Hermetically sealed packaging and neutron shielding for scintillation-type radiation detectors

A well logging instrument includes a source of high energy neutrons arranged to bombard a formation surrounding the instrument. A scintillator sensitive to gamma radiation resulting from interaction of the high energy neutrons with the formation is disposed in the instrument. A neutron shielding material surrounds the scintillator. A neutron moderator surrounds the neutron shielding material. An amplifier is optically coupled to the scintillator.

CROSS-REFERENCE TO RELATED APPLICATIONS

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of scintillation-type radiation detectors for use in well logging instruments. More specifically, the invention relates to various forms of radiation shielding for such detectors to reduce detection of radiation other than the specific radiation events intended to be detected.

2. Background Art

Various well logging instruments are known in the art that measure radiation phenomena originating from subsurface formations surrounding a wellbore. Such instruments may be inserted into the wellbore at the end of an armored electrical cable, on a pipe string or coiled tubing or other conveyance. Some forms of such instrument include a source of high energy neutrons. The source is configured to emit the neutrons into the formations surrounding the wellbore. The source may be a “chemical” source that includes a radioisotope, for example, americium-241 powder mixed with beryllium powder. Other sources are electrically controlled accelerators, such as deuterium-tritium or deuterium-deuterium accelerators that emit a continuous stream of neutrons or controlled duration “bursts” of neutrons into the formations.

Such well logging instruments include one or more radiation detectors, typically scintillation counters. A scintillation counter includes a crystal made from a material that is sensitive to radiation entering therein. Such material emits a small flash of visible, infrared or ultraviolet light upon interaction with radiation. Typically the amplitude of the flash is related to the energy of the deposited radiation. A converter and amplifier, such as a photomultiplier, is optically coupled to the crystal, and is arranged to generate a detectable electrical pulse corresponding to each radiation detection event. U.S. Pat. No. 7,084,403 issued to Srivastava et al. describes a variety of different materials used for scintillation detector crystals, including their respective advantages and disadvantages.

One particularly useful type of well logging instrument is known under the trademark RST which is a trademark of the assignee of the present invention. Such instruments and its more recent improved implementations thereof, include an accelerator type source of neutrons that emits controlled duration bursts of high energy neutrons into the formations surrounding a wellbore. One or more scintillation type radiation detectors are arranged in the instrument to detect gamma rays resulting from interactions of the neutrons with the surrounding formations. In order for such instruments to provide measurements that are closely representative of the properties of the surrounding formations, it is desirable to shield the one or more radiation detectors from both direct emission of neutrons from the source and from neutrons that interact with the formations and the materials in the wellbore. Such neutrons may cause events in the radiation detectors that are not related to the properties of the formations desired to be evaluated.

Certain materials for scintillation detectors, for example, those described in the above referenced '403 patent, have physical characteristics such as being hygroscopic and being susceptible to damage by mechanical shock and vibration that make it desirable to mount the scintillation crystal to reduce the effects thereon of moisture, shock and vibration. It is also desirable to provide such mounting with suitable radiation shielding properties so that the radiation detector is primarily sensitive to radiation events of interest, while being relatively insensitive to other radiation events.

SUMMARY OF THE INVENTION

A well logging instrument according to one aspect of the invention includes a source of high energy neutrons arranged to bombard a formation surrounding the instrument. A scintillator sensitive to gamma radiation resulting from interaction of the high energy neutrons with the formation is disposed in the instrument. A neutron shielding material surrounds the scintillator. A neutron moderator surrounds the neutron shielding material. An amplifier is optically coupled to the scintillator.

A method for neutron activation gamma ray well logging according to another aspect of the invention includes bombarding a subsurface formation with high energy neutrons. Neutrons emanating from the subsurface formation are moderated proximate a scintillator. The moderated neutrons are absorbed between the place of the moderating and the scintillator. At the scintillator gamma rays emanating from the subsurface formations resulting from interactions therewith of the neutrons are detected.

DETAILED DESCRIPTION

An example well logging instrument12is shown as it may be used in a wellbore inFIG. 1. The instrument12includes a generally elongated housing14configured to move along a wellbore22drilled through subsurface formations24. The housing14may be made from stainless steel, titanium or similar material having wall thickness selected to resist crushing under hydrostatic pressure of fluid (not shown) in the wellbore22. The instrument12may be moved along the wellbore22by an armored electrical cable15. The cable15may be extended and retracted by a winch26or similar spooling device known in the art. Electrical power to operate the instrument12may be provided by a recording unit28disposed at the Earth's surface30. The recording unit28may include equipment (not shown separately) for detecting, recording and interpreting signals transmitted from the instrument12as it moves along the wellbore22. The example device for conveying the instrument12along the wellbore (cable15and winch26) are only shown to provide an example of conveyance that may be used with an instrument according to the invention. Other devices known in the art, non-limiting examples of which include coiled tubing, drill pipe (including logging while drilling), production tubing and slickline may also be used to convey the instrument12along the wellbore22. Accordingly, the conveyance shown inFIG. 1is not intended to limit the scope of the present invention.

The instrument12may include a neutron source16within the housing14. The source16in the present example may be a chemical isotope or accelerator (pulsed or DC) source of high energy neutrons. A shield20may be disposed inside the housing14between the source16and at least one radiation detector10. The shield20may be made from a material having properties of substantially preventing direct movement of neutrons from the source16to the radiation detector10. Thus, neutrons from the source16generally interact with fluid in the wellbore22and the surrounding formations24, resulting in radiation events including gamma rays that may enter the at least one radiation detector10and be detected.

The radiation detector10in the present example may be configured to detect gamma rays emanating from the formations24in response to interaction between the neutrons from the source16and the various atomic nuclei in the formations24. Energy level and/or numbers of such gamma rays may be related to properties of interest in the formations24, including their chemical composition, fractional volume of pore space (“porosity”) and the composition of fluids present in the pore spaces.

The gamma rays detected by the radiation detector10may result in electrical pulses produced by the detector10(explained with reference toFIG. 2) in response to such detections. Such electrical pulses may be communicated to a pulse height analyzer and telemetry unit, shown generally at18and disposed within the housing14. The pulse height analyzer and telemetry unit18may impart signals to the cable15that correspond to the numbers of and energy levels of the detected gamma rays. Alternatively or additionally, the pulse height analyzer and telemetry unit18may include signal recording devices for storage of analyzed electrical pulses from the detector10for interrogation when the instrument12is withdrawn from the wellbore22.

An example of a radiation detector is shown in more detail inFIG. 2. The detector10may include a scintillation crystal32(or “scintillator”). The scintillation crystal32may be made from various materials known to emit flashes of light upon entry therein of gamma ray and/or x-ray radiation. Examples of such materials are described in U.S. Pat. No. 7,084,403 issued to Srivastava et al. The scintillation crystal32may be shaped generally as a cylinder and may be surrounded, other than on one longitudinal end, by an optically reflective material, shown as reflector34. The reflector34serves to cause light traveling in a direction other than toward the one longitudinal end of the scintillation crystal32to be reflected back into the crystal32, thus increasing the probability that such light will be directed toward a light detector, converter and amplifier48, such as a photomultiplier tube, coupled to the longitudinal end of the scintillation crystal32. Such amplifiers are known in the art. Suitable examples of amplifiers are set forth in U.S. Pat. No. 4,937,446 issued to McKeon et al. and assigned to the assignee of the present invention, or for example, in, G. Knoll,Radiation Detection and Measurement, J. Wiley (2000), ISBN-10: 0471073385, ISBN-13: 978-0471073383.

In the present example, the reflector34may be surrounded on its exterior by an x-ray shield36. The x-ray shield36may be a thin foil made from a high atomic number (Z) material such as lead. The x-ray shield36can reduce the number of ionizing radiation photons below a threshold energy level from entering the scintillation crystal32. By limiting entry of such low energy ionizing radiation photons, the number of detection signals generated that are unrelated to radiation events of interest may be substantially reduced. Notwithstanding the fact that the analyzer (18inFIG. 1) could otherwise identify such photons by the amplitude of the corresponding electrical pulses from the amplifier48, typically the amplifier48has a finite, non-zero recovery time after generating each electrical pulse in response to a scintillation from the crystal32. Limiting detection pulses to those radiation events of interest by suitable shielding therefore may provide the benefit of increased overall detector efficiency.

Other nuclear reaction products, including alpha particles, electrons (beta particles) and neutrons could enter the detector10and cause nuclear reactions therein resulting in flashes of light not related to the radiation of interest to be detected by the instrument (22inFIG. 1). Accordingly, radiation detectors according to the invention may include additional shielding devices to reduce the number of such reaction particles and other radiation from entering the detector (10inFIG. 1). In the present example, a neutron shield38may be disposed over the exterior of the x-ray shield36. The neutron shield38may be made from various materials that have a high neutron capture cross section, for example, boron (e.g., boron-10), lithium-6, cadmium and gadolinium, or compounds made therewith. Metallic lithium is highly chemically reactive, and so while having suitable neutron absorption properties, may be unsuitable for use as the neutron shield38. An alternative may be, for example, lithium-6 carbonate. An advantage of using lithium-6 containing materials for the neutron shield38as contrasted, for example, with boron containing materials is that lithium does not discharge a gamma ray upon neutron capture; it only emits alpha particles (helium nuclei). Such emissions may be readily stopped from entering the crystal32by the x-ray shield36(if used), or by a thin metal foil, such as copper, silver or aluminum, for example. The neutron shield38may be made from solid material containing the neutron absorbing material, for example, solid metallic cadmium, or solid lithium carbonate. Alternatively, particles of the neutron absorbing material may be mixed in a binder such as epoxy resin or silicone rubber.

If a high Z material such as lead is used for the x-ray shield36, then in some examples an additional foil layer, for example, an intermediate Z layer, may underlay the high Z material. For example, a lead outer foil may be underlain by a metallic silver inner foil. Such inner foil may absorb fluorescent x-rays that may be discharged by the lead foil by reason of, for example, absorption or scattering of higher energy radiation photons by the lead foil (characteristic x-rays). For purposes of the present example, “High Z” may be defined as Z being greater than 64. “Intermediate Z” may be defined as Z being between 35 and 66.

In some examples, the neutron shield38may be made from a dual layer material. For example a lithium-6 containing material may be used on the exterior of a dual layer neutron shield to absorb most entering thermal neutrons. An inner layer of boron-10 containing material may be used to absorb epithermal neutrons that otherwise pass through the lithium-6 containing material layer. The dual layer neutron shield may include solid layer materials or mixed materials as explained above with reference to the single layer neutron shield.

In some examples a neutron moderator40may surround the exterior of the neutron shield38. The neutron moderator40may be made from materials having high concentration of hydrogen nuclei, for example, high density polypropylene or titanium hydride. The neutron moderator40serves the purpose of reducing energy of any entering neutrons so that they may be captured by the neutron shield38instead of entering the scintillation crystal32. By excluding neutrons from the scintillation crystal32, and consequent emission of light therefrom and/or neutron-induced creation of radioisotopes in the scintillation crystal32(which itself may lead to scintillation unrelated to the radiation phenomena of interest) accuracy of measurement of gamma photon spectra from neutron activation may be improved.

In the example shown inFIG. 2, a longitudinal end of the crystal32may be covered by an optical coupling44of types known in the art. The optical coupling44is typically placed in contact with a faceplate46of the photomultiplier or an equivalent device48. The optical coupling material could also be loaded with a neutron absorbing material to improve the shielding, as long as the coupling material's optical transparency is substantially not affected.

In the example shown inFIG. 2, the reflector34may contain neutron absorbing materials. In this case the reflector acts simultaneously as a reflector of light from the scintillator32and as an absorber of neutrons.

In another example of a shielded scintillation crystal10A shown inFIG. 3, the reflector34may be surrounded by a metal foil50, such as made from aluminum, to prevent entry of charged particles, including alpha particles and electrons, into the scintillation crystal32.

In another example, a radiation detector may provide neutron shielding for a scintillation crystal and the capability of detecting epithermal and/or thermal neutrons using the same devices. The shielded detector10B inFIG. 4includes a reflector34surrounding the scintillation crystal32as in the previous examples. The reflector34may include a charged particle shield50and/or x-ray shield around the reflector34as in other examples. A neutron moderator40may surround the exterior of the shielded detector10B as in the previous examples. An annular space52between the moderator40and the reflector34(or charged particle or x-ray shield50) may be filled with helium 3 gas. The annular space52may also include electrodes54such that the gas and electrodes in combination form a helium-3 thermal neutron detector. Thus, neutrons may be simultaneously detected and stopped from entering the crystal32. As in the previous embodiments, the action of the gas in the annular space52may be supplemented by including a neutron absorbing layer (e.g.,38inFIG. 2) such as may be made from lithium, boron, cadmium and/or gadolinium containing materials as explained above. In some examples, a combination of charged particle shield and/or x-ray shield may be disposed, as shown at50inFIG. 4, on the interior of the annular space52. The shield50may be covered by an additional layer51of neutron absorbing material. In some examples, the neutron moderator40may be substituted by a thermal neutron absorbing material, such as cadmium or gadolinium, that is somewhat transparent to epithermal neutrons. In such examples, the helium-3 filled annulus with electrodes would serve to detect epithermal neutrons. Having a neutron absorbing layer such as may include lithium-6 inside the helium-3 filled annulus, as well as a charged particle/x-ray shield will in combination reduce entry of any nuclear reaction products from the helium-3 filled annulus into the scintillation crystal32.

In any of the foregoing examples, the performance of the radiation detector may be improved by providing neutron shielding around part or all of the amplifier. Referring toFIG. 5, a radiation detector10C may include a scintillator crystal32including external reflection and shielding materials110according to any of the previous examples. In the present example, part or all of the exterior of the amplifier48(e.g., photomultiplier) may be surrounded by a layer53made of neutron absorbing material. Such layer53may be applied during manufacture of the detector10C and may be made from a potting compound such as room temperature vulcanizing (“RTV”) silicone having mixed therein a neutron absorbing material as explained above with reference to the previous examples. In addition, or alternatively, the faceplate46A of the amplifier48may be made from glass including neutron absorbing material therein. In making such a faceplate including neutron absorbing material, the material selected to absorb neutrons should be chosen to avoid causing scintillation within the faceplate46A by reason of absorbing neutrons.

Any of the foregoing examples may be further improved by mounting the scintillation crystal32within a sealed, shock absorbing housing (e.g.42inFIGS. 2 and 3). Such housings are known in the art in particular where the crystal is made from hygroscopic material, and in the case of well logging instruments disposed in a drilling tool assembly (“logging while drilling” tools) for example, the crystal is mounted within the housing to isolate the crystal from excessive shock and vibration. See for example, U.S. Pat. No. 4,158,773 issued to Novak. Examples of such mounting according to the invention may be better understood by referring toFIGS. 6 and 7. InFIG. 6, the scintillation crystal32can be disposed within a reflector34as in the previous examples. The neutron shield40may be disposed externally to the reflector34as explained with reference to some examples above. A shock absorbing material66may include RTV silicone, cross-linked polymerizing gel agent dispersed in oil, or similar material that can cushion the crystal32. Typically the foregoing components are mounted in a hermetically sealed housing64. A longitudinal end of the crystal32opposite to the optical coupling44may be in contact with a pressure plate62. The pressure plate62may be urged against the end of the crystal32by a spring60or similar biasing device. In some examples, the shock absorbing material66may itself include neutron absorbing material mixed therewith. The example shown inFIG. 6includes the shock absorbing material66disposed externally to the neutron shield40. The reverse arrangement is shown inFIG. 7.

Radiation detectors and well logging instruments made according to the various aspects of the invention may have improved performance with respect to detection of gamma rays resulting from neutron interactions in Earth formations by reason of reduced background noise from unwanted scintillations caused by neutron interactions in the scintillator crystal.