IONIZING RADIATION DETECTOR

Abstract

A radiation detector may include an active structure having an input face and an output face for incident ionizing radiation. The active structure may include: a first organic scintillator including at least one neutron-absorbing material and enabling discrimination between fast neutrons, thermal neutrons, and photons; at least one second scintillator arranged in front of the first scintillator and capable of preferentially detecting the alpha and/or beta radiation, the two scintillators having, due to the choice of their constituents, different mean pulse decay constants, and the second scintillator having a thickness smaller than the first.

Claims

1. A radiation detector, comprising: an active structure having an input face and an output face suitable for incident ionizing radiation, wherein the active structure comprises a plastic scintillator assembly comprising (i) a first organic scintillator comprising a neutron capture material and allowing discrimination between fast neutrons, thermal neutrons, and photons, (ii) a second scintillator arranged in front of the first and capable of preferentially detecting alpha and/or beta radiation, and (iii) a third scintillator arranged in front of the second scintillator (ii), wherein the third scintillator (iii) is suitable for preferentially detecting alpha radiation, wherein the first scintillator (i) and the second scintillator (ii) have different mean photoluminescence decay constants due to a choice of their constituents, wherein the second scintillator (ii) is thinner than the first scintillator (i), and wherein a difference between refractive indices of two adjacent scintillators is not more than 0.5.

2. (canceled)

3. The detector of claim 1, further comprising: a gain photon-electron converter arranged behind the output face of the active structure to collect light emitted within the scintillators.

4. The detector of claim 1, wherein the scintillators increase in thickness going from inlet to outlet.

5. The detector of claim 1, wherein the first scintillator (i) has a thickness in a range of from 3 to 100 mm.

6. The detector of claim 1, wherein the second scintillator (ii) is suitable for preferentially detecting the beta radiation and has a thickness in a range of from 50 to 250 microns.

7. The detector of claim 1, wherein the third scintillator (iii) has a thickness in a range of from 1 to 50 microns.

8. (canceled)

9. The detector of claim 1, wherein a difference between the mean photoluminescence decay constants of two adjacent scintillators is in a range of from 25 to 250 ns.

10-11. (canceled)

12. The detector of claim 1, wherein the first scintillator (i) is a plastic scintillator comprising a primary fluorescent element, and a neutron capture element.

13. The detector of claim 9, wherein the primary fluorescent element comprises biphenyl, meta-terphenyl or 2,5-diphenyloxazole, and wherein the neutron capture element comprises lithium, boron, cadmium, or gadolinium.

14-15. (canceled)

16. The detector of claim 1, wherein the active structure comprises an inorganic scintillator that is capable of gamma spectrometry.

17. The detector of claim 1, wherein a difference between refractive indices of two adjacent scintillators is not more than 0.05.

18. A process for detecting and discriminating ionizing radiation, the method comprising: exposing the input face of the detector of claim 1 to a radiation source; and analyzing, at least on the basis of knowledge of the mean decay constants of the scintillators, an optical signal generated by the active structure of the detector thereby discriminating as to a nature of the radiation from among at least four different types of radiation, wherein the discriminating optionally comprises performing a pulse shape discrimination (PSD) method.

19. (canceled)

20. The process of claim 18, comprising: collecting an optical signal generated by the active structure by a gain photon-electron converter so as to be converted into an electrical pulse.

21. The process of claim 18, which uses a detector comprising an inorganic scintillator that is capable of gamma spectrometry, wherein the gamma component, when detected, is analyzed spectrometrically.

22. The detector of claim 9, wherein the first scintillator (i) further comprises a secondary fluorescent element.

23. The detector of claim 16, wherein the inorganic scintillator is based on silver-doped zinc sulfide (ZnS:Ag) or cerium-doped gadolinium pyrosilicate (Gd.sub.2Si.sub.2O.sub.7:Ce).

24. The detector of claim 16, wherein the inorganic scintillator is on the outlet side of the active structure.

25. The detector of claim 1, wherein the scintillators have a mean photoluminescence decay constant which decreases going from the inlet to the outlet.

26. The detector of claim 1, wherein the first scintillator has an emission spectrum whose maximum is at a wavelength in a range of from 400 to 630 nm.

27. The detector of claim 1, wherein the mean photoluminescence decay constant of the first scintillator is in a range of from 1 to 20 ns.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0076] FIG. 1 is a partial schematic representation of an example of a detector according to the invention,

[0077] FIG. 2 is a partial schematic representation of one embodiment example of a detector,

[0078] FIG. 3a, FIG. 3b and FIG. 3c illustrate detection results when the detector in FIG. 2 is exposed to a source of neutrons and gamma rays jointly emitted by californium-252,

[0079] FIG. 4a and FIG. 4b illustrate detection results when the detector in FIG. 2 is exposed to a carbon-14 beta radiation source,

[0080] FIG. 5a and FIG. 5b illustrate detection results when the detector in FIG. 2 is exposed to a plutonium-239 alpha/gamma radiation source,

[0081] FIG. 6a, FIG. 6b, FIG. 6c and FIG. 6d illustrate detection results when the detector in FIG. 2 is simultaneously exposed to two radiation sources, plutonium-239 and californium-252,

[0082] FIG. 7a, FIG. 7b, FIG. 7c and FIG. 7d illustrate detection results when the detector in FIG. 2 is simultaneously exposed to two radiation sources, carbon-14 and californium-252,

[0083] FIG. 8a, FIG. 8b and FIG. 8c illustrate detection results when the detector in FIG. 2 is simultaneously exposed to two radiation sources, carbon-14 and plutonium-239,

[0084] FIG. 9a, FIG. 9b, FIG. 9c, FIG. 9d and FIG. 9e illustrate detection results when the detector in FIG. 2 is simultaneously exposed to three radiation sources, carbon-14, plutonium-239, and californium-252,

[0085] FIG. 10 is a partial schematic representation of another implementation example of a detector,

[0086] FIG. 11a, FIG. 11b, and FIG. 11c illustrate detection results when the detector in FIG. 10 is exposed to a source of neutrons and gamma rays jointly emitted by californium-252,

[0087] FIG. 12a and FIG. 12b illustrate detection results when the detector in FIG. 10 is exposed to a carbon-14 beta radiation source,

[0088] FIG. 13a and FIG. 13b illustrate detection results when the detector in FIG. 10 is exposed to a plutonium-239 alpha/gamma radiation source,

[0089] FIG. 14a, FIG. 14b, FIG. 14c and FIG. 14d illustrate detection results when the detector in FIG. 10 is simultaneously exposed to two radiation sources, curium-244 and californium-252,

[0090] FIG. 15a, FIG. 15b and FIG. 15c illustrate detection results when the detector in FIG. 10 is simultaneously exposed to two radiation sources, carbon-14 and curium-244,

[0091] FIG. 16a, FIG. 16b, FIG. 16c and FIG. 16d illustrate detection results when the detector in FIG. 10 is simultaneously exposed to two radiation sources, carbon-14 and californium-252,

[0092] FIG. 17a, FIG. 17b, FIG. 17c, FIG. 17d and FIG. 17e illustrate detection results when the detector in FIG. 10 is simultaneously exposed to three radiation sources, carbon-14, plutonium-239, and californium-252,

[0093] FIG. 18 is a partial schematic representation of an example of a detector according to the invention, in hemispherical form,

[0094] FIG. 19a, FIG. 19b, FIG. 19c and FIG. 19d are partial schematic illustrations of different examples of a detector including four scintillators,

[0095] FIG. 20 is a partial schematic representation of another implementation example of a detector, and

[0096] FIG. 21a, FIG. 21b, FIG. 21c, FIG. 21d and FIG. 21e illustrate detection results when the detector in FIG. 19a is simultaneously exposed to three radiation sources, carbon-14, plutonium-239 and californium-252.

DETAILED DESCRIPTION

[0097] FIG. 1 shows an example of a radiation detector 1 according to the invention. Detector 1 comprises an active structure 2 having a radiation input face 2a and an output face 2b for radiation not entirely stopped by the active structure. The source of the radiation R to be detected is located on the side of the input face 2a.

[0098] In the example under consideration, the active structure 2 is in the form of a stack of three detection layers, each including a scintillator of a given thickness, the scintillators increasing in thickness from the detector input 2a to the output 2b.

[0099] Thus, scintillator 30, located at the front of the detector, has the finest thickness. Scintillator 20 is located directly behind scintillator 30 and is thicker than scintillator 30. Finally, scintillator 10 is located behind scintillator 20 and has the largest thickness of the three scintillators.

[0100] Such an active structure is, for example, generally cylindrical or parallelepipedal in shape.

[0101] The detector also includes a converter, such as a photomultiplier 5, located behind the active structure 2, i.e. on the output side 2b, which collects the light emitted by the various scintillators 10, 20 and/or 30 and converts the optical signal collected into an electrical signal. The electrical signal generated has pulses characteristic of the energy deposited by the ionizing radiation in the detector.

[0102] The scintillators of the active structure 2 are designed to have different response properties to ionizing radiation, which enables the different pulses to be sorted so as to identify the nature of the ionizing radiation(s) inducing scintillation.

[0103] Scintillator 10 is, for example, a plastic scintillator whose composition allows discrimination between fast neutrons, thermal neutrons and gamma rays, as described below.

[0104] Scintillator 20 is, for example, a plastic scintillator that is preferentially capable of detecting beta radiation, while scintillator 30 preferentially detects alpha radiation.

[0105] Thus, a fivefold discrimination between different types of ionizing radiation is possible with such a structure.

[0106] Each of these scintillators firstly has its own mean photoluminescence decay constant, which cannot be confused with that of any other scintillator, and secondly a thickness chosen to favor interactions with the type(s) of radiation to be detected.

Example 1

[0107] FIG. 2 shows an implementation example of a detector 1 according to the invention, including three plastic scintillators 10, 20 and 30.

[0108] In this example, scintillator 10 consists of a poly(styrene-co-methacrylic acid) polymer matrix including the fluorescent elements 2,5-diphenyloxazole (PPO) and 1,4-bis(5-phenyl-2-oxazolyl)benzene (POPOP), and a thermal neutron-absorbing element, lithium alpha-valerate, said compound being isotopically enriched in the lithium-6 isotope via the process described in patent application FR3083622.

[0109] Scintillator 10 is thus prepared independently of scintillators 20 and 30, by adding styrene, methacrylic acid, PPO, POPOP and lithium alpha-valerate to a one-necked flask. The solution is then saturated with neutral gas, followed by five cold vacuum degassing cycles. The solution is then packaged in an airtight glass mold, which gives the scintillator its final shape. The solution is heated at 80? C. for 8 weeks under an inert atmosphere. Scintillator 10 is removed from the mold, cut and polished.

[0110] Scintillator 10 thus obtained has a thickness e.sub.1 of about 7000 ?m and a mean photoluminescence decay constant ?.sub.1 of about 3.3 ns.

[0111] Scintillator 10 has, for example, a refractive index of about 1.58.

[0112] Scintillators 20 and 30 may be prepared directly one on top of the other. As a variant, scintillators 20 and 30 may be bonded together using an optical binder.

[0113] Plastic scintillator 20 may be produced via the process described in patent application FR3075977, which discloses a method for preparing plastic scintillators with mean decay constants of between 2 and 88 ns.

[0114] For example, a mixture of polystyrene, naphthalene, PPO and 9,10-diphenylanthracene is prepared, these elements being dissolved in toluene. The solution is poured onto a glass plate and spread using a doctor blade-type spreading device, then left under ventilation for the toluene to evaporate off, i.e. about 2 days, depending on the thickness.

[0115] In the example under consideration, but not limiting in the preparation, scintillator 20 thus obtained has a thickness e.sub.2 of about 150 ?m and a mean photoluminescence decay constant ?.sub.2 of about 35 ns.

[0116] Scintillator 20 has, for example, a refractive index of about 1.60.

[0117] The same method may be used to prepare plastic scintillator 30: a mixture of polystyrene, naphthalene, PPO and 9,10-diphenylanthracene dissolved in toluene is prepared, in proportions, for example, different from those used for preparing scintillator 30, for example those described in patent application FR3075977.

[0118] The solution thus obtained may be poured and spread directly onto scintillator 20 by means of the same spreading device (such as a doctor blade), then left under ventilation for a period of about two hours to evaporate off the toluene.

[0119] Scintillator 30 thus obtained has a thickness e.sub.3 of about 17 ?m and a mean photoluminescence decay constant ?.sub.3 of about 71 ns, and a refractive index of about 1.60.

[0120] Scintillator 10 is connected to scintillators 20 and 30 by means of an optical binder 50, for example an optical grease known to those skilled in the art, so that the three scintillators are arranged from the thinnest to the thickest going from the inlet 2a to the outlet 2b of the active structure, as illustrated in FIG. 2.

[0121] Other types of optical binder may be considered, for example an optical cement or a viscous transparent liquid with an optical index, also known as the refractive index, of about 1.6 at 425 nm, or any other material that is transparent to the emission wavelength of the various scintillators. For example, an optical binder with a thickness of 50 ?m is prepared.

[0122] In this example, scintillators 10, 20 and 30 have an emission spectrum whose maximum is at a wavelength of 425 nm for scintillator 10, and 435 nm for scintillators 20 and 30.

[0123] Detector 1 also includes a photomultiplier 5, for instance the Hamamatsu commercial photomultiplier tube (registered trademark) R6231, which is coupled to the output face 2b of active structure 2, for example by means of an optical grease 50.

[0124] The photomultiplier 5 is supplied with high voltage, for example 1070 V, and connected to a digitizer 6 so as to transform the electrical signal into a digital signal.

[0125] The digital signal thus generated may be used to represent the detection results in various ways.

[0126] For example, the two-dimensional spectrum representing, for each pulse, the ratio Q.sub.tail/Q.sub.tot of the delayed charge Q.sub.tail of the pulse to the total charge Q.sub.tot of the same pulse may be plotted as a function of the total charge Q.sub.tot, as illustrated for different radiation sources in FIGS. 3a, 4a, 5a, 6a, 7a, 8a and 9a.

[0127] The total charge Q.sub.tot is calculated by integrating the current carried by the pulse between its start and 780 ns.

[0128] The delayed charge Q.sub.tail is calculated as the integral of the current carried by the pulse between 42 and 780 ns.

[0129] The ratio Q.sub.tail/Q.sub.tot thus provides indications of the pulse's broader or narrower shape, and allows the emergence of pulse grouping zones by interaction type.

[0130] In order to determine zones of interest according to the different types of ionizing radiation, detector 1 is first subjected to a single source of radiation at a time. These defined zones may subsequently be used to discriminate between multiple types of ionizing radiation.

[0131] Detector 1 is thus subjected to a californium-252 neutron/gamma source located about 8 cm from the input face 2a.

[0132] Thermal neutrons are generated by partial thermalization of the californium-252 source, which is shielded by 5 cm of high-density polyethylene (HDPE) and 3 cm of lead, allowing the energy of the neutrons emitted by the source to be degraded until thermal neutrons are obtained.

[0133] The two-dimensional spectrum of the detection results for this source is represented in FIG. 3a. Pulse shape analysis (PSD) allows two grouping zones 100 and 110 to emerge: zone 100, which comprises the lower lobe of the spectrum, groups together pulses resulting from interactions with gamma rays, and zone 110, which lies above zone 100, groups together pulses resulting from interactions with neutrons.

[0134] Interactions of the detector with thermal neutrons may be discriminated from those with fast neutrons by separating zone 110 into two subzones (not shown), one subzone including the lobe extending widthwise for fast neutrons, and one subzone including the oval above this lobe for thermal neutrons.

[0135] These zones thus defined allow the number of events derived from each type of radiation to be isolated. The energy spectrum for each type of radiation can then be obtained, as shown in FIGS. 3b and 3c for neutrons and gamma rays, respectively.

[0136] These graphs are obtained by projecting the zone of interest of the two-dimensional spectrum on the x-axis: the x-axis on the graphs in FIGS. 3B and 3C represents the energy channels of the histogram, i.e. the total charge Q.sub.tot, and the y-axis the number of events counted for each total charge value.

[0137] The neutron energy spectrum represented in FIG. 3b has an exponential-type spread E1 and a peak P1, E1 extending between 0 and 35 000 channels and P1 between 5000 and 10 000 channels, corresponding to the interactions of fast neutrons and thermal neutrons with scintillator 10, respectively.

[0138] In the same manner as described above, detector 1 is subjected to a carbon-14 beta source, placed in contact with active structure 2. Energy deposition takes place in scintillators 10, 20 and 30.

[0139] Only the pulses received by scintillators 20 and 30 are counted, and represented on the two-dimensional spectrum in FIG. 4a, allowing a zone of interest 200 to be defined, grouping together the pulses resulting from the interactions of the detector with the beta radiation.

[0140] The corresponding beta energy spectrum is represented in FIG. 4b. It has a distinctive peak P200 between 0 and 5000 channels.

[0141] Detector 1 is subjected to a plutonium-239 alpha/gamma source, placed in contact with active structure 2 on the input side 2a. Alpha interaction energy is deposited in scintillator 30, while gamma interaction energy is deposited in scintillator 10.

[0142] The pulses received by scintillator 30 are counted, and a zone of interest 300 grouping together the pulses resulting from alpha interactions is defined on the two-dimensional spectrum, as shown in FIG. 5a. Pulses resulting from gamma interactions with scintillator 10 are grouped together in zone 100 defined above.

[0143] The alpha radiation energy spectrum is represented in FIG. 5b. It has a distinctive peak P300 between 5000 and 12 000 channels.

[0144] The grouping zones by radiation type are summarized in Table 1 below.

TABLE-US-00001 TABLE 1 x-axis (Q.sub.tot) y-axis (Q.sub.tail/Q.sub.tot) Alpha - Zone 300 5000-12000 0.4-0.62 Beta - Zone 200 0-5000 0.3-0.6 Neutrons - Zone 110 0-35000 0.19-0.33 Gamma - Zone 100 0-35000 0-0.1875

[0145] As mentioned previously, these zones thus defined allow discrimination between the different types of radiation detected simultaneously by detector 1.

[0146] By way of example, detector 1 is simultaneously subjected to two sources of ionizing radiation, namely a californium-252 source and a plutonium-239 source.

[0147] The detection results are shown in FIGS. 6a to 6d. As these figures show, detector 1 has the ability to discriminate between neutrons (FIG. 6b), gamma radiation (FIG. 6c) and alpha radiation (FIG. 6d).

[0148] In the example shown in FIGS. 7a to 7d, detector 1 is simultaneously subjected to a californium-252 source and a carbon-14 source, and has the ability to discriminate neutrons (FIG. 7b), gamma radiation (FIG. 7c) and beta radiation (FIG. 7d).

[0149] In the example shown in FIGS. 8a to 8c, detector 1 is simultaneously subjected to a californium-252 source and a plutonium-239 source, and has the ability to discriminate alpha radiation (FIG. 8b) from beta radiation (FIG. 8c).

[0150] Finally, in the example shown in FIGS. 9a to 9e, detector 1 is subjected simultaneously to the three abovementioned sources of ionizing radiation, namely a californium-252 source, a carbon-14 source and a plutonium-239 source.

[0151] The two-dimensional spectrum shown in FIG. 9a displays pulse groupings in the four zones defined above, meaning that detector 1 has detected alpha, beta, gamma and neutron ionizing radiation.

[0152] Detector 1 is thus capable of fivefold discrimination, i.e. it can discriminate between fast and thermal neutrons (peaks P1 and P2 in FIG. 9d, respectively), alpha radiation (FIG. 9b), beta radiation (FIG. 9c) and gamma radiation (FIG. 9e).

Example 2

[0153] FIG. 10 shows a variant of a detector 1 according to the invention, including two plastic scintillators 10, 20 and an inorganic scintillator 30.

[0154] In this example, the plastic scintillators 10 and 20 are prepared via a method similar to that which has just been described.

[0155] Plastic scintillator 10 preferentially detects neutrons and gamma rays, and has a thickness e.sub.4 of about 7000 ?m, a mean photoluminescence decay constant ?.sub.4 of about 3.3 ns, and a refractive index of about 1.58.

[0156] Plastic scintillator 20 preferentially detects beta radiation, has a thickness e.sub.5 of about 153 ?m, a mean photoluminescence decay constant ?.sub.4 of about 47 ns, and a refractive index of about 1.60.

[0157] To prepare inorganic scintillator 30, a mixture of silver-doped zinc sulfide (ZnS:Ag) and polystyrene is dispersed in toluene, the mixture being, for example, 80 m % ZnS:Ag and 20 m % polystyrene, m % denoting here the mass percentage.

[0158] The mixture is then poured and spread as a thin layer directly onto scintillator 20, for example by means of a doctor blade-type device, before being left under ventilation for a period of about two hours in order to evaporate off the toluene.

[0159] The ZnS:Ag preparation is obtained, for example, from a commercial fluorescent powder with a particle size of about 8 ?m.

[0160] Scintillator 30 thus obtained has a thickness e.sub.6 of about 43 ?m and a mean photoluminescence decay constant ?.sub.6 of about 200 ns.

[0161] As described previously, scintillator 10 is connected to scintillators 20 and 30 by means of an optical binder 50, for example an optical grease or cement.

[0162] In the example under consideration, scintillators 10, 20 and 30 have an emission spectrum whose maximum is at a wavelength of 425 nm for scintillator 10, 435 nm for scintillator 20 and 450 nm for scintillator 30.

[0163] Detector 1 thus obtained is first subjected to one source of radiation at a time, as described above, so as to determine the grouping zones allowing the pulses from each type of radiation to be isolated.

[0164] Detector 1 is subjected to a partially thermalized californium-252 source emitting both fast and thermal neutrons, and gamma rays.

[0165] Energy is deposited in scintillator 10, and two zones of interest 100 and 110 are determined on the two-dimensional spectrum represented in FIG. 11a, respectively grouping pulses resulting from gamma and neutron interactions.

[0166] Pulse shape analysis thus allows neutrons to be distinguished from gamma rays, as shown in FIGS. 11b and 11c.

[0167] The neutron energy spectrum in FIG. 11b shows an exponential spread P1 and a peak P2 corresponding to fast and thermal neutrons, respectively.

[0168] Similarly, detector 1 is subjected to a carbon-14 beta source, and the zone of interest 200 is determined, grouping together the pulses resulting from beta interactions with scintillator 20, as represented on the two-dimensional spectrum shown in FIG. 12a and the associated beta energy spectrum shown in FIG. 12b.

[0169] Detector 1 is also subjected to a plutonium-239 alpha source, for which energy is deposited in inorganic scintillator 30, allowing the zone of interest 300 to be defined, grouping together the pulses resulting from alpha interactions with this scintillator, as illustrated in the two-dimensional spectrum shown in FIG. 13a and the associated alpha energy spectrum shown in FIG. 13b.

[0170] The zones of grouping by radiation type are summarized for this example of detector design in Table 2 below.

TABLE-US-00002 TABLE 2 x-axis (Q.sub.tot) y-axis (Q.sub.tail/Q.sub.tot) Alpha - Zone 300 0-25000 0.8-1.0 Beta - Zone 200 0-5000 0.3-0.6 Neutrons - Zone 110 0-35000 0.19-0.33 Gamma - Zone 100 0-35000 0-0.1875

[0171] Similarly to what has been described previously, the detector thus produced can discriminate between several types of radiation, notably when it is irradiated simultaneously by several radiation sources.

[0172] For example, when detector 1 is subjected to a curium-244 source and a californium-252 source, neutrons (FIG. 14b), alpha radiation (FIG. 14c) and gamma radiation (FIG. 14d) can be discriminated by means of the dimensional spectrum shown in FIG. 14a.

[0173] In another example shown in FIGS. 15a to 15c, detector 1 is simultaneously subjected to a curium-244 source and a carbon-14 source, and has the ability to discriminate alpha radiation (FIG. 15b) from beta radiation (FIG. 15c).

[0174] In the example shown in FIGS. 16a to 16d, detector 1 is simultaneously subjected to a californium-252 source and a carbon-14 source, and has the ability to discriminate neutrons (FIG. 16b) from beta radiation (FIG. 16c) and gamma radiation (FIG. 16d).

[0175] Finally, in the example shown in FIGS. 17a to 17e, detector 1 is simultaneously subjected to all three sources of ionizing radiation, namely a partially thermalized californium-252 source, a carbon-14 source and a plutonium-239 source.

[0176] The two-dimensional spectrum represented in FIG. 17a shows pulse groupings in the four zones defined above, meaning that detector 1 has detected ionizing radiation of the alpha, beta, gamma and neutron types.

[0177] Detector 1 is thus capable of fivefold discrimination, i.e. it can discriminate between fast and thermal neutrons (exponential spread P1 and peak P2 in FIG. 17d, respectively), alpha radiation (FIG. 17b), beta radiation (FIG. 17c) and gamma radiation (FIG. 17e).

[0178] Other arrangements of the various scintillators are possible. The active structure may be in the form of a half-sphere, as shown in FIG. 18, or other convex shapes, with the thinnest scintillator on the outside of the structure and the thickest scintillator at its core.

[0179] The active structure 2 may also include an inorganic scintillator 60 that is capable of gamma spectrometry of any incident gamma radiation.

[0180] The inorganic scintillator 60 may be arranged in contact with, notably behind or alongside, the first scintillator 10, as illustrated in FIG. 19a for a parallelepipedal active structure 2, and in FIG. 19b for a hemispherical active structure 2.

[0181] The inorganic scintillator 50 may again be integrated into the first scintillator 10, as shown in FIG. 19b and FIG. 19d.

Example 3

[0182] FIG. 20 shows an implementation example of a detector 1 including such an inorganic scintillator 60.

[0183] Detector 1 includes, for example, three organic scintillators 10, 20 and 30, for example identical to those of the first implementation example described above and shown in FIG. 2.

[0184] The inorganic scintillator 60 is arranged behind the first organic scintillator 10.

[0185] It is, for example, of the BGO type and has a thickness e.sub.7 of about 3 cm.

[0186] As described previously, the detector also includes a photomultiplier 5 arranged behind the active structure 2, for example connected to the inorganic scintillator 60 by means of an optical binder 50.

[0187] Detector 1 thus obtained is subjected simultaneously to three sources of ionizing radiation, namely a californium-252 source, a carbon-14 source and a plutonium-239 source.

[0188] The two-dimensional spectrum represented in FIG. 21a shows pulse groupings in zones 300, 100, 110 corresponding to the detection of alpha (FIG. 21c), gamma (FIG. 21e) and neutron (FIG. 21d) ionizing radiation, respectively.

[0189] The gamma rays whose pulses appear in zone 100 come from interactions with the plastic scintillator 10.

[0190] A new zone 600 may be defined, corresponding to the interactions of gamma rays with the inorganic scintillator 60, allowing the energy spectrum shown in FIG. 21b to be obtained.

[0191] This spectrum has a characteristic profile which makes it possible to determine the emission energy value(s) of the gamma rays detected, and thus to identify the source(s) of radiation.

[0192] Needless to say, the invention is not limited to the examples that have just been described.

[0193] For example, radioactive sources other than those tested may be used to define the abovementioned zones of interest.

[0194] The combined preparation of scintillators 20 and 30 one on top of the other may be achieved via methods other than the use of a doctor blade device, notably via the autogenous coupling process described in patent application FR2983310.

[0195] Other molecules may be used for scintillator 10 as fluorescent elements; a person skilled in the art will know how to adapt the formulation of this scintillator according to the need.