Current status on plastic scintillators modifications.

DOI: 10.1002/chem.201404093

Review

& Plastic Scintillators

Current Status on Plastic Scintillators Modifications** Guillaume H. V. Bertrand, Matthieu Hamel,* and Fabien Sguerra[a]

Chem. Eur. J. 2014, 20, 15660 – 15685

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Review Abstract: Recent developments of plastic scintillators are reviewed, from 2000 to March 2014, distributed in two different chapters. First chapter deals with the chemical modifications of the polymer backbone, whereas modifications of the fluorescent probe are presented in the second chapter.

All examples are provided with the scope of detection of various radiation particles. The main characteristics of these newly created scintillators and their detection properties are given.

Introduction Protection of civilians and facilities against CBRN-E (chemical, biological, radiological, nuclear, and explosives) threats represent a true challenge due to the constant increase of world’s population movements. According to Dr. El Baradei (1997– 2009 IAEA Director General), terrorists who are unconcerned about exposing themselves to radiation could easily conceal a source in a truck or a suitcase. “The danger of handling powerful radioactive sources can no longer be seen as an effective deterrent, which dramatically changes previous assumptions. […] Security of nuclear and other radioactive material has taken on dramatically heightened [in IAEA’s work] significance in recent years.” As an example, a dramatic story happened in late 2013 in Mexico, in which a radioactive cobalt-60 source (3,000 Curies, 111 TBq) was stolen from its transportation truck. Fortunately, the material was safely recovered eight days later. In this context, numerous detectors could be used for nuclear and/or radiological (NR) detection. Among them, we will focus in this Review on plastic scintillators (hereafter abbreviated as PS; see abbreviations in the footnote). These materials can be defined as one or several fluorescent probes embedded in a polymer matrix, and the resulting system is able to produce light, while interacting with a radioactive source (Figure 1). For instance, a typical scintillation co*cktail is made from p-terphenyl and POPOP dissolved in polystyrene (PSt). The preparation of a PS was first described in the late 1950s.[2]

[a] Dr. G. H. V. Bertrand, Dr. M. Hamel, Dr. F. Sguerra CEA, LIST Laboratoire Capteurs & Architectures Electroniques CEA Saclay, 91191 Gif-sur-Yvette cedex (France) E-mail: [emailprotected] [**] Standard abbreviations used in this document: a-NPO: 2-(1-naphthyl)-5phenyloxazole; AIBN: aso-bisisobutyronitrile; BBO: 2,5-bis-(4-biphenylyl)1,3,4-oxadiazole; CL: cathodoluminescence; CQD: carbon quantum dot; DVB: divinylbenzene; FOM: figure of merit; HMPA: hexamethylphosphoramide; IBIL: ion beam induced luminescence; LiMA: lithium methacrylate; LiSal: lithium salicylate; MEH-PPV: poly[2-methoxy-5-(2-ethylhexyloxy)-1,4phenylene-vinylene; MPA: mercaptopropionic acid; MOF: metal–organic framework; NMP: 1-methyl-2-pyrrolidinone; p-T: p-terphenyl; PBBO: 2-(4-biphenylyl)-6-phenylbenzoxazole; PBD: 2-(4-biphenylyl)-5-phenyl-1,3,4-oxadiazole; PEG: poly(ethylene glycol); PL: photoluminescence; PBMA: poly(benzyl methacrylate); PMMA: poly(methyl methacrylate); POPOP: 1,4-bis(5-phenyl2-oxazolyl)benzene; PPO: 2,5-diphenyloxazole; PS: plastic scintillator; PSD: pulse shape discrimination; PSt: polystyrene; PVA: polyvinyl alcohol; PVK: polyvinylcarbazole; PVT: polyvinyltoluene; QD: quantum dots; SEM: scanning electron microscopy; SSD: spectral shape discrimination; St: styrene, TTA: triplet–triplet annihilation. Chem. Eur. J. 2014, 20, 15660 – 15685

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Figure 1. PSs displaying different emission wavelengths (excitation with UV lamp; copyright CEA).

To detect special nuclear materials, PSs present several advantages. They are cheap (especially interesting for large-size detection systems), sensitive to gamma rays, can be handled without any specification, reliable, stable, and can be prepared in large volumes. More particularly, the choice of the detector will become extremely important in the future due to the combination of the cheapness of PS ($ 2,000 for a 3.8 cm 36 cm 173 cm large PVT detector, compared to $ 6,000 for a 5 cm 10 cm 41 cm NaI(Tl) inorganic scintillator[3]) and the necessity for some countries to cover, at best, their borders with radiation portal monitors. However, some drawbacks have lead several groups to a renewed interest in the chemical development of PSs: they display a poor resolution, were presumed for a long time to be unable to perform fast neutron/gamma discrimination, afford relative low scintillation yields compared with inorganic scintillators and cannot give access to the full energy of an incident gamma. The basic principle of radiation/matter interaction is as follows: when an incoming radiation interacts with the polymer, it will lose a given part of its energy, depending on its nature. Alpha particles, which are highly ionizing but heavy, will thus interact within a few micrometres (e.g. an alpha particle of energy 5 MeV will penetrate within 35 mm). In contrast, a 1 MeV electron would need 4.3 mm and a 1 MeV gamma particle not less than 14 cm to release their energy. As explained, since the mean free path will be different for each particle, the response of the PS should vary with them. Actually, this is not often the case. So, when energy is deposited inside the PS, multiple interactions occur, and among others, the matrix will allow the release of UV photons. Following other interactions, the emerging photons will be transferred from UV to visible light, where it can be recovered with photodetectors, such as photomulti-

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Review Also, this Review is exclusively limited to PSs and their derivatives (composites, sol–gel, etc.). No data will be given regarding improvements on organic single crystals, optical (scintillating) fibers, liquid scintillators, and inorganic scintillators.

Modifications of Polymeric Matrix for Special Applications

Figure 2. From ionizing radiation to electric signal.

Poly(methyl methacrylate)-based scintillators plier tubes (Figure 2). This photon-to-electron conversion allows physical access to the radiation/matter interaction. Depending on their application, PSs display both advantages and drawbacks. On one hand, they are cheap, available in large dimensions and morphologies, doped with various elements, have a large choice of emission wavelengths and have a fast response. On the other hand, their light response is moderate (light yield of typically 10 000 ph per MeV of deposited energy), have a low density and effective atomic number, so are not efficient for gamma spectroscopy and display a poor resolution. We will see in this Review that chemical modifications of these materials can lead to differentiations in the particles’ responses. The goal of this Review is to define new developments of PSs. Despite the fact that many tools of detection devices have been improved (electronics, signal processing, etc.), most commercial PSs are those which were developed in the 1950s and 1960s. This Review will be limited to developments from 2000 to March 2014, unless particularly relevant data were published before this date. The reader can refer to other reviews in this field for previous improvements.[4, 5] This Review will be mainly focused on chemical developments, and is thus less oriented to the nuclear physics point of view, even if application examples are given when available. It is therefore structurally based on chemical developments (first part about the modification of the polymer; second part about the modification of the luminescent probe). However, as the goal is devoted to nuclear detection, materials will also be described in terms of photophysical characteristics, such as scintillation yield (or light output), decay time, scintillation wavelength, and so on. Also, pictures of PSs are given when available. Sometimes some publications deserve to be classified in several topics (e.g., metal loading of a specially designed polymer). They will be therefore referenced two or several times.

Abstract in French: Nous proposons dans cette revue les dveloppements les plus rcents sur la chimie des scintillateurs plastiques, plus prcisment depuis 2000 jusqu’en mars 2014. La revue est divise en deux sections : la premire concerne les modifications chimiques de la matrice polymrique, alors que la deuxime section prsente les modifications envisages sur les molcules fluorescentes. Tous les exemples dcrits sont fournis avec leurs proprits caractristiques de scintillation. Chem. Eur. J. 2014, 20, 15660 – 15685

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Poly(methyl methacrylate) (PMMA) occupied a special place in the transparent plastic for scintillation family. A review especially devoted to that topic was recently published by Salimgareeva and Kolesov[6] and is a very precise and complete overview of PMMA-based scintillator technology before 2005. We strongly recommend this publication as a complement of this part as we will focus on post-2005 developments.

Dr. Guillaume H. V. Bertrand studied chemistry at “ l’ENS Ulm ” in Paris and obtained his Ph.D. in chemistry at UPMC (Paris, France), working with Dr. Corinne Aubert on cobalt complexes for photovoltaic applications. He was then the recipient of “the foundation Monahan” research grant and spent one year in M.I.T. (Boston, USA) working on thiophene-based covalent organic frameworks with Dr. Mircea Dinca. In 2013 he joined the CEA (Gif-surYvette, France) as a post-doctoral fellow, supervised by Dr. Matthieu Hamel, developing bismuth doping in plastic scintillators.

Dr. Matthieu Hamel received his Ph.D. in organic chemistry from the University of Caen Basse-Normandie (France) in 2005. Since 2009, he holds a permanent position at the French Atomic Energy Commission (CEA) as an expert scientist. His research is based on the preparation of luminescent polymers for CBRN-E detection.

Dr. Fabien Sguerra received his engineer’s degree in organic chemistry from Ecole Europenne de Chimie, Polymres et Matriaux (Strasbourg, France) in 2009. The same year he joined the Laboratoire de Tectonique Molculaire at the University of Strasbourg (France) to pursue his Ph.D. under the supervision of Prof. Mir Wais Hosseini and Prof. Vronique Bulach, working on the synthesis of porphyrin ligands for the preparation of MOFs, sensitization of lanthanide emission, and surface modifications. He is currently a post-doctoral scientist at Commissariat l’Energie Atomique (Gif-surYvette, France).

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Review Unlike polystyrenes, polysiloxanes, or polyepoxides cited in this review, PMMA does not have any benzene rings in its backbone. Hence, it lacks of primary emitters but in return confers a very high near UV transparency. This compromise makes it a target of choice for scintillation application as the monomer methyl methacrylate (MMA) is quite cheap and its polymerization chemistry is known (Scheme 1). In PSt-based

five to ten times more resistant to radiation (rad-hard). This has been explained in the early 2000, with chain’s flexibility as the main argument.[15] It was stated that under ionizing conditions, radicals formed from hom*olitically cut chains can recombine or react without damaging the backbone or impeding on the physical characteristics at doses up to 3.5 MGy. Polysiloxane is a rather vague name as it defines a large family, but for scintillation applications, cross-linked copolymers containing phenyl groups are the system of choice. On a synthetic standpoint, this system is composed of vinyl-terminated diphenylsiloxane-dimethylsiloxane copolymer (resin A) and hydrideterminated methylhydrosiloxane-phenylmethylsiloxane copolymer (resin B) (Scheme 2). The rubber is synthesized using a plat-

Scheme 1. Monomer and general formulation of PMMA and PBMA.

PSs the energy deposited by an ionizing radiation is mainly absorbed by the polymer matrix, which can be efficiently transferred to the emitters by means of Fçrster energy exchange,[7] whereas PMMA-based scintillators have to rely on less efficient non-radiative exchanges. Primary emitters are frequently referred as the secondary solvent, because they account for a large part of the scintillator to maximize scintillation yield. Most of the research from the last century was performed with naphthalene[8] as the primary emitter. Classical characteristics of a PMMA-based scintillator are a scintillation yield close to 5000 ph MeV 1 (which means the number of photons created after the deposition of 1 MeV from the particle) and a decay time around 3 ns, for a sample size of 3 cm3. However, early 2000s research showed that naphthalene can be replaced by 1,1,3-trimethyl-phenylindan[9] as a reliable solution to achieve up to 12 000 ph MeV 1 scintillation yields. Different mixtures of secondary and tertiary emitters have been tried with success,[10] and once again all of this has been exhaustively described in Salimgareeva’s review. More recently Van Loef et al. evaluated the use of PBMA [poly(benzyl methacrylate); Scheme 1] for fast neutron/gamma discrimination purpose but pulse height experiments revealed very low scintillation yield (< 1000 ph MeV 1). They also conclude that PVT and PSt equivalents are more suited and give good result for the design application.[11] PMMA is also characterized by a refractive index which strongly differs from PSt; this property has been used for cladding scintillating plastic fibers.[12] Polysiloxanes Polysiloxanes were an early subject of study for scintillation applications.[13] They differ from traditional PSs as they are elastomers and are often referred to as silicon rubber. Their main attractive feature is their very flexible chain, which gives them a wide range of temperature stability[14] and irradiation tolerance. Polysiloxanes were early identified as an attractive alternative to PSt or PVT-based plastic sensors as they proved to be Chem. Eur. J. 2014, 20, 15660 – 15685

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Scheme 2. Standard composition of polysiloxane rubber precursor.[17b]

inum-catalyzed hydrosilylation. Karstedt’s catalyst enables room temperature vulcanization of the resin mix. Different ratios of diphenylsilyl in resin A have been explored; however, a ratio larger than 22 % has dramatic consequences on the sample’s transparency (transmission < 75 %). Classical characteristics of a polysiloxane-based scintillator are scintillation yields in the range 3000–6000 ph MeV 1 and decay times of about 5 ns for a typical sample size of 2 cm3. The main drawback of this family of polymer is the poor solubility of major fluorescent dyes. For instance PPO loading is always in the range 1–1.5 wt %.[16] It has been shown that the fluorescence quenching due to aggregation counteracts the increase of PPO concentration. In the same study by Quaranta et al.,[17] it was also proved that the phenyl emission intensity arising from the polysiloxane core decreases as the PPO concentration increases (Figure 3). This is a strong clue as to how the excited state can transfer from the backbone to the primary dye; hence the percentage of phenyl rings in the copolymer structure will have an impact on the scintillation yield. This has been recently highlighted[17c] with a work that describes optical and scintillation response of

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Review

Figure 3. Emission spectra in the UV range (lex = 250 nm) from the diphenylsiloxane unit of a standard siloxane matrix with different PPO concentrations. (Reproduced with permission from reference [17a]. Copyright 2010 from the authors.)

polysiloxanes with various phenyl contents synthesized using a blend of resin A and modified resin B to overcome the 22 % limit previously stated. The designed polymers did not give the expected result of increasing emission intensity with increasing quantity of phenyl groups in the blend. However, an increase in the phenyl–phenyl excimer formation was found as the number of phenyl groups increases. We can also note that higher quantity of phenyl decreases the stability of the device toward high doses. Another drawback for silicon rubber/PPO matrices is that they are more absorbent in the near UV than their PSt and PVT counterparts. This issue can be partially bypassed by adding a red emitter to the mix which gives a 17 % increase of the collected light when coupled with an appropriate photomultiplier (Figure 4).[17d] This last work also shows the capability of polysiloxane-based scintillators to achieve satisfying neutron detection. The low solubility of classic luminescent dyes in polysiloxane copolymers also hinders access to good intrinsic neutron/ gamma detection through high concentration techniques (see corresponding chapter), but it is possible to engineer this type of PS for neutron detection. To tackle this problem, research has been focused on incorporating elements with high neutron cross sections, such as boron or gadolinium,[16a] in the polysiloxane matrix. Gadolinium doping was briefly studied by

Figure 4. Polysiloxane-based red emitting scintillator irradiated by 380 nm lamp (left) and in daylight (right). Lv = Lumogen violet. Lr = Lumogen Red. (Reproduced with permission from reference [17d]. Copyright 2013 from the authors.) Chem. Eur. J. 2014, 20, 15660 – 15685

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Bell et al., who successfully incorporated trinitro or tris(tributylphosphate)gadolinium complexes into polysiloxane. Unfortunately they observed a reduced light output ( 50 %) with respect to the PVT equivalent. Loading with boron has been more investigated in order to detect thermal neutrons. closo-Carborane cages linked to base resins were used to achieve high boron loading—it was observed that ortho- and para-carboranes are less stable than meta-carborane.[16a] In most of the reported compositions boron concentration varies between 2 and 5 wt %, except for one report of a PS device prepared with up to 18 % by weight of boron.[18] The high nuclear cross section for neutron absorption and its low atomic number make boron a candidate of choice for neutron/gamma discrimination, although the presence of Si atoms in the matrix is known to make polysiloxane sensitive toward gamma ionization by Zeff increase. On a pure detection aspect, boron loading in polysiloxane gives the same performance as the commercial PVT counterpart.[19] It was also demonstrated that the boron loading has no effect on the internal quantum efficiency (no quenching of dye’s excited states), but slightly inhibits energy transfer from the matrix to the dyes[20] (decrease of the light output when the loading increases). Epoxyde resins Polyepoxydes, most commonly referred as epoxy or araldite, are a class of materials that cover a wide range of applications due to their easy chemical tuning. It is not surprising that transparent and easy-to-cast polyepoxyde matrix containing fluorophores for scintillation purposes are obtained. All epoxy resins share the same practical use: they are good glues and readily polymerize at room temperature. This is why the main application of epoxy in radiation detection is not as a scintillator matrix but as an optical glue.[21] Polyepoxydes are obtained by mixing two components: a prepolymer, the “binder”, and a curing agent, also called “hardener”. These two molecules, once mixed, give a highly cross-linked network (Scheme 3). Classical characteristics of an epoxy-based scintillator are scintillation yields reported around 50 % of a Pilot B scintillator, decay times of about 10 ns and a typical sample size of 3 cm3. Most of the recent work was done with a commercial optical epoxy[22] (EpoTek 305), which seems to be very close to standard epoxy glues. Early interest in the 1960s quickly showed major limitation: a very low UV transmission (Figure 5), and poor solubility of classic scintillating molecules.[23] Recent experiments have nonetheless made use of polyepoxyde as a matrix because of its easy handling that does not need any advanced chemistry knowledge. Encapsulation of inorganic fluor BaF2 :Ce was attempted[24] following in the footsteps of early 1990s technology.[25] Crystals, 50 nm in size, were successfully and hom*ogeneously trapped in the matrix after curing. The good refractive index match between the polymer and BaF2 :Ce was not sufficient to compensate the loss of transmitted light through the sample. Despite being evidently designed for radiation detection, no data are available concerning their use under ionizing radiation. Another recent

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Review Polyesters Polyethylene terephthalate (PET)[27] and polyethylene naphthalate (PEN)[28] (Scheme 4) have been proven to be radioluminescent without the addition of any fluorophores. PET scintillation yield is rather modest (2200 ph MeV 1), but higher scintillation output is claimed for its naphthalene counterpart

Scheme 4. Structures of poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN).

(10 500 ph MeV 1). Nevertheless PEN radioluminescence intensity seems to vary drastically with the polymer quality and the surface treatment of the samples.[29] Furthermore PEN and PET degradation is observed under laser and proton beam.[30] Loading a PEN matrix with 6LiF in order to achieve thermal neutron detection has been performed,[31] but the samples suffer from low transparency, even for 150 mm thicknesses, due to a lack of solubility of 6LiF in the polymer matrix, which is the opposite of the detection system goal (large systems are required for accurate neutron detection).

Scheme 3. An example of standard binder and hardener and their subsequent cured polyepoxyde.

Polyimides Carturan and Quaranta have extensively studied the use of polyimides for scintillation purposes.[32] Polyimide films are typically prepared from 4,4’-hexafluoroisopropylidenediphthalic anhydride/diaminobenzophenone solution in NMP, this solution being further doped with different amounts of Rhodamine B, followed by spin coating on silica plates and curing at various temperatures (Scheme 5). The formed polyimide is able to fluoresce and acts as a photon donor to Rhodamine B. Upon irradi-

Figure 5. Transmission percentage of a standard cured optical polyepoxide compared to an optical PMMA (samples 6.4 mm thick). (Reproduced with permission from reference [23]. Copyright 1968 Taylor & Francis.)

study[26] used polyepoxyde to, as the authors’ claimed, create long lifetime luminescent organic dye embedded in an organic matrix. Their objective was to use a long decay pulse in a PS/ optical fiber/detector set up to differentiate pulses from the plastic and from stray ionization of the fiber. They choose polyepoxyde for the purpose of immobilizing xanthene dyes in a transparent matrix. The authors of this paper assumed this immobilization would promote phosphorescence by decreasing vibrational or O2 quenching of triplet states and thus enable the observation of long luminescence lifetime (0.7 to 3.0 ms). No real proof of this effect is reported, that is, no clear count versus channel graph, to support this theory. Luminescence lifetime data were given without graphic showing time resolution of luminescence versus wavelength, which cast serious doubt about the attribution of the phosphorescence peak. Chem. Eur. J. 2014, 20, 15660 – 15685

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Scheme 5. Synthesis of polyimide (top) and red dyes used as dopants (bottom).

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Review ation with 5.478 MeV alpha particles, these films displayed a scintillation efficiency of 1400 ph MeV 1 (14 % of NE-102 PS). The authors also studied Nile Red as dye. Doped films were studied by ion beam induced luminescence (IBIL).[33] The samples were irradiated by a 2.0 MeV 4He + ion beam at fluences ranging between 1012 and 1015 ions per cm2. IBIL results indicated that the polyimide-based scintillator has better radiation hardness and good scintillation efficiency for high doses irradiations. In their last report,[33c] three different fluorinated polyimides were prepared and compared with respect to radiation hardness. With all these improvements, efficiencies ranging from 50 to 60 % relative to NE-102 have been reached (5000– 6000 ph MeV 1). To the best of our knowledge, this was the first use of rhodamine B as dye for scintillation purposes.

Table 2. Isotopes used for neutron capture.[34] Isotopes 6

Li B 113 Cd 155 Gd 157 Gd 10

Thermal cross section (Barns)

Isotope abundance [%]

940 2000 30 000 60 700 254 000

7.5 19.9 12.2 14.8 15.6

Loading with elements Metal loading The wide diversity of polymer matrices for scintillation applications enables chemists to use their arsenal of synthetic modifications to tune the properties of scintillators. Among them, inorganic chemistry is particularly attractive as it gives a complementary solution to a large range of scintillation applications (Table 1). We can differentiate two main ideas behind the use

Table 1. Metals and specific isotopes that are potential targets for plastic scintillation applications.[45] Elements/Isotopes 6

Li, B,

113

Cd,

155

Gd,

Figure 6. LiMA monomer and performance versus quantity of enriched and natural LiMA containing PS. Measurements were conducted with a thermal neutron beam. (Reproduced with permission from reference [36]. Copyright 2013 Elsevier B.V.)

Applications 157

176

Yb, 160Gd, 100Mo, 37Cl Pb 19 73 F, Ge 150 Nd, 160Gd, 100Mo, 130Te, 82Se Pb, Sn, W, Hg, Bi

neutron detectors, searching for neutrino oscillations detection of solar neutrinos detection of astrophysics neutrinos searching for dark matter searching for double b decay high-energy physics

of metals in PSs: the increase of density to increase gamma interaction with radiation, or target a nucleus known to have a large cross section towards a specific radiation type.

Lithium-6 is a naturally occurring isotope (7.5 wt %) with a very well-known chemistry. A recent study showed that it was possible to synthesize (natural or enriched) lithium methacrylate (LiMA 95 wt %).[36] Both natural and enriched LiMAs were successfully copolymerized with styrene up to a 1:10. As expected the enriched LiMA gave better performance toward thermal neutron detection (Figure 7). Another way to incorporate lithium in PSs is to blend in inorganic crystals. This approach can be used to combine different interesting elements. A typical example is lithium gadolinium borate (LGB) doped with cerium,[37, 38a] which forms scintillating microcrystals that can be blended in a PVT matrix (BC490), and can be used in neutron spectrometry.[39] This work

Lithium, boron, cadmium and gadolinium loadings for thermal neutron detection As it can be seen throughout this Review, recent focus has been made on neutron detection, and PMMA-based scintillators can be used as an alternative to PSt for such applications. Absorption of thermal neutrons (generated after multiple collisions with hydrogen atoms) is not natural for a standard PS and requires different elements other than C, H, N, S or O. The desired isotopes with large cross sections towards thermal neutrons are given in Table 2 (from reference [34]) with their natural abundances. In order to increase nuclear cross section towards neutrons, the monomer can be engineered to include lithium-6 atom (Figure 6).[35] Chem. Eur. J. 2014, 20, 15660 – 15685

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Figure 7. Photo of scintillator samples, 1.25 wt % enriched LiMA (left) and 10 wt % enriched LiMA (right). Diameters are 20 mm, thicknesses 10 mm. (Reproduced with permission from reference [36]. Copyright 2013 Elsevier B.V.)

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Review demonstrated an increased resolution with a sample (60 cm3) exposed to a monoenergetic proton beam, but also claimed that higher volumes do not increase the resolution[38b] due to the weak light output and the diffusive nature of the dispersed LGB. To the best of our knowledge, only a single publication refers to the use of 2-vinylnaphthalene doped with lithium as a monomer for PS.[40] The preparation of poly(2-vinylnaphthalene) is not described. Loaded with lithium-6 salicylate (Li-Sal) as a neutron capture reagent and fluorophore (lem max = 408 nm), it enables the detection of thermal neutrons. Li-Sal was readily prepared from the reaction between salicylic acid and 6LiOH in acetone/water at 60 8C (Scheme 6). Thin film samples (thickness 110 mm) doped with 25 % of LiSal were thus obtained and exposed to radiation flux. The authors mentioned that undoped poly(2-vinylnaphthalene) was insensitive to neutrons or gamma radiation. Unfortunately the material seems opaque even at low thickness (Figure 8).

Scheme 6. Preparation of 6Li-Sal.

prepared from the reaction of styrene and maleic anhydride in a mixture of toluene and diethyl ether (90:10), initiated with AIBN at 60 8C. 6LiOH is then added to give the deprotonated polymer; the authors assumed that lithium was ultimately transferred from the matrix to the salicylic acid fluorophore. The overall mixture was cast on acrylic disks to afford 200 mm thin films. Despite a visually transparent sample, reported transmission is rather low with a 78 % average value between 360 and 600 nm. The maximum 6Li loading obtained that resulted in a transparent film was 4.36 wt %. The authors report an average light yield of 360 photons per thermal neutron capture event in the presence of a shielded 252Cf source. In the early 2000s researchers started to anticipate the shortage of 3He. An alternative was to incorporate boron in PSt and PVT matrices for scintillation application. Boron-loaded scintillators could be sensitive to fast neutrons (recoil proton from neutron interaction) and thermal neutrons (boron-10, which after capture of a thermal neutron produces alpha and lithium particles). This duality pushed Normand et al. to synthesize and to characterize a new formula in 2002.[44] The scintillator composition is based on a classical 1.5 wt % p-terphenyl, 0.01 wt % POPOP in styrene, and with 5 wt % of boron, albeit without saying which molecule was used. This work aimed at surpassing the performances of the commercial BC-454 PS with a material that is four times cheaper. They indeed showed the feasibility of thermal neutron/gamma discrimination[44a] and application to waste drum assessment (Figure 9).[44c]

Figure 8. Example of a composite sample of thickness 110 mm and 48 mm diameter (Copyright Indraneel Sen).

A recent extension of this work consisted in biaxially stretching PEN films for thermal neutron detection.[41] Large scale (ca. 1 m2), 6LiF-loaded films were successfully prepared. The biaxially stretched composite poly(ethylene naphthalate) had 20 % higher neutron light yield as compared to unstretched composite film. The Istituto Nazionale di Fisica Nucleare is also currently working on lithium fluoride loading in composites, by entrapping 6LiF nanodiamonds in siloxane detectors.[42] Another use of Li-Sal has been described by Mabe et al. Transparent lithiated polymer films were obtained from another new matrix: poly(styrene-co-lithium maleate) with a 1:1 alternating ratio.[43] Rather unusually in the chemistry of PSs, this copolymer is not obtained as a bulk material, but is previously Chem. Eur. J. 2014, 20, 15660 – 15685

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Figure 9. Comparison of Birks’ fit between commercial (BC-454) and new boron loaded scintillator.[44b]

Boron can also be introduced through the use of carborane compounds in a PSt matrix. It has been shown that this method can also produce PS doped with 5 wt % of boron. This loading did not affect the light output (Figure 10), but gave a dramatic increase in thermal neutron detection (Figure 11).[45] On a side note this technology has been used in experiments for monitoring in situ neutron capture therapy.[46] Britvich et al.[47] reported the use of neutron absorbers for PS loading, either in the form of o-carborane (4 %), boric acid[48] (3 %), or 6LiF (0.1 %). Scintillation yields were measured to be 8600, 6200, and 9400 ph MeV 1 respectively.

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Figure 10. Left: Comparison of PL spectrum between standard and borondoped PSs. Right: Comparison of transmission spectrum between standard and boron-doped PSs. (Reproduced with permission from reference [45]. Copyright 2001 World Scientific Publishing Company.)

Scheme 7. Organogadolinium compounds soluble in classical monomers (MMA and styrene). General formula of a) gadolinium phenylcarboxylate;[51] b) gadolinium isopropoxide;[49] and c) gadolinium phenylpropionate.[55]

Figure 11. Evolution of neutron sensitivity with boron loading on a pulse height spectrum (left) and on a quantum yield versus number of 10B atom by volume spectra (right). (Reproduced with permission from reference [45]. Copyright 2001 World Scientific Publishing Company.)

Gadolinium has also been hosted in a polymer matrix. Until 2001, loading higher than 0.5 wt % had never been reached. However, in 2001 the use of HMPA as an additive was found to increase the solubility of gadolinium nitrate[49] and gadolinium chloride[50] in methyl methacrylate and gave access to a PS loaded with up to 3 wt % of Gd. This loading decreased the light output (estimated at 5000 ph MeV 1), but increased the sensitivity toward neutron detection (Figure 12). The same phenomenon was observed for gadolinium phenylcarboxylate (Scheme 7 a), phosphinate, and phosphine derivatives, which were found to increase solubility in styrene, and achieved a loading as high as 5 wt % for a 60 % light yield (photon yield not given) relative to the unloaded PS.[51] Some work has also been performed on gadolinium loading in plastic fibers[52] without clear presentation of the chemical

Figure 12. Evolution of neutron sensitivity with gadolinium loading on a light output versus mass fraction spectra (left) and on pulse height spectra (right). (Reproduced with permission from reference [45]. Copyright 2001 World Scientific Publishing Company.) Chem. Eur. J. 2014, 20, 15660 – 15685

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structure. Gadolinium loading in the form GdF3 nanoparticles has also been reported.[53] Organogadolinium compounds could also be used without additives to enhance the neutron cross section of PS; in 2009 Ovechkina et al. described the use of gadolinium isopropoxide (Scheme 7 b) blended in a PSt matrix at a concentration up to 3 wt %. This materials show good characteristics (a scintillation yield of 8500 ph MeV 1 for a 3 wt % Gd loading) and some promising neutron response.[54] Unfortunately these first results have not been exploited to their full potential. Another analysis was performed on gadolinium phenylpropionate in styrene (Scheme 7 c)[55] that only showed the negative influence of gadolinium doping on the light yield; no neutron response was reported. A solution to reach higher gadolinium loading was recently proposed.[56] The authors achieved very high metal loading using functionalized Gd2O3 nanocrystals to create a 3 mm thick nanocomposite with up to 40 wt % of capped Gd2O3 (which corresponds to a 22.7 wt % loading of Gd). On the one hand, this high Gd incorporation decreased significantly the transmission of the device, but it also enabled the observation of a photoelectric peak under 662 keV gamma irradiation (Figure 13) and a high scintillation yield (27 000 ph MeV 1). We note here that this study does not mention thermal neutron detection despite the high Gd loading. Finally lithium, boron, and cadmium can also be introduced after the polymerization by means of extrusion technics. Nanoparticles are mixed with PSt or PVT pellets in an extruder to form a mix that can be cast.[57] This will be further developed in the appropriate section. It is worth noting that other publications cited hereafter in this Review include lithium loading, but are classified upon other criteria.

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Review counts for up to 22 wt % of the total mass giving one of the densest PS ever published: 1.55 g cm 3. The first-generation of lead-doped systems gave modest scintillating materials (ca. 1000 ph MeV 1), but a major improvement was achieved recently, leading to light output > 5000 ph MeV 1.[60] Application has already been found for PS at the Laser Mgajoule facility in Bordeaux, France, to be implemented in a hardened X-ray imaging system in a high radiative background (Figure 14).[61]

Heavy metal loading Another topic of interest with regard to metal loading in a plastic matrix is to make it denser and to increase its effective Z (Zeff); however, heavy atoms tend to have a strong fluorescence quenching due to multiple vibrational relaxations. Nevertheless a compromise can be found between higher absorption and lower light output. A PSt-based scintillator was successfully prepared with 17 wt % of tetraphenyltin (4.7 wt % of Sn, Scheme 8a).[58]

Scheme 8. Chemical formulas of various organometallic compounds: a) tetraphenyltin b) tetraphenyllead; c) lead dimethacrylate; and d) triphenylbismuth.

This loading diminished the light output by 30 % (ca. 7000 ph MeV 1) compared to an unloaded scintillator; it was also noted that mechanical properties were lowered. The lack of popularity of tin loading can also be explained by the high toxicity of organotin compounds. Organolead compounds are also hazardous, but the possibility to achieve a high Zeff motivated further study. Tetraphenyl lead (Scheme 8 b) was a first candidate with very similar result as for tetraphenyltin.[58] As stated before, methacrylic acid is a good medium to incorporate Li atoms in the matrix, but methacrylic acid has also been used as a lead chelating agent, thus allowing loading a larger amount of lead in a PSt matrix (Scheme 8c).[59] Lead acChem. Eur. J. 2014, 20, 15660 – 15685

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Figure 14. Diagnostic implantation in the LMJ facility.

As a complementary note, PMMA can be chemically transformed to blend in inorganic compounds. A major application is to create transparent X-ray or neutron shields,[62] but this will not be discussed further in this review. Bismuth is also very attractive as it is the heaviest nonradioactive element and its organometallic derivatives are less noxious. Triphenylbismuth (Scheme 8 d) is fairly soluble in styrene and allows reaching high incorporation percentage. Recently Cherepy and Payne published a series of studies[63] on such a polymer with a 40 wt % loading of BiPh3 (19 wt % of Bi atom) in a PVK matrix. They evaluated the response of two different secondary fluorophores (PVK should be considered as a primary fluorophore): 9,10-DPA (9,10-diphenylanthracene) and fac-Irpic (fac-bis(2-phenylpyridine)(picolinato)iridium(III). They observed a distinct photoelectric peak and the escaping X-ray peak in the pulse height spectra with 137Cs (Figure 15), 22 Na, or 57Co gamma sources. Another team has focused on a different method to include bismuth in PSs by linking it to a monomer and then polymerizing it (Scheme 9).[64] Only BiIII complexes were used and most generaly their phenyl or bulky alkoxy derivatives due to their relative stability and their low absorption in the visible spectrum. However, it is still very difficult to obtain a clear PS as radical conditions tend to degrade Bi complexes and make the scintillator yellowish (absorption around 300–400 nm). In order to understand the intrinsic influence of bismuth doping and heavy atom fluorescence quenching, a systematic study on various bismuth complexes was performed by Hamel et al. Bismuth organometallics were synthesized by using two main paths: Grignard chemistry (Li and Mg), and acid–base

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Figure 15. 137Cs Pulse height spectra showing the bismuth loading influence on scintillation efficiency and the presence of photoelectric peak. (Reproduced with permission from reference [63d]. Copyright 2012 Europhys. Lett.)

Scheme 9. Bismuth complexes linked to a methacrylate moiety.[64]

ligand exchange (Figure 16 a). These complexes embedded in a PS matrix matched commercial lead-loaded scintillator performances and are a target of choice to achieve low-energy gamma spectrometry.[65] The Figure 16 b presents the evolution of an 241Am pulse area spectra versus bismuth loading, showing a clear increase of the photoelectric peak; each spectra were recorded for 15 seconds with a 20 kBq source, demonstrating this technology readiness for field deployment. Substitution of Pb by Bi answers the concern of toxicity, which can be an industrial prospect as Bi-doped PSs are not yet commercially available. The spectral and temporal characteristics of X-ray luminescence of composites consisting of microparticles (1–20 mm) of “heavy” components (oxides, fluorides and sulfates of Zn, Cd, Cs, Ba, La, Lu, Hf, Pb, Bi) and an organic polymer binder (typically PSt/PPO/POPOP) impurities have been investigated.[66] Among the composites studied, the highest light yield was achieved for the system consisting of LaF3 with PSt activated with PPO + POPOP. No information was given about the morphology of the corresponding PS. Fluorinated scintillators A collaboration between Polish and French teams has led to a proof-of-concept of fast neutron/gamma discrimination using the energy threshold of the reactions 19F(n,a)16N or 19 F(n,p)19O.[67] Indeed, the energetic gap allows the discrimination of neutrons with E > 2.5 MeV from less energetic neutrons and gamma radiation. To reach this, PSt was replaced by poly(2,3,4,5,6-pentafluorostyrene). A very high density was observed (1.56), equal to the Pb-loaded PS cited above.[59d] Light output was estimated to be close to 3100 ph MeV 1, which is Chem. Eur. J. 2014, 20, 15660 – 15685

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Figure 16. a) Synthetic pathways towards organobismuth complexes. b) Pulse area spectra of 241Am, breaking down the loading effect on the light yield.

similar to its liquid scintillator counterpart, with a decay time of 3.0 ns, and preliminary results for n/g discrimination of a PuBe radioactive source were modest (Figure 17), as due to small dimensions of the sample, energy deposition of a highly energetic electron from beta decay of 16N and 19O is rather poor. Cross-linked polymers Some applications may present harsh working conditions, as for instance high temperature. It is known that the glass transition temperature of PSt, a principal component of PSs, is close to 100 8C and thus prevents it from being used close to this

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Figure 17. Neutron/gamma discrimination when irradiating the sample with a PuBe source.

temperature. Cross-linked polymers can improve this thermal stability. Preliminary examples were found in the Russian literature.[47] A low quantity of divinylbenzene (DVB, 0.01–1 %) was added in the scintillator preparation, affording cross-linked PS with relative light yield equal to commercial samples (97–102 % light yield of BC-404, close to 10 000 ph MeV 1). Other examples of cross-linked scintillators can be found in patent publications.[68] Stability up to 175 8C is claimed (probably under neutral atmosphere) with PSs incorporating up to 20 % of DVB, associated with p-tert-butylstyrene and usual (PBD, PPO, BBO, p-terphenyl and PPBBO, a-NPO, POPOP, preferred compounds in bold font) or unusual (p-vinylbiphenyl) fluorophores; < 1 % of DVB is also used for Gd-loaded PSs.[16a] The French Atomic Energy Commission (CEA) has extensively studied cross-linking for phoswich applications. A phoswich (i.e., “phosphor sandwich”) is a combination of two scintillators showing different responses upon irradiation, usually with a fast and a slow decay time. More specifically, the challenge was to combine two cross-linked PSs without the use of optical cement or glue, for beta/gamma discrimination.[69] Thus, the DE/E discrimination gives the access to the full energy of the b particle. So, a 150–700 mm thin, 3 ns fast PS was coupled to a 1 mm thick, 80 ns slow PS, all of them displaying scintillation wavelengths centered at 420 nm (Figure 18). This technology has been successfully applied as a proof-ofconcept for a complete contamination meter (Figure 19).

Side chain modification Besides using the carboxylic group to incorporate inorganic elements, simple chemistry can be performed to link organic dyes to the monomer. It enables the design of polymer with more or less proximity between the dyes. In some cases when linkers such as stilbene or 1,8-naphthalimide are used, delay fluorescence due to a bimolecular interaction can be observed. It has been proven that neutron give rise to more localized energy deposits enabling delayed fluorescence, so this delayed bimolecular fluorescence can be used to discriminate between neutron/gamma ionization. Delayed fluorescence gives Chem. Eur. J. 2014, 20, 15660 – 15685

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Figure 18. Principle of b/g discrimination in phoswich scintillator. Beta particles appear in the pink zone in the two-dimensional spectrum (on the bottom).

Figure 19. Results of b/g discrimination in a phoswich scintillator with a 700 mm thin layer.

a longer pulse and can be separated by means of pulse shape discrimination (PSD). Hamel et al. attempted this, with the synthesis of a 1,8-naphthalimide dye (Scheme 10 a) linked to different polymer chains,[70] including PMMA derivatives. Unfortunately, PSD was not significant. The same approach was performed with the synthesis of stilbene-substituted polymer[71] (Scheme 10b), which is a single-component PS, but the results did not show clear response. Side chain modification can also be a tool for more fundamental experiments. A study by Adadurov et al. shows that partial substitution of the methyl with a benzyl group (i.e., benzyl methacrylate) can be used to probe the formation of excimers and their impact on the scintillation yield.[72] We also note that MMA substituted with fluorescent probe was used to monitor the polymerization rate under ionizing condition,[73] but this is outside the scope of this paper. Finally, as already mentioned, the patent from Simonetti et al. concerns the use of p-tert-butylstyrene instead of styrene for polymerization of PSs.[68]

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Scheme 10. Synthesis of emitters linked to a methacrylic monomer: a) 1,8-naphthalimide dye[70] and b) stilbene dye.[71]

Sol–gels Although sol–gel materials can be considered as inorganic structures, one has to admit that they are prepared from organic molecules. Since the 2000s, identical developments have been performed, mainly by the collaboration of Dai and Wallace’s groups. They all include loading sol–gel materials with thermal neutron-sensitive elements, such as lithium, gadolinium or boron. Table 3 references the main characteristics of each scintillator. Both boron- or lithium-enriched sol–gel scintillators have been produced.[74] A gel containing enriched 10 B(OH)3 was not discernibly sensitive to thermal neutrons (originating from thermalized AmBe of 252Cf sources) at all, in contrast to lithium-loaded materials. All these scintillators were also tested under alpha irradiation. The europium-doped gadolinium oxide sol–gel was irradiated with 14 keV X-rays. Imaging of a 30 mm thick tungsten wire was possible (Figure 20).[75] The work performed by Kesanli et al. deals with the preparation of a hybrid PSt–silica nanocomposites in the presence of

Table 3. Various sol–gel scintillators with dopants, fluorophores and characteristics. Ref.

Loading elements

Fluorophores

Typical morphology

Comments

[74]

salicylic acid PPO POPOP

Ø 25.4 mm h < 1 mm

two series produced: one “standard” and one doped with PEG-400

europium complex PPO/POPOP as scintillation co*cktail

n.d. cylindrical monolith dimension n.d. Ø 25.4 mm h 100’s mm Ø 25 mm h mm n.d. 8–700 mm xerogel

high density [7.1 g cm 3] hybrid organic-inorganic material

[75] [76]

LiOH·H2O B(OH)3 10 B(OH)3 11 B(OH)3 Gd 6 Li- salicylate

[77]

various

[78]

B(OMe)3

butyl-PBD

[79] [81]

europium complex CQDs

LiOH·H2O

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Review an arene-containing alkoxide precursor through room temperature sol–gel processing, affording transparent monoliths.[76] 6 Li-Sal was used as the neutron sensitizer, with different 6Liloadings, ranging from 0.5 to 1.0 wt %. The key molecule is 3(trimethoxysilyl)propyl polystyrene, which allows efficient scintillation energy transfer while being integrated inside the sol– gel matrix. Surprisingly the fluorophores were added as a liquid scintillation co*cktail. Neutron detection results are discussed in terms of pulse height spectra, relative to commercial neutron-sensitive inorganic scintillator KG2. In their following publication, various fluorescent compositions were tested (two organic and seven inorganic), following the same lithium loadings (1.5 wt %).[77] The light emission spectra of the scintillators and their pulse shapes were measured, but no clear statement was given as to which composite scintillator would be the best. Boron-loading was performed by Koshimizu et al.[78] Boron was directly linked to the main chain through the condensation of trimethyl borate B(OMe)3. The scintillation characteristics were examined under He irradiation, and it was found that the scintillation intensity increased with the concentration of poly(ethylene glycol) (PEG). It also increased linearly with the concentration of butyl-PBD. The authors specified that approximately half of the prepared samples could be successfully fabricated into monoliths without cracking. A second use of luminescent europium cations in sol–gels has been described in the literature for the detection of g rays.[79] The behavior of the luminescence spectra of the excited states of rare earth metals indicates a strong, linear dependence with gamma radiation doses, up to 400 Gy. Similar results were also obtained for Tb-doped silica gel. Another modification of the fluorescent probe includes a 2,5-diphenyloxazolederived molecule, covalently linked to the sol–gel macrostructure by either a hydroxymethyl or a urethane linkage at the 4position.[80] Results were compared with standard PPO entrapped in the sol–gel matrix. First the PPO derivatives were compared by liquid scintillation counting, and then they were compared altogether by detecting low-energy b radiation emitted by tritium. Preparation of sol–gel scintillators afforded transparent, fracture-free colorless materials. Surface-functionalized carbon quantum dots (CQDs) entrapped in sol–gel xerogels were used for the first time by Quaranta et al.[81] CQDs were obtained from thermal decomposition of citric acid in the presence of hot [3-(2-aminoethylamino)propyl]trimethoxysilane, giving rise to materials with decay time estimated close to 4 ns and 26 % quantum yield. These materials were not tested so far for ionizing radiation detection. Although interesting, in particular for their high density suitable for g spectroscopy, all these materials suffer from a lack of possibility to prepare large volumes of scintillators. They are therefore limited to the detection of particles with low linear energy transfer.

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Miscellaneous Extrusion and molding Whereas probably 99 % of the worldwide production of PSs is performed by bulk, thermally initiated polymerization, the group of Pla-Dalmau has studied the potential of extruded PS for optical fibres (Figure 21).[57, 82] Starting from PSt or PVT pel-

Figure 21. Extruded PS fluorescing under UV inspection lamp at Fermilab for the MINERvA (Main INjector ExpeRiment Neutrino-A) project (Copyright United States Department of Energy).

lets or powders, an extension of this work includes loading with inorganic powders, such as lithium or boron, for thermal neutron detection.[57] Quality of materials and fluorophores, as well as the conditions for extrusion (air vs. neutral atmosphere) are discussed. They claim a production cost 5–9 times cheaper compared with cast scintillators, albeit with slightly reduced optical properties. Almost at the same time, the Institute of High Energy Physics in Russia reported molded and extruded PS.[47, 83] Melting was performed around 200 8C and the melt was extruded through a spinneret. Extrusion was realized with various organic dyes and (in)organic fillers. The authors even tried combining two polymers, for example, polystyrene with polyethylene, polyisobutylene, or polypropylene, giving rise to scintillators with low levels of optical transmission. Application of such extruded PS has been recently reported for measuring antiproton annihilations.[84] Composite scintillators It is known that single crystals of aromatic molecules are particularly efficient scintillators. However, their high price and the difficulty in preparing large detectors limit their use. In this context, a strategy based on grinding single crystals and incorporating them into an inert matrix (usually Sylgard) has been developed by the Institute for Scintillation Materials in Ukraine.[85] So-called composite scintillators, made from p-terphenyl or stilbene have been successfully tested for fast neutron/gamma discrimination (Figure 22).[86]

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Figure 23. Left: Photographs of the capillary arrays; Right: A partial image (8 mm 8 mm) of a capillary array illuminated by neutrons. (Reproduced with permission from reference [88b]. Copyright 2007 IEEE.)

Good light yields were observed for PSt/PPO/POPOP/o-carborane, in the range 7000–11 000 ph MeV 1. Very good resolution was observed when the scintillator was illuminated by neutrons. Almost at the same time, this technology has also been developed at the French Atomic Energy Commission for the formation of PS fibers loaded with metals.[59a, 89] The same technology to potentially image X-ray sources has been applied to Laser Mgajoule facility. Figure 22. Top: 2D plot of zero crossing time vs. energy measured with composite p-terphenyl-based scintillator (50 25 mm) under irradiation of a shielded PuBe source. (Reproduced with permission from reference [86c]. Copyright 2012 IOP Publishing.) Bottom: snapshot of one sample. (Copyright from the author.)

These composite scintillators are usually compared with their parent materials, single-crystal scintillators, in terms of decay time, light output, and other performances. Large-area, stilbene composite scintillators have recently been reached (Ø 200 mm h 20 mm),[85h] but diffusion issues seem to limit the scintillation properties of these materials while being prepared with a useful thickness. A composite scintillator made from stilbene single crystals embedded in Sylgard is currently available via Proteus Inc. (USA). A dual fast neutron/thermal neutron detector based on a phoswich-based strategy with combined stilbene and cerium-doped gadolinium silicate (Ce:GSO) composite layers has also been reported by the same institute.[87] Structured scintillators Radiation monitoring devices have introduced the possibility to create PSs embedded in a fiber optic array.[88] Already known for fibers filled with liquid scintillators, the challenge herein proposed is to be able to polymerize liquid monomers inside the capillary while preserving optical transmission. Thus, a 36 cm2 hexagonal prism made from Ø 100 mm capillaries is sunk into the monomer and polymerization is processed directly into the capillaries (Figure 23). Some methods include loading with o-carborane for thermal neutron detection.[88c] Chem. Eur. J. 2014, 20, 15660 – 15685

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Microspheres and polymer dots Commercial PS microspheres are available from Saint-Gobain or Detec-Rad, but they can also be prepared by evaporation/ extraction process. Typically an organic phase containing the polymer and the dyes is slowly added to a vigorously stirred aqueous solution. The organic phase is slowly evaporated and the polymer spheres are then filtered. Depending on the polymer concentration, spheres from few nanometers (polymer dots) to micrometer (microspheres) scale can be prepared. Such polymers appear to be an alternative to liquid scintillation for the quantification of alpha and beta emitters because it does not generate mixed wastes after the measurement.[90] Scintillation properties of microspheres prepared from PSt doped with various classical fluorophores—for example, p-terphenyl, PPO, POPOP, bis-MSB and naphthalene—with a typical diameter of about 130 mm were investigated. Detection efficiency values obtained with these synthesized microspheres for 3 H, 14C, 90Sr/90Y, and 241Am sources are better than those obtained using commercial plastic scintillation microspheres. Osakada et al. prepared water-soluble polymer dots (26– 35 nm diameter) doped with an iridium complex in order to achieve X-ray computed tomography.[91] A poly(vinylcarbazole)mixed PSt graft of ethylene oxide functionalized with a carboxylic end group (to enhance the water solubility of the polymer dots) and an iridium complex dye were prepared as described above. Under X-ray radiation, the iridium polymer dots dissolved in water are five times more luminescent than iridium complex in THF or non-doped polymer dots in water.

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Review Poly(ether sulfone) The same group who extensively studied PEN and PET polyesters, recently reported the first example of poly(ether sulfone) as scintillation material.[92] Again, the polymer scintillates by itself. An extremely high refractive index of 1.74 and a density of 1.37 g cm 3 are shown (1.58 and 1.04 g cm 3, respectively, for PSt). Scintillation yield was in the range 3000– 5000 ph MeV 1. Despite its amber color, the material is able to fluoresce at 350 nm. In contrast to PET and PEN, a scintillator of significant size could be prepared, with dimensions 31 31 5 mm3 (Figure 24). This material was tested with alpha radionuclides such as 241Am and 252Cf.

Figure 24. A 31 31 5 mm poly(ether sulfone) (PES) plate. PES is an amber transparent resin (top) and is also shown under ultra-violet light (bottom). (Reproduced with permission from reference [92]. Copyright 2013 Elsevier B.V.)

Modification of the Luminescent Probe Quantum dots Quantum dots (QD) offer numerous advantages such as tunable emission wavelength, fast response time, thermal, and chemical stability, and thus could be used as fluorophores for radiation detection. Nevertheless, the incorporation of QDs in a polymer matrix is challenging, as the QD aggregation must be avoided in order to prevent luminescence self-quenching. Chem. Eur. J. 2014, 20, 15660 – 15685

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Several strategies such as chemical modification of the QD’s surface or incorporation in a prepolymer have been developed. Vancso et al. have published a review[93] dealing with the design of QD-polymer hybrid material and this aspect will not be discussed herein. The first example involving the utilization of QDs for radiation detection was published by Ltant and Wang in 2006.[94] Nanoporous glass was impregnated with a CdSe-ZnS QD solution. The obtained QD–glass hybrid material offers an excellent energy resolution (2 % at 59 keV) compared with standard inorganic NaI(Tl) (6–7 % at 662 keV), but the acquisition time required is very long due to the low amount of photons emitted by the system. A Rhodamine B based scintillator shows a scintillation output significantly higher than its QD counterpart. These results have been attributed by the authors to the larger Stokes shift of rhodamine compared to cadmium QDs. QDs usually suffer from small Stokes shift (20 to 40 nm) leading to photon reabsorption and contributing to lower the quantum efficiency of the scintillators. Even though nanoporous QD– glass scintillators are easily accessible, these materials have major drawbacks. Energy transfer from the glass to the QDs is not an efficient process and thus high QD concentration is required in order to observe radioluminescence. Furthermore the preparation of large-scale nanoporous glass is rather complicated. For these reasons, QD–polymer scintillators are a better alternative. Rogers et al.[95] described the preparation of transparent PMMA scintillators loaded with 0.5 wt % CdTe QDs. The authors evaluated different QD sizes, emitting from 520 to 600 nm and different polymer volumes, geometries and thicknesses, but only observed very low count rates (8 to 10 counts per minute with a 241Am source). They also attributed this lack of efficiency to small Stokes shift of the QDs and the small quantum efficiency of the photomultiplier at the QD emission wavelength. In order to prevent self-absorption, QDs can be used not as emitters but as antenna. Campbell and Crone[96] have published the first example of radiation detection using QDs in a polymer matrix. CdSe/ZnSe QDs were surface-passivated with hexadecylamine and incorporated in the semiconductor polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV). The polymer was dissolved in chloroform with a QD volume fraction from 0 to 21 % and was spin casted on sapphire substrate. The absorption and photoluminescence (PL) spectrum (Figure 25 a) of the polymer are slightly affected by the QD incorporation in polymer matrix. QDs have higher ionization energy than the polymer; radiation thus mainly produces excitation of the inorganic QDs that subsequently transfer their energy to the organic polymer matrix by means of Fçrster energy transfer. The cathodoluminescence (CL) was measured using 3 keV electrons at a current of 30 pA and normalized to the pure MEH-PPV emission (Figure 25 b). The CL increased linearly with the QD concentration for volume fraction between 0 and 15 %, but it dramatically decreased for the 21 % QD-doped film. Optimum CL intensity was expected by the authors for a QD volume fraction of 40 %, but phase separation was observed by scanning electron mi-

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Review Table 4. Composition of the PSt PPO/QD-based scintillators.

Figure 25. a) Absorption and PL spectrum of MEH-PPV film (solid) and MEHPPV QD 0.15 doped film (dashed). b) CL intensity as a function of QD volume fraction. (Reproduced with permission from reference [96]. Copyright 2006 Wiley-VCH.)

croscopy (SEM) for a QD volume fraction higher than 15 %, responsible for the CL decrease. Polymers doped with a primary fluorophore have been investigated in order to enhance the light output of QD-based PSs.[97] Optimal radioluminescence intensity was found for a PSt matrix loaded with 0.4 wt % of PPO and 0.1 wt % of CdSe QDs. QDs are good candidates for the enhancement of the spatial resolution of X-ray detection. Water-soluble CdTe QDs[98] capped with mercaptopropionic acid (MPA) were dispersed in polyvinyl alcohol (PVA) and thin films were spin-coated on a glass substrate. It was possible to obtain a transparent 30 mm film with 2 wt % of QDs. Transparency decreased when the thickness of the film or the amount of QDs increased, owing to segregation of the QDs. Transparent bulk samples of PMMA doped with 0.5 wt % QDs with a thickness between 10 and 20 mm were prepared. Hybrid PMMA/QD polymers were tested for X-ray detection (Figure 26), and showed better spatial resolution (5 lines per mm) than the commercial Gd2O2S/Tb screen (2.8 lines per mm). The possibility to load PSt/CdSe scintillators with the commonly used PPO was studied by Lawrence et al.[99] The authors have described the preparation of PSt loaded with PPO and/or

Figure 26. a) X-ray image of CdTe and CdSe QD solution samples in containers (left: CdTe; right: CdSe). b)–d) X-ray imaging resolution study on a CdTe/ PVA nanocomposite film; a 0.1 mm thick Pb mask was used; b) area with resolution of 1.8–3.1 lines per mm; c) low resolution area with 0.6 lines per mm; d) enlarged image of higher resolution area with 4.3–5.0 lines per mm. (Reproduced with permission from reference [98]. Copyright 2011 American Institute of Physics.) Chem. Eur. J. 2014, 20, 15660 – 15685

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Sample

PPO [%]

CdSe QDs [%]

X-ray induced fluorescence intensity [a.u.]

PS PS/PPO PS/QD PS/PPO/QD

– 0.2 – 0.2

– – 0.2 0.2

5 4 7 23

QS (Table 4). Under X-ray radiation, no significant enhancement of the scintillation was observed when the PSt was doped with PPO or QDs. However, the scintillation intensity was four times higher when both PPO and QD were included in the polymer matrix. The same trend was observed by the authors for 241Am alpha-induced scintillation. Organometallic complexes Ionization of a polymer matrix statistically leads to the formation of 25 % of singlet excitons and 75 % of triplet excitons.[100] Only the singlet excitons are collected in standard organic scintillators. Organometallic phosphorescent complexes are very promising chromophores for scintillation applications as the heavy-atom-induced spin-orbit coupling allows them to collect both singlet and triplet excitons and convert both of them into photons. The utilization of iridium complexes in OLEDs already allows the preparation of systems with quantum efficiencies close to 100 %. Campbell and Crone[101] published examples of 5 mm drop-casted PVT and PVK films doped with ([Ir(mppy)3]; mppy = 2-(p-tolyl)pyridinato) complex. The amount of the iridium complex in the films varied from 1 to 35 wt % in PVT and from 0.04 to 10 wt % in PVK. The scintillation response of these films as a function of wavelength (Figure 27 a) were obtained by the excitation of the samples with a 10 keV electron gun. A mono-exponential decay of approximately 850 ns is observed for both matrices. The spectrally integrated scintillations yield were measured using EJ-232 (8400 ph MeV 1) PS as a reference. Figure 27 b shows the estimated scintillation yield as a function of [Ir(mppy)3] wt % in plastic, the scintillation yield is enhanced as the amount of complex increases in the plastic. The authors claim a scintillation yield of 30 000 ph MeV 1 for an [Ir(mppy)3] content of 20 wt % in PVT and 4 wt % in PVK. The scintillation is assumed to be more efficient in PVK matrix due to a better energy matching between the polymer and the organometallic fluorophore. Cherepy et al.[63] described a system composed of a PVK matrix incorporating an iridium complex ([Ir(ppy)2(pic)]; ppy = 2-phenylpyridine ; pic = picolinate) or 9,10-diphenylanthracene (DPA) and doped with high-Z triphenyl bismuth (BiPh3). Most of the bismuth-loaded scintillators are made of PVT and incorporation of a high amount of heavy atoms mainly leads to a decrease of the scintillators light yield. However, PVK has a low excited state energy and its use prevents quenching of the light yield induced by BiPh3. When the amount of BiPh3 increases from 0 to 40 % the beta radioluminescence decreases for DPA-based scintillators and

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Review Table 5. Composition and scintillation properties of the samples.

Figure 27. a) Scintillation response as a function of wavelength for PVT (upper panel) and PVK (lower panel) incorporating the indicated weight percentages of [Ir(mppy)3]; b) Integrated scintillation yield for PVT (open circles) and PVK (solid squares) plastics as a function of [Ir(mppy)3] wt % (bottom axis) and Ir wt % (top axis). (Reproduced with permission from reference [101]. Copyright 2007 American Institute of Physics.)

slightly increases for the [Ir(ppy)2(pic)]-containing scintillators. This behavior can be explained by an augmentation of the triplet population induced by the bismuth. Gamma-ray spectra were acquired with 137Cs and 241Am, were fit with a spectrum simulator and scintillation yields were determined by comparison with EJ-208 Compton edge (Table 5). The best scintillation yield was achieved by the 3 % [Ir(ppy)2(pic)]/40 % BiPh3 scintillator; however, the results are still lower than commercial EJ-208 PS. A better energy resolution and 25-fold increase of the photoelectric peak height is observed for the bismuth-loaded samples. Another advantage of organometallic-based PS is that they can achieve detection of fast neutrons and gamma rays by Chem. Eur. J. 2014, 20, 15660 – 15685

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Sample Matrix Fluorophore

BiPh3 Relative gamma yield at 662 keV

Resolution FWHM [%] at 662 keV

1a 2a

PVK PVK

40 % 0.66 40 % 0.78

9 6.8

PVK

–

0.73

EJ-208

PVT

–

3 % DPA 3 % [Ir(ppy)2(pic)] 3 % [Ir(ppy)2(pic)] unknown

PSD. Signals for gamma rays can be attributed to the fluorescence of the PSs (arising for the singlet state of the fluorophore) and the fast neutron response is assigned to delayed emission caused by triplet–triplet annihilation (TTA). An example of PSD achieved with an iridium complex in a polymeric matrix was described by Feng et al.[102] The authors prepared a PVT polymer doped with the 0.1–0.2 wt % iridium complex [Ir(ppy-F2)2(F2-pic)] (ppy-F2 = 2-(4,6-diphenyl)pyridine ; F2-pic = 3,5-difluoropicolinate). When ionized by an AmBe source, the scintillator shows a bi-exponential decay with a prompt and a delayed signal (assigned to the gamma and the fast neutrons response, respectively). In order to access the PSD efficiency of the iridium doped scintillator, measurements of the PSD figure of merit (FOM, the separation between the neutron and the gamma events divided by the sum of the FWHM values for the distributions) were performed. The PVT/Ir scintillators have a PSD capability (FOM = 1.4) lower than the commercial liquid scintillator EJ-301 (FOM = 2.1). The scintillator also has a low gamma rejection ratio—when the trigger level is set to an energy threshold of 400 keVee, 98 % of the fast neutrons are detected. Using EJ-204 PS as reference (10 000 ph MeV 1), the light yield of this system was estimated at 7 400 ph MeV 1. Polyvinylcarbazole (PVK) was doped with 0.025 to 0.2 wt % of [Ir(ppy)2(acac)] (acac = acetyl acetonate) and the CL of the different samples (Figure 28 a) shows an augmentation of the iridium-centered luminescence at 515 nm and only a small decrease of the PVK emission at 420 nm when the iridium concentration is increased. In the same paper, the authors propose a hitherto unseen feature: spectral shape discrimination (SSD). Samples were irradiated with 20 keV electron beam to simulate the scattered electron generated gamma ionization and with 2 MeV proton beam to simulate the recoil proton generated by fast neutron ionization (Figure 28 b). In the case of gamma ionization, both luminescence of the matrix host (PVK) and the guest (iridium complexes) can be observed, whereas in the case of fast neutron ionization, only the iridium-centered luminescence is observed. On the same basis, the group of Adadurov reported the use of two different fluorophores in PS, one for collecting singlet states [1,4-dimethyl-9,10-diphenylanthracene, with addition of the wavelength shifter 1-phenyl-5-(4-methoxyphenyl)-3-(1,8naphthoenyl-1’,2’-benzimidazole)-2-pyrazoline] and [Eu(dbm)3(phen)] (dbm = dibenzoylmethane; phen = 1,10-phenanthro-

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Scheme 11. Synthetic procedure and ligand used for the preparation of the iridium(III) complexes.[104]

Figure 28. a) Unnormalized CL spectra for PVK samples doped with 0.025 % (black), 0.05 % (red), and 0.20 wt % (blue) of [Ir(ppy)2(acac)]. b) Steady-state CL and proton radioluminescence spectra for PVK doped with 0.2 wt % of [Ir(ppy)2(acac)]. The spectra are normalized to the maximum intensity peak at 515 nm to highlight the particle-specific response. (Reproduced with permission from reference [102b]. Copyright 2012 IEEE.)

line) for triplet states.[103] So herein TTA is not used for n/g discrimination, but the large decay time difference between the two dyes is exploited. Regarding the application of fast neutron/gamma discrimination, the properties are described in the Table 7. Hamel et al.[104] incorporated iridium complexes in PSt-based cross-linked copolymer instead of the poly(vinylcarbazole) matrix commonly used in the examples cited above. Samples with 29 different iridium complexes were thus prepared (Scheme 11). Theses samples exhibited modest scintillation yields (in the range 400–1500 ph MeV 1) due to the low loading of iridium complexes within the matrix (0.02–0.05 wt %) and an unoptimized Fçrster energy transfer. Higher amounts of complexes did not improve scintillation yields; this behavior could be explained by the low solubility of most of the organometallic complexes in the monomers and high absorption of the samples when increasing organometallic concentration. Scintillators doped with [Ir(piq)2(acac)] (piq = phenylisoquinoline) showed unexpected thermoluminescence (Figure 29). Whereas most of the work is focused on the incorporation of iridium complex in a polymer matrix, Adadurov et al. published a study on the preparation of europium-based scintillators.[105] Thin PSt films (50 to 150 mm) loaded with the europiChem. Eur. J. 2014, 20, 15660 – 15685

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Figure 29. Cross-linked scintillators with increasing loading of [Ir(piq)2(acac)] (0–0.1 wt %).

um complex [Eu(dbm)3(phen)] in the range 0.5 to 4 wt % were prepared. Under alpha radiation, the scintillation yield increases with the increase in europium concentration, reaching a maximum of 5650 ph MeV 1 for the 4 wt % film. Scintillation yield of film incorporating other europium complexes[105b] ([Eu(bza)3(phen)] (bza = benzoylacetonate), [Eu(bpa)3(phen)] (bpa = biphenoylacetonate), and [Eu(acac)3(phen)]; Scheme 12) showed lower values (91, 85 and 2 %, respectively) with respect to the [Eu(dbm)3(phen)] emission intensity. Unfortunately the utilization of these complexes in PSs requires long integration time (ca. 0.25 s) because europium has a very long emission lifetime (ca. 500 ms) compared to classical organic fluorophores (few nanoseconds). Europium and terbium complexes were also claimed as efficient fluorescent probes for scintillation in a patent.[106] The global structure of EuIII complexes is made from di-ketones and phosphine oxide. These compounds, embedded in a polymer matrix, such as polystyrene or polysiloxane, seemed to be sensitive to tritium.

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Review

Scheme 12. Europium complexes studied.

Ionic liquids This new family of scintillators is close to the border, since it could be seen more as a composite, fully organic scintillator. The technology provides a single molecule that is able to act as both the matrix and the fluorophore. Chemical engineering can offer materials with finely tuned properties, such as liquid/ solid, glass transition, emission wavelength, light yield, and so forth. Thus, a collaboration of physicists and chemists from Strasbourg claimed the use of a fluorescent molecule, usually a diphenyloxazole, biphenyl, carbazole, fluorene, or anthracene derivatives, linked to an ionic liquid moiety (an imidazolium group).[107] Typical anions Y counterbalancing the imidazolium salt are Br , PF6 C12H25OSO3 , C16H33OSO3 and (CF3SO2)2N . A global structure of PPO-functionalized materials is shown in Figure 30. It was demonstrated that spectroscopic properties are governed by the oxazole group, with e values in the range 60– 95 % that of PPO, and emission spectra centered around 370 nm. Scintillation decay times are in the range 1.3–1.8 ns under 2 MeV proton irradiation, and slight differences occur when the anion is changed. Used as such, this system is able to perform fast neutron/gamma discrimination when exposed to an AmBe radioactive source. Metal–Organic Frameworks Metal–organic frameworks (MOF) are crystalline periodic structures obtained by the combination of a metal ion or a cluster coordinated by a rigid polydentate organic molecule. Allendorf et al.[108, 109] were the first team to study the radioluminescent properties of these materials. Organic scintillators carboxylate derivatives (Scheme 13), such as 2,6-naphthalenedicarboxylic acid (NDC), benzene-1,3,5-trisbenzoate (BTB), 5,5’-(naphthalene-2,6-diyl)diisophthalic acid (DPNTC), 5,5’-(anthracene-9,10diyl)diisophthalic acid (DPATC), or stilbene dicarboxylic acid (SDC) were used as organic linkers. Chem. Eur. J. 2014, 20, 15660 – 15685

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Figure 30. Top: general structure of ionic liquids allowing PSD between fast neutrons and gamma. Middle: scintillation decay profiles of OxImC16-PF6 (x = 16; Y = PF6) under gamma (blue), 2 MeV proton (red) and 2 MeV alpha (green) excitation. Bottom: example of a PSD biparametric spectrum obtained from the same compound. (Reproduced with permission from reference [107b]. Copyright 2011 from the Author.)

Cathodoluminescence (Table 6) was performed by IBIL irradiation of a single crystal and the emission intensity was compared to anthracene or trans-stilbene emission. The CL intensity is on the same order of magnitude or even greater than the anthracene and trans-stilbene references (9 to 124 % of the CL intensity). The MOF CL emission decay can be fitted with a bi-exponential decay composed of a short component (tCL) originating from the linker fluorescence and a rather long component caused by TTA. This observation is promising as MOF materials could be able to perform PSD. Furthermore, MOFs are extremely radiation resistant compared to anthracene and transstilbene crystals used as reference; indeed when irradiated no new emissive species and no deformation of the crystal structure was observed. Based on this preliminary work, the authors have determined a few parameters (prevent the utilization of breathing or interpenetrating MOFs, favored rigid organic linkers, etc.) that could lead to the preparation of more efficient radioluminescent MOFs.

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Review Table 6. Luminescence properties of MOFs. Linker

PLex [nm]

PLem [nm]

CLem [nm]

Stokes Shift [nm]

tPL [ns]

tcL [ns]

Relative CL intensity

NDC/BTB NDC NDC

364 361 365

381 380 384

390 425 440

26 64 75

15 11 8

0.79[a] 1.24[a] 0.55[a]

DPNTC

353

381

410

0.64[a]

DPATC

397

438

475

0.39[a]

SDC SDC

375 410

390 441

474 449

99 39

15 (100 %) 11 (100 %) 11 (98 %) 1 (2 %) 11 (92 %) 4 (8 %) 1 (86 %) 514 (14%) n.d. n.d.

n.d. n.d.

0.09[b] 0.22[b]

[a] Relative to trans-stilbene crystals. [b] Relative to anthracene crystals.

the creation rate of triplets should increase and therefore their TTA could occur efficiently for PSD to be achieved. This assumption was successfully demonstrated with small, laboratory-scale radioactive sources such as 252Cf or AmBe. A formula-derived PS from Zaitseva’s synthesis is currently sold by Eljen Technology under the trade name EJ-299-33.[113] Currently, numerous research teams use this scintillator as a benchmark under various experimental conditions.[114] However, many investigations are still underway, since not all fluorophores, even at high concentration, are able to attain “suitable” triplet states’ vicinity for high TTA rates, and the commercial scintillator seems to display instability due to nucleation of PPO and morphology deformations (Figure 31).

Scheme 13. Organic scintillators carboxylate derivatives used to build the MOFs.

Nevertheless, this method still presents some limitations, a lot of synthetic MOFs are unstable under ambient conditions and crystals of a bigger size (few cm3) are required for counting applications.

High concentration Another important breakthrough is not really a modification of the fluorophore inside the PS, but rather addition to very high concentration. It was assumed for a long time that PSs are not able to perform PSD between fast neutrons and gamma. Probably based on the previous work of Brooks,[110] two groups noticed that the TTA, which is at the genesis of n/g PSD, was not probable enough in plastics compared with liquids.[111, 112] Thus, by increasing drastically the loading of primary fluorophore, Chem. Eur. J. 2014, 20, 15660 – 15685

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Figure 31. Pictures of NE-150 (left) and EJ-299-33 (right) after whitening and/or dishing of the morphology. (Copyright F. Brooks for left picture. Copyright CEA for right picture.)

Very recently Zaitseva et al. combined lithium[115] or boron[116] loading with a primary fluorophore at high concentration to perform thermal neutron/fast neutron/gamma triple discrimination. This was made possible by adding the lithium salt of 3phenylsalicylic acid (range 5–10 % to the total weight of the material, either natural or isotopically enriched to 6 Li, Figure 32) or carborane to their previous PPO-loaded PS. As one can see, neutron/gamma discrimination is an extremely hot topic and numerous laboratories try to develop their own technology. The following Table gives an overview of the most relevant results. As one can see, there are two main strategies, namely adding the first fluorophore at a high concentration and triplet harvesting organometallics. Both present advantages and

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Review Table 7. Different strategies and results for neutron/gamma discrimination in PSs. Ref.[a]

Strategy

Biggest size [Ø, h]

FOM (at a given energy in [keVee])

Decay Scintillation yield [ph MeV 1] time [ns]

[85c]

stilbene single crystals in silicone

200 mm 20 mm

1.00 (500)

n.d.

4.5 410 (assumption) (assumption)

252

[86b]

p-T or stilbene single crystals in Sylgard

50 mm 50 mm

1.41 (p-T, 600) 1.19 (stilbene, 600)

9900 (p-T) 5700 (stilbene)

n. d.

420 (p-T); 395 (stilbene)

10 cm diffusion issues Pb-shielded which lowers 500 mCi PuBe performances for big samples

[110]

1st fluo highly concentrated

25 mm 25 mm

n.d.

n. d.

420 (assumption)

PoBe

ageing and whitening

[111e]

1st fluo highly concentrated

103 mm 114 mm 1.25 (305 mm, 200)

3400–4500

420

AmBe

low scintillation yield; large volume available

[112b]

1st fluo highly concentrated

50 mm 50 mm

n.d.

430

5.1 cm Pb-shielded 252 Cf

precursor of EJ-299-33 (see line below)

[113]

EJ-299-33

127 mm 150 mm –

8600

n.d.

420

–

from commercial brochure

[114c]

EJ-299-33

50 mm 50 mm

0.8 (200)

8600

n.d.

420

252

[11]

1st fluo highly concentrated

100 cm3

1.6 (500)

up to 13 000 (sizedependent)

6.0–9.5 ns ([PPO]dependent)

385–440 ([PPO]dependent)

AmBe

similar to [112b]

[102b] organometallics 25 mm 15 mm

1.4 (400)

7300

800

515

AmBe

elevated cost?

[103b] organometallics 16 mm 10 mm

1.37 (250)

n.d.

370 000

590–620

1 cm Pb-shielded PuBe

independent from TTA

[107]

ionic liquids

micrometers

n.d.

< 50 380 (assumption) (assumption)

AmBe

currently limited to small sizes only

[117d]

polysiloxanes

30 mm 10 mm

n.d.

4000

n.d.

610

n.d.

–

[119]

n.d.

390 cm3

2.25 (1000)

13 000

< 10

440

AmBe

FOM dependent of the electronic system used for PSD

3.31 n.d. (2525 mm, 480)

Emission Source wavelength [nm] Cf

Comments

–

[a] Some references refer to the first published document, whereas some others refer to benchmark of scintillators. Reference [110] is added for better comparison.

drawbacks, but for example Eljen Technology decided to industrialize the high concentration strategy. All the reported FOMs are in the same range, close to 1.3. However, it is still difficult to conclude which product is the best from the multiple setup experiments described, especially in the nature and a possible shielding of the n/g radioactive source. Usually the scintillation yields are lower than standard PSs, such as BC-400 (SaintGobain), EJ-200 (Eljen Technology), or SP32 (Envinet a. s.), which are known to deliver about 10 000 ph MeV 1. CEA succeeded in preparing a large PS with dimensions > 100 mm (Figure 33). As expected, discrimination properties are lower with such size. Chem. Eur. J. 2014, 20, 15660 – 15685

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However some points for investigation remain and the race is not over.[117] Photophysics will probably be the final tool to tune the best available PSs for PSD.[111f] As an example, Blanc et al. were able to simulate either fast neutron or gamma interaction with a PS by using a femtosecond laser. TTA was achieved when high power densities were deposited inside the matrix of the scintillator. Experiments conducted on liquid sample BC-501A, commercially available NE-102, and labproduced neutron/gamma discriminating PSs confirmed these results. In the long search (over 50 years) to find a suitable substitute of 3He in neutron detection portal monitors, this example

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Review occurred in the 1980s and why the 2000s are so interesting. It is worth noting that the breakthrough initiated with EJ-299-33, a neutron/gamma discriminating PS developed at LLNL, will incite global companies to fund chemists to find scintillators of the future. An easy comparison could be performed with LED and Si-based photovoltaic systems, which are slowly but surely substituted by OLEDs and organic-photovoltaic systems. It is thus evident that new composite materials will replace standard scintillators. It is just a question of time. A combination of various methodologies should also give access to new materials, for example, the first use of 3D printing techniques for the manufacture of PSs.[118] Among all the examples cited in this text, emerging solutions for the replacement of 3He (neutron detectors) seem a good goal for international laboratories. Gamma spectrometry is also one of the highest challenge chemists have to achieve in the near future. Many possibilities exist also for detection on the small scale, that is, mainly for alpha and beta detection. Polyimides, sol–gels, and other highly-absorbing or fragile materials can be really useful for this purpose.

Acknowledgements

Figure 32. 252Cf PSD patterns obtained with Li-loaded PSD plastics: a) 5 % of Nat Li-3-PSA (containing 0.01 % of natural abundance 6Li), 9 mm HDPE moderation; b) 5 % of 6Li-3-PSA with 9 mm HDPE moderation. (Reproduced with permission from reference [115b]. Copyright 2013 Elsevier B.V.)

We thank the French Direction Gnrale de l’Armement and Secrtariat Gnral de la Dfense et de la Scurit Nationale for the grant to G.H.V.B. We also thank the Agence Nationale de la Recherche for the grant to F.S. Keywords: dyes · fluorescence polymers · radiation detector

of recent development of PSs is certainly the most impressive, coming from the strong competition in advances in the field over the last three years.

Conclusion It is really surprising to admit that > 90 % of commercial PSs available from worldwide suppliers, such as Saint–Gobain, Eljen Technology, or Envinet have been developed probably 30– 40 years ago. This is probably why almost no developments Chem. Eur. J. 2014, 20, 15660 – 15685

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plastic

scintillators

[1] http://www.iaea.org/newscenter/news/2013/mexicoradsource4.html (last access Jan 21st, 2014). [2] M. G. Schorr, F. L. Torney, Phys. Rev. 1950, 80, 474. [3] D. C. Stromswold, E. R. Siciliano, J. E. Schweppe, J. H. Ely, B. D. Milbrath, R. T. Kouzes, B. D. Geelhood, IEEE Nucl. Sci. Symp. Conf. Rec. 2003, 2, 1065 – 1069. [4] F. D. Brooks, Nucl. Instrum. Methods 1979, 162, 477 – 505. [5] S. W. Moser, W. F. Harder, C. R. Hurlbut, M. R. Kusner, Radiat. Phys. Chem. 1993, 41, 31 – 36. [6] V. N. Salimgareeva, S. V. Kolesov, Instrum. Exp. Tech. 2005, 48, 273 – 282. [7] J. S. Aspler, C. E. Hoyle, J. E. Guillet, Macromolecules 1978, 11, 925929. [8] H. P Kallmann, .M. Furst, F. H. Brown, American Patent Application US3068178, 1962. [9] a) V. N. Salimgareeva, R. M. Polevoi, V. A. Ponomareva, N. S. Sannikova, S. V. Kolesov, G. V. Leplyanin, Russ. J. Appl. Chem. 2003, 76, 1655 – 1658; b) V. N. Salimgareeva, R. M Polevoi, S. V. Kolesov, S. S. Ostakhov, V. A. Ponomareva, G. V. Leplyanin, Russ. J. Appl. Chem. 2003, 76, 804-808. [10] T. Inagaki, R. Takashima, Nucl. Instrum. Methods 1982, 201, 511 – 517. [11] E. V. van Loef, G. Markosyan, U. Shirwadkar, K. S. Shah, IEEE Trans. Nucl. Sci. 2014, 61, 467 – 471. [12] P. Rebourgeard, F. Rondeaux, J. P. Baton, G. Besnard, H. Blumenfeld, M. Bourdinaud, J. Calvet, J.-C. Cavan, R. Chipaux, A. Giganon, J. Heitzmann, C. Jeanney, P. Micolon, M. Neveu, T. Pedrol, D. Pierrepont, J.-C. Thvenin, Nucl. Instrum. Methods Phys. Res. Sect. A 1999, 427, 543 – 567. [13] a) M. Bowen, S. Majewski, D. Pettey, J. Walker, R. Wojcik, K. Zorn, IEEE Trans. Nucl. Sci. 1989, 36, 562 – 566; b) V. M. Feygelman, .J. K. Walker, J. P. Harmon, Nucl. Instrum. Methods Phys. Res. Sect. A 1990, 295, 94 – 98. [14] G. Deshpande, M. E. Rezac, Polym. Degrad. Stab. 2002, 76, 17 – 24.

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