Plastic scintillators that discriminate between alpha and beta rays emitted by a radioactive medium, and a method for discriminating between alpha and beta rays using these scintillators

Abstract

A plastic scintillator discriminating between alpha and beta rays emitted by a radioactive medium, said plastic scintillator being made of a plastic scintillator material capable of discriminating alpha radiation from beta radiation by the shape of the scintillation pulse that these radiations create, characterized in that it has a surface area S likely to be in contact with the radioactive medium / total volume V of the scintillator (S / V) greater than or equal to 7 cm⁻¹, and a surface area S likely to be in contact with the radioactive medium greater than or equal to 10 cm². A method for preparing this plastic scintillator. A detection device comprising this plastic scintillator.A method for detecting a signal due to alpha rays or a signal due to beta rays, or for discriminating a signal due to alpha rays from a signal due to beta rays, in radiation comprising alpha rays and / or beta rays emitted by a radioactive medium, wherein the plastic scintillator is exposed to said radioactive medium, and the signal due to alpha rays is separated from the signal due to beta rays by pulse shape discrimination. Figure for the abstract: Figure 2C.

Description

Title of the invention: PLASTIC SCINTILLATORS DISCRIMINATING ALPHA RAYS FROM RAYS BETA RAYS EMITTED BY A RADIOACTIVE MEDIUM AND METHOD FOR DISCRIMINATING ALPHA RAYS FROM BETA RAYS USING THESE SCINTILLATORS Technical field

[0001] The invention relates to a plastic scintillator or scintillation detector which discriminates alpha rays from beta rays emitted by a radioactive medium.

[0002] It will be understood that if the plastic scintillator is exposed only to alpha rays then it can detect only alpha rays, and gives an “alpha response”.

[0003] In the same way, it will be understood that if the plastic scintillator is exposed only to beta rays then it allows only beta rays to be detected, and gives a “beta response”.

[0004] Finally, if the plastic scintillator is exposed to both alpha rays and beta rays, then it allows the two rays to be detected separately and gives a separate response for these two rays.

[0005] The radioactive medium may be a gaseous medium or a liquid medium containing one or more alpha ray emitters and / or one or more beta ray emitters and / or one or more mixed alpha ray and beta ray emitters such as Radon-222, and optionally in addition one or more gamma ray emitters such as those resulting from natural radioactivity.

[0006] The gaseous or liquid medium may in particular be water or an aqueous solution such as sea water or brackish water.

[0007] These emitters can be in the form of a radioactive gas or a radioactive aerosol with, for example, elements from the descendants of radon, alpha emitters.

[0008] Let us specify that by aerosol, we mean a medium comprising particles suspended in a gas such as air, these particles being solid and / or liquid particles, generally having a negligible fall speed.

[0009] A plastic scintillator or scintillation detector comprises a plastic scintillator material - or scintillation material - namely a material comprising a polymer matrix in which at least one fluorescent compound is incorporated.

[0010] The invention further relates to a method for discriminating alpha rays from beta rays emitted by a radioactive medium using these scintillators.

[0011] It should be noted that, throughout the present description, the term "plastic scintillator" or "scintillation detector" means an apparatus comprising a scintillator material placed in the appropriate form for its use.

[0012] The technical field of the invention can, in general, be defined as that of nuclear instrumentation, in particular that of the detection of ionizing radiation, such as ionizing radiation emitted by a medium in a gaseous form or in the form of aerosols.

[0013] More precisely, the technical field of the invention can be defined as that of the selective detection of alpha radiation or particles compared to beta radiation.

[0014] The invention finds its application in particular in the detection of radon (222Rn) which may be found in a gaseous radioactive medium such as air or a liquid radioactive medium such as water.

[0015] Radon (222Rn) is a naturally occurring radioactive gas. Particularly in France, it is the second leading cause of lung cancer after smoking. Its quantification has recently been added to the diagnosis on the state of easements "risks" and information on soils (ESRIS). European Directive 2013 / 59 / Euratom has reduced the reference detection threshold from 400 to 300 Bq / m3. In buildings open to the public, its control is mandatory every 10 years. Due to its partially granitic geology, France is particularly concerned by this European directive. STATE OF THE PRIOR ART

[0016] Among the techniques for detecting radioactive gases or aerosols, it is first necessary to distinguish between so-called "continuous" techniques and so-called "integrated" techniques.

[0017] Continuous techniques implement prolonged exposure of the sensor to the radioactive gas to allow accumulation of the interaction. The sensor response is then read after a determined exposure time, which prohibits any real-time determination of the volume activity of the gas being investigated. In other words, in continuous techniques, the measurement, such as liquid scintillation, is carried out in the laboratory and not in the space to be analyzed.

[0018] On the contrary, integrated techniques can be deployed directly in the space to be analyzed.

[0019] Many "integrated" techniques exist for the real-time detection of radioactive gases or aerosols. These techniques generally use passive detectors, trace detectors, for the identification of the presence or absence of these radioactive gases or aerosols, or active detectors for measuring volume activity in the air.

[0020] These active detectors are based on different principles. They can be silicon semiconductor detectors, proportional counters using a carrier gas (Argon-methane) and a purification system, wire chambers for measurement in air, gamma spectrometry detectors (encapsulated semiconductor or scintillator), and finally detectors with bubbling type sampling and remote measurement in the laboratory.

[0021] The vast majority of these integrated techniques use the counting of ionizing particles, that is to say that a radiation / matter interaction is counted numerically by shock.

[0022] As a result, these techniques do not provide access to information relating to the energy of the particle, or even to its nature.

[0023] This is particularly penalizing in the case of Radon (222Rn), because the decay pattern of 222Rn is complex and gives rise to numerous descendants, some of which have similar decay properties and which can be confused with those of radon.

[0024] Indeed, reading [Fig.l] shows that in a closed system, and after approximately O 1 Q 01 / 1 four hours, radon is in equilibrium with the following radionuclides: Po and Po 222 214 214 which are alpha emitters (such as Rn) and Pb and Bi which are beta emitters.

[0025] Some devices do use alpha spectroscopy, therefore energy recognition, but this technique has low detection sensitivity and therefore does not allow low volume activities to be achieved in a reasonable measurement time.

[0026] If we therefore wish to access spectrometric information (in other words information relating to the energy, and therefore to the nature of the ionizing radiation), two other problems appear:

[0027] (1) it is necessary to be able to distinguish by whatever method a radiation / matter interaction from an alpha emitter or a beta emitter.

[0028] (2) it is also necessary to be able to differentiate between radiation / matter interactions of the same family, in other words between two or more alpha emitters of different energy and / or between two or more beta emitters of different energy.

[0029] It therefore appeared that it would be relevant, in particular in the context of radon detection, to have sensors allowing discrimination between particles, in particular between particles which constitute alpha radiation and particles which constitute beta radiation.

[0030] This discrimination between particles can be achieved using scintillator materials, i.e. materials which have the property of emitting light when subjected to ionizing radiation.

[0031] These scintillator materials have photophysical properties that allow particle discrimination. More precisely, it is by pulse shape discrimination (PSD) that the separation of the signals is achieved.

[0032] Currently, the scintillator material can be chosen either from monocrystalline organic scintillator materials (for example trans-stilbene), or from plastic scintillator materials.

[0033] Plastic scintillator materials comprise fluorescent molecules imbedded in an organic polymer containing only non-metallic atoms, e.g., carbon, hydrogen, nitrogen, oxygen, and sulfur. Plastic scintillator materials possess "PSD" properties.

[0034] But a final problem arises, particularly concerning the detection of alpha and beta emitters.

[0035] Indeed, the difficulty in detecting alpha and beta emitters lies, among other things, in the short path of alpha and beta radiation in matter, unlike, for example, neutrons and gamma radiation.

[0036] In particular, the alpha radiation emitted by 222Rn has a path of the order of 50 mm in air, and only a few tens of micrometers in water.

[0037] The interaction with sensors, such as plastic scintillators, can therefore be likened to a purely surface interaction, which is the case for radioactive aerosols but not necessarily for gases. For example, radon can diffuse into the material but this diffusion is very slow.

[0038] This short path therefore leads to a very limited interaction between the alpha or beta radiation and the sensor material, such as a plastic scintillator material, which makes the detection of alpha and beta emitters very difficult.

[0039] Known plastic scintillators generally have a large cylinder shape.

[0040] Since the interaction of the scintillator with alpha and beta radiation is weakly penetrating (i.e. the penetration distance of the radiation into the scintillator is small), a large cylindrical scintillator, relative to the penetration distance, is unsuitable for two reasons:

[0041] - a significant volume of the scintillator is not exposed to alpha radiation and beta, so it's useless.

[0042] - on the contrary, this large volume of scintillator turns out to be sensitive to the rays gamma rays in particular, which come from natural radioactivity. This response to the background noise of gamma rays therefore results in a reduction in the detection limit.

[0043] This is particularly the case for radioactive aerosols and gases in the air, and even more so for radioactive aerosols and gases in water.

[0044] There is therefore a need for a plastic scintillator making it possible to improve the very limited interaction between the alpha or beta radiation emitted by a radioactive medium and the material of the plastic scintillator, and thus to ensure detection of this radiation which is sensitive and selective.

[0045] In particular, there is a need for activity metrology for the precise, reliable and real-time determination of radon activity in the air. Disclosure of the invention

[0046] This aim, and others still, are achieved, in accordance with the invention by a plastic scintillator discriminating alpha rays from beta rays emitted by a radioactive medium, said plastic scintillator being constituted by a plastic scintillator material capable of discriminating alpha radiation from beta radiation by the shape of the scintillation pulse that these radiations create, characterized in that it has a surface area S capable of being in contact with the radioactive medium / total volume V of the scintillator (S / V) ratio greater than or equal to 7 cm ', preferably greater than or equal to 8 cm 1 , and a surface area S capable of being in contact with the radioactive medium greater than or equal to 10 cm2, preferably greater than or equal to 50 cm2.

[0047] As already stated above, it will be understood that if the plastic scintillator is exposed only to alpha rays then it can detect only alpha rays, and gives an “alpha response”.

[0048] In the same way, it will be understood that if the plastic scintillator is exposed only to beta rays then it allows only beta rays to be detected, and gives a “beta response”.

[0049] Finally, if the plastic scintillator is exposed to both alpha rays and beta rays, then it allows the two rays to be detected separately and gives a separate response for these two rays.

[0050] In this document, the terms ray and radiation are sometimes used interchangeably.

[0051] The plastic scintillator according to the invention has never been described in the prior art and comprises a combination of three characteristics which has never been described in the prior art.

[0052] According to a first, essential characteristic of the plastic scintillator according to the invention, the latter comprises a specific plastic scintillator material capable of discriminating alpha radiation from beta radiation by the shape of the scintillation pulse that they create, in other words having “PSD” properties, with respect to these two radiations.

[0053] Plastic scintillator materials having “PSD” properties are materials well known to those skilled in the art and have already been defined above. Those skilled in the art can easily determine which plastic scintillator materials, having “PSD” properties, are capable of discriminating alpha radiation from beta radiation by their pulse shape (by the shape of the scintillation pulse they create).

[0054] Such plastic scintillator materials are described in particular in documents WO-A1-2014 / 135640 and WO-A2-2012 / 142365, to the description of which reference may be made.

[0055] The photophysical characteristics of the plastic scintillator material are those of a pulse shape discriminating plastic scintillator material.

[0056] Advantageously, the plastic scintillator material may have an emission wavelength, of photoluminescence, which is located in the visible spectrum, namely from 360 nm to 650 nm, preferably from 380 to 450 nm.

[0057] Advantageously, the plastic scintillator material may have a scintillation decay constant for singlet states of 0.5 to 300 ns, preferably of 1 to 100 ns, more preferably of 1 to 20 ns.

[0058] Advantageously, the plastic scintillator material may have a scintillation efficiency of 100 to 20,000 ph / MeV, preferably of 5,000 to 15,000 ph / MeV, more preferably of 7,000 to 11,000 ph / MeV.

[0059] Advantageously, the plastic scintillator material is capable of separating the signal due to alpha radiation from the signal due to beta radiation emitted by the radioactive medium by their resulting pulse shape with a figure of merit value (more simply called alpha / beta Figure of Merit (Fdm)) greater than 1, preferably greater than 1.2, more preferably greater than 1.5 (unitless) at 600 keVee (“kilo electron Volts electron equivalent”).

[0060] This value of 600 keVee is the value of the abscissa at which this value of Fdm is calculated.

[0061] Thus, the plastic scintillator material which constitutes the scintillator according to the invention can comprise a matrix made of an organic polymer (for example linear or branched) or of a crosslinked organic polymer, in which at least one fluorescent compound is incorporated, called the first fluorescent compound or primary fluorophore.

[0062] By organic polymer is meant a polymer comprising only non-metallic atoms (for example comprising only atoms chosen from carbon, hydrogen, nitrogen, oxygen and sulfur atoms).

[0063] Similarly, the crosslinked organic polymer is a polymer comprising only non-metallic atoms (for example comprising only atoms chosen from carbon, hydrogen, nitrogen, oxygen and sulfur atoms).

[0064] Advantageously, the organic polymer may be capable of being obtained by polymerization of an aromatic monomer, and the crosslinked organic polymer may be capable of being obtained by polymerization of a mixture comprising an aromatic monomer and a monomer acting as a crosslinking agent.

[0065] Advantageously, the aromatic monomer may be chosen from styrene; styrenes substituted by one or more alkyl group(s) such as t-butylstyrene and its isomers, vinyltoluene and its isomers, and vinylxylene and its isomers; vinylnaphthalene optionally substituted by one or more alkyl group(s); α-vinylcarbazole; and mixtures thereof; and the monomer acting as a crosslinking agent may be chosen from alkyl diacrylates, alkyl dimethacrylates, such as 1,4-butanediyl dimethacrylate, divinylbenzene, and mixtures thereof.

[0066] In the present, when mention is made of an alkyl group and, unless otherwise specified, it is generally understood to mean a linear or branched alkyl group generally from 1 to 20C, preferably from 1 to 10C, more preferably from 2 to 6C, better still from 2 to 4C such as a methyl, ethyl, isopropyl, n-propyl, n-butyl, isobutyl or tert-butyl group; or a cyclic alkyl group generally from 3 to 10C, preferably from 3 to 6C such as a cyclopropyl, cyclobutyl, cyclopentyl, or cyclohexyl group.

[0067] By aromatic monomer is generally meant, in the present, a monomer comprising at least one aromatic ring, and in addition a polymerizable group.

[0068] Said polymerizable group is generally chosen from groups comprising an ethylenically unsaturated carbon-carbon double bond such as the vinyl group.

[0069] Generally, said at least one fluorescent compound is an organic compound.

[0070] Preferably, said at least one fluorescent compound is incorporated into a mass concentration of 10% to 30% by mass of the total mass of the plastic scintillator material.

[0071] The plastic scintillator material may comprise one or more first fluorescent compound(s) or primary fluorophore(s), optionally further a second fluorescent compound, and optionally further a third fluorescent compound.

[0072] The plastic scintillator material may therefore comprise only one fluorescent compound (primary fluorophore), or two fluorescent compounds or three fluorescent compounds or even more.

[0073] Advantageously, the first fluorescent compound may be chosen from biphenyl, 2,5-diphenyloxazole and substituted derivatives thereof, fluorescent compounds of the oxadiazole family, molecules having intrinsic properties of generating triplet states under dense ionizing radiation and their triplet-triplet annihilation, meta-terphenyl, ort / zo-terphenyl, para-terphenyl, and mixtures thereof; the second fluorescent compound may be chosen from bis-methylstyrylbenzene (bis-MSB), 1,4-di-[2-(5-phenyloxazolyl)]benzene (POPOP), 9,10-diphenylanthracene, 4-ethoxy-A-(2',5'-di-t-butylphenyl)-1,8-naphthalimide, and 3-hydroxyflavone; and the third fluorescent compound may be selected from perylene, 4-butylamino-A-(2',5'-di-Z-butylphenyl)-1,8-naphthalimide, and compounds of the coumarin family, for example coumarins 1, 6, 30, 102, 151, 314, 343 and acridine yellow.

[0074] According to a second, essential characteristic of the plastic scintillator according to the invention, the latter has a surface area S capable of being in contact with the radioactive medium / total volume V of the scintillator (S / V) ratio greater than or equal to 7 cm1, preferably greater than or equal to 8 cm *.

[0075] By surface S of the scintillator likely to be in contact with the radioactive medium, or surface S potentially in contact with the radioactive medium, is meant the surface which, during the actual implementation of the scintillator in a radioactive medium, is in contact with the radioactive medium, is exposed to this radioactive medium. The surface S can also be called more simply the active surface.

[0076] According to the invention, the S / V has a specific value which maximizes the radiation / matter interaction and ensures efficient signal processing.

[0077] According to a third, essential, fundamental characteristic of the plastic scintillator according to the invention, the latter has a surface S capable of being in contact with the radioactive medium greater than or equal to 10 cm2, preferably greater than or equal to 50 cm2.

[0078] Indeed, it has been demonstrated that below this value of the active surface S, the radiation / matter interaction is not quantitatively sufficient to allow effective signal processing with regard to the normative detection threshold even if the S / V ratio is greater than or equal to 7 cm1. This detection threshold was established for Rn-222 at 100 Bq / m3 by the WHO (World Health Organization), and at 300 Bq / m3 in France by the IRSN (Institute for Radioprotection and Nuclear Safety).

[0079] The value of the active surface S of the plastic scintillator according to the invention ensures sufficient detection to meet detection standards.

[0080] It has been demonstrated, surprisingly, according to the invention, that a plastic scintillator combining the three fundamental and essential characteristics set out higher allowed both sensitive and selective detection of alpha and beta radiation.

[0081] Detection combining sensitivity and selectivity is not obtained if the plastic scintillator does not possess both of these three fundamental and essential characteristics.

[0082] In other words, a plastic scintillator, made of a material having “PSD” properties, which is in a volume form - defined by specific S and S / V values ​​- allowing access to high surfaces, ensures detection which is both sensitive and selective.

[0083] These two criteria for detection are not satisfied without the combination of these three characteristics.

[0084] The invention further relates to a method for preparing the plastic scintillator as described above.

[0085] According to a first embodiment of the method for preparing the plastic scintillator according to the invention, the plastic scintillator material is firstly put in the form of a monolith (which does not have the values ​​of S and S / V of the plastic scintillator according to the invention), then said monolith is machined, cut to the final shape of the plastic scintillator according to the invention which has a surface area S capable of being in contact with the radioactive medium / total volume V of the scintillator (S / V) ratio greater than or equal to 7 cm1, preferably greater than or equal to 8 cm1, and a surface area S capable of being in contact with the radioactive medium greater than or equal to 10 cm2, preferably greater than or equal to 50 cm2.

[0086] The monolith may be a parallelepiped, a cube or a cylinder, for example a right circular cylinder.

[0087] The monolith can be machined, cut by any cutting, machining technique, making it possible to obtain a geometry satisfying the conditions relating to the values ​​of S and S / V of the plastic scintillator according to the invention. This cutting, machining technique can be chosen from among the cutting, machining techniques which implement a water cutter or a three-dimensional drill or any other mechanical tool making it possible to achieve the desired cutting precision.

[0088] According to a second embodiment of the method for preparing the plastic scintillator according to the invention, the plastic scintillator material is put, during its preparation, directly into the final form of the plastic scintillator according to the invention, for example by molding in a mold in the final form of the scintillator according to the invention, or by 3D printing.

[0089] In this second embodiment, the shaping can therefore be carried out during the preparation of the material, for example by preparing a liquid mixture of the aromatic monomer, the optional crosslinking agent (the aromatic monomer and the optional crosslinking agent having previously been suitably purified to remove their radical inhibitors), the fluorescent compound(s), then pouring this mixture into a mold in the shape of the desired scintillator. Polymerization is then carried out in the mold, generally by heating the mixture. After cooling, the scintillator thus obtained is removed from the mold.

[0090] The demolded scintillator can then generally be ground, for example on a lathe, then at least one of its surfaces is polished in order to obtain a surface condition compatible with the desired applications.

[0091] The invention also relates to a detection device comprising the plastic scintillator according to the invention and a display device allowing an operator to be aware of the ambient radioactivity level by reading the number of counts per second, and possibly the nature of a source of radioactivity, for example radon-222.

[0092] The invention also relates to a method for detecting a signal due to alpha rays or a signal due to beta rays or for discriminating a signal due to alpha rays from a signal due to beta rays, in radiation comprising alpha rays and / or beta rays emitted by a radioactive medium, in which the plastic scintillator according to the invention is exposed to said radioactive medium, and the signal due to alpha rays is separated from the signal due to beta rays by pulse shape discrimination (PSD).

[0093] In the absence of alpha rays in the radiation, the method is capable of detecting only the signal due to beta rays.

[0094] Similarly, in the absence of beta rays in the radiation, the method is capable of detecting only the signal due to alpha rays.

[0095] When the radiation comprises alpha rays and / or beta rays, the method is capable of discriminating the signal due to alpha rays from a signal due to beta rays.

[0096] The radioactive medium to be a gaseous radioactive medium such as air, a liquid radioactive medium such as water.

[0097] The radiation comprising alpha rays and / or beta rays may be radiation emitted by a mixed source.

[0098] The mixed source may be Radon-222 and its radioactive descendants.

[0099] This “PSD” can be achieved for example either by integration of charges, or by the technique known as zero-crossing. There are other sub-techniques for achieving this “PSD”.

[0100] The invention will be better understood upon reading the detailed description which follows, in particular of particular embodiments provided in the form of examples. This detailed description is made in relation to the attached drawings. Brief description of the drawings

[0101] [Fig.l] is a diagram showing the decay of 222Rn, and its descendants.

[0102] [Fig.2A],

[0103] [Fig.2B] and

[0104] [Fig.2C] show a plastic scintillator according to the invention, the shape of which is generally that of a right circular cylinder, in which the plastic scintillator material, in cross-section, is in the form of a circular spiral.

[0105] - [Fig.2A] is a side view of this plastic scintillator.

[0106] - [Fig.2B] is a cross-sectional view of this plastic scintillator according to the line AA of [Fig.2A], at scale 1:1.

[0107] - [Fig.2C] is a perspective view of this scintillator.

[0108] [Fig.3A],

[0109] [Fig.3B] and

[0110] [Fig.3C] show a plastic scintillator according to the invention, the shape of which is generally that of a right circular cylinder, in which the plastic scintillator material is in the form of curved fins (or curved petals) arranged along the radius of the cylinder around its center.

[0111] - [Fig.3A] is a side view of this plastic scintillator.

[0112] - [Fig.3B] is a cross-sectional view of this plastic scintillator according to the line AA of [Fig.3A], at scale 1:1.

[0113] - [Fig.3C] is a perspective view of this scintillator.

[0114] [Fig.4A],

[0115] [Fig.4B] and

[0116] [Fig.4C] show a plastic scintillator according to the invention, the shape of which is generally that of a right circular cylinder, in which the plastic scintillator material, in cross-section, is in the form of a plurality of ten-petal flower-shaped elements.

[0117] - [Fig.4A] is a side view of this plastic scintillator.

[0118] - [Fig.4B] is a sectional view of this plastic scintillator along line AA of [Fig.4A], at scale 1:1.

[0119] - [Fig.4C] is a perspective view of this scintillator.

[0120] [Fig.5A],

[0121] [Fig.5B] and

[0122] [Fig.5C] show a plastic scintillator according to the invention, the shape of which is generally that of a right circular cylinder, in which the plastic scintillator material which forms this cylinder is axially perforated by through-orifices of circular right section.

[0123] - [Fig.5A] is a side view of this plastic scintillator.

[0124] - [Fig.5B] is a sectional view of this plastic scintillator along line AA of [Fig.5A], at scale 1:1.

[0125] - [Fig.5C] is a perspective view of this scintillator.

[0126] [Fig.6A],

[0127] [Fig.6B] and

[0128] [Fig.6C] show a plastic scintillator according to the invention, the shape of which is generally that of a right circular cylinder, in which the plastic scintillator material which forms this cylinder is axially perforated by through-holes of hexagonal or honeycomb cross-section.

[0129] - [Fig.6A] is a side view of this plastic scintillator.

[0130] - [Fig.6B] is a sectional view of this plastic scintillator along line AA of [Fig.6A], at scale 1:1.

[0131] - [Fig.6C] is a perspective view of this scintillator.

[0132] [Fig.7A],

[0133] [Fig.7B] and

[0134] [Fig.7C] show a plastic scintillator according to the invention, the shape of which is generally that of a right circular cylinder, in which the plastic scintillator material, in cross-section, is in the form of two spirals, respectively dextrorotatory and levorotatory.

[0135] - [Fig.7A] is a side view of this plastic scintillator.

[0136] - [Fig.7B] is a sectional view of this plastic scintillator along line AA of [Fig.7A], at scale 1:1.

[0137] - [Fig.7C] is a perspective view of this scintillator.

[0138] [Fig.8A],

[0139] [Fig.8B] and

[0140] [Fig.8C] show a plastic scintillator according to the invention, the shape of which is generally that of a right circular cylinder, in which the plastic scintillator material, in cross-section, is in the form of spokes arranged according to the radius of the cylinder around its center.

[0141] - [Fig.8A] is a side view of this plastic scintillator.

[0142] - [Fig.8B] is a sectional view of this plastic scintillator along line AA of [Fig.8A], at scale 1:1.

[0143] - [Fig.8C] is a perspective view of this scintillator.

[0144] [Fig.9] is a photograph of a plastic scintillator according to the invention, the shape is broadly that of a right circular cylinder, in which the plastic scintillator material, in cross-section, is in the form of a circular spiral.

[0145] [Fig. 10] is a graph showing the figure of merit of the acquisition chain in the presence of Radon 222 in equilibrium with its descendants.

[0146] The channels are plotted on the abscissa and the number of strokes is plotted on the ordinate.

[0147] [Fig. 11] is a graph showing the total emission spectrum of Radon 222 in balance with his descendants.

[0148] The channels are plotted on the abscissa and the number of strokes is plotted on the ordinate.

[0149] [Fig. 12] is a graph showing the spectrum of beta emissions.

[0150] The channels are plotted on the abscissa and the number of strokes is plotted on the ordinate.

[0151] [Fig. 13] is a graph showing the spectrum of alpha emissions.

[0152] The channels are plotted on the abscissa and the number of strokes is plotted on the ordinate.

[0153] DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0154] The plastic scintillator according to the invention is constituted by a specific plastic scintillator material capable of discriminating alpha radiation from beta radiation by their pulse shape, in other words having “PSD” properties, with respect to these two radiations.

[0155] The plastic scintillator material of the scintillator according to the invention has already been described above in detail.

[0156] Plastic scintillator materials having “PSD” properties are materials well known to those skilled in the art and have already been defined above. Those skilled in the art can easily determine which plastic scintillator materials, having “PSD” properties, are capable of discriminating alpha radiation from beta radiation by their pulse shape.

[0157] Such plastic scintillator materials are described in particular in documents WO-A1-2014 / 135640 and WO-A2-2012 / 142365, to the description of which reference may be made.

[0158] Generally, the plastic scintillator material comprises a matrix made of an organic polymer (which may be, for example, linear or branched) or of a crosslinked organic polymer, in which at least one fluorescent compound, called the first fluorescent compound or primary fluorophore, is incorporated.

[0159] At least one fluorescent molecule or compound, generally organic, is incorporated into the polymer matrix.

[0160] Thus, the following may be incorporated into the polymer matrix:

[0161] - at least one first fluorescent molecule, generally organic, the spectral properties exhibit different characteristics;

[0162] - possibly a second fluorescent molecule, generally organic, of which spectral properties exhibit different characteristics;

[0163] - if the intended application requires it, possibly a third molecule fluorescent, generally organic, whose spectral properties have different characteristics.

[0164] Polymeric matrix.

[0165] During the interaction between any radiation and matter, the latter must be capable of properly transferring the deposited energy.

[0166] In the case of a plastic scintillator material, the material is none other than the polymer matrix.

[0167] Concretely, the polymer of the polymer matrix of the scintillator material can possess a large number of aromatic groups which will be ionized after radiation-matter interaction.

[0168] Along the ion's path, the different molecules will be ionized and then recombinations will take place, themselves resulting in de-excitations. The de-excitation will occur radiatively by emission of a fluorescence photon.

[0169] For this, the organic polymer can mainly have units derived from the polymerization of an aromatic monomer.

[0170] The organic polymer may therefore be capable of being obtained by polymerization of an aromatic monomer, and the crosslinked organic polymer may therefore be capable of being obtained by polymerization of a mixture comprising an aromatic monomer and a monomer acting as a crosslinking agent.

[0171] As already specified above, the term “aromatic monomer” is generally understood herein to mean a monomer comprising at least one aromatic ring. This or these aromatic ring(s) may be chosen from aromatic carbon rings, or heteroaromatic rings, each of these rings may comprise from 3 to 10 atoms, for example from 6 to 8 atoms. The heteroatom(s) may be chosen from N, O, P or S. The aromatic monomer may, for example, be chosen from monomers comprising a benzene ring, a naphthyl group or a carbazole ring. The aromatic ring(s) may also be substituted by one or more group(s) chosen from alkyl groups.

[0172] According to the invention, this monomer further comprises a polymerizable group.

[0173] Said polymerizable group is generally chosen from groups comprising an ethylenically unsaturated carbon-carbon double bond such as the vinyl group.

[0174] Preferably, the aromatic monomer may be chosen from styrene; styrenes substituted by one or more alkyl group(s) such as t-butylstyrene and its isomers, vinyltoluene and its isomers, and vinylxylene and its isomers; vinylnaphthalene optionally substituted by one or more alkyl group(s); N-vinylcarbazole; and mixtures thereof, and the optional monomer acting as crosslinking agent is chosen from alkyl diacrylates, alkyl dimethacrylates, such as 1,4-butanediyl dimethacrylate, divinylbenzene and mixtures thereof.

[0175] When the polymer is prepared from a single aromatic monomer without a crosslinking agent, it can then be referred to as a homopolymer (non-crosslinked).

[0176] When the polymer is prepared from several different aromatic monomers, without crosslinking agent, we can then speak of a copolymer (non-crosslinked).

[0177] When the polymer is prepared from a single aromatic monomer and the crosslinking agent, we can then speak of a polymer crosslinked by the crosslinking agent.

[0178] When the polymer is prepared from several different aromatic monomers, and the crosslinking agent, we can then speak of a copolymer crosslinked by the crosslinking agent.

[0179] Crosslinking is carried out using molecules which can have at least two polymerizable double bonds.

[0180] According to the invention, very good results are observed when the crosslinking is carried out using as crosslinking agent alkyl diacrylates corresponding to the following general formula (I) or alkyl dimethacrylates corresponding to the following general formula (II):

[0181] [Chem.l]

[0182] [Chem.2] d (GH?L O d~OV (ü)

[0183] where n is an integer generally from 1 to 20, preferably from 2 to 6. n can take any value between 1 and 20, for example 1, 2, 3, 4, 5 or 6.

[0184] Preferably, 1,4-butanediyl dimethacrylate is used.

[0185] A range of 10% to 50% by mole of crosslinking agent in the polymerization mixture and a range of 90% to 50% of aromatic monomer are particularly suitable for preparing the scintillator materials according to the invention.

[0186] Preferably, the percentage of crosslinking agent used is between 15% and 20% by mole of the polymerization mixture. In this range, the desired properties of transparency, stability, and discrimination potential are further enhanced.

[0187] The polymer matrix must be as transparent as possible at the emission wavelength of the incorporated fluorophore(s); typically the scintillator material must be transparent at wavelengths greater than 400 nm.

[0188] A gain in transparency is obtained through the use of alkyl diacrylates and even more so alkyl dimethacrylates which are known to be transparent in the near UV.

[0189] Among these diacrylates and dimethacrylates, 1,4-butanediyl dimethacrylate is preferably used because of its shrinkage coefficient which is the lowest among all the compounds which could be tested.

[0190] By way of example, the observed density of the scintillator material according to the invention, such as the material of example 1, is 1.08.

[0191] The optionally crosslinked polymer or copolymer of the scintillator material according to the invention can be prepared by any polymerization process known to those skilled in the art.

[0192] However, in order for the polymer to have the minimum of impurities, the polymerization is preferably carried out by thermal initiation.

[0193] It is generally not necessary to add a radical initiator known to those skilled in the art such as AIBN or benzoyl peroxide.

[0194] However, if the polymerization is difficult to complete, it is then possible, in this case, to use a radical initiator chosen for example from the two aforementioned radical initiators, at a mass concentration generally of 0.02% to 0.2%, preferably of 0.05% to 0.1% of the total mass of the material.

[0195] First fluorescent molecule.

[0196] The at least one first fluorescent molecule (first fluorescent compound), generally organic, can be chosen so as to have an absorption spectrum, the maximum absorption intensity of which is located between 250 and 350 nm, for example this maximum absorption intensity can be centered on 300 nm.

[0197] Once excited, this molecule is then capable of emitting photons whose corresponding wavelength is located between 340 and 400 nm, for example, whose maximum emission intensity can be centered on 360 nm.

[0198] Furthermore, the first fluorescent molecule, since it is generally incorporated into the matrix at a high mass content, must advantageously have a high solubility constant in apolar solvents.

[0199] In terms of mass concentration, a mass concentration of at least 10% by mass of this first fluorescent molecule relative to the total mass of the material is generally used so that the scintillator material has satisfactory n / y discrimination capabilities.

[0200] Advantageously, incorporation rates of between 10% and 30% by mass of the total mass of the material may be used. Typically, a scintillator material composed of a polymer matrix and a first fluorescent molecule or fluorescent compound at 16.4% by mass of the total mass of the material constitutes an example of a scintillator material according to the invention.

[0201] The first fluorescent compound may be chosen from biphenyl, 2,5-diphenyloxazole and substituted derivatives thereof, fluorescent compounds of the oxadiazole family, molecules exhibiting intrinsic properties of generating triplet states under dense ionizing radiation and their triplet-triplet annihilation, meto-terphenyl, ort / zo-terphenyl, para-terphenyl, and mixtures thereof.

[0202] In the context of the invention, it has been found that biphenyl, in particular, has all of the aforementioned characteristics.

[0203] Para-terphenyl behaves as a "first fluorescent compound" "first fluorescent molecule" but cannot be added in large quantities due to its low solubility. On the other hand, its addition in a range between 0.5 and 2.5% generally allows for improved discrimination.

[0204] A mixture of para-terphenyl and biphenyl can therefore be used as the at least one first fluorescent compound.

[0205] Second fluorescent molecule.

[0206] In addition to the first fluorescent molecule, a second fluorescent molecule, generally organic, can be incorporated into the polymer matrix.

[0207] The second possible fluorescent molecule may be chosen so as to have an absorption spectrum capable of covering the emission spectrum of the first fluorescent molecule, in particular, to optimize the energy transfers between the two fluorescent molecules, which means, in other words, that the second fluorescent molecule is capable, on the one hand, of absorbing the photons of wavelengths belonging to the emission spectrum of said first fluorescent molecule, and on the other hand, following this absorption, of emitting photons, such that the emission spectrum of said second fluorescent molecule ranges from 350 to 650 nm with a maximum emission intensity of between 400 and 600 nm.

[0208] The content at which the second fluorescent molecule is incorporated into the polymer matrix is ​​much lower than that of the first fluorescent molecule.

[0209] Typically, a concentration of second fluorescent molecule of between 0.01% and 1% by mass of the total mass of the material is used, preferably from 0.01% to 0.3% by mass of the total mass of the material.

[0210] The second fluorescent compound may be selected from bis-methylstyrylbenzene (bis-MSB), 1,4-di-[2-(5-phenyloxazolyl)]benzene (POPOP), 9,10-diphenylanthracene, 4-ethoxy-iV-(2',5'-di-t-butylphenyl)-1,8-naphthalimide, and 3-hydroxyflavone.

[0211] The second preferred fluorescent compound is POPOP.

[0212] Third fluorescent molecule.

[0213] In addition to the first fluorescent molecule and the second fluorescent molecule, a third fluorescent molecule, generally organic, may be incorporated into the polymer matrix.

[0214] This third fluorescent molecule can be chosen so as to have an absorption spectrum capable of covering the emission spectrum of the second fluorescent molecule, in particular, to optimize the energy transfers between the two fluorescent molecules (i.e. the second and third fluorescent molecules) and improve the intrinsic efficiency of the scintillation detector.

[0215] Which means, in other words, that the third fluorescent molecule is capable of absorbing photons of wavelengths belonging to the emission spectrum of said second fluorescent molecule, said third fluorescent molecule being capable, following this absorption, of emitting photons, in such a way that the spectrum emission of said third fluorescent molecule has a maximum emission intensity of between 500 and 600 nm.

[0216] The content at which the third fluorescent molecule is incorporated into the polymer matrix is ​​much lower than that of the first fluorescent molecule.

[0217] Typically, a concentration of third fluorescent molecule of 0.001% to 0.1% by mass of the total mass of the material, preferably 0.002% to 0.05% by mass, for example 0.005% by mass of the total mass of the material, is used.

[0218] The third fluorescent compound is chosen from perylene, 4-butylamino-V-(2',5'-di-t-butylphenyl)-1,8-naphthalimide, and compounds of the coumarin family, for example coumarins 1, 6, 30, 102, 151, 314, 343 and acridine yellow.

[0219] An example of a scintillator material that may constitute the scintillator according to the invention may comprise biphenyl, para-terphenyl and POPOP as fluorescent compounds in a polystyrene matrix.

[0220] From the scintillator material according to the invention, a scintillator - or scintillation detector - according to the invention is prepared by shaping the plastic scintillator material that has just been described. This scintillator must satisfy the conditions relating to S and S / V, stated above, which characterize the plastic scintillator according to the invention.

[0221] According to a first embodiment of the method for preparing the plastic scintillator according to the invention, the plastic scintillator material is firstly put into the form of a monolith (which does not have the values ​​of S and S / V of the plastic scintillator according to the invention), then said monolith is machined, cut to the final shape of the plastic scintillator according to the invention which therefore has a surface area S capable of being in contact with the radioactive medium / total volume V of the scintillator (S / V) ratio greater than or equal to 7 cm ', preferably greater than or equal to 8 cm ', and a surface area S capable of being in contact with the radioactive medium greater than or equal to 10 cm2, preferably greater than or equal to 50 cm2.

[0222] The monolith can be a parallelepiped, a cube or a cylinder, for example a right circular cylinder.

[0223] The monolith can then be machined, cut by any cutting or machining technique, making it possible to obtain a geometry satisfying the conditions relating to the values ​​of S and S / V of the plastic scintillator according to the invention. This cutting or machining technique can be chosen from among the cutting or machining techniques which use a water cutter, or a three-dimensional drill, or any other mechanical tool making it possible to achieve the desired cutting precision. as long as the optical quality of the material is not affected, for example by intense heating in the cutting area.

[0224] The cutting and machining technique can also be chosen from laser cutting techniques.

[0225] According to a second embodiment of the method for preparing the plastic scintillator according to the invention, the plastic scintillator material is put, during its preparation, directly into the final form of the plastic scintillator according to the invention, for example by molding in a mold in the final form of the scintillator according to the invention, or by 3D printing.

[0226] In this second embodiment, the shaping can therefore be carried out during the preparation of the material, for example by preparing a liquid mixture of the aromatic monomer, the optional crosslinking agent (the aromatic monomer and the optional crosslinking agent having been previously purified suitably to remove their radical inhibitors), the fluorescent compound(s), then by pouring this mixture into a mold in the shape of the desired scintillator. Polymerization is then carried out in the mold, generally by heating the mixture. After cooling, the scintillator thus obtained is removed from the mold.

[0227] The demolded scintillator can then generally be ground, for example on a lathe, then at least one of its surfaces is polished in order to obtain a surface condition compatible with the desired applications.

[0228] The scintillator according to the invention can have numerous shapes chosen so that the value of S / V is greater than 7 cm ', and its volume, in accordance with the invention, must also be greater than 10 cm2.

[0229] Examples of these shapes are shown in Figures 2A, 2B, 2C, 3A, 3B, 3C, 4A, 4B, 4C, 5A, 5B, 5C, 6A, 6B, 6C, 7A, 7B, 7C, 8A, 8B, and 8C.

[0230] These examples of shape are absolutely not limiting, and the plastic scintillator according to the invention can have any shape that can be obtained by cutting, machining, in particular by water jet, or by 3D printing or by molding. The only limits as to the shape that the scintillator can have are imposed for example by the resolution of the machine, the precision of the movement and the diameter of the water jet.

[0231] Other examples of shapes for the plastic scintillator (not shown) are those of a right circular cylinder, in which the plastic scintillator material, in cross-section, is in the form of a plurality of snowflake-shaped elements, or Koch flakes or other derived fractals, such as the Cesaro fractal, quadratic fractals, the "sphereflake" fractal, etc.

[0232] The scintillators of these figures can be prepared for example by first manufacturing a monolith of plastic scintillator material. This monolith can have for example the shape of a cylinder, preferably a right circular cylinder, a parallelepiped or a cube. In the case of a cylinder with a diameter of 6 cm and a height of 1.5 cm, the S / V ratio is then 1.33 cm'1.

[0233] The monolith is then cut, for example with a water jet, to give the scintillator according to the invention having the shape shown in Figures 2A, 2B, 2C, 3A, 3B, 3C, 4A, 4B, 4C, 5A, 5B, 5C, 6A, 6B, 6C, 7A, 7B, 7C, 8A, 8B, and 8C with an S / V ratio greater than 7 cm '.

[0234] In these figures, the solid shapes, solid volumes, are constituted by the plastic scintillator material and these solid shapes define empty shapes, empty volumes in which the radioactive medium, such as a radioactive gas like air in which a mixed emitter is diluted (such as Radon 222) or a radioactive liquid such as water in which a mixed emitter is diluted can penetrate and come into contact with the surface of the plastic scintillator material.

[0235] Again, all examples of shapes shown and not shown are absolutely not limiting.

[0236] Figures 2A, 2B and 2C show a plastic scintillator according to the invention, the shape of which is generally that of a right circular cylinder, in which the plastic scintillator material, in cross-section, is in the form of a circular spiral, in other words a spirally wound turn. The spiral defines channels or interstices into which the radioactive medium, such as a radioactive gas such as air in which a mixed emitter is diluted or a liquid such as water, can penetrate and come into contact with the surface of the plastic scintillator material.

[0237] [Fig.9] is a photograph of a “spiral” plastic scintillator such as shown in Figures 2A, 2B and 2C.

[0238] Table 1 gives the main characteristics of a “spiral” plastic scintillator according to the invention as shown in Figures 2A, 2B and 2C and in [Fig.9].

[0239] [Tables 1] Table 1. Main characteristics of a “spiral” plastic scintillator according to the invention. Characteristic Value range Height (cm) 0.1-5 Diameter (cm) 1-10 Thickness of a turn (cm) 0.01 - 0.5 Width of a gap (cm) 0.01-0.1 Active surface (cm*) > = 10, preferably > = 50 Volume (cm3) > = 1.42 Surface / Volume ratio (cnt1) > = 7, preferably > = 8 wavelength. de sdndliation (nm) From 360 nm to 650 nm, preferably from 380 to 450 nm. Scintillation decay constant (ns) From 0.5 to 300 ns, preferably from 1 to 100 ns, more preferably from 1 to 20 ns Scintillation efficiency (ph / MeV) From 1000 to 20000 ph / MeV, preferably from 5000 to 15000 ph / MeV, more preferably from 7000 to 11000 ph / MeV Figure of merit alpha / beta (unitless) >1.0, preferably >1.2, more preferably >1.5

[0240] Figures 3A, 3B and 3C show a plastic scintillator according to the invention, the shape of which is generally that of a right circular cylinder, in which the plastic scintillator material is in the form of curved fins, curved petals, arranged according to the radius of the cylinder around its center.

[0241] Figures 4A, 4B and 4C show a plastic scintillator according to the invention, the shape of which is generally that of a right circular cylinder, in which the plastic scintillator material, in cross-section, is in the form of a plurality of ten-petal flower-shaped elements.

[0242] Figures 5A, 5B and 5C show a plastic scintillator according to the invention, the shape of which is generally that of a right circular cylinder, in which the plastic scintillator material which forms this cylinder is axially perforated by through-orifices of circular right section.

[0243] Figures 6A, 6B and 6C show a plastic scintillator according to the invention, the shape of which is generally that of a right circular cylinder, in which the plastic scintillator material which forms this cylinder is axially perforated by through-holes of hexagonal or honeycomb cross-section.

[0244] Figures 7A, 7B and 7C show a plastic scintillator according to the invention, the shape of which is generally that of a right circular cylinder, in which the plastic scintillator material, in cross-section, is in the form of two spirals, respectively dextrorotatory and levorotatory.

[0245] Figures 8A, 8B and 8C show a plastic scintillator according to the invention, the shape of which is generally that of a right circular cylinder, in which the material plastic scintillator, in cross section, is in the form of rays arranged according to the radius of the cylinder around its center.

[0246] The scintillator according to the invention can in particular be integrated into a detection device comprising the plastic scintillator according to the invention, a photon-electron converter such as for example a photomultiplier tube, signal acquisition electronics and a display device displaying for example the alpha and / or beta emission counting rate, and possibly the recognition of the radioactive isotope(s) having interacted with said plastic scintillator.

[0247] The intended applications for the scintillators according to the invention include in particular the detection of alpha by eliminating the presence of other types of de-excitation, such as for example beta or gamma de-excitation. The example of radon 222 given in [Fig.l] shows that an accurate alpha count resulting from the decay of radon 222 must take into account possible alpha or beta decays of the radon 222 descendants.

[0248] When beta radiation interacts with an organic medium, it directly ionizes the scintillator and a photonic signal is recorded, born from a so-called fast fluorescence component.

[0249] When alpha radiation interacts with an organic medium, a high ionization density in a small volume leads to recombinations of specific excited states (triplet states), leading to a photonic signal born from a so-called slow fluorescence component.

[0250] It is therefore by pulse shape discrimination (PSD) that the separation of the signals is carried out, more particularly here by comparison of the slow component of the signals attributed to the beta or to the alpha, the beta signal generally having a slow component weaker in time than the alpha signal.

[0251] The system, in addition to the scintillator, includes one or two photomultipliers (PM), which must be adapted to the emission length of the organic scintillator. The system, once under high voltage, is isolated from the light to minimize background noise. The signal at the output of the photomultiplier is then processed in a suitable electronic chain including in particular a QDC (“Charge to Digital Converter”) allowing the integration of charges and the separation of signals.

[0252] The invention will now be described with reference to the following examples, given for illustrative and non-limiting purposes. Examples#:

[0253] Example 1.

[0254] In this example, a plastic scintillator according to the invention is prepared.

[0255] Pure styrene (82.2% by mass in the mixture, 136 g), biphenyl (16.4% by mass in the mixture, 27.18 g), para-terphenyl (1.2% by mass in the mixture, 2.038 g) and POPOP (0.08% by mass in the mixture, 136 mg), are mixed under an inert atmosphere in a dry flask and brought to a temperature above the boiling point of styrene. A refrigerant is therefore suitable for this flask.

[0256] Typically, the bottle used in this example and which acts as a mold is a cylindrical glass bottle with a capacity of 250 mL.

[0257] Once the mixture is partially polymerized, the vial is sealed with a gas-tight cap and then placed in an oven to complete and terminate the polymerization and obtain the plastic scintillator material.

[0258] After polymerization, the plastic scintillator material obtained, in the form of a monolithic cylinder, is demolded.

[0259] This monolithic cylinder is then cut into several smaller cylinders.

[0260] Each of these cylinders is ground on the lathe so that it has plane faces parallel to each other. Then these faces are polished until a surface condition is obtained that is optically compatible with the desired applications.

[0261] This plastic scintillator material is then cut into the desired shape in accordance with the invention using a water cutter.

[0262] In this example, the plastic scintillator material is cut so as to obtain a plastic scintillator, in accordance with the invention, which is in the form of a circular spiral.

[0263] In this scintillator, the plastic scintillator material is wound to form a spiral wall or coil defining channels or interstices in which the radioactive gas can flow.

[0264] A photograph of this spiral plastic scintillator is shown in [Fig.9].

[0265] The main geometric, photophysical and physical characteristics of this spiral scintillator are given in Table 2 below.

[0266] [Tables2] Table 2. Main characteristics of the plastic "spiral" stimulator according to the invention, prepared in Example 1. Characteristic Value Height (cm) 1.5 Diameter (cm) 6 Thickness of a turn (cm) 0.2 Width of a gap (cm) 0.08 Active surface (cm2) 189.3 Volume (cm3) 21.523 Surface / Volume ratio (cm1) 8.8 Scintillation wavelength (nm) 425 Scintillation decay constant (ns) 10 Scintillation efficiency (ph / MeV) 8000 Figure of merit αβ / β (unitless) 1.50 at 500 keVee ± 80 keVee

[0267] For the purpose of measurement, this spiral scintillator according to the invention is then cut again with a saw to have the following dimensions: length 4 mm, width 3 mm, and height 5 mm.

[0268] This plastic scintillator according to the invention is then integrated into an experimental assembly in order to show that it makes it possible to obtain discrimination of the signals due to alpha rays from the signals due to beta rays emitted by a mixed source, namely a radon 222Rn atmosphere (See [Fig.l]).

[0269] Firstly, the plastic scintillator is exposed to an atmosphere of radon 222Rn for 240 h, then in a second stage, it is left in the open air for 168 h to reach the so-called secular equilibrium.

[0270] The plastic scintillator according to the invention is then integrated into an experimental assembly comprising the scintillator and a photomultiplier adapted to the emission wavelength of the scintillator.

[0271] The system, once under high voltage, is isolated from the light to minimize background noise and not damage the photomultiplier.

[0272] The signal output from the photomultiplier is then processed by a signal digitizer which notably includes a “QDC” (“Charge to Digital Converter” in English) allowing the integration of the charge and the separation of the signals relating to alpha or beta events.

[0273] It is by pulse shape discrimination (“Pulse Shape Discrimination” or “PSD” in English) that the separation of the signals is carried out, for example here by charge integration, by comparison of the slow component of the signals attributed to alpha rays or beta rays, the alpha signal generally having a slow component more pronounced temporally than the beta signal.

[0274] These signals are then sorted according to the shape of the slow component of the pulse.

[0275] This processing and the following ones are carried out in particular with a QDC having the pulse shape discrimination function, such as for example the NanoPSD module of the LabZY or the QDC CAEN DT5730B.

[0276] These sorted pulses appear in the form of two populations depending on the nature of the interaction (alpha or beta) as shown in [Fig. 10].

[0277] In this embodiment, [Fig. 10] shows the timeless signature of the pulse shape (“Time-Invariant Pulse Shape spectrum” or “TIPS” in English). Concretely, the betas are sorted in the low channels (on the left) while the alphas are sorted in the high channels (on the right).

[0278] Then, each stand can be separated from the other and the associated energy spectrum can then be constructed.

[0279] Figures 11 to 13 therefore present respectively the total emission spectrum of Radon in equilibrium with its descendants ([Fig. 11]), the spectrum of beta emissions due to the Pb and Bi emitters ([Fig. 12]) and the spectrum of alpha emissions due to the three emitters 222Rn 218Po and 214Po. ([Fig. 13]).

[0280] More precisely, in [Fig. 13], we can distinguish from left to right, the peak OOO O 1 Q Rn emission partly overlapping the Po peak (1500 - 2500 channels) and the 214Po peak (2600 - 3500 channels).

[0281] In view of these Figures, we note the absence of beta signals in the alpha spectrum, and vice versa, and we see that it is possible to separate the spectral contributions of the three alpha emitters.

[0282] From these measurements and figures, several conclusions can be drawn:

[0283] - the ability to discriminate alpha from beta leads to a spectral separation of two natures of almost perfect de-excitation. There is almost no alpha in the beta spectrum and very little beta overlap in the alphas. These betas can then be separated by analysis of regions of interest in the alpha spectrum.

[0284] - the energy resolution of the scintillator studied in this example facilitates the processing of qualitative data such as pulse height analysis (PHA). This PHA processing performed after the PSD further improves the rejection of beta versus alpha. This energy resolution also makes it possible to define two regions of interest in the alpha spectrum: an area comprising the contributions emanating from the de-excitation of the Rn and Po emitters, and an area exclusive to Po. When Rn is in secular equilibrium with its descendants, the comparison of these areas of interest provides information on the activity of radon and its descendants.

[0285] - the separate analysis of these two areas of interest is interesting for the application preferential target targeted by the person skilled in the art, since 222Rn is in equilibrium with the

[0286]

[0287]

[0288] O 1 Q 01 / 1 Po after 12 minutes and with all its short-lived descendants (notably Po) after about 4 hours. The ability to analyze these three radionuclides independently allows access to real-time analysis of radon, and essentially of any other alpha / beta or alpha / gamma emitting gas or aerosol. - finally, the resolution of the Bateman equations of these radionuclides at their secular equilibrium gives a ratio (222Rn + 218Po) / 214Po = 1.98252. - On the other hand, the areas of interest described previously give: • 222Rn + 218Po = 74385 shots • 214Po = 37290 shots • or a Ratio: (222Rn + 218Po) / 214Po = 1.989. This latter ratio is therefore in perfect agreement with the ratio given above obtained from Bateman's equation.

Claims

Claims

1. Plastic scintillator discriminating alpha rays from beta rays emitted by a radioactive medium, said plastic scintillator being constituted by a plastic scintillator material capable of discriminating alpha radiation from beta radiation by the shape of the scintillation pulse that these rays create, characterized in that it has a surface area S capable of being in contact with the radioactive medium / total volume V of the scintillator (S / V) ratio greater than or equal to 7 cm4, preferably greater than or equal to 8 cm4, and a surface area S capable of being in contact with the radioactive medium greater than or equal to 10 cm2, preferably greater than or equal to 50 cm2;the plastic scintillator material comprising a matrix in which at least one fluorescent compound, called the first fluorescent compound or primary fluorophore, is incorporated, the matrix being: - made of an organic polymer comprising only atoms chosen from carbon, hydrogen, nitrogen, oxygen and sulfur atoms and being obtained by polymerization of an aromatic monomer which comprises at least one aromatic ring and a vinyl group; or - made of a crosslinked organic polymer comprising only atoms chosen from carbon, hydrogen, nitrogen, oxygen and sulfur atoms and being obtained by polymerization of a mixture comprising an aromatic monomer which comprises at least one aromatic ring and a vinyl group and a monomer acting as a crosslinking agent.;

2. A plastic scintillator according to claim 1, wherein the plastic scintillator material has a photoluminescence emission wavelength of 360 nm to 650 nm, preferably 380 to 450

3. 11111. A plastic scintillator according to any preceding claim, wherein the plastic scintillator material has a scintillation decay constant for singlet states of 0.5 to 300 ns, preferably 1 to 100 ns, more preferably 1 to 20 ns.

4. A plastic scintillator according to any preceding claim, wherein the plastic scintillator material has a scintillation efficiency of 100 to 20000 ph / MeV, preferably 5000 to 15000 ph / MeV, more preferably 7000 to 11000 ph / MeV.

5. A plastic scintillator according to any preceding claim wherein the plastic scintillator material has a figure of merit greater than 1, preferably greater than 1.2, more preferably greater than 1.5, at 600 keVee.

6. A plastic scintillator according to any one of claims 1 to 5, wherein the aromatic monomer is selected from styrene; styrenes substituted by one or more alkyl groups such as t-butylstyrene and its isomers, vinyltoluene and its isomers, and vinylxylene and its isomers; vinylnaphthalene optionally substituted by one or more alkyl groups; N-vinylcarbazole; and mixtures thereof; and the monomer acting as a crosslinking agent is selected from alkyl diacrylates, alkyl dimethacrylates, such as 1,4-butanediyl dimethacrylate, divinylbenzene, and mixtures thereof.

7. A plastic scintillator according to any one of claims 1 to 6, wherein the plastic scintillator material comprises one or more first fluorescent compound(s) or primary fluorophore(s), optionally further a second fluorescent compound, and optionally further a third fluorescent compound.

8. A plastic scintillator according to claim 7, wherein the first fluorescent compound is selected from biphenyl, 2,5-diphenyloxazole and substituted derivatives thereof, fluorescent compounds of the oxadiazole family, molecules having intrinsic properties of generating triplet states under dense ionizing radiation and their triplet-triplet annihilation, meta-terphenyl, ortho-terphenyl, para-terphenyl, and mixtures thereof; the second fluorescent compound is selected from bis-methylstyrylbenzene (bis-MSB), 1,4-di-[2-(5-phenyloxazolyl)]benzene (POPOP), 9,10-diphenylanthracene, 4-cthoxy-A^-(2',5'-di-t-butylphenyl)-1,8-naphthalimide, and 3-hydroxyflavone; and the third fluorescent compound is selected from perylene, 4-butylamino-iV-(2',5'-di-t-butylphenyl)-l,8-naphthalimide, and compounds of the coumarin family, e.g., coumarins 1, 6, 30, 102, 151, 314, 343 and acridine yellow.

9. A method of preparing the plastic scintillator according to any one of claims 1 to 8, wherein the plastic scintillator material is first formed into a monolith (which does not have the S and S / V values ​​of the plastic scintillator), and then said monolith is machined, cut into the final shape of the plastic scintillator.

10. A method of preparing the plastic scintillator according to any one of claims 1 to 8, wherein the plastic scintillator material is, during its preparation, directly put into the final form of the plastic scintillator, for example by molding in a mold into the final form of the plastic scintillator, or by 3D printing.

11. Detection device comprising the plastic scintillator according to any one of claims 1 to 8 and a display device allowing an operator to be aware of the ambient radioactivity level by reading the number of counts per second, and possibly the nature of a source of radioactivity, for example radon-222.

12. Method for detecting a signal due to alpha rays or a signal due to beta rays or for discriminating a signal due to alpha rays from a signal due to beta rays, in radiation comprising alpha rays and / or beta rays emitted by a radioactive medium, in which the plastic scintillator according to any one of claims 1 to 8 is exposed to said radioactive medium, and the signal due to alpha rays is separated from the signal due to beta rays by pulse shape discrimination (PSD).

13. The method of claim 12, wherein the radioactive medium is a gaseous radioactive medium such as air, or a liquid radioactive medium such as water.

14. A method according to any one of claims 12 and 13, wherein the radiation comprises alpha rays and beta rays emitted by a mixed source such as Radon-222 and its radioactive daughters.

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