EP2452206A2 - Détection de rayonnements au moyen d un scintillateur - Google Patents

Détection de rayonnements au moyen d un scintillateur

Info

Publication number
EP2452206A2
EP2452206A2 EP10797891A EP10797891A EP2452206A2 EP 2452206 A2 EP2452206 A2 EP 2452206A2 EP 10797891 A EP10797891 A EP 10797891A EP 10797891 A EP10797891 A EP 10797891A EP 2452206 A2 EP2452206 A2 EP 2452206A2
Authority
EP
European Patent Office
Prior art keywords
scintillator material
response function
aspects
matrix
derivative
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP10797891A
Other languages
German (de)
English (en)
Other versions
EP2452206A4 (fr
Inventor
Markus Berheide
James A. Grau
Bradley Albert Roscoe
Christian Stoller
Peter Wraight
Matthieu Simon
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Services Petroliers Schlumberger SA
Gemalto Terminals Ltd
Schlumberger Holdings Ltd
Prad Research and Development Ltd
Schlumberger Technology BV
Original Assignee
Services Petroliers Schlumberger SA
Gemalto Terminals Ltd
Schlumberger Holdings Ltd
Prad Research and Development Ltd
Schlumberger Technology BV
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Services Petroliers Schlumberger SA, Gemalto Terminals Ltd, Schlumberger Holdings Ltd, Prad Research and Development Ltd, Schlumberger Technology BV filed Critical Services Petroliers Schlumberger SA
Publication of EP2452206A2 publication Critical patent/EP2452206A2/fr
Publication of EP2452206A4 publication Critical patent/EP2452206A4/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
    • G01T1/16—Measuring radiation intensity
    • G01T1/20—Measuring radiation intensity with scintillation detectors
    • G01T1/202—Measuring radiation intensity with scintillation detectors the detector being a crystal
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V5/00—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity
    • G01V5/04—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity specially adapted for well-logging

Definitions

  • This patent specification relates to improved scintillator based radiation detection. More particularly, this patent specification relates to methods and systems for using improved energy calibration and resolution monitoring using intrinsic radiation sources.
  • Scintillation detectors featuring a scintillator crystal and a photodetector are widely used different industries, and in particular in the field of oilfield services.
  • a common problem in the use of scintillation detectors for nuclear spectroscopy or similar energy sensitive measurements is that the detector response function changes for example with changing environmental conditions.
  • the sensitivity of the photodetector element will vary with time (drift) and with changing environmental conditions such as temperature and magnetic fields.
  • the gain of the detectors can be stabilized by using a control circuit that keeps the peak of an external stabilization source signal in the same channel of the multi-channel analyzer.
  • a disadvantage of this method is the need to supply an external stabilization source. Additionally, the external source is often only irradiating part of the crystal which may not give average results. Other techniques are used were the stabilization is based on a measured signal of an external radiation source. Such an external source may not primarily be used for stabilization. Such techniques may use thresholds, windows ratios, or more complex algorithms.
  • the disadvantage is that the radiation may be weak or absent at least part of the time which can result in stabilization loss in particular with changing source strength.
  • Scintillator materials are widely used to build detectors for measuring X- ray and ⁇ -radiation. Dense materials with high atomic numbers are preferred to measure ⁇ -rays, since the stopping power of the materials increases with these parameters and thus the size of the detector can be reduced without loss of sensitivity.
  • many of the heavier scintillator materials have an intrinsic background radioactivity due to the presence of radioactive isotopes in the heavier elements of the crystal matrix.
  • Lutetium has been found to be a valuable constituent in scintillator materials, but suffers from the presence of a radioactive isotope. In large detectors this background count rate might contribute significantly to the maximal achievable count rate and thus negatively affect the precision and accuracy of the measurement.
  • lutetium oxyorthosilicate has been established as a useful scintillator for medical imaging, but its intrinsic radioactivity affects the count rate in large scintillator crystals.
  • LuAP:Ce and LuAG:Pr have been used as matrix materials for scintillators.
  • Typical intrinsic count rates for material containing a large fraction of Lu are around a few hundred counts per second per cubic centimeter (cm-3s-l).
  • a 2" x 4" crystal contains about 200 cm 3 of material, so that the count rate reaches around 50,000 s "1 . This is about 5-10% of the count rate capability of a fast conventional detector and thus creates a loss in statistical precision of several %.
  • Intrinsic radioactivity is therefore conventionally regarded as a disturbance.
  • a system for the downhole detection of nuclear radiation includes a tool body adapted to be deployed in a wellbore and a scintillator material that intrinsically generates radiation.
  • the scintillator material is mounted within the tool body.
  • a photodetection system is coupled to scintillator material, and mounted within the tool body.
  • the photodetection system is adapted to generate electrical signals based on light emitted from the scintillator material according to a response function.
  • a processing system is adapted and programmed to receive the electrical signals and determine one or more aspects of the response function, for example, that are susceptible to variations due to changing downhole environmental conditions, based on electrical signals from the intrinsically generated radiation.
  • the determined aspects of the response function can include energy or variation of resolution of the radiation detector system at a given incident energy. Additionally, gain, offset and/or non-linearity may be parameters describing such aspects of the response function of the radiation detector system. Some embodiments may look at even higher order parameters of the response function.
  • the processing system can be programmed to calculate based on the electrical signals a spectrum, and a derivative of the spectrum. Based on the calculated derivative, the processing system can monitor energy resolution and/or make an energy calibration.
  • the derivative can be a first, second or higher order derivative.
  • the scintillator material is a matrix a substantial part of which is an element that contains one or more naturally occurring radioactive isotopes.
  • a substantial part of the scintillator material matrix is one or more metallic elements, preferably rare-earth elements, more preferably, lanthanides.
  • the scintillator material is Lutetium aluminate essentially in the perovskite or garnet phases.
  • the scintillator material may be doped with an activator such as Cerium or Praseodymium.
  • the tool body is adapted to be deployed as part of a drilling operation.
  • FIG. 1 is a block diagram of a gamma-ray spectroscopy system in accordance with some embodiments
  • Fig. 2 is a spectrum chart showing spectra for a PR:LuAG crystal
  • FIG. 3 is a flowchart describing a technique for stabilising gain of a gamma-ray spectroscopy system, according to some embodiments
  • Fig. 4 is a chart showing a differentiated spectrum, according to some embodiments.
  • Fig. 5 is a chart showing differentiated spectrum smoothed using a smoothing filter, according to some embodiments.
  • Fig. 6 is a chart showing an example of a fit function used to approximate the peak in a differentiated spectrum.
  • individual embodiments may be described as a process which is depicted as a flowchart, a flow diagram, a data flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process may be terminated when its operations are completed, but could have additional steps not discussed or included in a figure. Furthermore, not all operations in any particularly described process may occur in all embodiments. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc. When a process corresponds to a function, its termination corresponds to a return of the function to the calling function or the main function.
  • embodiments of the subject matter disclosed in the application may be implemented, at least in part, either manually or automatically.
  • Manual or automatic implementations may be executed, or at least assisted, through the use of machines, hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof.
  • the program code or code segments to perform the necessary tasks may be stored in a machine readable medium.
  • a processor(s) may perform the necessary tasks.
  • features in a spectrum associated with a scintillation material's intrinsic radioactive decay is used for the determination of one or more parameter's of the detector response function.
  • An advantage of some such embodiments is that the gain of the detector can be stabilized without an external stabilization source and in absence of any other external sources of radiation.
  • An additional benefit of some embodiments is that the intrinsic material is evenly distributed within the detector material and therefore is not affected by source movement with respect to the crystal. Furthermore, according to some embodiments, it is possible to monitor the resolution of the detector over time without using an external source. In some cases, the internal spectrum contains more than one feature. In this case, according to some embodiments, multiple parameters such as but not limited to gain and/or offset and/or non-linearity can be determined at the same time. In certain cases, an estimate of higher order terms in the energy calibration and detector resolution can be provided.
  • Fig. 1 is a block diagram of a gamma-ray spectroscopy system in accordance with some embodiments.
  • Fig. 1 illustrates a gamma-ray spectroscopy system 110 configured for use in nuclear well logging operations.
  • the gamma-ray spectroscopy system 110 may provide spectroscopic analysis of gamma-rays or x-rays from a surrounding geological formation or borehole to determine, among other things, a general composition of the formation.
  • the gamma-ray spectroscopy system 110 may employ a scintillator 112 having a natural radioactivity. Using techniques described below, the gamma-ray spectroscopy system 110 may stabilize the gain of the system using the natural radioactivity of the scintillator 112.
  • the scintillator 112 may represent any scintillator having a natural radioactivity.
  • the scintillator 112 may represent, for example, a scintillator based at least in part on Lutetium Silicate (LSO), or Lutetium Aluminum Perovskite (LuAP), or Lutetium Aluminum Garnet (LuAG), or Lanthanum Bromide (LaBr 3 ) or Lanthanum Chloride (LaCl 3 ).
  • LSO Lutetium Silicate
  • LiAP Lutetium Aluminum Perovskite
  • LuAG Lutetium Aluminum Garnet
  • LaBr 3 Lanthanum Bromide
  • LaCl 3 Lanthanum Chloride
  • the scintillator 112 may represent any other scintillator containing a naturally occurring radioactive isotope such as, for example, Bismuth Germanium Oxide (BGO) containing 207 Bi. According to some embodiments the scintillator 112 is a pure crystal such as undoped BGO. According to some other embodiments the scintillator material such as LuAP or LuAG is doped with a material such as Cerium, Praseodymium or other similar activators.
  • BGO Bismuth Germanium Oxide
  • LuAP or LuAG is doped with a material such as Cerium, Praseodymium or other similar activators.
  • the scintillator material can be of a common type, for example an oxide or a halide (e.g. containing Cl, Br, I), which is additionally doped with a radioactive material.
  • a radioactive doped scintillators can suffer from disadvantages, such as non-uniform distribution of the radioactive material. It may also negatively impact the luminescence properties of the scintillator material.
  • the scintillator material rather than doping with a radioactive material, contains an element that is substantially part of the scintillator material matrix and incidentally also contains a fraction of naturally occurring radioactive isotopes.
  • the scintillator material is selected from the group containing metallic elements. According to some more preferred embodiments, the scintillator material is selected from materials containing rare-earth elements. Even more preferably, the scintillator material is selected from materials containing lanthanides.
  • the energy deposited by the gamma-ray may be converted into light and received by a photodetector such as a photomultiplier 114 or any other device suitable for converting light into an electrical signal like an avalanche photodiode (APD).
  • a photodetector such as a photomultiplier 114 or any other device suitable for converting light into an electrical signal like an avalanche photodiode (APD).
  • APD avalanche photodiode
  • Gamma-rays detected by the scintillator 112 may arise from external radiation or from the internal radioactivity of the scintillator 112.
  • an external reference source of radiation may be avoided for the purpose of stabilizing the gain of the gamma-ray spectroscopy system 110.
  • the source of radioactivity within the scintillator 112 may be uniformly distributed throughout the scintillator 112. As such, the corresponding response of the scintillator 112 to the internal radiation source may be insensitive to non
  • the photomultiplier 114 may convert the light from the scintillator 112 into an electrical signal 116.
  • the gamma- ray spectroscopy system 110 may alternatively employ multi-channel plate multipliers, channeltrons, or solid state devices such as Avalanche Photo Diodes in lieu of the photomultiplier 114.
  • the electrical signal 116 may be amplified by amplification circuitry 118, which may provide an amplified signal 120 to signal processing circuitry 122.
  • the signal processing circuitry 122 may include a general or special-purpose processor, such as a microprocessor or field programmable gate array, and may perform a spectroscopic analysis of the electrical signal, which may include the gain stabilization techniques described herein.
  • the signal processing circuitry 122 may additionally include a memory device or a machine-readable medium such as Flash memory, EEPROM, ROM, CD-ROM or other optical data storage media, or any other storage medium that may store data or instructions for carrying out the following techniques.
  • the signal processing circuitry 122 may stabilize the gain of the amplified signal 120. Stabilizing the gain of the amplified signal 120 may ensure a consistent gain across variable conditions, such as variances in temperature or the age of the gamma-ray spectroscopy system 110, i.e. the electrical signal will have the same pulse height for a given amount of energy deposited in the scintillation crystal independent of temperature, age, detector count rate and other factors that can affect the total gain of the system.
  • the gain stabilization approaches employed by the signal processing circuitry 122 may rely not on an external radiation source, but rather the natural radioactivity of the scintillator 112.
  • the scintillator 112 may include a naturally radioactive material that may serve as a reference source of radiation.
  • the scintillator 112 may be a Lutetium Aluminum Perovskite (LuAP) or Lutetium Aluminum Garnet (LuAG) scintillator.
  • the LuAP (or LuAG) scintillator may have a natural radioactivity as a certain isotope of Lutetium decays within the LuAP (or LuAG) scintillator.
  • the decay of the Lutetium generates beta and gamma radiation that may interact with the scintillator 112 to generate a corresponding scintillation signal, and the resulting energy spectrum may be used to stabilize the gain of the gamma-ray spectroscopy system 110.
  • LuAP and LuAG are non-hygroscopic, and have very high stopping power due to their high density and effective Z. Additionally, LuAP and LuAG have excellent temperature characteristics and show very little loss (or even gain) of light output with temperature.
  • Fig. 2 is a spectrum chart showing spectra for a PR:LuAG crystal.
  • a 100s spectrum 210 of a 50mmxl00mm PnLuAG crystal overlaid with a spectrum 212 that has undergone a 5% gain shift.
  • Fig. 3 is a flowchart describing a technique for stabilising gain of a gamma-ray spectroscopy system, according to some embodiments.
  • a processing system is used to compare a previously recorded 'standard' spectrum SPCa with a gain (G) and/or offset (O) corrected measured spectrum SPCb(G,O).
  • SPCb(G,O) indicates symbolically that the original shape of the measured spectrum SPCb is modified by applying gain and offset and thus the resulting spectrum SPCb(G,O) is a function of these two parameters.
  • a 'standard' spectrum is generated using a much longer acquisition to minimize statistical errors.
  • a current spectrum is measured, which is not yet gain or offset corrected.
  • the spectrum is then is normalized to the acquisition time of the measured spectrum. This normalization makes use of the fact that the intrinsic activity is constant over time within the limits of statistical variation.
  • a residual res is then be computed between the 'standard' spectrum and the current measured spectrum where a gain and/or offset is applied to the measured spectrum.
  • step 318 the gain and/or offset corrections are determined.
  • a minimization routine would be applied to determine the gain (Gmin) and possibly also offset (Offmin) at which the residual is minimal.
  • step 320 the gain is then be stabilized with the methods known from prior art.
  • the interfering radiation remains essentially constant over a time period that is significantly longer then the time used to update the stabilization circuit (which is typically seconds), such that the described minimization technique can be carried out.
  • the measured radiation is described by a set of standards, and the optimization is extended to a set of gains and relative amplitudes of the standards.
  • the differentiated spectrum of the internal radiation is used. Differentiation is a very simple mathematical process, which can easily be handled in most acquisition systems. The resulting differentiated spectrum may have more prominent and localized features than the original spectrum.
  • Fig. 4 is a chart showing a differentiated spectrum, according to some embodiments.
  • Spectrum 410 is a differentiated spectrum computed from spectrum 210 in Fig. 2.
  • the differentiated spectrum 410 of this PnLuAG detector show three prominent peaks 412, 414 and 416.
  • the detector gain stabilization uses any of the peaks 412, 414 and 416, in combination with a known gains stabilization technique for stabilization sources.
  • An advantage of using a differentiated spectrum is that the peaks can be easily separated from other spectral features that may interfere with the intrinsic background. Note that a full energy or escape peak from a gamma source would show up as a bipolar peak in the differentiated spectrum and a Compton edge appears as a negative peak. According to some embodiments, higher order differentials are used, such as second derivative or third derivative spectra.
  • Fig. 5 is a chart showing differentiated spectrum smoothed using a smoothing filter, according to some embodiments. The effect of 1 A - 1 A - 1 A filtering on the differentiated spectrum 410 of Fig. 4 is shown in the smoothed spectrum 510 of Fig. 5.
  • the internal spectrum from the intrinsic radiation of the scintillator material can also be used to accurately monitor the intrinsic detector resolution. This can be important for some measurement applications. For example in geochemical logging, environmental parameters such as temperature have an impact on the detector response function and indirectly influence the accuracy of the determined elemental concentrations.
  • Fig. 6 is a chart showing an example of a fit function used to approximate the peak in a differentiated spectrum.
  • the data points, shown as "+" signs, such as mark 610 are the differentiated spectrum 510 shown in Fig. 5.
  • the fit curve 612 is shown as resulting from a fit function.
  • the width of the fit curve 612 is used to estimate a detector resolution.
  • the variation of resolution with peak position is estimated.
  • the estimated variation is used to determine problems such as noise or deterioration of the detector.
  • a more complex intrinsic spectrum is used to determine a non-linear energy calibration.
  • the differentiated spectrum 410 of the PnLuAG detector in Fig. 4 shows three prominent peaks 412, 414 and 416.
  • the detector gain stabilization uses all of the peaks 412, 414 and 414 and optimizes the parameters gain offset and non-linearity until a measured spectrum is best fitted to a previously determined reference spectrum. Since differentiation is a very simple mathematical process, which can easily be handled in most acquisition systems, the use as described herein are particularly well- suited for downhole applications where processing power may be very limited. For example the described techniques are particularly well-suited for applications carried out during and as part of a drilling operation such as MWD and LWD operations.
  • a second derivative of the spectrum is combined with a search for roots based on linear fitting around the intersections of the second derivative with the axis.
  • Such root finding algorithms are known in the art. For example they may search the function for sign changes and then do a local linear interpolation of the data which gives a first order approximation of the root value.
  • the intrinsic radioactivity of certain scintillator materials in a radiation detector is used as a count rate reference or 'intrinsic pulser' for the apparatus containing the radiation detector.
  • Using the scintillator material's intrinsic radioactivity as a count rate reference thus avoids some disadvantages associated with an external count rate reference.
  • the count rate reference can be used to measure a number of properties of the detection system.
  • the count rate reference or intrinsic pulser is used for testing functionality of the system without adding additional parts, such as an electronic pulser, or other extrinsic source.
  • the count rate reference or intrinsic pulser is used for precision dead time corrections. This is possible because the intrinsic count rate is perfectly random, but overall very stable in energy and average count rate with only the statistical spread from the total number of decay events in a given time period. With a large detector delivering an average count rate of about 50,000 s "1 one has to acquire only 20s of data to get to a statistical precision of 0.1%.
  • the internal radiation can be used as input for pileup simulation. The randomness of the intrinsic events makes it equivalent to random external radiation events resulting in real pileup events.
  • corrections for pileup are based on characterization measurements made in a calibration facility. Characterization measurements could include variable count rate by changing external source strength, controlled variations in the environment, etc.
  • An advantage over using a conventional external source as a count rate reference is that the intrinsic radioactivity is uniformly distributed throughout the system and therefore the count rate is produced at the same location as the signal. Therefore there are no geometrical or shielding effects that would change the count rate in the system.
  • the dead time of the system under exposure to external radiation can be estimated from a comparison with dead time of a reference spectrum of the intrinsically generated radiation.
  • the comparison may be based on evaluating count rates in different regions of the spectrum. For example, if only peaks need to be clearly distinguished, then an integration of counts over the region of each peak can be performed without any fitting.
  • the comparison is based in part on spectral fitting that includes the use of standard spectra of the intrinsically generated radiation and at least one component form an external radiation source.
  • this type of characterization of the dead time of the reference spectrum is performed on a prototype or calibration system well before performing the well log. For example it could be done during the engineering phase of product development.
  • the characterization is performed as a calibration step on each system, and can be occasionally repeated in a local workshop.

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  • Physics & Mathematics (AREA)
  • High Energy & Nuclear Physics (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • General Physics & Mathematics (AREA)
  • General Life Sciences & Earth Sciences (AREA)
  • Geophysics (AREA)
  • Chemical & Material Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Health & Medical Sciences (AREA)
  • Molecular Biology (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Measurement Of Radiation (AREA)

Abstract

L'invention concerne des procédés et des systèmes apparentés pour la détection de rayonnements nucléaires. Le système peut comprendre un corps d’outil prévu pour être déployé dans un puits de forage et un matériau scintillateur générant intrinsèquement un rayonnement. Le matériau scintillateur est monté à l’intérieur du corps d’outil. Un système de photo-‌détection est couplé au matériau scintillateur et monté à l’intérieur du corps d’outil. Des traits caractéristiques d’un spectre associé à la désintégration radioactive intrinsèque du matériau scintillateur sont utilisés pour déterminer un ou plusieurs paramètres de la fonction de réponse du système détecteur de rayonnements.
EP10797891.8A 2009-07-10 2010-07-09 Détection de rayonnements au moyen d un scintillateur Withdrawn EP2452206A4 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US22463509P 2009-07-10 2009-07-10
PCT/US2010/041476 WO2011006038A2 (fr) 2009-07-10 2010-07-09 Détection de rayonnements au moyen d’un scintillateur

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EP2452206A2 true EP2452206A2 (fr) 2012-05-16
EP2452206A4 EP2452206A4 (fr) 2017-03-29

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CA (1) CA2743051C (fr)
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US20170168192A1 (en) * 2015-12-14 2017-06-15 Baker Hughes Incorporated Scintillation materials optimization in spectrometric detectors for downhole nuclear logging with pulsed neutron generator based tools

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Publication number Publication date
WO2011006038A3 (fr) 2011-05-05
WO2011006047A2 (fr) 2011-01-13
WO2011006038A2 (fr) 2011-01-13
WO2011006047A3 (fr) 2011-04-07
EP2452206A4 (fr) 2017-03-29
CA2743051A1 (fr) 2011-01-13
CA2743051C (fr) 2014-09-30

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