating radiation.

4. An apparatus for detecting penetrating radiation that comprises in combination a sealed system, an ionizable medium in said system, means for circulating said medium, said circulating means being adapted to impart surges to said medium separated by uniform time intervals, means in the path of said medium for de-ionizing said medium, means for separating positive and negative ions formed by subjecting the deionized medium to penetrating radiation, and means for recording the alternating component of the current produced by collecting one group of the separated ions as an indication of the intensity of the penetrating radiation.

5. A detector of radioactive emanations comprising in combination a closed circulatory system; an ionizable medium in said system; means for circulating said medium in said system; said circulating means being adapted to impart surges to said medium that are spaced from each other by uniform time intervals; said circulatory system having in succession in the path of the circulating ionizable medium, a de-ionizing zone, an ionizing zone, and a second ole-ionizing zone; deionizing means in each of said de-ionizing zones; means in said ionizing zone for separating the positive and negative ions produced by the action of radioactive emanations of said ionizable medium; means for collecting one group of the separated ions; and means for recording the alternating component of the electrical current produced by the collection of one group of ions as a measure of the radioactive emanations.

6. An apparatus for detecting penetrating radiation that comprises in combination a sealed system, an ionizable medium in said system, a pump in said system for circulating said medium in surges that are spaced from each other by uniform time intervals, means in the path of said medium for de-ionizing said medium, means for separating positive and negative ions formed by subjecting the de-ionized medium to the pene- 7 trating radiation, means for collecting the negative ions, and means for recording the alternating component of the current produced by collecting the separated negative ions as an indication of the intensity of the penetrating radiation.

7. An apparatus for detecting penetrating radiation that comprises in combination a sealed system, an ionizable medium in said system, means in said system for circulating said medium in said system, means in the path of said medium for separating positive and negative ions formed by subjecting the medium to the penetrating radiation, means forming a part of the sealed system defining a medium path that shunts the separating means, means for intermittently diverting the circulating medium to the shunt path, means for collecting the separated negative ions, and, means for recording the alternating component of the current produced by collecting the separated negative ions as an indication of the intensity of the penetrating radiation.

8. An apparatus for detecting penetrating radiation that comprises in combination a sealed system, an ionizable medium in said system, said sealed system having means defining two circuits for said ionizable medium, means for alternately circulating said ionizable medium through the circuits, separate means in each circuit for separating positive and negative ions formed by subjecting the ionizable medium in each circuit to the penetrating radiation, means for collecting the negative ions produced in each circuit, and means for conducting the current produced by collecting the separated negative ions in each circuit to opposite sides of a push-pull amplifier, and means for recording the output alternating current from the amplifier as an indication of the intensity of the penetrating radiation.

9. An apparatus for detecting penetrating radiation that comprises in combination a sealed system, an ionizable medium in said system, means for circulating said medium, said circulating means being adapted to impart surges to said medium separated by uniform time intervals, means in the path of said medium for deionizing said medium, means composed of eleotrically conducting matter with small openings in it through which the ionizable medium circulates,such means to receive the radiation which it is desired to measure, and to which means the free electrons produced by the ionization will difiuse, but to which, because of lack of suflicient time, the positive ions will not diffuse the deionized medium to penetrating radiation, and means for recording the alternating component of the current produced by collecting one group of the separated ions as an indication of the intensity of the penetrating radiation.

ROBERT E. FEARON.

No references cited.