PLASTIC SCINTILLATOR, DETECTOR, ASSOCIATED MANUFACTURING PROCESS AND SCINTILLATION MEASUREMENT PROCESS

Abstract

Material for plastic scintillation measurement comprising: a polymeric matrix; a primary fluorophore incorporated in the polymeric matrix and composed of N-(2-ethylhexyl)carbazole, the monomer form of the N-(2-ethylhexyl)carbazole being spontaneously in physicochemical equilibrium with the exciplex form; and, a secondary fluorophore. A plastic scintillator can be manufactured in a simplified manner with the material of the invention, while having optimized properties for the plastic scintillation measurement. The invention also relates to the process for the manufacture of the material, to a part comprising the material and to the associated measurement device, and also to the process for measurement by plastic scintillation using the material.

Claims

1. A composition for plastic scintillation measurement, comprising: a polymeric matrix; a primary fluorophore incorporated in the polymeric matrix and composed of N-(2-ethylhexyl)carbazole, the monomer form of the N-(2-ethylhexyl)carbazole being spontaneously in physicochemical equilibrium with the exciplex form; and, a secondary fluorophore.

2. A composition according to claim 1, wherein the polymeric matrix is completely or partially composed of at least one polymer comprising repeat units resulting from the polymerization of a monomer comprising at least one aromatic, (meth)acrylic or vinyl group.

3. A composition according to claim 2, wherein the monomer is at least one monomer selected from the group consisting of styrene, vinyltoluene, vinylxylene, vinylbiphenyl, vinylnaphthalene, vinylcarbazole, methyl (meth)acrylate, (meth)acrylic acid and 2-hydroxyethyl (meth)acrylate.

4. A composition according to claim 3, wherein the monomer is styrene or vinyltoluene.

5. A composition according to claim 1, wherein the polymeric matrix is constituted, completely or partially, of at least one crosslinked polymer.

6. A composition according to claim 1, wherein the composition comprises from 1% by weight to 50% by weight of the primary fluorophore.

7. A composition according to claim 6, wherein the composition comprises from 1% by weight to 5% by weight of the primary fluorophore.

8. A composition according to claim 6, wherein the composition comprises from 10% by weight to 50% by weight of the primary fluorophore.

9. A composition according to claim 1, wherein the composition comprises at least one neutron absorber.

10. A composition according to claim 9, wherein the neutron absorber is selected from the group consisting of at least one organometallic lithium, boron, gadolinium or cadmium complex and a mixture of these complexes.

11. A composition according to claim 10, wherein the neutron absorber is an organometallic gadolinium complex selected from the group consisting of gadolinium tris(tetramethylheptanedionate), a gadolinium tricarboxylate and gadolinium tris(acetylacetonate).

12. A composition according to claim 10, wherein the neutron absorber is an organometallic gadolinium complex present in the material at a concentration between 0.2% by weight and 2.5% by weight of gadolinium.

13. A composition according to claim 12, wherein the polymeric matrix comprises from 0.002% by weight to 0.2% by weight of the secondary fluorophore.

14. A composition according to claim 12, wherein the secondary fluorophore is selected from the group consisting of 1,4-di[2-(5-phenyloxazolyl)]benzene, 1,4-bis(2-methylstyryl)benzene, 1,4-bis(4-methyl-5-phenyl-2-oxazolyl)benzene, 9,10-diphenylanthracene and their mixtures.

15-16. (canceled)

17. A device for plastic scintillation detection comprising a part comprising a composition as defined by claim 1.

18. (canceled)

19. A process for the manufacture of the composition as defined by claim 1, the process comprising the following steps: a) having available a polymerization medium comprising: monomers, oligomers or their mixtures intended to form at least one constituent polymer of a polymeric matrix; a primary fluorophore composed of N-(2-ethylhexyl)carbazole; and, a secondary fluorophore; b) polymerizing the polymerization medium in order to obtain the material.

20. A process according to claim 19, wherein the polymerization medium comprises a neutron absorber, a crosslinking agent, a polymerization initiator or their mixtures.

21. (canceled)

22. A process according to claim 19, wherein steps a) and b) are carried out in a mold.

23. A process according to claim 19, comprising a step c) during which the composition or the preform of the part is machined.

24. A plastic scintillation measurement process, comprising: i) at least one composition as defined claim 1 is brought into contact with ionizing radiation or an ionizing particle in order for the material to emit radioluminescent radiation; and ii) the radioluminescent radiation is measured.

25-26. (canceled)

Description

BRIEF DESCRIPTION OF THE FIGURES

[0091] FIG. 1 represents the light absorption spectra of plastic scintillators according to the invention. The absorbance, expressed in an arbitrary unit, is a function of the wavelength of the light absorbed, expressed in nanometers.

[0092] FIG. 2 represents the fluorescence emission spectrum of plastic scintillators according to the invention. The intensity, expressed in a standardized unit, is a function of the wavelength of the light emitted, expressed in nanometers.

[0093] FIG. 3 represents the light yield of plastic scintillators according to the invention comprising an increasing concentration of EHCz. The light yield, expressed in an arbitrary unit, is a function of the percentage by weight of the EHCz molecule in each plastic scintillator.

[0094] FIG. 4 represents the energy spectra of three plastic scintillators when the EHCz is used alone at the concentration of 3 molar %, mixed in addition with the secondary fluorophore POPOP and mixed with the secondary fluorophore Bis-MSB. The light yield obtained on the ordinate is expressed in an arbitrary unit.

DESCRIPTION OF SPECIFIC EMBODIMENTS

[0095] The examples are carried out at atmospheric pressure and ambient temperature.

1. Synthesis of the EHCz Molecule

[0096] The EHCz molecule is available commercially under the CAS registry number 187148-77-2.

[0097] It can be obtained by nucleophilic reaction of carbazole, deprotonated beforehand, with 2-ethylhexyl bromide, according to the following reaction scheme:

##STR00002##

[0098] The carbazole is added portionwise to a suspension of sodium hydride, cleaned beforehand with pentane of its mineral oil, in dry N,N-dimethylformamide (DMF) in a 500 ml round-bottomed flask maintained under an argon atmosphere. After stirring for 30 minutes, the 2-ethylhexyl bromide is slowly added.

[0099] The mixture obtained is stirred at ambient temperature for 16 hours.

[0100] Water is added to the mixture and then the crude product is extracted with ethyl ether. The organic phase is dried, concentrated and then purified by chromatography on silica gel.

[0101] The EHCz molecule obtained is a colorless oil. The molar yield is approximately 73% for 20 g of EHCz synthesized.

[0102] The characteristics of the proton NMR spectrum of the EHCz molecule are as follows: .sup.1H NMR (CDCl.sub.3, 500 MHz) 0.86 (t, 3H, .sup.3J 7.3); 0.91 (t, 3H, .sup.3J 7.3); 1.21-1.43 (m, 8H); 2.07 (sep, 1H, .sup.3J 6.7); 4.10-4.21 (m, 2H); 7.39 (d, 4H, .sup.3J 8.2); 7.45 (dt, 4H, .sup.3J 6.9, .sup.4J 1.1).

2. Physicochemical Characteristics of the EHCz Molecule

[0103] In its pure form, the properties of EHCz are as follows: [0104] colorless transparent liquid; [0105] refractive index of 1.64 at 404 nm; [0106] fluorescence centered at approximately 420 nm; [0107] high flash point of 170 C.; [0108] viscous liquid state at ambient temperature; [0109] good stability with regard to temperature, time and oxygen.

3. Manufacture of Plastic Scintillators

[0110] Different plastic scintillators are manufactured according to the characteristics specified in Table 1: they differ in the EHCz content and the composition of the polymeric matrix, which can comprise styrene (St) and/or 1,4-butanediyl dimethacrylate (1,4) (with indication of the proportion by weight of each monomer when they are both present in the matrix), and also in the possible presence of a secondary fluorophore, such as 1,4-bis[2-(5-phenyloxazolyl)]benzene (POPOP) or 1,4-bis(2-methylstyryl)benzene (Bis-MSB), which are represented below. The EHCz content and the secondary fluorophore content are expressed as percentage by weight of the plastic scintillator, the remainder being constituted by the percentage by weight of the polymeric matrix.

TABLE-US-00001 TABLE 1 [00003] [00004] Composition of the matrix (proportion by weight in the case of a EHCz Secondary mixture) [% by weight (% by fluorophore # of the mixture] weight) (% by weight) 1 St/1, 4 (80:20) [99] 1 2 St/1, 4 (80:20) [98] 2 3 St/1, 4 (80:20) [97] 3 4 St/1, 4 (80:20) [96] 4 5 St/1, 4 (80:20) [95] 5 6 St/1, 4 (80:20) [96.97] 3 POPOP (0.03) 7 St/1, 4 (80:20) [96.97] 3 Bis-MSB (0.03) 8 St [90] 10 9 St [80] 20 10 St/1, 4 (80:20) [70] 30 11 St/1, 4 (80:20) [60] 40

[0111] By way of representative example for all the plastic scintillators of Table 1, the manufacture of the reference plastic scintillators 10 and 7 is described in detail hereinafter.

3.1. Example 1 of Manufacture of a Plastic Scintillator Devoid of Secondary Fluorophore (Reference 10)

[0112] A mixture composed of purified EHCz (20% by weight) and distilled styrene (80% by weight) is introduced under an inert atmosphere composed of argon into a round-bottomed flask predried under vacuum. The mixture is degassed according to the freeze-pump-thaw degassing method. Having returned to ambient temperature, the solution is poured into a mold intended to give the form of the scintillator. This mold is sealed under an inert atmosphere and then heated at 65 C. for 10 days. Once the polymerization is complete, the mold is broken in order to recover the crude plastic scintillator, which is polished to give it its final form.

3.2. Example 2 of Manufacture of a Plastic Scintillator Comprising a Secondary Fluorophore (Reference 7)

[0113] A mixture composed of purified EHCz (3% by weight), POPOP (0.03% by weight), styrene (77.58% by weight) and 1,4-butanediyl dimethacrylate (19.39% by weight) is introduced under an inert atmosphere composed of argon into a round-bottomed flask predried under vacuum. The mixture is degassed according to the freeze-pump-thaw degassing method. Having returned to ambient temperature, the solution obtained is poured into a mold intended to give the form of the plastic scintillator. This mold is sealed under an inert atmosphere and then heated at 65 C. for 10 days. Once the polymerization is complete, the mold is broken in order to recover the crude plastic scintillator, which is polished to give it its final form.

4. Photophysical Properties of a Plastic Scintillator Comprising the EHCz Molecule

4.1. Properties in the Absence of Irradiation

[0114] FIG. 1 shows the absorption spectra of the reference plastic scintillator 8 having 10% by weight of EHCz (continuous line) and the reference plastic scintillator 9 having 20% by weight of EHCz (dotted line). It shows the advantage of incorporating EHCz at a high concentration in order for the exciplex formed to emit as far as possible luminescence in the region of transparency of the material.

[0115] FIG. 2 shows the fluorescence emission spectra of the plastic scintillators 1 to 5 and 8 to 11. In order to make it easier to compare them, the intensity of these spectra is standardized by arbitrarily assigning the value 1 to the value of greatest intensity of each spectrum.

[0116] FIG. 2 illustrates the fact that the fluorescence emission spectrum is shifted toward the higher wavelengths when the EHCz concentration increases in the plastic scintillator: this hypsochromic shift reflects the increase in the concentration of EHCz molecules in the exciplex form to the detriment of the monomer form with which it is in physicochemical equilibrium. The proportion of the excimer form becomes particularly high, in particular for concentrations of EHCz of greater than 30%.

4.2. Properties Under Irradiation by a Gamma Source

[0117] In an environment protected from light, the plastic scintillators 1 to 5 and 8 to 11 are successively coupled optically using a Rhodorsil optical grease to a photomultiplier supplied with high voltage. A cobalt-60 gamma source irradiates each plastic scintillator, which then emits scintillation photons. An electronic acquisition device converts the scintillation pulse into an electronic signal which is subsequently amplified by a photomultiplier and then recorded and digitized by virtue of an electronic acquisition board.

[0118] The signal obtained is subjected to the following processing sequence: inversion of the signal in order to render it positive, smoothing, integration of the signal with respect to time, distribution of the value by histogram, followed by subtraction of the signal obtained under the same conditions without plastic scintillator in order to eliminate the residual background noise.

[0119] This histogram makes it possible to obtain an energy spectrum, which then gives the light yield obtained for each plastic scintillator, as illustrated by FIG. 3.

[0120] This figure shows that, for EHCz concentrations of less than 20%, there exists an optimum light yield centered at 4%. Beyond approximately 20%, the light yield again increases because the response of the plastic scintillator is shifted toward the high wavelengths by virtue of the higher concentration of the excimer form of the EHCz.

5. Influence of the Presence of a Secondary Fluorophore in the Plastic Scintillator of the Invention

[0121] The plastic scintillators 6 and 7, respectively comprising POPOP and Bis-MSB as secondary fluorophore, are compared with the plastic scintillator 3 comprising the same proportion of EHCz (3% by weight). The energy spectrum histograms obtained according to the protocol described in example 6 are illustrated by FIG. 4. This figure shows that the EHCz can behave as a primary fluorophore suited to the scintillation. Moreover, if no secondary fluorophore is added to the plastic scintillator, the weak pulses obtained are reflected by a squashing of the spectrum towards the left which corresponds to the low output energies, indicating that the plastic scintillator is not luminous enough. This is explained by the fact that the emission wavelength for 3% by weight of EHCz is not the most suitable for the photomultiplier used and that the plastic scintillator absorbs a portion of the light which it emits.

[0122] The presence of a secondary fluorophore in the material for the plastic scintillation measurement according to the invention is generally particularly advantageous for the purpose of improving the quality of the measurement. The percentage by weight of EHCz in the plastic scintillator can then preferably be comprised between 0.002% and 0.2%, indeed even between 0.01% and 0.1%.

6. Example of Qualitative or Quantitative Plastic Scintillation Measurement of a Radioactive Substance According to the Measurement Process of the Invention

6.1. Measurement Protocol

[0123] A plastic scintillator comprising EHCz and a secondary fluorophore is connected to a photomultiplier tube by means of optical grease.

[0124] Subsequent to its exposure to the radioactive substance, the plastic scintillator emits scintillation photons which are converted into an electrical signal by the photomultiplier tube supplied with high voltage.

[0125] The electrical signal is subsequently acquired and then analyzed with an oscilloscope, spectrometry software or an electronic acquisition board.

[0126] This analysis results in an energy spectrum histogram representing, on the abscissa, the channels (derived from an output energy) and, on the ordinate, the number of counts. After calibration with a gamma-emitting source of known energy, the energy of the radioactive substance to be measured is determined.

6.2. Quantitative Measurement with the Plastic Scintillator 5

[0127] On the basis of this measurement protocol, a quantitative measurement is carried out with the reference plastic scintillator 5 of Table 1 containing 5% by weight of the EHCz molecule and manufactured according to the manufacturing process described in detail in example 3.

[0128] The plastic scintillator is coupled using the Rhodorsil RTV141A optical grease to a photomultiplier (Hamamatsu H1949-51 model) supplied with a high voltage (Ortec 556 model). The signal leaving the photomultiplier is recovered and then digitized by an electronic board specific to the inventors. This board can be replaced by another equivalent electronic board (for example CAEN DT5730B model) or an oscilloscope (for example Lecroy Waverunner 640Zi model).

[0129] In a first step, an energy calibration of the system (scintillator+photomultiplier) is carried out by means of 2 radioactive sources: one emitting gamma rays in the [0-200 keV] range and the other in the [500-1.3 MeV] range. This energy calibration is carried out by locating the channel corresponding to 80% of the amplitude of the Compton edge. For example, if the ordinate of the Compton edge corresponds to 100 counts, the abscissa on the falling slope of the Compton edge at 80 counts associates the energy of the Compton edge (in keV) with the channel.

[0130] In a second step, this calibration having been carried out, a chlorine-36 beta source (mean energy 251 keV, 2n activity equal at most to 3 kBq) is joined to the upper face of the plastic scintillator. The analysis of the energy spectrum gives a read activity of 2.1 kBq (and thus an intrinsic efficiency of 70%) and a photoelectric peak centered at approximately 250 keV.

6.3. Quantitative Measurement with the Plastic Scintillator 7

[0131] On the basis of the same measurement protocol, a quantitative measurement is carried out with the reference plastic scintillator 7 of Table 1 containing 3% by weight of the EHCz molecule and 0.03% by weight of the Bis-MSB molecule and manufactured according to the manufacturing process described in detail in example 3.

[0132] The plastic scintillator is coupled using the Rhodorsil RTV141A optical grease to a photomultiplier (Hamamatsu H11284 MOD model) supplied with a high voltage (CAEN N1470 model).

[0133] The recovery and then the digitization of the signal leaving the photomultiplier, and also the energy calibration of the system, is in accordance with example 6.2.

[0134] This calibration having been carried out, a chlorine-36 beta source (mean energy 251 keV, 2n activity equal at most to 3 kBq) is joined to the upper face of the plastic scintillator. The analysis of the energy spectrum gives a read activity of 2.8 kBq (and thus an intrinsic efficiency of 96%) and a photoelectric peak centered at approximately 250 keV.

[0135] The present invention is not limited to the embodiments described and represented, and a person skilled in the art will know how to combine them and to contribute thereto with his general knowledge of numerous alternative forms and modifications.

[0136] The invention is applicable to the fields where scintillators are used, in particular: [0137] in the industrial field, for example for the measurement of physical parameters of parts during manufacture, for the nondestructive inspection of materials, for the monitoring of radioactivity at the entrance and exit points of sites and for the monitoring of radioactive waste, [0138] in the geophysical field, for example for the evaluation of the natural radioactivity of soils, [0139] in the field of fundamental physics and in particular nuclear physics, [0140] in the field of the safety of goods and people, for example for the safety of critical infrastructures, the monitoring of moving goods (luggage, containers, vehicles, and the like), and also for the protection from radiation of workers in the industrial, nuclear and medical sectors, [0141] in the field of medical imaging.

REFERENCES CITED

[0142] [1] Principles and practice of plastic scintillator design, Radiat. Phys. Chem., 1993, Vol. 41, No. 1/2, 31-36. [0143] [2] Current status on plastic scintillators modifications, Chem. Eur. J., 2014, 20, 15660-15685. [0144] [3] Non-toxic liquid scintillators with high light output based on phenyl-substituted siloxanes, Opt. Mater., 2015, 42, 111-117. [0145] [4] WO 2013076281. [0146] [5] Techniques de l'ingnieur, Mesures de radioactivit par scintillation liquide, Rfrence p2552, publication du 10/03/2004 [Techniques of the Engineer, Measurements of radioactivity by liquid scintillation, Reference p 2552, publication of Oct. 3, 2004].