AU2015293890B2

AU2015293890B2 - Device and method for measuring the radioactivity of a material

Info

Publication number: AU2015293890B2
Application number: AU2015293890A
Authority: AU
Inventors: Ludovic BOURVA; Herve Toubon
Original assignee: Areva Mines SA
Current assignee: Orano Mining SA
Priority date: 2014-07-22
Filing date: 2015-07-22
Publication date: 2020-03-26
Anticipated expiration: 2035-07-22
Also published as: CA2954587A1; FR3024242A1; AU2015293890A1; EP3172592A1; EP3172592B1; WO2016012499A1; FR3024242B1

Abstract

The measurement device is designed for measuring the radioactivity of a material when same is being transported by a conveyor, and comprises a gamma spectrometry device (7) having at least one gamma spectrometry detector (9) and designed to be disposed on the path of the conveyor in order to carry out spectrometry measurements on the material when same is being transported by the conveyor (4), said measurements being carried out by detecting the photons emerging from the flow of transported material, and a processing unit for processing the measurement signals supplied by the gamma spectrometry device (7).

Description

Device and method for measuring the radioactivity of a material

Technical Field of the Invention The present invention relates to the field of measurement of radioactivity of a material.

Background Art During the operation of a radioactive ore mine or for sorting contaminated soils, it may be desirable to analyze the radioactivity of the relevant material. To do this, it is possible to conduct in the laboratory radioactivity measurements on samples taken from the material. Nevertheless, these one-off measurements may not give the possibility of analyzing the whole of the material (ore, material from dismantlement or from the cleaning-up of nuclear installations, soils contaminated with different types of radionuclides). Further, obtaining results from analysis may take a certain time. GB 2,017,294A discloses a device for sorting ore comprising a conveyer belt for transporting the ore, a counting device able to carry out a counting measurement of the number of gamma rays emerging from the ore and inferring therefrom a total radioactivity level of the material, and a splitting device for sorting the different areas of the ore according to their radioactivity level. Nevertheless, these measurements exclusively give the possibility of measuring the total radioactivity level of the material but do not allow identification of the radionuclides present in the material, nor of determining the origin or the distribution of the total radioactivity of the material if there are several radionuclides present in the material. It may be desirable for there to be a device for measuring radioactivity, that in some embodiments, may give the possibility of identifying and quantifying the radionuclides present within a material transported on a conveyer, notably in the case of a radioactive ore. It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

Summary of the Disclosure

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In a first aspect of the present invention is proposed a device for measuring radioactivity of a material during its transport by a conveyor, the device for measuring radioactivity comprising a gamma spectrometric device having at least one gamma spectrometry detector and provided so as to be positioned on the path of the conveyor in order to conduct spectrometric measurements on the material during its transport by the conveyor, these measurements being conducted by detecting photons emerging from the flow of transported material, and a processing unit for processing measurement signals provided by the gamma spectrometric device. The device for measuring radioactivity can be configured for carrying out spectrometric measurements on the material during its transport by the conveyor, each spectrometric measurement being carried out over a spectrometry period, during which the photons are detected by associating with each detected photon an energy, in order to determine the population of photons versus their energy, and for carrying out, during a spectrometric measurement, several gamma counting measurements on the material during its transport by the conveyor, by means of the gamma spectrometric device, each gamma counting measurement being conducted over a counting period, at the end of which is determined the total number of detected photons during the counting period, independently of the energy of the photons. According to particular embodiments, the device for measuring radioactivity may comprise one or several of the following features: - it may be configured for calculating the activity, the mass and/or the content of at least one radionuclide present in the material according to a spectrometric measurement; - it may be configured for carrying out several gamma counting measurements on the material during its transport by the conveyor, during a spectrometric measurement; - it may be configured for calculating the activity, the mass and/or the content of at least one radionuclide according to a counting measurement and a measurement of the content of this radionuclide calculated according to a previous spectrometric measurement; - it may comprise a device for measuring the height of material on the conveyor in register with the gamma spectrometric device; - it may comprise a weighing device for measuring the mass of the material transported by the conveyor;

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- it may be configured for calculating the activity, the mass and/or the content of at least one radionuclide according to detection efficiency of photons by the gamma spectrometric device depending on the energy of these photons, on the height of material on the conveyor in register with said or each gamma spectrometry detector and/or to the mass of the material transported by the conveyor; - the height of material on the conveyor may be comprised between 5 cm and 60 cm; - the gamma spectrometric device may comprise several gamma spectrometrydetectors. The invention also proposes, in a further aspect, a system for measuring radioactivity, comprising a conveyor for transporting a material and a device for measuring radioactivity as defined above, the gamma spectrometric device of which is positioned on the path of the conveyor. According to particular embodiments, the system may comprise one or several of the following features: - the conveyor may be a belt conveyor comprising an endless belt for transporting the material; - the height of material on the conveyor may be comprised of between 5 cm and 60 cm; - the gamma spectrometric device may comprise several gamma spectrometry detectors. In yet a further aspect of the present invention, the invention also proposes a method for measuring radioactivity of a material transported by a conveyor, notably a conveyor with an endless belt, comprising the carrying out of gamma spectrometric measurements on the material during its transport by the conveyor, by means of a gamma spectrometric device comprising at least one gamma spectrometry detector laid out on the path of the conveyor in order to detect the photons emitted by a flow of material during its transport by the conveyor. Each spectrometric measurement is carried out over a spectrometry period, during which the photons are detected by associating with each detected photon an energy, in order to determine the population of photons versus their energy. The method also comprises the achievement, during a spectrometric measurement, of several gamma counting measurements on the material during its transport by the conveyor, by means of the gamma spectrometric device used for spectrometric measurements, with each gamma counting measurement being conducted over a

12168552_1 (GHMatters) P104999.AU counting period, at the end of which is determined the total number of detected photons during the counting period, independently of the energy of the photons. According to particular embodiments, the method may comprise one or several of the following features: - it may comprise the calculation of the activity, of the mass and/or of the content of at least one radionuclide present in the material according to a spectrometric measurement; - it may comprise the conducting of gamma counting measurements on the material during its transport by the conveyor, by mean of the gamma spectrometric device used for spectrometric measurements; - several counting measurements may be conducted during a spectrometric measurement; - it may comprise the calculation of the activity, of the mass and/or of the content of at least one radionuclide according to a counting measurement and a measurement of the content of this radionuclide calculated according to a previous spectrometric measurement; - it may comprise the measurement of the height of the material on the conveyor in register with the gamma spectrometric device; - it may comprise the measurement of the mass of the material transported by the conveyor; - it may comprise the calculation of the activity, of the mass and/or of the content of at least one radionuclide depending on a detection efficiency of photons by the gamma spectrometric device according to the energy of these photons, to the height of material in register with said or each gamma spectrometry detector, and/or on the measurement of the mass of the material transported by the conveyor; - it may comprise the verification of a spectrometric measurement and/or calculations carried out from this spectrometric measurement by comparing activity, mass and/or content of at least one radionuclide, in particular bismuth 214 (Bi214), calculated according to main spectral lines of energies characteristic of this same radionuclide; - it may comprise the calculation of the activity, of the mass and/or of the content of uranium 235 (U235) and/or uranium 238 (U238) according to the results of a spectrometric measurement for protactinium 234 (Pa234); - it may comprise the calculation of the activity, of the mass and/or of the content of protactinium 234, the calculation of the activity, of the mass and/or of the

12168552_1 (GHMatters) P104999.AU content of bismuth 214 (Bi214) and/or the calculation of the activity, of the mass and/or of the content of radium 226 (Ra226), according to a spectrometric measurement; - it may comprise the calculation of the activity, of the mass and/or of the content of uranium 235 (U235) and/or the calculation of the activity, of the mass and/or of the content of uranium 238 (U238) according to a spectrometric measurement; - it may comprise the calculation of the activity, of the mass and/or of the content of uranium 235 and/or the calculation of the activity, of the mass and/or of the content of uranium 238 which is carried out from the activity, the mass and/or the content calculated for protactinium 234, bismuth 214 and/or radium 226; - the height of material on the conveyor may be comprised of between 5 cm and 60 cm; - the gamma spectrometric device may comprise several gamma spectrometry detectors.

Brief Description of the Figures The invention and its advantages will be better understood upon reading the description which follows, only given as a non-limiting example, and made with reference to the appended drawings, wherein: - Fig. 1 is a schematic side view of a system for measuring radioactivity comprising a conveyor and a device for measuring radioactivity; - Fig. 2 is a schematic cross-sectional view of the system of Fig. 1, along II-Il in Fig. 1; - Fig. 3 is a schematic view of the device for measuring radioactivity of the whole of Figs. 1 and 2; and - Figs. 4 are graphs illustrating a spectrum resulting from a gamma spectrometric measurement.

Detailed Description of Embodiments of the Invention The system 2 for measuring radioactivity of Figs. 1 and 2 comprises a conveyor 4 provided for conveying a material, for example a radioactive ore extracted from a mine, and a device for measuring radioactivity 5 adapted for measuring the radioactivity of the flow of material during the transport of the material by the conveyor 4, notably by carrying out spectrometric measurements.

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The conveyor 4 comprises an endless conveying member for transporting the material. The conveyor 4 is here a belt conveyor, the conveying member is a endless conveyer belt 6. The device for measuring radioactivity 5 is adapted for conducting both spectrometric measurements during which the device for measuring radioactivity 5 identifies and quantifies the different radionuclides contained in the material during its transport by the conveyor 4, and counting measurements during which the device for measuring radioactivity 5 counts the photons emitted by the material during its transport by the conveyor in order to determine the total radioactivity of the material. The device for measuring radioactivity 5 comprises a gamma spectrometric device 7 adapted for detecting the gamma photons emitted by the material during its transport by the conveyor 4 and measuring the energy of the detected photons, and a data processing unit 8 for processing the corresponding measurement signals provided by the gamma spectrometric device 7. The gamma spectrometric device 7 is positioned on the path of the conveyor 4 so as to be able to detect the gamma photons emitted by the material during its transport by the conveyor 4. The gamma spectrometric device 7 comprises at least one gamma spectrometry detector, or gamma spectrometer 9, for conducting gamma spectrometric measurements on the material during its transport by the conveyor 4. The gamma spectrometric device 7 comprises several gamma spectrometers 9, typically two to four gamma spectrometers, here three gamma spectrometers 9 (Fig. 1). Each gamma spectrometer 9 detects photons emitted by the material located on the belt 6 and emerging from this material, and measures the energy of each detected photon. Each gamma spectrometer 9 provides at the output a signal representative of the number of emitted photons and of the energy associated with each detected photon. Such a signal then gives the possibility of determining a gamma spectrum indicating, over a given period of time, the number of detected photons versus the energy. The gamma spectrometric device 7 comprises a shielding 10 surrounding the gamma spectrometers 9 and laid out for allowing the photons emerging from the material transported by the conveyor 4 and located in register with the gamma spectrometers 9 to attain the gamma spectrometers 9, while protecting the gamma spectrometers 9 from photons emitted by other photon sources. The shielding 10 is

12168552_1 (GHMatters) P104999.AU made in a material strongly attenuating radiations, for example in lead or in tungsten. Optionally, the device for measuring radioactivity 5 comprises a height sensor 12 for measuring the height of the material transported by the conveyor 4 in register with the gamma spectrometric device 7. The height sensor 12 is for example a laser telemeter positioned above the belt 6 for measuring the distance between the telemeter and the upper surface of the material transported by the belt 6, and calculating the height of material according to the distance from the telemeter to the conveyor 4 in the absence of material on the conveyor 4. The height of the material transported by the belt 6 is typically comprised between 5 cm and 60 cm. Optionally, the system 2 comprises a jig for adjusting the level of the material on the belt 6. The height of the material on the belt 6 is then substantially constant. Optionally, the device for measuring radioactivity 5 comprises a weighing device 14 for measuring the mass of the material transported by the conveyor 4 in register with the gamma spectrometric device 7 during a given period of time. The height sensor 12 and the weighing device 14 are positioned in register with the gamma spectrometric device 7 or shifted upstream or downstream relatively to the gamma spectrometric device 7 along the conveyor 4. In the case of an upstream or downstream shift, by knowing the displacement of the conveyor 4, it is then possible to associate each height and/or mass measurement with the corresponding spectrometric measurement. The belt 6 has a transport strand 6A ensuring the transport of the material and (circulating to the left in Fig. 1) and a return strand 6B (circulating to the right in Fig. 1). The gamma spectrometric device 7 is positioned so as to conduct measurements on the material transported by the belt 6, more specifically by the transport strand 6. The gamma spectrometric device 7 is positioned below the transport strand 6A, while being contiguous to the transport strand 6A. Alternatively, the gamma spectrometric device 7 is positioned above or on the sides of the transport strand 6A. The conveyor 4 comprises guiding assemblies 16 with rollers distributed under the transport strand 6A for guiding it and supporting it. The gamma spectrometric device 7 is positioned between two adjacent guiding assemblies 16,

12168552_1 (GHMatters) P104999.AU so as to be placed as close as possible to the belt 6. The guiding assemblies 16 optionally comprise guiding rollers 17 which are laid out for raising the side portions of the transport strand 6A. The belt 6 is wound on two return rollers 18, so that the transport strand 6A and the return strand 6B each circulate from one roller 18 to the next, in opposite directions. At least one of the return rollers 18 is motor-driven for driving the belt 6. Optionally, in the case of excessive room temperatures, the device for measuring radioactivity 5 comprises a system for ventilation and/or conditioning laid out for maintaining the spectrometric device (spectrometers) under standard operating conditions for this type of equipment (-10OC < T < +300C). As illustrated in Fig. 3, each gamma spectrometer 9 comprises a photon detector 22 able to detect photons attaining the detector and to provide for each detected photon a measurement signal representative of the energy of this photon. Each detected photon is therefore counted and characterized by its energy. The photon detector 22 is for example a scintillator detector comprising a crystal able to emit a light signal when it is hit by a photon, which is then converted into an electric signal. The crystal is for example made in lanthanum bromide (LaBr). Alternatively, the photon detector 22 is a detector with a semi-conductor comprising a semi-conducting substrate able to emit an electric signal when it is hit by a photon. The semi-conducting substrate is for example made in germanium (Ge). Each gamma spectrometer 9 comprises an amplifier 24 and an analog/digital converter 26. The amplifier 24 is for example an electronic amplifier or a photomultiplier. The amplifier 24 amplifies the measurement signal provided by the photon detector 22. The converter 26 receives the amplified measurement signals and provides a digital output signal forming the output signal of the gamma spectrometer 9. It may be advantageous that the gamma spectrometer 9 is a gamma spectrometer with a median resolution or a high resolution over an energy range comprised between 50keV and 3MeV. The absolute resolution of the gamma spectrometer 9 at a given energy is the full width at mid-height (or FWHM) of a peak of a spectrum provided by the spectrometer at this given energy. The relative resolution is expressed as a percentage, which is equal to the ratio of the full width at mid-height over the energy multiplied by 100. An average relative resolution is comprised between 1% and 5%. A high relative resolution is less than 1%. Thus, the

12168552_1 (GHMatters) P104999.AU gamma spectrometer 9 has a relative resolution of less than 5% (case of medium resolution) or of less than 1% (case of high resolution). In some embodiments, the gamma spectrometer 9 is a scintillator detector with lanthanum bromide, which generally has an average resolution, or a germanium (Ge) detector, which generally has a high resolution. The processing unit 8 is a computer, for example a personal computer. It comprises a processor and a memory for the storage of software applications which may be executed by the processor for processing the measurements provided by the gamma spectrometry device 7. The processing unit 8 is connected to the gamma spectrometric device 7 for receiving the output signal of each gamma spectrometer 9. The processing unit 8 receives the output signal of each gamma spectrometer 9 individually. The processing unit 8 has an input associated with each gamma spectrometer 9. Alternatively, the processing unit 8 has an input receiving a signal containing the output signals of different gamma spectrometers 9. The processing unit 8 is configured for recording each output signal of each gamma spectrometer 9 and establishing, over a given measurement period, a gamma spectrum of the population of detected photons over the measurement period depending on the radiation energy of the detected photons. In the case when the gamma spectrometric device 7 has several gamma spectrometers 9, the processing unit 8 is configured for adding the photons detected by the different gamma spectrometers 9 over each measurement period. The implantation of several gamma spectrometers 9 gives the possibility of increasing the number of photons taken into account over a same measurement period. The gamma spectrometric device 7 and the processing unit 8 are separated and distant from each other. The gamma spectrometric device 7 is contiguous with the belt 6 while the processing unit is distant from the belt 6. Alternatively, the gamma spectrometric device 7 and the processing unit 8 are gathered in a same detection and processing unit. When operating, a flow of material is transported by the conveyor 4 and runs facing the gamma spectrometric device 7 which conducts measurements of the gamma radiations emitted by the different radionuclides contained in the material transported by the conveyor 4. The measurements are conducted while the material is transported by the conveyor 4 at a non-zero speed.

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The device for measuring radioactivity 5 is adapted for applying a measurement method with view to characterizing the material transported by the conveyor 4. In particular, the processing unit 8 is programed for applying this method according to the measurement signals provided by the gamma spectrometric device 7. The characterization of the material comprises the identification and quantification of the different radionuclides contained in the material. It may be preferable for it to comprise the calculation of the activity, of the mass and/or of the content of one or several radionuclide(s) according to the range established by the measurements given by the gamma spectrometric device 7. The measurement method comprises the achievement of spectrometric measurements, each achieved over a spectrometry period, during which the photons are detected by associating with each detected photon an energy, in order to determine the population of photons versus their energy. At the end of a spectrometric measurement, a spectrum of the population of photons is established depending on their energy over the spectrometric period. The result of a spectrometric measurement is a spectrum. Each spectrometric measurement is achieved from measurement signals provided by the gamma spectrometric device 7 over a spectrometry period. The spectrometric periods have the same duration d. The spectrometry measurements are conducted over distinct successive spectrometryperiods. The measurement method comprises the calculation of the activity, of the mass and/or of the content of one or several radionuclides of interest according to each spectrometry measurement. The calculation is performed at the end of the spectrometry period. Fig. 4 illustrates a gamma energy spectrum of a radioactive material. The spectrum comprises peaks around characteristic energies of particular radionuclides. A radionuclide may emit photons at one or several energies and thus be associated with one or several peaks of the spectrum. The decrease of a radionuclide may result in one or several other radionuclides moving down with different characteristic energies. The characterization of certain radionuclides therefore gives the possibility of characterizing one or several ascending or descending radionuclides. The detection efficiency e(E) of a photon detector at a radiation energy E is the ratio between the number of photons detected at the radiation energy E and the

12168552_1 (GHMatters) P104999.AU number of photons emitted at the radiation energy E by the radioactive products contained in the material. The detection efficiency is a unit less number strictly less than 1. The detection of photons of strong energy is more efficient than the detections of photons of low energy since the latter will be strongly absorbed even within the thickness of the crossed material. The device for measuring radioactivity 5 is calibrated by determining its detection efficiency versus the radiation energy E. The detection efficiency versus the radiation energy E is determined for example by physical or modeled calibration, by means of commercial tools (of the type ISOCS or MERCURAD - PASCALYS marketed by CANBERRA) The measurement method comprises the measurement of the radioactive activity of a radionuclide emitting photons at a radiation energy E depending on the measurement of the net area M of the peak of the spectrum at the radiation energy E, according to the following equation: Ax(E) = M(E) / (e(E)*x(E)) wherein Ax is the radioactive activity (number of disintegrations per second) of the radionuclide X at the energy E in Becquerels (Bq), M is the net area of the peak of the spectrum located at the radiation energy E in counts per second (c/s), and Ix(E) is the intensity of the line at the radiation energy E for the radionuclide X (number of photons emitted per Bq). The intensity Ix(E) is issued from nuclear data tables, for example a table of the JEFF type. The intensityIx(E) is without any unit. The measurement method comprises the calculation of the mass of at least one radionuclide X detected by the gamma spectrometric device 7 during a spectrometry period depending on a predetermined proportionality factor and on the radioactive activity Ax of this radionuclide X. The mass is determined from the following equation: Ax(E) =Ax*N*mx/Mmolx wherein Ax is the activity of the radionuclide X at energy E in Becquerels (Bq), A is the radioactive period of the radionuclide X (s1), N is the Avogadro number of the radionuclide X, mx is the mass of the radionuclide X in grams (g), and Mmolx is the molar mass of the radionuclide X in grams per mole (g/mol). The predetermined proportionality factor is thus Ax*N/Mmolx which is a constant value for the radionuclide X. The measurement method comprises the calculation of the content T of the material in one or several radionuclides.

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The content Tx of a radionuclide X in the material is calculated according to the mass of this radionuclide, from the average density of the material (in g/cm3 ) and on the volume of material conveyed by the belt 6 facing the gamma spectrometric device 7 during the spectrometry period. The material mass is measured by means of the weighing device 14. The volume of the material is calculated from the displacement of the conveyor 4 and from the area of a cross-section of the material flow on the conveyor 4. This variable area is calculated according to the height of material on the conveyor 4, calculated according to the measurement of the height 12 by the sensor. Alternatively, if the height is calibrated, it is possible to consider that this area is constant. The measurement method comprises the achievement of counting measurements, each conducted over a counting period, at the end of which is determined the total number of detected photons during the counting period, independently of the energy of the photons. The counting periods have the same duration p. The counting measurements are achieved by means of the measurements provided by the gamma spectrometric device 7. The counting of photons over a counting period of a determined duration, gives the possibility of calculating the overall radioactivity of the material. The counting of the set of photons emerging from the material during a counting period, regardless of the energy of the photons, is for example carried out by integrating the spectrum provided by the gamma spectrometric device 7 during the counting period. The measurement method comprises the achievement of several counting measurements during the achievement of a same spectrometric measurement. The duration p of the counting periods is less than those d of the spectrometry periods. Each spectrometry period is subdivided into n counting periods, n being an integer strictly greater than 1. Because of the statistical nature of photon measurements, it is necessary to carry out a spectrometric measurement over a sufficient period in order to have a representative sample of photons, allowing a reliable analysis of the spectrum, notably by comparing the number of photons at different energies. It is possible to conduct counting measurements over a more reduced period in so far that the results of the counting measurement are only used as a whole for determining the radioactivity of the material as a whole and not for characterizing

12168552_1 (GHMatters) P104999.AU one or several radionuclides from the number of photons detected at different energies. For a strongly radioactive material, the duration of a counting measurement is for example of the order of one second and the duration of a spectrometric measurement is of the order of 30 seconds. For a weakly radioactive material, the duration of a counting measurement is for example of the order of thirty seconds and the duration of a spectrometric measurement is of the order of 30 minutes. The measurement method may advantageously comprise the calculation of the activity, of the mass and/or of the content of at least one radionuclide of the material at the end of a counting measurement, depending on the counting measurement on the one hand and on the activity, the mass and/or the content of this radionuclide calculated from a completed previous spectrometric measurement, and it may be preferable in some embodiments to be the last conducted spectrometric measurement. This intermediate characterization is based on the assumption that the mass ratios of the different radionuclides in the material remain constant between two consecutive spectrometric measurements. At the end of a spectrometry period Dprec, the spectrometric measurement carried out over this spectrometry period Dprec gives the possibility of calculating the average content of a radionuclide X Tx(Dprec) and the overall count S(Dprec) of the photons detected over this spectrometric period, regardless of their energy. During the following spectrometry period Dext, the average content of a radionuclide X Tx(Pnxt) over a counting period Pext included in this current following spectrometry period Dext is for example determined according to the following equation: Tx(Pnext) = Tx (Dprec) *n*s(Pext)/S(Dprec) wherein n is the integer number of counting periods in a spectrometry period; s(Pnext) is the overall count of photons over the relevant Pext counting period; Tx (Dprec) is the average radionuclide X content calculated from the spectrometric measurement of the preceding spectrometry period Dprec; and S(Dprec) is the overall count of the photons over the preceding spectrometry period Dprec.

The calculation of the content is thus carried out for each counting period included in the current spectrometry period, depending on the content and on the overall count of the preceding spectrometry period.

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At the end of each spectrometry period, the average radionuclide X content and the overall count of the photons are updated and used for the calculations of the average radionuclide X content of each counting period of the next spectrometry period. The measurement method comprises the generation of files of measurements which may be consulted by an operator. In an embodiment, the processing unit 8 is configured for, for example generating at the end of each counting period and/or at the end of each spectrometry period, a measurement file indicating the content and the mass of one or several radionuclides of interest. The processing unit 8 may advantageously comprise a database storing the spectra of measurements and the measurement files. The results of the spectrometric measurements may vary according to the height of material on the belt. Indeed, the probability that a photon emitted by a layer of the material at a distance from the gamma spectrometry device 7 is absorbed by a lower layer of the actual material before attaining the gamma spectrometric device 7 is higher for a low energy photon than for a high energy photon. Thus, the variation of the height of the material modifies the relative detection efficiencies of the photons according to their energy. The measurement method comprises the measurement of the height of material on the belt, the determination of the detection efficiency associated with an energy and with the measured height, and the achievement of the calculations of activity, of mass and/or of content of at least one radionuclide depending on the determined detection energy. In an embodiment, the measurement method comprises the determination of the detection efficiency by consulting predetermined tables indicating the detection efficiency versus the energy and the height. The tables are for example recorded in a memory of the processing unit 8. The system 2 comprising the conveyor 4 and the device for measuring radioactivity 5 may be used for characterizing different types of radioactive or potentially radioactive materials. The material is for example an ore or a material stemming from a dismantlement or clean-up of nuclear installations, or of potentially contaminated soils. The material is a bulk product with variable grain size (block, granulate, powder). The material is for example an ore, concrete, cement, earth. The system 2 gives the possibility of detecting the presence in the conveyed material of any radionuclide emitting gamma photons during its radioactive decay.

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As an example, the system 2 gives the possibility of detecting uranium, thorium, cesium or plutonium. The device for measuring radioactivity 5 may be used for characterizing the uranium, for example contained in an ore. More than 90% of the gamma radiations emitted during the radioactive decay of uranium is related to the end of the decay chain, in particular lead 214 (Pb214) and bismuth 214 (Bi214). Thus, if one of the descendants of uranium has the possibility of migrating differently from uranium, the sum of the gamma radiations of the whole of the descendants of the uranium is not proportional to the amount of uranium present. Uranium 238 (U238) is not directly an emitter of gamma radiations but it may be perfectly correlated with its second descendant protactinium 234 (Pa234m) which emits a line at 1,001keV. Indeed, Pa234m is always in equilibrium with U238 under geological conditions, since Pa234m has a very short period and is in equilibrium with U238 in less than a year. The activity of U238 is considered to be equal to the activity of Pa234. The measurement method comprises the calculation of the activity of U238 as indicated above, from the net measurement of the area of the peak at the energy of 1,001keV: Au238(1,001keV) = Apa 2 34 m(1,001keV) = M(1,001keV)

/ (e(1,001keV)*1u 23 8 (1,001keV)) wherein Au238(1,001keV) is the activity of U238 at the energy level of 1,001keV; Apa 2 34 m(1,001keV) is the activity of Pa234m at the energy level of 1,001keV; M(1,001keV) is the net area of the peak of the spectrum under the peak of energy at 1,001keV, e(1,001keV) is the efficiency of the gamma spectrometric device for photons of energy 1,001keV and u238(1,001keV) is the intensity of the radiation at the energy of 1,001keV for U238. Bismuth 214 (Bi214) emits photons at four main energies i.e.: 934 keV, 1,120 keV, 1,509 keV or 2,448 keV. The measurement method comprises the calculation of the activity of Bi214 from each of the four energy peaks corresponding to Bi214: ABi214(EBi) = M(EBi) / (e( EBi)*u238(EBi)), for each energy peak of level Ei associated with Bi214. The measurement method comprises the control of the spectrometric measurement by comparing the activities of Bi214 with each other and/or with their

12168552_1 (GHMatters) P104999.AU average. A too large deviation may be the sign of an error, notably on the density or the volume of the relevant material during the spectrometry period. In an embodiment, the measurement method comprises the emission of a non-blocking alert signal if there exists a deviation greater than a threshold value (for example 30%) between at least two of the activities of Bi214 and/or the average of the activities of Bi214. The measurement method comprises the calculation of the ratio between the activity of U238 and the average of the activities of Bi214. This allows determination of the disequilibrium level of the ore noted as the U/Bi ratio. It may be advantageous that the measurement method comprises the calculation of activity of U235 depending on the different characteristic energy of U235, except for the line at 186keV which also corresponds to the emission line of radium 226 (Ra226). The measurement method comprises the calculation of the number of counts due to U235 in the peak at 186keV depending on one or several activities of U235 and/or on the average of the activities of U235, from the following equation: Mu235(186keV )= Au235 * e(l86keV) * lu23(186keV) wherein Au235is an activity of U235 measured from a peak different from the 186keV peak or an average of activities of U235 measured from several peaks different from the peak at 186keV. The measurement method comprises the calculation of a ratio between the activities of U235 and of U238, which allows determination of the U235/U238 ratio and verification of the enrichment in U235. The measurement method comprises the calculation of the activity of radium 226 (Ra226) like the substraction of the net total area MT of the 186 keV peak reduced by the number of counts due to U235 evaluated earlier, according to the following equation: AR 22 e(186keV) = (M(186keV)- Mu23s(186keV)) / (e(l86keV) * lu238(186keV)) wherein M(186keV) is the total net area of the peak at 186 keV and Mu2 3 s(186keV) is the number of counts due to U235. The ratio between Au238 and ARa226 gives the possibility of determining the disequilibrium level of the ore, noted as the U/Ra ratio. For U235, a longer acquisition period may be required for calculating measurements of significant activity from spectrometric measurements provided by the spectrometric device. The measurement of the activity of U235 is conducted for

12168552_1 (GHMatters) P104999.AU example after a number N of spectrometry periods, N being an integer strictly greater than 1. For example N is equal to 5. By means of the invention, it is possible to conduct significant on-line measurements on the material during its transport on the conveyor 4. The spectrometric measurements allow determination of the activities, masses and/or contents of one or several particular radionuclides, for fine characterization of the material. The spectrometric measurements carried out over long periods and counting measurements carried out during the spectrometric measurements, on shorter periods, allow determination of a relatively reliable content and mass of one or several particular radionuclides between two spectrometric measurements. The spectrometric measurements and the counting measurements are carried out by means of the same gamma spectrometric device. Thus, the number of components is limited and the measurement method may be applied reliably in difficult environments (dust, temperatures...). The device for measuring radioactivity is passive. It is configured for conducting a passive measurement of the intrinsic radioactivity of the material conveyed by the conveyor 4, without subjecting the material to excitation radiation. The device for measuring radioactivity does not proceed with an active measurement consisting of subjecting the material to an excitation radiation, in particular neutron radiation, generated by a radiation source, in order to then detect the induced radioactivity. The characterization of the ore for example gives the possibility of achieving an output balance of the workshop located upstream from the conveyor or an input balance for the workshop located downstream from the conveyor. This measurement for example gives the possibility of verifying the radionuclide content of the ore produced in a mine, for example a uranium or thorium mine, and of verifying the consistency with measurements carried out in an ore processing plant. For example in the case of earths, contaminated cement or concretes, this measurement gives the possibility of sorting batches of potentially contaminated materials. In some embodiments, the method for measuring radioactivity is applied on a given amount of a material (or batch) which is desired to be characterized as a whole, by determining the activity, the content and/or the mass of one or several

12168552_1 (GHMatters) P104999.AU radionuclides in this relevant batch as a whole. The batch is gradually conveyed on the conveyor 4 while applying the method for measuring radioactivity, and the characterization is determined at the end of the passing of the whole of the batch, by establishing an average characterization for the whole of the batch. The method for measuring radioactivity is not applied for sorting the batch by separating different portions of the batch according to the activity, the content and/or the mass of one or several radionuclides in such or such portion of the batch. The characterization of the whole of a batch is notably useful during decontamination operations for characterizing different batches. In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

12168552_1 (GHMatters) P104999.AU

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1.- A device for measuring the radioactivity of a material during its transport by a conveyor, the device for measuring radioactivity comprising a gamma spectrometric device having at least one gamma spectrometry detector and provided so as to be positioned on the path of the conveyor in order to conduct spectrometric measurements on the material during its transport by the conveyor, these measurements being conducted by detecting photons emerging from the flow of transported material, and a processing unit for processing measurement signals provided by the gamma spectrometric device, the device for measuring radioactivity being configured for carrying out spectrometric measurements on the material during its transport by the conveyor, each spectrometric measurement being carried out over a spectrometry period, during which the photons are detected by associating with each detected photon an energy, in order to determine the population of photons versus their energy, and for carrying out, during a spectrometric measurement, several gamma counting measurements on the material during its transport by the conveyor, by means of the gamma spectrometric device, each gamma counting measurement being conducted over a counting period, at the end of which is determined the total number of detected photons during the counting period, independently of the energy of the photons. 2.- The device for measuring radioactivity according to claim 1, wherein the processing unit is programed for calculating the activity, the mass and/or the content of at least one radionuclide present in the material depending on a spectrometric measurement. 3.- The device for measuring radioactivity according to claim 1 or 2, configured for calculating the activity, the mass and/or the content of at least one radionuclide depending on a counting measurement and on the content of this radionuclide calculated according to a previous spectrometric measurement. 4.- The device for measuring radioactivity according to any one of the preceding claims, comprising a device for measuring the height of material on the conveyor in register with the gamma spectrometric device and/or a weighing device for measuring the masse of the material transported by the conveyor. 5.- The device for measuring radioactivity according to any one of the preceding claims, configured for calculating the activity, the mass and/or the content

12168552_1 (GHMatters) P104999.AU of at least one radionuclide depending on a detection efficiency of photons by the gamma spectrometric device according to the energy of these photons, to the height of material on the conveyor in register with said or each gamma spectrometry detector, and/or on the mass of the material transported by the conveyor. 6.- A system for measuring radioactivity, comprising a conveyor for transporting a material and a device for measuring radioactivity according to any one of the preceding claims, the gamma spectrometric device of which is positioned on the path of the conveyor. 7.- The system according to claim 6, wherein the conveyor is a belt conveyor comprising an endless belt for transporting the material. 8.- A method for measuring radioactivity of a material transported by a conveyor, notably a conveyor with an endless belt, comprising the achievement of gamma spectrometric measurements on the material during its transport by the conveyor, by means of a gamma spectrometric device comprising at least one gamma spectrometry detector laid out on the path of the conveyor for detecting the photons emitted by a flow of material during its transport by the conveyor, each spectrometric measurement being carried out over a spectrometry period, during which the photons are detected by associating with each detected photon an energy, in order to determine the population of photons versus their energy, the method comprising the achievement, during a spectrometric measurement, several gamma counting measurements on the material during its transport by the conveyor, by means of the gamma spectrometric device used for spectrometric measurements, each gamma counting measurement being conducted over a counting period, at the end of which is determined the total number of detected photons during the counting period, independently of the energy of the photons. 9.- The measurement method according to claim 8, comprising the calculation of the activity, of the mass and/or of the content of at least one radionuclide present in the material depending on a spectrometric measurement. 10.- The method according to claim 8 or 9, comprising the calculation of the activity, of the mass and/or of the content of at least one radionuclide depending on a counting measurement and on the content of this radionuclide calculated according to a previous spectrometric measurement. 11.- The method according to any one of claims 8 to 10, comprising the measurement of the height of material on the conveyor in register with the gamma

12168552_1 (GHMatters) P104999.AU spectrometric device and/or the measurement of the mass of the material transported by the conveyor. 12.- The method according to any one of claims 8 to 11, comprising the calculation of the activity, of the mass and/or of the content of at least one radionuclide depending on a detection efficiency of photons by the gamma spectrometry device according to the energy of these photons, to the height of material in register with said or each gamma spectrometry detector, and/or of the measurement of the mass of the material transported by the conveyor. 13.- The method according to any one of claims 8 to 12, comprising the verification of a spectrometric measurement and/or of calculations carried out from this spectrometric measurement by comparison of activity, of mass and/or of content of at least one radionuclide, in particular bismuth 214, calculated according to the main spectral lines of characteristic energies of this same radionuclide. 14.- The method according to any one of claims 8 to 13, comprising the calculation of the activity, of the mass and/or of the content of uranium 235 and/or of uranium 238 depending on the results of a spectrometric measurement for protactinium 234. 15.- The measurement method according to any one of claims 8 to 14, comprising the calculation of the activity, of the mass and/or of the content of protactinium 234, the calculation of the activity, of the mass and/or of the content of bismuth 214 and/or calculation of the activity, of the mass and/or of the content of radium 226, depending on a spectrometric measurement. 16.- The measurement method according to any one of claims 8 to 15, comprising the calculation of the activity, of the mass and/or of the content of uranium 235 and/or the calculation of the activity, of the mass and/or of the content of uranium 238 depending on a spectrometric measurement. 17.- The measurement method according to claims 15 and 16, wherein the calculation of the activity, of the mass and/or of the content of uranium 235 and/or the calculation of the activity, of the mass and/or of the content of uranium 238 is carried out from the activity, from the mass and/or from the content calculated for protactinium 234, bismuth 214 and/or radium 226.

12168552_1 (GHMatters) P104999.AU