533570046
Preparation and characterization of highly lead-loaded red plastic scintillators
under low energy X-rays.
2
Matthieu Hamel,1, Grégory Turk2, Adrien Rousseau2,
Stéphane Darbon2, Charles Reverdin2 and Stéphane Normand1
1. CEA, LIST, Laboratoire Capteurs et Architectures Électroniques,
F-91191 Gif-sur-Yvette Cedex, France
2. CEA, DIF, Bruyères-le-Châtel, F-91297 Arpajon, France
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Abstract
To the aim of development of a spatially resolved x-ray imaging system intended for Inertial Confinement
Fusion (ICF) at the Laser Mégajoule (LMJ) facility, new plastic scintillators have been designed. The main
characteristics are the following: fast decay time, red emission and good X-rays sensitivity in the range 10-40
keV. These scintillators are prepared by copolymerisation of different monomers with an organometallic
compound. In this matrix are embedded two fluorescent compounds, allowing to shift the energy from the UV to
the near IR spectrum. Different 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%w. Thus, small cylinders were prepared and their performance under X-ray
beam was studied and compared with inorganic Cerium-doped Yttrium Aluminium Garnet scintillator
(Y3Al5O12:Ce3+). Eventually, such new scintillators or their next generation could replace expensive and brittle
inorganic scintillators, inducing a strong industrial potential.
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Keywords: Plastic scintillator; X-rays; red fluorescence; fast decay time; lead loading.
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1. Introduction
The development of 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, which can destroy 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 10-40 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
(28.000 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 10-40 keV X-ray energies due to their low photoelectric cross section in this range.
The photoelectric effect probability Ppe is approximately proportional to a power of effective
atomic number Z, roughly between Z4 and Z5. For non-relativistic X-ray photon (E < 511
keV), we can use the following approximation that can be expressed as in equation (1):

Corresponding author. Tel: +33 1 69 08 33 25; fax: +33 1 69 08 60 30. E-mail address:
matthieu.hamel@cea.fr.
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533570046
Z 4.35
(1)
Ex3
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 10-40 keV is of prime importance, since we
want to avoid X-ray absorption by Compton 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 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 lead-loaded 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 extend this method for the
preparation of plastic scintillators and the results will be presented herein.
Ppe 
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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-(2’,5’-di-t-butylphenyl)-4-butylamino-1,8naphtalimide 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 spectrofluorimeter. 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. 2 inch diameter and a few mm thick.
4 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-naphtalimide 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 methacrylate. The preparation of these scintillators
was patented [9].
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3. Results and discussion
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3.1. Choice of the fluorophores
The combination of radiation hardness, nanosecond fluorescence lifetime, high Stokes shift,
quantum yield and good physical properties for a red fluorescent molecule is still a challenge
for chemists [10]. To the best of our knowledge, only a single publication explained what
composes a fast and red plastic scintillator [11]. 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 5 ns 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 [12] or optical fibre 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 [13]. They are drawn in Figure 1.
Figure 1: N-(2’,5’-di-t-butylphenyl)-4-butylamino-1,8-naphtalimide
3,4,9,10-perylenetetracarbodiimide 2 and Nile Red 3.
1,
bis-N-(2,5-di-t-butylphenyl)-
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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-(2’,5’-di-t-butylphenyl)-4-butylamino-1,8-naphtalimide 1. The photophysical
characteristics of the three compounds are resumed in Table 1.
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Table 1: Spectroscopic data of compounds 1-3.
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(nm)
Compound a
max
 (L mol-1 cm-1)
A
417
15,100
1
528
109,200
2
524
33,400
3
a
Spectra recorded in spectroscopic toluene at the concentration 10-5 M.
b
Stokes shift (   1 / max
in cm-1).
 1/ max
A
F
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c
(nm)
max
F
485
540
570
 (cm-1) b
3,362
421
1,540
F (%) c
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72
60
Absolute quantum yield of fluorescence.
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1,8-Naphthalimide molecule 1 displayed excellent properties as its role of first fluorophore
with a nearly quantitative 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 quantum yield was good (72%) and the molar extinction
coefficient very high, allowing thus to dope the scintillator with very low concentrations.
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Nevertheless, scintillation prepared with the second system allowed us reaching scintillation
wavelengths above 600 nm, which is of interest for avoiding Čerenkov Effect (see below).
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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%w p-terphenyl and 0.05%w POPOP). As a matter,
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. Figure 2 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 scintillator 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 can be
found in the published works of Cho et al [14] in the 70’s.
Figure 2 (color online): 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 10-40 keV is highlighted in blue.
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3.3. Preparation of the scintillators
Based on the different possibilities of fluorophores mixtures, fluorophores concentrations and
lead percentage, various scintillators have been prepared. The matrix was a ternary system
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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 polymer displays an excellent refractive index [15] of
1.61 at 580 nm which can balance the low refractive index of methacrylic acid [6] (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  × Z4 [16], which allows to estimate by rule of thumb X-ray photoelectric absorption.
As can be seen in Table 2, whereas Zeff increases rapidly, the density  does not change
dramatically. As a result, typical values of  × Z4 are located around 3 – 3.2 × 106 for 10%w
loaded plastic scintillator. For comparison, YAG:Ce displays a higher but close value of 4.77
× 106.
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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 experimental, 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.
Table 2: General properties of the lead-loaded plastic scintillators. These data will be discussed along the
paper.
Ref.
%w
Zeff
Decay
time Determination

Zeff4 1st fluorophore 2nd fluorophore max

F
3
6
Pb
(g/cm )
(/10 )
(%w)
(%w)
#1
10.9
1.20
40.26
3.15
1 (0.5)
2 (0.02)
(nm)
586
#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
1 (0.05)
1 (0.05)
1 (0.05)
1 (0.05)
1 (0.05)
1 (0.5)
1 (1)
1 (1)
2 (0.002)
2 (0.002)
2 (0.002)
2 (0.002)
2 (0.002)
3 (0.02)
2 (0.04)
3 (0.05)
579
578
580
591
578
632
544
630
(ns)
coefficient
τ1 12.0, I1 0,98
τ2 46.0, I2 0.02
13.26
12.78
11.28
9.22
12.09
n.d.
n.d.
n.d.
0.997
0.989
0.992
0.996
0.997
0.993
n.d.
n.d.
n.d.
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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/2 versus wavelength [17] To this aim,
various compositions of scintillators were studied so as to shift the maximum of emission to
longer wavelengths (Figure 3). Thus, we succeeded in preparing a plastic scintillator with the
first fluorophore absorbing in the UV region and a wavelength shifter allowing fluorescence
close to 590 nm. This has been possible by using molecules with good Stokes shift, such as
1,8-naphthalimides [18].
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Figure 3 (color online): normalized scintillation emission of various scintillators compared with Čerenkov
effect (normalized to wavelength of minimal spectral sensitivity of considered CCD camera) and the
relative CCD spectral sensitivity.
Scintillation performances were evaluated under X-rays excitation. A tungsten-anticathod
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 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 fibre derived from the trajectory of the X-ray beam. Three different
scintillators were tested, depending on their concentration of lead: scintillators #3, #1 and #6
with 5.4, 10.9 and 27.4 %w, respectively. Data are represented in Figure 4. It showed that
according to Figure 2, 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
resumed 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 better intensity was observed for the highest loaded plastic scintillator.
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Figure 4: emission spectra under 35 keV X-rays for scintillators #3, #1 and #6 (ca. 5, 10 and 27 %w-Pb)
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Except Čerenkov spectrum, the other interest of emission spectrum of scintillator is the
spectral adaptation towards its detector, represented by the spectral matching factor SMF [19].
It is a number without dimension between 0 and 1 (1 meaning a perfect fit), which is
calculated with the formula (2):
SMF 
 E ( ) S

CCD
( )d
(2)
 E ( )d

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Where E() is the normalized emission spectrum of the scintillator and SCCD() is the spectral
sensitivity of the detector, a CCD camera in our case.
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Table 3: Spectral Matching Factor of various scintillators towards the Pixis CCD camera.
Scintillator
YAG:Ce
#1
#2
#3
#4
#5
#6
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SMP
0.993
0.966
0.999
0.940
n.d.
0.989
0.983
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.
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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 [20].Decay
times of our 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 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, allowing analyzing the signal with precision of
100 picoseconds. The temporal signal of the excitation pulse was recorded and all the decay
signals of scintillators were deconvoluted from excitation. Resulting signals are drawn in
Figure 5. This Figure showed the decay time of the 10.9%w lead-loaded plastic scintillator
#1, along with well-known standard scintillators measured in the same conditions. Among the
nine lead-loaded plastic scintillators tested, only one (sample #1) was presented a doubleexponential fit. Determination 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 (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 to 10 times faster than YAG:Ce which was estimated between
and 70 [21] and 119 ns [22] (at the maximum emission wavelength of 530-550 nm), and close
to the commercial red scintillating plastic scintillator BC430 (16.8 ns at 580 nm, data not
shown in Figure 5). Different inorganic scintillators suitable for X-ray spectrometry such as
BGO obviously did not display an appropriate decay time, according to Figure 5. Finally, no
relevant afterglow was detected since after 100 ns only 0.1% of the initial light intensity was
remaining.
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Figure 5 (color online): Normalized luminous intensity of various scintillators relative to time.
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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%w) 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 [23].
Another point is the concentration of fluorophores, which is also a trick for reducing the
decay time [24], 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%.
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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.MeV-1). As can be seen
of Figure 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):
100  t
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
 e  dt
0

e
t

 10 4 h .MeV 1
(3)
dt
0
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The curve of merit expresses that the scintillator emits as much as 104 hν.MeV-1 in less than
100 ns. The region of interest where ideal scintillators are located is over this curve. Being
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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 8,000 ph/MeV at peak wavelength of 550 nm under X-ray irradiation [25].
They were measured by analyzing the CCD image intensity of scintillators irradiated by an Xray source term peaking at 40 keV. The tested scintillators were placed at 40 cm of the X-ray
source and imaged through a 4 meters-long optical relay system to a CCD camera. The X-ray
source term being known by previous measurements with Amptek CdTe detector and
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 on Figure 6.
Figure 6: Experimental setup of relative scintillating efficiency measurement
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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.
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Figure 7 (color online): scintillation efficiencies of different organic (round points) and inorganic (square
points) scintillators. The color of the points indicate the scintillation wavelengths, i.e. 400-450 nm for
violet-blue, 520-550 nm for green and 560-590 nm for yellow-orange.
3.4. Discussion
The goal of this project was the preparation of plastic scintillators sensitive to 10-40 keV Xrays 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.
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: 260-300 and
360-470 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 [26] 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 10-40 keV X-rays and enough fluorescent to afford good scintillation efficiency.
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4. Conclusion
Due to the harsh conditions 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
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scintillating fast plastic scintillators. New developments permitted to load 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 ameliorate this efficiency from 2.5% to almost 25%, relative to YAG:Ce.
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Acknowledgements
We greatly thanks Gilles Ledoux and Christophe Dujardin from LPCML, Université de Lyon
1, for their precious know-how and their measuring chains that allowed to extract decay times
and spectral emissions of our scintillators.
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