- Email: [email protected]
Preparation and characterization of highly lead-loaded red plastic scintillators under low energy x-rays
Nuclear Instruments and Methods in Physics Research A 660 (2011) 57–63
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Preparation and characterization of highly lead-loaded red plastic scintillators under low energy x-rays Matthieu Hamel a,n, Gre´gory Turk c, Adrien Rousseau b, Ste´phane Darbon b, Charles Reverdin b, Ste´phane Normand a ´ lectroniques, F-91191 Gif-sur-Yvette Cedex, France CEA, LIST, Laboratoire Capteurs et Architectures E CEA, DAM, DIF, F-91297 Arpajon, France c LCPMR, UPMC, CNRS UMR 7614, 11 rue Pierre et Marie Curie, F-75231 PARIS Cedex 5, France a
b
a r t i c l e i n f o
abstract
Article history: Received 22 July 2011 Received in revised form 31 August 2011 Accepted 31 August 2011 Available online 7 September 2011
To the aim of development of a spatially resolved x-ray imaging system intended for Inertial Confinement Fusion (ICF) experiments at the Laser Me´ga Joule (LMJ) facility, new plastic scintillators have been designed. The main characteristics are the following: fast decay time, red emission and good x-rays photoelectric absorption in the range 10–40 keV. These scintillators are synthesized by copolymerization of different monomers with an organometallic compound. In this matrix two fluorescent compounds are embedded, allowing to shift the energy from the UV to the near IR spectrum. Several parameters were studied: fluorophores concentration, nature of the secondary fluorophore and lead concentration. An outstanding effective atomic number of 53 has been reached, for a loading of lead corresponding to 29 wt%. Thus, small cylinders were prepared and their performances under x-ray beam studied and compared with those of inorganic Cerium-doped Yttrium Aluminum Garnet reference scintillator (Y3Al5O12:Ce3 þ ). Eventually, such new scintillators or their next generation could replace expensive and brittle inorganic scintillators, inducing a strong industrial potential. & 2011 Elsevier B.V. All rights reserved.
Keywords: Plastic scintillator X-rays Red fluorescence Fast decay time Lead loading
1. Introduction The development of spatially resolved x-ray imaging system within 10–40 keV has to take into account hard radiative environment induced by ICF in the LMJ experiment chamber. Indeed, the image acquisition is difficult due to highly energetic particle and beaming emission resulting directly or indirectly from deuterium–tritium fusion reaction. This can destroy equipment located close to the experiment chamber [1]. After 100 ns, the dose rate of neutron and gamma rays (created by neutron interactions with the chamber environment and structure) reaches instantaneously 108 rad(equiv.Si).s 1 and generates a high noise level on the image. Despite the high dose rate, the integrated dose is only 2rad (equiv.Si). Hence any x-ray imaging system in those conditions has to be as less vulnerable as possible. To this aim, the perfect scintillator (whether organic or inorganic) should display the following requirements: a fast characteristic decay time (below more than 99% of the generated light emitted in less than 50 ns), a scintillation wavelength shifted as far as possible to the red wavelengths so as to eliminate Cherenkov blue light by means of optical filtering and a good
n
Corresponding author. Tel.: þ33 1 69 08 33 25; fax: þ 33 1 69 08 60 30. E-mail address: [email protected] (M. Hamel).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.08.062
photoelectric absorption of 10–40 keV x-rays. So far, the best choice should be YAG:Ce single crystal, which is known for its excellent properties towards x-rays imaging properties but suffers from a temperature dependent response [2], a peak scintillation wavelength located at 550 nm and a decay time too long (exponential characteristic decay time of 70 ns if single crystal [3] and 130 ns if polycrystalline [4]) for our applications. Lutetium Sulfide scintillator Lu2S3:Ce seemed to gather all the required scintillating properties (28.000 photons per MeV, maximum scintillation peak at 592 nm and decay time of 32 ns) but the authors were limited by the small volume of the single crystal, which was approximately 1 mm3 [5]. Also of interest, common organic scintillators (such as BC-400) are not absorbent enough in the range 10–40 keV x-ray energies due to their low photoelectric cross-section in this range. The photoelectric effect probability Ppe is estimated by rule of thumb proportional to a power of effective atomic number Z, roughly between Z4 and Z5. For non-relativistic x-ray photons (E o511 keV), we can use the following approximation: Ppe pZ 4:35 =E3x
ð1Þ
Common plastic scintillators have a low effective atomic number (ca. 5) compared to inorganic scintillators (ca. 30–40). The choice of high effective atomic number components is of prime importance since we want to avoid x-ray absorption within
58
M. Hamel et al. / Nuclear Instruments and Methods in Physics Research A 660 (2011) 57–63
10–40 keV by the Compton effect and to favor photoelectric effect. Indeed Compton absorption scatters impinging x-rays, resulting in increased absorption thickness, which degrades spatial resolution of the whole imaging system. Therefore a loading of plastic scintillators with heavy elements could increase the photoelectric cross–section in our range of interest and could decrease the thickness of scintillators to absorb sufficient amount of x-ray energy to acquire a resolved x-ray image of ICF target. Since the pioneering work of Pichat et al. [6] dealing with the addition of heavy metals in plastic scintillators, most of the research in this field has been produced mainly in the 1950s and the 1960s [7]. Since then, only a few publications dealt with this issue [8]. Commercial lead-loaded plastic scintillators cannot satisfy all the LMJ requirements already explained before: EJ-256 from Eljen Technology displays a blue wavelength and a loading ranging from 1 to 5%. It seems that loading up to 10% should be possible, but the manufacturer does not recommend it. Bicron BC-452 is available either with 2, 5 or 10 wt% Pb-loading but its emission peak is also centered around 420 nm. RP-452 from Rexon is produced at 5 wt% loading. Amcrys-H is able to reach 12 wt% of lead but is not precise in the emission wavelength. It is noteworthy that all these plastic scintillators suffer from a dramatic decrease of their light output, due to the loading of lead. All these discrepancies prompted us to develop our own home-made plastic scintillators, that would satisfy all the requirements. A decade ago an efficient method for the production of optical resins doped with lead was described [9], consisting of a matrix prepared from styrene, methacrylic acid and lead dimethacrylate. We decided to extend this method for the preparation of plastic scintillators and the results will be presented herein.
2. Experimental Bis-N-(2,5-di-t-butylphenyl)-3,4,9.10-perylenetetracarbodiimide and Nile Red were used as received from Sigma Aldrich. Lead dimethacrylate was purchased from Fox Chemicals and used as received. Vinyltoluene, 2-hydroxyethyl methacrylate and methacrylic acid were distilled from calcium hydride. The synthesis of N-(20 ,50 -di-t-butylphenyl)-4-butylamino-1,8-naphtalimide was already described by some of us [10]. Absorption spectra were recorded with a Jenway 6715 spectrophotometer. Fluorescence spectra and quantum yields of fluorescence were obtained with a Horiba Jobin Yvon Fluoromax-4 spectrofluorimeter. Absolute quantum yields were obtained following this procedure [11] with an integration sphere developed by Horiba Jobin Yvon. Decay times were observed under UV excitation of the scintillator. Effective atomic numbers were estimated with XmDAT software [12]. Typical scintillators developed in this experiment are of ca. 2 in. diameter and a few mm thick; 4 different concentrations of lead have been studied: 5, 10, 20 and 27 wt%. The primary fluorophore was N-(20 ,50 -di-t-butylphenyl)-4-butylamino-1,8-naphtalimide whereas the wavelength shifter was either bis-N-(2,5-di-t-butylphenyl)-3,4,9,10-perylenetetracarbodiimide or Nile Red. Primary and secondary fluorophores were solvated in the appropriate amounts of vinyltoluene, methacrylic acid and lead methacrylate. The preparation of these scintillators was patented [13]. Decay times of our scintillators were measured on a singlephoton counting chain. The excitation was produced by electrical discharges in a lamp containing gaseous hydrogen, which resulted in UV bursts impinging on the tested scintillator. The light emitted by the scintillator was filtered around the principal wavelength emission, so as to eliminate stray light, and then collected by an XP2020 photomultiplier tube, whose anode signal was redirected to an electronic chain of temporal discrimination,
Fig. 1. Experimental setup of relative scintillating efficiency measurement.
allowing analyzing the signal with precision of 100 ps. The temporal signal of the excitation pulse was recorded and all the decay signals of scintillators were deconvoluted from excitation. Scintillation performances were evaluated under x-rays excitation. A tungsten-anticathode Philips RX FFL tube powering at 40 kV and 40 mA delivered an X-ray bremsstrahlung emission with a maximum of fluence close to 35 keV and stopping at 40 keV. The beam was shuttered, leading to a square section of 1 cm2 on the scintillator. Scintillation photons were collected with an optical fiber derived from the trajectory of the x-ray beam. Scintillation yields were estimated following this procedure: the tested scintillators were placed at 42 cm from the x-ray source and imaged through a 4 m-long optical relay system to a CCD camera. The x-ray source term being known by previous measurements with Amptek CdTe detector, knowing the spectral sensitivity of the CCD camera and knowing the scintillating efficiency of YAG:Ce, we can deduce the scintillating efficiency of scintillators. The experimental setup is shown in Fig. 1.
3. Results and discussion 3.1. Choice of fluorophores The combination of radiation hardness, nanosecond fluorescence lifetime, large Stokes shift, quantum yield and good physical properties for a red fluorescent molecule is still a challenge for chemists [14]. To the best of our knowledge, only a single publication explained what composes a fast and red plastic scintillator [15]. This system used a long series of fluorophores, i.e. butyl-PBD, dimethyl-POPOP, perylene and rubrene. The initial 35 ns decay time was then reduced to 5ns by exposing the polymerized sample to large irradiation doses, leading to an important decrease of the light output. Some other recipes exist for liquid scintillation [16] or optical fiber applications but compounds such as rhodamine B or ammonium salts are too polar to be conveniently dissolved into solid solutions of polymers derived from polystyrene. We decided therefore to develop our own fluorophores, and two systems were considered after several tests [17]. They are drawn in Fig. 2. Actually the difference between the two combinations concerns the second fluorophore, which is in the first system bis-N(2,5-di-t-butylphenyl)-3,4,9,10-perylenetetracarbodiimide and in the second system Nile Red. Both wavelength shifters’ absorption spectra fit well with the emission of N-(20 ,50 -di-t-butylphenyl)-4butylamino-1,8-naphtalimide 1. The photophysical characteristics of the three compounds are given in Table 1. 1,8-Naphthalimide molecule 1 displayed excellent properties as its role of first fluorophore with a fluorescence quantum yield
M. Hamel et al. / Nuclear Instruments and Methods in Physics Research A 660 (2011) 57–63
59
Fig. 2. N-(20 ,50 -di-t-butylphenyl)-4-butylamino-1,8-naphtalimide 1,bis-N-(2,5-di-t-butylphenyl)-3,4,9,10-perylenetetracarbodiimide 2 and Nile Red 3.
Table 1 Spectroscopic data of compounds 1–3. Compound 1 2 3
a
1 1 1 max lmax (nm) e (L mol cm ) lF (nm) Dn (cm ) A
417 528 524
15,100 109,200 33,400
485 540 570
3,362 421 1,540
b
FF (%)
c
96 72 60
a
Spectra and fluorescence quantum yields recorded in spectroscopic toluene at the concentration of 10 5 M. max max b Stokes shift (Dn ¼ 1=lA 1=lF in cm 1). c Absolute quantum yield of fluorescence.
near 100% and a good Stokes shift. The choice of the second fluorophore was more tedious and balanced in favor to perylenediimide 2. Despite a very low Stokes shift, the quantum yield was good (72%) and the molar extinction coefficient very high, thus allowing doping of the scintillator with very low concentrations. Nevertheless, scintillation prepared with the second system allowed us to reach scintillation wavelengths above 600 nm, which was interesting for avoiding the Cherenkov effect (see below). 3.2. Loading of the scintillators Common plastic scintillators are known to have inefficient x-ray absorption over 5–10 keV energies, owing to their low effective atomic number Zeff (5.7 according to a polystyrenebased plastic scintillator doped with 1.5 wt% p-terphenyl and 0.05 wt% POPOP). As a matter of fact, doping plastic scintillators with different lead concentrations should increase the effective atomic number and as a consequence the photoelectric cross section in the 10–40 keV x-rays range. Fig. 3 shows a calculation of x-ray mass attenuation coefficient against photons energies for 3 scintillators: YAG:Ce, an undoped plastic scintillator and a plastic scintillator doped with 12 wt% Pb (Zeff E40; undoped and doped scintillators were simulated with the same composition feed except for lead). Results indicate that absorption at 20 keV was 20 times as high as for loaded plastic scintillators compared with their unloaded cousins, and nearly the same absorption was obtained for some energies of interest between loaded plastic scintillator and YAG:Ce. We therefore focused our efforts on the loading of scintillators with ca. 10 wt% lead. Lower and higher lead loading rates were also tested for comparison. Different loadings with tin were also investigated but will not be
discussed in this paper. Relevant information concerning tin loading can be found in the published works of Cho and Tsai [18] in the 1970s. 3.3. Preparation of scintillators Based on different possibilities of fluorophores mixtures, fluorophores concentrations and lead percentage, various scintillators have been prepared. The matrix was a ternary system composed of vinyltoluene, methacrylic acid and lead dimethacrylate. In the case of the heaviest scintillator a binary system 2-hydroxyethyl methacrylate/lead dimethacrylate was used. Vinyltoluene was chosen since its homopolymer displays an excellent refractive index [19] of 1.61 at 580 nm, which can balance the low refractive index of methacrylic acid [7] (1.50). Relevant properties of these samples are resumed in Table 2. Another important criterion is the x-rays absorption efficiency, which is proportional to the value rZ4 [20], which allows estimation (by the rule of thumb) of the x-ray photoelectric absorption. As can be seen in Table 2, whereas Zeff increases rapidly, the density r does not change dramatically. As a result, typical values of rZ4 are located around 3–3.2 106 for 10 wt% loaded plastic scintillator. For comparison, YAG:Ce displays a higher but close value of 4.77 106. For all scintillators the percentage of lead was expressed a priori from the starting feed of the preparation. Two elemental analyses were performed on samples #2 and #5 at the Laboratoire Central d’Analyses du CNRS, Solaize, France. The experimental values slightly differed from the theoretical, but towards the upper values, with %Pb measured at 12.34 and 29.51%, instead of 11.0 and 27.4%, respectively. This gap was explained by a slight evaporation of volatile solvents while heating the scintillator. 3.4. Physical results 3.4.1. Scintillation wavelength Cherenkov light intensity in the sense of number of photons per material unit length and per wavelength width interval is known to decrease with 1/l2 [21]. To this aim, various compositions of scintillators were studied so as to shift the maximum of emission to longer wavelengths (Fig. 4). Thus, we succeeded in preparing a plastic scintillator with the first fluorophore absorbing in the UV region and a wavelength shifter allowing
60
M. Hamel et al. / Nuclear Instruments and Methods in Physics Research A 660 (2011) 57–63
Fig. 3. Calculation of absorption for three different scintillators (standard plastic scintillator: green curve; 12 wt% Pb-loaded plastic scintillator: red curve; YAG:Ce: black curve) obtained with XmDAT. The area of interest 10–40 keV is highlighted in blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 2 General properties of the lead-loaded plastic scintillators. These data will be discussed along the paper. wt Pb%
r (g/cm3)
[1]
10.9
1.20
40.26
3.15
[2] [3] [4] [5] [6] [7] [8] [9]
11.0 5.4 21.9 27.4 21.9 10.9 10.9 10.9
1.18 1.12 1.54 1.55 1.38 1.12 1.16 1.12
40.31 32.93 49.60 53.11 49.60 40.25 40.19 40.19
3.11 1.32 9.32 12.33 8.35 3.01 3.03 2.92
Ref.
Zeff
r Z4eff (/106)
1st fluorophore (wt%)
2nd fluorophore (wt%)
lmax F
1 (0.5)
2 (0.02)
586
1 (0.05) 1 (0.05) 1 (0.05) 1 (0.05) 1 (0.05) 1 (0.5) 1 (1) 1 (1)
2 2 2 2 2 3 2 3
579 578 580 591 578 632 544 630
Fig. 4. Normalized scintillation emission of various scintillators compared with Cherenkov effect (normalized to wavelength of minimal spectral sensitivity of considered CCD camera) and the relative CCD spectral sensitivity.
(0.002) (0.002) (0.002) (0.002) (0.002) (0.02) (0.04) (0.05)
(nm)
Decay time (ns)
Correlation coefficient
t1 12.0, I1 0,98 t2 46.0, I2 0.02
0.997
13.26 12.78 11.28 9.22 12.09 n.d. n.d. n.d.
0.989 0.992 0.996 0.997 0.993 n.d. n.d. n.d.
fluorescence close to 590 nm. This has been possible by using molecules with good Stokes shift, such as 1,8-naphthalimides [22]. Three different scintillators were tested with various lead concentrations: scintillators #3, #1 and #6 with 5.4, 10.9 and 27.4 wt%, respectively. Data are represented in Fig. 5. They showed that according to Fig. 3, x-rays were not well-absorbed when a small amount of lead was incorporated inside the matrix: almost no scintillation could indeed be detected for the lowest doped plastic scintillator #3. The two other scintillators displayed a blue-shifting emission of 6 and 11 nm for #1 and #6, respectively, compared with UV excitation (data not drawn but presented in Table 2). Nevertheless, only the emission relative to the second fluorophore 2 was detectable, which means that the cascade of wavelength-shifting is efficient. Under the same conditions, a higher intensity was observed for the most loaded plastic scintillator. Except for the Cherenkov spectrum, the other interest of emission spectrum of scintillator is the spectral adaptation towards its detector, represented by the spectral matching factor
M. Hamel et al. / Nuclear Instruments and Methods in Physics Research A 660 (2011) 57–63
61
Fig. 5. Emission spectra under 35 keV x-rays for scintillators #3, #1 and #6 (ca. 5, 10 and 27 %wt Pb).
Table 3 Spectral matching factor of various scintillators towards the Pixis CCD camera. Scintillator
SMP
YAG:Ce #1 #2 #3 #4 #5 #6
0.993 0.966 0.999 0.940 n.d. 0.989 0.983
SMF [23]. It is a number without dimension between 0 and 1 (1 meaning a perfect fit), which is calculated with the formula: R EðlÞSCCD ðlÞdl SMF ¼ Dl R ð2Þ Dl EðlÞdl where E(l) is the normalized emission spectrum of the scintillator and SCCD(l) is the normalized spectral sensitivity of the detector, a CCD camera in our case. It is clear from Table 3 that all plastic scintillators display a good adaptation for their use with this CCD camera, owing to their principal emission wavelength higher than 550 nm.
3.4.2. Decay time For the purpose of the project, it was mandatory to find the fastest scintillators so as to meet the requirement of limited time of image acquisition of ICF target at LMJ facility [24]. Resulting signals are drawn in Fig. 6. This figure showed the decay time of 10.9 wt% lead-loaded plastic scintillator #1, along with wellknown standard scintillators measured in the same conditions. Among the nine lead-loaded plastic scintillators tested, only one (sample #1) was characterized by a double-exponential fit. Correlation coefficients for deconvoluted signals are excellent (Table 2), which mean that the fit is in perfect agreement with the measured curve. As can be seen, loaded plastic scintillator presented a slower decay (characteristic exponential decay time between 9.2 and 13.2 ns, depending on the feed composition, see Table 2) than unloaded blue scintillating plastic scintillator NE102 but remained fast enough for x-ray imaging with limited time during ICF experiments at LMJ facility. This decay time was 7 –10 times faster than that of YAG:Ce which was estimated between 70 and 119 ns (at the maximum emission wavelength of 530–550 nm), and close to that of the commercial red scintillating plastic scintillator BC-430 (16.8 ns at 580 nm, data not shown in Fig. 6). Another inorganic scintillator suitable for x-ray spectrometry such as BGO obviously did not display an appropriate decay time, according to Fig. 6. Finally, no relevant afterglow was detected since after 100 ns only 0.1% of the initial light intensity was remaining.
Fig. 6. Normalized decay curves of NE102, plastic scintillator #1, YAG:Ce and BGO.
It is noteworthy that the presence of lead has a strong influence on the decay time of the scintillator. As one can see, the fastest scintillator is also the heaviest (Table 2, entry #5, 27.4 wt%), which is the only scintillator presenting a decay time below 10 ns. Quenchers are known to reduce not only the scintillation yield but also the slow component of the signal, and thus the decay time. They are commonly used for the preparation of ultra-fast plastic scintillators [25]. Another point is the concentration of fluorophores, which is also a trick for reducing the decay time [26], despite again in low scintillation yield. In our case, we indeed observed a slight decrease of the decay time by comparing entries #1 and #2, which have exactly the same composition except the concentration of fluorophores, which is 10 times higher for #1. As a result, the decay time was reduced by nearly 25%. 3.4.3. Scintillation efficiency One of the main factors describing a scintillator is the scintillation efficiency (expressed in visible photons emitted per absorbed MeV of x-ray energy, i.e. in ph.Me V 1). As can be seen in Fig. 7, scintillation efficiency is represented versus characteristic decay time. The points corresponding to scintillators are graphically situated with respect to a merit curve expressed as in the relation (3) R 100 t=t e dt Z R01 t=t ¼ 104 hnMeV 1 ð3Þ dt 0 e The curve of merit expresses that the scintillator emits as much as 104hnMeV 1 in less than 100 ns. The region of interest where ideal scintillators are located is over this curve. In addition, this curve of merit admits a physical limit at 105hnMeV 1. Given
62
M. Hamel et al. / Nuclear Instruments and Methods in Physics Research A 660 (2011) 57–63
4. Conclusions
Fig. 7. Scintillation efficiencies of different organic (round points) and inorganic (square points) scintillators. The color of the points indicates the scintillation wavelengths, i.e. 400–450 nm for violet-blue, 520–550 nm for green and 560– 590 nm for yellow-orange. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
that no known scintillating efficiency exceeds 105ph.MeV 1, the region of interest is limited down by the merit curve and up by the maximal known efficiency [27]. The relative scintillation efficiencies of our materials were compared with that of YAG:Ce, known to deliver nearly 8000 ph/ MeV at peak wavelength of 550 nm under x-ray excitation [28]. They were measured by analyzing the CCD image intensity of scintillators irradiated by an x-ray source term peaking at 40 keV. Our best lead-loaded scintillator, namely #1, has its emission spectrum peaking at 580 nm under x-rays excitation with a scintillating efficiency of 200 ph/MeV. This has been considered as the main drawback by the authors, compared to YAG:Ce and other known compositions, and will be improved in the future. To be fully operational for our main application this fluorescence yield should be improved by a factor of 100. 3.5. Discussion The goal of this project was the preparation of plastic scintillators sensitive to 10–40 keV x-rays, which presented a red scintillation, a fast decay time and good scintillation efficiency. Among these four properties, three of them have been fulfilled. Only the scintillation efficiency has to be improved; a progression of one decade is expected for the next generation of scintillators. Two issues could be pointed out. The first one is probably a weak energy transfer from the matrix, where interactions between radiation and matter occur, to the primary fluorophore. It is admitted that the absorption spectrum of the primary fluorophore has to fit correctly with the emission spectrum of the matrix. Molecule 1 presents two absorption bands: 260–300 and 360–470 nm. It could be possible that the energy cascade is not good enough. We will therefore consider the use of another fluorescent molecule able to cover the emission spectrum of the matrix and transfer the energy close to the absorption of compound 1. The second aspect is related to the influence of lead, which could act as a quencher of the scintillation process. Quenching of scintillation by tetraphenyl lead and other organolead molecules has already been discussed [29]. The authors explained that the quenching should occur by energy transport to the triplet level of the quencher, possibly by dipole–dipole interactions. Anyway, the best solution in our case seems finding the best compromise so as to be both sensitive to 10–40 keV x-rays and fluorescent enough to afford good scintillation efficiency.
Due to the harsh conditions that will be encountered at the LMJ facility, YAG:Ce, which is so far the best compromise for x-ray imaging in ICF conditions presents also many drawbacks. We propose therefore herein the preparation and some characterizations of highly lead-loaded red scintillating fast plastic scintillators. On the basis of four main parameters (red scintillation wavelength, fast decay time, sensitivity towards 10–40 keV x-rays and scintillation efficiency), three of them have been reached. New developments permitted loading plastic scintillators with very high concentrations of lead without dramatic loss of optical properties. Calculations of the x-rays absorption performances showed that our matrix is comparable with YAG:Ce. A smart combination of fluorescent molecules allowed obtaining pretty fast plastic scintillators with monoexponential decay times close to 10 ns. From this first generation of plastic scintillators, only the scintillation efficiency had to be increased. The challenge is now to enhance this efficiency from 2.5% to almost 25%, relative to YAG:Ce.
Acknowledgements We thank Gilles Ledoux and Christophe Dujardin from LPCML, Universite´ de Lyon 1, for their precious know-how and their measuring chains that allowed extraction of decay times and spectral emissions of our scintillators.
References [1] (a) J.L. Bourgade, V. Allouche, J. Baggio, C. Bayer, F. Bonneau, C. Chollet, S. Darbon, L. Disdier, D. Gontier, M. Houry, H.P. Jacquet, J.P. Jadaud, J.L. Leray, I. Masclet-Gobin, J.P. Negre, J. Raimbourg, B. Villette, I. Bertron, J.M. Chevalier, J.M. Favier, J. Gazave, J.C. Gomme, F. Malaise, J.P. Seaux, V.Y. Glebov, P. Jaanimagi, C. Stoeckl, T.C. Sangster, G. Pien, R.A. Lerche, E.R. Hodgson, Review of Scientific Instruments 75 (2004) 4204; (b) J.L. Bourgade, R. Marmoret, S. Darbon, R. Rosch, P. Troussel, B. Villette, V. Glebov, W.T. Shmayda, J.C. Gomme, Y.L. Tonqueze, F. Aubard, J. Baggio, S. Bazzoli, F. Bonneau, J.Y. Boutin, T. Caillaud, C. Chollet, P. Combis, L. Disdier, J. Gazave, S. Girard, D. Gontier, P. Jaanimagi, H.P. Jacquet, J.P. Jadaud, O. Landoas, J. Legendre, J.L. Leray, R. Maroni, D.D. Meyerhofer, J.L. Miquel, F.J. Marshall, I. Masclet-Gobin, G. Pien, J. Raimbourg, C. Reverdin, A. Richard, D. Rubin de Cervens, C.T. Sangster, J.P. Seaux, G. Soullie, C. Stoeckl, I. Thfoin, L. Videau, C. Zuber, Review of Scientific Instruments 79 (2008) 10F301. [2] T. Yanagida, T. Itoh, H. Takahashi, S. Hirakuri, M. Kokubun, K. Makishima, M. Sato, T. Enoto, T. Yanagitani, H. Yagi, T. Shigetad, T. Ito, Nuclear Instruments and Methods A 579 (2007) 23. [3] /www.detectors.saint-gobain.comS (last access 08/29/2011). [4] E. Miho´kova´, M. Nikl, J.A. Mareˇs, A. Beitlerova´, A. Vedda, K. Nejezchleb, K. Blazˇek, C. D’Ambrosio, Journal of Luminescence 126 (2007) 77. [5] J.C. van’t Spijker, P. Dorenbos, C.P. Allier, C.W.E. van Eijk, A.R.H.F. Ettema, G. Huber, Nuclear Instruments and Methods in Physics Research: Beam Interactions with Materials and Atoms 134 (1998) 303. [6] L. Pichat, P. Pesteil, J. Cle´ment, Journal of Chemical Physics 50 (1953) 26. [7] J. Dannin, S.R. Sandler, B. Baum, International Journal of Applied Radiation and Isotopes 16 (1965) 589. [8] (a) B.K. Schabes, Kalamazoo College Report, 2010, p. 28; (b) W.G. Lawrence, S. Thacker, S. Palamakumbura, K.J. Riley, V.V. Nagarkar, IEEE Nuclear Science Symposium Conference Record (2010) 242; (c) B.K. Cha, J.H. Shin, J.Y. Kim, H. Jeon, J.H. Bae, C.h. Lee, S. Chang, H. Kim, B.J. Kim, G. Cho, IEEE Nuclear Science Symposium Conference Record (2008) 1232. [9] Q. Lin, B. Yang, J. Li, X. Meng, J. Shen, Polymer 41 (2000) 8305. [10] M. Hamel, A.M. Frelin-Labalme, V. Simic, S. Normand, Nuclear Instruments and Methods A 602 (2009) 425. ˚ [11] L. Porre s, A. Holland, L.O. Palsson, A.P. Monkman, C. Kemp, A. Beeby, Journal of Fluorescence 16 (2006) 267. [12] /http://www-nds.iaea.org/publications/iaea-nds/iaea-nds-0195.htmS (last access 08/29/2011). [13] M. Hamel, S. Darbon, S. Normand, G. Turk, French Patent Application 2010/ 12/21. [14] P.A. Cahill, Radiation Physics and Chemistry 41 (1993) 351. [15] I.B. Berlman, Y. Ogdan, Nuclear Instruments and Methods 178 (1980) 411.
M. Hamel et al. / Nuclear Instruments and Methods in Physics Research A 660 (2011) 57–63
[16] J.M. Flournoy, C.B. Ashford, in: Liquid Scintillation Counting and Organic Scintillators (1989) 83. [17] M. Hamel, V. Simic, S. Normand, Unpublished experimental results. [18] Z.H. Cho, C.M. Tsai, IEEE Transactions on Nuclear Science NS-22 (1975) 72. [19] T. Depireux, F. Dumont, A. Watillon, Journal of Colloid Interface and Science 118 (1987) 314. [20] A. Koch, C. Raven, P. Spanne, A. Snigirev, Journal of the Optical Society of America A 15 (1998) 1940. [21] D.E. Groom, S.R. Klein, European Physical Journal C15 (2000) 163. [22] M. Hamel, V. Simic, S. Normand, Reactive and Functional Polymers 68 (2008) 1671.
63
[23] E.H. Eberhardt, Applied Optics 7 (1968) 2037. [24] G. Turk, C. Reverdin, D. Gontier, S. Darbon, C. Dujardin, G. Ledoux, M. Hamel, V. Simic, S. Normand, Review of Scientific Instruments 81 (2010) 10E509. [25] P.B. Lyons, S.E. Caldwell, L.P. Hocker, D.G. Crandall, P.A. Zagarino, J. Cheng, G. Tirsell, C.R. Hurlbut, IEEE Transactions on Nuclear Science 24 (1977) 177. [26] T. Marroda´n Undagoitia, F. von Feilitzsch, L. Oberauer, W. Potzel, A. Ulrich, J. Winter, M. Wurm, Review of Scientific Instruments 80 (2009) 043301. [27] M.D. Birowosuto, P. Dorenbos, Physica Status Solidi A 206 (2009) 1. [28] G. Blasse, B.C. Grabmaier (Eds.), Luminescent Materials, Springer, 1994. [29] E.A. Andreeshchev, V.S. Viktorova, S.F. Kilin, K.A. Kovyrzina, Y.P. Kushakevich, I.M. Rozman, V.M. Shoniya, Optics and Spectroscopy 57 (1984) 624.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Preparation and characterization of highly lead-loaded red plastic scintillators under low energy x-rays Matthieu Hamel a,n, Gre´gory Turk c, Adrien Rousseau b, Ste´phane Darbon b, Charles Reverdin b, Ste´phane Normand a ´ lectroniques, F-91191 Gif-sur-Yvette Cedex, France CEA, LIST, Laboratoire Capteurs et Architectures E CEA, DAM, DIF, F-91297 Arpajon, France c LCPMR, UPMC, CNRS UMR 7614, 11 rue Pierre et Marie Curie, F-75231 PARIS Cedex 5, France a
b
a r t i c l e i n f o
abstract
Article history: Received 22 July 2011 Received in revised form 31 August 2011 Accepted 31 August 2011 Available online 7 September 2011
To the aim of development of a spatially resolved x-ray imaging system intended for Inertial Confinement Fusion (ICF) experiments at the Laser Me´ga Joule (LMJ) facility, new plastic scintillators have been designed. The main characteristics are the following: fast decay time, red emission and good x-rays photoelectric absorption in the range 10–40 keV. These scintillators are synthesized by copolymerization of different monomers with an organometallic compound. In this matrix two fluorescent compounds are embedded, allowing to shift the energy from the UV to the near IR spectrum. Several parameters were studied: fluorophores concentration, nature of the secondary fluorophore and lead concentration. An outstanding effective atomic number of 53 has been reached, for a loading of lead corresponding to 29 wt%. Thus, small cylinders were prepared and their performances under x-ray beam studied and compared with those of inorganic Cerium-doped Yttrium Aluminum Garnet reference scintillator (Y3Al5O12:Ce3 þ ). Eventually, such new scintillators or their next generation could replace expensive and brittle inorganic scintillators, inducing a strong industrial potential. & 2011 Elsevier B.V. All rights reserved.
Keywords: Plastic scintillator X-rays Red fluorescence Fast decay time Lead loading
1. Introduction The development of spatially resolved x-ray imaging system within 10–40 keV has to take into account hard radiative environment induced by ICF in the LMJ experiment chamber. Indeed, the image acquisition is difficult due to highly energetic particle and beaming emission resulting directly or indirectly from deuterium–tritium fusion reaction. This can destroy equipment located close to the experiment chamber [1]. After 100 ns, the dose rate of neutron and gamma rays (created by neutron interactions with the chamber environment and structure) reaches instantaneously 108 rad(equiv.Si).s 1 and generates a high noise level on the image. Despite the high dose rate, the integrated dose is only 2rad (equiv.Si). Hence any x-ray imaging system in those conditions has to be as less vulnerable as possible. To this aim, the perfect scintillator (whether organic or inorganic) should display the following requirements: a fast characteristic decay time (below more than 99% of the generated light emitted in less than 50 ns), a scintillation wavelength shifted as far as possible to the red wavelengths so as to eliminate Cherenkov blue light by means of optical filtering and a good
n
Corresponding author. Tel.: þ33 1 69 08 33 25; fax: þ 33 1 69 08 60 30. E-mail address: [email protected] (M. Hamel).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.08.062
photoelectric absorption of 10–40 keV x-rays. So far, the best choice should be YAG:Ce single crystal, which is known for its excellent properties towards x-rays imaging properties but suffers from a temperature dependent response [2], a peak scintillation wavelength located at 550 nm and a decay time too long (exponential characteristic decay time of 70 ns if single crystal [3] and 130 ns if polycrystalline [4]) for our applications. Lutetium Sulfide scintillator Lu2S3:Ce seemed to gather all the required scintillating properties (28.000 photons per MeV, maximum scintillation peak at 592 nm and decay time of 32 ns) but the authors were limited by the small volume of the single crystal, which was approximately 1 mm3 [5]. Also of interest, common organic scintillators (such as BC-400) are not absorbent enough in the range 10–40 keV x-ray energies due to their low photoelectric cross-section in this range. The photoelectric effect probability Ppe is estimated by rule of thumb proportional to a power of effective atomic number Z, roughly between Z4 and Z5. For non-relativistic x-ray photons (E o511 keV), we can use the following approximation: Ppe pZ 4:35 =E3x
ð1Þ
Common plastic scintillators have a low effective atomic number (ca. 5) compared to inorganic scintillators (ca. 30–40). The choice of high effective atomic number components is of prime importance since we want to avoid x-ray absorption within
58
M. Hamel et al. / Nuclear Instruments and Methods in Physics Research A 660 (2011) 57–63
10–40 keV by the Compton effect and to favor photoelectric effect. Indeed Compton absorption scatters impinging x-rays, resulting in increased absorption thickness, which degrades spatial resolution of the whole imaging system. Therefore a loading of plastic scintillators with heavy elements could increase the photoelectric cross–section in our range of interest and could decrease the thickness of scintillators to absorb sufficient amount of x-ray energy to acquire a resolved x-ray image of ICF target. Since the pioneering work of Pichat et al. [6] dealing with the addition of heavy metals in plastic scintillators, most of the research in this field has been produced mainly in the 1950s and the 1960s [7]. Since then, only a few publications dealt with this issue [8]. Commercial lead-loaded plastic scintillators cannot satisfy all the LMJ requirements already explained before: EJ-256 from Eljen Technology displays a blue wavelength and a loading ranging from 1 to 5%. It seems that loading up to 10% should be possible, but the manufacturer does not recommend it. Bicron BC-452 is available either with 2, 5 or 10 wt% Pb-loading but its emission peak is also centered around 420 nm. RP-452 from Rexon is produced at 5 wt% loading. Amcrys-H is able to reach 12 wt% of lead but is not precise in the emission wavelength. It is noteworthy that all these plastic scintillators suffer from a dramatic decrease of their light output, due to the loading of lead. All these discrepancies prompted us to develop our own home-made plastic scintillators, that would satisfy all the requirements. A decade ago an efficient method for the production of optical resins doped with lead was described [9], consisting of a matrix prepared from styrene, methacrylic acid and lead dimethacrylate. We decided to extend this method for the preparation of plastic scintillators and the results will be presented herein.
2. Experimental Bis-N-(2,5-di-t-butylphenyl)-3,4,9.10-perylenetetracarbodiimide and Nile Red were used as received from Sigma Aldrich. Lead dimethacrylate was purchased from Fox Chemicals and used as received. Vinyltoluene, 2-hydroxyethyl methacrylate and methacrylic acid were distilled from calcium hydride. The synthesis of N-(20 ,50 -di-t-butylphenyl)-4-butylamino-1,8-naphtalimide was already described by some of us [10]. Absorption spectra were recorded with a Jenway 6715 spectrophotometer. Fluorescence spectra and quantum yields of fluorescence were obtained with a Horiba Jobin Yvon Fluoromax-4 spectrofluorimeter. Absolute quantum yields were obtained following this procedure [11] with an integration sphere developed by Horiba Jobin Yvon. Decay times were observed under UV excitation of the scintillator. Effective atomic numbers were estimated with XmDAT software [12]. Typical scintillators developed in this experiment are of ca. 2 in. diameter and a few mm thick; 4 different concentrations of lead have been studied: 5, 10, 20 and 27 wt%. The primary fluorophore was N-(20 ,50 -di-t-butylphenyl)-4-butylamino-1,8-naphtalimide whereas the wavelength shifter was either bis-N-(2,5-di-t-butylphenyl)-3,4,9,10-perylenetetracarbodiimide or Nile Red. Primary and secondary fluorophores were solvated in the appropriate amounts of vinyltoluene, methacrylic acid and lead methacrylate. The preparation of these scintillators was patented [13]. Decay times of our scintillators were measured on a singlephoton counting chain. The excitation was produced by electrical discharges in a lamp containing gaseous hydrogen, which resulted in UV bursts impinging on the tested scintillator. The light emitted by the scintillator was filtered around the principal wavelength emission, so as to eliminate stray light, and then collected by an XP2020 photomultiplier tube, whose anode signal was redirected to an electronic chain of temporal discrimination,
Fig. 1. Experimental setup of relative scintillating efficiency measurement.
allowing analyzing the signal with precision of 100 ps. The temporal signal of the excitation pulse was recorded and all the decay signals of scintillators were deconvoluted from excitation. Scintillation performances were evaluated under x-rays excitation. A tungsten-anticathode Philips RX FFL tube powering at 40 kV and 40 mA delivered an X-ray bremsstrahlung emission with a maximum of fluence close to 35 keV and stopping at 40 keV. The beam was shuttered, leading to a square section of 1 cm2 on the scintillator. Scintillation photons were collected with an optical fiber derived from the trajectory of the x-ray beam. Scintillation yields were estimated following this procedure: the tested scintillators were placed at 42 cm from the x-ray source and imaged through a 4 m-long optical relay system to a CCD camera. The x-ray source term being known by previous measurements with Amptek CdTe detector, knowing the spectral sensitivity of the CCD camera and knowing the scintillating efficiency of YAG:Ce, we can deduce the scintillating efficiency of scintillators. The experimental setup is shown in Fig. 1.
3. Results and discussion 3.1. Choice of fluorophores The combination of radiation hardness, nanosecond fluorescence lifetime, large Stokes shift, quantum yield and good physical properties for a red fluorescent molecule is still a challenge for chemists [14]. To the best of our knowledge, only a single publication explained what composes a fast and red plastic scintillator [15]. This system used a long series of fluorophores, i.e. butyl-PBD, dimethyl-POPOP, perylene and rubrene. The initial 35 ns decay time was then reduced to 5ns by exposing the polymerized sample to large irradiation doses, leading to an important decrease of the light output. Some other recipes exist for liquid scintillation [16] or optical fiber applications but compounds such as rhodamine B or ammonium salts are too polar to be conveniently dissolved into solid solutions of polymers derived from polystyrene. We decided therefore to develop our own fluorophores, and two systems were considered after several tests [17]. They are drawn in Fig. 2. Actually the difference between the two combinations concerns the second fluorophore, which is in the first system bis-N(2,5-di-t-butylphenyl)-3,4,9,10-perylenetetracarbodiimide and in the second system Nile Red. Both wavelength shifters’ absorption spectra fit well with the emission of N-(20 ,50 -di-t-butylphenyl)-4butylamino-1,8-naphtalimide 1. The photophysical characteristics of the three compounds are given in Table 1. 1,8-Naphthalimide molecule 1 displayed excellent properties as its role of first fluorophore with a fluorescence quantum yield
M. Hamel et al. / Nuclear Instruments and Methods in Physics Research A 660 (2011) 57–63
59
Fig. 2. N-(20 ,50 -di-t-butylphenyl)-4-butylamino-1,8-naphtalimide 1,bis-N-(2,5-di-t-butylphenyl)-3,4,9,10-perylenetetracarbodiimide 2 and Nile Red 3.
Table 1 Spectroscopic data of compounds 1–3. Compound 1 2 3
a
1 1 1 max lmax (nm) e (L mol cm ) lF (nm) Dn (cm ) A
417 528 524
15,100 109,200 33,400
485 540 570
3,362 421 1,540
b
FF (%)
c
96 72 60
a
Spectra and fluorescence quantum yields recorded in spectroscopic toluene at the concentration of 10 5 M. max max b Stokes shift (Dn ¼ 1=lA 1=lF in cm 1). c Absolute quantum yield of fluorescence.
near 100% and a good Stokes shift. The choice of the second fluorophore was more tedious and balanced in favor to perylenediimide 2. Despite a very low Stokes shift, the quantum yield was good (72%) and the molar extinction coefficient very high, thus allowing doping of the scintillator with very low concentrations. Nevertheless, scintillation prepared with the second system allowed us to reach scintillation wavelengths above 600 nm, which was interesting for avoiding the Cherenkov effect (see below). 3.2. Loading of the scintillators Common plastic scintillators are known to have inefficient x-ray absorption over 5–10 keV energies, owing to their low effective atomic number Zeff (5.7 according to a polystyrenebased plastic scintillator doped with 1.5 wt% p-terphenyl and 0.05 wt% POPOP). As a matter of fact, doping plastic scintillators with different lead concentrations should increase the effective atomic number and as a consequence the photoelectric cross section in the 10–40 keV x-rays range. Fig. 3 shows a calculation of x-ray mass attenuation coefficient against photons energies for 3 scintillators: YAG:Ce, an undoped plastic scintillator and a plastic scintillator doped with 12 wt% Pb (Zeff E40; undoped and doped scintillators were simulated with the same composition feed except for lead). Results indicate that absorption at 20 keV was 20 times as high as for loaded plastic scintillators compared with their unloaded cousins, and nearly the same absorption was obtained for some energies of interest between loaded plastic scintillator and YAG:Ce. We therefore focused our efforts on the loading of scintillators with ca. 10 wt% lead. Lower and higher lead loading rates were also tested for comparison. Different loadings with tin were also investigated but will not be
discussed in this paper. Relevant information concerning tin loading can be found in the published works of Cho and Tsai [18] in the 1970s. 3.3. Preparation of scintillators Based on different possibilities of fluorophores mixtures, fluorophores concentrations and lead percentage, various scintillators have been prepared. The matrix was a ternary system composed of vinyltoluene, methacrylic acid and lead dimethacrylate. In the case of the heaviest scintillator a binary system 2-hydroxyethyl methacrylate/lead dimethacrylate was used. Vinyltoluene was chosen since its homopolymer displays an excellent refractive index [19] of 1.61 at 580 nm, which can balance the low refractive index of methacrylic acid [7] (1.50). Relevant properties of these samples are resumed in Table 2. Another important criterion is the x-rays absorption efficiency, which is proportional to the value rZ4 [20], which allows estimation (by the rule of thumb) of the x-ray photoelectric absorption. As can be seen in Table 2, whereas Zeff increases rapidly, the density r does not change dramatically. As a result, typical values of rZ4 are located around 3–3.2 106 for 10 wt% loaded plastic scintillator. For comparison, YAG:Ce displays a higher but close value of 4.77 106. For all scintillators the percentage of lead was expressed a priori from the starting feed of the preparation. Two elemental analyses were performed on samples #2 and #5 at the Laboratoire Central d’Analyses du CNRS, Solaize, France. The experimental values slightly differed from the theoretical, but towards the upper values, with %Pb measured at 12.34 and 29.51%, instead of 11.0 and 27.4%, respectively. This gap was explained by a slight evaporation of volatile solvents while heating the scintillator. 3.4. Physical results 3.4.1. Scintillation wavelength Cherenkov light intensity in the sense of number of photons per material unit length and per wavelength width interval is known to decrease with 1/l2 [21]. To this aim, various compositions of scintillators were studied so as to shift the maximum of emission to longer wavelengths (Fig. 4). Thus, we succeeded in preparing a plastic scintillator with the first fluorophore absorbing in the UV region and a wavelength shifter allowing
60
M. Hamel et al. / Nuclear Instruments and Methods in Physics Research A 660 (2011) 57–63
Fig. 3. Calculation of absorption for three different scintillators (standard plastic scintillator: green curve; 12 wt% Pb-loaded plastic scintillator: red curve; YAG:Ce: black curve) obtained with XmDAT. The area of interest 10–40 keV is highlighted in blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 2 General properties of the lead-loaded plastic scintillators. These data will be discussed along the paper. wt Pb%
r (g/cm3)
[1]
10.9
1.20
40.26
3.15
[2] [3] [4] [5] [6] [7] [8] [9]
11.0 5.4 21.9 27.4 21.9 10.9 10.9 10.9
1.18 1.12 1.54 1.55 1.38 1.12 1.16 1.12
40.31 32.93 49.60 53.11 49.60 40.25 40.19 40.19
3.11 1.32 9.32 12.33 8.35 3.01 3.03 2.92
Ref.
Zeff
r Z4eff (/106)
1st fluorophore (wt%)
2nd fluorophore (wt%)
lmax F
1 (0.5)
2 (0.02)
586
1 (0.05) 1 (0.05) 1 (0.05) 1 (0.05) 1 (0.05) 1 (0.5) 1 (1) 1 (1)
2 2 2 2 2 3 2 3
579 578 580 591 578 632 544 630
Fig. 4. Normalized scintillation emission of various scintillators compared with Cherenkov effect (normalized to wavelength of minimal spectral sensitivity of considered CCD camera) and the relative CCD spectral sensitivity.
(0.002) (0.002) (0.002) (0.002) (0.002) (0.02) (0.04) (0.05)
(nm)
Decay time (ns)
Correlation coefficient
t1 12.0, I1 0,98 t2 46.0, I2 0.02
0.997
13.26 12.78 11.28 9.22 12.09 n.d. n.d. n.d.
0.989 0.992 0.996 0.997 0.993 n.d. n.d. n.d.
fluorescence close to 590 nm. This has been possible by using molecules with good Stokes shift, such as 1,8-naphthalimides [22]. Three different scintillators were tested with various lead concentrations: scintillators #3, #1 and #6 with 5.4, 10.9 and 27.4 wt%, respectively. Data are represented in Fig. 5. They showed that according to Fig. 3, x-rays were not well-absorbed when a small amount of lead was incorporated inside the matrix: almost no scintillation could indeed be detected for the lowest doped plastic scintillator #3. The two other scintillators displayed a blue-shifting emission of 6 and 11 nm for #1 and #6, respectively, compared with UV excitation (data not drawn but presented in Table 2). Nevertheless, only the emission relative to the second fluorophore 2 was detectable, which means that the cascade of wavelength-shifting is efficient. Under the same conditions, a higher intensity was observed for the most loaded plastic scintillator. Except for the Cherenkov spectrum, the other interest of emission spectrum of scintillator is the spectral adaptation towards its detector, represented by the spectral matching factor
M. Hamel et al. / Nuclear Instruments and Methods in Physics Research A 660 (2011) 57–63
61
Fig. 5. Emission spectra under 35 keV x-rays for scintillators #3, #1 and #6 (ca. 5, 10 and 27 %wt Pb).
Table 3 Spectral matching factor of various scintillators towards the Pixis CCD camera. Scintillator
SMP
YAG:Ce #1 #2 #3 #4 #5 #6
0.993 0.966 0.999 0.940 n.d. 0.989 0.983
SMF [23]. It is a number without dimension between 0 and 1 (1 meaning a perfect fit), which is calculated with the formula: R EðlÞSCCD ðlÞdl SMF ¼ Dl R ð2Þ Dl EðlÞdl where E(l) is the normalized emission spectrum of the scintillator and SCCD(l) is the normalized spectral sensitivity of the detector, a CCD camera in our case. It is clear from Table 3 that all plastic scintillators display a good adaptation for their use with this CCD camera, owing to their principal emission wavelength higher than 550 nm.
3.4.2. Decay time For the purpose of the project, it was mandatory to find the fastest scintillators so as to meet the requirement of limited time of image acquisition of ICF target at LMJ facility [24]. Resulting signals are drawn in Fig. 6. This figure showed the decay time of 10.9 wt% lead-loaded plastic scintillator #1, along with wellknown standard scintillators measured in the same conditions. Among the nine lead-loaded plastic scintillators tested, only one (sample #1) was characterized by a double-exponential fit. Correlation coefficients for deconvoluted signals are excellent (Table 2), which mean that the fit is in perfect agreement with the measured curve. As can be seen, loaded plastic scintillator presented a slower decay (characteristic exponential decay time between 9.2 and 13.2 ns, depending on the feed composition, see Table 2) than unloaded blue scintillating plastic scintillator NE102 but remained fast enough for x-ray imaging with limited time during ICF experiments at LMJ facility. This decay time was 7 –10 times faster than that of YAG:Ce which was estimated between 70 and 119 ns (at the maximum emission wavelength of 530–550 nm), and close to that of the commercial red scintillating plastic scintillator BC-430 (16.8 ns at 580 nm, data not shown in Fig. 6). Another inorganic scintillator suitable for x-ray spectrometry such as BGO obviously did not display an appropriate decay time, according to Fig. 6. Finally, no relevant afterglow was detected since after 100 ns only 0.1% of the initial light intensity was remaining.
Fig. 6. Normalized decay curves of NE102, plastic scintillator #1, YAG:Ce and BGO.
It is noteworthy that the presence of lead has a strong influence on the decay time of the scintillator. As one can see, the fastest scintillator is also the heaviest (Table 2, entry #5, 27.4 wt%), which is the only scintillator presenting a decay time below 10 ns. Quenchers are known to reduce not only the scintillation yield but also the slow component of the signal, and thus the decay time. They are commonly used for the preparation of ultra-fast plastic scintillators [25]. Another point is the concentration of fluorophores, which is also a trick for reducing the decay time [26], despite again in low scintillation yield. In our case, we indeed observed a slight decrease of the decay time by comparing entries #1 and #2, which have exactly the same composition except the concentration of fluorophores, which is 10 times higher for #1. As a result, the decay time was reduced by nearly 25%. 3.4.3. Scintillation efficiency One of the main factors describing a scintillator is the scintillation efficiency (expressed in visible photons emitted per absorbed MeV of x-ray energy, i.e. in ph.Me V 1). As can be seen in Fig. 7, scintillation efficiency is represented versus characteristic decay time. The points corresponding to scintillators are graphically situated with respect to a merit curve expressed as in the relation (3) R 100 t=t e dt Z R01 t=t ¼ 104 hnMeV 1 ð3Þ dt 0 e The curve of merit expresses that the scintillator emits as much as 104hnMeV 1 in less than 100 ns. The region of interest where ideal scintillators are located is over this curve. In addition, this curve of merit admits a physical limit at 105hnMeV 1. Given
62
M. Hamel et al. / Nuclear Instruments and Methods in Physics Research A 660 (2011) 57–63
4. Conclusions
Fig. 7. Scintillation efficiencies of different organic (round points) and inorganic (square points) scintillators. The color of the points indicates the scintillation wavelengths, i.e. 400–450 nm for violet-blue, 520–550 nm for green and 560– 590 nm for yellow-orange. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
that no known scintillating efficiency exceeds 105ph.MeV 1, the region of interest is limited down by the merit curve and up by the maximal known efficiency [27]. The relative scintillation efficiencies of our materials were compared with that of YAG:Ce, known to deliver nearly 8000 ph/ MeV at peak wavelength of 550 nm under x-ray excitation [28]. They were measured by analyzing the CCD image intensity of scintillators irradiated by an x-ray source term peaking at 40 keV. Our best lead-loaded scintillator, namely #1, has its emission spectrum peaking at 580 nm under x-rays excitation with a scintillating efficiency of 200 ph/MeV. This has been considered as the main drawback by the authors, compared to YAG:Ce and other known compositions, and will be improved in the future. To be fully operational for our main application this fluorescence yield should be improved by a factor of 100. 3.5. Discussion The goal of this project was the preparation of plastic scintillators sensitive to 10–40 keV x-rays, which presented a red scintillation, a fast decay time and good scintillation efficiency. Among these four properties, three of them have been fulfilled. Only the scintillation efficiency has to be improved; a progression of one decade is expected for the next generation of scintillators. Two issues could be pointed out. The first one is probably a weak energy transfer from the matrix, where interactions between radiation and matter occur, to the primary fluorophore. It is admitted that the absorption spectrum of the primary fluorophore has to fit correctly with the emission spectrum of the matrix. Molecule 1 presents two absorption bands: 260–300 and 360–470 nm. It could be possible that the energy cascade is not good enough. We will therefore consider the use of another fluorescent molecule able to cover the emission spectrum of the matrix and transfer the energy close to the absorption of compound 1. The second aspect is related to the influence of lead, which could act as a quencher of the scintillation process. Quenching of scintillation by tetraphenyl lead and other organolead molecules has already been discussed [29]. The authors explained that the quenching should occur by energy transport to the triplet level of the quencher, possibly by dipole–dipole interactions. Anyway, the best solution in our case seems finding the best compromise so as to be both sensitive to 10–40 keV x-rays and fluorescent enough to afford good scintillation efficiency.
Due to the harsh conditions that will be encountered at the LMJ facility, YAG:Ce, which is so far the best compromise for x-ray imaging in ICF conditions presents also many drawbacks. We propose therefore herein the preparation and some characterizations of highly lead-loaded red scintillating fast plastic scintillators. On the basis of four main parameters (red scintillation wavelength, fast decay time, sensitivity towards 10–40 keV x-rays and scintillation efficiency), three of them have been reached. New developments permitted loading plastic scintillators with very high concentrations of lead without dramatic loss of optical properties. Calculations of the x-rays absorption performances showed that our matrix is comparable with YAG:Ce. A smart combination of fluorescent molecules allowed obtaining pretty fast plastic scintillators with monoexponential decay times close to 10 ns. From this first generation of plastic scintillators, only the scintillation efficiency had to be increased. The challenge is now to enhance this efficiency from 2.5% to almost 25%, relative to YAG:Ce.
Acknowledgements We thank Gilles Ledoux and Christophe Dujardin from LPCML, Universite´ de Lyon 1, for their precious know-how and their measuring chains that allowed extraction of decay times and spectral emissions of our scintillators.
References [1] (a) J.L. Bourgade, V. Allouche, J. Baggio, C. Bayer, F. Bonneau, C. Chollet, S. Darbon, L. Disdier, D. Gontier, M. Houry, H.P. Jacquet, J.P. Jadaud, J.L. Leray, I. Masclet-Gobin, J.P. Negre, J. Raimbourg, B. Villette, I. Bertron, J.M. Chevalier, J.M. Favier, J. Gazave, J.C. Gomme, F. Malaise, J.P. Seaux, V.Y. Glebov, P. Jaanimagi, C. Stoeckl, T.C. Sangster, G. Pien, R.A. Lerche, E.R. Hodgson, Review of Scientific Instruments 75 (2004) 4204; (b) J.L. Bourgade, R. Marmoret, S. Darbon, R. Rosch, P. Troussel, B. Villette, V. Glebov, W.T. Shmayda, J.C. Gomme, Y.L. Tonqueze, F. Aubard, J. Baggio, S. Bazzoli, F. Bonneau, J.Y. Boutin, T. Caillaud, C. Chollet, P. Combis, L. Disdier, J. Gazave, S. Girard, D. Gontier, P. Jaanimagi, H.P. Jacquet, J.P. Jadaud, O. Landoas, J. Legendre, J.L. Leray, R. Maroni, D.D. Meyerhofer, J.L. Miquel, F.J. Marshall, I. Masclet-Gobin, G. Pien, J. Raimbourg, C. Reverdin, A. Richard, D. Rubin de Cervens, C.T. Sangster, J.P. Seaux, G. Soullie, C. Stoeckl, I. Thfoin, L. Videau, C. Zuber, Review of Scientific Instruments 79 (2008) 10F301. [2] T. Yanagida, T. Itoh, H. Takahashi, S. Hirakuri, M. Kokubun, K. Makishima, M. Sato, T. Enoto, T. Yanagitani, H. Yagi, T. Shigetad, T. Ito, Nuclear Instruments and Methods A 579 (2007) 23. [3] /www.detectors.saint-gobain.comS (last access 08/29/2011). [4] E. Miho´kova´, M. Nikl, J.A. Mareˇs, A. Beitlerova´, A. Vedda, K. Nejezchleb, K. Blazˇek, C. D’Ambrosio, Journal of Luminescence 126 (2007) 77. [5] J.C. van’t Spijker, P. Dorenbos, C.P. Allier, C.W.E. van Eijk, A.R.H.F. Ettema, G. Huber, Nuclear Instruments and Methods in Physics Research: Beam Interactions with Materials and Atoms 134 (1998) 303. [6] L. Pichat, P. Pesteil, J. Cle´ment, Journal of Chemical Physics 50 (1953) 26. [7] J. Dannin, S.R. Sandler, B. Baum, International Journal of Applied Radiation and Isotopes 16 (1965) 589. [8] (a) B.K. Schabes, Kalamazoo College Report, 2010, p. 28; (b) W.G. Lawrence, S. Thacker, S. Palamakumbura, K.J. Riley, V.V. Nagarkar, IEEE Nuclear Science Symposium Conference Record (2010) 242; (c) B.K. Cha, J.H. Shin, J.Y. Kim, H. Jeon, J.H. Bae, C.h. Lee, S. Chang, H. Kim, B.J. Kim, G. Cho, IEEE Nuclear Science Symposium Conference Record (2008) 1232. [9] Q. Lin, B. Yang, J. Li, X. Meng, J. Shen, Polymer 41 (2000) 8305. [10] M. Hamel, A.M. Frelin-Labalme, V. Simic, S. Normand, Nuclear Instruments and Methods A 602 (2009) 425. ˚ [11] L. Porre s, A. Holland, L.O. Palsson, A.P. Monkman, C. Kemp, A. Beeby, Journal of Fluorescence 16 (2006) 267. [12] /http://www-nds.iaea.org/publications/iaea-nds/iaea-nds-0195.htmS (last access 08/29/2011). [13] M. Hamel, S. Darbon, S. Normand, G. Turk, French Patent Application 2010/ 12/21. [14] P.A. Cahill, Radiation Physics and Chemistry 41 (1993) 351. [15] I.B. Berlman, Y. Ogdan, Nuclear Instruments and Methods 178 (1980) 411.
M. Hamel et al. / Nuclear Instruments and Methods in Physics Research A 660 (2011) 57–63
[16] J.M. Flournoy, C.B. Ashford, in: Liquid Scintillation Counting and Organic Scintillators (1989) 83. [17] M. Hamel, V. Simic, S. Normand, Unpublished experimental results. [18] Z.H. Cho, C.M. Tsai, IEEE Transactions on Nuclear Science NS-22 (1975) 72. [19] T. Depireux, F. Dumont, A. Watillon, Journal of Colloid Interface and Science 118 (1987) 314. [20] A. Koch, C. Raven, P. Spanne, A. Snigirev, Journal of the Optical Society of America A 15 (1998) 1940. [21] D.E. Groom, S.R. Klein, European Physical Journal C15 (2000) 163. [22] M. Hamel, V. Simic, S. Normand, Reactive and Functional Polymers 68 (2008) 1671.
63
[23] E.H. Eberhardt, Applied Optics 7 (1968) 2037. [24] G. Turk, C. Reverdin, D. Gontier, S. Darbon, C. Dujardin, G. Ledoux, M. Hamel, V. Simic, S. Normand, Review of Scientific Instruments 81 (2010) 10E509. [25] P.B. Lyons, S.E. Caldwell, L.P. Hocker, D.G. Crandall, P.A. Zagarino, J. Cheng, G. Tirsell, C.R. Hurlbut, IEEE Transactions on Nuclear Science 24 (1977) 177. [26] T. Marroda´n Undagoitia, F. von Feilitzsch, L. Oberauer, W. Potzel, A. Ulrich, J. Winter, M. Wurm, Review of Scientific Instruments 80 (2009) 043301. [27] M.D. Birowosuto, P. Dorenbos, Physica Status Solidi A 206 (2009) 1. [28] G. Blasse, B.C. Grabmaier (Eds.), Luminescent Materials, Springer, 1994. [29] E.A. Andreeshchev, V.S. Viktorova, S.F. Kilin, K.A. Kovyrzina, Y.P. Kushakevich, I.M. Rozman, V.M. Shoniya, Optics and Spectroscopy 57 (1984) 624.