CA1243783A - Well logging tool with an accelerator neutron source - Google Patents

Abstract

A Well Logging Tool With An Accelerator Neutron Source Abstract In the illustrative embodiments of the invention disclosed, a neutron porosity well logging tool includes a D-T neutron accelerator, a fast-neutron monitor for monitoring accelerator output, near and far epithermal neutron detectors for deriving a primary porosity measurement, a neutron detector array intermediate to the near and far epithermal detectors for providing enhanced spatial resolution, and a far-spaced thermal neutron detector. The epithermal neutron detectors are shielded and spaced relative to the accelerator to optimize tool response to formation porosity, whereby the tool affords accurate porosity sensitivity over the entire range of porosities of interest. The source monitor may additionally be employed to measure capture gamma ray intensity vs.
time between neutron bursts for deriving formation sigma. Also, the detector array may include both epithermal and thermal neutron detectors to afford an epithermal/thermal comparison measurement. The specially shielded near-epithermal is also independently useful as a monitor of the output of high energy neutron sources.

Description

3~3 Description A Well Logging Tool With An Accelerator Neutron Source Field of the Invention The invention relates to well logging tool for investigating an earth formation surrounding a borehole and determining the characteristics of the earth formation. More particularly, the invention permits a more accurate determination of formation porosity and other characteristics, such as the macroscopic capture cross section, by using an accelerator neutron source. It also relates to improved apparatus for monitoring the output of an accelerator neutron source.

Descr~tion of the Prior Art Knowledge of the porosity of an earth formation surrounding a borehole is important in the petroleum industry to identify possible oil and gas producing regions and to calculate the maximum producible oil index of a formation, as well as other important parameters. Generally, known well logging tools for measuring porosity use a chemical neutron source, e.g., Cf252, AmBe, or PuBe, and two or more neutron detectors spaced at different distances from the sourceO Tools of this type are described in detail in U.SO patent number 3,483,376 issued to S. Locke on December 9, 1969, and U.S. patent number 3,566,117 issued to M. Tixier on February 23, 1971.
Chemical neutron sources, however, are subject to problems of handling, shipment and storage which seriously hinder their use. Indeed, because of the `~:
~, ~

~z4~33 concern about radiation safety, regulations applying to chemical sources are becoming more restrictive and may, in the future, prohibit their use altogether~
~urthermore, chemical sources have limitations with respect to output intensity, typically being on the order of 4 x 107 neutrons per second or less. The use of larger chemical sources, even if possible within the constraints of well tool design, would merely exacer~ate the aforementioned radiation safety probl~ms.
Various types of accelerator neutron sources are also available for possible use in well logging tools, e.g. an accelerator based on the deuterium-tritium (D-T) reaction that produces 14-~eV neutronsr an acce-lerator based on the deuterium-deuterium (D-D) reac-tion that produces 2.5-MeV neutrons, and an accele-rator ba~ed on the tritium-tritium (T-T) reaction that produce~ a spectrum of 1- to 10-MeV neutrons, with the average energy of the neutrons near 5 MeV~ An example of a D-T accelerator neutron source is shown in ~.S.
patent number 3,775,216, issued to A. Frentrop on November 27, 1973, and assigned to the assignee o~
this application.
As an accelerator neutron source may be switched

2~ on and off as desired, the radiation pro~lems arislng from the uce o' a chemical source do not exist wi~h aocelerator sources. A further advantage of accele-rator sources is the increased neutron source strength possible. Thus neutron outputs of 5 x 108 neutrons per second or greater are readily available, which is an order of magnitude greater than of chemic~l sources. Because of the greater neutron output inten-sity, accelerator sources also afford increased sta-tistical accuracy and permit logging operations to be

3~ performed more rapidly. Additionally, the higher so~rce strength increases design flexibility regard-ing shielding and spacing between different detectors and spacing between a particular detector and the ~2~37~3 source, so that the performance of the sonde may be --improved. For example, larger detector spacings lead to smaller borehole effect~.
It has been found, h~wever, that the direct sub-stitution of an accelerator source for a chemicalsource in a known porosity sonde does not provide a viable well logging tool. This results because the response of known poxosity tools is governed mainly by the high energy neutron transport to the vicinity of the detector(s) and by the slowing down through energy degrading collisions to lower energies locally where detection takes place. Source neutron energy affects porosity response through the total cross sections seen by the neutrons in their transport to the detec-15. tor vicinity. Por a D-T source, which produces 14 MeY
neutrons, the formation cross sections seen by the neutrons are quite different than those for the typi-cal chemical source, e.g. 4-MeV average neutrons for AmBe source neutrons. Consequently, the effects of a variation in porosity on the neutron detectors, and therefore the output signals of the detectors, are different ,or the D-T accelerator source than for a chemical source. ~or instance, one substantial dis-advantage of a direct substitution of a D-T accele-rator source for a chemic21 source is ~ lack of poro-sity sensitivity above apDroximately 25% porosity.
Theoretic211y, logging devices using a single detector with a 3,T accelerator would a~ford good sensitivity to change in porosity over the full range of porosities of interest. ~owever, careful control or measurement of the accelerator neutron output is essential in order to derive an accurate determination of porosity where only a single detector is used.
Prior a~tempts at developing single-detector porosity tools have suffered from inaccuracies because the accelera~or neutron output has not been successfully controlled. Purt~er, it has heretofore been di~ficult in practice to make direct measurements of 14 Mev `
~ _4~ 37~3 neutrons which are sufficiently unaf~ected by the contents of the borehole environment, or wi~h suffi-cient psecision and reproducibility, to be useful for porosity-determination purposes.
Accordingly, a need exists for a porosity tool that uses an accelerator neutron source, thereby eli-minating the radiation problems associated with the use of a chemical neutron source, and that, at the same time, affords accurate porosity sensitivity over lO the full range of interest in well logging applica- !u tions. Also, a need exists ~or a porosity logging tool that uses a high-strength accelerator neutron source which provides better statistical accuracy, t~
permits faster logging, increases design flexibility, -15 and makes a better poro~ity determination that is less affected by borehole effects.
A need correspondingly exists for a monitor capable of measurements of the neutron output of a high energy neutron accelerator, but which is substantially insensitive to the borehole environment.
Knowledge of the intensity of a neutron source ~~-used in a well logging tool is important ~or a number of additional reasons as well. With chemical sources, which are lnherently stable, the int~nsity of the 2~ source is known and output calibration may be readily aocomplished prior to a logging operation. Obviously, however, cali~ration of the logging measu;ements relative to source strength is also required. In the case of accelerator-type sources, which are inherently unstable, accurate knowledge of source intensity is even more important. In order to o~tain useful log-ging measurements with an accelerator source, it is essential that source intensity be accurately moni-tored concurrently with the logging measurements, or steps must otherwise be taken, such as using plural detectors and forming ratio measurements, to compen-sate ~or variations in source strength. Ideally the source monitor should be responsive only to source ., ~2~ 33 strength and should be insensitive to other variables, e.g. formation matrix and fluid, borehole fluid, borehole size, tool offset, etc., which influence borehole measurements.
S Various devices for monitoring the output of neutron generators have been suggested. Typically, such devices include means for distinguishing source neutrons, i.e., neutrons emitted directly by the source that have not interacted with any nuclei, from other radiation, e.g., lower-energy, scattered neutrons and naturally occuring and induced gamma rays. An example of such a neutron source intensity monitor is disc}osed in U.S. patent number 4,268,749 to Mills. In that monitor, a fast neutron detector, - 15 ~preferably of the helium-3 type with high helium-3 gas pressure, is used to detect neutrons, and signals from the detector are transmitted to a discrlminator that is biased to cou~t helium-3 recoils from source neutrons and discriminate against nonsource neutrons from the formation.
Another example of a neutron source intensity monitor is disclosed in ~.S. patent number 4,271,361 to Jacobs. In the '361 monitor, an arsenic layer i5 employed to emit gamma rays of ~pproximately 17 milliseconds half life when excited by incident fast neutrons. A gamma ray detector adjacent to the arsenic layer detects the ga~ma rays and zpplies sig-nals representative thereof to an energy-selective counting circuit, which counts the arsenic-originatins gamma ray events as an indication of source intensity.
Still another example Df a neutron source inten-sity monitor is a device having a scintillator, a photomultiplier, and a pulse shape discriminator for distinguishing source neutrons from other radiation.
Several problems exist with the known devices for monitoring neutron source output, especially in a borehole environment. ~irstly, the prior devices tend not to be borehole compatible, i.e., they are affected -6- ~37~3 by scattered-back neutrons, naturally occurring radia-tion, and other borehole effects resulting from the borehole contents, size, formation matrix, and the like. Secondly, the high temperatures and other harsh conditions typically encountered in a borehole often necessitate complex gain compensation circuitry or other measures, such as cryogenic flasks, to sta bilize the monitor. Thirdly, pulse shape discrimina-tors, in addition to having very complex elec~ronics, work well only with liquid scintillators, which makes high-temperature operations difficult, and are compa-ratively slow, which prevents their use in applica-tions where counting rates are high.

-Summarv of t-he Invention The foregoing and other re~uirements of the prior art are met, in accordance with one aspect of the invention, by the provision of a well logging tool that includes an accelerator neutron source, a neutro~
source monitor, a near epithermal neutron detector, a 20 f ar epithermal neutron detector, a special neutron shield for the near epithermal detector, and, optionally, an array of neutron detectors located intermediately of the near and far epithermal detectors and/or a far thermal neutron detector.
The neutron source monitor is located so that it ls responsive primarily to u~moderated neutrons that are emitted from the source, i.e., neutrons that have not interacted with the formation or the borehole.
T~e monitor preferably comprises an organic scintillator having a characteristic dimension that is small relative to the average range of gamm~ ray-induced (Compton scattering) electrons that would produce light pulses of ~agnitudes comparable to those resulting from recoil protons produced by source neutrons, while it is large relative to the average range of such recoil protons. The scintillator is coupled via a photomultiplier to a pulse height ~2~37B3 ~

discriminator circuit that passes pulses in the higher energy portion of the flat portion of the scintillator pulse height spectrum where the spectrum is dominated by source ne-ltrons. Consequently, the neutron source monitor produces a signal that is proportional to the output strength, or intensity, of the neutron source. The source monitor has good inherent gain stability, but iE needed active gain compensation can be provided through, for example, the use of another pulse height differential discriminator to pass a second range of pulses and by comparing a ratio of the count rates from the two pulse ranges to a reference value.
Also, the source monitor may be used to measure lS the decay of the thermal neutron population in the earth formation surrounding the borehole for purposes of deriving formaton sigma. To that end, capture gamma rays detected by the monitor during periods between neutron bursts are detected and sorted in a timing multiscaler.
The near epithermal neutron detector is placed cl~se to the neutron source. The shield for the near epithermal detector preferably raises its low energy neutron de~ection threshold ~o approximately 10 eV to 100 eY, while maintaining sensitivity to higher neutron energies, and suitably comprlses an annulus of neutron moderating-neutron absor~ing materials surrounding the near detector. ~hls arrangement affords increased ratio porosity sensitivity over the entire range (0%-40%j of interest. Prefera~ly, no high density shielding is located between the neutron source and the near epithermal neutron detector, which permits very close spacing of the near detector to the source. Such close source-detector spacing contributes to enhanced ratio porosity sensitivity.
Because of the increased average detection energy of the near epithermal neutron detector, it has a severely decreased sensitivity to the formation poro -8~ 37~3 sity but remains sensitive to borehole environmental effects and tool standoffO Consequently, it is useful for correcting for borehole environmental effects and tool standoff when porosity is determined by either the ratio technique or the crossplot techniquel each of which is discussed in more detail below. In accordance with still a further feature of ~he invention, the novel shielded near-spaced neutron detector structure may be employed independently as a monitor of high energy neutron flux. In this mode of operation, it is particularly useful to monitor the source strength of D~T-type neutron accelerators.
The far epithermal detector is positioned with respect to the neutron source so ~hat the detector remains fully sensitive to the fo mation porosity, and preferably is eccentered in the sonde and rear-shielded to further enhance its sensitivity to the formation and decrease its sensitivity to borehole neutrons. In another embodiment, the far epithermal 2V detector is neither eccentered nor shielded and retains its normal porosity sensitivity.
The porosity of the formation can be determined by taking a râtio of the ou~put signal from the near epithermal detec~or to ~he output signal from the far epithermal detector, i.e., by the ratio technique, or ~y usi~g a plot of the sisn21 from the near epithermal detectsr normalized by the signal from the ~eutron source monitor versus the signal from the far epit~ermal neutron detector normalized by the signal from the neutron source monitor, i~e., by the crossplot technique. With either technique, a porosity determination comparable to or better than that afforded by prior art chemical source tools is obtained, while a~ the same time avoiding the safety and intensity limitations associated with the prior art tools.
The intermediately-spaced neutron detector array preferably includes a pair of axially-spaced neutron - -9~ 7~33 detectors of like energy sensitiYi~y (epithermal or thermal) to provide greater sp~tial resolution in the porosity measurement. This is particularly useful for thin bed de~inition. To enhance sensitivity to the formation, the detector pair is preEerably eccentered in the sonde and back-shielded to rleduce borehole effects. A third neutron detector in the array may have a substantially different neutron energy sensitivity, i.e., thermal if the axially-spaced pair is epithermal sensitive and epithermal if the pair is thermal sensitive, and is preferably locate~ at the same source-detector spacing as the more closely spaced of the detector pair of the array. The third detector is pre~erably transversely offset from the detector pair and backshieided to reduce borehole effects. The epithermal/thermal c~unt rate ratio of the two like-spaced detectors can ~e used to provide a local, derived formation si~ma and for comparison of the epithermal and thermal neutron fluxes.
The optional far thermal detector is located farther from the neutron source than the far eplthermal neutron detector, but close to it 'o enhance the s~a.istical accuracy of ~he me2su~emenLs.
- ~he therm21 neu.ron detector is preSe-2bly eccen sred ln the sonde and shielded on .he side acins the borehole so that borehole environmen.21 ef ec~s a~e reduce~. The provision o~ a therrual neu_ron de_ec_o~
in the tool permits determination of forr.ation characteristics dependent upon thermzl absorption, such as the formation capture cross sec_ion ~, but also has good porosity sensitivity and statistic21 precision.
Preferably, the accelerator neutron source is a D-T souroe, although other types of accelera.ors ~ay be used if desired. Suita~le neutron de.ectors are helium-three (~e-3) proportional counters, with the -10~ 3 ~3 two detectors closest to the neutron source, i.e., the near epithermal neutron detector and the far epither-mal neutron detector, being covered by a thin cadmium layer in order to render the detector~ insensitive to neutrons with energies below approximately 0.5 eV.

rief Description of the Drawinqs The objects and advantages of the present inven-tion may be better understood by reference to the : following detailed description of exemplary embodi-ments thereof, taken in con junction with the accom-panying drawings r in which:.
FI~URE 1 is a cross-sectional view of one embodi-ment of a well logging tool in accordance with the invention; . .
FIGURÆ 2 is a cross-sectional view of the well logging tool em~odiment shown in Figure 1 taken along line 2-2 and illustrates the thermal neutron detector and the shielding therefor;
FIGURE 3 is a cross-sectional view of the well logging tool embodiment shown in ~igure 1 taken along line 3-3 and illustrates the far epithermal neutron detector and the shielding therefor;
~ IGURE 4 is a cross-secticnal view of the embodi-ment shown in ~igure 1 taken along the line 4-4 and illustrates the ~e-3 detector array and the shielding therefor;
~IG~RE S is a schematic diagram of the detector signal processing circuitry of the well tool embodi-ment of ~igure l;
~IG~RE 6 is a plot of the near epithermal-far epithermal detector count rate ratio versus porosity;
~IGURE 7 is a crossplot of the far epithermal detector count rate versus the near epithermal detector count rate for various ~ormation matrices;
FIGURE 8 is a schematic cross-sectional view of another embodiment of a well iogging tool for deter-mining the porosity of a subterranean formation in 2~7~33 t accordance with a further aspect of the present inven-tion, showing one embodiment of a novel shielded detector system and source monitor of the invention;
FIGURE 9 is a pictorial view o~ another embodi-ment of the detector shielding structure of thepresent invention;
FIGURE 10 illustrates another embodiment of the detector shielding structure of the present invention;
FIGURE 11 is still another embodiment of the detector shielding structure of the present invention;
FIGURE 12 is a graphical representation of the comparative effectiveness of the present invention as a porosity indicator relative to the two-detector, unshielded neutron porosity technique;
1~. FIGURE 13 is a schematic diagram of a neutron source intensity monitor in accordance with the invention being used with a system for investigating an earth formation surrounding a mud-filled borehole;
~ IGURE 14 is a graph showins curves of count rates for neutron-induced events and ~or gamma ray-induced events versus energy for a neutron source intensity monitor in accordance with the invention and ~IGU~E 15 is a linear plot of the pulse height spectrum (counts/channel v. channel) and shows a linear approximation to the spectrum shape in the region of the discriminator window for monitoring source strength.

Illustrative embodiments of an apparatus based on the principles of the invention are shown in the figures, in which like reference numerals designate like components. In ~igure 1, a well loggins tool, or sonde, 10 for investiga~ing the porosity, thermal neutron capture cross section and other parameters of an earth formation surrounding a borehole is shown.
Tne sonde 10 includes an accelerator neutron source -12~ 3~3 s;
12, a neutron source monitor 14, a near epithermal neutron detector 16, a thermal/epithermal detector array 17, a far epithermal neutron detector 1~, and a thermal neutron detector 20. Shields 22, 24, and 26 S are provided to shield the detectors 16, 18, and 20, respectively. A shield ~8 is located ~etween the near epithermal neutron detector 16 and the far epithermal neutron ~etector lB and also serves to shield the thermal/epithermal detector array 17.
The tool 10 is intended to be a sidewall tool, and a bow spring t indicated sche~atically at 11, or other conventional device may be provided on the tool to urge it against the borehole wall. Although the tool i~ primarily intended for open hole logging, it ma~ be used in cased holes andr if desired, may be sized for through-tubing use.
The neutron accelerator 12 may be of any suitable type, but preferably is a D-T type (14 MeV) source having an output on the order of ~ x 10 n/sec or greater for enhanced detector statistics and logging spe~d. ~lthough not shown in Figure 1, it will be understood that the accelerator package includes the necessary high-voltage power supply znd firing cir-cuits incident to accelerator operation, and these circuits may also be conventional. For purposes of the present invention, the accelerator may be operated i~ either the continuous (d.c.) mode or the pulsed mode. If the la~ter mode is used, the accelerator package would of course also include the necessary pul~ing circuits, ~s is knownO It will also be understood that suitable power supplies (not shown) are likewise providPd in the tool 10 to drive the detectors 16, 17, 18 and 20 and other downhole electronics.
In a preferred embodiment, the neutron source monitor 14 includes a scintillator 30, prefer bly plastic~ although other types may be used, which detects the fast neutrons emitted by the source, and -13- ~z~37~3 .

a photomultiplier 32, which amplifies the signals ~' produced by the scintillator 30. As descri~ed more ~~
fully hereinafter in connection with Figures 13~
the monitDr 14 may typically comprise a 0.~ inch (1 3 cm) diameter by 0.5 inch (1.3 cm) long plastic scintillator (NE102A, BC-438, etc ) and should be located so that its response during and immediately following a neutron burst is dominated by unmoderated, high-energy neutrons coming directly from the source and the effects of scattered neutrons and gamma rays during such time period are minimized Preferably, the monitor 14 is located closely adjacent to, or on, the radius of the accelerator neutron source 12, with ~-the'scintillator 3D at the position of the ~arget 34 o~ the accelerator 12 ~o~ever, because o~' design constraints (space limitations) r the monitor may have to be located coaxially with the saurce; for instance, in some cased-hole tools. In the absence o~ heavy shielding between the source 12 and the monitor 14, 2D the scintillator 30 may be spaced 2S ~ar 25 30 cm from the source 12, as is described more fully hereinbelow With the neutron source monitor ~4 a~pro?riately sized and loc2ted, it is res~onsive during and i~me-aiately following the bu-st ~ri~2rily to ur~ode-a'_ec neu.rons that are inciaent on it directly -rom the neu~ron sou~ce, thereDy reliably aetec.ing ch2nges in ,he output in.ensity of .he source 12, 2nd is rel2-tively unaffected during such ~eriod by ch~nges in borehole or formation characteristics Consecuently, the output signal of the neutron source monitor 14 is useful, as described hereinafter, in norm21izing the 'out~ut signals of the epitherm~l and thermal neutron detectors for sou~ce strength fluctuation ~he moni-tor 14 has good inherent gain stability, bu', as illustrated schematically herein in ~igures 5 an~ 13, where still greater gain stability is need, it can be used in a circuit for feedback control of the ' --14~ 37~ ~

photomultiplier high-voltage power supply for the monitor.
According to another feature of the invention, the monitor 14 may also be used to detect capture gamma rays as a function of time after the burst for the purpose of determining the formation sigma. The data detection and processing steps for obtaining for-mation sigma are described in more detail in con-nection with Figure 5.
The near epithermal neutron detector 16 and the far epithermal neutron detector 18 shown in Figure 1 are preferably helium-three (He-3) proportional counters covered by a thin, e.g., 0.020 inch (0.05 cm), cadmium shield ~o make the detectors insensitive to neutrcns having energies bel~w the epithermal range, i.e., below about 0.~ eV. Other types of neutron detectors, such as boron triflouride (BF3) detectors, may of course be used. Detectors 1~ and lB
provide the primary porosity measurement.
~he sensitive, or active, volume of the near detector 16 should be located in close proximity to the source 12, preferably, although not necess-:ily, without intervening high density shielding. Por example, for a 1 inch (2.5 cm) diame~er by 3 inch 2~ ~7.6 cm) long cylindriczl detector, a typical detector s ze r a suitaDle spacing would be o~ the order of 8 to 10 inches (20 cm to 25 cm) be~ween the center of the sensitive volume to the target of a D-T accelerator.
Such close source-detector sp2cing enhances the ratio porosity sensitivity as compared to a longer-spaced detector with intervening shielding. ~s will be understood, the optimum source-detector spacing will vary with a number of factors, e.g. detector size, pressure, so-lrce intensity, a~celerator type, and the 3S like.
The shield 22 for the near detector 16 is preferably annular in shape and encircles the sen-sitive volume of the near detector and, as described -15- ~z~37~3 more fully hereinbelow, is designed to raise the low energy neutron detection threshold of the near detec-tor to a level at which the detector efficiency is nomlnally maximum for higher energy ranges. To that end, the detector threshold is raised to at least approximately lO eV and preferably to on the order of - lO0 eV. With the shield 22 designed thusly and a near detector located in close proximity to the SO'I~Ce without intervening hish density shielding, the near detector is relatively insensitive to changes in the porosity of the formation since ~eutrons that have interacted with the formation will generally have energies below the low energy neutron detection threshol~ of the detector. ~owever, the near detector will rema n sensitive to borehole environmental and tool standoff effects since neutrons that have inter-acted only with the contents of the borehole will generally have energies above lO eV. Consequently, the output signal from the near detector may be used to compensate other detector sign ls for borehole environmental and tool standoff effects. Appar2tus and techniques useful for that purpose are described in the commonly-owned U.S. Patents No. 4,423,323 issued December 27, 1983 t~ Ellis et al. and No.

4,~24,274 issued June 18, 1985 to Scott, the pertinent portions of which are here~y incorporated herein.
Fùrther details regarding the configuration and spacing and function of the near detector 16 and the constru~tion of the shield 22 are described herein-below in connection with Figures 8-12.
~ he spacing of the far detector 18 from the source 12 is preferably selected to maximi2e statis-tical precision, i.e., counting sensitivityr while minimizing the effects of such environmental factors as tool standoff and the like. Generally, the stand-off effect is reduced at greater spacings, but only at the cost of reduced counting rates. For a 1.5 inch (3.8 cm) diameter by 6 inch (1502 cm) long cylindrical -16~ 37~3 detector, another typical detector size, a suitable spacing from the accelerator tar~et 34 to the center of the sensitive volume of the far detector lB, shielded as described hereinafter, would be on the order of 23-27 inches (58 cm-68 cm). As with the near detector, the optimum spaeing will vary depending upon the size of the active volume, the pressure of the detector, etc.
For greater formation sensitivity, the far epi-thermal detector 18 i6 preferably eccentered t~ oneside of the sonde 10 and oriented towards the forma-tion by shielding. As shown in Figure 3, the shield 24 for the ~ar epithermal detector is cylindrical in shape wit~ a slot 36 therein, with the far detector 18 being located in the slot. The shield 24 thereby shields the sensitive volume o~ the far detector from neutrons incident on it from the side of the sonde towards the borehole, i.e., the side of the sonde away from th~ formation. As will be understood, the effect of such eccentering and rear-shielding of the far detector is to decrease the sensitivity of the far detector to borehole environmental effects and thereby increase its sensitivity to changes in the porosity OL
the form~tion.
~he comparatively long spacing between the source 12 and the far detector 18 makes the far detector 18 relatively insensitive to source neutrons. ~owever, an additional shield 28 may be provided between the near detector 16 and the far detector 18 to further reduce the sensitivity o~ the far detector to source neutrons.
According to another feature of the invention, the shield 28 also surrounds and shields the thermal/epithermal detector array 17 located between the near and far epithermal detectors 16 and 1~. The array 17 includes a plurality, e.g. three, relatively small ~e-3 detectors 17ar 17b and 17c. The detectors 17a and 17b are preferably aligned axially of the tool 1;2~37B3 10 in end-to-end relation. Where space limitations do not permit such axial alignment, th~ detectors 17a and 17~ m~y ~e cicumferentially offset, but preferably so as to be symmetrically positioned on either side of the line of contact of the toDl 10 with the borehole (or casing) wall. In either case, the detectors 17a and 17b provide measurements of the neutron flux at the two detector locations The difference ~etween - the countin~ rates at the two detec~ors (or other 1~ suitable count rate comparison) affords enhanced spatial resolution o~ por~sity and other formation parameters. This is particularly useful for thin bed definition.
The detectors 17a ar~ 17b may be sensitive to lS either epithermal neutrons or thermal neutrons, to provide either an epithermal porosity measurement or a thermal porosity measurement. As shown in Pigure 4, they are preferably located in an axial slot 31 in the shield 28 so as to enhance sensitivity to the formation and tD reduce sensitivity to the borehole.
The third detector 17c is preferably located at the same sp2cing from the source 12 as the detector 17a. I. ~lso ls a ~e-3 neu~ron aetec.o-, DU. Wi _h a di'ferent eneray sensitivi~y than 'he detecto_s 17a and 17b. ~hus if the Qetectors 17a and l?b z_e epithe-~zl aetectors, the Qe.ec_o~ 17c should ~e a therm21 detector, and vice versa. ~he epitherma~/
thermal flux ratio from the two like-spaced detecto-s 17a and 17c can ~e used to provide a loc21, de_ived formation sisma measurement and for compa-lson ~etween epithermal and thermal porosity ~easurements. The epithermal/thermal count rate ratio can be expected to incre2se as formation sigma increases and to decre2se as sigma decreases.
3~ As illustrated in Pigure 4, the detector 17c is advantageously circumferentially and `ransversely o'fset from detector 17a, and may conveniently ~e 2~37~33 ~I t located within the same slot 31 as the d~tectors 17a and 17b.
The arrangement and configuration of the ~e-3 detector array 17 are shown in ~igures 1 and 4 in an idealized, i.e., illustrative, manner, and other arrangements and configurations may of course be employed. The principal purposes of the array are to provide enhanced spatial resolution in the porosity measurement, a local sigma measurement, and an epithermal/thermal flux comparison. The particular detector sizes, pressures, and source spacings employed may be selected as desired to optimize detection performance.
The sensitive volume of the optional thermal neutron detector 20 is lo^ated farther from the source than that of the far epithermal detector lB, but preferably is located 25 close 25 practical to the far detector to maximize counting rates. As with the detectsrs 16, 17 and lB, the location of the thermal detector can be changed depending upon such factors as the size of the active volume and the pressure of the detector. The sensitive volume may ~e of 2ny ropriate size commer.sura'e wi.h achieving des~red count -a.e statis.ics, e.g., 2.25 inch (~.7 cm) 2~ diame'er by 4 inch (10.2 cm) long. ?refera~ly, the .he-~l neutron detec~o- is also eccentered in the sonde towards the forma.i3n siae oS the son~e.
~ s depicted in ~igure 2, a shield 26 shields the sensitive volume of the ~hermal neutron detector 20 3D from neutrons incident on it from the side of the sonde towards the borehole, i~ the same manner that the shields 24 and 2~ shleld the far e~ithermzl detector 18 and the detector array i7, respectively, zs indicated above. As a result of being eccentered and ~ecause of the shielding 26, the thermal neu.ron detec.or is comparatively less sensi~ive to borehole - environmental e~fects and, thereby, comparatively more sensitive to changes in formation characteristics, -19~ L37~3 `-e.g., the formation macrvscopic capture cross section ~. The shield 26 is prefera~ly a thin, e.g., 0.02D
inch (0.05 cm), cadmium shield that has an arcuate shape with an approximately 180 arc, but other S neutron absorbing materials and other shapes may be used if desired.
The shields 22, 24, and 28 may be made from any suitable material, or combination of materials, that has both neutron moderating and neutron absorbing properties. A neutron moderator is necessary in order to slow down faster neutrons, i.e., those which have undergone few, if any, formation interactions, and a -~
neutron absorber is necessary in order to absorb thermal neutrons, i.e., those that have been thermalized by the moderating material or by the borehole contents or formation.
The neutron ~bsorber is preferably a l/v type such as boron or the like, where v is neutron velocity. A suita~le shielding material is boron 2~ carbide (34C) distributed in an epoxy binder or some other hydrogenous binding medium, e.g. 6~% B4C by weisht ln epoxy. TAe shlelds 22, 24, and 28 should each be thick enough to accom?lish i.s intended - pu-pose with ? he shielding r.ateri21 selectea. ?o.
2~ excmple, i the shield 22 i~ r.ade rom .he aforementioned 6~% B C-epoxy materialt a 'hickness of roughly 1 lnch (2.~ cm) has been ound satis~actory ~n order to raise the low energy neutron detection threshold of the near detector from 0.~ eV to approximately 1~ eV and higher.
The function of the shielding materi~l i5 discussed more fully hereinbelow with refer~nce to the embodiment o~ Figures 8-12. The same shielding material is pre~erably, though not necessarily, used 35 f or all three shields 22, 24 and 28.
~ ith the neutron source monitor 14 and the neutron detectors 16, 17, 18, and 2~ appropriately spaced and shielded, improved determinations of 20- ~2437B3 .

formation porosity can be made. Since, as explained previously, the near detector 16 is relatively less sensitive to changes in porosity but relatively more sensitive to borehole environmenta} and tool standoff effects and the far detector 18 is relatively more sensitive to changes in porosity, an improved porosity determination can be made using either the ratio technique or, following normalization of the individual detector count rates by use of ~he neutron monitor intensity measurement, by a cross plot of the near detector 16 and far detector 18 count rates. As mentioned, the outputs of the array detectors 17a, 17b and 17c may be employed to obtain porosity measurements having enhanced vertical resolution, and also to provide a comparison between epithermal and thermal porosity measurements.
~ urthermore, since the porosity response of the thermal neutron detector 20 is relatively less sensitive to borehole size and tool standoff effects than the porosity response of the epithermal detectors 16 and 18, its output may be used to derive an additional porosi'y measurement. Such a thermal neutron porosity measurement may be of particular value where the thermal detector response is free Oc signi~icant influence by neutron absorbers in the formation and boreh~le environment. Also, the thermal detector output allows the macroscopic capture cross section ~ and other formatlon capture characteristics to be determined in the same tool with the epithermal porosity measurement. Where, as described hereinafter, the neutron monitor 14 is used to detect capture gamma rays following the neutron bursts, the far-spaced thermal detector 20 may be omit_ed and the capture cross section and other thermal neutron-related parameters of the formation may be obtainedfrom the monitor ga~ma ray data.
The outpu~ signals from the He-3 eplthermal detectors 16 and 18 may be amplified and counted ln -21- ~437~3 .
any suitable way to derive a count rate (N) for the near detector 16 and a count rate (F) for the ~ar detector 18. For example, as shown in Figure 5, the detector signals from the detectors 16 and 18 may be S fed to charge sensitive preamplifiers 35 and 37 and thence to pulse amplifiers 38 and 40, with the spectrum of amplified pulses from each detector then being sent to a leading edge discriminator 42 and 44 whose output drives a scaler 46 and 48. The scaler outputs are applied to signal processing circuitry 50, which may comprise a suitably programmed ~igital computer, mieroprocessor or other data processing device, for generation of the ratio N/F of the near detector ~caler counts (N) to the far detector scaler counts (~) as an indicator of porQsity. This indicator has been found to be subject to relatively small environmental (mudcake, borehole size, etc.) effects and tool standoff effects, and thus to provide an accurate, reliable porosity measurement. ~he ~/~
ratio signal may be applied in conventional fashion to a recorder/plotter 52 for recording as a function of tool depth.
~ igure 6 shows the near/far (N~) ratio response 2S measured in llmestone formations cf porosities of 0, 13.2, ,29.3 and 40.6 % with an 8-inch, uncased borehole~ As will be appreciated, the porosity sensitivity of the N/~ ratio is good over the entire 0 to 41~ range.
If desired, a porosity determination may also be made by means cf a crossplot of the near and far detector count rates. As will be understood, the cross~lot technique allows for correction for environmental effects.
~igure 7 is a near-far (N-~) crossplot for 0 to 41% porosities in sandstone, limestone and dolomite formations, and illustrates the effect of matrix change on the crossplot. The data plotted are for an 8-inch, uncased borehole.

~ ~37~33 ~ .
When the crossplot technique is used to determine porosity, the N and F count rates are first normalized by the output of the neutron flux monitor 14. For that purpose, the detector scaler 46 and 48 output signals are each divided into the count rate (intensity~ output signal ~Window A) from the monitor 14, as indicated in Figure 5 by the outputs A/N and A/F from the signal processing circuitry 50. The manner in which the intensity output signal A is generated by the monitor 14 and its associated circuitry (54-60 in Figure ~) is des ribed hereinbelow in connection with Figures 13-15.
The normalized far detector count rate F is also hichly porosity sensitive and may be used if desired to derive porosity information. It is, however, somewhat more susceptible to environmental effects than the N/~ ratio, but this susceptibility can be decreased by use of higher strength accelerators together with greater source-to-detector spacings.
The far ther~al detector output signals may likewise be similarly amplified, discriminated and scaled (components 62-68 in ~igure 5) to provide a scaler output count rate T for ~he thermal detector 20. ~his signal t too, is preferably norr.alized by ~he monitor output signal A, and may be used to derive a measurement of the formation macroscopic ca?!ure cross section ~ . It may also be Itsed, either alone or in conjunction with one or more of the normalized epithermal detector count rate signals A/N and A/F, to 3~ derive porosity information or to evaluate the influence of neutron absorbers on the porosity mezsurement. Any or all of these functions may be readily implemented in the signal processing circuits 50 in any suitable manner.
The output from each of the detectors in the array 17 is alsu su.itably amplified, discriminated and scaled (components 70-76 in Figure 5), and the count rates therefrom Ar 1, Ar 2 and Ar 3, respectively, are -23~ 371~3 r.-transmitted to the surface signal processing circuits 50. For ease of illustration, the circuitry f~r the detectors 17a, 17b and 17c is shown collectively in Fig. 5, bu~ it will be understood that each detector will have the appropriate individual components. At the surface, the count rates Ar 1, Ar 2 and Ar 3 are recorded as a function of tool depth. The difference in count rates between detectors 17a and 17b (Ar 1-Ar 2) may also be formed and recorded vs. depth to provide the aforementioned enhanced spatial information for greater vertical resolution of the porosity measurement. These count rates may first be normalized by reference to the output signal A from tbe monitor 14 to minimize the effect of source strength fluctuation on the individual count rates Ar 1 and Ar 2 . The epithermal/thermal ratio, e.g.
Ar l/Ar 3, may also be developed and recorded vs.
depth as a further porosity indicator. In addition, a local formation sigma may be derived and recorde~, ,as indicated in ~igure 5.
Where, in accordance with 'he invention, the neutron monitor 14 is to be used to detecl capture gamm2 rays following the neutron bursts, a second channel 7~ ~see Pig. 5) is provided in the monitor output circuitry to couple the detec,or output pulses via a leading edge discriminator 80 to a ,iming multiscaler 82. The threshold level of the discriminator 80 is set at a relatively low level, e.g., în the range of 50 KeV (eauivalent gamma ray energy), to enhance count rate statistics. The multiscaler 82 sorts the pulses into time bins and transmits the count per bin data Gi, where i is the bin number, to the surface in digital format.
Preferably a sufficient number of time bins, e.g. 256, is employed to record the entire count vs. time curve both during and after-each neutron burst, but at least during substantially the full period between bursts.
The count data ~i are processed at the surface to `
-24- ~Z~3~83 C ~
~ ;
derive formation sigma in any suitable manner. For example, ~ormation sigma may be determined in accordance with the technique disclosed in U. S. '' Patent No. 4,292,51a issued September 29, 1931 to C. W. Johnstone. Because of the close proximity of the monitor 14 to the source 12, a dlffusion correction is preferably made to the sigma value determined therefrom. The capture gamma ray data Gi are preferably both deadtime and background corrected. --Background measurements are preferably made during periodically repeated intervals during which the neutron bursts are suppressed, in accordance with the techniques disclosed in U.S. ~atent No. Re. 2~,477 i sued July 8, 1975 to W. B. Nelligan. ;
~n the embodiment of Figure 8, the sonde lO is shown as sus,pended in a borehole 84 by a cable 86 for investigating the porosity of a subterranean formation 88. The borehole is illustrated as an open hole and as containing a well fluid 89. In the case of a ," 20 comple~ed, producing well, the sonde 10 may be sized for through ~ubing use, as aforementioned. The conventional hoist and depth-recording devices (not shown) would also be employed, as will be evident to those skilled in the art. The sonde will also be ~5 understood to include a bow spring or other conventional device (see 11 in ~igure 1) for urging the sonde against the borehole wall as shown.
As in the embodiment of Fi~ure 1, the sonde 10 includes a neutron accelerator 12, a near-spaced 30 neutron detector 16 and a far spaced neutron detector ,~
18. The accelerator and detectors may be of the types described in connection with ~igure 1 The embodiment of Figure 8 is otherwise generally similar to that of Figure 1, except that the detector array 17 and the thermal detector 20 have been omitted.
Electrical power for the sonde lD is supplied over the cable 86 from a power supply (not shown) at the surface. It will be understood that suitable -25- ~2~37~3 power supplies (not shown3 are likewise provided in --the sonde 10 to drive the detectors 16 and 18 and other d~wnhole electronics.
The detectors 16 and 18 may be conventional and preferably are of the He-3 gas-filled type. A
suitable gas pressure for purposes of the invention is ten atmospheres for the near detector 16 and fifteen atmospheres for the far detector 18, but these may be varied as desired to optimize the energy response of the detector. According to one feature of the invention, described more fully hereinafter, the near-detector 16 is surrounded by a novel shielding structure to modify the energy response of the near de'ector system as a whole (detector 16 plus shield st ucture) so as to render it predominantly sensitive to neutrons having energies greater than 10-100 eV and above incident thereon directly from the accelerator 12 or its immediate vicinity.
Signals generated by the detectors 15 and 18, and representative of the number of neutrons detected, are applied in the conventional way over the conductors 90 to preamplifier and discrimlnator circuits 92, which, illustrativelyr may include multi-channel amplifiers, scaling circuits and pulse height discriminator circuits ac shown in Figure 5, preparatory to being applied to cable driver circuits 94 for transmission to the surface over the cable 86.
At the ~urface, the signals are received by the signal-conditioning circuits 96, where they are shaped 3D or otherwise converted or restored for further processing as required and are applied to a multi-channel counter or rate meter, including a f irst channel 98 for the far detector 1~ counts and a second channel 100 for the near detector 16 counts. The counts from the two detector are accumulated for a desired time interval, e.s. one or two seconds or more, which may conveniently be rela~ed to the logging speed so as to provide an output for each incremental - -26- ~4~7~3 interval of depth along the borehole. The counter channels are then read out to a porosity circuit 102 where a suitable ratio function of the two total counts or count rates is formed as an indicator of formation porosity. For the reasons given hereinafter, this porosity determination is sensitive to formation porosity over the entire porosity range from 0 to 100 p.u. tporosity units) of interest in oil explorations, of which the range is typically from 0 to about 40 p.u. or higher.
As shown in Figure 8, the near detector 16 is located close to the accelerator 12. For example, with a detector having an active (sensitive) volume of approximately 1 inch x 3 inch (2.5 cm x 7.6 cm), a centerline spacing from the target of the ac~elerator in the range of 6-10 inches ~15-26 cm) has been found suitable. The far detector, which is larger for better statistics, e.g. 1 inch x 6 inch (2.~ cm x 15 cm), is suitably spaced with its center within the range of 20-26 inches (50 cm-66 cm) from the accelerator target~ Both detectors may be clad in caomium to raise the detec'ion sys~em detection thresholds to epithermal or higher energy levels, or only one or neither may be so clad. The cadmium cladding 104 is illustrated in ~igure 8 on the near detector only and is shown exaggerated in thickness for clarity. An approximate thic~ness is on the order of 0;05 cm~ Where sensitivity to thermal neutrons is desired, either or both detect~rs may be unclad.
In accordance with the invention, the near detec-tor 16 is additionally surrounded by an annular shield 106 co~posed of boron carbide (B~C) in an epoxy binder or some other hydrogenous binding medium. The shield 106 is annular in cross section and is formed with a central boxe 108 for receipt of the cadmium-clad detector 16. The shield 106 thus performs a dual function, acting both as a neutron moderator, by virtue of the hydrogenous binder, and as a neutron -27~ 37~3 absorber, by virtue of the boron carbide. To that end, suitable proportions for the shield may be approximately 47% by volume of B4C and 53% by volume of binder.
S The result of this ~ombined shielding of the near detector is markedly to reduce the sensitivity uf the near detector system-to formation porosity. That is to say, the number of neutrons detected by the near detector changes much less with porosity than does the number detected by the far detector, and the effect of this alteration of the near detector porosity sensi-tivity is to enhance the porosity resolution of the sonde. The far detector, of course, remains fully sensitive to porosity, as before.
l~ The reasons for the change of near detector system porosity sensitivity are thought to be two-fold. First, the combined B4C-epoxy shield structure moderates the energies of those neutrons directly incident on the detector 16 from the source 12 to levels more readily detected by the ~e-3 detector 16, and second, it shifts the rela~ive sensitivity of the near detector system towards higher energy neutrons and away from lower energy neutrons. What this means in practical terms is that the signal produced by detector 16 has a relatively smaller component indicative of the characteristics of the formation and a relatively larger component indicative of the ini-tial neutron flux intensity from the source. This may be bett~r understood by reference to the following description of a theoretical, somewhat simplified ~odel of the principal neutron interactions which occur in the region of the sonde lO, 25 indicated diagrammati~ally by neutron paths a - e in ~igure 8.
The accelerator may be regarded for present pur-3~ poses as aa essentially isotropic neutron source, with neutrons emanating in all directions. ~he neutrons following paths a and b may be considered representa-tive of those which travel through the sonde 10, the ~Z~37~3 -2~-~.
well fluid ~9, and enter the fsrmation 88, undergoing scattering reactions along their path lengths, and which are then scattered back to the ~onde in the region of the near detector system. Such neutrons are hereinafter referred to as 'Ifar field" (formation~
neutrons. Due to the energy loss resulting from the - scattering reactions, the far ield neutrons are statistically less likely to successfully traverse the B4C-epoxy shield 106 and are more likely to be absorbed therein. Neutrons scattered back from the borehole contents are more likely to have higher energy than those illustrated by paths a and b and therefore might traverse the outer B4C-epoxy shield 106. These neutrons are likely to be 50 moderated in energy in the process as to have a high pr~bability of being absorbed in the cadmium cladding 104. This is illustrated by the path c in ~ig. 1. These neu-trons are referred to hereinafter as ~Inear field"
neutrons, It will be appreciated that neutrons - -20 impinging directly on the outer shield 106, such as those represented by path d, as well 2S other neutrons having undergone relatively less energy loss through either few major scatterings or many sli~ht scatter-ings, are statistically most likely to p2SS through both shields 106 and 104 and reach the sensitive volume of the detector. At least some, however, will be sufficiently moderated in energy by the B4C-epoxy --shield to be within the sensitive range of the ~e-3 detector 16. These neutrons are referred to herein-after as "source" neutrons.
~ ence the combined effect of the shields 106 and 104 is to render the near detector 16 largely respon-sive to neutrons which carry little information from the formation and borehole environment. Since the scattering and absorption processes involved are sta-tistical in nature, there will of course be some overlap between the categories of neutrons in this model that are detected, i.e., most, but not all, far ~;~437~33 field neutrons reaching the near detector will be stopped by the shielding and not counted. So the near detector will retain a slight porosity sensitivity. A
larger, but still not predominant, proporti~n of the total neutrons counted will be near field neutrons, and the remaining predominant category of the neutrons counted will be source neutrons.
On the other hand, neutrons reaching the forma-tion 88 along the path e and scattered back to the sonde in the region of the far detector 18 will be unimpeded by any moderating shield such as 106 and will thus reach the sensitive volume of the detector and be counted. The far detector, therefore, remains fully sensitive to formation porosity.
As mentioned, the cadmium cladding raises the detection threshold of the detectors to epithermal energy or higher, e.g. to 0.5 ev and above. The thickness of the ~4C-epoxy shield 106 can be selected to provide the desired threshold level for the detec-tor. Generally, the level should be selected at or about the point where the detector is at the maximum nominal efficiency for detectiQn of the higher energy source neutrons, i.e., at about the enersy level at which the source neutrons will begin to be filtered out! e.g~, at least approximately 10 eV and preerably on the order of 100 eV.
Althoush uniform mixing of the moderating mate-rial (epoxy resin) and the absorbing material (B4Cl is depicted in the embodiment of Figure 8, it may be advantageous in certain circumstances to provide separate annuli of different materials. Thus in the embodiment of ~igure 9, the outer shield 106 is shown as composed of two annuli, the outer one 110 of a hydrogenous material such as polyethylene and the inner one 112 of B4C. The shielding materials may also be arranged in other ways. ~or instance, in the embodiment of ~igure 10, the absorber material, e~s.
C, is shown arranged in circumferentially-spaced ~ _30~ 3~3 sectors 114A, 114B, 114C and 114D interleaved by alternate sect~rs 116Ai 116B, 116C and 116D of mod-erator material~ e.g., epoxy resin. This particular construction is useful in reducing the sensitivity of

5 the detector shield system to the spatial distribution of neutron energies. The energy response of the detector system could also be altered by removing the cadmium claddins or ~y using another material having a dlfferent energy absorption cross section for the cladding, such as indium, gadolinium or silver.
In a like vein, the embodiment of ~igure 11, wherein the moderating material or materials are arranged in layers 118A, 118B, etc., between alternate axially-spaced layers 120A, 120B, etc. of absorbing mate:ial or materials, is useful for applications where it is desired to take advantage of or compensate ~or the axial distribution of neutrons along the length of the detector. ~he same moderating materi21 and absorbing materi21 may be used throughout, or, 25 2D indicated in ~igure 11, different types of m~derating materizls M~ , etc., and different ~ypes of absorb-ing rate_i21s P.l, A2, etc., may be employed.
?igure 12 illus rates the comp2ra.ive porosi.y sensitivity o~ the emDodlment of ~igure B rela'ive -o 2j ~he prior art 14 ~ev wo-de.ec.or porosity technigue.
m~ he~ solld-line graph 12Z represen-_s the plot o .he N/~ ratio as a unc.ion of porosity in the p,ior art tool. As previously mentioned, this graph shows little, if any, porosi.y response above about 20 p.u.
By comparlson, the porosity res~onse of the N/~ ratio ~ormed using the shielded-detector technique of the present invention, represented by the dotted-line graph 124, shows signi icant change with porosity over the full range up to and above 40 p.u.
3~ Where reference is made to B4C as an element of the detector shielding, it will be understood that either B10 or B10 enriched natural boron may be used.
- B10 has the greater absorption cross section, but a -- -31- ~Z~37~3 l~wer material concentration. Thus a possible shielding c~mpositi~n might be B4C enriched in 31 .
Similarly, other high cross section l/v-like absorbers could be used, such as lithium carbonate - ~ in an epoxy resin binder. In this case, the Li2Co3 could advantageously be enriched in Li . Similarly, ~inder materials other than epoxy resin or hydrogenous materials other than polyethylene may be used as moderators in accordance with the invention.
In the embodiment of Figure 13, the neutron source intensity monitor 14 of the invention is shown in use in a more generalized-borehole logging tool 10.
The disposition of the tool within the borehole ~4 is essentially the same as in ~igure 8, except that the 15 sonde is not eccentered and a mudcake 126 is shown as formed on the borehole wall.
~s in the embodiment of ~igure l, the sonde 10 is depicted as containing a neutron source 12, the moni-tor 14, a near-s~aced detector 16, and a fa~-spaced 20 detector 18. The two detectors shown are intended to be îllustrative of a typic21 detector arrangement in a sonde, and the sonde might contain only one or seve~al such de~ecto~s. The detecto_s themselves may be Oc any sui~able tv~e. The sonae r.ight also contain 25 shielding, 2S shown in ~igures 1 2nd 8 'or exa~?le, 'or the neu'ron source an~/or ~o_ ~he aetectors.
The neutron source 12 could be either a chemical - source or an accelerator source. In accord2nce with ~he invention, however, t~e neutron source intensity }
30 monitor is pre~erably used with an accelerator source, and ls particularly advantaaeous in connection with high-energy monoenergetic sources of the D-T type (14 MeV neutrons) or the D-D type ~2.5 ~eV neutrons). ~or convenience, the invention is described hereinafter as 35 being used with a 14 MeV D-T accelerator source.
The scintillator 30 of the monitor 14 is shown 2S
being adjacent to the source 12, but it could ~e co-axial with the source depending upon design con-~ - ~ ;
-32- ~2~3~83 straints. What is si~nificant, however, is that the sci~tillator 30 be located relative to the source so that the response of the scintillator is dominated, at least during and immediately following a neutron burst in the case of pulsed operation, by high energy neutrons coming directly from the source, rather than lower energy, scattered neutrons or g~mma rays. In the absence of heavy shielding between the source and the scintillator, it has been found that the scintil-lator 30 may be spaced as far as 30 cm from the sourceand still function as a detector of source neutrons.
The scintillator 30 is also shown as being in contact with the photomultiplier 32. As will be understood, however, the scintillator may be optically coupled to the photomultiplie~ in any suita~le way e.g., a light pipe, fiber optics, or a system of lenses and mirrors, that will conduct the flashes of light in the scintillator to the photomultiplier, and need not be physically connected thereto.
Generally, the scintillator 30 may comprise any type of hydrogen-containing scintillator, e.g., liquid, plastic, or crystal, that detects scintilla-tions resulting from ~roton-recoil events. Such scintillators are generally known in the art 25 2~ "organic" scintillators, and will be referred to herein in that sense. Suitable organic scintillators include, for example, NE-213 (liquid), NE-102 & N~-162 (plastic), and stilbene (crystal). Plastic scintilla-tors have been found to be particularly advantageous.
Organic scintillators have very short decay times and, consequently, the maximum counting rate can be very - large, whlch permits them to be located relatively close to the neutron source. The close proximity o, the scintillator to the source maximizes the neutron flux from the source intercepted by the scintillator, thereby affording a high signal-to-noise ratio in the detector output, and also reduces the susceptibility of the monitor to scattered-back neutrons. As will be _33- ~43783 discussed more fully below, the latter feature contributes to the inherent gain stability of the monitor. ~urther, organic scintillators are proportional in their response to both electrons and protons, e.g. approximately twice as great for electrons as for protons in the 5-10 MeV energy range, which allows many low energy gamma ray-induced (Comp-ton scattering) scintillations to be distingui~hed from the neutron-induced (proton recoil) scintilla-tions of interest on the basis of pulse height.
If high temperatures are anticipated during bore-hole logging operations, the scintillator may be located in a dewar flask or may be otherwise thermally insulated. It is an advantage of the invention, however, t:hat such thermal insulation may be omitted by use of an appropriate hightemperature plastic scintillator.
The signals from the detectors 16 and 18 and the photomultiplier 32 are transmitted in a known manner with known equipment to a surface data processing system 128 by insulated electrical conductors, not shown, located in the armored ca~le 86. At the surface, following any necessary preliminary decoding, pul~e shaping, amplification or the like, the signals are applied to signal processing circuits 50 that carry out the desired computations, etc., and provide outputs ~o a plotter-recorder 52. The number of out-puts shown is exemplary, and the actual number and types of outputs provided will depend upon the number and type of detectors in the sonde and the type of information being obtainedO Two examples of such signal processing circuits, in conjunction with which the source intensity monitor of the invention has pplication, are described in the aforementioned U.S.
Patents No. 4,423,323 to Ellis et al. and No.
4,52~,274 to Scott.
. While the sonde 10 i5 being moved through the borehole 84, an indication of the depth of the sonde ~`` ~34~ ~2~37~3 h in the borehole is provided by a depth determining apparatus, generally indicated by reference numeral 130, which is responsive to the movement of the cable 86 as it is let out and reeled in by a winch, not shown. The depth determining apparatus 130 is con-nected to the plotter-recorder 52 by a conventional cable-following mechanical linkage 132.
In Figure 13, the box 134 depicts in more detail the components 54-60 of Figure 5 for processing the output of the monitor 14 for purposes of monitoring source intensity. The signal pulse train from the photomultiplier 32 is supplied, after being conven-tionally amplified and otherwise processed ~amplifier 54 in Pigure 5), to a first pulse height differential discriminator circuit 136 and, where active gain stabilization is desired as discussed below, to a second pulse height differential discrimination circuit 138. Jointly these two discriminator circuits comprise the contents of component 56 in Figure 5.
The discriminator circuits are shown in ~igure 13 2S
~eing located in the downhole electronics 134, but they could if desired ~e included in the surface electronics 128. Each pulse height differential dis-c-iminator clrcuit 136 and 138 passes sig~als having magnitudes, i.e., pulse heights, within a selected range and attenuates 211 other signals. The location of these-selected ranges with respect to the pulse height spectrum of the scintillator is discussed below in conjunction with ~igure 14. The signals passed by the pulse height differential discriminator circuits 136 and 138 are supplied to first and second scaler circuits 140 and 142, respectively, t~at generate outputs Nl and N2 indicative of the num~ers of neutron interactions detected by the scintillator 30 (over a time period of specified dura~ion) within the energy ranges associated with the respective discriminator circuits. The scalers 140 and 142 comprise component 58 in Figure S.

-- ~ y ~Z43783 E
As explained hereinafter, the output signal from the scaler circuit 140 is proportional to the number o~ 14 MeV neutrons emitted by the neutron source 12 or, in other words, to the neutron source intensity. This signal i5 preferably sent over the cable 86 to the surface signal processing circuits 50 and is recorded by the plotter-recorder 52 as a measurement of neutron source intensity.
The outputs from the first scaler circuit 140 and the second scaler circuit 142 are Also supplied to a gain stabilizer circuit 60 (Figures 5 and 13). The gain stabilizer circuit forms the ratio of the output Nl from scaler circuit 140 to the output N2 from scaler circuit 142, i.e., N1/N2, and in conventional fashion derives a control signal for the high voltage supply 144 of the photomultiplier 32 based on a comparison of this ratio with a constant reference value. The gain stabilizer circuit then controls the high voltage supply, again in a manner known in the art, to increase or decrease photomultiplier gain, as the case may be, to maintain the ratio at the reference value.
Due to the inheren~ gain stability of the monitor of the invention, as desrribed in detail below, active gain compensation is unnecessary in many applications.
In such a situation, only the first pulse height di~ferential discriminator circuit 136 and the fir~t scaler circuit 140 are needed ~o monitor the neutron souroe intensity, and the entire active gain sta~i-lizer circuit, including the discriminator circuit138, the second scaler circuit 142, and ~he gain stabilizer circuit 60 are unnecessary and can be eliminated.
In ~igure 14, count rate is plotted on the vertical axis and pulse height (recoil proton energy for curve 146 and Compton elec~ron energy for curve 148) is plotted on the horizontal axis. The curve ` designated by reference numeral 146 is a curve of an -36- ~24~783 observed scintillator pulse height spectrum for neutron-induced recoil proton events in a neutron-gated 3/4 inch x 3/4 inch ~1.9 cm x 1.9 cm) NE-213 liquid scintillator. The source was a 14 MeV D-T
accelerator.
The curve designated by reference numeral 14B is a curve of an observed scintillator pulse height spec-trum for gamma ray-induced Compton electron events in the same scintillator, but gated for gammas. The total scintillator pulse height spectrum is obtained by adding the curves 146 and 148. The curves 146 and 148 were obtained by placing the scintillator about 1.~ inches (4.6 cm) from the target of a D-T
accelerator neutron source. A pulse shape discriminator was used in order to separate the neutron induced events from the gamma ray-induced events. As the composition of NE-213 is similar to that of plastic, the curves of Figure 14 may be taken as representative of plastic scintillator spectra as well.
As noted, the curve 146 is essentially a pulse height spectrum for recoil protons, with the corres-ponding recoil proton energy shown along the hori-zontal axis. Because neutron-proton soattering is basically isotropic in the center-of-mass system at the energies involved in ~igure 14, the undistorted recoil proton pulse height spectrum is rectangular in shape. ~owever, the observed pulse height spectrum is nonrectangular due to the influence of several factors, including multiple neutron scattering, escape of recoil protons from the scintillator volume, non-linear lisht response of the scintillator-photo-multiplier, and the resolution function of the equip-ment. The curve 146 has essentially two portions: a first portion that remains su~stantially flat or decreases sligh~ly at hi~her energies, i.e., energies from the discriminator threshold level at about 3-4 MeV up to 12-13 MeV, as energy increases, and a second _37_ ~Z~3t7~3 i portion that decreases rather rapidly at still higher energies, i.e., energies above 13-14 ~eV, as energy increases.
As is apparent from Figure 14, the neutron-induced spectrum 145 is much higher, usually by atleast two orders of magnitude, than the gamma ray-induced spectrum 148. Therefore, the total scintil-lator pulse height spectrum, which is obtained by adding these curves, has essentially the same shape as the curve 146 in the region of interest: 10 ~eV or greater recoil proton energy.
Reference numeral 150 designates the range, i.e., the pulse height interval or window, of the signals that are passed by the first pulse height differentia discriminator circuit 136, and reference numeral 152 designates the range of signals that are passed by the second pulse height differential discriminator circ~it 133. ~he pulse height interval 150 is selected so that it is in the higher-energy section of the flat portion of the curve 146, where the effects of 14-MeV
neutrons dominate and where, as is dicussed in more detail below, the effects of lower-energy, scattered n~utrons and gamma rays are insignificant. The window 150 should be wide en~ugh to provide sufficiently hish counting rates for satisfactory statistical precision.
As illustrated in Figure 14, the pulse height window 150 is preferably cnosen so that signals due to recoil proton energies greater than about 10 MeV are passed. In this region, the effects of lower-energy, scattered neutrons and gamma rays do not apprecia~ly affect the ccunting rate due to 14-MeV neutrons when the total scintillator pulse height spectrum is measured, which of course is what is measured by the monitor during normal operations. Lower-enerqy, scattered neutrons do not appreciably affect the counting rate because the scintillator 30 is located suffic~ently close to the source so that most of the neutrons reaching the scintillator are source ., ~ ~ . !
J
-3B- 12 4 3783 f .

neutrons~ i.e., directly incident thereon from the source and not back-scattered, and because the dis-criminator 136 blocks pulses due to lower-energy, scattered neutrons. Similarly, gamma rays do not appreciably affect the counting rate during the period of recoil neutron production because the discriminator 136 blocks pulses due to lower-energy gamma rays ~nd because the small scintillator size substantially eliminates the effects of higherenergy ga~ma rays.
Thus, the window 150 ef~ectively passes signals induced substantially only by higher-energy, source neutrons. Consequently, the signal passed by the window 150, and counted by the scaler circuit 140, is proportional to and primarily representative of the 14-MeV neutron source intensity.
In addition to placing the energy window 150 high enough in the pulse height spectrum to eliminate low-e~ergy recoil protons and gamma rays, as described above, the following procedure is preferably followed in settinq the window 150. ~irst, the pulse height spectrum is accumulated. Second, with reference to ~igure 15, the limits Xl and X2 of the energy window l5D are preferably set so as to substantially satisfy the following relationship:
~N = 1 ~ 1 = 0 --N~g 1 X3 ~ g where:
~ is the total num~er of counts in the window 3~ 152;
hg is the change from g=l in gain g of the spectrum;
~ N is the change in N with ~g;
X3 is the X-axis intercept of the linear approxl-mation 154 to the shape of the spectrum 156 in theregion of the window 150; and Xav9 is (Xl + X2)/~
Third, Xl and X2 should stay in the linear region of the spectrcm 156. If necessary or desirable, the -39- ~4~78~

scintillator size may be modified to give the desired spectrum shape.
Inasmuch as the effects of both higher-energy and lower-energy ga~ma rays are substantially eliminated in a neutron source intensity monitor in accordance with the invention, a pulse shape discriminator, which is complex and comparatively slow, is unnecessary.
The pulse height interval 152 for the second pulse height differential discriminator circuit 138 is preferably selected so that it is at or higher than the knee of the curve 146, where the curve decreases markedly a5 energy increa~es. With the window 152 selected thusly, if the monitor system gain changes, the curve 146 will shift significantly and the count-ing rate in the window 152 will change accordingly.If the high ~oltage increases, the curve will shift to the right, and the counting rate in the window will increase. If the high voltage decreases, the curve will shift to the left, and the counting rate in the window will decrease. Consequently, the counting rate N2 in the window 1~2 is sensitive to fluctuations in gain and may be used, preferably in conjunction with the counting rate Nl as described above, for gain control.
As indicated previously, active gain stabiliza-tion will be unnecessary in many applications, in which ~ase the second pulse height differential dis-criminator circuit 138, the second scaler circuit 142, and the gain stabilizer circuit 60 can be eliminated.
Actlve gain s,abilization will be unnecessary, for instance, where the inherent gain stability of the monitor system, e.g., first order gain changes, l.e., 10-20%r cause only second order changes, i.e., 1-2%, in the measured source intensity, affords acceptable precision.
In order to achieve this inherent stability, the scintillator must be sized, on the one hand, to mini-mize gamma ray-induced (Compton scattering) events _40~ 3~83 and, on the other hand, to maximize recoil proton-induced events in the energy range of interest~ In the case of a 14 MeV D-T accelerator, where the pulse height window 150 is set at 10 MeV and above, the S scintillator size should therefore be selected to minimize the number of Compton electron events within the scintillator which would produce light pulse mag-nitudes comparable to those produced by 10 MeV and above recoil protons. Electron energy loss in a scin-tillator is approximately 2 MeV/gm/cm . A typicalplastic scintillator might have a density of approxi-mately 1 gm/cm3, so that, in a plastic scintillator, the electron energy loss would be about 2 MeV/cm. At the energies of interest here, however, the light output produced in the scinti~lator by an electron energy loss event is approximately twice that produced by a recoil proton energy loss event. ~ence, a 5 MeV
electron would result in approximately the same scin-- tillator output as a 10 MeV proton.
- 20 Accordingly, for an energy window 150 beginning at 10 MeV, the number of 5 MeV and above electron energy loss events in the scintillator must be kept small. And as elecLron energy loss is approximately 2 MeV/cm in a plastic scintillator, the upper limit ~or the size of the scintillator in any dimension, referred to herein as the "characteristic dimension,"
is approximately 2.5 cm or about 1 inch, for a volumetric upper limit of approximately 16 cm3 or 1 in3. A suitable characteristic dimension for the organic scintillators mentioned herein, when used to detect 14 MeY source neutrons, has been found to be on the order of 1/2" (1.3 cm) or 3/4" (1.9cm). This choice of scintillator size is consistent both with providing for inherent gain stabilization and with providing for sufficiently high count rates for good statistical precision.
If lower energy windows are used, which would be the case for a 2.5 MeV D-D accelerator, for example, -41- ~Z43783 the scintillator mus~ be smaller because many more lower energy eleotrons would lose all of their ener-gies in a l inch (2.5 cm) crystal. Moreover, the relationship of light output per ener~y loss is less favorable at lower energies, so that electron events at a given energy appear as recoil proton ~neutron-induced) events at more th~n twice ~he electron energy.
~ith the scintillator appropriately sized, higher-energy gamma ray-induced signals will be sub-stantially eliminated since the Compton electrons produced in the scintillator will escape from the scintillator and not produce signals, whereas recoil protons produced in the scintillator by the source neutrons will produce signals that are picked up by the photomultiplier. The resulting scintillator pulse height spectrum will be similar to the curve 146 in Fi~ure 14. Also, the scintillator should be located with respect to the neutron source so that most of the neutrons reaching the scintillator come directly from the neutro~ source and are not lower-energy, scattered neutrons. If these conditions obtain (scintillator size and placement relative to the source) and if the pulse height window l;0 ls chosen as indicated above, i.e., so that the effects of lower-energy, scattered neutrons and lower-energy gamma rays are insigni-ficant, a simple neutron source intensity monitor that has inherent gain stability results.
Although the neutron source monitor of the inven-tion has been described herein in connection with aborehsle log~ing tool, it will be understood that it can be used as well in other applications where high energy neutrons sources are utilized.
Although the invention has been described herein with respect to specific embodiments thereof, it will be understood that various modifications and varia-tions may be made thereto without departing from the inventive concepts disclosed. All such modifications -42- ~Z~37~3 and varia~ions, therefore, are intended to be in-cluded within the spirit and scope of the appended claims.

.....

Claims (35)

We claim:

1. A well logging tool for investigating an earth formation surrounding a borehole comprising: an accelerator neutron source comprising a substantially monoenergetic D-T source of 14-MeV neutrons; neutron source monitoring means responsive primarily to unmoderated neutrons incident thereon directly from said neutron source for monitoring the output thereof, the sensitive volume of said monitoring means being located externally of but closely adjacent to the neutron source; first neutron detection means for detecting epithermal neutrons, said first neutron detection means including a sensitive volume spaced from said accelerator neutron source and being substantially insensitive to neutrons below approximately 0.5 eV in energy, the sensitive volume of said first neutron detection means being located close to said neutron source without substantial intervening high density shielding; first shielding means, having both neutron moderating and neutron absorbing properties, for shielding the sensitive volume of said first neutron detection means so as to increase the low energy neutron detection threshold of said first neutron detection means to at least approximately 10 eV; second neutron detection means for detecting epithermal neutrons, said second neutron detection means including a sensitive volume located farther from said accelerator neutron source than the sensitive volume of said first neutron detection means and eccentered towards one side of the well logging tool, said second neutron detection means being substantially insensitive to neutrons below approximately 0.5 eV in energy; and second shielding means, having both neutron moderating and neutron absorbing properties, for shielding the sensitive volume of said second neutron detection means from neutrons incident thereon from a side thereof away from said one side of the well logging tool.

2. The well logging tool of claim 1 wherein said first shielding means has a substantially annular shape and is located around said first neutron detection means and comprises an hydrogenous material that moderates neutrons having a neutron absorbing material dispersed therein.

3. The well logging tool of claim 2 wherein the neutron absorbing material is boron carbide.

4. The well logging tool of claim 1 wherein said second shielding means comprises an hydrogenous material that moderates neutrons having neutron absorbing boron carbide material dispersed therein.

5. The well logging tool of claim 1 additionally comprising third shielding means located between said first neutron detection means and said second neutron detection means and comprises an hydrogenous material that moderates neutrons having a neutron absorbing material dispersed therein.

6. The well logging tool of claim 5 wherein the neutron absorbing material is boron carbide.

7. The well logging tool of claim 1 wherein said neutron source monitoring means includes an organic scintillator, comprising said sensitive volume, and a photomultiplier, said scintillator and said photomultiplier being optically coupled.

8. The well logging tool of claim 7 wherein said scintillator is a plastic scintillator.

9. The well logging tool of claim 1 wherein said first and second neutron detection means each comprises an 3He proportional counter covered by a thin cadmium layer.

10. The well logging tool of claim 1 additionally comprising: means for deriving a signal indicative of the ratio of the output signal of said first neutron detection means to the output signal of said second neutron detection means as an indication of formation porosity.

11. The well logging tool of claim 1 additionally comprising: means for deriving a first ratio signal indicative of the ratio of the output signal of said neutron source monitoring means to the output signal of said first neutron detection means; and means for deriving a second ratio signal indicative of the ratio of the output signal of said neutron source monitoring means to the output signal of said second neutron detection means.

12. The well logging tool of claim 11 additionally comprising: means for combining said first and second ratio signals to derive a signal indicative of the porosity of the formation.

13. A well logging tool for investigating an earth formation surrounding a borehole comprising: a substantially monoenergetic D-T accelerator source of 14-MeV neutrons; neutron source monitoring means responsive primarily to unmoderated neutrons incident thereon directly from said neutron source for monitoring the output thereof, the sensitive volume of said monitoring means being located externally of but closely adjacent to the neutron source;
a first 3He proportional counter spaced from but close to said neutron source, without substantial intervening high density shielding, and being substantially insensitive to neutrons below approximately 0.5 eV in energy;
first shielding means, having both neutron moderating and neutron absorbing properties, for shielding said first 3He proportional counter so as toincrease the low energy neutron detection threshold of said first 3He proportional counter to at least approximately 10 eV; a second 3He proportional counter located farther from said neutron source than said first 3He proportional counter, said second 3He proportional counter being eccentered towards one side of the well logging tool and being substantially insensitive to neutrons below approximately 0.5 eV in energy; second shielding means, having both neutron moderating and neutron absorbing properties, for shielding said second 3He proportional counter from neutrons incident thereon from a side thereof away from said one side of the well logging tool; and third 3He proportional counter located farther from said neutron source than said second 3He proportional counter and being eccentered towards said one side of the well logging tool and shielded on the side thereof away from said one side of the well logging tool by a neutron absorbing material.

14. The well logging tool of claim 13 additionally comprising: means for deriving a first ratio signal indicative of the ratio of the output signal of said neutron source monitoring means to the output signal of said third 3He proportional counter; and means for deriving a second ratio signal indicative of the ratio of the output signal of said neutron source monitoring means to the output signal of said second 3He proportional counter.

15. The well logging tool of claim 14 additionally comprising: means for combining said first and second ratio signals to derive a signal indicative of the macroscopic capture cross section of the formation.

16. The well logging tool of claim 7 wherein said photomultiplier produces signals representative of the energies of the recoil protons produced by detected neutrons and further comprises first pulse height discriminator means coupled to said photomultiplier for passing photomultiplier output signals having magnitudes corresponding to a range of the scintillator pulse height spectrum located on the flat region of the spectrum and sufficiently high therein comparatively to eliminate scintillator outputs produced by scattered-back neutrons and by gamma rays; and means for counting the signals passed by said first pulse height discriminator means as an indication of neutron source intensity.

17. The well logging tool of claim 16 further comprising: second pulse height discriminator means coupled to said photomultiplier for passing photomultiplier output signals having magnitudes corresponding to a range of the pulse height spectrum located on a comparatively rapidly varying region of the spectrum above said flat region thereof; means for counting the signals passed by said second pulse height discriminator means; and gain stabilizer means coupled to said first and second counting means for taking a ratio of the counting rates from said respective spectrum ranges and for deriving a gain-compensation signal so as to maintain said ratio at a substantially constant value.

18. The well logging tool of claim 9 wherein: said neutron source is a substantially monoenergetic D-T source of 14-MeV neutrons; the range of signals passed by said first pulse height discriminator corresponds to a range of the pulse height spectrum located above about 10-MeV; said scintillator comprises a plastic organic scintillator; and said characteristic dimension is such that the scintillator volume is on the order of 1 in3 (16.4 cm3) or less.

19. The well logging tool of claim 7 wherein said neutron source monitoring means further comprises: means coupled to said photomultiplier for measuring, as a function of time during periods when said neutron is not operating, the intensity of capture gamma rays in the formation surrounding the tool; and means for processing said intensity measurements to derive a measurement of the thermal neutron capture cross section of the earth formation.

20. The well logging tool of claim 19 wherein said capture gamma ray intensity measuring means comprises means for measuring at least substantially the entire intensity vs. time decay curve during said periods when said neutron source is not operating.

21. The well logging tool of claim 1 further comprising means defining an array of neutron detectors, located between the sensitive volumes of said first and second neutron detector means, for detecting the neutron flux in the formation surrounding the borehole at at least two different axially-spaced points from said neutron source.

22. The well logging tool of claim 21 wherein: said detector array means for detecting the neutron flux at said at least two points comprises two detectors of substantially the same neutron energy sensitivity, and said detector array defining means further comprises a third neutron detector of a neutron sensitivity substantially different from that of said two detectors.

23. The well logging tool of claim 22 wherein said two detectors are sensitive to one of thermal neutron energies and epithermal neutron energies and said third detector is sensitive to the other of said energies.

24. The well logging tool of claim 21 wherein the sensitive volumes of said two detectors are (1) eccentered towards one side of the tool and (2) axially-spaced from one another lengthwise of the tool, and the sensitive volume of said third detector is (1) located at substantially the same distance from the neutron source as the more closely spaced of said two detector sensitive volumes and (2) transversely spaced from said more closely spaced sensitive volume.

25. A neutron source monitor for monitoring the output of a high-energy neutron source, comprising: an organic scintillator for detecting high-energy neutrons incident thereon substantially directly from the neutron source and for producing outputs related to the incident energies thereof, said scintillator having a characteristic detector dimension that is large relative to the average range of recoil protons produced in said scintillator by substantially source-strength neutrons and small relative to the average range of those Compton-scattering electrons produced in said scintillator which would appear to the scintillator to have energies within the energy range of said recoil protons; a photomultiplier optically coupled to said scintillator and responsive to the outputs thereof for producing signals representative of the energies of the recoil protons produced by detected neutrons; first pulse height discriminator means coupled to said photomultiplier for passing photomultiplier output signals having magnitudes corresponding to a range of the pulse height spectrum located on the flat region of the scintillator spectrum and sufficiently high thereon comparatively to eliminate scintillator outputs produced by scattered-back neutrons and by gamma rays; and means for counting the signals passed by said first pulse height discriminator means as an indication of neutron source intensity.

26. The neutron source monitor of claim 25 wherein: said neutron source is a substantially monoenergetic D-T source of 14-MeV neutrons, and the range of signals passed by said first pulse height discriminator corresponds to a range of the pulse height spectrum located above about 10-MeV.

27. The neutron source monitor of claim 26 wherein said characteristic dimension is such that the scintillator volume is on the order of 1 in3 (16.4 cm3) or less.

28. The neutron source monitor of claim 27 wherein the limits of the range of signals passed by said first pulse height discriminator means are selected such that the relationship .DELTA.N/N.DELTA.g = 0 is substantially satisfied; where N is the number of counts within said range, .DELTA.g is the change in gain of the pulse height spectrum, and .DELTA.N is the change in N with .DELTA.g.

29. The neutron source monitor of claim 25 wherein said scintillator comprises a plastic organic scintillator.

30. The neutron source monitor of claim 25 further comprising: second pulse height discriminator means coupled to said photomultiplier for passing photomultiplier output signals having magnitudes corresponding to a range of the pulse height spectrum located on a comparatively rapidly varying region of the spectrum above said flat region thereof; means for counting the signals passed by said second pulse height discriminator means; and gain stabilizer means coupled to said first and second counting means for taking a ratio of the counting rates from said respective spectrum ranges and for deriving a gain-compensation signal so as to maintain said ratio at a substantially constant value.

31. The neutron source monitor of claim 30 wherein the range of signals passed by said second pulse height discriminator means corresponds to a range of the pulse height spectrum located in the region of the knee of the spectrum.

32. The neutron source monitor of claim 31 wherein said gain-compensation signal is applied to the high voltage power supply for said photomultiplier.

33. A neutron detector system for selectively monitoring the output of a high energy neutron source in a borehole logging tool, comprising: a neutron detector for detecting neutrons emitted by said neutron source and for generating a signal representative thereof, said detector normally being comparatively sensitive to low energy neutrons and comparatively insensitive to high energy neutrons; and neutron shielding means surrounding said detector for (1) moderating high energy neutrons incident thereon to intermediate energy levels within the sensitive range of said detector and for moderating intermediate energy neutrons incident thereon to comparative low energy levels and (2) absorbing comparatively low energy neutrons incident thereon or moderated therein, whereby said detector output signal is predominately indicative of high energy neutrons emanating substantially directly from said neutron source, as distinct from intermediate energy and low energy neutrons scattered back to the detector from the borehole environment or earth formations surrounding the borehole.

34. The detector system of claim 33 wherein said neutron shielding means comprises a member encircling the sensitive volume of said detector, said member being composed of a substantially uniformly distributed mixture of neutron moderating hydrogenous material and neutron absorbing boron carbide material.

35. The detector system of claim 34 further comprising a cladding of material on said detector for absorbing low-energy neutrons, said cladding being located inside of said neutron shielding means.