BAKE FOR THE DETECTION OF A RADIATION SOURCE WITH MULTIPLE SCINTILLATOR DETECTORS

Description

DOMAINE TECHNIQUE

[0001] The technical field of the invention is the detection of irradiating sources using a monitoring beacon. ART ANTERIEUR

[0002] Access points to nuclear facilities, as well as to certain facilities that may contain radioactive sources, are generally equipped with detection beacons. The purpose is to detect the presence of a radioactive source carried by an individual or placed in a vehicle. Such beacons may also be installed at border crossings. This type of beacon typically consists of one or more large plastic scintillators. The operating principle is based on the continuous measurement of a count rate, representing the number of pulses detected per second. When the measured count rate deviates significantly from a background noise value, a detection alarm is triggered.

[0003] The information provided by this type of beacon is limited to the detection or non-detection of a counting rate exceeding a certain threshold. It does not include information allowing for the preliminary characterization of a detected source.

[0004] US2018 / 329089 describes a device for forming an image showing the position of a neutron or gamma-ray photon source. The device comprises two detectors spaced apart and configured to detect neutrons with energies greater than 1 MeV, or photons with energies greater than 300 keV.

[0005] Document EP3401706 describes a portable measuring device for the simultaneous detection of neutrons and gamma photons. The device may include a scintillator material of the NalL (lithium-doped sodium iodide) type.

[0006] Document US9477005 describes a device for detecting nuclear matter that decays by spontaneous fission. The detection of spontaneous fission is based on a difference in the propagation speed of fast neutrons and photons.

[0007] The invention described below aims to supplement the information obtained from a monitoring beacon. The objective is to obtain more information regarding the type of radiation source detected. EXPOSE DE L'INVENTION

[0008] A first object of the invention is a detection beacon, according to claim 1, intended to detect the presence of a radiating source in an environment extending around said beacon, the radiating source being capable of emitting ionizing photons, of the X or gamma type, and / or neutrons, the beacon comprising: at least one first detector, comprising a three-dimensional organic scintillator material, the first detector being configured to form a pulse following an interaction of a neutron or a photon in the organic scintillator material and to generate a first detection signal from each pulse thus formed; The detection beacon is characterized in that it comprises: at least one second detector, comprising a three-dimensional inorganic scintillator material, having been modified by the addition of a capture isotope, suitable for capturing thermal neutrons, the second detector being configured to form a pulse following an interaction of a neutron or a photon in the inorganic scintillator material, and to generate a second detection signal from each pulse thus formed, the smallest dimension of the second detector being greater than 1 cm; a processing unit, connected to each first and each second detector, the processing unit being configured to perform a characterization of the irradiating source detected by the beacon from the first detection signal and the second detection signal.

[0009] Several embodiments are defined by the dependent claims.

[0010] A second object of the invention, according to claim 10, is a method for detecting a radiating source present in an environment extending around a beacon according to the first object of the invention, comprising the steps of: a) analysis of at least one first detection signal and at least one second detection signal, such that when the first detection signal crosses a first threshold or when the second detection signal crosses a second threshold, an alert signal is given; b) following the emission of an alert signal, characterization of the radiation emitted by the source using the first detection signal and / or the second detection signal.

[0011] The invention will be better understood by reading the explanation of the examples of embodiment presented, in the continuation of the description, in connection with the figures listed below. FIGURES

[0012] There figure 1A Diagram a first example of a beacon according to the invention. figure 1B diagram shows a second example of a beacon according to the invention. figure 2 It shows components that can be included in a beacon processing unit, in order to process signals resulting from the beacon's detectors. figures 3A, 3B et 3C They respectively represent a first, second, and third test configuration. figure 4 shows energy spectra resulting from the second detector, obtained by implementing each test configuration, using different sources. figure 5 This diagram schematically represents a pulse shape resulting from the interaction of a neutron or photon in a scintillator detector doped with a capture isotope. figures 6A, 6B And 6Care diagrams showing the shape parameter as a function of the energy of pulses detected by the second detector, implementing the first configuration, respectively with sources of 137< Cs, 60< Co and 252< Cf. figure 6D And 6E These are diagrams showing the shape parameter as a function of the energy of pulses detected by the second detector, implementing the second configuration and the third configuration using a 252 < Cf. The figure 7A is a parameter diagram of the shape / energy of pulses detected by the second detector, obtained according to the first configuration, without an irradiating source. figure 7B is an energy spectrum resulting from the second detector, obtained in the first configuration, without an irradiating source. figure 8A and the figure 8B are energy spectra formed from detection signals from two different organic scintillator detectors, the scintillators being exposed to a source of 137 < Cs, or 60 < Co or 252 < Cf. The figure 9 is a Rossi-alpha histogram, corresponding to a histogram of time-shifted pulses resulting from rays emitted in temporal coincidence. figure 10A This diagram illustrates the movement of a source along different detectors forming a beacon. figure 10B shows an evolution in the count rate of detectors constituting the beacon as a function of the source's position. figure 11 diagram of the processing steps that can be performed by the beacon. EXPOSE DE MODES DE REALISATION PARTICULIERS

[0013] There figure 1A Figure 1 represents an example of a beacon designed to detect the presence of a radiating source in the vicinity of said beacon. A radiating source is defined as a source emitting ionizing radiation. Ionizing radiation may include ionizing photons, generally gamma or X-ray photons, and / or neutrons. The vicinity is defined as within a radius of a few meters around the beacon.

[0014] The beacon comprises a first detector 10 or several first detectors 10. Each first detector comprises a first detection material 11, of the organic scintillator type, and a first detection circuit 12. The detection material is designed to interact with the ionizing radiation emitted by the source. As is known in the field of scintillators, each interaction generates a light pulse, which is converted into an electrical pulse by a detection circuit. The detection circuit 12 comprises a photodetector, connected to the detection material, and configured to emit electrical pulses whose amplitude is correlated with the amount of energy deposited by the ionizing radiation during the interaction in the detection material.Thus, upon interaction with ionizing radiation, in this case a neutron or a gamma or X-ray photon, the first scintillator material 11 emits a light pulse, the intensity of which depends on the energy deposited by the neutron or the X or gamma photon during the interaction. The first detection circuit 12 detects the light pulse and generates an electrical pulse whose amplitude depends on the intensity of the light pulse, which in turn depends on the energy deposited by the neutron or the X or gamma photon during the interaction.

[0015] The first detector 10 is intended for the detection of X-ray or gamma photons and / or neutrons. The volume of the first detection material 11 must be sufficient to ensure acceptable detection sensitivity. Thus, the thickness, or diameter, and more generally, the smallest dimension of the volume, is greater than 1 cm, and preferably greater than 2 cm or 5 cm. An advantage of organic scintillators is that it is possible to obtain large detection volumes, typically greater than 1 dm³ or even 10 dm³ or several tens of dm³.

[0016] The beacon includes a second detector 20, or several second detectors 20. Each second detector 20 comprises a second detection material 21, of the inorganic scintillator type, and a second detection circuit 22. The second detection circuit 22 includes a photodetector, as described in relation to the first detection circuit. The detection material 21 of each second detector 20 consists of an inorganic scintillator-type material, having previously undergone the addition of a capture isotope, suitable for neutron capture, resulting in the emission of a charged particle. The neutron capture can, in particular, be of the (n,α) type. The capture isotope can, for example, be 6 < Li. More generally, the added capture isotope allows the emission of a charged particle in the inorganic scintillator, under the effect of neutron capture.The mass fraction of the capture isotope is preferably between 1% and 20%. The higher the mass fraction, the more sensitive the detector is to neutrons.

[0017] The second detector 20 is intended for the detection of X-ray or gamma photons and / or neutrons. The volume of the second detection material 21 must be sufficient to ensure acceptable detection sensitivity. Therefore, the thickness, or diameter, and more generally, the smallest dimension of the volume of the second detector, is greater than 1 cm, and preferably greater than 2 cm or 5 cm. The smallest dimension is understood to be the smallest of the three dimensions of the volume of the second detection material 21.

[0018] In the example described, the second detection material 21 is sodium iodide doped with 6<Li, hereafter referred to as NaIL. The second detection material may include another type of scintillator, for example CLYC (Cs2LiYCl6:Ce) or CLLB (Cs2LiLaBr6).

[0019] The addition of the capture isotope enables the detection of thermal neutrons (energy less than 0.025 eV), epithermal neutrons (energy between 0.5 and 50 keV), as well as intermediate neutrons with energies ranging from 50 keV to several hundred keV, up to approximately 1 MeV. The detection properties of thermal and intermediate neutrons by the inorganic scintillator are described below, in connection with the figures 6A à 6E .

[0020] One of the advantages of the beacon is the proximity between the organic and inorganic detection materials. Thus, the distance between a second detector 20 and at least one first detector 10 is preferably less than 30 cm, and preferably less than 20 or 15 cm. As described later, placing a first organic detection material 11 near a second inorganic detection material 21 doped with 6Li or 10B increases the sensitivity of the second detector 20 to neutrons emitted by a source in the vicinity of the beacon.

[0021] There figure 1B represents another configuration, in which the second detector 20 is surrounded by four first detectors 10, each first detector 10 being located less than 30 cm, or less than 20 cm or 15 cm from the second detector 20.

[0022] Depending on other configurations, the use of several second detectors is possible.

[0023] Each detection circuit 12, 22 is configured to form a usable electrical pulse from light pulses generated in the scintillator material to which it is connected. Each detection circuit may include a preamplifier, and optionally an amplifier and a pulse-shaping circuit.

[0024] The beacon includes a processing unit 30, designed to process the electrical signals resulting from the various detection circuits. The processing unit 30 may include analog electronic circuits or digital electronic circuits. It may also include a microprocessor or another type of circuit, for example an FPGA (Field Programmable Gate Array), implementing a program for processing and / or interpreting the measurements resulting from each detector.

[0025] A known advantage of inorganic scintillators is their compatibility with spectrometric measurements, in which the inorganic scintillator is coupled to a spectrometric measurement circuit. Preferably, the processing unit 30 includes a multi-channel analyzer 32, usually designated by the acronym MCA (Multi-Channel Analyzer), to process the pulses resulting from a detector, particularly a second detector. The MCA is known to establish an amplitude spectrum of the pulses detected during a measurement period. During the measurement period, the second detector is exposed to photons and / or neutrons. The multi-channel analyzer 32 forms the amplitude spectrum of the pulses detected by the detector. The amplitude spectrum is usually referred to as the Sp energy spectrum, the amplitude of each pulse being correlated with the energy deposited in the detector material by the interaction that generated said pulse.The Sp energy spectrum is usually represented as a histogram, showing the number of pulses detected for each amplitude channel, with each amplitude channel corresponding to an energy band.

[0026] The MCA 32 may be preceded by a pulse amplification and shaping circuit 31. The processing unit 30 may include several MCAs, each MCA being connected to the detection circuit 12, 22 of a detector.

[0027] Organic scintillators are known to be less suitable for spectrometric applications than inorganic scintillators. This is because they are composed of chemical elements with low atomic numbers, making them less conducive to the formation of photoelectric interactions. In this type of material, the predominant interaction during the detection of X-ray or gamma photons is Compton scattering. However, one possibility is that at least a first detection circuit is connected to a magnetic compound analog (MCA), allowing for the acquisition of an amplitude spectrum of the detected pulses. This possibility is more widely discussed in connection with... figures 8A et 8B .

[0028] The processing unit 30 may also include a pulse shape analyzer 33, coupled to a discriminator 34, the discriminator classifying the pulses according to their shape. The pulse shape analyzer and the discriminator are described in relation to the figures 5 as well as 6A to 6E. The pulse shape analyzer and discriminator are adapted to process detection signals resulting from the one or each of the second detectors, of the doped inorganic scintillator type.

[0029] The processing unit 30 may also include a coincidence analysis circuit 35, configured to determine the occurrence of detected interactions within the same time interval and to establish a time lag between said interactions. An example of a temporal coincidence analysis circuit is described in connection with the figure 9 .

[0030] The processing unit includes an interpretation module 36, programmed to characterize a detected source. This characterization provides information about the detected source, in addition to simply detecting it. The characterization may include: a type of irradiation produced by the source: radiation or neutron; an identification of an isotope when the source emits gamma or X-rays; an identification of a type of source when the source emits neutrons, in particular the presence of an isotope undergoing spontaneous fission decay; a movement of the source.

[0031] THE figures 3A, 3B et 3C show three configurations that were tested by the inventors, in order to highlight the effect of proximity between one or more organic scintillators and an inorganic scintillator doped with 6< Li: Regarding the configuration of the figure 3A (first configuration), an inorganic scintillator 20 made of NaIL, cylindrical in shape and with a volume of 3 inches (diameter) by 3 inches (height), was placed at a distance d = 30 cm from a source S. The source S could be 137Cs (activity 460 kBq), 60Co (activity 314 kBq), or 252Cf (neutron emission rate of 2.7 × 10⁴ ns⁻¹). Regarding the configuration of the figure 3B (second configuration), the inorganic scintillator 20 was placed in the figure 3A between two parallelepiped-shaped organic scintillators 10, made of polyvinyl toluene (PVT) with a volume of 10 cm x 10 cm x 10 cm. These were EJ-200 type scintillators - manufactured by Eljen Technology. A source S of 252 < Cf was placed 30 cm from the inorganic scintillator NaIL. Each organic scintillator 10 was placed at a distance D' equal to 10 cm from the inorganic scintillator 20. On the configuration of the figure 3C (Third configuration), an organic scintillator 10, shaped like a third of a hollowed and truncated crown, was placed on each side of the inorganic scintillator 20. Each scintillator described a crown segment with an angle of 120°, a height of 13 cm, a thickness of 10 cm, and an inner radius of 4 cm. The distance d between the 252< Cf source and the NaIL scintillator was 30 cm.

[0032] In each of the configurations described above, the inorganic scintillator 20 was connected to a processing unit 30, enabling the establishment of an energy spectrum. figure 4 represents energy spectra established respectively: using the first configuration, the source being 137< Cs (curve a), 60< Co (curve b) and 252< Cf (curve c); using the second configuration, the source being 252< Cf: curve d; using the third configuration, the source being 252< Cf: curve e.

[0033] Curves a) and b) show that the inorganic scintillator allows the acquisition of energy spectra, on which the peaks correspond to the emission energies of 137 < Cs (662 keV), 60 < Co (1173 keV and 1332 keV). The peak observed at the energy 1460 keV corresponds to 40 < K, naturally present in certain materials, for example in soil or concrete. A peak is also observed at the energy 2614 keV, which corresponds to the radioelement 208 < Tl, the latter being a decay product of the naturally occurring radioactive isotope 232 < Th.

[0034] On curve c), we observe a peak centered at 3500 keV, which corresponds to thermal or intermediate neutrons emitted by the source, or thermalized in the environment. The energy at 3500 keV corresponds to the energy of the α particle emitted following neutron capture. The rest of the signal on this curve corresponds essentially to prompt or delayed gamma photons emitted by the spontaneous fission of 252 < Cf.

[0035] On curves d) and e), we observe that the peak centered at 3500 keV has a higher amplitude than on curve c), this effect being more pronounced on curve e) than on curve d). The increase in the amplitude of the peak at 3500 keV is due to a scattering effect, by the organic detector(s) 10, of the neutrons emitted by the source S. Each first detector 10 contains an organic detection material, conducive to the thermalization and scattering of fast neutrons emitted by the source. Thus, the proximity of at least one detector containing an organic scintillator material significantly improves the neutron component in the spectrum detected by the inorganic scintillator doped with 6Li. The third configuration ( figure 3C ), is particularly favorable. The scattering effect is all the more pronounced the closer the first detector 10 is to the second detector 20, and the larger the organic material forming the first detector. The possibility of creating large-volume organic scintillator materials can be exploited: this increases the detection sensitivity of the organic scintillator detector, while simultaneously increasing the neutron scattering effect, which in turn increases the neutron detection sensitivity of the adjacent inorganic scintillator detector.

[0036] Another advantage of an inorganic scintillator doped with 6Li is the ability to discriminate between pulses generated by interactions of X-ray or gamma photons and those generated by thermal or intermediate neutrons. Each detected interaction, generated by a neutron or a photon, gives rise to an electrical pulse whose shape depends on the radiation that interacted with the detector material. figure 5 diagram shows a pulse detected following the interaction of a photon in the scintillator material (curve in trait plein) or following a neutron interaction in the scintillator material (dashed curve). In the latter case, the pulse is affected by a "tail," or "trail," usually referred to by the English term "tail." For each detected pulse, a shape parameter can be determined, for example, a ratio Qtot − Qshort Qtot , Or : Q tot corresponds to the total charge detected, that is to say the integral of the pulse; Q short corresponds to the proportion of Q tot corresponding to the impulse without the tail: cf. hatched part of the figure 5 .

[0037] The measurement of the parameters Q tot And Qtot − Qshort Qtot , of each interaction allows discrimination between gamma photons and neutrons. Gamma photons form an energy-distributed region ( Q tot variable), with a ratio Qtot − Qshort Qtot high. This reflects the fact that the pulses resulting from interactions of γ photons in an inorganic scintillator doped with 6< Li are relatively symmetrical, and strongly distributed in amplitude.

[0038] The pulses resulting from interactions of thermal or intermediate neutrons in the 6<Li doped inorganic scintillator are more asymmetrical and less amplitude distributed. The ratio Qtot − Qshort Qtot is weaker than that of the pulses resulting from an interaction of a gamma photon.

[0039] In order to efficiently separate the photon and neutron contributions in each spectrum, the processing unit includes a shape analyzer 33, allowing for the determination of the total charge Q tot detected by each pulse as well as the part Q short corresponding to the impulse without the trail. The shape analyzer allows the shape parameter to be determined. Qtot − Qshort Qtot of each pulse. Based on Q tot and Qtot − Qshort Qtot The impulse is assigned either to a photon or to a neutron by the discriminator 34 described previously.

[0040] THE figures 6A à 6E These are figures representing the number of detected pulses as a function of energy (x-axis - keV) and the shape parameter, i.e., the ratio Qtot − Qshort Qtot resulting from analyzer 33 (y-axis). The gray level corresponds to the number of pulses detected. Each figure corresponds to an acquisition time of 10 minutes.

[0041] THE figures 6A et 6B were obtained according to the first configuration, shown on the figure 3A using the 131< Cs and 60< Co sources respectively. The emission peaks at 662 keV, 1173 keV and 1332 keV are clearly marked: for example, 344392 counts ( i.e pulses detected) at 662 keV and 295,619 counts at 1173 keV. A region of interest, corresponding to the pulses attributed to neutrons, has been circled. The signal in this region of interest is representative of the ambient background noise: 47 counts on the figure 6A and 55 shots on the figure 6B .

[0042] THE figures 6C, 6D And 6E were obtained according to the first, second and third configurations respectively represented on the figures 3A, 3B et 3C Using the 252< Cf. source, we count 1408, 2310, and 4778 shots attributed to neutrons, respectively. The comparison of figures 6D And 6E with the figure 6C This demonstrates the advantage of placing an organic material near the inorganic scintillator doped with 6<Li. This creates a thermalizing environment, which increases the number of neutrons detected by the inorganic scintillator.

[0043] On the figures 6C, 6D And 6E , we can also appreciate the detected pulses, outside the circled region of interest correspond to prompt or delayed gamma photons resulting from the spontaneous fission of 252< Cf.

[0044] To evaluate the detection limit, an inorganic scintillator 20 was used, according to the first configuration ( figure 3A ), in the absence of a source. The figure 7A represents the detected pulses according to an Energy / Shape Parameter diagram Qtot − Qshort Qtot . There figure 7B shows an energy spectrum. The figures 7A And 7B These values ​​are representative of the background noise. The background noise values ​​allow us to calculate a detection limit for an acquisition time of 10 minutes. For 137 < Cs and 60 < Co, the detection limit is 12.9 counts per second and 9.78 counts per second, respectively. Regarding neutron detection, in the peak centered at 3500 keV, the detection limit is 0.05 counts per second.

[0045] The results presented on the figures 6A à 6E These values ​​were established using sources with known photon or neutron emission. A sensitivity coefficient k can thus be established, allowing a correlation to be made between the activity of the source, located 30 cm from the inorganic scintillator, and the counting rate (counts per second), for an acquisition time of 10 minutes.

[0046] Regarding neutron detection, the sensitivity coefficient k differs for the three tested configurations. The emission rate of the source (252 < Cf) used can be converted to a mass equivalent of 240 < Pu, allowing the sensitivity coefficient to be expressed in counts / s per gram as 240 < Pu.

[0047] The detection limit, expressed in Bq for 137< Cs or 60< Co, or g of 240< Pu for neutron measurements, is obtained by dividing the detection limit, expressed in counts per second (or number of pulses detected per second) by a sensitivity coefficient k.

[0048] Table 1 shows, for each tested configuration: the background noise, determined in the spectral regions of interest 662 keV (ROI1) for 137< Cs, 1173 and 1332 keV (ROI2) for 60< Co and ROI3 for neutrons, on the figure 7A . the raw signals measured, on the ROI1 of the figure 6A for 137< Cs, ROI2 of the figure 6B for 60< Co, ROI3 of figures 6D, 6D And 6E for neutron measurements: the calculated net signals; for neutron measurements, the different detection efficiencies and the equivalent mass 240< Pu corresponding to each net count rate; the sensitivity coefficients, allowing a correspondence to be established between count rates and activity values ​​for 137< Cs, 60< Co, from the photon count rates, or an equivalent mass 240< Pu from the neutron count rates; the detection limits expressed both in counts.s -1< or per unit activity (Bq) for the 137< Cs and 60< Co sources or in equivalent mass 241< Pu for the neutron source.

[0049] In the "background noise" and "raw count rate" lines, the figure on which the indicated value was measured has been entered. Table 1 137< Cs 60< Co 240< Pu- fig. 3A 240< Pu - fig. 3B 240< Pu - fig. 3C Bruit de fond [coups] 3252 ( fig. 7A ) 1863 ( fig. 7A ) 28 ( fig. 7A ) 28 ( fig. 7A ) 28 ( fig. 7A ) Signal brut [coups] 344392 ( fig. 6A ) 295619 ( fig. 6B ) 1408 ( fig. 6C ) 2310 ( fig. 6D ) 4778 ( fig. 6E ) Signal net [coups] 341140 293756 1380 2282 4750 Taux de comptage [coups.s -1< ] 568.56 489.59 2.300 3.803 7.917 Rendement de détection neutron 8.519 10 -5< 1.409 10 -4< 2.932 10 -4< Masse équivalente 240< Pu [g] 26.48 26.48 26.48 LD [coups / s] 12.9 9.78 0.05 0.05 0.05 k [coups.s-1 / Bq] ou [ / g 240< Pu] 1.24 10 -3< 1.56 10 -3< 0.0868 0.1436 0.2988 LD [Bq] ou [g. 240< Pu] 10453 6279.5 0.5632 0.3405 0.1636

[0050] We observe that the configurations of figures 3B et 3C These arrangements allow the detection limit of 240< Pu to be significantly lowered. They also increase the sensitivity of the beacon to neutron-emitting isotopes.

[0051] THE figures 8A et 8B show respectively spectra obtained on each of the organic scintillators 10, arranged according to the third configuration ( figure 3C ), using successively a source of 137 < Cs (curve a), a source of 60 < Co (curve b) or a source of 252 < Cf (curve c). The spectra were obtained by connecting each detector to a spectrometric processing unit.

[0052] As previously mentioned, with this type of detector, the predominant interaction of gamma photons is inelastic scattering, or Compton scattering. Given the negligible probability of photoelectric interaction, the characteristic emission peaks of ¹³⁷Cs, ¶⁰Co, or the peak corresponding to a neutron detection are not detectable. However, the shape of the spectrum is correlated with the nature of the detected radiation: Spectra formed using ¹³⁷Cs and ¶⁰Co exhibit a front, the position of which depends on the energy of the detected photons. Thus, the detection of a front indicates the presence of a photon-emitting source, and the position of the front can provide information about the energy of the photons emitted by the source, and therefore about the nature of the gamma-emitting isotopes. The absence of a front in the spectrum indicates that the detected source primarily emits neutrons.Thus, organic scintillators can be used to establish a first indication of the type of radiation emitted by a source detected by the beacon.

[0053] A quantitative analysis can be performed by subtracting the measured signal from a signal corresponding to background noise, acquired in the absence of an irradiating source. On the figures 8A et 8B The background noise signal corresponds to curve d). Taking the background noise into account and subtracting it from the measured signal provides quantitative information specific to a potential irradiating source. This quantitative information can be combined with the spectral information resulting from the inorganic scintillator. This improves the confidence level in detecting a source, particularly when the detection from the inorganic scintillator is close to the detection limit.

[0054] One advantage of organic scintillators is their ability to be produced in large volumes, resulting in high sensitivity and enabling rapid detection of neutron or photon radiation emitted by a source. In the event of a suspected detection, the processing unit can acquire a spectrum of the pulses detected by the inorganic scintillator, allowing for more precise identification of the source type. The larger the volume of the organic scintillators, the greater their neutron moderation effect, thus improving the neutron detection sensitivity of the inorganic scintillator.

[0055] Furthermore, organic scintillators are sensitive to fast neutrons, unlike the inorganic scintillator doped with 6Li, which is sensitive to thermal or thermalized neutrons. The sensitivity of organic scintillators to both fast neutrons and photons can be exploited to obtain information about the presence of spontaneously fissioning isotopes in the detected source.

[0056] The signals detected by an organic scintillator can be analyzed by a time-coincidence circuit to establish a Rossi-alpha curve. A Rossi-alpha curve is a histogram of interactions detected within a time window following the detection of an interaction. The width of the time window is typically a few hundred nanoseconds. Rossi-alpha curves have been established using the configuration described in connection with the figure 3B We successively used the sources of 137 < Cs, 60 < Co, and then 252 < Cf. Each detected pulse was assigned a time label corresponding to its detection time. Rossi alpha curves were obtained in post-processing, based on the time labels assigned to each pulse. The initial pulse is the pulse that initiated the opening of the time window. After a sufficient exposure time (10 minutes), a histogram of the measured time shifts was established. The initial pulse and each pulse detected within the same time window are considered coincident, up to the time shift.

[0057] There figure 9 represents the number of coincident interactions (y-axis) for different time lags (x-axis - unit [ns]). On the figure 9 Curves a), b), and c) correspond respectively to the 137 < Cs source, the 60 < Co source, and the 252 < Cf source. Curves a) and b) are typical of photon-emitting radiating sources. On curve c), the time-shift histogram shows a peak centered at 30 ns. This histogram corresponds to (γ, n) coincidences resulting from the spontaneous fission of 252 < Cf. Indeed, spontaneous fission generates the simultaneous emission of γ photons and fast neutrons. Fast neutrons propagate through air at a lower speed than photons, which explains the observed time shift. The time shift corresponding to the observed peak depends on the distance between the source and the detectors. The greater the distance, the greater the time shift.

[0058] Thus, when one or more scintillators are connected to a time-coincidence detection circuit, obtaining a histogram of time-shifts of pulses considered coincident (i.e., detected within a narrow time window) allows the type of source to be identified. More precisely, the appearance of a peak, corresponding to a time shift of a few tens of nanoseconds, between two coincident interactions, indicates the potential presence of a spontaneous fission neutron source.

[0059] It is noted that such a peak does not appear when a neutron source detected by the beacon consists of a mixture of an alpha-emitting isotope with a target material that allows for an (α,n) reaction: such a source is usually referred to as an (α,n) source. Thus, the use of a Rossi-Alpha histogram allows for the differentiation between a spontaneous fission neutron source and an (α,n) type neutron source, for example, an Am-Li source.

[0060] This embodiment uses the fact that the organic scintillator or each organic scintillator is sensitive to fast neutrons, unlike the doped inorganic scintillator, which is sensitive to thermal or intermediate neutrons.

[0061] THE figures 10A et 10B illustrate a possible use of a beacon with multiple scintillators, as shown on the figure 3B . There figure 10A It shows the arrangement of the scintillators forming the beacon, as well as a path followed by a source. The source's path is indicated by an arrow. Five positions of the source, numbered 1 to 5, have been identified. figure 10B represents a count rate (ordinate axis) measured by the three detectors: NaIL, PVT1 (organic scintillator) and PVT2 (another organic scintillator), as a function of the position of the source (abscissa axis).

[0062] To obtain comparable values, the count rates of PVT1 and PVT2 were divided by 100. It can be observed that the time evolution of the count rates makes it possible to determine if a source is moving and to estimate the direction of its movement. This is an advantage of using different independent detectors in the same beacon.

[0063] There figure 11 summarizes the main processing steps that can be implemented by the processing unit.

[0064] During step 100, the beacon is placed in monitoring mode. The detection signals from each detector are representative of the background noise, within statistical fluctuations. When the count rate of at least one detection signal exceeds an alert threshold, the beacon switches to alert mode. The alert threshold can be predefined. For example, it could be an average of signals, such as a moving average, plus n times the standard deviation.

[0065] Following the switch to alert mode, the processing carried out by the beacon aims to characterize the source from the detection signals resulting from the different detectors.

[0066] The treatment may include one or more of the following operations: 111: Formation of an energy spectrum from a first detection signal, resulting from an organic scintillator, and estimation of the presence of neutrons or photons. 121: Formation of an energy spectrum from a second detection signal, resulting from the inorganic scintillator, in order to identify gamma isotopes and neutron emission. 122: Discrimination of the pulses of the second detection signal according to their shape and energy in order to distinguish a photon component and a neutron component of the detected radiation. 112: Establishment of a Rossi-alpha histogram, in order to identify a potential spontaneous fission reaction in the source. Step 112 can be carried out if a neutron component is confirmed following steps 122 and / or 121 and / or 111. 130: Comparison of the count rates resulting from each detector and estimation of source displacement.

[0067] The invention is based on a certain complementarity in the detection capabilities of two types of detectors: Each first detector is an organic scintillator: it exhibits high detection sensitivity to photons and fast neutrons, making it ideal for use as an early warning detector. Each second detector is a doped inorganic scintillator: it can be used for finer characterization, particularly for identifying gamma isotopes or discriminating between neutron and photon components of the radiation produced by the source. Proximity to at least one first detector significantly increases sensitivity to neutrons. Each first detector can be used to detect the occurrence of spontaneous fission in the source.

Claims

1. Detection beacon (1) for detecting the presence of a radiation source in an environment extending around said beacon, the radiation source being capable of emitting ionizing photons, such as X-rays or gamma rays, and / or neutrons, the beacon comprising: - at least one first detector (10), comprising a three-dimensional organic scintillator material (11), the first detector being configured to form a pulse following an interaction of a neutron or a photon in the organic scintillator material and to generate a first detection signal from each pulse thus formed; the detection beacon being characterized in that it comprises: - at least one second detector (20), comprising a three-dimensional inorganic scintillator material (12) to which a capture isotope conducive to thermal neutron capture has been added, the second detector being configured to form a pulse following an interaction of a neutron or a photon in the inorganic scintillator material, and to generate a second detection signal from each pulse thus formed, the smallest dimension of the second detector being greater than 1 cm; - a processing unit (30), connected to each first and second detector, the processing unit being configured to characterize the radiation source detected by the beacon based on the first detection signal and the second detection signal.

2. Beacon according to claim 1, wherein the distance between at least one organic scintillator material of a first detector of said beacon and at least one inorganic scintillator material of a second detector of said beacon is less than 20 cm.

3. Beacon according to any of the preceding claims, wherein a plurality of organic scintillator materials of a plurality of respective first detectors of said beacon are arranged around an inorganic scintillator material of a second detector.

4. Beacon according to any of the preceding claims, wherein a plurality of organic scintillator materials of a plurality of respective first detectors of said beacon are arranged on either side of an inorganic scintillator material of a second detector.

5. A beacon according to any of the preceding claims, comprising: - a multichannel analyzer (32), programmed to determine an amplitude of each pulse resulting from at least one second detector (20) of said beacon, and to form an energy spectrum , representing a number of pulses detected as a function of their respective amplitude; - the processing unit (30) being configured to identify γ-emitting isotopes as a function of the energy spectrum.

6. Beacon according to any of the preceding claims, comprising: - a multichannel analyzer (32), programmed to determine an amplitude of each pulse resulting from at least one second detector of said beacon; - a pulse shape analyzer (33), programmed to analyze a shape of the pulses resulting from the second detector and determine a shape parameter ( Qtot − Qshort Qtot ) of each analyzed pulse; - a discriminator (34), so as to discriminate, in the second detection signal, pulses resulting respectively from an interaction of a photon or a neutron in the second detector; - the processing unit is programmed to determine a photonic contribution and a neutron contribution of the radiation produced by the source.

7. Beacon according to any of the preceding claims, wherein: - the beacon comprises a temporal coincidence analysis circuit (35), programmed to establish, from the or each first detection signal, a histogram of the temporal interval separating two consecutive pulse detections; - the processing unit being programmed to determine the occurrence of a spontaneous fission reaction in the source from the histogram.

8. Beacon according to any of the preceding claims, wherein: - the beacon comprises a multichannel analyzer (32), programmed to determine an amplitude of each pulse resulting from at least one first detector (10) of said beacon, and to form an energy spectrum representing a number of pulses as a function of their respective amplitudes; - the processing unit is configured to identify γ-emitting isotopes as a function of the energy spectrum.

9. Beacon according to any of the preceding claims, wherein the capture isotope is 6Li or 10B.

10. A method for detecting a radiation source present in an environment extending around a beacon (1) according to any of the preceding claims, comprising the steps of: a) analyzing at least a first detection signal and at least a second detection signal, such that when the first detection signal crosses a first threshold or when the second detection signal crosses a second threshold, an alert signal is given; b) following the emission of an alert signal, characterizing the radiation emitted by the source using the first detection signal and / or the second detection signal.

11. Method according to claim 10, wherein: - the beacon comprises a multichannel analyzer (32), programmed to determine an amplitude of each pulse resulting from at least one second detector, and to form an energy spectrum representing a number of pulses as a function of their respective amplitudes; - step b) comprises identifying γ-emitting isotopes from the energy spectrum.

12. Method according to any of claims 10 or 11, wherein the beacon comprises a multichannel analyzer programmed to determine an amplitude of each pulse detected by at least one second detector, and wherein step b) comprises the sub-steps of: - analyzing the shape of the pulses generated by the second detector, so as to assign a shape parameter ( Qtot − Qshort Qtot ) to each pulse; - classifying each pulse according to the shape parameter and energy, so that each pulse is assigned to a photon or a neutron.

13. Method according to any of claims 10 to 12, wherein: - the beacon comprises a time coincidence analysis circuit (35), programmed to establish, from at least a first detection signal from said beacon, a histogram of the time interval between two consecutive pulse detections; - step b) comprises detecting a spontaneous fission reaction as a function of the histogram.

14. Method according to claim 13, in which the temporal coincidence analysis circuit is connected to two first detectors of said beacon or to a first detector of said beacon and a second detector of said beacon.

15. Method according to any of claims 10 to 14, wherein: - the beacon comprises a multichannel analyzer, programmed to determine an amplitude of each pulse detected by at least a first detector of said beacon, and to form an energy spectrum representing a number of pulses detected as a function of their respective amplitudes; - step b) comprises identifying γ-emitting isotopes from the energy spectrum.

16. Method according to any of claims 10 to 15, wherein step b) comprises comparing each first detection signal and each second detection signal at different times, so as to estimate a movement of the detected source.