# Highly Lead-Loaded Red Plastic Scintillators as an X-Ray Imaging System for the Laser Méga Joule

Matthieu Hamel, Stéphane Normand, Grégory Turk, Stéphane Darbon

► To cite this version:

Matthieu Hamel, Stéphane Normand, Grégory Turk, Stéphane Darbon. Highly Lead-Loaded Red Plastic Scintillators as an X-Ray Imaging System for the Laser Méga Joule. : International Conference on Advancements in Nuclear Instrumentation, Measurement Methods and their Applications, Jun 2011, Gant, Belgium. pp.1268-1272, 10.1109/TNS.2012.2194305 . cea-05497693

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# Highly Lead-loaded Red Plastic Scintillators as an X-ray Imaging System for the Laser Méga Joule

Matthieu Hamel, Stéphane Normand, Grégory Turk, and Stéphane Darbon

**Abstract**— The scope of this project intends to record spatially resolved images of core shape and size of a DT microballoon during Inertial Confinement Fusion (ICF) experiments at Laser Méga Joule facility (LMJ). We need to develop an X-ray imaging system which can operate in the radiative background generated by an ignition shot of ICF. The scintillator is a part of the imaging system and has to gather a compromise of scintillating properties (scintillating efficiency, decay time, emission wavelength) so as to both operate in the hard radiative environment and to allow the acquisition of spatially resolved images. Inorganic scintillators cannot be used because no compromise can be found regarding the expected scintillating properties, most of them are not fast enough and emit blue light. Organic scintillators are generally fast, but present low X-ray absorption in the 10 to 40 keV range, that does not permit the acquisition of spatially resolved images.

To this aim, we have developed highly lead-loaded and red-fluorescent fast plastic scintillators. Such a combination is not currently available via scintillator suppliers, since they propose only blue-fluorescent plastic scintillators doped with up to 12%w Pb.

Thus, incorporation ratio up to 27%w Pb has been reached in our laboratory, which can afford a plastic scintillator with an outstanding Zeff close to 50. X-rays in the 10 to 40 keV range can thus be detected with a higher probability of photoelectric effect than for classic organic scintillators, such as NE102. The strong orange-red fluorescence can be filtered, so that we can eliminate residual Čerenkov light, generated by y-ray absorption in glass parts of the imaging system. Decay times of our scintillators evaluated under UV excitation were estimated to be in the range 10 to 13 ns.

## I. INTRODUCTION

The development of spatially resolved X-ray imaging system within 10 to 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 particles and beaming emission resulting directly or indirectly from deuterium-tritium fusion reaction, which can deteriorate equipments close to the experiment chamber [1]. Hence any X-ray imaging system in those conditions has to be as less vulnerable as possible.

To this aim, the perfect scintillator (whatever organic or inorganic) should gather the following requirements: a fast characteristic decay time (below 50 ns); a scintillation wavelength shifted as far as possible to the red wavelengths so as to eliminate Čerenkov blue light by means of optical filtering and a good photoelectric absorption of 10 to 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 suffer from a response dependent with temperature [2], a peak scintillation wavelength located at 550 nm and a decay time too long (70 ns if single crystal and 130 ns if polycrystalline) for our applications. Lutetium Sulfide scintillator Lu2S3:Ce seemed to gather all the researched scintillating properties (28000 photons per MeV, maximum scintillation peak at 592 nm and decay time of 32 ns) but the authors were limited with the small volume of the single crystal which was approximately 1 mm³ [3].

Also of interest, common organic scintillators (such as BC400) are not enough absorbent in the range 10 to 40 keV X-ray energies due to their low photoelectric cross section in this range. Common plastic scintillators have a low effective atomic number (ca. 5) compared to inorganic scintillators (ca. 30 to 40). Maximizing the photoelectric cross section by choosing high effective atomic number components, within 10 to 40 keV is of prime importance, since we want to avoid X-ray absorption by Compton effect. Therefore a loading of plastic scintillators with heavy elements could increase the photoelectric cross section in our range of interest and could decrease the necessary 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. [4] dealing with the addition of heavy metals in plastic scintillators, most of the research in this field has been produced mainly in the 50's and the 60's [5]. Since then, to the best of our knowledge we were not able to detect any important work. Commercial leadloaded plastic scintillators cannot respect 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%w Pb-loading but its emission peak is also located around 420 nm. Amcrys-H is able to reach 12%w of lead but do not precise 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 home made plastic scintillators which would embrace all the requirements. A decade ago an efficient method for the production of optical resins doped with lead was described [6], consisting of a matrix prepared from styrene methacrylic acid and lead dimethacrylate. We decided to adapt this method for the preparation of plastic scintillators and some results will be presented herein.

## II. EXPERIMENTAL

Nile Red and Bis-N-(2,5-di-t-butylphenyl)-3,4,9,10- perylenetetracarbodiimide were used as received from Sigma Aldrich. Lead dimethacrylate was purchased from Fox Chemicals and used as received. Vinyltoluene and methacrylic acid were distilled under vacuum from calcium hydride. The synthesis of N-(2',5'-di-t-butylphenyl)-4-butylamino-1,8-napthalimide was already described by some of us [7]. 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 spectrofluorometer. Absolute quantum yields were obtained with an integration sphere. Decay times were observed under UV excitation of the scintillator. Effective atomic numbers were estimated with XμDAT software [8].

Typical scintillators developed in this experiment are ca. 50 mm diameter and a few mm thick. Four different concentrations of lead have been studied: 5, 10, 20 and 27%w. The primary fluorophore was N-(2',5'-di-t-butylphenyl)-4- butylamino-1,8-napthalimide whereas the wavelength shifter was either bis-N-(2,5-di-t-butylphenyl)-3,4,9,10- perylenetetracarbodiimide or Nile Red. These two fluorophores were
solvated in the appropriate amounts of vinyltoluene, methacrylic acid and lead dimethacrylate. From different matrix and different fluorophores concentrations, approximately 25 scintillators were prepared and tested. The preparation of these scintillators was patented [9].

### III. RESULTS AND DISCUSSION

#### A. Choice of the Matrix

Common plastic scintillators are known to have insufficient X-ray absorption over 5 to 10 keV energies, owing to their low effective atomic number Zeff (5.7 according to a polystyrene-based plastic scintillator doped with 1.5%w pterphenyl and 0.05%w POPOP). As a matter, doping plastic scintillators with high lead concentrations should increase the effective atomic number and as a consequence the photoelectric cross-section in the 10 to 40 keV X-rays range. Fig. 1 is 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%w Pb (Zeff ≈ 40; undoped and doped scintillators were simulated with the same composition feed except for lead). Results indicate that absorption at 20 keV was 20 times higher 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%w lead. Fewer and higher compositions were also tested for comparison. Different loadings with tin were also investigated

but will not be discussed in this paper. Relevant information about tin loading can be found in the published works of Cho et al [10] in the 70's.

Based on the 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. Vinyltoluene was chosen since its polymer displays an excellent refractive index [11] of 1.61 at 580 nm which can balance the low refractive index of methacrylic acid (1.50) [6].

Another important criterion is the X-rays absorption efficiency which is proportional to the value $ρ × Z^4$ [12], it allows to estimate by rule of thumb X-ray photoelectric absorption. As a result, typical values of $ρ × Z^4$ are located around 3 to $3.2 × 10^6$ for 10%w loaded plastic scintillator. For comparison, YAG:Ce displays a higher but close value of $4.77 × 10^6$.

For all scintillators the percentage of lead was expressed a priori from the starting feed of the preparation. Two examples of elemental analyses were performed at the Laboratoire Central d'Analyses du CNRS, Solaize, France. The experimental values slightly differed from the experimental, but towards the upper values, with %Pb measured at 12.34 and 29.51%, instead of waited values of 11.0 and 27.4%, respectively. This gap was explained by a slight evaporation of volatile solvents while heating the scintillator.

#### B. Choice of the Fluorophores

The choice of the fluorophores has to fulfill two criteria: fast decay time (ca. 10 ns) and energy transfer from UV to red wavelength. To this aim, we developed two different fluorophores combinations and compared them. They are drawn in Fig. 2 and their spectroscopic data are resumed in Table I.

These fluorophores were chosen for their excellent photophysical properties and their chemical stability. 1,8-Naphthalimide molecule 1 displayed excellent properties as its role of first fluorophore with a maximal quantum yield of fluorescence and a good Stokes shift. The choice of the second fluorophore was more tedious and balanced in favour to perylenediimide 2. Despite a very low Stokes shift, the fluorescence quantum yield was good (about 72%) and the molar extinction coefficient very high, allowing thus to dope the scintillator with very low concentrations.

Decay times of some scintillators were measured on a single-photon counting chain. The excitation was produced by electrical discharges in a lamp containing gaseous hydrogen, which resulted in UV bursts striking 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, allowing analyzing the signal with precision of 100 ps. The temporal signal of the

<figure><img src="image_2.png" /><figcaption>Fig. 1. Simulation of absorption for three different scintillators (standard plastic scintillator: green curve; 12%w Pb-loaded plastic scintillator: red curve; YAG:Ce: black curve) obtained with XµDAT. The area of interest 10 to 40 keV is highlighted in blue.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Fig. 2. N-(2',5'-di-t-butylphenyl)-4-butylamino-1,8-naphthalimide 1, bis-N-(2,5-di-t-butylphenyl)-3,4,9,10-perylenetetracarbodiimide 2 and Nile Red 3.</figcaption></figure>

<table><caption>TABLE I
SPECTROSCOPIC DATA OF COMPOUNDS 1-3.</caption><thead><tr><th>Compound <sup>a</sup></th><th>λ<sub>max</sub><br>(nm)</th><th>ε<br>(L mol<sup>-1</sup> cm<sup>-1</sup>)</th><th>λ<sub>r</sub><sup>max</sup><br>(nm)</th><th>Δν<br>(cm<sup>-1</sup>)<sup>b</sup></th><th>Φν<br>(%)<sup>c</sup></th></tr></thead><tbody><tr><td>1</td><td>417</td><td>15100</td><td>485</td><td>3362</td><td>96</td></tr><tr><td>2</td><td>528</td><td>109200</td><td>540</td><td>421</td><td>72</td></tr><tr><td>3</td><td>524</td><td>33400</td><td>570</td><td>1540</td><td>60</td></tr></tbody></table>

<sup>a</sup>Spectra recorded in spectroscopic toluene at the concentration $10^{-5}$ M.

<sup>b</sup> Stokes shift ($Δν = 1/λ_{A}^{max} - 1/λ_{r}^{max}$ in cm<sup>-1</sup>).

<sup>c</sup> Absolute quantum yield of fluorescence.
excitation pulse was recorded and all the decay signals of scintillators were deconvoluted from excitation. An example of resulting signals is drawn in Fig. 3. This Figure showed the decay time of the 10.9%w lead-loaded plastic scintillator named #1 on the Figure, along with well-known standard scintillators measured in the same conditions. Generally, loaded plastic scintillator presented a slower decay (between 9.2 and 13.2 ns, depending on the feed composition) 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 to 10 times faster than YAG:Ce which was estimated between 70 [13] and 119 ns [14] (at the maximum emission wavelength of 530 to 550 nm), and close to the commercial red scintillating plastic scintillator BC430 (16.8 ns at 580 nm, data not shown in Fig. 3). Different inorganic scintillators suitable for X-ray spectrometry such as BGO obviously did not exhibit an appropriate decay time, according to Fig. 3. Finally, no relevant afterglow was detected since after 100 ns only 0.1% of the initial light intensity was remaining.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Normalized luminous intensity of various scintillators relative to time. #1 is our 10.9%w Pb-loaded plastic scintillator.</figcaption></figure>

It is noteworthy that the presence of lead has a strong influence on the decay time of the scintillator. Indeed, the fastest scintillator also the heaviest (27.4%w Pb and a decay time 9.22 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 [15].

Another point is the concentration of fluorophores, which is also a trick for reducing the decay time [16], despite again in lowering scintillation yield. In our case, we indeed observed a 25% decrease of the decay time by increasing by 10 fold the concentration of the fluorophores.

### C. 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.MeV⁻¹). As can be seen of Fig. 5, 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 this relation:

$$ \eta = \frac{\int_{0}^{100} e^{-\tau} dt}{\int_{0}^{\infty} e^{-\tau} dt} = 10^{4} \text{h} \text{v.MeV}^{-1} $$

The curve of merit expresses that the scintillator emits as high as 104 hν.MeV⁻¹ in less than 100 ns. The region of interest where ideal scintillators are located is over this curve. Being given that no known scintillating efficiency exceeds 105 hν.MeV⁻¹, the region of interest is limited down by the merit curve and upper by the maximal known efficiency.

The relative scintillation efficiencies of our materials were compared with YAG:Ce, known to deliver nearly 8000 ph/MeV at peak wavelength of 550 nm under X-ray irradiation [17]. They were measured by analyzing the CCD image intensity of scintillators irradiated by an X-ray source term peaking at 40 keV. The tested scintillators were placed at 42 cm from the X-ray source and imaged through 4 meterslong optical relay system to a CCD camera. The X-ray source term being known by previous measurements with Amptek CdTe detector and knowing both the spectral sensitivity of the CCD camera, and the scintillating efficiency of YAG:Ce, we can deduce the scintillating efficiency of scintillators. The experimental setup is shown on Fig. 4.

<figure><img src="image_5.png" /><figcaption>Fig. 4. Experimental setup of relative scintillating efficiency measurement</figcaption></figure>

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.

The goal of this project was the preparation of plastic scintillators sensitive to 10 to 40 keV X-rays which presented a red scintillation, a fast decay time and a 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.

<figure><img src="image_6.png" /><figcaption>Fig. 5. Scintillation efficiencies of different organic (round points) and inorganic (square points) scintillators. Our scintillators are quoted in references #1 to #5. The color of the points indicates the average scintillation wavelengths, i.e. 400 to 450 nm for violet-blue, 520 to 550 nm for green and 560 to 590 nm for yellow-orange.</figcaption></figure>
Two issues could be pointed out. First 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 to 300 and 360 to 470 nm. It could be possible that the energetic 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.

Second is 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 [18] The Authors explained that the quenching should occur by energy transport to the triplet level of the quencher, possibly by dipole-dipole interactions. Whatever happens, the best solution in our case seems to find the best compromise so as to be both sensitive to 10 to 40 keV X-rays and enough fluorescent to afford good scintillation efficiency.

## IV. CONCLUSION

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. New developments permitted to load plastic scintillators with very high concentrations of lead without dramatic loss of optical properties. Computations of X-rays absorption performances showed that our matrix is comparable with YAG:Ce. A smart combination of fluorescent molecules allowed to obtain 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 improve this efficiency from 2.5% to almost 25%, relative to YAG:Ce.

## ACKNOWLEDGMENT

We greatly thanks Gilles Ledoux and Christophe Dujardin from LPCML, Université de Lyon 1, for their precious knowhow and their measuring chains that allowed to extract decay times and spectral emissions of our scintillators.

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