EP0886759A1 - Jauge d'epaisseur de paroi de tuyau a retrodiffusion compton employant un collimateur a concentration et un detecteur annulaire - Google Patents

Jauge d'epaisseur de paroi de tuyau a retrodiffusion compton employant un collimateur a concentration et un detecteur annulaire

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Publication number
EP0886759A1
EP0886759A1 EP97908911A EP97908911A EP0886759A1 EP 0886759 A1 EP0886759 A1 EP 0886759A1 EP 97908911 A EP97908911 A EP 97908911A EP 97908911 A EP97908911 A EP 97908911A EP 0886759 A1 EP0886759 A1 EP 0886759A1
Authority
EP
European Patent Office
Prior art keywords
detector
pipe
photons
thickness
collimator
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Withdrawn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
EP97908911A
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German (de)
English (en)
Other versions
EP0886759A4 (fr
Inventor
Robert Gould
Edward S. Kenney
Saifullah Khan
Xiangjun Xu
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.)
Penn State Research Foundation
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Penn State Research Foundation
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Publication date
Application filed by Penn State Research Foundation filed Critical Penn State Research Foundation
Publication of EP0886759A1 publication Critical patent/EP0886759A1/fr
Publication of EP0886759A4 publication Critical patent/EP0886759A4/fr
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B15/00—Measuring arrangements characterised by the use of electromagnetic waves or particle radiation, e.g. by the use of microwaves, X-rays, gamma rays or electrons
    • G01B15/02—Measuring arrangements characterised by the use of electromagnetic waves or particle radiation, e.g. by the use of microwaves, X-rays, gamma rays or electrons for measuring thickness
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
    • G21K1/00—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating
    • G21K1/02—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating using diaphragms, collimators
    • G21K1/025—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating using diaphragms, collimators using multiple collimators, e.g. Bucky screens; other devices for eliminating undesired or dispersed radiation

Definitions

  • This invention is directed toward the measure of the thickness of material using nuclear techniques. More particularly, the invention is directed toward apparatus and methods for measuring the thickness of pipe wall, from the outside of the pipe, using an annular detector, concentric conical detector collimation, and backscatter gamma radiation.
  • One common type of measurement is to place a gamma ray source on one side of the pipe wall, and a gamma ray detector on the opposite side of the pipe wall, and to measure the attenuation of transmitted gamma radiation.
  • the amount of attenuation of gamma radiation can be related to the thickness of the pipe.
  • An obvious disadvantage of this technique is that either the gamma ray source, or the gamma ray detector, must be placed within the pipe. This can be extremely impractical, and sometimes virtually impossible, if the pipe contains flowing fluids, especially when at high temperature and high pressure.
  • Other nuclear techniques have been used to measure the thickness of pipe from within the pipe.
  • Nuclear backscatter techniques have been used in the prior art to measure the wall thickness of pipe from the outside of the pipe. The most common of these systems is based upon the measure of scattered radiation at approximately 90° with respect to the incident beam. For scattering angles ranging fro 90° to 180°, Houlong. Lee in "A High-Speed Wide Aperture Compton Scatter Imaging Technique - A Computational Study With Application to a Pipe Inspection System", The Pennsylvania State University. Ph. D. Thesis, 1991 , showed that scattering probability is near a minimum at 90° and a maximum at 180°.
  • An object of the present invention is to provide apparatus and methods for measuring the thickness of pipe wall from the outside of the pipe.
  • An additional object of the invention is to provide apparatus and methods for measuring the thickness of a pipe wall while fluid is flowing within the pipe. Yet another object of the present invention is to provide apparatus which can be used to scan the wall thickness of the pipe, and to provide methodology for converting these scans into images of the thickness of the pipe wall thereby immediately exposing any dangerous wall thinning. Another object of the invention is to provide apparatus and methods for measuring wall thickness of pipe without having to remove any insulation or scuff guard on the outside of the metal pipe.
  • An additional object of the invention is to provide apparatus for which the sensitivity and spatial resolution can be varied depending upon operational conditions including whether the pipe is filled with liquid, or filled with gas, or filled with a combination of liquid and gas, or contains or does not contain outer insulation, or contains liquid of varying density.
  • Still another object of the present invention is to provide an apparatus which is relatively insensitive to positioning with respect to the wall of the pipe to be measured.
  • This disclosure is directed toward apparatus and methods for measure of the thickness of pipe wall, from the outside of the pipe, using nuclear techniques.
  • the disclosed measurement system employs a source of gamma radiation, a gamma ray detector, and Compton backscattering as the basic nuclear reaction in order to obtain a pipe thickness measurement. While Compton backscattering for thickness measurement is well known in the prior art, the present invention system is novel in the application of a unique detector and detector collimator to improve sensitivity in water filled pipes. More specifically, without the detector and detector collimation arrangement of the present invention, the measure of pipe thickness in an operating flow system is highly impractical.
  • the detector collimation can be changed, and can be removed to yield accurate thickness measurements of gas filled pipes with improved statistical precision.
  • By moving the apparatus around the periphery of the pipe and along the axis of the pipe a complete image of the pipe thickness can be generated.
  • the present invention utilizes Compton scattering at a scattering angle range near 180° for improved efficiency. It was also shown by Lee that the use of a dual detector arrangement provides excellent insensitivity to positioning errors. Such errors can dramatically affect results in a conventional Compton imaging system For even greater efficiency the invention employs a single annular scmtillator crystal. This also reduces system complexity by using a single photomultiplier to collect scintillation pulses. With only one detector element, calibration is simplified.
  • the present invention uses an annular gamma ray detector coupled to a conical detector focusing collimator.
  • the gamma ray detector is preferably a scintillation detector crystal made of thallium activated sodium iodide (Nal(TI)), bismuth germinate (BGO) or any other suitable material which can be formed into the annular geometry.
  • the system has been designed for the use of a relatively low energy gamma ray source to generate the desired backscatter
  • An isotopic source such as mercury-203 (Hg ), which emits a photon in the gamma ray energy of 279 keV, is preferred.
  • Hg mercury-203
  • photon and gamma rays will be used interchangably in this disclosure, but it should be understood that the referenced photons are within the gamma ray energy range
  • the axes of the annular gamma ray detector and the conical detector collimators are preferably coincident with the axis of a source collimator aperture, and are, therefore, perpendicular to the major axis of the pipe and to the surface of the pipe upon which the collimated beam impinges
  • the system is, therefore, configured to detect backscatter radiation at essentially 180 degrees.
  • This detector and collimator arrangement is unique, effective, and straightforward, and can be used with disclosed measurement methods to image the wall thickness of pipes containing air, steam, any type of liquid and combinations of gas and liquid. Because of the nature of Compton imaging, physical contact with the pipe wall is unnecessary and the measurement can be made outside of the pipe while fluid is flowing within the pipe. With the proper choice of detector collimator geometry, the disclosed system can image pipe walls without removing insulation or scuff guards. The wide aperture design allows acceptable counting rates to be obtained with relatively low intensity sources thereby reducing the weight of required collimation and personnel shielding and thereby reducing the overall weight of the apparatus. With the use of available robotic motion control systems, the system can be used to make automated scans or images of the wall thickness of a pipe in a matter of minutes.
  • Fig. 1 illustrates the basic Compton scatter reaction which is utilized in the invention
  • Fig. 2 shows a prior art Compton scatter measurement system
  • Fig. 3 illustrates the normalized differential scattering coefficient for Compton scattered photons, for an incident photon energy of 279 keV;
  • Fig. 4 shows the relationship between gross count rate recorded by the system detector as a function of detector position, for three different pipe wall thicknesses
  • Fig. 5 is a cross sectional view of the thickness measurement instrument
  • Fig. 6 is a perspective sectional view of the detector and detector collimator elements of the system
  • Fig. 7 is an illustration of a single backscattering by a slab of material
  • Fig. 8 illustrates the geometry used to calculate the response of the system
  • Fig. 9 shows the geometric response of the system
  • Fig. 10 is a graphical calculation of pipe thickness of bare pipe measured by the system, using a source which emits 279 keV gamma radiation;
  • Fig. 11 shows system response to insulated, empty pipe using a 279 keV gamma ray source
  • Fig. 12 shows system response to water filled pipe using a source emitting 279 keV gamma radiation
  • Fig. 13 illustrates the system and pipe geometry used in Monte Carlo calculation of the system response
  • Fig. 14 illustrates system response as a function of pipe thickness for pipes filled with varying densities of fluids, and using detector collimation and no detector collimation;
  • Fig. 15 shows pipe thickness measurements made with the system compared with known pipe thicknesses, where the pipe is both insulated and bare;
  • Fig 16 a shows the response of the system to the thickness of a pipe which is partially filled with water
  • Fig. 16b is a cross sectional illustration of the pipe partially filled with water
  • Fig. 17 illustrates the system response to the thickness of aluminum plate
  • Fig. 18 illustrates a means for conveying the measurement system around the circumference of a pipe to be gauged
  • Fig. 19 illustrates apparatus for measuring an image of pipe wall thickness
  • Fig. 20 illustrates a hypothetical pipe wall thickness image measured with the invention.
  • Equation (1) where mo is the rest mass of an electron 10, E' is the energy of the scattered photon, E ⁇ is the incident photon energy, c is the speed of light and ⁇ is the scatter angle 12.
  • Equation (1) Using equation (1), if the incident energy E ⁇ is known, and ⁇ , the detection angle 12 is fixed, returning photons at the predicted energy can be counted. If scatter angle ⁇ is set and the ratio of scattered to incident photons is compared, the thickness of an object can be measured as will be illustrated subsequently. Usually, the scattered photons at energy E' are detected using one of two techniques.
  • the first technique allows the detector to measure the gross gamma ray flux, and the flux at energy E' is spectroscopically separated from the gross flux.
  • the second technique coliimates the detector such that it only "sees" a focal point of scatter activity at the angle ⁇ , therefore (ideally) all gamma radiation detected is at a single energy, namely E' .
  • the present invention need not always employ spectroscopy thereby simplifying the system, reducing cost and weight, and increasing reliability. Simple spectroscopy with relatively simple electronics is one useful option.
  • Equation (1) and Fig. 1 show the basis for all Compton scattering measurements.
  • ⁇ 90° scattering angle
  • FIG. 5 A cross sectional schematic representation of the major elements of the measurement system is shown in Fig. 5.
  • the backscatter instrument is identified as a whole by the numeral 100.
  • a preferably Hg 203 source 14 is mounted within a source housing 40 which is affixed to a source plug 42 which can be removed from a shield 44.
  • the shield 44 is preferably lead (the preferred heavy metal) and functions primarily to shield personnel from the source 14.
  • a portion of the gamma radiation at 279 keV, emitted by the source 14 passes through a passage 48 in the shield 44 and through a source shutter 46, through an aperture 18 in the source collimator 16, which is positioned within the annulus of an annular detector 50, thereby forming a beam of incident gamma radiation indicated schematically by the arrow 15.
  • the source shutter 46 is made of gamma ray absorbing material and can be used to block the passage 48 thereby terminating the beam 15.
  • the pencil beam 15 emerges from the aperture 18 and impinges upon the pipe to be scanned.
  • a pipe 70 is illustrated with a layer of insulation 62 and a scuff guard 60.
  • the system is designed such that scatter events occurring within an inspection volume 22 will be detected by the annular detector 50.
  • the detector is preferably a scintillation crystal such as Nal(TI) or bismuth germinate (BGO) and is hermetically enclosed by a housing 51 which usually incorporates a reflecting material.
  • the annular detector preferably comprises a disk shaped crystal with a concentric, cylindrical annulus. It should be understood, however, that variations of this geometry can be employed, such as an annular detector which does not encompass the entire 360 degrees, but perhaps only 300 or 320 degrees.
  • the system as shown employs a scattering angle range near
  • a single photomultiplier tube 52 or other light sensing device is optically coupled to the detector crystal 50. Signals from the photomultiplier tube 50 are input into preferably a computer (not shown in this Figure). Power is supplied to the photomultiplier tube 52 by a suitable power supply (not shown in this Figure).
  • a focusing collimator allows the detector to sense scatters from only within a relatively small inspection volume 22 and thereby greatly increases the response of the system to pipe thickness when the pipe is filled with water.
  • the focusing collimator comprises a plurality of conical annular cones 26 which are aligned around the annular crystal 50. The apex of each cone is truncated and faces the pipe to be inspected. The base of each cone is positioned against the face of the crystal 50.
  • the function of the conical focusing collimator is unique, effective, and straightforward, as will be described in more detail in following sections.
  • the cooperating annular detector and conical cone detector collimator permits pipes containing air, steam, liquid, or a combination of gas or liquid to be effectively imaged for thickness.
  • the flexibility, accuracy and precision of the measurement system significantly advances the art of pipe thickness imaging.
  • the system has been designed for the use of a relatively low
  • 203 gamma ray energy source 14 preferably Hg with a gamma ray energy of 279 keV.
  • This low energy combined with a high efficiency detector 50 with annular geometry, allows the use of a relatively low activity source (as low as 100 milliCuries (mCi)), even with use of the detector collimator 26. These factors require minimum shielding 44 which, in turn, give rise to a very portable system weighing less than 20 pounds.
  • Fig. 6 is a perspective, cutaway view of the detector collimatmg cones 26, the detector 50, the source collimator 16 and the source aperture 18. and perhaps better illustrates the arrangement of these elements with respect to the photomultiplier tube 52 and the pipe 70.
  • the system is designed such that the gross count rate, which is greater than the count rate in a specified energy region corresponding to 180° backscattering, can be employed.
  • a gross counting rate method has several advantages when compared to the spectral measurement technique. These advantages include reduced requirements for system components and simpler data analysis. All subsequent discussion of the system response assumes that the data comprise gross count rate data from the detector 50.
  • the directional preference of a scattered photon suggests that, for high energy incident photons, more Compton scattered photons appear in the forward direction, at a scattering angle 0of less than 90 degrees, than at greater degrees of scattering. Solely from the point view of a scattering distribution, it is more efficient to set a detector to measure scatter radiation at relatively small or "forward scatter" scatter angles ⁇ . However, the coherent scattering and primary beam are also forward oriented thereby creating additional gamma rays which would be detected by the detector, and which would be considered as "noise". Stated another way, the forward scatter gamma radiation would have to be separated from these other types of radiation in order to obtain the desired results. Therefore, a measurement can be easily complicated if a detector is set up to measure the forward scattering. In addition, in many cases one-side access to a test object is preferred and even necessary as discussed previously. Therefore forward direction scatter measurements are not used in the present invention.
  • the wide-aperture detector embodiment of the present invention accepts all the radiation which reaches the detector front surface.
  • the effects of the concentric, conic collimator used in another embodiment of the invention will be discussed in great detail in subsequent sections.
  • the present invention, 5 embodied with no detector collimation will be used to better understand the geometric factors associated with the present measurement.
  • the removal of the prior art type collimator from the apparatus of the present invention has a great impact on the measuring process. A simple example can be used to demonstrate some characteristics of scatter measurements. Attention is
  • the slab 82 is illuminated by a pencil beam 15 perpendicular to the slab surface.
  • a solid angle ⁇ which the detector element 3 5 subtends from a point along the radiation beam in the slab, changes very little.
  • This saturation is determined predominantly by one property of the test material, namely, the ratio of Compton scattering to the total cross section of the tested material for a given energy.
  • This feature can be conveniently used to identify an unknown material as, for example, in the fields of oil or coal well logging, since the scatter measurement is made within a pipe surrounded by earth formation, and the physical extent of the earth formation is "infinite" with respect to the depth of investigation of the measurement. Note that the selection of photon energy becomes very important since the cross section ratio is energy dependent.
  • the electron density in the region defined by the system is proportional to the number of observed singly scattered photons, and the Compton scattering cross section is usually the dominant factor affecting recorded counts.
  • the Compton-to-attenuation ratio turns out to be a more important factor than the Compton scattering cross section alone.
  • multiple scattering will have a more dramatic influence on the measurement process.
  • the response of the wide-aperture detector used in the present invention is quite different from that of conventional prior art systems.
  • equation (4) one can calculate the saturation thickness s of a steel slab for different incident photon energies.
  • the saturation thickness is assumed to be the thickness at which the detected intensity reaches its 90 percent maximum intensity. As the incident photon energy increases, the saturation thickness increases. It can be shown, however, that the maximum backscattered photon energy is about 250 keV.
  • the saturation thickness will also reach a limit when the incident photon energy is high enough, because the backscattered photons are absorbed within the material and therefore can not reach the detector. For a very low photon energy, this effective range is too small to cover certain desired pipe thicknesses as noted in this disclosure.
  • the observed intensity of singly scattered photons as shown in equation (4) is the accumulated result of scattering and attenuation. These two effects involve the differential scattering cross section and the exponential attenuation factor through which the photon is transported.
  • the scattering term F presents the scattered photons.
  • the attenuation term for the incident beam of energy Eo has the effect of reducing the incident beam intensity along its path, while the one for scattered photons is seen to reduce the possibility of the photons being observed by the detector 80.
  • Another factor governing the detector response is the system geometry. The system's geometric influence on a measurement is implicitly expressed in the terms of integral paths and the solid angles that a detector subtends. This influence will be identified as the "geometric response" or "geometric effect”.
  • a first technique for determining material thickness using the previously described apparatus which is suggested in a simple slab geometry case shown in Fig. 7 and as discussed above, is to fully accumulate scattering information.
  • the data measured is related to the material thickness and the ratio of Compton-to-attenuation coefficients.
  • a second technique for determining material thickness using the previously described apparatus is to use the attenuation effect in the scattering process. Since scattered photons traverse various paths to the detector element 80, and these paths are relatively independent of incident beam path 15, although their origins are the primary beam path 15. These scattered photons experience attenuation as they traverse material on their way to the detector element 80.
  • This process can be seen to be comparable to conventional radiography in certain aspects, wherein photon attenuation is directly related to the density of the material being traversed by the photon.
  • the scattering medium then can be regarded as a radiation source which is activated by the incident beam 15.
  • a detector represented by the detector element 80 in Fig. 7, can then be used to observe the attenuated photons arising from this activation, which can be regarded as emerging from the medium 82.
  • the energy of the incident radiation beam is very important for Compton scatter pipe thickness measurement system presented in this disclosure.
  • choosing proper photon energy which falls within the gamma ray energy portion of the photon energy spectrum, involves many system considerations, such as the source preparation and manufacturing, the behavior of the selected energy photons in the system, and system weight.
  • There are several constraints on the selection of radiation sources such as the limited available suitable radioactive isotopes, the low weight shield required for system portability, and the higher energy photons desired for a better Compton-to-attenuation ratio as discussed above.
  • Hg 203 mercury-203
  • Hg source are that it yields a good gamma ray energy for Compton imaging and for reduction of system shielding weight, has suitable half-life for generating high intensity gamma rays, and emits a simple energy emission spectrum with the predominant gamma ray emission at 279 keV. Calculations of relative saturation intensity as a function of source energy also suggests that higher photon utilization efficiency can be reached using 279 keV gamma ray emission from Hg 203 . There are additional benefits produced by Hg 203 that will be discussed in subsequent sections of this disclosure . The disadvantages of
  • Hg 203 using Hg as a source are that it is unavailable commercially and it requires a long irradiation time or a high neutron flux to make the source due to the low abundance of Hg 202 (29.9%) and small neutron absorption cross section (4 barns)
  • the following model was used to calculate the response of the pipe thickness measuring system.
  • the model was used to optimize design parameters of the system, and was further used to calibrate the operating thickness imaging system as will subsequently be described in detail
  • An annular detector 50 with inner radius r, as identified by the dimension 82, and outer radius r 2 , as identified by the dimension 81, is used to detect scattered photons, which are preferably in the gamma ray energy region.
  • a collimated incident gamma ray flux with 279 keV energy from a Hg 203 source 14 is defined by a source collimator 16 and passes through the aperture 18 (see Fig. 5) thereby forming a beam 15 which is treated as a pencil beam along the detector center line. The beam 15 penetrates the regions associated with the inspected pipe.
  • These regions can be any combination of pipe 70, water 80 filling the interior of the pipe, heat insulation 62, and scuff guard 60 which is typically aluminum, ail being aligned as shown in Fig. 8. These regions are simplified as slabs for modeling purposes, but yield results which closely match the response of the measuring device shown in Figs 5 and 6.
  • the surface of the detector 50 is parallel to the surfaces of the slabs 60, 62, 70 and 80.
  • the above simplification has the effect of producing a large reduction of computation time because of the symmetry of the model.
  • the distance from the detector to the front surface of the objects is Tair, over which only air exists.
  • the slab thicknesses of heat insulation, pipe wall and water are noted as Tjns. Tpjpe, and Tw. respectively.
  • Typical composition parameters of the regions are listed in Table A, and are used in computing attenuation coefficients and other reaction parameters required for the model predictions.
  • the singly scattered photon intensity can be computed.
  • theoretical calculations can provide a more insightful view of the measurement process.
  • the contribution by higher order scatterings (beyond order 2) will not be considered, because of the complexity of computation and the relatively minor influence of such scattering on the measured results.
  • the actual calculations are numerical in nature, using the Romberg integration method, but will not be presented for reasons of brevity.
  • the first question to ask, when testing a pipe, is where one should place the detector relative to a test object.
  • the complete geometric response is associated with all geometric factors in the system. To understand this effect, it is helpful to consider a simple case, as shown in Fig. 9.
  • An annular detector 50 has again an inner radius r t and outer radius r 2 identified by the numerals 82 and 81 , respectively.
  • the distance from a point P, which can be thought of as the previously defined inspection volume 22 of the measurement, on the central or "beam" line 15 to the detector surface is defined as T.
  • the solid angle that the detector front surface subtends is expressed as
  • This equation can be used to estimate the distribution of geometric response functions for various distance T, inner detector radius r, and outer detector radius r 2 .
  • the detector dimensions and Tair are seen to be the basic parameters for the system arrangement and optimization, and these components were optimized using the model calculations. Results of the design parameters of the system will be summarized below.
  • T a ir set at around the maximum of the geometric response
  • the measurement will have better stability and better efficiency, and the scattering from the pipe wall can also be enhanced. This enhancement can also reduce the necessary gamma ray source strength thereby reducing shielding requirements.
  • the maximum geometric response for bare pipe occurs at a value of T a j r about 1 cm, a practical distance for a detector setup for measuring pipes.
  • the detector responses in terms of relative counting rate to the pipe wall thickness were computed and are shown in Fig. 10 for different detector outer radii r z of 1.385 cm, 2.655 cm, and 4.56 cm represented by curves 110, 112, and 114, respectively. It is clear that the responses demonstrate that the measurement system has good sensitivity to the pipe wall thickness ranging up to about 1.4 cm, with the best sensitivity being obtained with the largest outer detector radius.
  • Increasing the detector annular area or "window size" influences the measurement of the system in several ways in that: (1) increases geometric efficiency; (2) spreads the scattered radiation spectrum; (3) increases the detection of multiply scattered radiation; and (4) changes the detectable radiation field.
  • the geometric response function is changed when a larger detector size is applied. More exactly, the deeper portions of the pipe wall gain more positional weight than the outer surface portions. That is why better sensitivity and resolution appear for the larger detector setup.
  • the geometric response function is used to improve system resolution to the pipe thickness as well as detection efficiency.
  • the optimal position for the insulated pipe appears to be as close as possible to the detector, and a large detector window size has been found to improve both geometric efficiency and resolution of the measurement.
  • the detector responses to the pipe wall thickness with 2.54 cm and 5.08 cm thick insulation, were computed and are shown as curves 122 and 120, respectively, in Fig.
  • the filling water in an uninsulated or insulated pipe has a severe impact on the scattering process.
  • Water has a high Compton-to-photoelectric ratio. This is a good effect since fewer photons are lost by photoelectric abso ⁇ tion.
  • the region over which the filling water exists is generally large. All of these features result in less resolution to the pipe thickness.
  • these features of the filling water can be exploited if one adopts a slight modification of the previously discussed technique.
  • the geometric factor has a strong effect in the measurement system. The photons scattered by the water can be made to gain more importance by judiciously reshaping the geometric response. Fig.
  • this source is quite different from a conventional radiography source in the aspects of spatial distribution, directional anisotropy and energy spectrum, it is similar in the sense of where the radiation originates.
  • the view of the detector clearly has different weights for different scattered source positions along the beam due to geometric response functions.
  • the pipe wall is positioned in a lower geometric weight position in direct view.
  • the weight for the filling water is increased as if there was a stronger radiation source behind the pipe wall, in the water.
  • the high Compton-photoelectric ratio and small attenuation coefficient of water enhance its role as a radiation source. The problem then becomes similar to conventional radiography, meaning we are left with an attenuation measurement along the scatter path to the detector.
  • the response function can be shaped by varying the aperture 18 size of the source collimator 16 (see Fig. 5) so that scattering information enhanced by the response function can be obtained.
  • a desirable geometric function for a system can be further shaped in addition to varying the source collimator aperture.
  • the large area annular detector of the present invention it is possible to implement significantly different geometric response functions by implementing the detector collimation in front of the detector surface.
  • This detector collimator embodiment provides increased potential for applications of this system design to various inspection or imaging situations.
  • the present system is designed such that no detector collimation can be used, or alternately different types of detector collimation can be used.
  • system performance with empty pipes is quite good without any type of detector collimation, and the absence of any collimation increases the measured count rate and therefore improves the statistical significance of the measurement.
  • Detector collimation design was optimized using the MCNP (Monte Carlo Neutron Photon) computer code, which is a widely accepted code utilizing the MCNP (Monte Carlo Neutron Photon) computer code, which is a widely accepted code utilizing the MCNP (Monte Carlo Neutron Photon) computer code, which is a widely accepted code utilizing the MCNP (Monte Carlo Neutron Photon) computer code, which is a widely accepted code utilizing the MCNP (Monte Carlo Neutron Photon) computer code, which is a widely accepted code utilizing the MCNP (Monte Carlo Neutron Photon) computer code, which is a widely accepted code utilizing the MCNP (Monte Carlo Neutron Photon) computer code, which is a widely accepted code utilizing the MCNP (Monte Carlo Neutron Photon) computer code, which is a widely accepted code utilizing the MCNP (Monte Carlo Neutron Photon) computer code, which is a widely accepted code utilizing the
  • FIG. 13 is a illustrates a typical MCNP geometric layout used to optimize detector collimator design.
  • the source 14 is Hg 203 and the source collimation 44 is lead.
  • a pencil beam 15 is formed and impinges upon a pipe 70 filled with water 80 and wrapped with insulating material 62.
  • the plurality of shielding cones are conic, as illustrated previously, each with a truncated apex directed toward the pipe, and a base coaxially aligned with the center axis of the angular detector 50.
  • each cone is identified by the dimension 190
  • the spacings between the cones at the detector face is identified by the dimension 192
  • the distance between the truncated apex of the cone and the surface of the insulation around the pipe is defined by the dimension 192.
  • Cone lengths are preferably, but not necessarily equal.
  • Cone spacings are preferably, but not necessarily, equal.
  • Hg 203 14 source is in the form of an oxide pellet of length and diameter roughly equal to 0.5 cm, it can be assumed to behave like a point source.
  • the shield design 44 collimates the gamma rays through an aperture 18 which is 3/16 in. in diameter. This aperture is 3 inches long and is seen to extract roughly a pencil beam 15 from the isotropic gamma emissions source 14. This pencil beam 15 is assumed to be 3/16 in wide in the MCNP simulation with uni-directional gammas. Both these approximations are shown to produce accurate results when compared to the actual experiments run in the laboratory setup.
  • collimator dimensions such as cone length, cone spacing, the number of cones, and the like.
  • annular detector 50 dimensions such as diameter, width, inner hole diameter, and the like. Additional investigations may also be performed on varying the distance between the collimator and the pipe wall. For brevity, however, the results of the computations used to optimize the detector collimator design will be given. In order to scan pipes with thermal insulation, the collimator length would typically need to be greater than the insulation thickness. Referring again to Fig.
  • the collimator can then be placed at a distance 192 from the insulation equal to the difference between the collimator focal region and the insulation thickness, where again the focal region is defined as the position of primary response that the system "sees”. This would then place the focal point of the collimator at the wall of the pipe 70.
  • the theoretical location of the collimator is defined as the point where the concentric cones 26 are designed to focus.
  • the focal length is located between the theoretical focal length of the collimator and the focal length (the region of maximum recorded counts) of the bare crystal without a collimator. The region of maximum counts for this detector is seen to occur roughly 1/2 inch from the detector.
  • the focal length will shift to some value between 1/2 inch and the focal length of the collimator.
  • a cone spacing 194 of 0 i.e. an infinite number of cones
  • the practical focal length will equal the theoretical focal length.
  • the focal length will equal that without a collimator, in this case roughly 1/2 inch.
  • a large cone spacing would lessen the effect of a collimator and the focal length would be closer to that of a detector with no collimator.
  • the collimator length 190 is also expected to change the focal length.
  • the focal length is therefore expected to be closer to a detector 50 without a collimator as a result of the reduced collimating effect.
  • the longer the collimator the greater the collimating effect, and the closer the practical focal length will be to the theoretical focal length.
  • a longer collimator length results in a greater focal length while a shorter collimator gives a shorter focal length. It is clear that as the collimator length is decreased, backscatter from closer locations are able to reach the detector. This tendency therefore shifts the focal length closer to the detector.
  • the addition of a focusing collimator to the front of the detector 50 has proved to be invaluable when scanning water filled pipes. While the effect of multiple scattering is greatly diminished, the number of total counts recorded by the detector is also seen to be significantly reduced. This reduction in the number of recorded counts can be attributed to two major components.
  • the collimator cones 26 naturally block a fraction of counts which are meaningful to the count spectrum (backscatter from the pipe wall) as well as counts which are insignificant, like backscatter from insulation and water. In addition to blocking out meaningful counts from the spectrum, the addition of the collimator cones 26 to the detector 50 naturally means the detector will be further away from the pipe wall 70.
  • This greater distance 192 plus 190 results in a smaller solid angle of backscatter which can interact with the detector, hence a lower number of counts.
  • the pipe wall thicknesses was varied from 1/16 to 1/2 inches. In each case it was seen that there is a significant drop in recorded counts recorded by the detector 50 as the pipe wall 70 was moved away from the detector. It was also seen that the maximum number of counts were obtained with the detector roughly 1/2 inch from the pipe wall. In order to minimize this inherent loss of recorded counts, one may use a collimator of reduced length 190 so as to lessen the distance between the detector and the pipe wall. One may also lessen the spacing 194 between the collimator cones, hence causing less interference for pipe wall backscatter.
  • the collimator In addition to the trade off between resolution and recorded counts, it is also desirable to make the collimator as insensitive to distance 192 as possible. This effect would provide a greater margin for error when scanning pipes as the distance 192 + 190 between the pipe wall and the detector can not be kept perfectly constant. The degree of this distance variation will depend on the physical uniformity of the thermal insulation 62 as well as the scanning system connected to the detector. Using the MCNP model, it was found that the collimator with cone spacing 194 of 3/16 inch shows the greatest sensitivity to distance and will likely generate significant errors should it be used to conduct a pipe scan. The collimator with cone spacing 194 of 3/4 inch displays the greatest insensitivity to position errors but also gives the worst resolution.
  • the length 190 of the collimator is an extremely important design parameter for two reasons. It plays an important role in the trade off between system sensitivity and the number of recorded counts.
  • the collimator length 190 naturally determines the closest possible distance between detector and pipe wall. This effect is extremely important to maximize the total number of detected counts. As the detector is placed further away from the pipe wall, solid angle of backscatter counts becomes increasingly small. This reduced solid angle translates to a lower number of detected counts.
  • This parameter was investigated in the MCNP code with collimator lengths between 0.5 and 1 inches. While the collimator length was varied, the remaining collimator design parameters were kept constant. The cone spacing 194, for example was kept at 3/8 in. and the outer radius of the detector 50 was kept at 1.612 inches. In order to fully investigate the effect of changing collimator length 190, two types of runs were performed.
  • the first run deals with the resolution of the backscattered photons to pipe wall thickness. This was accomplished by simulating an insulated, water filled pipe and varying the pipe wall thickness. This simulation showed how backscatter counts change for different collimators as a result of pipe wall change.
  • the second run performed dealt with taking counts as collimator distance was varied. This simulation was performed by placing a 3/16 inch metal sheet at different collimator distances and measuring the amount of detected backscatter. This experiment was performed with different collimator lengths, hence providing information on how detected counts varied as collimator length 190 was changed. It also showed the sensitivity of counts to changes in distance from the collimator.
  • the change in cone spacing 194 gives similar results to changes in collimator length.
  • the distance between the collimator and detector is not dependent on cone spacing.
  • the number of counts is therefore not expected to vary as dramatically as with collimator length 190, as there is no change in solid angle with varying cone spacing.
  • the expected focal region becomes smaller, hence causing greater sensitivity to distance changes.
  • this smaller focal region would also more effectively eliminate multiple scatters from the water, thereby increasing the system resolution.
  • a smaller cone spacing 194 would also be expected to eliminate a greater number of scatters from the water as well as scatters from the pipe 70. A lower count rate is therefore expected.
  • the MCNP code using the geometry shown in Fig. 13 was also used to determine the optimum number of detector collimator cones 26. It may at first be thought that the number of cones would be determined by the cone spacing 194 as greater cone spacing allows fewer cones 26. This dependency is, however, only valid for fixed detector size. By maintaining the same cone spacing 194 and collimator length 190, adding additional cones 26 would naturally require a larger detector radius X 2 - Unlike the design parameters of collimator length 190 and cone spacing 194, the number of cones does not depend on a trade off between counts and system resolution. As additional cones 26 are added, it was found that the system resolution does not drastically change.
  • Additional cones 26 would however reduce the size of the focal region and therefore cause greater sensitivity to change in distance from the collimator. As additional cones are added resulting in a larger detector, the expected number of counts increases, but at the expense of a more cumbersome system pipe thickness measurement system. MCNP calculation were made maintaining the previously discussed optimum cone spacing 194 of 3/8 inch and the previously discussed optimum collimator length 190 at 1.0 inch. These values required a detector radius of 4.05 inches for 4 cones, 5 inches for 5 cones and slightly over 6 inches for 6 cones. Calculations showed that as the number of cones 26 is increased, the number of counts increases. The smallest detector 50 considered, which is one using a collimator with 3 cones, gave the greatest counts per unit area.
  • the density of fluid within a pipe plays an extremely important role in the resolution of pipe wall thickness.
  • the invention embodied without detector collimation can be effectively used to measure the wall thickness of low density, gas filled pipe.
  • changes in pipe wall thickness with water filled pipes is virtually undetectable. Even with liquid such as water of relatively low density, for example 0.7 specific gravity, the change in pipe wall thickness is clouded by multiple scatter in the water. While the addition of a focusing collimator greatly diminishes the effect of multiple scatters, it does not completely block out scatters from the water. As a result, changes in water density are seen to affect the total counts as well as resolution to pipe wall thickness.
  • Fig. 13 In order to investigate the effect of water density, the model shown in Fig. 13 was again set up in the MCNP code using a collimator with cone spacing 194 of 3/8 inch and lengths 190 of 1.0 inch.
  • the detector 50 outer radius X2 was maintained at 1.613 inches
  • the water 80 within the pipe 70 was varied from 0.7 to 1.0 specific gravity
  • the wall thickness of the pipe 70 was varied between 1/16 and 1/2 inches.
  • the results using this collimator were then compared to a similar MCNP calculations using a bare detector with no collimation. in both cases, the pipe was covered with 1.75 inches of thermal insulation 62. This setup would indicate the change in counts as well as the loss in resolution to pipe wall thickness that is expected from denser water.
  • Fig. 14 is a plot of measured counts as a function of pipe wall thickness.
  • the family of curves identified by the numeral 202 represents counts measured with detector collimation for varying densities of fluid within the pipe 70 ranging from 0.1 to 1.0 grams/cubic centimeter (gm/cc).
  • Curves 206, 208, 210 and 212 represent results with no detector collimation for fluid within the pipe 70 of densities 0.1 , 0.8, 0.9 and 1.0 gm/cc, respectively.
  • the collimator increases thickness resolution by blocking out counts which are not a result of Compton scattering in the pipe.
  • curves within the family of curves 202 essentially overlay at pipe thicknesses greater than 0.2 - 0.3 cm, with the exception of curve 204 which represents a low density fluid such as light gas.
  • curve 204 represents a low density fluid such as light gas.
  • Fig. 14 presents a graphical illustration of the previously stated facts that either a bare or collimated detector can be used to effectively measure the thickness of gas filled pipe, but only a collimated detector yields acceptable results if the pipe is filled with liquid.
  • the bare detector embodiment is possibly preferred in that the measured counts are greater thereby yielding a more statistically precise answer.
  • the distance 192 of the collimator cones 26 from the wall of the pipe 70 is an extremely important thickness scanning parameter.
  • the detector 50 In order to maximize the number of recorded counts, the detector 50 should be placed as close to the pipe wall as possible. As the detector is moved closer to the pipe wall, the detected solid angle increases, hence increasing total recorded counts. However, as the collimator 26 is moved closer to the pipe wall, the focal point moves into the pipe interior. With water filled pipes, this interior region consists of water 80. A larger amount of scattering within the water is therefore expected, along with a loss in resolution from single and multiple scatters. As a result of these extra recorded counts, the total number of counts is expected to increase.
  • Fig. 15 illustrates thickness measurements of an actual eroded pipe specimen as a function of the azimuthal angle q around the circumference of the pipe.
  • Curve 267 which connects "diamond” measured data points, represent thickness measurements of the specimen with no insulation.
  • Curve 269 which connects rectangular measured data points, represents thickness measurements of the same specimen through 1 inch of insulation. Agreement between the two sets of measurements is quite good thereby demonstrating the invention's ability to accurately determine the thickness of both insulated and uninsulated pipes.
  • the data shown in Fig 15 also demonstrates precision in the measurement in that repeatability is quite good.
  • the solid data points 268 represent "true' or "actual" pipe thickness values at the indicated angles q.
  • Figs. 16a and 16b demonstrate the system's performance in a partially filled pipe situation.
  • the pipe 70 is shown in Fig 16b in cross section, and is partially filled with water 80 and air 315.
  • the counts recorded by the detector are plotted as a function of the angle q in Fig. 15a.
  • the notch at 10° is clearly indicated by the drop in count rate 302 to approximately 2500 counts per measure point.
  • the use of thickness measurement system is not limited to measuring the wall thickness of steel pipe, but can be used to measure the thickness of any material which possess nuclear characteristics falling within the bounds previously discussed.
  • One such example is aluminum plate or sheet material.
  • Fig. 17 shows the system response for thickness measurement of aluminum plate.
  • Curve 322 represents the counts per measurement interval recorded in the system detector as a function of the thickness of the aluminum plate.
  • Fig. 18 illustrates the system configured to make scans around the circumference of a pipe 70.
  • the backscatter instrument 100 is connected to a track or rail 381 by a bracket 379.
  • the track 381 fits around the circumference of the pipe 70.
  • the track is positioned around the pipe 70 by expanding about a hinge 383, and subsequently clamping by means of a clamp 377.
  • the backscatter instrument 100 is then conveyed circumferentially around the pipe on the track 381 by a suitable drive mechanism (not shown).
  • Counts in the system detector can be taken with the instrument 100 positioned at interval stations around the pipe, or can be collected and recorded as the instrument is moved continuously around the circumference of the pipe 70. It should be understood that the mounting and instrument conveyance means shown in Fig. 18 is only one embodiment, and that there are numerous alternate means for mounting and conveying the instrument about the structure to be measured.
  • the present invention can be used to generate images of the thickness of a structure.
  • the system embodied to produce an image of pipe wall thickness is shown in Fig. 19.
  • the instrument 100 is mounted so that it can scan a pipe circumference, at an axial position along the z axis of the pipe, following a path indicated by the broken line 270 by rotation about an angle q.
  • Count data from the photomultiplier tube 52 are measured as a function of the angle q and the axial position along the z axis and input into a computer 290.
  • the instrument is then moved axially along the z axis, and another circumferential measurement is made by rotation about the angle q following a path 272.
  • counts recorded as a function of q and z are input into the computer 290. This process is repeated sequentially for the paths 274, 276, and other paths not shown.
  • the resulting count data recorded as a function of q and z are then used to generate an image or map of the thickness of the pipe wall, and output to a suitable recording device 292 which can produce a hard copy, or a digital recording of the image.
  • Power is supplied to the photomultiplier 52 by a power supply 284.
  • a clock 286 and control electronics 288 also cooperate with the power supply
  • Fig. 19 illustrates the instrument as moving around the circumference of the pipe in a series of discrete, circular paths 270, 272, 274, and 276.
  • An alternate embodiment of the system would employ a mounting and drive mechanism (not shown) such that the instrument is conveyed along the z axis in a helix path.
  • Fig. 20 illustrates a hypothetical pipe wall thickness scan generated with the system embodied as shown in Fig. 19.
  • the counts recorded at each position are converted to an absolute thickness measurement using calibration methods discussed in the following section. The results can be displayed as a two dimensional 8 x 9 measurement array of numerical thickness measurements.
  • a "gray" scale can be used to convert these measurements to a more visual scan as shown in Fig. 20.
  • a shade of "gray” is assigned to each (q,z) pixel 282 generated by the scan.
  • the darkest shade can be used to represent the maximum or "gauge" pipe thickness. Thicknesses less than the maximum gauge thickness can be assigned proportionally lighter shades of gray, where no metal (i.e. zero thickness) is represented by white.
  • the data can be displayed in alternate ways, such as a three dimensional cylinder.
  • the gray scale can be replaced with a color scale, wherein the color indicates pipe thickness.
  • the thickness measurement system such that the measured quantity, namely counts recorded by the detector, can be converted to the quantity of interest, namely material thickness.
  • the relationship between the measured count data and quantities of interest is, as discussed in detail above, a function of numerous factors such as the type of material being measured, the fluid type behind the material being measured, the source strength, the detector efficiency, the type of source and detector collimation used, count intervals, and the like.
  • "environmental” calibrations, “computed” calibrations, or a combination of both types of calibrations can be employed. Stated simply, environmental calibrations use the response of the instrument as measured in known environmental conditions. A calibration relation is then derived to relate instrument response to thickness measurements made under these known conditions.
  • Computed calibrations use various models, such as the previously discussed MCNP code, to calculate calibration relations. Since it is usually impractical to environmentally simulate every condition in which an instrument is to be used, a combination of environmental and computed calibrations are usually used, wherein a relatively large number of computed calibrations are "normalized” to a relatively limited number of environmental calibrations. The combination of environmental and computed calibrations is the preferred means for calibrating the present thickness measuring system.
  • a system has been developed to measure the thickness of pipe wall, from the outside of the pipe, using nuclear apparatus and methods.
  • the system comprises a photon source, such as Hg 203 , which emits 279 keV gamma radiation, and an annular gamma ray detector which are positioned on the outside of the pipe wall.
  • Pipe wall thickness is determined from the response of the annular detector to gamma radiation from the source which first passes through the annulus of the detector, and which is subsequently Compton backscattered by the pipe wall.
  • Concentric, conical detector collimation is used in one embodiment of the system to enhance the thickness measurement of the walls of pipe which are filled with relatively dense fluids such as water.
  • the system can also be used to measure, nondestructively, the thickness of materials in other types of structure.
  • the system In measuring pipe wall thickness, the system has two major benefits over conventional, prior art ultrasonic pipe wall thickness measurement systems. More specifically, the system eliminates the need to remove insulation which results in great savings in the person-hours required to perform pipe inspections. In addition, the system can image pipe walls in empty, steam-filled, or liquid-filled pipes even while in operation, e.g. at normal elevated operating temperatures or flow velocities.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • General Physics & Mathematics (AREA)
  • Length-Measuring Devices Using Wave Or Particle Radiation (AREA)
  • Analysing Materials By The Use Of Radiation (AREA)
  • Measurement Of Radiation (AREA)

Abstract

L'invention concerne la mesure de l'épaisseur d'un matériau à l'aide des techniques nucléaires. Plus particulièrement, elle concerne un dispositif et un procédé permettant de mesurer l'épaisseur de la paroi d'un tuyau depuis l'extérieur de celui-ci (70) à l'aide d'un détecteur annulaire (50), d'une collimation par détecteur conique concentrique (26) et d'un rayonnement gamma rétrodiffusé. Une source de rayons gamma (14), de préférence à une énergie de 279 KeV émise par le mercure 203, est collimatée en un faisceau crayon et bombardée perpendiculairement sur l'extérieur de la paroi du tuyau à mesurer. Le rayonnement à rétrodiffusion Compton venant de la paroi est mesuré par un détecteur de photons concentrique qui comporte de préférence un cristal, tel qu'un iodure de sodium, optiquement couplé à un seul tube photomultiplicateur. L'épaisseur du tuyau est déterminée à partir de la réponse du détecteur à l'aide d'une relation prédéterminée obtenue durant l'étalonnage du système. La collimation par détecteur conique concentrique est utilisée pour obtenir des mesures précises quand le tuyau est rempli de liquide.
EP97908911A 1996-03-04 1997-03-03 Jauge d'epaisseur de paroi de tuyau a retrodiffusion compton employant un collimateur a concentration et un detecteur annulaire Withdrawn EP0886759A4 (fr)

Applications Claiming Priority (3)

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US12755 1987-02-09
US1275596P 1996-03-04 1996-03-04
PCT/US1997/003516 WO1997033141A1 (fr) 1996-03-04 1997-03-03 Jauge d'epaisseur de paroi de tuyau a retrodiffusion compton employant un collimateur a concentration et un detecteur annulaire

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EP0886759A1 true EP0886759A1 (fr) 1998-12-30
EP0886759A4 EP0886759A4 (fr) 2000-11-22

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AU (1) AU2070397A (fr)
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US5896429A (en) * 1997-09-15 1999-04-20 Massachusetts Institute Of Technology Method for measurement of blast furnace liner thickness
US8767912B1 (en) 2013-04-09 2014-07-01 King Abdulaziz University System for inspection and imaging of insulated pipes and vessels using backscattered radiation and X-ray fluorescence
US9151722B2 (en) 2014-01-15 2015-10-06 King Abdulaziz University Systems for determining and imaging wax deposition and simultaneous corrosion and wax deposit determination in pipelines
WO2015112118A1 (fr) 2014-01-21 2015-07-30 Halliburton Energy Services, Inc. Système de diagraphie de fond de trou avec sensibilité azimutale et radiale
JP6299033B2 (ja) * 2014-05-12 2018-03-28 一般財団法人電力中央研究所 非破壊検査方法およびその装置
US8976936B1 (en) 2014-05-22 2015-03-10 King Abdulaziz University Collimator for backscattered radiation imaging and method of using the same
CN103969270A (zh) * 2014-05-27 2014-08-06 东莞理工学院 基于背散射成像技术的发射装置
US20160274039A1 (en) 2014-09-25 2016-09-22 King Abdulaziz University System for determining and imaging wax deposition and corrosion in pipelines
WO2016053344A1 (fr) * 2014-10-02 2016-04-07 Halliburton Energy Services, Inc. Imagerie tomographique de fond de trou
WO2016058095A1 (fr) * 2014-10-14 2016-04-21 Inversa Systems Ltd. Procédé d'inspection de zone dégradée de structure métallique recouverte d'un composite de réparation, et procédé de mesure d'épaisseur de paroi restante d'une structure composite
US9020099B1 (en) 2014-12-03 2015-04-28 King Abdulaziz University Miniaturized pipe inspection system for measuring corrosion and scale in small pipes
WO2017062011A1 (fr) * 2015-10-08 2017-04-13 Halliburton Energy Services, Inc. Détermination de l'épaisseur de la paroi d'un tubage
FR3043247B1 (fr) * 2015-10-30 2020-06-19 Commissariat A L'energie Atomique Et Aux Energies Alternatives Collimateur pour la spectrometrie de diffraction x, dispositif associe et son utilisation
CN111694046B (zh) * 2020-07-24 2022-06-07 中国工程物理研究院核物理与化学研究所 一种单能γ装置
CN117686533B (zh) * 2023-12-13 2024-05-17 中国特种设备检测研究院 一种检测管壁缺陷、相邻连续缺陷及绘制管壁缺陷图像的方法

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CA2248145A1 (fr) 1997-09-12
AU2070397A (en) 1997-09-22
EP0886759A4 (fr) 2000-11-22
WO1997033141A1 (fr) 1997-09-12

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