Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
  • G01T1/16—Measuring radiation intensity
  • G01T1/167—Measuring radioactive content of objects, e.g. contamination
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
  • G01T1/36—Measuring spectral distribution of X-rays or of nuclear radiation spectrometry

Definitions

• the technical field of the invention relates to a technique for non-destructive measurement of 238 U or U activity.
• Gamma spectrometry is one of the most commonly used passive non-destructive nuclear measurement techniques to obtain qualitative and quantitative information on gamma-emitting radionuclides.
• pure P emitters their in situ radiological characterization is made difficult by the very short path of electrons in dense media and is usually carried out in the laboratory through destructive analyzes of samples taken in the field. These destructive laboratory measurements are, however, accompanied by certain disadvantages: questions about the representativeness of the samples taken, cost and analysis time.
• Uranium 235 and Uranium 238 can be measured by gamma spectrometry.
• gamma spectrometry is quite complex to implement, and assumes certain assumptions as to the depth and gradient of contamination, particularly in porous objects such as concrete walls. These assumptions are not necessarily valid.
• the acquisition time can be long, due to the low intensity of the gamma emissions from the 238 U descending radioelements.
• the inventor proposes an alternative method for estimating Uranium activity on civil engineering type structures, equipment or samples taken, for example concrete cores. This is a method based on a device that is inexpensive and simple to implement.
• a first object of the invention is a method for estimating the uranium, or uranium 238, activity of an object, using a detector configured to form pulses under the effect of a exposure to P particles, the detector being connected to a spectrometric measurement circuit, configured to establish a spectrum, the spectrum corresponding to a histogram of the amplitude of the pulses formed during a measurement period, the method comprising the following steps:
• step c application of a transfer function to the first spectral value determined during step c), so as to estimate a uranium or 238 U activity of the object.
• Step b) may include the sub-steps: - bl) interposition of a screen between the detector and the object and acquisition of a background spectrum, the screen being configured to absorb the P particles emitted by the object;
• the screen used during sub-step bl) can be an aluminum screen whose thickness is greater than or equal to 3 mm or 4 mm.
• the first energy band can extend up to a first maximum energy, the first maximum energy being greater than or equal to 1000 keV or 1500 keV or 2000 keV.
• the method may comprise the steps: f) determination of a second spectral value, in a second energy band, extending between a second minimum energy and a second maximum energy, the second maximum energy being less than 300 keV;
• the distance between the detector and the object can be:
• the method comprises:
• step b) includes a correction of the measured spectrum using the spectrum P of the natural activity of the object resulting from (ii), to obtain the corrected spectrum.
• Step (i) can result from a gamma spectrometry measurement carried out on the object.
• the detector includes an organic scintillator material.
• the detector may include a semiconductor material.
• a second object of the invention is a measuring device, configured to determine a Uranium activity of an object, the device comprising:
• a detector comprising a detector material configured to form pulses under the effect of exposure to P particles;
• a spectrometry circuit connected to the detector, and configured to form a histogram of the amplitude of the pulses detected by the detector during an acquisition period;
• a processing unit connected to the spectrometry circuit, and configured to implement steps b) to d) of a method according to the first object of the invention.
• the detector comprises an organic scintillator material, the thickness of the organic scintillator material being preferably between 3 mm and 10 mm.
• the organic scintillator detector can extend over an area of at least 20 cm on each side.
• the detector can be covered with an envelope configured to absorb particles with energy greater than or equal to 4.5 MeV or 5 MeV.
• the detector may include a semiconductor material.
• the device may include a movable screen, configured to move from a closed configuration, in which the screen is interposed between the detector material and the object, to an open configuration in which the screen leaves a space between the detector material.
• Figure 1 schematically shows a measuring device.
• Figure 2 shows an evolution of the energy deposited in the detector material of the device by P particles as a function of the thickness of the latter.
• Figure 3 shows an evolution of the energy deposited in the detector material by particles P as a function of the thickness of an aluminum screen interposed between the detector material and the object
• Figure 4A shows a spectrum of the energy of particles P detected by the detector material as a function of different distances between the detector material and the object, the uranium contamination being assumed to be surface.
• Figure 4B shows a spectrum of the energy of particles P detected by the detector material as a function of different distances between the detector material and the object, the uranium contamination being assumed to be volumetric, at a thickness of 2 mm.
• Figure 5 shows energy spectra of P particles detected by the detector material in the presence of an aluminum screen, without the aluminum screen. We also represent a corrected spectrum resulting from the subtraction of the spectra modeled with and without screen.
• Figure 6 shows a spectrum of the energy of P particles detected by the detector material due to the natural radioactivity of the object, as well as spectra of the energy of P particles detected by the detector material with and without subtraction of the spectrum of natural activity.
• Figure 7 shows simulations of energy spectra of P particles detected by the detector material in different configurations.
• Figure 8A shows spectral simulations of the energy of P particles detected by the detector material resulting from different radioelements, for a low enrichment of 235 U.
• Figure 8B shows simulations of energy spectra of particles P detected by the detector material resulting from different radioelements, for a strong enrichment of 235 U.
• Figure 9 shows a spectrum of the energy of particles detected by the material detector.
• Figure 10 shows an energy spectrum of P particles detected by the detector material.
• Figure 11 shows the evolution of a transfer function, making it possible to convert a spectral value into an activity level, for different contamination depths.
• Figures 12A, 12B and 12C show experimental maps carried out with a device according to the invention.
• Figure 13A shows the evolution of a ratio between a first spectral value and a second spectral value as a function of the enrichment in 235 U, taking into account surface contamination.
• Figure 13B shows the evolution of a ratio between a first spectral value and a second spectral value as a function of the enrichment in 235 U, taking into account volume contamination.
• Figure 14 shows the main steps of a method for determining the U activity by implementing the invention.
• Figure 1 represents a measuring device allowing measurement of the uranium activity of an object 2.
• the device comprises a scintillator detector 10, comprising a scintillator material 11 preferably organic, preferably based on Polyvinyltoluene (PVT) .
• PVT Polyvinyltoluene
• Light pulses are formed. These light pulses are converted into electrical pulses by one or more photodetectors 12.
• the electrical pulses are then processed by a spectrometry circuit.
• the spectrometry circuit 13 is configured to form a amplitude histogram of the pulses detected by the organic scintillator during an acquisition period.
• an organic scintillator detector is suitable for carrying out charged particle spectrometry, type P".
• An organic scintillator is also sensitive to ionizing photons of type X or y.
• the materials forming an organic scintillator have a low number atomic, which makes them not very conducive to the formation of photoelectric interactions.
• an organic scintillator is considered not suitable for X or y spectrometry type applications.
• the scintillator detector is covered with an optically sealed envelope 14, of thin thickness, for example an aluminized PET (Polyethylene Terephthalate) film, 18 ⁇ m thick to ensure sealing against ambient light.
• the low thickness makes it possible to minimize the attenuation of radiation p.
• the term P particle designates a P- particle.
• the thickness of the envelope can reach 30 pm to stop alpha particles with energies of 4.5 MeV or 5 MeV.
• the thickness e of the scintillator detector is 4 mm. Details as to the thickness of the scintillator will be given in connection with Figure 2.
• An organic scintillator material has the advantage of being insensitive to gamma radiation, due to a low atomic number. In addition, this type of scintillator limits the backscattering phenomenon of p particles.
• Another advantage is a certain stability of the response of the scintillator material with respect to thermal variations. The response, in terms of light intensity produced, when exposed to the same radiation, is stable from 0° to 50°C, which is appropriate for field use conditions.
• the measuring device comprises a processing unit 20, configured to implement spectra processing steps described below.
• the processing unit 20 is programmed to execute instructions coded in a memory, connected to the processing unit by wired or wireless connection.
• the processing unit 20 may in particular include a microprocessor.
• the detector may comprise a semiconductor material, suitable for spectrometry p. It may for example be a silicon type semiconductor, for example planar silicon.
• Another advantage of an organic scintillator material is the ability to be manufactured in different dimensions and shapes. When the object is a sample taken, the shape can be adapted to the shape of the sample.
• the detector 10 comprises a movable screen 15, acting as a shutter, configured to be arranged: in a closed position, between the detector 10 and the object to be measured 2; or in an open position, freeing the space between the detector 10 and the object 2, as shown in Figure 1.
• the device is mainly dedicated to the radiological monitoring of walls.
• the surface of the detector material in a plane perpendicular to its thickness, is 50 cm x 50 cm. This makes it possible to address a large surface area during each measurement, while being compact and light enough to be easily handled.
• the screen 15 is movable in translation in a plane parallel to the detector material 11.
• the screen is for example made of aluminum, the thickness being preferably equal to 4 mm. Details relating to the thickness of the screen 15 are given in connection with Figure 3.
• the screen 15 can be connected to the detector material by a slide system, allowing translation of the screen relative to the detector material.
• Object 2 is an object to be controlled, likely to present mass or surface activity of Uranium, the isotopia being known or unknown.
• the object is a concrete wall.
• the invention is based on detection, by detector 10, of P particles emitted by isotopes descending from Uranium isotopes. These include 234m Pa, which is in radioactive equilibrium with 238 U, and emits P particles up to a maximum energy EPmax equal to 2269 keV.
• EPmax 198 keV.
• Figure 2 represents a quantity of energy (ordinate axis - unit MeV.Bq -1 .g), deposited by P particles emitted by a homogeneous distribution of 234m Pa in a depth of 2 mm concrete.
• the abscissa axis corresponds to the thickness (unit mm) of the scintillator material 11.
• the depth of 2 mm corresponds to the maximum path of the P particles with energy 2269 keV (maximum emission energy of 234m Pa) in the concrete.
• the simulation was carried out using the MCNP6 calculation code (Monte Carlo N Particles ), which is a reference code in the field of modeling interactions between ionizing radiation (P, y, neutrons) and matter
• Figure 3 represents a quantity of energy (ordinate axis - unit MeV.Bq -1 .g)), deposited by P particles emitted by a homogeneous distribution of 234m Pa in a concrete depth of 2 mm.
• the abscissa axis corresponds to the thickness (unit mm) of the aluminum screen 15.
• Figures 4A and 4B address the question of the distance between the detector and the object to be controlled.
• the controlled object being potentially contaminating, it is preferable to prevent the detector 10 from being in direct contact with the object. A slight step back is useful, so as to allow the screen 15 to move from the open position to the closed position.
• the x-axis corresponds to the energy (MeV) and the y-axis corresponds to the counting rate (s 1 . Bq 1 . cm 2 ).
• the spectrum is not significantly modified. The difference between the spectra acquired considering distances of 7 mm and 10 mm is negligible.
• Figure 4B shows simulations of the same type as those described in connection with Figure 4A.
• Figure 4B we considered a homogeneous volume distribution of 234m Pa over a thickness of 2 mm of concrete.
• the difference between the spectra acquired considering distances of 2.5 mm and 10 mm is negligible.
• the inventor considers that the optimal distance between object 2 and detector 10 is between 5 mm and 10 mm. Beyond 10 mm, there is a risk of a loss of efficiency, that is to say a reduction in the number of P particles detected. Below 5 mm, handling the screen becomes complex, the latter being too close to the object, with risks of contact between the screen and the object. Subsequently, the distance between the detector and the object is equal to 7.5 mm.
• Figure 5 shows the effect of blocking the detector by the screen 15 on the spectrum measured by the detector.
• Figure 5 represents spectra measured experimentally facing a wall contaminated with uranium.
• Y axis counting rate (s 1 ) - x axis: energy (MeV).
• s 1 counting rate
• s 1 counting rate
• x axis energy (MeV)
• the spectra were measured using the same acquisition duration of 120 seconds.
• the screen was a 4 mm thick aluminum plate.
• the screened spectrum denoted Spbdf
• the background noise y is due to the emitting radioelements y present in the object (for example natural radioelements) or in the environment of the detector.
• Subtracting the spectrum with Spbdf screen from the spectrum without SpPy screen makes it possible to obtain a spectrum corrected for background noise y.
• the corrected SpP spectrum, or raw P spectrum is considered only representative of the P particles emitted by the object.
• radioelements potentially present in the object, let us cite for example 40 K, as well as descendants of 232 Th and 238 U. Such radioelements are for example present in concrete objects. Background noise can also result from artificial radioactivity present in the detector's environment.
• the raw P spectrum SpP includes a natural component due to P particles emitted by natural radioelements. This natural component can, in a first approach, be neglected because it is generally quite weak.
• Table 1 shows the different P-emitting radionuclides of natural and artificial origin likely to be encountered in a concrete wall potentially contaminated with uranium.
• Figure 6 shows the corrected P spectrum Sp, as described in connection with Figure 5 (raw P spectrum), as well as a P spectrum, also corrected for background noise y, measured facing an uncontaminated concrete wall ( natural background noise spectrum or RN background noise spectrum, denoted SpRN).
• the uncontaminated concrete wall was located in the same facility as the contaminated wall, on which the corrected P spectrum was formed.
• the spectrum P of background noise RN denoted SpRN
• Figure 6 shows a net P spectrum, denoted SpP', obtained by subtracting the natural background P spectrum SpRN from the raw P spectrum.
• the net P spectrum SpP' is thus representative of the artificial P activity in the measured wall.
• Figure 7 represents P spectra simulated with the MCNP6 calculation code, in a detector material as previously described (50 cm x 50 cm x 4 mm), taking into account specific activities of 232 Th, 238 U and 40 K measured by gamma spectrometry on a wall considered uncontaminated. Measurement by gamma spectrometry made it possible to estimate activity levels of 15 Bq/Kg for 232 Th, 22 Bq/Kg for 238 U, and 485 Bq/Kg for 40 K. These mass activities make it possible to simulate the natural component in the P spectrum measured by the detector.
• the distance between the wall and the detector was considered equal to 7.5 mm, and the density of the concrete was considered equal to 2.3 g.crrr 3 .
• the natural component of the P spectrum is dominated by 40 K, up to the energy 1311 keV, which corresponds to the maximum energy of the P particles emitted by 40 K; the measurement of the P spectrum resulting from 234m Pa is much greater than the natural contribution at low enrichment as well as at high enrichment, beyond 1 MeV; it is possible to model the natural component in a P spectrum measured by the detector, taking into account the mass activities, established a priori or measured, of the main natural P emitting radioelements.
• Figures 8A and 8B show simulations of P spectra measured by a detector as described in Figure 1, taking into account a homogeneous surface activity of uranium, on a wall element of 50 cm side, taking into account respectively an enrichment in 235 U of 1% and 82.55% by mass.
• surface activity we mean an activity distributed over a depth of 10 pm.
• the contributions of 234m Pa, 234 Th and 231 Th are represented.
• 234m Pa and 234 Th are descendants of 238 U, while 231 Th is a descendant of 235 U.
• the distance between the detector and the wall was considered equal to 7.5 mm.
• the ordinate axis corresponds to a counting rate (s 1 . Bq 1 . cm 2 ) and the abscissa axis corresponds to an energy level (MeV).
• Figures 8A and 8B show that whatever the level of enrichment, the spectrum is dominated by the contribution of 234m Pa, the latter being weaker at high enrichment (figure 8B) than at low enrichment (figure 8A).
• a first energy range AE1 delimited by a first minimum energy Elmin, greater than or equal to 300 keV we observe that by considering a first energy range AE1 delimited by a first minimum energy Elmin, greater than or equal to 300 keV, and that by summing the spectrum over this first energy range AE1, the spectral value obtained does not depend than the activity of 234m Pa.
• spectral value we mean a value measured from the spectrum. This may in particular be a counting rate or a counting in the energy range.
• the spectral value obtained on the first energy range AE1 is a first spectral value denoted NI.
• the first energy range AE1 extends up to a first maximum energy Elmax greater than or equal to 2269 keV, the latter corresponding to the maximum emission energy of a particle P per 234m Pa.
• the first energy range AE1 is [300 keV; 2500 keV], Such an energy range makes it possible to collect the entire useful spectral content of the P activity of 234m Pa.
• the first maximum energy Elmax can be equal to 2300 keV, or be less than 2000 keV or 1500 keV or 1000 keV.
• the first maximum energy Elmax is preferably less than or equal to 2500 keV, or even 2300 keV.
• the first optimal energy range is [300 keV; 2500 keV]
• a more restricted energy range in this interval, may be suitable.
• An energy range extending from 300 keV, or from a first minimum Elmin value greater than or equal to 300 keV, and less than or equal to 1000 keV, or 1500 keV may be suitable.
• the first spectral value NI, in the first energy range AE1 [Elmin, Elmax] previously defined, is then intended to be processed by a transfer function, so as to be able to estimate an activity level in 238 U.
• L The establishment of the transfer function is described below, in connection with Figure 11. Taking into account the enrichment rj then makes it possible to convert the activity in 238 U into activity in U.
• Figures 8A and 8B show another interesting aspect of the invention: we can define a second energy range AE2, between 0 keV and 300 keV, in which the spectrum P is influenced by 231 Th, this radioelement descending from 235 U.
• a second energy range AE2 [E2min, E2max], between a second minimum energy E2min>0 and a second maximum energy E2max ⁇ 300 keV, in which the spectral value N2, called the second value spectral, depends on 235 U, in particular at high enrichment.
• N1/N2 ratio of the first and second spectral values depends on the enrichment q.
• the experimental measurement of this ratio can be used to estimate the enrichment q of uranium.
• the first spectral value NI makes it possible to estimate 238 U, using the transfer function FT. Indeed, the spectral content, in the first energy range, does not depend on the enrichment. The ratio of the first and second spectral values makes it possible to estimate the enrichment q.
• the 238 U activity can then be used to estimate the U activity.
• the 235 U enrichment can also be determined experimentally by gamma spectrometry.
• Uranium contains ⁇ -emitting isotopes, first and foremost 234 U or 238 U.
• a 244 Cm source with an activity of 2800 Bq, was placed at 3 mm from the detector material.
• 244 Cm emits a particles with energies greater than 5700 keV, which is much higher than the energies of the a particles emitted by isotopes of U, for example 4775 keV for 234 U.
• Figure 9 shows the spectrum measured over a period d acquisition of 900s (ordinate axis: number of pulses detected - abscissa axis: energy (keV)).
• keV energy
• the particles a detected by the detector may be capable of influencing the spectral value N2 in the second energy band AE2, or even the spectral value NI in the first spectral band AE1. This is particularly the case when the detector comprises a semiconductor material, for example Si.
• the envelope 14 is advantageously dimensioned to stop a particles with energy 4.5 MeV or 5 MeV .
• the envelope is made of aluminized PET (Polyethylene Terephthalate)
• a thickness of 30 ⁇ m makes it possible to stop the particles a, while absorbing, in a manner considered negligible, the particles.
• the few hits observed at an energy above 270 keV correspond to y photons after subtracting the background noise, resulting from statistical fluctuations.
• Figure 10 shows a P spectrum measured experimentally on a concrete wall contaminated with 1% enrichment uranium (ordinate axis: number of pulses detected - abscissa axis: energy (keV)).
• ordinate axis number of pulses detected - abscissa axis: energy (keV)).
• keV energy
• the detector was an EJ200 detector (Eljen Technology), with a sensitive surface area of 2430 cm 2 (493 mm side).
• the energy calibration of the spectrum that is to say the correspondence between the value of the pulse amplitudes and the energy, was carried out using a 207 Bi source, which emits electrons, at values of discrete energy, by internal conversion.
• Uranium contamination can be considered as surface (for example on metal objects), or having diffused into the object, for example in the case of liquid contamination of porous objects, such as a floor or a concrete wall.
• Figure 11 shows the result of digital modeling of a detector as previously described, placed at a distance of 7.5 mm from a concrete wall, density 2.3, whose uranium 238 U activity was distributed at different depths. We considered a unit activity of 238 U (1 Bq/g or 1 Bq/cm 2 in the hypothesis of surface contamination).
• the detector model was validated in the laboratory by comparing P spectra respectively measured and modeled by exposing the detector to a standard source of 207 Bi.
• the ordinate axis corresponds to the value of the transfer function FT (unit s 1 . Bq 1 . cm 2 ) and the abscissa axis corresponds to the contamination depth considered.
• the transfer function corresponds to the first spectral value NI, in an energy range of [300 keV - 2500 keV], for a unit activity of lBq.g 1 .
• the detector described above was used on a uranium enrichment installation by gas diffusion.
• the detection limit was such that:
• FT transfer function, expressed in Bq.g 1 or in Bq. cm -2
• the detection limit is expressed in Bq.g 1 or in Bq. cm -2 .
• Tables 2 and 3 represent detection limit values respectively expressed in surface activity and in specific activity. The values are calculated from expressions (2) and (3), considering a measurement duration equal to the duration of the background noise measurement. We also took into account a contribution from natural activity in the spectrum, described in connection with Figure 7. The activities of the natural isotopes taken into account were 15 Bq/Kg for 232 Th, 22 Bq/Kg for 238 U, and 485 Bq/Kg for 40 K. Each measurement includes a measurement with the aluminum screen and a measurement without the aluminum screen, of the same durations. LD in Bq.cnr 2
• the detection limits presented in Tables 2 and 3 concern 238 U via the detection of the P spectrum of 234m Pa. As the enrichment increases, the quantity of 238 U decreases relative to the total quantity of uranium. Also, if we wish to maintain a low detection limit for uranium, we must increase the acquisition duration.
• Figure 12B shows the corresponding 238 U activity levels (Bq/m 2 ).
• the enrichment in 235 U being known, we estimated activity levels in U, the latter being represented in Figure 12C (Bq/m 2 ).
• the enrichment is not known, it can be determined by usual methods, for example by gamma spectrometry.
• the enrichment rj in 235 U can be evaluated from a ratio between the first spectral value NI and the second spectral value N2.
• the second energy band AE2 is between 0 keV and 250 keV.
• the second minimum limit E2min of 150 keV was determined in order to limit the influence of gamma radiation: between 150 keV and 300 keV, it was found that the contribution of gamma radiation in the P spectrum is stable, and can be easily subtracted taking into account the measurement with the aluminum screen.
• a second minimum energy E2min less than 150 keV the contribution of radiation gamma in the P spectrum may fluctuate more, due to the higher sensitivity of the plastic scintillator material to low energy photons.
• Figure 13A shows the evolution of a ratio between the first and second spectral values.
• the first energy band is [300 keV - 2500 keV] and the second energy band is [150 keV - 300 keV].
• the contamination is assumed to be surface, i.e. considered to be distributed over a thickness of 10 pm.
• Figure 13B shows the evolution of a ratio between the spectral values (count rate) in the first energy band [300 keV - 2500 keV] and in the second energy band [150 keV - 300 keV] in the case of mass contamination, distributed over a thickness of 2 mm.
• Figures 13A and 13B the ordinate axis corresponds to the N1/N2 ratio and the abscissa axis corresponds to the enrichment in 235 U, expressed in%.
• Figures 13A and 13B were established on the basis of modeling with the MCNP6 calculation code.
• FIG. 13A and 13B show the establishment of enrichment calibration functions, respectively for a surface activity and a volume activity.
• the use of a background noise spectrum, obtained according to the closed configuration, with the screen 15, is not necessary. Indeed, if one wishes to carry out a rough check, measurements carried out only in the open configuration, ie without a screen, may prove sufficient to carry out a first level check.
• the coupling between the open configuration and the closed configuration makes it possible to subtract the background noise, essentially due to the y photons, which makes it possible to obtain more precise measurements: more precise quantification of the 238 U activity or value of the enrichment more accurate.
• Figure 14 summarizes the main steps of a process according to the invention.
• Step 100 arrangement of a detector facing the object to be characterized
• Step 110 acquisition of a spectrum, in the open configuration: this is the measured spectrum Sp.
• Step 120 acquisition of a background noise spectrum Spbdf, in the closed configuration, or taking into account a spectrum acquired in the closed configuration.
• Step 130 subtraction of the spectrum resulting from step 120 from the spectrum resulting from step 110.
• the spectrum resulting from this step is a raw P spectrum Sp.
• Step 140 taking into account the natural activity P of the object. This step includes the following sub-steps:
• Sub-step 141 taking into account a level of natural activity of the object: the level of natural activity may have been estimated by a measurement on a comparable object, considered uncontaminated.
• Sub-step 142 estimation of a natural P SpRN spectrum (or P RN spectrum) resulting from step 141.
• step 140 includes a sub-step 143 of measuring a raw spectrum P carried out on an object considered to be representative of the measured object, and not contaminated. This involves implementing steps 110 and 120 on the object considered representative, which makes it possible to obtain the natural P spectrum
• Step 140 is preferable, but optional.
• Step 150 correction of the raw P spectrum, resulting from step 130, taking into account the natural P spectrum resulting from step 140. This step is optional. Step 150 makes it possible to obtain a net P spectrum (or corrected P spectrum) denoted SpP'.
• Step 160 determination of a first spectral value NI in the first energy band AE1 from the net P spectrum SpP' resulting from step 150 or from the raw P spectrum resulting from step 130.
• Step 170 application of the transfer function FT to the first spectral value so as to estimate an activity of the object in 238 U denoted A[ 238 U]
• Step 180 taking into account an enrichment rate q in mass of 235 U.
• the enrichment is either known, because it depends on the installation, or unknown, in which case it can result from a non-destructive measurement of the type gamma spectrometry.
• the enrichment can also be determined from the spectrum from the corrected P spectrum resulting from step 150 or from the net P spectrum resulting from step 130. For this, step 190 is implemented.
• Step 190 includes the following substeps:
• Substep 191 determination of a second spectral value N2 in the second energy band AE2 from the net spectrum P resulting from step 150 or from the raw spectrum P resulting from step 130.
• Substep 192 application of the enrichment calibration function to the N1/N2 ratio to estimate an enrichment q.
• the enrichment calibration function depends on hypotheses as to the nature of the activity: surface activity (see Figure 13A) or volume activity (see Figure 13B).
• Step 200 from the enrichment q, resulting from step 180, and the activity of 238 U resulting from step 170, determination of the uranium activity A[U],
• steps 180 to 200 are not implemented.
• Steps 130 to 200 can be implemented by the processing unit.
• the invention could be implemented on fuel enrichment or manufacturing installations.

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