US2835821A - Method of observing and classifying the spectrum of gamma rays - Google Patents

Description

May 20, 1958 R. E. FEARON 2,835,821 METHOD 0E oBsEEvTNG AND cLAssTETTNG THE SPECTRUM oT GAMMA RAYs 2 Sheets-Sheet 1 Filed June 22, 1951 2 7 @fm/MEA HT'ORNEY MaLyr 20, 1958 R. E. FEARON METHOD OF OBSERVING AND CLASSIFYING THE SPECTRUM 0F GAMMA RAYS Filed June 22, 1951 2 Sheetsf-Sheec 2 A INVENToR. ierzz afa/p United States Patent f l'i/il'li- PED @F AND CLASSFYING THE SPECTRUM GF GAMMA RAYS Robert E. Fearon, Tulsa, Gkla., assigner to Well Surveys, Incorporated, Tuisa, Okla., a corporation of Delaware Application `lune 22, 1951, Serial No. 233.004

1'7 Cla-ixus. (Cl. Z50-71) This invention relates in general to the art of detecting radioactivity and more particularly to the art of detecting radioactivity at elevated temperatures such as those encountered in well logging.

In the prior art it is well known to use scintillation counters comprising fluorescent media and photosensitive detectors for the detection of radioactivity. However, the scintillation counters have been incapable of effective operation at elevated temperatures such as are often encountered in deep boreholes. The fluorescent media heretofore used in scintillation counters comprise crystals and liquids. These media do not scintillate eiiiciently when hot. The photosensitive detector heretofore used comprise photomultipliers for converting the scintillations of light into electron ow and multiplying the electron flow to yield an appreciable signal. The photosensitive surfaces of such photomultipliers of the prior art emit excessive dark current (thermionic emission of electrous) at elevated temperatures; indeed the photosensitive material may melt and flow ott the cathode. Thermionic emission also occurs from the dynodes of the electron multiplier of the photomultiplier.

The invention corrects the unsuitability of the fluorescent media by doing away with crystals or liquids and using instead gas which will scintiliate at high temperatures. The photosensitive surface is made of material that does not melt at borehoue temperatures and that has a high eiectronic work function so that it does not emit excessive dark current at such temperatures. Thermionic emission from the dynodes is eliminated by doing away with the electron multiplier and using gas ampiiiication instead.

Therefore, the principal object of this invention is to provide a method and apparatus for detecting `radioactivity at the elevated temperatures often encountered in well logging. Another object is to provide a scintillation counter adapted to operate at high temperatures. Still another object is to provide a method and apparatus for determining the energy of gamma rays. Other objects and advantages of the present invention will become apparent from the following detailed description when considered with the accompanying drawings, in which:

Figure l is a diagrammatic illustration of a radioactivity well logging operation;

Figure 2 is a verticalsectional view of one form of radiation detector utilizing a scintillation counter;

Figure 3 is a vertical sectional view'of a modified form of the radiation detector utilizing coincidence counting by a pair of scintillation counters;

Figure l is a horizontal sectional view taken along section d-d of Figure 3;

Figure 5 is a vertical sectional View of a furthertmodi tication of the radiation detector using magnetic de ection and coincidence .counting by three scintillation counters;

estantes may au,

Figure 6 is a vertical sectional View of the magnetic circuit used for the detector illustrated in Figure 6; and

Figure 7 is a vertical sectional view of a still further modified form of the radiation detector utilizing a lens and mask to focus certain scintillations in a scintillation counter.

Although this invention may be used for any detection or measurement of radioactivity under conditions of high temperatures, it will be described as used in the art of radioactivity well logging, for this invention has particular application to such well logging where high borehole temperatures are encountered. Referring to the drawings in detail particularly Figure l, there is illus trated a radioactivity well surveying operation employing this invention. A well 10 penetrates the earths surface 11 and may or may not be cased. Disposed within the well is subsurface instrument 12 of the well logging system. instrument 12, shown diagrammatically in vertical section, comprises detector 13, powered from power supply t4, and ampliiier 15, powered from power supply 16, and may or may not comprise a source 17 of neutrons and/or high energy gamma rays with obstacle 1S to shield detector 13 from direct radiation from the source 17. if natural radiation from the formations surrounding the borehole are to be measured, source 17 is omitted. lf source 17 is a source of both neutrons and gamma rays but only neutrons are desired, source 17 is surrounded by appropriate shielding material 19 of heavy metal, such as lead, which permits only the neutrons to pass freely through to the formations near the borehole.

Cable 2h suspends the instrument in the well and electrically connects the instrument with the surface appara tus. The cable is wound on or unwound from drum 21 in raising and lowering instrument 12 to traverse the well. Through sliprings 32 and brushes 23 on the end of the drum, the cable is electrically connected to amplier 24 which is in turn connected through pulse height discriminator 25, wave shaper 26, and pulse rate conversion circuit 27 to recorder 28. Recorder 28 is driven through a transmission 29 by measuring reel 30 over which cable 20 is drawn so that recorder 27 moves in correlation with depth as instrument l2 traverses the well. The elements areshown diagrammatically, and it is to be `understood that the necessary associated circuits and power supplies are provided in a conventional manner. 1t: is to be further understood that the power supplies 14 and 16 may be replaced by suitable transformers and rectiers which are supplied with power through the cable 20 from the surface of the earth. lt is also to be understood that the instrument housing will be constructed to withstand the pressures and mechanical and thermal abuses encountered in logging a deep borehole and yet provide adequate space within it to house the necessary apparatus and permit the transmission of radiation through it. A preferred form of the detector 13 is shown in Figure 2. Detector 13 is shown in the form of a detachable sub. Steel jacket 31, forming the outer wall of detector 13, forms part of the casing of instrument 12 and is threaded on both ends so that detector 13 may be screwed to `steel wall 32 of the instrument housing above the derector and so that the steel wall 33 of the source housing may be screwed to the detector. Steel walls 34 and 35 form the other outside walls of detector 13. The detector chamber 36 is iilled through tube 37 and valve 38 with acompressed gas (approximately 3000 pounds per square inch) that will emit light when exposed to ionizing radiation. Hollow sapphire crystal hemisphere 39 is sealed -to steel wall 35 by annealed copper gasket 4t) in order to seal chamber 41 from chamber 36. The lining of chamber 41 comprises photocathode 42 which emits electrons when struck by photons of light from the gas of spencer chamber 36. The photosensitive surface is of two kinds; the inner surface of sapphire crystal hemisphere 39 is self-silvered with photosensitive metal while the surface of steel Wall 35 may be thickly plated. By halfsilvered is meant coated, not necessarily with silver, so thinly that a substantial amount of light may pass through. it is necessary that hemisphere 39 be transparent and only half-silvered in order that light may reach the photosensitive surface, i. e., the exposed surface of the coating on hemisphere 42 and. the exposed surface ot' the plating on steel wall 35. Disposed within chamber 41 is anode 43 which is supplied with Voltage by lead wire 44 through insulating plug 45. Chamber 41 is filled with low pressure (approximately 0.1 atmosphere) gas that will effect gas amplification, that is, at proper voltage, electrons being accelerated from photocathode 42 to anode 43 will ionize the gas and produce an avalanche of electrons passing to the anode.

Although the mechanics above described is particularly adapted for operation at high temperature, particular materials are also desirable for such operation. Prior art fluorescent media used in scintillation counters are, as noted above, ineffective at the elevated temperatures of some deep boreholes. This is due to rearrangement of filled energy levels of the macroscopic crystal or liquid. This rearrangement at high temperature leaves vacant energy states which favor thermal degradation of an excitation rather than radiative degradation, i. e., a scintillation. The monatomic gases have no energy states, except atomic ones, for the atoms of such gases are substantially'independent of one another. Consequently, thermal degradation in a monatomic gas does not increase substantially with temperature; in fact, thermal degradation is unlikely at any temperature, making radiative degradation almost a certainty. If monatomic gases are used as scintillation media, decrease in eciency with rising temperature is avoided.

The monatomic gases which may be used singly or in combination at appreciable density under practical pressures (of the order of three thousand pounds per square inch) at temperatures of the borehole (up to 400 F.) are mercury vapor and the inert gases. The most suitable inert gases are neon, argon, krypton and xenon. Helium, although an inert gas7 is poorer on account of its low density. Radon, although inert also, is not included because it is radioactive. All of these gases emit light when in an excited state; hence, they scintillate. Mercury vapor is favored because of its strong emission line at 2537 Angstrom units in the ultra-violet. Krypton and xenon are favored because of their high density, hence, high stopping power at practically attainn able pressures. For example, a tive million volt electron may be stopped in less than two inches in krypton at 3,000 pounds per square inch and in approximately twothirds this distance in xenon at the same pressure. This pressure is Within the range best adapted for well logging instruments, providing suitable density of suitable volume without excessive steel required to withstand the pressure. These gases have a further necessary feature in that they transmit and not absorb their own radiations. All of these gases emit light when in an excited state; hence, they scintillate when exposed to ionizing radiation. These gases all emit light to an appreciable extent in the ultra-violet portion of the spectrum and are therefore particularly adapted for use with the photo-cathodes best suited for high temperature measurements, as described below.

Prior art scintillation counters operated for the most part in the visible spectrum, but photocathodes sensitive in this region are more rare and difficult to manufacture than photocathodes responding exclusively to the ultraviolet spectrum, below 4000 Angstrorns. Furthermore, photocathodes whose `threshold wave length are in the ultra-violet region do not generally emit as many dark current pulses, i. e., do notV thermionically emit at low` temperatures, as do photocathodes sensitive to visible light. This is because the threshold depends upon the electronic work function which also determines the temperature Vat which thermionic emission sets in. The work function is the energy that must be imparted to an electron to permit it to escape from the photosurface. To impart this energy to an electron the light photon impinging on the photosurface must have at least this energy; hence, the photon must correspond with a frequency associated with photon energy nearly as great as the Work function of a higher energy. A high work function requires light of high frequency and short wave length, To thermionically emit electrons, the photosurface must be heated until the thermal energy of some of the electrons approaches the work function; therefore a high `vork function requires high temperature for emission. rfhus, a photosurface sensitive only to ultraviolet light has a high work function and. is therefore particularly adapted for use as a photoemitter at high temperatures. The photocathode of this invention is made of materials photosensitive only to light of short wave length because such materials are easier to find and Work with than those of the prior art and are better for use at high temperatures. Satisfactory material should have a threshold wave length below 4000 Ang- Stroms.

The threshold must not be less than 1500 Angstroms, for the sapphire hemisphere is not transparent to light of shorter wave length, and light of shorter Wave length cannot reach the photocathode to be detected. The reason for making hemisphere 39 of sapphire is Sapphires strength and its transparency. Fused quartz is not quite as strong as sapphire and is likewise not transparent below 1500 Angstroms.

Another undesirable quality of photosurfaces of the prior art is low melting point, for such surfaces melt off at the high temperatures encountered in some boreholes. The photocathode of this invention is therefore made of materials that are solid at borehole temperatures.

Photosensitive materials meeting the above requirements of solidity at 400 F. and .threshold between 1500 Angstroms and 4000 Angstroms are copper, nickel, platinum, gold, silver, or tungsten. Although these metals are to be used for the photocathode, they are not necessarily to be used in pure form but may be activated in a manner well-known in the art. Activation generally consists in forming on the surface a composite layer of dissimilar materials at least one of which is a semi-conductor.

Prior art scintillation counters have employed electron multipliers in photomultipliers for amplifying the electron stream emitted from the photocathode. The work function of the surfaces of the dynodesiof electron multipliers must be low to be ecient, and therefore these surfaces emit excessive\ electrons thermionically at borehole temperatures often encountered. Amplification at the point where the electrons are first released is necessary to increase the signal above noise level. Hence the electron multiplier must be replaced and not merely omitted. In this invention, gas amplification is ernployed. Chamber 41 is filled with any low pressure (of the order of 0.1 atmosphere) proportional counting gas such as hydrogen, argon, helium, or other noble gases mentioned above as suitable scintillation media. No quenching agent is needed since it is often preferred to operate in the proportional counting region. For Geigertype photon counting, one may use a suitable quenching vapor, such as petroleum ether or chlorine. Gas amplification is effected by applying suitable voltage between anode 43 and photocathode i2 whereby an electron avalanche is produced upon photoemission of electrons from photocathode 42.

In the operation of this invention, ionizing radiation from the formations surrounding the borehole (natural assumed radiation or secondary radiation resulting from bombarding the formations with emanations from source 17) enter chamber 36 through the steel walls and produce scintillations in the scintillation medium which fills chamber 36. These scintillations are photons of light some of which pass through transparent crystal 39 to photocathode 42 where they cause photoemission of electrons from the photocathode surface. These electrons are attracted to anode 43; enroute the electron stream is amplified by gas amplification, producing an appreciable signal which may be further amplified by amplifier 15' and sent via cable 20 to the surface. There the signal may be again amplified by amplifier 24. Pulse height discriminator 25 rejects pulses not of the desired height and thereby eleminates the small dark current pulses from the signal. Shaper 26 shapes the pulses in the signal so that all pulses are of the same shape; this is necessary in order that each pulse may have the same effect as any other on pulse rate conversion circuit 27 which translates the number of pulses per second into a direct current signal proportional thereto which may be recorded on recorder 28 as a function of depth.

In Figures 3 and 4 as illustrated a modification of detector 13 wherein two photosensitive devices 46 and 47 are electrically connected in a conventional manner so that they provide output pulses only when they both produce pulses coincidently. The modification as shown in Figures 3 and 4 is identical with the detector 13 as shown in Figure 2 except for photosensitive devices 46 and 47 used in place of the photosensitive device of Figure 2. Photosensitive devices 46 and 47 comprise hollow sapphire crystal cylinders 48 and 49, respectively, closed by gaskets 50 and 51, respectively, and steel discs 52 and S3, respectively. Photocathodes 54 and 5S are Iformed on part of the inside wall of cylinders 48 and 49, respectively. Devices 46 and 47 also comprise anodes 56 and $7. Disposed about cylinders 43 and 49 are opaque masks 58 and 59, respectively. Each mask is slit to admit light to a photocathode from a certain direction. The masks are oriented so that photosensitive device 46 detects `light in one region of chamber 36 above plane a shown in Figure 4, and photosensitive device 47 detects light in another'region of chamber 37, below plane b shown in Figure 4. An output pulse will result only when light arises coincidently in both regions. This will occur when ionizing particles have suflicient energy to pass from one region to the other and cause scintillation in both regons at very nearly the same time. Other scintillations will not result in an Voutput pulse except when by chance there are coincident scintillations in both regions. This makes possible the detection of only those incident radiations above the energy required to produce light in both regions.

in Figure 5 is illustrated another modification of detector 13 adapted to classify radiation energy. Radiation from the formations around the borehole produces electrons of energy dependent upon incident radiation energy. Classification of the energy of these electrons classifies energy of the incident radiation. Photosensitive devices 6ft, 62, and 63 `are constructed similarly to devices 46 and 47 of Figures 3 and 4 and are similarly masked. The masks and devices are disposed to detect light along a curved path. A magnetic field of proper strength is provided in a conventional manner parallel to the axes of the photosensitive devices so that certain electrons having a narrow range of velocity entering at the beginning of the curved path, will continue in that curve-d path and produce scintillations detected by all three photosensitive devices, the outputs of which are electrically connected in a conventional manner so that they provide overall output pulses only upon such triple coincidents of pulses from all three devices. Electrons with velocities outside the proper range will not travel in the curved path by the photosensitive devices but will strike obstacle 64 or the masks of the devices and will '6 not produce an output pulse. This detector 13 as shown in Figure 5 will indicate the number of electrons having a certain velocity range and hence may be used to classify energy of the incident radiation. Varying the magnetic field or disposition of photosensitive devices varies the velocit-y range detected.

The magnetic field of Figure 5 may be produced, as shown in Figure 6, by Alnico magnet 65 having soft iron pole pieces 66 and 67, between which detector 13 is disposed.

The optical efficiency (portion of light detected) with which a photosensitive device detects a scintillation depends upon the solid angle subtended by the device, i. e., the nearer a scintillation is to the device, the greater the proportion of the light detected by the device, and hence the greater the effect of the scintillation upon the output of the photosensit'ive device. Since optical efficiency depends upon the position of a'scintillation, detector 13 as shown injFigure 2 may not be accurately used to measure the intensity of a scintillation. lt is desired to measure this intensity, for it is indicative of the energy of incident radiation. To measure this intensity, it is necessary that the optical efficiency be nearly the saine for each scintillation detected. This may be accomplished as shown in Figure 7 wherein the photosensitive device is masked by opaque mask 63 except for a small hole 69 through which lens 70 focuses light arising within recess 71 in steel wall 34. With such an arrangement, only scintillations arising in recess 7l will be detected. Since all of these will have very nearly the same optical efiiciency, detector 13 may be used for measuring the intensity of scintillations and hence energy of incident radiation.

it is to be understood that `this invention is not to be limited to the .specific modifications described Vbut is to Vbe limited only by the following claims.

I claim:

1. A method of detecting radiation that comprises the steps of subjecting a confined non-radioactive monatomic scintillation gas to radiation, and photoelectrically deriving electrical signals systematically related to scintilla- `tions produced in all said gas by the radiation impinging thereupon.

2. A. method of making a radioactivity log of a Well in which regions thereof are found which have temperatures which may be as high as 400 F. that comprises the steps of subjecting a confined non-radioactive monatomic gas to radiation emanating from the walls of the well, photoelectrically deriving electrical signals systematically related to scintilations produced in all said gas by the radiation impinging thereupon, transmitting the derived signals to the surface of the earth, and there recording the derived signals in correlation with the depth in the well at which the signals originated.

3. A method of making a radioactivity log of a well that comprises the steps of subjecting a confined nonradioactive monatomic gas to radiation emanating from the walls of the well, detecting scintillations pro-duced in a first region of said gas by the radiation impinging thereupon by subjecting a first photosensitive cathode to the photons of light produced in said first region or' the gas by the scintillations, detecting scintillations produced in a second region of said gas by the radiation impinging thereupon by subjecting a second photosensitive cathode to the photons of light produced in said second region of the gas by the scintillations, separately multiplying the electrons emitted by said first and second photosensitive cathodes, combining the resultant electrical signals to provide output pulses when said first and second photosensitive cathodes emit electrons coincidentally thereby dcriving an output signal which is a measure of the intensity of radiation having sniiicient energy to produce coincident scintillations in said first and second regions of said gas, amplifying the combined signal, transmitting the amplified signal to the surface of 7 the earth, and there recording the amplified `signal in correlation with the depth in the well at which the signals originated.

4. A detector of radioactivity comprising in combination means dening a chamber, a non-radioactive monatomic gas within said chamber, a photocathode for detecting scintillations in the gas by emitting electrons, means defining a second chamber and enclosing said photocathode, an ionizable medium within said second chamber adapted to be ionized by the electrons emitted by the photocathode, a collector anode also disposed within said second chamber, means for impressing a potential between said anode and photocathode, whereby electrons emitted by the photocathode will be accelerated toward the anode to produce an electron avalanche in the ionizable medium that will be collected by the anode.

5. A detector of radioactivity comprising in combination means defining a chamber, a non-radioactive monatomic gas within said chamber, and means comprising a photosensitive pulse-type ionization chamber for detecting scintillations in t'ne gas by producing proportionally related electrical signals said photosensitive pulse-type ionization chamber being optically connected to all portions of the defined chamber.

6. In a subsurface radioactivity well logging unit, a radiation detector, amplifier, and transmission means, said radiation detector comprising in combination means defining a chamber, a non-radioactive monatomic gas within said chamber, a photocathode for detecting scintillations in the gas by emitting electrons, means defining a second chamber and enclosing said photocathode, an ionizaole medium within said second chamber adapted to be ionized by the electrons emitted by the photocathode, a collector anode also disposed within said second chamber, means for impressing a potential between said anode and photocathode whereby electrons emitted by the photocathode will be accelerated toward the anode to produce an electron avalanche in the ionizable medium that will be collected by the anode.

7. A detector of radioactivity comprising in combination a non-radioactive monoatornic gas a photocathode for detecting scintillations in said gas by emitting electrons, and means for multiplying the electrons to provide an amplified signal, said photocathode being formed of a material stable at temperatures up to 400 F. and having a threshold for wave lengths that are less than 4000 A. but greater than 1500 A.

8. A detector of radioactivity comprising in combination means dening a chamber, a nonradioactive monatomic gas within said chamber that is adapted to scintillate when subjected to radiation, a photocathode for detecting scintillations in said gas by emitting electrons, said photo-cathode being optically connected to all portions of the chamber, and a pulse-type ionization charnber enclosing said photocathode and adapted to produce gas amplification of electrons emitted by said photocathode.

9. In a subsurface unit adapted for making a radioactivity log of a well in which regions thereof are found which have temperatures which may be as high as 400 F., a radiation detector, amplier, and transmission means,

said radiation detector comprising in combination means defining a chamber, a non-radioactive monatomic gas within said chamber that is adapted to scintillate when subjected to radiation, a photocathode for detecting scintillations in said gas by emitting electrons, said photocathode being formed of a material stable up to 400 F., and having a threshold for wave lengths that are less than 4,000 A. but greater than 1500 A., and a pulsetype ionization chamber enclosing said photocathode and adapted to produce gas amplification of electrons emitted by said photocathode.

10. A detector of radioactivity comprising in combination means dening a chamber, a non-radioactive monatomic gas coniined within said chamber, photoelectric means for deriving electrical signals systematically related to light impinging thereon, said photoelectric means being optically connected to all portions of the chamber, and means for measuring said electrical signals.

l1. n a detector of radiation, the combination of a sealed'container confining a gasiform luminophor comprising a non-radioactive noble gas capable of interacting with said radiation and emitting light, and means associated with said container for measuring said light.

l2. A detector according to claim 11, wherein argon is the luminophor.

13. A detector of penetrative radiation comprising a gasiforrn luminophor including a non-radioactive noble gas capable of converting individual units of mixed kinds of penetra-tive radiation, such as individual photons and quanta, into scintillations of different intensities for said different kinds with the most intense scintillations resulting from a different one of said kinds of radiation than would produce the moet intense scintillations in predetermined solid and liquid luminophors; a photo-electric device to convert scintillations originating in said gasiorm luminophor into electrical pulses having diterent amplitudes in accordance With different intensities thereof; and a utilization device for selectively responding to ones of said pulses in accordance with their amplitudes.

14. A detector according to claim 11 wherein neon is the luminophor.

15. A detector according to claim 11 wherein krypton is the luminophor.

16. A detector according to claim 11 wherein Xenon is the luminophor.

17. A detector according to claim 11 wherein helium is the luminophor.

References Cited in the tile of this patent UNITED STATES PATENTS 1,961,717 Thomas June 5, 1934 2,143,095 Thomas ian. 10, 1939 2,221,374 Farnsworth Nov. 12, 1940 2,351,028 Fearon June 13, 1944 2,534,932 Sun Dec. 19, 1950 2,759,107 Armistead et al Aug. 14, 1956 OTHER REFERENCES Electrons and Nuclear Counters, by S. A. Korf, published by D. Van Nostrand Co., in April 1946, pp. 34, 35.

Claims (1)

10. A DETECTOR OF RADIOACTIVITY COMPRISING IN COMBINATION MEANS DEFINING A CHAMBER, A NON-RADIOACTIVE MONATOMIC GAS CONFINED WITHIN SAID CHAMBER, PHOTOELECTRIC MEANS FOR DERIVING ELECTRICAL SIGNALS SYSTEMATICALLY RELATED TO LIGHT IMPINGING THEREON, SAID PHOTOELECTRIC MEANS BEING OPTICALLY CONNECTED TO ALL PORTIONS OF THE CHAMBER, AND MEANS FOR MEASURING SAID ELECTRICAL SIGNALS.

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