US2768307A - Scintillometers - Google Patents

Description

A. L. TIRICO SCINTILLOMETERS Filed July 26; 1952 2 Sheets-Sheet 2 O m g y Mu E m N 5 m4 R A. p o WW W /V T nwflu w.

-occurs deeply within a thick cathode element. over this cannot be remedied by simply making the cathode very thin since this will reduce the likelihood that United States Patent SCINTILLOMETERS Arthur L. Tirico, Glen Ridge, N. .L, assignor to The Texas Company, New York, N. Y., a corporation of Delaware Application July 26, 1952, Serial No. 301,019

Claims. (Cl. 250-71) This invention relates to improvements in detectors of penetrative radiation. More particularly it relates to improvements in scintillometer types of detectors.

As is known, scintillometers have much greater efiiciencies than other types of detectors, such as Geiger- Mueller tubes, in which the generation of an electrical pulse is started by the injection into an electric discharge space of charged-particles which are provided quite directly by the radiation either because it actually comprises charged-particles or because it produces them as direct by-products of interactions. In the case of a scintillometer the radiation is first converted into light as a preliminary step and later into charged-particles (electrons) through the action of that light on a photoemitter. The greater efficiencies of scintillometers which are exemplified in their detection of gamma radiation can be explained as follows:

In both of Geiger-Mueller tube and a scintillometer a gamma ray must be made to sustain an interaction before it becomes effective to produce an electrical pulse. In a Geiger-Mueller tube the interaction is made to occur within the cathode of the tube to the end that a chargedparticle will be projected therefrom into the intense electric field between the cathode and anode. When such a particle is caught and accelerated in a Geiger-Mueller electric field it usually gives rise to an avalanche of electrons which, when collected on the anode, is sufficient to constitute a measurable pulse. In the case of a scintillometer the interaction is made to occur in a luminophor to the end that a light impulse will be produced which in turn can produce charged-particles, i. e., electrons, by irradiating a photo-electric element. When these particles are duly multiplied in number, either by the use of a cascade ,of secondary emitter electronmultipliers or by using them to start a Townsend avalanche in a gas tube (see copending U. S. application,

Ser. No. 138,341, filed January 13, 1950, now Patent No. 2,694,152) they also will attain sufficient numbers to constitute a measurable pulse.

Accordingly, whereas particles must be able to escape from any interaction point within the cathode of a Gei- This limited whereby they will not escape if the interaction Morethe average gamma ray will sustain any interaction in it at all. Thus in Geiger-Mueller detectors of the directly-actuated type (as distinguished from the photosensitive type) the cathode wall thickness, however it may be selected, will limit the attainable detection efficiency either predominantly for one or the other of two the case of a scintillometer is to make the luminophor of transparent material. Once this is done this element may be made very massive, for affording a good incidence of gamma interactions, before its size begins to materially impede the scape of the kind of energy, light, into which it converts the gamma radiation, this being true even if the light is generated deeply within the lumi nophor.

Thus it is seen that the great advantage of scintillom eters lies in the fact that the interaction element, i. e., the luminophor or detector head in which the penetra tive radiation undergoes its first change in form, can be massive without absorbing the usable energy which it produces in response to the radiation. Because of this a luminophor of a given volume (a solid detector head) may produce a much higher percentage of useful scintillations for agiven flux density ,of intercepted radiation than the number of useful interactions sustained in a a Geiger-Mueller tube (a hollow detector head) of equal volume. Of course, this goes a long way toward making the scintillometer a very eflicient detector. However high overall efiiciency for the detector will not be realized unless its other components are capable of converting a high percentage of the scintillations into electrical impulses. I have found that according to prior art practices the optimum percentage has not been attained because the best utilization has not been made of the small number of photo-electrons which can be provided .by one scintillation.

In a photo-multiplier tube these electrons are accelerated to about volts and then projected upon a secondary emitter electron multiplier. In effect the function of this secondary emitter, i. e., the efiect of the first dynode of the multiplier .tube is to convert the, say, 2

or 3 100 volt primary photo-electrons into say 4- to 10 low-energy secondary electrons. This multiplication process is thereafter repeated over and over again by use of a cascade of secondary emitters untila total gain of perhaps a /2 million has been obtained. The principal weakness in this arrangement is incurred in the first electron conversion, that is in the conversion of the few primary photo-electrons provided by a scintillation into secondary electrons emitted by the first dynode, and is due to the fact that the secondary emission ratio is only reliable statistically rather than absolutely. Thus if the dynode has a secondary emission ratio of 3 its average output can be depended upon to be about three times as great as the average input to it. However this does not mean that it will invariably emit exactly 3 electrons each time that it is bombarded with one primary electron. Instead in specific individual cases of impingement by primary electrons, the secondary emission ratio is subject to random variations, whereby the dynode may sometimes emit more, or even considerably more, than 3 secondary electrons whereas at others it may emit less secondary electrons, or even none at all (an emitted secondary electron being one which not only is freed within the lattice of the dynode but in addition escapes from the surface thereof into the free space and electric field between the first and second dynodes). Because of this and due to the smallness of the number of primary electrons which are available for each received scintillation some counts will invariably be lost at the first .dynode with the result that the efficiency of the counter will be impaired, in proportion to their number. Moreover this weakness will be aggravated in any case Where the first dynode of the photo-multiplier tube does not have its intended secondary emission ratio, a condition which is very possible since this characteristic of a dynode is difficult to standardize and to stabilize. Nor can this situation be corrected by making the bombarding electrons more energetic. As is Well-known the highsuflicient ranges to attain the surface and escape.

emissive backing-material and/or that any secondary electrons which they do free within the lattice have in- In any case whatever may be the correct reason it is known that the secondary emission ratio will not be enhanced by imparting additional energy to the bombarding primary electrons.

In the case of a photo-sensitive Geiger-Mueller tube having a foraminous cathode (see Figs. 1 and 2 in the above-mentioned copending application) some or all of the small number of photo-electrons which are provided by a single scintillation are emitted on the outside surface of the cathode. While it is true that there is always a possibility that frequently most of them will become caught in the fringing anode field which penetrates -through the foramina of the cathode and that therefore they may enter the discharge space of the tube and be- -come effectual to start an avalanche, it is also true that some of them, and occasionally all of them, may not do so and instead may simply return to the outside surface of the cathode thereby resulting in a lost count.

Accordingly it is an object of the present invention to provide an improvement for scintillometers which affords more certain utilization of the small number of photo-electrons which can be provided by a single scintillation thereby to reduce the number of lost electron counts and increase-the etficiency of the detector.

In general these and other objects are attained by utilizing said electrons in a device, such as a light amplifier tube, wherein each of them may be accelerated to a very high potential such as 30,000 volts and their increased energy may be utilized to good avail. In a light amplifier tube the accelerated electrons are made to bombard a luminescent target, rather than a secondary emitter dynode, which will produce a new scintillation whose intensity is related to the increased energy of the electrons. By this use of a light amplifier to couple the luminophor to a photo-electric device such as a photo-multiplier tube or a photo-sensitive Geiger- Mueller tube, the number of photo-electrons which will be emitted in the latter device for each scintillation generated in the luminophor may be so increased as to greatly reduce the likelihood of any count being lost in the ways described above. Moreover the use of a light amplifier for coupling a luminophor to a photo-electric device offers a number of incidental advantages and novel features which will be apparent from the detailed description of the invention which follows:

In the drawing:

Fig. 1 represents a scintillometer embodying the present invention, its light amplifier and luminophor portions being shown in longitudinal cross-section;

Fig. 2 is a modification of the embodiment of Fig. 1; and

Fig. 3 represents another type of scintillometer embodying the present invention, its light amplifier and luminophor portions also being shown in longitudinal crosssection.

The scintillometer shown in Fig. 1 comprises as its photo-electric device and luminophor respectively a photo-multiplier tube, 10, and a so-called honeycomb phosphor, 11, of the kind disclosed and fully described in the copending application, Serial No. 244,883, filed September 4, 1951, now Patent No. 2,667,259. In the Fig. 1 detector these two elements are coupled together by a light amplifier tube, 12, rather than by direct exposure of the cathode of the photo-electric device to the lumiof the dynode that their energy is given up in the less nophor. The light amplifier tube 12 comprises a vacuum envelope 13 having a cylindrical side wall 14 and input and output endwalls 15 and 16 respectively carrying a photo-emissive coating, 17, and a fluorescent target 18 on their inside surfaces whereby they directly face each other across the discharge space extending lengthwise of the tube. Since according to the present invention emission from the coating 17 should occur only in response to scintillations originating in the luminophor 11, it is essential that a means be provided for preventing any feed-back of light from the fluorescent target 18 to the coating 17. If this were not prevented a regenerative process could be set up whereby the amplifier tube 12 would sustain a continuous discharge. Accordingly the surface of the fluorescent target 18 which faces the coating 17 consists of a metallic film 19 which is permeable to electrons but not to light. This film may consist of a thin evaporated-on layer of aluminum through which high energy electrons can readily pass but which will act as an excellent reflector of the spectral light originating in the target 18. Thus this target comprises a coating 20 of fluorescent material, which is first applied to the inside surface of the end wall 16, and the metallic film 19 which is subsequently applied over the fiourescent coating. Processes for making such targets are well-known in the art of television picture tubes and therefore it is deemed unnecessary to describe any of them in detail herein.

The photo-emissive coating 17 and the fluorescent target 18 are connected to respective terminal pins 21 and 22 which extend to the exterior of the envelope 13 where they, in turn, are connected to the negative and positive poles of a source of potential 23 for establishing and maintaining a strong electron-accelerating electric field between these two end-wall electrodes of the tube. If it is necessary the coating 17 may be applied to the end wall 15 over a previously deposited transparent conductive film to the end that all parts of the photo-emissive coating may be maintained to a uniform potential. Suitable transparent coatings are well-known in the art and include, for example: (1) a monomolecular layer of tungsten and, (2) a coating of the material known as Nesa which is manufactured by the Pittsburgh Plate Glass Company of Pittsburgh, Pennsylvania.

The light amplifier tube 12 is provided with a focus coil 24 which when energized with direct current from a source 25 will set up a magnetic field whose flux lines extend axially through the tube along the discharge space etween its end walls. Actually this element is not essential, since, as distinguished from the requirements of image amplification there is no necessity of preserving an electron image intact while it is being accelerated from a photo-emitter to a fluorescent screen. However it may be advantageous to use a focus coil for either or both of two reasons: (1) that it will play a part :in assuring that each emitted photo-electron will necessarily move from the input end wall 15 to the output end wall 16 where it can serve a useful purpose. Thus if an electron be emitted near the periphery of the emissive coating 17 with an appreciable transverse velocity which might tend to propel it against this wall, the combined effect of the axial focusing flux and the axial accelerating field will be to prevent such an excursion of the electron and to compel it to move axially along the tube to a point of impingement on the fluorescent screen 18; (2) that it is a wayof keeping the 4 or 5 photo-electrons which are emitted from the coating 17 for any single scintillation closely grouped together, as they are transferred to the target 18 for impingement thereon, if tests reveal, as theoretically is a possibility, that an increase in the conversion efiiciency of the target 18 may be gained as a function of the density of the electron bombardment. On the other hand if a focusing coil 24 is employed care must be taken to prevent the fringing magnetic field at the output end of the tube from disturning the electron optics of the hoto-multi lier tube I03 e.- g'., by adequately spacing the end wall- 17 from the phot'o eathode of the multiplier tube to avoid disturbing effects doing so, if need be, with the help of a' light pipe for coupling the elements together; or by using a low reluctance cor'e member, near the output end of the coil 24', to control the return path of its fringing flux for affording it an easy route through such a member.

The source 23 must be a high voltage source so that the accelerating field between the coating 17 and the target 18 can sufiice to cause the amplifier tube to manifest substantial gain. However its current capability may be very small since the current drawn by this tube will be of negligible proportions.

During operation of the Fig. 1' scintillometer each scintillation produced by the luminophor 11 will cause the coating 17 to emit approximately the same number of photo-electrons as would the cathode of the photomultiplier tube 10 if it were similarly directly coupled to the luminophor. However in' the case of the tube 12, in contrast to the tube 10, these few electrons can be made unfailingly eifective, for the purpose of eventually producing an electrical pulse, by the expedient of simply greatly accelerating them. It is because of this that provision is made, by the arrangement shown herein, for accelerating the electrons away from the coating 17 and onto the target 18 to final velocities of the order of, say, 30,000 to 50,000 volts. In this way it is possible for some of the relatively copious energy available at the source 23' to be absorbed by the electrons and conveyed by them to the target 18, whereby a strong pulse of fluorescent light will be generated constituting, in effect, areproduction of the original scintillation greatly multiplied in intensity. Evenif the gain of the tube 12 is no more than 4 or 5 it should have a very pronounced effect from the total efficiency of the detector. However, judging by the performance of comparable tubes which have existed in the prior art, gains very much greater than 4 or 5 may be expected. Thus, whereas in some image amplifier tubes which use only moderate amounts of acceleration, perhaps because of the neces sity of maintaining precise electron optics accurately to preserve an image, gains of only 3 to 5 per stage are attained, in certain other image amplifier tubes, which are used in connection with fluorescopy and in which relatively low definition capabilities are acceptable, attained multiplications of light intensity have reached magnitudes of the order of over 100. Of course in the case of the light amplifier tube 12 there is no necessity at all of the preservation of an electron image as such.

' The embodiment of Fig. 2 includes a modified light amplifier tube 12 which is arranged not only to amplify the light received from the luminophor but further elfectively to converge it, by gathering input light from a relatively large area of a luminophor and producing output light on a relatively small area adjacent the cathode o a ph to l tric device.

Thus the light amplifier tube 12, performs a function similar to that of a light condensing optic used for coupling a large luminophor to a small photo-electric device. While in the past it has been proposed to use passive eondensi g optics, such as refractive lenses and tapered I lit pipes, such proposals have not been put into large s'dale use because of the substantial attenuation which the already very limited amount of light available in a single scintillation will sustain in its transmission through a unit volume of any suitable light-conductive solid. However in the arrangement} shown in Fig. 2- an effect'ivel'y equivalent function is performed by an element whichnot only does not attenuate the light but actually amplifies it. I The luminophor, part of which is shown at 11' of Fig. 2' is of unusually massive proportions, such as can be easily attained in accordance with the teachings of U. S. Patent 2,559,219. According to that patent a gag-sot s luminophor may inexpensively and easily be made unusu ally large by comprising it either of fine fluorescent par-' ticles carried in a massive transparent plastic matrix, or of a fluorescent solute dissolved in a quantity of transparent liquid solvent. Where a great disparity exists between the size of a luminophor and that of the cathode of any normally available photo-electric device, some such expedient is essential as: (1) using a plurality of the photo-electric devices; (2) using a condensing optic between the output surface of the luminophor and the cathone the photo-electric device; or (3) entirely surrounding the luminophor with a good reflector, e. g., an aluminum coating applied over the entire outside surface of the luminophor, except for a window portion adjacent the cathode of the photo-electric device to the end that light which does not radiate directly toward the cathode, from any point where a scintillation occurs within the luminophor, will be reflected back and forth inside it between inwardly facing surfaces of the reflector enough times to afford the possibility of its eventually reaching said window over a many-times folded path. As noted above the use of a large refractive condensing light optic involves attenuation of light therein. Similarly, in the case of a reflector covered luminophor, although it is very desirable to increase it in size to increase its ability to intercept radiation, nevertheless a" point of diminishing returns is soon reached at which the scintillations as received in the photo-electric device will have become too weakened to reliably actuate the photo-electric device because much of that part of the light which must travel multiple-folded paths within the luminophor in order to reach its window will be markedly attenuated therein. Since according to the present invention, light may be gathered from a larger output area (or window) of the luminophor, less internal back and-forth reflection is needed to bring about a substantial escape of light and therefore less attenuation is occasioned. Thus the present arrangement not only avoids the attenuation occasioned in condensing optics themselves, but in addition makes it possible to reduce the attenuation sustained in massive luminophors.

The tube 22' preferably should include a velocity filter Stl, such as one of the filters described in U. S. Patent 2,548,118, for rejecting the undesirable, but fortunately relatively slower, thermal electrons which unavoidably will be emitted from the photo-emissive coating 17 of the tube. It is more important for the tube 12 than for the tube 12 to include such a filter because of the fact that the number of thermal electrons emitted, i. e., the number of electrons constituting its noise or dark current, will increase with the area of its photo-emissive coating, all other things being equal. Since it is entirely possible that the'individual scintillations occurring in the luminophor 11 will irradiate only localized areas of the coating 17 due to its large size, then it follows that the signal-to-noise ratio will tend to deteriorate as the photo-emissive coating is made larger. However, since the thermal electrons can be selectively rejected by the use of a velocity filter this disadvantage of increasing thesize of the coating 17" can be avoided.

The fluorescent target l8 employed in the amplifier tube 12", like the target 18 in Fig. 1, includes a fluorescent coating (Zfi) and a reflector coating (19'), the only difference being that they are deposited upon a differently shaped surface. As will be apparent from the drawing, the inside surfaces of the input and output ends (15, 16) of the envelope (13) of the tube 12are concentric spherical surfaces. This geometry is used to cause the force lines of any electric field established between the coating 17 and the target 18 to converge onto the target 18 so that in the operation of this device they will constitute an electrostatic electron optic for propelling and directing any photo-electrons emitted by the large coating 17 in convergent directions toward the small target 18. While such an optic should be quite adequate for the purposes herein, a magnetic focusingcoil 24, having an associated source of current 25 for energizing it, are shown as optional additional parts of the arrangement.

The source of accelerating high voltage for the tube 12' is shown to include a principal section 31 for polarizing the target 18' at a very positive potential with respect to the coating 17 and another section 32 for polarizing the velocity filter at a slightly (and adjustably) negative potential with respect thereto, in accordance with the teachings of the above-mentioned patent, 2,548,il8.

The light amplifier tube used in the Fig. 3 embodiment (12) is of toroidal configuration and of relatively intermediate size so that it can serve to couple a similarly shaped rnassive-liquid-luminophor (11) of relatively large diameter to a cylindrical photo-sensitive Geiger- Mueller tube (10) of relatively small diameter. Since the structural details of the photo-electric device 16' do not themselves constitute novel features of the present invention, they are not fully disclosed herein, and instead it is merely noted they may be in accordance with any suitable art which has already been developed or which may be developed in the future, e. g., the prior art exemplified by the teachings of copending application Serial No. 138,341, filed January 13, l950, now Patent No. 2,694,152.

The scintillometer (10, 11", 12") of Fig. 3 constitutes part of a bore hole logging electrode which is shown therein and which further comprises a sealed hollow shell 41 containing the scintillometer and, along with it, its power supply, and its output electrical circuitry, this being of various possible suitable kinds depending on the manner in which it is intended to employ the scintillometer. Since the present invention does not purport to provide novel power supplies for a scintillometer nor novel output circuitry therefore none of trese components are being shown or described herein in detail but rather are being generally represented by the single block 44.

It will be understood by those familiar with the art that unless an entirely portable source of power is supplied in the logging electrode 40, for example, storage batteries or the like, then the cable 43 must aitord means for supplying the logging tool with power from a source thereof at the head of the bore hole, tiat is to say, the cable must afford means for carrying electrical energy down to the logging tool electrode 4% at the same time that it affords a transmission medium for carrying information up to the surface from the tool.

One reason why the use of a light amplifier may afford an additional advantage in an apparatus like that shown in Fig. 2 is that but for its use it might not be possible for certain of its elements to be made with optimum values for certain of their dimensions without entailing the sacrifice of using less-than-optimum values for other dimensions. For example consider the following: To start with, the diameter of the bore hole 42 is usually selected primarily to favor other factors than what diameters may be best for the radiation-detecting logging tool, e. g., it may be selected because a particular size of drill rod or of steel tube casing is most conveniently available. Once this dimension is fixed, a certain diameter for the outside of the logging tool and of the toroidal luminophor mounted therewithin will be optimum, i. e., preferable to any other on the basis that larger diameters will interfere with free movement of the electrode along the bore hole and smaller diameters will reduce the interception of the radiation flux, that is for a luminophor of fixed length. Since, as is known, the diameter of a Geiger-Mueller tube and the radial thickness of the toroidal liquid luminophor also have their optimum values, situations may occur in which rigid adherence to the use of all of these values will result in a large slack space between the inner cylindrical periphery of the luminophor and the cathode of the Geiger-Mueller tube. In such a situation a toroidal light amplifier tube 12", in addition to amplifying the intensity of each scintillation, also performs the useful function of coupling the Geiger-Mueller tube to the luminophor across the otherwise determinal slack space.

In the amplifier tube 12" its photo-emissive coating (17") is carried on the inside surface of outer cylindrical Wall 45 and its fluorescent target (18) is carried on the inside surface of inner cylindrical wall 46. While the showing of a velocity filter is omitted from Fig. 3 for the purpose of simplifying the drawing, such a filter should preferably be used in this type of embodiment because of the large area of its coating 17 While means have been shown herein which can be adequately effective for preventing light feed-back from the fluorescent target of a light amplifier tube to its photoemissive cathode, whereby each output pulse of light will correspond in duration to an input scintillation, it may be that in practice it will be advantageous to use an electronic switch, corresponding to the type of electronic quenching circuits used in Geiger-Mueller practice, to assur the avoidance of any continuous discharge. It may be advantageous to do this since it is possible for some light, albeit an extremely small amount thereof, to be generated in residual gas, remaining after the tube has been evacuated, due to ionization thereof by the high energy electrons moving therethrough and to the ensuing recombinations occurring during de-ionization.

Obviously, many modifications and variations of the invention, as hereinbefore set forth, may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

I claim:

1. A scintillometer comprising a massive, fast luminophor for responding to a substantial number of any impinging quanta of certain penetrative radiation to convert them into respective scintillations; a light amplifier tube for receiving light from a scintillation originating in the luminophor to convert the same into a pulse of light of greater intensity than that received; and a photo-electric device for receiving pulses of light from the light amplifier tube and converting them into electrical pulses said light amplifier tube comprising a photo-emissive cathode adapted to receive light from said luminophor and a fluorescent target adapted to receive accelerated photo-electrons from said photo-emissive cathode; and energy-level filtering means located at an intermediate position between said cathode and said target and responsive to polarization at a predetermined constant lower potential than said cathode for preventing electrons within a selected range of initial energies from reaching said target,

2. A scintillometer as in claim 1 in which said photoelectric device is a photo-electron multiplier tube.

3. A scintillometer as in claim 1 in which said photoelectric device is a photo-sensitive Geiger-Mueller tube.

4. A scintillometer comprising a massive, fast luminophor for responding to a substantial number of any impinging quanta of certain penetrative radiation to convert them into respective scintillations; an electron discharge device for converting input light from a scintillation originating in said luminophor into a pulse of output light; a photo-electric device for receiving pulses of output light from said discharge device and converting them into electrical pulses; said discharge device comprising a phcto-emissive cathode adapted to receive light from said luminophor and a fluorescent target adapted to receive accelerated photo-electrons from said photo-emissive cathode; said cathode being substantially larger than said target; means for establishing an electron optic between said relatively large cathode and said relatively small target to cause electrons from the former to converge as they move toward the latter and energy-level filtering means including a filter-grid located at an intermediate position between said cathode and said target and responsive to polarization at a predetermined constant lower potential than said cathode for preventing electrons within a selected range of initial velocities from reaching said target.

5 A scintillometer as in claim 1 in which the light amplifier comprises: an hermetically sealed envelope including concentric end walls one having a substantially larger area than the other and both having substantially the same center of curvature and a side wall of flared out configuration toward the larger of the two end walls, sudh as a frusto-conical or truncated-pyramidal configuration; a photo-emissive cathode carried on the inside surface of one of the end walls and a fluorescent target carried on th inside surface of the other; terminal means connectable to an external source of potential to establish an electric field, between said cathode and said target, the force References Cited in the file of this patent UNITED STATES PATENTS 2,297,478 Kallrnann Sept. 29, 1942 2,523,132 Mason et al Sept. 19, 1950 2,555,423 Sheldon June 5, 1951 2,555,545 Hunter et al. June 5, 1951 2,559,219 Ludeman July 3, 1951 2,612,610 Marshall et al. Sept. 30, 1952

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