OA18483A - Device and method for measuring the radioactivity of a material. - Google Patents

Abstract

The measuring device is intended for measuring the radioactivity of a material during its transport by a conveyor, and comprises a gamma spectrometry device (7) having at least one gamma spectrometry detector (9) and intended to be placed on the path of the conveyor to perform spectrometry measurements on the material during its transport by the conveyor (4), these measurements being carried out by the detection of the photons emerging from the flow of material transported, and a processing unit to process the signals measurements provided by the gamma spectrometry device (7).

Description

The present invention relates to the field of measuring the radioactivity of a material.

When operating a radioactive ore mine or sorting contaminated soil, it is desirable to analyze the radioactivity of the material in question.

To do this, it is possible to carry out radioactivity measurements in the laboratory on samples taken from the material.

However, these one-off measurements do not make it possible to analyze all of the material (ore, material resulting from the dismantling or remediation 10 of nuclear installations, land contaminated by different types of radionuclides). In addition, it may take some time to obtain scan results.

GB2017294A discloses an ore sorting device comprising a conveyor belt for transporting the ore, a counting device suitable for carrying out a count measurement of the number of gamma rays emerging from the ore and deducing therefrom a total radioactivity level of the material, and a counting device. switching device to sort the different zones of the ore according to their level of radioactivity.

However, these measurements only make it possible to measure the total radioactivity level of the material but do not make it possible to identify the radionuclides present in the material, nor to determine the origin or the distribution of the total radioactivity of the material if there is. several radionuclides present in the material.

One of the aims of the invention is to provide a device for measuring radioactivity making it possible to identify and quantify the radionuclides present in a material transported on a conveyor, in particular in the case of a radioactive ore.

To this end, the invention provides a device for measuring the radioactivity of a material during its transport by a conveyor, the device for measuring radioactivity comprising a gamma spectrometry device having at least one gamma spectrometry detector and designed to be arranged on the path of the conveyor to perform spectrometry measurements on the material during its transport by the conveyor, these measurements being carried out by the detection of the photons emerging from the flow of material transported, and a processing unit for processing signals from measurement provided by the gamma spectrometry device.

According to particular embodiments, the device for measuring radioactivity comprises one or more of the following characteristics:

- it is configured to calculate the activity, mass and / or content of at least one radionuclide present in the material according to a spectrometry measurement; .

f

- it is configured for performing several gamma counting measurements on the material during its transport by the conveyor, during a spectrometry measurement;

- it is configured to calculate the activity, mass and / or content of at least one radionuclide according to a count measurement and the content of this radionuclide calculated according to a previous spectrometry measurement;

- It includes a device for measuring the height of material on the conveyor in line with the gamma spectrometry device;

- it includes a weighing device for measuring the mass of the material transported by the conveyor;

- It is configured to calculate the activity, mass and / or content of at least one radionuclide as a function of an efficiency of detection of photons by the gamma spectrometry device as a function of the energy of these photons, of the height of material on the conveyor in line with the or each gamma spectrometry detector and / or the mass of the material transported by the conveyor;

- the height of material on the conveyor is between 5 cm and 60 cm;

- the gamma spectrometry device comprises several gamma spectrometry detectors.

The invention also provides a radioactivity measurement system, comprising a conveyor for transporting a material and a radioactivity measuring device as defined above, the gamma spectrometry device of which is placed on the path of the conveyor.

According to particular embodiments, the system comprises one or more of the following characteristics:

- the conveyor is a belt conveyor comprising an endless belt for transporting the material.

- the height of material on the conveyor is between 5 cm and 60 cm;

- the gamma spectrometry device comprises several gamma spectrometry detectors.

The invention also proposes a method for measuring the radioactivity of a material transported by a conveyor, in particular an endless belt conveyor, comprising the performance of gamma spectrometry measurements on the material during its transport by the conveyor, to the using a gamma spectrometry device comprising at least one gamma spectrometry detector arranged in the path of the conveyor to detect the photons emitted by a flow of material during its transport by the conveyor.

According to particular modes of implementation, the method may include one or more of the following characteristics:

- it includes the calculation of the activity, mass and / or content of at least one radionuclide present in the material according to a spectrometry measurement;

- It includes performing gamma counting measurements on the material during its transport by the conveyor, using the gamma spectrometry device used for the spectrometry measurements;

- several count measurements are carried out during a spectrometry measurement;

- it includes the calculation of the activity, the mass and / or the content of at least one radionuclide according to a count measurement and the content of this radionuclide calculated according to a previous spectrometry measurement ;

- it includes the measurement of the height of material on the conveyor in line with the gamma spectrometry device;

~ it includes the measurement of the mass of the material transported by the conveyor;

- it includes the calculation of the activity, mass and / or content of at least one radionuclide as a function of an efficiency of detection of photons by the gamma spectrometry device as a function of the energy of these photons , the height of material in line with the or each gamma spectrometry detector, and / or the measurement of the mass of the material transported by the conveyor;

- it includes the verification of a spectrometry measurement and / or calculations carried out from this spectrometry measurement by comparison of activity, mass and / or content of at least one radionuclide, in particular bismuth 214 ( Bi214), calculated as a function of main spectral lines of characteristic energies of this same radionuclide;

- it includes the calculation of the activity, mass and / or content of uranium 235 (U235) and / or uranium 238 (U238) based on the results of a spectrometry measurement for protactinium 234 (Pa234);

- it includes the calculation of the activity, the mass and / or the content of protactinium 234, the calculation of the activity, the mass and / or the content of bismuth 214 (BÎ214) and / or the calculation the activity, mass and / or content of radium 226 (Ra226), as a function of a spectrometry measurement;

- it includes the calculation of the activity, the mass and / or the content of uranium 235 (U235) and / or the calculation of the activity, the mass and / or the content of uranium 238 (U238) as a function of a spectrometry measurement;

- it includes the calculation of the activity, the mass and / or the content of uranium 235 and / or the calculation of the activity, the mass and / or the content of uranium 238 is carried out from ( activity, mass and / or content calculated for protactinium 234, bismuth 214 and / or radium 226;

- the height of material on the conveyor is between 5 cm and 60 cm;

* the gamma spectrometry device includes several gamma spectrometry detectors.

The invention and its advantages will be better understood on reading the description which follows, given solely by way of non-limiting example, and made with reference to the accompanying drawings, in which:

- Figure 1 is a schematic side view of a radioactivity measuring system comprising a conveyor and a radioactivity measuring device;

- Figure 2 is a schematic cross-sectional view of the system of Figure 1, according to II-II in Figure 1;

- Figure 3 is a schematic view of the radioactivity measuring device of all of Figures 1 and 2; and

- Figures 4 is a graph illustrating a spectrum resulting from a gamma spectrometry measurement.

The radioactivity measuring system 2 of Figures 1 and 2 comprises a conveyor 4 provided for conveying a material, for example a radioactive ore extracted from a mine, and a radioactivity measuring device 5 suitable for measuring the radioactivity of the material flow. during the transport of the material by the conveyor 4, in particular by performing spectrometry measurements.

The conveyor 4 comprises an endless conveyor member for transporting the material. The conveyor 4 is here a belt conveyor, the conveying member of which is an endless conveyor belt 6.

The radioactivity measuring device 5 is suitable for performing both spectrometry measurements during which the radioactivity measuring device 5 identifies and quantifies the various radionuclides contained in the material during its transport by the conveyor 4, and measurements counting during which the radioactivity measuring device 5 counts the photons emitted by the material during its transport by the conveyor to determine the total radioactivity of the material.

The radioactivity measuring device 5 comprises a gamma spectrometry device 7 suitable for detecting the gamma photons emitted by the material during its transport by the conveyor 4 and measuring the energy of the photons detected, and a data processing unit 8 for processing the corresponding measurement signals supplied by the gamma spectrometry device 7. The gamma spectrometry device 7 is placed 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 spectrometry device 7 comprises at least one gamma spectrometry detector, or gamma spectrometer 9, for carrying out gamma spectrometry measurements on the material during its transport by the conveyor 4. The gamma spectrometry device 7 preferably comprises several spectrometers gamma 9, typically two to four gamma spectrometers, Here three gamma spectrometers 9 (Figure 1).

Each gamma spectrometer 9 detects photons emitted by the material located on band 6 and emerging from this material, and measures the energy of each photon detected. Each gamma spectrometer 9 provides at output a signal representative of the number of photons emitted and of the energy associated with each photon detected. Such a signal then makes it possible to determine a gamma spectrum indicating, over a given period of time, the number of photons detected as a function of energy.

The gamma spectrometry device 7 comprises a shield 10 surrounding the gamma spectrometers 9 and arranged to allow the photons emerging from the material transported by the conveyor 4 and located in line with the gamma spectrometers 9 to reach the gamma spectrometers 9, while protecting the spectrometers gamma 9 of photons emitted by other sources of photons. The shielding 10 is made of a material which strongly attenuates radiation, for example lead or tungsten.

As an option, the radioactivity measuring device 5 comprises a height sensor 12 for measuring the height of the material transported by the conveyor 4 in line with the gamma spectrometry device 7.

The height sensor 12 is for example a laser range finder placed above the strip 6 to measure the distance between the range finder and the upper surface of the material transported by the strip 6, and calculate the height of material as a function of the distance from the rangefinder to conveyor 4 in the absence of material on conveyor 4.

The height of the material transported by the strip 6 is typically between 5 cm and 60 cm.

Optionally, the system 2 includes a jig for leveling the level of the material on the strip 6. The height of the material on the strip 6 is then substantially constant.

Optionally, the radioactivity measuring device 5 comprises a weighing device 14 for measuring the mass of the material transported by the conveyor 4 in line with the gamma spectrometry device 7 during a given period of time.

The height sensor 12 and the weighing device 14 are arranged in line with the gamma spectrometry device 7 or offset upstream or downstream with respect to the gamma spectrometry device 7 along the conveyor 4. In the case of an offset in upstream or downstream, knowing the movement of the conveyor 4 then makes it possible to associate each height and / or mass measurement with the corresponding spectrometry measurement.

The band 6 has a transport strand 6A ensuring the transport of the material and (circulating to the left in Figure 1) and a return strand 6B (flowing to the right in Figure 1).

The gamma spectrometry device 7 is arranged so as to perform measurements on the material transported by the band 6, more specifically by the transport strand 6. The gamma spectrometry device 7 is preferably placed below the transport strand 6A, in preferably being attached to the transport strand 6A. As a variant, the gamma spectrometry device 7 is disposed above or to the sides of the transport strand 6A.

The conveyor 4 comprises guide assemblies 16 with rollers distributed under the transport strand 6A to guide and support it. The gamma spectrometry device 7 is disposed between two adjacent guide assemblies 16, so as to be placed as close as possible to the strip 6. The guide assemblies 16 optionally include guide rollers 17 which are arranged to raise the side parts. of the transport strand 6A.

The web 6 is wound on two return rollers 18, so that the transport strand 6A and the return strand 6B each circulate from one roll 18 to the other, in opposite directions. At least one of the return rollers 18 is motorized for driving the strip 6.

As an option, in the case of excessive ambient temperatures, the radioactivity measuring device 5 comprises a ventilation and / or air conditioning system arranged to maintain the spectrometry device (spectrometers) under standard operating conditions for this type of equipment ( -10'C <T <+30 ^e C).

As illustrated in FIG. 3, each gamma spectrometer 9 comprises a photon detector 22 capable of detecting photons reaching the detector and of providing, for each photon detected, a measurement signal representative of the energy of this photon. Each photon detected is therefore counted and characterized by its energy.

The photon detector 22 is for example a scintillation detector comprising a crystal capable of emitting a light signal when it is struck by a photon, which is then converted into an electrical signal. The crystal is for example made from lanthanum bromide (LaBr).

As a variant, the photon detector 22 is a semiconductor detector comprising a semiconductor substrate capable of emitting an electrical signal when struck by a photon. The semiconductor substrate is for example made of germanium (Ge).

Each gamma spectrometer 9 comprises an amplifier 24 and an analog / digital converter 26. Amplifier 24 is, for example, an electronic amplifier or a photomultiplier. Amplifier 24 amplifies the measurement signal supplied by photon detector 22.

The converter 26 receives the amplified measurement signals and supplies a digital output signal forming the output signal of the gamma spectrometer 9.

Advantageously, the gamma spectrometer 9 is a medium resolution or high resolution gamma spectrometer over an energy range of between 50keV and 3MeV. The absolute resolution of the gamma 9 spectrometer at a given energy is the full width at half height (or FWHM for "full width at mid height") of a peak of a spectrum supplied by the spectrometer at that given energy. Relative resolution is expressed as a percentage, is equal to the ratio of full width at half height to energy multiplied by 100. An average relative resolution is between 1% and 5%. High relative resolution is less than 1%. Thus, the gamma spectometer 9 has a relative resolution of less than 5% (case of medium resolution) or less than 1% (case of high resolution).

In preferred embodiments, the gamma spectrometer 9 is a lanthanum bromide scintillation detector, which generally has medium resolution, or a germanium (Ge) detector, which generally has high resolution.

The processing unit 8 is a computer, for example a personal computer. It comprises a processor and a memory for storing software applications that can be executed by the processor for processing the measurements supplied by the gamma spectrometry device 7.

The processing unit 8 is connected to the gamma spectrometer device 7 to receive the output signal from each gamma spectrometer 9. The processing unit 8 receives the output signal from each gamma spectrometer 9 individually. The processing unit 8 has an input associated with each gamma spectrometer 9. As a variant, the processing unit 8 has an input receiving a signal containing the output signals of the various gamma spectrometers 9.

The processing unit 8 is configured to record each output signal of each gamma spectrometer 9 and establish, over a given measurement period, a gamma spectrum of the population of photons detected over the measurement period as a function of the energy of radiation of the photons detected.

In the case where the gamma spectrometry device 7 has several gamma spectrometers 9, the processing unit 8 is configured to add the photons detected by the different gamma spectrometers 9 over each measurement period. The installation of several gamma spectrometers 9 makes it possible to increase the number of photons taken into account over the same measurement period.

The gamma spectrometry device 7 and the processing unit 8 are separate and distant from each other. The gamma spectrometry device 7 is attached to the band 6 while the processing unit is remote from the band 6. As a variant, the gamma spectrometry device 7 and the processing unit 8 are combined in the same detection unit. and processing.

In operation, a flow of material is transported by the conveyor 4 and scrolls past the gamma spectrometry device 7 which measures the gamma radiation emitted by the various radionuclides contained in the material transported by the conveyor 4. The measurements are carried out during that the material is transported by the conveyor 4 at a non-zero speed.

The radioactivity measuring device 5 is suitable for implementing a measurement method for the characterization of the material transported by the conveyor.

4. In particular, the processing unit 8 is programmed for the implementation of this method according to the measurement signals supplied by the gamma spectrometry device 7.

The characterization of the material includes the identification and quantification of the different radionuclides contained in the material. It preferably comprises the calculation of the activity, mass and / or content of one or more particular radionuclide (s) according to the spectra established from the measurements provided by the gamma spectrometry device 7.

The measurement method includes performing spectrometry measurements, each performed over a spectrometry period, during which the photons are detected by associating each detected photon with an energy, to determine the population of photons as a function of their energy. At the end of a spectrometry measurement, a spectrum of the population of photons is established as a function of their energy over the spectrometry period. The result of a spectrometry measurement is a spectrum. Each spectrometry measurement is carried out from the measurement signals supplied by the gamma spectrometry device 7 over a spectrometry period. The spectrometry periods have the same duration d. The spectrometry measurements are carried out over distinct successive spectrometry periods.

The measurement process includes calculating the activity, mass and / or content of one or more radionuclide (s) of interest according to each spectrometry measurement. The calculation is performed at the end of the spectrometry period.

FIG. 4 represents an energy gamma spectrum of a radioactive material. The spectrum includes peaks around characteristic energies of particular radionuclides. A radionuclide can emit photons at one or more energies and thus be associated with one or more peaks of the spectrum.

The decay of a radionuclide can result in one or more other radionuclides descending with different characteristic energies. The characterization of certain radionuclides therefore makes it possible to characterize one or more ascending or descending radionuclide (s).

The detection efficiency e (E) of a photon detector at radiation energy E is the ratio between the number of photons at radiation energy E detected and the number of photons at radiation energy E emitted by radioactive products contained in the material. The detection efficiency is a unitless number strictly less than 1. The detection of high energy photons is more efficient than the detection of low energy photons because the latter will be strongly absorbed within the same thickness of the material crossed. .

The radioactivity measuring device 5 is calibrated by determining its detection efficiency as a function of the radiation energy E. The detection efficiency as a function of the radiation energy E is determined for example by physical or modeled calibration, at using commercial tools (ISOCS or MERCURAD PASCALYS type marketed by the CANBERRA company)

The measurement method comprises measuring the radioactive activity of a radionuclide emitting photons at radiation energy E as a function of the measurement of the net area M of the peak of the spectrum at radiation energy E, following l 'following equation:

Ax (E) = M (E) / (e (E) 'lx (E)) where Αχ is the radioactive activity (number of decays per second) of the radionuclide X at energy E in Becquerel (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 lx (E) is the line intensity at the radiation energy E for the radionuclide X (number of photons emitted by Bq).

The intensity l _x (E) comes from nuclear data tables, for example a table of type

JEFF. The intensity l _x (E) has no unit.

The measurement method comprises calculating the mass of at least one radionuclide X detected by the gamma spectrometry device 7 during a spectrometry period as a function of a predetermined proportionality factor and the radioactive activity A _x of this radionuclide X.

The mass is determined from the following equation:

A _X (E) = À _x * N * m _x / Mmol _x where A _x is the activity of radionuclide X at energy E in Becquerel (Bq), λ is the half-life of radionuclide X (s' ¹ ) , N is the Avogadro number of radionuclide X, m _x is the mass of radionuclide X in grams (g), and Mmol _x is the molar mass of radionuclide X in grams per mole (g / mol).

The predetermined proportionality factor is thus λ _x * N / Mmol _x which is a constant value for the radionuclide X.

The measurement method comprises calculating the content T of the material in one or more radionuclide (s).

The content T _x of a radionuclide X in the material is calculated as a function of the mass of this radionuclide, the average density of the material (in g / cm ³ ) and the volume of material conveyed by strip 6 opposite the device gamma spectrometry 7 during the spectrometry period.

The mass of material is measured using the weighing device 14. The volume of the material is calculated from the displacement of the conveyor 4 and 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 sensor 12. As a variant, if the height is calibrated, it is possible to consider that this area is constant.

The measurement method includes performing count measurements, each performed over a count period, after which the total number of photons detected during the count period is determined, regardless of the energy of the photons. The counting periods have the same duration g. The count measurements are carried out using the measurements provided by the gamma spectrometry device 7.

Counting photons over a counting period of determined duration makes it possible to calculate the overall radioactivity of the material.

The counting of all the photons emerging from the material during a counting period, regardless of the energy of the photons, is performed, for example, by integrating the spectrum supplied by the gamma spectrometry device 7 during the counting period.

The measurement method comprises performing several count measurements during the performance of the same spectrometry measurement. The duration p of the counting periods is less than that of the spectrometry periods. Each spectrometry period is subdivided into n counting periods, n being an integer strictly greater than 1.

Due to the statistical nature of photonic measurements, it is necessary to perform a spectrometry measurement over a sufficient period of time to have a representative sample of photons, allowing reliable analysis of the spectrum, in particular by comparison of the number of photons at different energies.

It is possible to perform the count measurements over a shorter period since the results of the count measurement are used only as a whole to determine the radioactivity of the material as a whole and not to characterize one or more radionuclides at from the number of photons detected at different energies.

For a highly radioactive material, the duration of a count measurement is for example of the order of one second and the duration of a spectrometry 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 spectrometry measurement is of the order of 30 minutes.

The measurement method advantageously comprises the calculation of the activity, the mass and / or the content of at least one radionuclide of the material at the end of a counting measurement, as a function, on the one hand, of the count measurement and, on the other hand, the activity, the mass and / or the content of this radionuclide calculated from a previous spectrometry measurement completed, preferably the last spectrometry measurement carried out.

This Intermediate characterization is based on the assumption that the mass ratios of the various radionuclides in the material remain constant between two consecutive spectrometry measurements.

At the end of a period of spectrometry D ^, the spectrometry measurement carried out over this period of spectrometry D _prec makes it possible to calculate the content of a radionuclide X mean T _x (D _{pn! C} ) and the overall count StD ^) photons detected over this period of spectrometry, whatever their energy.

During the following D ^ spectrometry period, the content of a radionuclide

X mean ΤχίΡ _{ηβ) (Ι} ) over a counting period P ^ included in this current Dnext spectrometry period in progress is determined for example according to the following equation:

ΤχίΡη, χ,) = Tx (D _prac ) ^ SÎP ^ rtJ / SiDpœ) where n is the integer number of counting periods in a spectrometry period; s (Pnext) is the global count of photons over the Pnext count period considered; Τχ (D _prec ) is the average radionuclide X content calculated from the spectrometry measurement of the preceding D _prec spectrometry period; and SiDp ^) is the global count of photons over the previous Dp ^ spectrometry period.

The content is thus calculated for each counting period included in the current spectrometry period, as a function of the content and of the overall count of the previous spectrometry period.

At the end of each spectrometry period, the average radionuclide X content and the overall photon count are updated and used for the average radionuclide X content calculations for each count period of the following spectrometry period.

The measurement method comprises the generation of measurement files that can be consulted by an operator. In one embodiment, the processing unit 8 is configured to generate, for example 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 more radionuclide (s) of interest. The processing unit 8 advantageously comprises a database storing the measurement spectra and the measurement files.

The results of spectrometry measurements may vary depending on the height of material on the strip. Indeed, the probability that a photon emitted by a layer of the material distant from the gamma spectrometry device 7 is absorbed by a lower layer of the material itself before reaching the gamma spectrometry device 7 is higher for a photon of low energy than for a high energy photon. Thus, the variation in the height of the material modifies the relative detection efficiencies of the photons as a function of their energy.

The measurement method includes measuring the height of material on the web, determining the detection efficiency associated with an energy and the measured height, and performing the activity, mass and / or grade calculations. into at least one radionuclide depending on the determined detection energy.

In one embodiment, the measurement method includes determining the detection efficiency by looking up predetermined tables indicating the detection efficiency as a function of energy and height. The tables are for example saved in a memory of the processing unit 8.

The system 2 comprising the conveyor 4 and the radioactivity measuring device 5 can be used to characterize different types of radioactive or potentially radioactive materials. The material is, for example, an ore or a material resulting from the dismantling or remediation of nuclear installations, or potentially contaminated soil. The material is a loose product of variable particle size (block, aggregate, powder). The material is for example an ore, concrete, cement, earth.

System 2 makes it possible to detect the presence in the conveyed material of any radionuclide emitting gamma photons during its radioactive decay. For example, System 2 can detect uranium, thorium, cesium or plutonium.

The radioactivity measuring device 5 can be used to characterize the uranium, contained for example in an ore.

More than 90% of the gamma radiation emitted during the radioactive decay of uranium is linked to the end of the decay chain, in particular lead 214 (Pb214) and bismuth 214 (BÎ214).

Thus, if one of the descendants of uranium has the possibility of migrating differently from uranium, the sum of the gamma radiation of all of the descendants of uranium is not proportional to the amount of uranium present.

Uranium 238 (U238) is not a direct emitter of gamma radiation but it is perfectly correlable with its second descendant, protactinium 234 (Pa234m) which emits a line at 1001keV. This is because Pa234m is always in equilibrium with U238 under geological conditions, because Pa234m has a very short half-life and is in equilibrium with U238 in less than a year. The activity of U238 is considered equal to the activity of Pa234.

The measurement method includes calculating the activity of U238 as indicated above, from the net measurement of the peak area at 1001keV energy;

Au23e (1001keV) = Ap ^ _Mm (1001keV) = M (1001keV) / (e (1001keV) 'l _U238 (1001keV)) in which Au ₂₃₈ (1001keV) is the activity of U238 at the energy level 1001 keV;

Ap ^ 4m (1001keV) is the activity of Pa234m at the 1001keV energy level; M (1001keV) is the net area of the peak of the spectrum under the energy peak at 1001keV, e (1001keV) is the efficiency of the gamma spectrometry device for photons of energy 1001keV and <u23a (1001keV) is l radiation intensity at the energy of 1001kev for U238 ..

Bismuth 214 (Bi214) emits photons at four main energies namely:

934 keV, 1120 keV, 1509 keV or 2448 keV.

The measurement process includes calculating the activity of Bi214 from each of four energy peaks corresponding to BÎ214:

Aei ₂ i4 (E _B i) = M (E _B i) / (e (E _BÎ ) * l _U23a (E _Bi )), for each energy peak of level E _B) associated with BÎ214,

The measurement method comprises checking the spectrometry measurement by comparing the activities of BII214 with each other and / or with their average. Too great a difference can be a sign of an error, especially in the density or volume of material considered during the spectrometry period.

In one embodiment, the measurement method comprises transmitting a non-blocking alert signal if there is a difference greater than a threshold value (for example 30%) between at least two of the activities of the BÎ214. and / or the average of the activities of BÎ214.

The measurement method includes calculating the ratio between the activity of U238 and the average activity of BÎ214. This makes it possible to determine the level of imbalance of the ore noted as the U / Bi ratio.

Advantageously, the measurement method comprises calculating the activity of U235 as a function of the different characteristic energies of U235, except for the 186keV line which also corresponds to the emission line of radium 226 (Ra226).

The measurement method includes calculating the number of hits due to U235 in the 186keV peak as a function of one or more activities of U235 and / or the average of the activities of U235, from the following equation:

M _U235 (186keV) = A ^ * e (186keV) * l _U235 (186keV) where Au235 is an activity of U235 measured from a different peak from the peak at 186keV or an average of U235 activities measured from several different peaks from the peak at 186keV.

The measurement method includes calculating a ratio between the activities of U235 and U238, which makes it possible to determine the U235 / U238 ratio and to verify the enrichment in U235.

The measurement method comprises the calculation of the radium 226 activity (Ra226) as the subtraction of the total net area M _T from the peak at 186 keV minus the number of hits due to U235 evaluated previously, according to the following equation:

AR, _22e (186keV) = (MT (186keV) - M _U235 (186keV)) / (e (186keV) * l _U238 (186keV)) where M _T (186keV) is the total net area of the peak at 186 keV and Mu23s (186keV) is the number of hits due to U235.

The ratio between Au238 and Af ^ e makes it possible to determine the level of imbalance of the ore denoted as ratio U / Ra.

For U235, a longer acquisition time may be required to calculate meaningful activity measurements from the spectrometry measurements provided by the spectrometry device. The measurement of the activity of U235 is carried out, for example, after a number N of spectrometry periods, N being an integer strictly greater than 1. N is for example equal to 5.

Thanks to the invention, it is possible to take meaningful in-line measurements on the material during its transport on the conveyor 4.

Spectrometry measurements make it possible to determine the activities, masses and / or contents of one or more particular radionuclides, for a detailed characterization of the material.

Spectrometry measurements carried out over long durations and counting measurements carried out during spectrometry measurements, over shorter durations, make it possible to determine a relatively reliable content and mass of one or more particular radionuclides between two spectrometry measurements.

The spectrometry measurements and the count measurements are carried out using the same gamma spectrometry device. Thus, the number of components is limited and the measurement method can be implemented reliably in difficult environments (dust, temperatures, etc.).

The radioactivity measurement device is passive. It is configured to perform a passive measurement of the intrinsic radioactivity of the material conveyed by the conveyor 4, without subjecting the material to excitation radiation.

The radioactivity measuring device does not perform an active measurement consisting in subjecting the material to excitation radiation, in particular neutron, generated by a radiation source, in order then to detect the induced radioactivity.

The characterization of the ore makes it possible, for example, to carry out an output assessment from the workshop located upstream of the conveyor or an input assessment for the workshop located downstream of the conveyor.

This measurement makes it possible, for example, to verify the radionuclide content of the ore produced in a mine, for example a uranium or thorium mine, and to verify the consistency with measurements carried out in an ore processing plant.

For example in the case of contaminated soil, cement or concrete, this measure makes it possible to sort out batches of potentially contaminated material.

In a preferred embodiment, the method for measuring radioactivity is applied to a given quantity of a material (or batch) which it is desired to characterize as a whole, by determining the activity, the content and / or the mass of one or more radionuclides in this batch considered as a whole. The batch is gradually conveyed on conveyor 4 while implementing the radioactivity measurement method, and the characterization is determined after passing the entire batch, establishing an average characterization for the entire batch

The radioactivity measurement method is not used to sort the batch by separating different parts of the batch according to the activity, content and / or mass of one or more radionuclides in this or that part of the batch.

The characterization of an entire batch is useful especially during decontamination operations, to characterize different batches.

Claims (17)

1. - Device for measuring the radioactivity of a material during its transport by a conveyor, the device for measuring radioactivity comprising a gamma spectrometry device (7) having at least one gamma spectrometry detector (9) and provided for be placed on the path of the conveyor to perform spectrometry measurements on the material during its transport by the conveyor (4). these measurements being carried out by the detection of the photons emerging from the flow of material transported, and a processing unit (8) for processing measurement signals supplied by the gamma spectrometry device (7), the radioactivity measurement device being configured to carrying out, during a spectrometry measurement, several gamma counting measurements on the material during its transport by the conveyor (4), using the gamma spectrometry device. 2. - Radioactivity measuring device according to claim 1, wherein the processing unit (8) is programmed for the calculation of the activity, the mass and / or the content of at least one radionuclide present in the material as a function of a spectrometry measurement. 3, - Device for measuring radioactivity according to claim 1 or 2, configured to calculate the activity, mass and / or content of at least one radionuclide as a function of a count measurement and the content of this radionuclide calculated based on a previous spectrometry measurement. 4, - Radioactivity measuring device according to any one of the preceding claims, comprising a device for measuring the height of material on the conveyor (4) in line with the gamma spectrometry device (7) and / or a weighing device (14) to measure the mass of the material transported by the conveyor (4). 5. - Device for measuring radioactivity according to any one of the preceding claims, configured to calculate the activity, the mass and / or the content of at least one radionuclide as a function of an efficiency of photon detection by the device. gamma spectrometry (7) as a function of the energy of these photons, the height of material on the conveyor (4) in line with the or each gamma spectrometry detector (7), and / or the mass of the material transported by the conveyor (4). 6, - Radioactivity measuring system, comprising a conveyor (4) for transporting a material and a radioactivity measuring device (5) according to any one of the preceding claims, in which the gamma spectrometry device is arranged on the path of the conveyor (4). 7. - System according to claim 6, wherein the conveyor is a belt conveyor comprising an endless belt (6) for transporting the material. 8. - A method of measuring the radioactivity of a material transported by a conveyor (4), in particular an endless belt conveyor, comprising carrying out gamma spectrometry measurements on the material during its transport by the conveyor, to the using a gamma spectrometry device (7) comprising at least one gamma spectrometry detector (9) arranged in the path of the conveyor (4) to detect the photons emitted by a flow of material during its transport by the conveyor (4) ), the method comprising carrying out, during a spectrometry measurement, several gamma counting measurements on the material during its transport by the conveyor (4), using the gamma spectrometry device (7) used for the measurements spectrometry. 9. - The measurement method according to claim 8, comprising calculating the activity, mass and / or content of at least one radionuclide present in the material as a function of a spectrometry measurement. 10. - The method of claim 8 or 9, comprising calculating the activity, mass and / or content of at least one radionuclide as a function of a count measurement and the calculated content of this radionuclide. based on a previous spectrometry measurement. 11. - Method according to any one of claims 8 to 10, comprising measuring the height of material on the conveyor (4) in line with the gamma spectrometry device (7) and / or measuring the mass of the material transported. by the conveyor (4). 12. - Method according to any one of claims 8 to 11, comprising calculating the activity, the mass and / or the content of at least one radionuclide as a function of an efficiency of detection of photons by the. gamma spectrometry device (7) as a function of the energy of these photons, of the height of material in line with the or each gamma spectrometry detector (9), and / or of the measurement of the mass of the material transported by the conveyor (4). 13. - Method according to any one of claims 8 to 12, comprising verifying a spectrometry measurement and / or calculations carried out from this spectrometry measurement by comparison of activity, mass and / or content. of at least one radionuclide, in particular bismuth 214 (BÎ214), calculated as a function of main spectral lines of energies characteristic of this same radionuclide. 14. - Method according to any one of claims 8 to 13, comprising calculating the activity, mass and / or content of uranium 235 (U235) and / or uranium 238 (U238) based on the results of a spectrometry measurement for protactinium 234 (Pa234). 15. - Measurement method according to any one of the preceding claims 8 to 14, comprising calculating the activity, the mass and / or the content of 5 protactinium 234, the calculation of the activity, mass and / or content of bismuth 214 (Bi214) and / or the calculation of the activity, mass and / or content of radium 226 (Ra226 ), depending on a spectrometry measurement. 16. - Measurement method according to any one of claims 8 to 15, comprising calculating the activity, the mass and / or the content of uranium 235 (U235) 10 and / or the calculation of the activity, the mass and / or the content of uranium 238 (U238) as a function of a spectrometry measurement. 17. - Measurement method according to claims 15 and 16, wherein the calculation of the activity, the mass and / or the uranium 235 content and / or the calculation of the activity, the mass and / or of the uranium-238 content is determined from the activity, mass 15 and / or the content calculated for protactinium 234, bismuth 214 and / or radium 226.