Radiation sensor

A radiation sensor includes a radiation source, at least two scintillators having distinguishable scintillation-light-emission-decay characteristics, and shielding configured to provide radiation-paths of different shielding between the source and the at least two scintillators. The shielding is configured such that the scintillators are primarily sensitive to radiation passing through different paths. The signals produced from scintillation events can be sorted by the scintillator that is the source of the event. A source-scintillator-based comparison of the signals may be used to provide information about the material in one or more of the radiation paths. The sensor may be disposed within a borehole to measure the density of the fluid in the borehole.

BACKGROUND

This invention pertains generally to technology for sensing and measuring electromagnetic radiation (photons) using scintillator devices. More particularly, the invention pertains to a configuration of scintillation material, shielding, and radiation source that may be used to measure properties of material in or around a wellbore (borehole). For example, the invention may be used to efficiently measure the density of fluid in the borehole when deployed in a wireline or other logging tool.

Prior-art systems for the measurement of fluid density in producing oil and gas wells are based on a sensor with a radio-isotope source such as241Am. Radiation from the source traverses the borehole fluid in an open sample cell and the counts received by a detector on the opposing end of the sample cell are dependent on the fluid density. In these prior-art systems the use of radio-isotopes cause concerns for health and safety of personnel and security of radio-isotopes.

Miniature x-ray tubes are viable alternate radiation sources if the output of the tube can be accurately measured. In laboratory and industrial applications, a separate output-monitor detector is added to monitor tube output. Use of a separate output-monitor detector for in situ measurements in a borehole is problematic due to space constraints that constrain sensors to a small diameter and long aspect.

SUMMARY

This invention uses the geometry of the sample cell to focus certain radiation paths on certain parts of a multi-faceted detector. Each facet of the detector is a certain scintillation crystal with a specific emission time, thus the light received by a photomultiplier tube can be sorted by species and related to a specific path of the radiation. Depending on sample cell design and detector faceting, the invention can be configured to measure: tube output, transmission through the wellbore fluid, and back-scattering of radiation by the wellbore fluid surrounding the sensor.

In one aspect of the invention, a photon sensor includes at least two scintillators (e.g., YSO, CaF2), each having scintillation-light-time characteristics distinguishable from scintillation-light-time characteristics of the other scintillators. These scintillators are each coupled to a common photodetector (e.g., photomultiplier tube). The signals generated by the photodetector (and support electronics) have timing characteristics reflective of the scintillation-light-time characteristics and are thereby distinguishable. The photon sensor further includes shielding configured to enhance the sensitivity of the scintillators to photons travelling along certain paths as compared to photons traveling other paths. Different scintillators are sensitive to different paths. The photon sensor further includes a pulse-shape discriminator capable of distinguishing signals based on the scintillator in which the signal originates.

In another aspect of the invention, a fluid-density logging tool includes at least two scintillators (e.g., YSO, CaF2), each having scintillation-light-time characteristics distinguishable from scintillation-light-time characteristics of the other scintillators. These scintillators are each coupled to a common photodetector (e.g., photomultiplier tube). The signals generated by the photodetector (and support electronics) have timing characteristics reflective of the scintillation-light-time characteristics and are thereby distinguishable. The logging tool further includes a means for enhancing the sensitivity of the different scintillators to different radiation paths. At least one scintillator is sensitive to a path through material of a known density and at least one scintillator is sensitive to a path through material of an unknown density. The logging tool further includes a means for distinguishing signals based on the scintillator in which the signal originates.

In another aspect of the invention, a method for determining the density of a material includes detecting photons with at least one of multiple scintillators, each scintillator having a characteristic scintillation-light-decay time that is distinguishable from the characteristic scintillation-light-decay time of the other scintillators. The method further includes determining which scintillator detected the photon based on the characteristic scintillation-light-decay time. The method further includes combining the number of events detected based on the detecting scintillator (e.g., dividing the number of events in one scintillator by the number of events in a second scintillator). The method further includes applying a calibration to the combination to provide a measure of the density of the material through which the photons detected by at least one of the scintillators passed.

DETAILED DESCRIPTION

In the summary above, and in the description below, reference is made to particular features of the invention in the context of exemplary embodiments of the invention. The features are described in the context of the exemplary embodiments to facilitate understanding. But the invention is not limited to the exemplary embodiments. And the features are not limited to the embodiments by which they are described. The invention provides a number of inventive features which can be combined in many ways, and the invention can be embodied in a wide variety of contexts. Unless expressly set forth as an essential feature of the invention, a feature of a particular embodiment should not be read into the claims unless expressly recited in a claim.

Except as explicitly defined otherwise, the words and phrases used herein, including terms used in the claims, carry the same meaning they carry to one of ordinary skill in the art as ordinarily used in the art.

Because one of ordinary skill in the art may best understand the structure of the invention by the function of various structural features of the invention, certain structural features may be explained or claimed with reference to the function of a feature. Unless used in the context of describing or claiming a particular inventive function (e.g., a process), reference to the function of a structural feature refers to the capability of the structural feature, not to an instance of use of the invention.

Except for claims that include language introducing a function with “means for” or “step for,” the claims are not recited in so-called means-plus-function or step-plus-function format governed by 35 U.S.C. § 112(f). Claims that include the “means for [function]” language but also recite the structure for performing the function are not means-plus-function claims governed by § 112(f). Claims that include the “step for [function]” language but also recite an act for performing the function are not step-plus-function claims governed by § 112(f).

Except as otherwise stated herein or as is otherwise clear from context, the inventive methods comprising or consisting of more than one step may be carried out without concern for the order of the steps.

The terms “comprising,” “comprises,” “including,” “includes,” “having,” “haves,” and their grammatical equivalents are used herein to mean that other components or steps are optionally present. For example, an article comprising A, B, and C includes an article having only A, B, and C as well as articles having A, B, C, and other components. And a method comprising the steps A, B, and C includes methods having only the steps A, B, and C as well as methods having the steps A, B, C, and other steps.

Terms of degree, such as “substantially,” “about,” and “roughly” are used herein to denote features that satisfy their technological purpose equivalently, though perhaps not identically, to a feature that is “exact.” For example, a component A is “substantially” perpendicular to a second component B if A and B are at an angle such as to equivalently satisfy the technological purpose of A being perpendicular to B.

Except as otherwise stated herein, or as is otherwise clear from context, the term “or” is used herein in its inclusive sense. For example, “A or B” means “A or B, or both A and B.”

Electromagnetic radiation (photons) such as x-rays and gamma-rays, as is well known in the art, interacts with material through a number of mechanisms, such as Rayleigh scattering, Compton scattering, the photoelectric effect, pair production, and photonuclear absorption. The combined effect of these interactions on a beam of photons passing through a layer of material can be represented by the following equation:
I=I0e−μρx
Here, I is the intensity of the photon beam exiting the material, I0is the intensity of the photon beam incident on the material, μ is the mass-attenuation coefficient of the material, ρ is the density of the material, and x is the thickness of the layer of material. The mass-attenuation coefficient is a function of the material and of the energy of the incident photon. For materials consisting of homogenous mixtures or compounds of atomic constituents, the mass-attenuation coefficient of the mixture or compound is the weighted sum of the mass-attenuation coefficients of the constituents:
=
Here, wiis the fraction by weight of the ithconstituent and is the mass-attenuation coefficient of the ithconstituent. Thus, a measure of the attenuation of a photon beam travelling through a material provides information about the density and makeup of the material:

Electromagnetic radiation such as x-rays or gamma-rays, as is well known in the art, may be provided by a chemical source or by a generator. Chemical sources provide the photons through radioactive decay of an isotope. The intensity and energy distribution of the photons is a function of the isotope. Generators, typically comprising a tube and control circuitry, provide photons by accelerating a charged particle (e.g., an electron) through an electric potential (a voltage difference) to collide with a target in the tube. The kinetic energy of the charged particle is converted in part to photonic energy. The intensity of the photon beam produced at the target is a function of the number of accelerated charged particles (the current) and the maximum energy of the photons in the beam is a function of the potential through which the particles are accelerated (the voltage). Thus, by controlling the current and voltage of the generator, an operator may control the intensity and energy distribution of the photon beam.

Scintillators, as is well known in the art, are materials that produce light when excited by ionizing radiation. For example, ionizing electromagnetic radiation such as x-ray or gamma-ray photons interact with the material to place the material into an excited energy state. Deexcitation of the material produces light (e.g., photons in the visible or near-visible range). The characteristics of the excitation and deexcitation process, and thus the characteristics of the light emission, depend in part on the scintillation material. For example, the amount of scintillation light produced by the scintillator per unit of energy deposited by the ionizing radiation varies from scintillator material to scintillator material. Similarly, the time variance of the scintillation light produced by the scintillator varies from scintillator material to scintillator material. For many materials, this time variance may be approximated as short period of excitation followed by a longer period in which the intensity (amount) of scintillation light produced decays exponentially:

IS⁡(t)=IS⁢_⁢0⁡(e-tτD-etτB)
Here, Is(t) indicates the intensity of scintillation output as a function of time (t), IS_0indicates the maximum intensity, τDindicates the characteristic light-decay time, and τBindicates the characteristic excitation time. Thus, a scintillator may be distinguished from another scintillator based on the shape or width of the scintillation light pulse. Common inorganic scintillators include, for example, NaI(Tl), CsI(Tl), BGO, CaF2(Eu), BaF2, YAP(Ce), YSO(Ce), LYSO(Ce), LSO(Ce), and LaCl3(Ce). There are also a variety of organic scintillators.

Electronic photodetectors, or light sensors, as is well-known in the art, are devices that convert light (photons in the visible or near-visible range) into electronic signals (e.g., voltage or current pulses). Photomultiplier tubes and photodiodes are examples of electronic photodetectors.

Pulse-shape discriminators (PSDs), including pulse-width discriminators (PWDs), refer to a well-known class of signal-processing structures. These structures may be implemented in hardware or software or some combination thereof. These structures use timing characteristics of a signal, such as rise time, decay time, and width, to categorize a signal. For example, a pulse-shape discriminator may categorize all signals according to the duration of the signal's decay tail. And a pulse-width discriminator may categorize signals according to the full width at half maximum of the signal. Signals with a common timing characteristic falling within a particular range of durations are considered of the same category.

Single-channel analyzers (SCAs) and multi-channel analyzers (MCAs) refer to a class of well-known signal-processing structures. These structures may be implemented in hardware or software or some combination thereof. These structures use magnitude characteristics of a signal to categorize a signal. A single-channel analyzer counts all signals above a lower threshold and below and upper threshold as a single category. A multi-channel analyzer will divide the range of magnitudes within the upper and lower thresholds into multiple categories (ranges of magnitudes) and separately count the signals according to the magnitude range they fall within.

An exemplary fluid-density instrument incorporating an exemplary x-ray sensor according to an embodiment of the invention is shown inFIG. 1. The instrument is depicted attached to a wireline19and disposed in a borehole defined by a casing17and a borehole fluid15. A scintillator assembly14is optically coupled to a photomultiplier tube16awhich is electrically or magnetically coupled to pulse-processing circuitry16b. The scintillator assembly14is positioned within a section of a housing11and spaced apart from an x-ray source10b. The x-ray source10b, here an x-ray generator tube, is controlled through control circuitry10a. The x-ray source10band control circuitry10aare positioned within a section of the housing11. A measurement cage comprising a tube12and support member13is positioned between the scintillator assembly14and the x-ray source10b. The cage includes a volume between the support member13and the outside surface of the tube12that is open to borehole fluid15. The inside surface of the tube12defines a volume that excludes the borehole fluid15(but may include material of a known density such as boron carbide or nitrogen). X-rays originating at the x-ray source10btravel through the various materials defining the borehole and instrument environment. Some x-rays travel through the fluid-excluding volume of the tube12to the scintillator assembly14where they are detected. Other x-rays travel through the fluid-filled volume of the cage to the scintillator assembly14where they are detected. Measurement of characteristics of the detected x-rays (e.g., number of x-rays detected per unit time and the energy distribution of the detected x-rays) can provide information regarding characteristics of the borehole environment, such as the density of the borehole fluid.

The exemplary scintillator assembly14can be understood with reference toFIG. 2a(a view of section A-A′ ofFIG. 1) andFIG. 3(an exploded view of a variant of the assembly14). The scintillator assembly14includes a first scintillator element14aand a second scintillator element14b. The first scintillator element14ais configured in a ring shape (roughly a disk with a hole through the center) such that the second scintillator element14bfits within the hole defined by the first scintillator element14b. Scintillator assembly14is a coaxial assembly of the first and second scintillator elements14a,14b. An x-ray shield14cmay optionally be coaxially disposed between the first scintillator element14aand the second scintillator element14b.

The materials of the first and second scintillator elements14a,14bdiffer with respect to the time characteristics of the scintillation light output. The idealized plots of scintillation light intensity versus time (light pulses)31,32depicted inFIG. 3show scintillation materials that differ primarily with respect to light decay time. The “fast” scintillation material of the second scintillator element14bproduces all its scintillation light corresponding to a scintillation event (an interaction of an x-ray with the scintillation material) in a shorter period of time than does the scintillation material of the first scintillator element14a. This scintillation-time-characteristic distinction between the first scintillator element14aand the second scintillator element14ballows for distinguishing events in the first scintillator element14afrom events in the second scintillator element14bbased on light-pulse shape. Thus, as shown inFIG. 4(a partially exploded view of the coupled scintillator assembly14and photomultiplier tube16a), a common photomultiplier tube16acan be used to convert scintillation light from the assembly14to electronic signals for processing by pulse-processing circuitry16bwhile preserving the ability to identify the source of the scintillation event as either in the first scintillator element14aor the second scintillator element14b.

The materials of the tube12, support members13, and housing11are chosen and structured to shield portions of the scintillator assembly14from x-rays that travel through one path less than from x-rays that travel through different paths. For example, as depicted inFIGS. 2a, 2b, and 2c(sectional views of section A-A′, B-B′, C-C′ ofFIG. 1, respectively) andFIG. 5(a view of a portion ofFIG. 1), the housing11and tube12may be structured such that: (1) the first scintillator element14ais more sensitive to x-rays traveling through borehole fluid15than to x-rays traveling through the fluid-excluding volume12bof the tube12and (2) the second scintillator element14bis more sensitive to x-rays traveling through the fluid-excluding volume12bof the tube12than to x-rays traveling through the borehole fluid15. The support member13comprises three support-member elements13a,13b,13cthat provide the necessary structural support while allowing the borehole fluid15to occupy the space between the casing17and the outer surface of the tube12. The tube12comprises the annular material12aand the fluid-excluding volume12b. The portion11aof the housing11positioned between the source10band scintillator assembly14includes several separated longitudinally-thinner sections (along the wireline tool's longitudinal axis) that define further volumes15a,15b,15copen to borehole fluid15. The portion11bof the housing11positioned coaxially around the scintillator assembly14is of uniform radial thickness. The combination of the wall12aof the tube12, the portion11aof the housing11, and the portion11bof the housing11provides the first scintillator element14awith less shielding from x-rays that travel from the x-ray generating target10cof the source10bthrough the volume of borehole fluid15radially within the support member13volume than from x-rays that travel other paths. The same combination provides the second scintillator element14bwith less shielding from x-rays that travel from the x-ray generating target10cof the source10bthrough the fluid-excluding volume12bof the tube12than from x-rays that travel other paths. Thus, (1) events in the first scintillator element14aprimarily indicate interactions of x-rays with the borehole fluid, and (2) events in the second scintillator element14bprimarily indicate the output of the source. The rates of events (intensity) in the two scintillator elements14a,14bmay be combined to provide an indication of x-ray attenuation, and thus borehole-fluid density.

The operation of the fluid-density instrument ofFIG. 1may be understood with reference to the exemplary flow depicted inFIG. 6. When the instrument is in the desired location in the borehole, the measurement is started60. X-rays that travel from the source10bto the scintillator assembly14may interact with the scintillator assembly14to create a scintillation event in the first scintillator element14aor the second scintillator element14b. The light pulse of the scintillation event is collected62awhen the photomultiplier tube16aconverts the scintillation light pulse to an electronic signal having time characteristics reflective of those of the light pulse. The processing circuitry16b, via a pulse-width discriminator, categorizes63the signal according to its width (and thus categorizes the event as in the first scintillator element14aor the second scintillator element14b). The processing circuitry16b, via a single-channel analyzer (or, optionally, a multi-channel analyzer), then increments the appropriate counter64a,64b. A single-channel analyzer for the first scintillator element14a(the “slow” scintillation material) will count64ball events in the first scintillator element14athat also yield a pulse height within a range defined by a lower-level discrimination threshold and an upper-level discrimination threshold. A single-channel analyzer for the second scintillator element14b(the “fast” scintillation material) will count64aall events in the second scintillator element14bthat also yield a pulse height within a range defined by a lower-level discrimination threshold and an upper-level discrimination threshold. Multi-channel analyzers may be used to count events as a function of pulse height. The collection of pulses and counting of events repeats62bfor some interval of time or depth in the borehole.

When counting for a given interval is complete, the fast counts and slow counts are combined65. For example, the total count of events in the first scintillator element14amay be divided by the total count of events in the second scintillator element14bto generate a ratio. If a multi-channel analyzer is used, this ratio may be generated for various pulse-height ranges. Other combinations are possible. For example, the difference between the counts may be used. A calibration based on the borehole configuration is applied66and an answer is provided 67 for the interval. For example, the ratio of first-scintillator-element counts to second-scintillator-element counts may be linearly related to the density of the borehole fluid15, with the parameters of the linear relationship (slope and intercept) based on the inner diameter of the casing17. Other relationships between ratio and fluid density may be appropriate (e.g., a quadratic relationship, or a relationship with a first linearity over a first range of densities and a second linearity of a second range of densities). Calibrations are typically determined through a combination of computer modeling (e.g., Monte Carlo simulation of instrument response) and physical modeling (e.g., measurement of various known fluid densities in various known borehole environments). Once the appropriate calibration is applied to the ratio, the fluid density may be provided by, for example, recording or displaying the density for the interval.

Once the desired measurement is complete, the process stops69.

The flow ofFIG. 7is similar to the flow ofFIG. 6. The difference between the flows is that theFIG. 7flow allows for adjustment of the x-ray source10b. That is, the collecting step72a, categorizing step73, counting steps74a,74b, interval-check step72b, combining step75, calibration step76, and answer step77are similar to, respectively, the collecting step62a, categorizing step63, counting steps64a,64b, interval-check step62b, combining step65, calibration step66, and answer step67described with reference toFIG. 6. The difference being that theFIG. 7flow may account for and utilize control over the intensity or energy of the x-rays generated by the x-ray source10bwhile theFIG. 6flow does not. Thus, the flow ofFIG. 6is appropriate for a chemical source of x-rays and a generator-source of x-rays that is not controlled. The output of the x-ray source10bis set to a specific intensity or energy by providing the specification of current and voltage to the x-ray source control circuitry10a. Thus, the answer provided 77 in theFIG. 7flow may include specification of the intensity or energy of the x-rays generated by the source10b. For example, based on the count of events in the second scintillator element14b(the “fast” count), the intensity of x-rays produced by the source10bmay be adjusted up (e.g., to increase count rate and statistical accuracy) or down (e.g., to decrease pulse pile up due to a count rate too high for the scintillator elements or processing circuitry).

A variant of the measurement cage and shielding is shown inFIGS. 8a-8d. In this variant, the portion11a′ of the housing11positioned between the source10band scintillator assembly14includes a disk-shaped longitudinally-thinner section (along the wireline tool's longitudinal axis) that defines a volume11c′ of reduced shielding from x-rays travelling through the borehole fluid15as compared to the variant shown inFIGS. 2a-2c. And the portion11b′ of the housing11positioned coaxially around the scintillator assembly14has separate sections of radially-thinner material to reduce the shielding from x-rays travelling through the borehole as compared to the variant shown inFIGS. 2a-2c. The shielding effects are similar to that described with reference toFIGS. 2a-2c: (1) events in the first scintillator element14aprimarily indicate interactions of x-rays with the borehole fluid, and (2) events in the second scintillator element14bprimarily indicate the output of the source. The rates of events (intensity) in the two scintillator elements14a,14bmay be combined to provide an indication of x-ray attenuation, and thus borehole-fluid density.

A variant of the measurement cage, shielding, and scintillator assembly is shown inFIGS. 9a-9d. In this variant, the scintillator assembly14includes three concentric scintillator elements14a′,14b′,14d′, each with a distinctive scintillation-light-decay time (and thus each with a distinctive width of scintillation light pulse). The three elements14a′,14b′,14d′ may be separated by shields similar to the shield14cdepicted inFIG. 3. The portion11a″ of the housing11positioned between the source10band scintillator assembly14includes several separated longitudinally-thinner sections (along the wireline tool's longitudinal axis) that define further volumes15a″,15b″,15c″ open to borehole fluid15. These volumes15a″,15b″,15c″ are radially positioned so that the third scintillator element14d′ is less shielded from x-rays traveling through the borehole fluid in the measurement cage than is the first scintillator element14a′ or the second scintillator element14b′. And the portion11b″ of the housing11positioned coaxially around the scintillator assembly14has separate sections of radially-thinner material to reduce the shielding from x-rays travelling through the borehole as compared to the variant shown inFIGS. 2a-2c. Thus, (1) events in the first scintillator element14a′ primarily indicate interactions of x-rays with the borehole fluid in the annulus between the fluid-density instrument and the casing17, (2) events in the second scintillator element14b′ primarily indicate the output of the source, and (3) events in the third scintillator element14d′ primarily indicate interactions of x-rays with the borehole fluid within the measurement cage.

A variant of the measurement cage, shielding, and scintillator assembly is shown inFIGS. 10a-10d. In this variant, the scintillator assembly14includes three scintillator elements14a″,14b″,14d″, each with a distinctive scintillation-light-decay time (and thus each with a distinctive width of scintillation light pulse). The first scintillator14a″ and the third scintillator element14d″ are two halves of a ring positioned around the second scintillator element14b″. The three elements14a″,14b″,14d″ may be separated by shields. The portion11a′″ of the housing11positioned between the source10band scintillator assembly14is of a substantially uniform thickness. And the portion11b′″ of the housing11positioned coaxially around the scintillator assembly14has separate sections of radially-thinner material to reduce the shielding from x-rays travelling through the borehole as compared to the variant shown inFIGS. 2a-2c. These thinner sections are positioned to align with the azimuthal position (relative to the instrument's longitudinal axis) of the first scintillator element14a″ and the third scintillator element14d″. Thus, (1) events in the first scintillator element14a″ primarily indicate interactions of x-rays with the borehole fluid in the annulus between the fluid-density instrument and the casing17and on one side of the instrument, (2) events in the second scintillator element14b″ primarily indicate the output of the source, and (3) events in the third scintillator element14d″ primarily indicate interactions of x-rays with the borehole fluid in the annulus between the fluid-density instrument and the casing17and on the other side of the instrument. The rates of events (intensity) in the first and second scintillator elements14a″,14b″ may be combined to provide an indication of x-ray attenuation, and thus borehole-fluid density, for paths on one side of the instrument. The rates of events (intensity) in the third and second scintillator elements14d″,14b″ may be combined to provide an indication of x-ray attenuation, and thus borehole-fluid density, for paths on the other side of the instrument.

This may be useful, for example, for detecting multiphase borehole fluid (e.g., water and natural gas) when the phases separate. For instance, the instrument may be oriented in a deviated borehole such that the first scintillator element14a″ is facing down (i.e., oriented (at least partially) along the gravitational force) and the third scintillator element14d″ is facing up (i.e., oriented (at least partially) against to the gravitational force). For borehole fluid comprising natural gas and water, the natural gas may separate and rise to the upper side of the borehole and the water will fall to the lower side of the borehole. Thus, the instrument may provide separate measures of natural gas density and water density.

The operation of a three-scintillator-element sensor in a fluid-density instrument may be understood with reference to the exemplary flow depicted inFIG. 11. This flow is similar to the flow depicted inFIG. 7. The flow ofFIG. 11differs in that it discriminates among three different pulse shapes (fast, medium, slow)73′ and separately counts scintillation events for each pulse shape (and thus each of the three scintillator elements)74a′,74b′,74c′. These counts may be combined75′ in various ways (e.g., determining a ratio of slow to fast and a ratio of medium to fast) and the various combinations may, through application of calibrations76′, be used to generate information about the borehole environment (e.g., borehole fluid densities for each phase in a multiphase fluid).

An exemplary single-channel analyzer is depicted in the block diagram ofFIG. 12a. The single-channel analyzer includes two comparators121,122, an AND gate123, and a counter124. The first comparator121compares the signal from the detector (“signal,” in the figure) to an upper-level reference voltage (Vup, the upper threshold). If the amplitude of the signal exceeds Vup, then the first comparator121output (Vout_1) is a digital “high” or “1” and if the amplitude of the signal is less than Vup, then Vout_1is a digital “low” or “0.” The second comparator122compares the signal to a lower-level reference voltage (Vlow, the lower threshold). If the amplitude of the signal exceeds Vlow, then the second comparator122output (Vout_2) is a digital “high” or “1” and if the amplitude of the signal is less than Vlow, then Vout_2is a digital “low” or “0.” The AND gate123combines an inverted Vout_1with Vout_2. If Vout_2is low (the signal amplitude is below Vup) and Vout_2is high (the signal amplitude is above Vlow), then the AND gate123output (VSCA) is high, otherwise, VSCAis low. The counter124counts the number of VSCApulses in a given time interval, keeps those events that have only one count in the interval, and thus counts the number of signal pulses having a maximum amplitude that falls within the voltage “channel” defined by Vupand Vlow. The time interval is a function of the width of the detector signal.

Optionally, the upper-level comparator may be omitted and the analyzer may process all events greater than the lower-level threshold. Also optionally, counts may be accepted or rejected based on a measurement of the signal width (e.g., to be accepted, the count event must correspond to a signal having a width within a predetermined range).

Multiple single-channel analyzers (or multiple counters in a single single-channel analyzer), each with a distinct counting time interval, may be used to discriminate detector signals based on the width of the detector signal. Thus, scintillation events may be distinguished based on the scintillator that is the source of the event.

This single-channel analyzer may be equivalently implemented in software operating on digitized detector signals. As is well-known in the art, the detector signal may be digitized by an analog-to-digital converter. The digitization may include periodic sampling (e.g., every 10 ns) to capture the signal pulse amplitude at various points in time and the software may be configured to determine the maximum amplitude (e.g., by comparing sample amplitudes with other sample amplitudes to determine the highest value sample amplitude). Alternately, the digitization captures only a single sample indicative of the pulse signal's maximum amplitude. This maximum amplitude may then be compared to threshold values encoded in the software (e.g., as a software variable). If the maximum amplitude is greater than the lower-level threshold and less than the higher-level threshold, a software counter (e.g., a software variable) is incremented.

A plot of two detector signal pulses, superimposed in time solely for sake of convenience, is shownFIG. 12b. In the plot, the maximum amplitude of the lower-amplitude pulse (shown as a dashed line) is less than Vlow(shown as a dotted horizontal line). Thus, the lower-amplitude pulse will not be counted. The higher-amplitude pulse (shown as a solid line) has a maximum amplitude greater than Vlowand less than Vup(shown as a dotted horizontal line). Thus, the higher-amplitude pulse will be counted.

An exemplary multi-channel analyzer is depicted the block diagram ofFIG. 13a. The multi-channel analyzer includes three comparators131,132,133, two AND gates134,135, and two counters136,137. The first comparator131compares the signal from the detector (“signal,” in the figure) to an upper-level reference voltage (Vup). If the maximum amplitude of the signal exceeds Vup, then the first comparator131output (Vout_1) is a digital “high” or “1” and if the maximum amplitude of the signal is less than Vup, then Vout_1is a digital “low” or “0.” The second comparator132compares the signal to a mid-level reference voltage (Vmid). If the amplitude of the signal exceeds Vmid, then the second comparator132output (Vout_2) is a digital “high” or “1” and if the amplitude of the signal is less than Vmid, then Vout_2is a digital “low” or “0.” The third comparator133compares the signal to a lower-level reference voltage (\how). If the amplitude of the signal exceeds Vlow, then the third comparator133output (Vout_3) is a digital “high” or “1” and if the amplitude of the signal is less than Vlow, then Vout_3is a digital “low” or “0.” The first AND gate134combines an inverted Vout_1with Vout_2. If Vout_1is low (the signal amplitude is below Vup) and Vout_2is high (the signal amplitude is above Vmid), then the AND gate134output (VCH1) is high, otherwise, VCH1is low. The second AND gate135combines an inverted Vout_2with Vout_3. If Vout_2is low (the signal amplitude is below Vmid) and Vout_3is high (the signal amplitude is above \how), then the AND gate134output (VCH2) is high, otherwise VCH2is low. The first counter136counts the number of VCH1pulses in a given time interval, keeps those events that have only one count in the interval, and thus counts the number of signal pulses having a maximum amplitude that falls within the voltage “channel” defined by Vupand Vmid. The second counter137counts the number of VCH2pulses in a given time interval, keeps those events that have only one count in the interval, and thus counts the number of signal pulses having a maximum amplitude that falls within the voltage “channel” defined by Vmidand Vlow. The time interval is a function of the width of the detector signal. More voltage channels may be added by adding more comparators, AND gates, and counters.

Multiple multi-channel analyzers (or multiple counters in a single multi-channel analyzer), each with a distinct counting time interval, may be used to discriminate detector signals based on the width of the detector signal. Thus, scintillation events may be distinguished based on the scintillator that is the source of the event.

This multi-channel analyzer may be equivalently implemented in software operating on digitized detector signals. As is well-known in the art, the detector signal may be digitized by an analog-to-digital converter. The digitization may include periodic sampling (e.g., every 10 ns) to capture the signal pulse amplitude at various points in time and the software may be configured to determine the maximum amplitude (e.g., by comparing sample amplitudes with other sample amplitudes to determine the highest value sample amplitude). Alternately, the digitization captures only a single sample indicative of the pulse signal's maximum amplitude. This maximum amplitude may then be compared to threshold values encoded in the software (e.g., as a software variable). If the maximum amplitude is greater than the lower-level threshold and less than the mid-level threshold, a lower-channel software counter is increments. If the maximum amplitude is greater than the mid-level threshold and less than the higher-level threshold, a higher-channel software counter is incremented. More channels may be added by using more thresholds and counters.

A plot of two detector signal pulses, superimposed in time solely for sake of convenience, is shownFIG. 13b. In the plot, the maximum amplitude of the lower-amplitude pulse (shown as a dashed line) is greater than Vlow(shown as a dotted horizontal line) and less than Vmid(shown as a dotted horizontal line). Thus, the lower-amplitude pulse will be counted in the lower channel. The higher-amplitude pulse (shown as a solid line) has a maximum amplitude greater than Vmidand less than Vup(shown as a dotted horizontal line). Thus, the higher-amplitude pulse will be counted in the higher channel.

An exemplary signal-processing circuit is depicted inFIG. 14. The detector signal (“signal,” in the figure) is provided as input to two comparators141,142and an analog-to-digital converter (ADC)146. An upper-level threshold voltage (Vup) is provided as input to the first comparator141and a lower-level threshold voltage (Vim) is provided as input to the second comparator142. The outputs (Vout_1and Vout_2) of the comparators141,143are combined in an AND gate143as described with reference toFIG. 12above. The output of the AND gate (VSCA) is provided to a timer144that measures the duration between the low-to-high transition of VSCA(the rising edge) and the following high-to-low transition of VSCA(the falling edge). This duration, the VSCApulse width, is provided to a processor145(a digital signal processor in this example). The VSCAsignal is provided as input to the processor145as an ADC read trigger. When VSCAis high, the processor145clocks the ADC data (the digitized detector signal) into processor145memory (e.g., random access memory). (Optionally, the detector signal may be continuously digitized and read and the pulse-amplitude and pulse-width discrimination functions may be entirely implemented in the processor145.)

The processor145acquires a number of samples from the ADC146sufficient to distinguish the detector signals based on the scintillator source. For example, a fast-scintillator event may yield a detector signal with an amplitude greater than the lower threshold for around 300 ns and a slow-scintillator event may yield a detector signal with an amplitude greater than the lower threshold for around 3000 ns. In this example, 50 ADC samples at 10 ns intervals would be sufficient to distinguish the fast signal from the slow signal based on the detector-signal width. Similarly, a fast-scintillator event may yield a detector signal with a rise time of around 15 ns and a decay time of around 70 ns and a slow-scintillator event may yield a detector signal with a rise time of around 15 ns and a decay time of around 900 ns. In this example, 120 ADC samples at 10 ns intervals would be sufficient to distinguish the fast signal from the slow signal based on the detector-signal decay time.

The processor categorizes the digitized detector signal according to time characteristics of the signal, such as width, rise time, or decay time. For example, given the set of ADC samples (voltage/time pairs) from a signal, a measure of the signal width (e.g., full-width-at-half-maximum) can be provided by determining the number of samples, n, between voltages at some predetermined level (e.g., half the maximum amplitude) and knowing the sampling interval, T:
width=(n+1)×T.
Once the width is determined, the signal can be associated with a scintillator and thus the locus of the signal event is determined. (It may be sufficient to determine that the signal width is simply greater than some value to determine that the signal is sourced in the slowest scintillator in the instrument. Thus, it may not be necessary to have ADC samples spanning the entire duration of the signal.) The decay time of the signal may also be used to identify the source scintillator. This may be determined by applying a linear-least-squares fit to the natural log of the ADC samples of the signal tail and assuming that the detector decays exponentially. And the processor may simply use the information provided by timer144as a measure of pulse width used to correlate a signal with an event in a specific scintillator.

The signal-processing circuit ofFIG. 14has the timer144and ADC146as external to the processor145. These components may equivalently be integrated into the processor145. Likewise, the ADC is depicted as read into memory integrated into the processor145but may equivalently be read into memory external to the processor145.

An exemplary processor145processing flow is depicted inFIG. 15. The flow is run (or continued) at the start150. The read trigger line is monitored151to determine when to read the ADC to capture a detector event. (Equivalently, the read trigger may generate an interrupt.) When triggered, the ADC values are read into processor memory152until some determined number of samples are collected152a(this number is based on the range of detector-signal widths for the instrument and the ADC sampling interval). The SCA pulse width provided by a timer is read153and compared153awith the acceptable range of pulse widths for the instrument (this range is based on the range of detector-signal widths for the instrument). Once the ADC samples are collected and the SCA pulse width is determined to be within the acceptable range, the decay rate of the detector signal is determined through a curve fit154. For example, a straight line may be fit to the natural logarithm of the ADC voltage samples of the tail through a linear least squares fitting procedure. The slope of this line is related to the detector-signal decay. The derived detector-signal decay is compared to the predetermined values for the scintillators155,156and the appropriate counter is incremented155a,156a. This process continues until stopped.

FIGS. 16a-16ddepict exemplary timing diagrams for the multi-channel analyzer depicted inFIG. 13with a counting interval of 400 ns from the start of the detector signal. This counting interval may begin when the detector signal reaches the lower-level threshold (Vlow), i.e., Vout_3is high, but the inputs to the counter136,137may be delayed so that the interval effectively begins at the start of the detector signal. InFIG. 16a, the detector signal161anever reaches the lower-level threshold (Vlow). Therefore, no count is recorded in either channel. InFIG. 16b, the detector signal161breaches the lower-level threshold but never exceeds the mid-level threshold (Vmid). Therefore, the MCA generates one VCH2pulse162b(long-dashed line) and the event is recorded in the MCA's lower-energy channel. InFIG. 16c, the detector signal161creaches the mid-level threshold but never exceeds the upper-level threshold (Vup). Therefore, the MCA generates one VCH1pulse163c(solid line) and two VCH2pulses162c(long-dashed line) and the event is recorded in the MCA's upper-energy channel. InFIG. 16d, the detector signal161dexceeds the upper-level threshold. Therefore, and MCA generates two VCH1pulses163d(solid line) and two VCH2pulses162d(long-dashed line) and the event is not recorded in either channel.

FIG. 17adepicts an exemplary timing diagram for the single-channel analyzer depicted inFIG. 12, or equivalently, the front end to the signal processing circuit depicted inFIG. 14.FIG. 17bdepicts an exemplary straight-line linear-least-square fit to the tail of a digitized detector signal to determine the detector-signal decay (and thereby determine in which scintillator the event occurred). InFIG. 17a, the detector signal171reaches the lower-level threshold (Vlow) but never exceeds the upper-level threshold (Vup). Therefore, the SCA generates one VSCApulse172(solid line). For theFIG. 12SCA, the event is counted in counter124. For theFIG. 14front end, the width of the VSCApulse172(the time from the rising edge to the falling edge) is determined by timer144to be read by processor145. InFIG. 17b, the natural logarithm of the tail of the digitized detector signal voltage171′ (square markers) is fit with a straight line173according to a linear-least-squares approach. For the idealized detector signal depicted inFIGS. 17aand 17b, the slope (−0.005) corresponds to a decay time of 200 ns (1/0.005).

FIGS. 18a-18bdepict exemplary detector responses for an exemplary two-scintillator fluid-density instrument. Here, the slow scintillator (the outer ring scintillator14ainFIG. 2) is CaF2and the fast scintillator (the inner cylinder14binFIG. 2) is YSO.FIG. 18adepicts the counts in a given time interval (equivalently, count rate) for the scintillators as a function of the density of the fluid15inFIG. 1. The long-dashed line181is the CaF2response. The solid line182is the YSO response.FIG. 18bdepicts the ratio183of CaF2to YSO counts as a function of fluid density. Note that for this instrument configuration there is ambiguity in determining fluid density based on either individual detector response because the curves are double valued at some points (two fluid densities associated with a given count). This ambiguity vanishes when the ratio of the CaF2to YSO counts is considered. Thus, the fluid-density measurement is improved by being able to distinguish the events based on the scintillator-source of the event.

While the foregoing description is directed to the preferred embodiments of the invention, other and further embodiments of the invention will be apparent to those skilled in the art and may be made without departing from the basic scope of the invention. And features described with reference to one embodiment may be combined with other embodiments, even if not explicitly stated above, without departing from the scope of the invention. For example, the invention is not necessarily limited to tools deployed via wireline and is not necessarily limited to measuring borehole-fluid density. Rather, the invention may be incorporated in instruments that are deployed by, for example, slick line, tubing, or drill string. And the invention may be configured to provide a measure of the attenuation of photons through a path passing through other than borehole fluid (e.g., a path through the casing or formation outside the casing). The scope of the invention is defined by the claims which follow.