Radiant energy – Invisible radiant energy responsive electric signalling – Neutron responsive means
Reexamination Certificate
2001-03-14
2002-12-17
Porta, David (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Neutron responsive means
C250S390010, C250S269400, C250S269500
Reexamination Certificate
active
06495837
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is directed toward an improved fast neutron detector, and more particularly directed toward the optimization of the detector efficiency when used in logging of earth formations penetrated by a borehole and for a variety of applications.
2. Background of the Art
In the context of this disclosure, “logging” is defined as the measure of a parameter of material penetrated by a borehole, as a function of depth within the borehole.
There are many types or classes of borehole logging systems. These classes include, but are not limited to, electromagnetic, acoustic and nuclear systems. Each class of logging system typically comprises a “source” which emits energy into the surrounding formation, and one or more “detectors” which measure energy returning from the formation. Detector responses, when properly analyzed and processed, yield formation and borehole parameters of interest.
Any type or class of logging system typically comprises a source and detector system with sufficient depth of investigation to penetrate the logging instrument housing, penetrate the immediate borehole region, enter the surrounding earth formation, interact with the formation, and induce some type of response which returns to the borehole and the logging instrument to be detected and analyzed. Nuclear logging systems typically involve the use and measure of gamma radiation and neutron radiation. These types of radiation have greater depths of penetration in comparison to protons, alpha particles and beta particles. As a result, nuclear logging instruments typically comprise a source of neutrons, or a source of gamma radiation, one or more neutron detectors, or one or more gamma ray detectors, or some combination of these different types of sources and detectors.
Logging instruments are typically conveyed along a borehole by means of a wireline or drill string thereby creating a “log” of formation parameters as a function of depth within the borehole. Borehole conditions are harsh in that temperatures and pressures are high. Components within a logging instrument, such as detectors, are subjected to these environmental conditions as well as vibration and impacts resulting from the conveying of the instrument along the borehole. As an example, nuclear detectors used in logging applications must be able to withstand these harsh conditions of the borehole environment including temperatures which can reach 175 degrees Centigrade (° C.) or higher.
All nuclear logging systems involve the measure of statistical nuclear processes. As a result, statistical significance of the measurement is of prime importance since it directly affects the statistical precision of one or more parameters of interest computed from the measurement. Statistical precision improves as the number of detector events increases. It is therefore very desirable to maximize the efficiency of nuclear detectors used in borehole logging operations. Furthermore, space is often limited in downhole instrumentation making it of utmost importance to maximize detector efficiency for a given geometry allowed in the design of the instrument.
Attention will now be directed toward prior art neutron detectors. Liquid scintillators have been used to detect high energy or “fast” neutrons. These scintillators also respond to impinging gamma radiation. Neutron and gamma ray “events” generate different pulse shape responses from liquid scintillators. Pulse shape discrimination methods therefore provide means for separating fast neutron and gamma ray induced responses in liquid scintillator detectors. Fast neutron and gamma ray measurements can be made with a single liquid scintillator detector. Liquid scintillators are relatively efficient. Unfortunately, liquid scintillators consist of flammable mixtures, and some mixtures have very low flash points. For these reasons, liquid scintillators are not desirable for high temperature, high pressure downhole applications.
Gas filled detectors, such as detectors containing relatively high pressures of helium-4 (
4
He), have been used as fast neutron detectors. These detectors are relatively rugged, and can withstand relatively high temperatures encountered within the borehole. Because the detectors are gas filled rather than liquid or solid, their detection efficiency is relatively low, and therefore not particularly desirable for downhole applications where statistical significance of measured detector response is of prime importance.
Plastic scintillators are relatively efficient neutron detectors, rugged in construction, and able to operate at temperatures of at least 175° C. These detectors are, however, responsive to both fast neutrons and gamma radiation. Neutron and gamma ray events can not be delineated by the shape or amplitude of pulses produced by the detector. The crystal anthracene, a hydrocarbon, is another type of solid material used in fast neutron detectors but, like the plastics, can not separate fast neutron from gamma ray events using pulse shape or pulse amplitude discrimination.
Stilbene and p-terphenyl crystals are fast neutron detectors and are reported to produce pulses which can be separated into fast neutron and gamma ray events. This class of detector does not have the flammability of the liquid scintillators. The crystals are, however, not rated as operable at temperatures of 175° C. The crystals are also difficult to fabricate, and availability is questionable with the only known source being Russia.
A fast neutron detector potentially suitable for downhole applications is an activated zinc sulfide scintillator combined with a nonscintillating plastic. The activated dopant is preferably silver (Ag) but other elements, such as copper (Cu) may be suitable or even better activators depending on the application of the detector. Activated zinc sulfide will be denoted by the symbol “ZnS” in the remainder of this disclosure, with the understanding that the dopant can consist of a variety of materials. The non scintillating plastic can be any hydrogen rich material that is optically transparent and that possesses suitable mechanical properties.
Geometrically, the detector is constructed with a ZnS cylindrical core surrounded by alternating and concentric cylinders of plastic and ZnS. The scintillator detector was first introduced by Emmerich in 1954 (W. S. Emmerich.,
Review of Scientific Instruments
, vol. 25, page 69 (1954)). Neutron and gamma ray events can be separated by pulse amplitude discrimination. Fast neutron detectors of this type are offered commercially by the Bicron division of Saint-Gobain International Ceramics, Inc. The material in not flammable, and it is thought that the detector can meet a 175° C. temperature rating with some modifications. The main disadvantage of this type of detector for borehole applications is the relatively small volume, with corresponding reduction in detector efficiency. Furthermore, efficiency is not maximized for specified detector volumes, and in particular for specified detector geometry of diameter D and length L. Detector volume is restricted by the lack of light transparency of ZnS, with scintillations within the ZnS element only being able to reach an optically coupled photomultiplier (PM) tube through the transparent plastic component of the detector. The plastic component of the detector contains hydrogen (H). As with other H containing fast neutron detectors, the material responds to fast neutrons impinging upon the detector by the proton recoil process, with recoil protons generating scintillations within the ZnS component of the detector. Detector response is further enhanced by a threshold (n,p) reaction with
32
S as reported by Birks (J. B. Birks,
The Theory and Practice of Scintillation Counting
, Pergamon Press, page 548, Oxford, 1964). This reaction introduces additional neutron induced proton flux within the ZnS scintillation material thereby increasing the efficiency of the detector.
Measures of fast neutrons are used in many prior art well logging systems to determ
Odom Richard C.
Tiller Donald E.
Wilson Robert D.
Computalog U.S.A, Inc.
McCollum Patrick H.
Moran Timothy
Porta David
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