Taking Advantage of Disorder: Small-Molecule Organic Glasses for Radiation Detection and Particle Discrimination

Article abstract of DOI:10.1021/jacs.7b03989

A series of fluorescent silyl-fluorene molecules were synthesized and studied with respect to their photophysical properties and response toward ionizing neutron and gamma-ray radiation. Optically transparent and stable organic glasses were prepared from these materials using a bulk melt-casting procedure. The prepared organic glass monoliths provided fluorescence quantum yields and radiation detection properties exceeding the highest-performing benchmark materials such as solution-grown trans-stilbene crystals. Co-melts based on blends of two different glass-forming compounds were prepared with the goal of enhancing the stability of the amorphous state. Accelerated aging experiments on co-melt mixtures ranging from 0% to 100% of each component indicated improved resistance to recrystallization in the glass blends, able to remain fully amorphous for >1 month at 60 °C. Secondary dopants comprising singlet fluorophores or iridium organometallic compounds provided further improved detection efficiency, as evaluated by light yield and neutron/gamma particle discrimination measurements. Optimized singlet and triplet doping levels were determined to be 0.05 wt % 1,4-bis(2-methylstyryl)benzene singlet fluorophore and 0.28 wt % Ir³⁺, respectively.

Full text of DOI:10.1021/jacs.7b03989

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Article
Taking Advantage of Disorder: Small-Molecule Organic
Glasses for Radiation Detection and Particle Discrimination
Joseph S. Carlson, Peter Marleau, Ryan A Zarkesh, and Patrick L Feng
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 20 Jun 2017
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Taking Advantage of Disorder: Small-Molecule Organic Glasses for
Radiation Detection and Particle Discrimination
Joseph S. Carlson, Peter Marleau, Ryan A. Zarkesh, Patrick L. Feng*
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Sandia National Laboratories, 7011 East Ave., Livermore, CA 94550
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ABSTRACT: A series of fluorescent silyl-fluorene molecules were synthesized and studied with respect to their photo-
physical properties and response toward ionizing neutron and gamma-ray radiation. Optically transparent and stable or-
ganic glasses were prepared from these materials using a bulk melt-casting procedure. The prepared organic glass mono-
liths provided fluorescence quantum yields and radiation detection properties exceeding the highest-performing bench-
mark materials such as solution-grown trans-stilbene crystals. Co-melts based on blends of two different glass-forming
compounds were prepared with the goal of enhancing the stability of the amorphous state. Accelerated aging experiments
on co-melt mixtures ranging from 0-100% of each component indicated improved resistance to recrystallization in the
glass blends, able to remain fully amorphous for >1 month at 60oC. Secondary dopants comprising singlet fluorophores or
iridium organometallic compounds provided further improved detection efficiency, as evaluated by light yield and neu-
tron/gamma particle discrimination measurements. Optimized singlet and triplet doping levels were determined to be
0.05 wt. % 1,4-bis(2-methylstyryl)benzene (bis-MSB) singlet fluorophore and 0.28 wt. % Ir3+, respectively.
kinetics of the produced light pulses. These differences
are evident in the relative fraction of light produced via
prompt singlet fluorescence versus delayed triplet-triplet
annihilation (TTA). In practice, nuclear recoils from fast
neutron interactions produce a greater proportion of de-
layed luminescence than gamma-rays and can be identi-
fied by their characteristic pulse shape. This is due to a
phenomenon known as ionization quenching, which
leads to a reduction in the relative proportion of prompt
fluorescence for neutron versus gamma-ray events.2 This
technique for identifying the type of incident particle is
known as pulse-shape discrimination (PSD).3 In mixed
fluorophore systems, such as plastics and liquids, Förster
resonant energy transfer also plays a role.
■ INTRODUCTION
Luminescent organic materials have attracted signifi-
cant recent attention due to their role in a variety of func-
tional devices. One particular area of interest comprises
organic light emitting diodes (OLEDs), which operate
based on charge injection and radiative recombination in
multi-layer devices.1 A related but more specialized appli-
cation that is based on similar principles of charge
transport and exciton mobility involves organic materials
for the detection of ionizing radiation. This application
serves as the basis for nuclear non-proliferation detection
of illicit nuclear materials such as highly enriched urani-
um or plutonium. By analogy to OLEDs, the performance
of an organic scintillator is controlled by the efficiency
and kinetics of radiative decay processes that are associ-
ated with ion recombination and exciton transport. The
ideal scintillator material can be characterized as having:
While this pulse-shape discrimination technique is
effective in some materials such as trans-stilbene single
crystals or liquid scintillation mixtures, there are several
limitations that preclude their use in critical applications
such as radiation portal monitors used at border crossings
and ports-of-entry. First, PSD is easily disrupted by the
presence of disorder or impurities. This is primarily due
to a reliance upon TTA to provide the delayed emission
component. TTA is a bimolecular recombination process
that requires Dexter electronic interaction between two
triplet excited states, the probability of which decreases
exponentially as a function of distance. The presence of
disorder or impurities decreases the effective triplet exci-
ton lifetime due to a higher density of trapping sites that
compete with TTA. Indeed, Arulchakkaravarthi et al. used
X-ray rocking curve analysis to correlate degradation in
the light yield and PSD with decreased structural order in
melt-grown single crystals.4 Related work by Carman et
al. has shown a sensitive dependence of the PSD upon the
purity and degree of crystallographic perfection in solu-
tion-grown trans-stilbene.5 Knowledge gained from these
studies have led to commercialization of high-quality
trans-stilbene single crystals, although several limitations
remain. Among these are low fracture toughness, high
1) minimal optical self-absorption
2) fast emission timing (fast counting)
3) emission wavelength matching to photodetector (typi-
cally 300 to 500 nm)
4) high conversion of radiation energy to visible light
(light yield)
5) discrimination between different types of radiation
6) energy resolution/spectroscopy
7) amenable to large sizes and various form factors, low
cost, strength, stability
For these reasons, we describe here our efforts to improve
the radiation detection performance and long-term stabil-
ity of organic-based scintillators via control over the mo-
lecular and bulk properties of transparent organic glasses.
Organic scintillators possess the unique ability to dis-
criminate ionization caused by fast neutron recoils on
nuclei from that caused by Compton scattering of gam-
ma-rays on electrons, owing to differences in the emission
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cost, limited size, and highly anisotropic scintillation re-
sponse.6
The combination of these limitations has led to signifi-
osition and therefore an artificially lower light yield
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measurement. Due to its higher mass, the secondary pro-
ton produced from neutron nuclear recoils will deposit ca.
two orders of magnitude more of its energy over a given
distance and as a result travel 1/100th the distance as an
electron of similar energy and is therefore more likely to
completely deposit its energy, even in small samples.2
cant interest in non-crystalline organic scintillators based
on polymers and organic liquids.2 Various strategies have
been employed to achieve neutron/gamma PSD in these
materials, although the obtained scintillation light yields
and discrimination performance have been found to be
distinctly inferior to single crystals.7,8 In this work, we
report on a new class of amorphous organic scintillator
that exceeds the scintillation performance of trans-
stilbene while enabling rapid fabrication via a bulk melt-
casting procedure.
Casting amorphous thin films with long term re-
sistance to crystallization is a challenge frequently en-
countered in the OLED field that directly impacts the
lifetime and conditions under which the device can be
operated. To the best of our knowledge, small-molecule-
based organic glasses have never been implemented in
large scale applications (ca. > 1 cm3).13 To make an accu-
rate comparison between our formulation and the state of
the art technology, samples had to be prepared on similar
sizes to a reference material. To this end, a 2-gram glass
sample of 1 was prepared to coincide with the size of
benchmark materials on hand. After several weeks at
room temperature the sample slowly began to turn
opaque on the surface and eventually the entire sample,
resulting from crystallization that was confirmed by pow-
der X-ray diffraction (XRD) (See Supporting Information).
Delayed crystallization would make organic glasses ineli-
gible for practical use in the field. The observation of a
glass transition temperature (Tg) and lack of solid to liq-
uid transition (Tm) after multiple heating and cooling cy-
cles in a DSC experiment is frequently used to qualitative-
ly assess the stability of a glassy material. It is apparent
that this is not true over long durations due to potential
kinetic instability of the glassy state.
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■ RESULTS AND DISCUSSION
Organic Glass Scintillators: Stability and Perfor-
mance on Practical Sizes
Considering that the photophysics of organic light-
emitting diode (OLED) emitters1 are strikingly similar to the
principles of radioluminescence in scintillators, we evaluated
fluorene as a potential fluorophore group. Despite possessing
high quantum yields with a blue (ca. 420 nm) emission pro-
file, fluorene and its derivatives have not been widely ex-
plored in the context of scintillators. Fluorene possesses an
advantage in that it can be readily derivatized at the benzylic
position to provide oxidative stability and enable facile modi-
fication of properties.9
The glass to crystal transition can proceed by two
modes: a glass-to-crystal transition originating from the
bulk or originating from the surface.14 There are known
inhibition methods for both types of growth. For example,
surface growth can be inhibited by applying a coating,15
while bulk growth can be inhibited by polymer additives16
or by blending compounds of similar structure.17 In-
creased stability can also be gained when a glass is formed
via vapor deposition.18
Figure #. Glass forming compounds 1 – 3.
We found that compounds 1 – 3 based on fluorene
chromophores were not only capable of providing greater
scintillation light yields and PSD performance than trans-
stilbene, but also were capable of being melted and cast
into a transparent glasses in sizes relevant to radiation
detection.10,11 These finding provided the opportunity to
explore several key objectives: (1) ability to circumvent
single-crystal growth procedures as a route towards larger
radiation detector elements, (2) ability to improve the
photophysical properties via control over secondary do-
pant concentrations, (3) ability to achieve isotropic opti-
cal, mechanical, and transport properties.12
Table 1. Co-melt formulation effect on Tg and stabil-
ity
1 : 2 (w/w)a
100 : 0
Tg (°C)b Transparency at 80 0Cc
72.6
75.2
80.2
81.0
89.4
95.1
98.9
< 24 hours
> 4 weeks
> 4 weeks
> 4 weeks
> 4 weeks
< 72 hours
< 24 hours
90 : 10
70 : 30
50 : 50
30 : 70
10 : 90
Co-melt Formulation for Indefinite Stability
Comparing scintillators of varying sizes can be chal-
lenging due to the fact that some performance variables
are size dependent. For example, with overlap of the exci-
tation and emission bands of the glass matrix, re-
absorption of the scintillation light can decrease perfor-
mance as a sample becomes larger, making small samples
appear artificially higher performing. Alternatively, small
samples have a greater chance of the secondary particle
escaping the sample, resulting in incomplete energy dep-
0 : 100
b
a200 mg samples. Tg value at the onset of the transition.
cEvaluated by observation using a jeweler’s loop.
At this juncture we instead sought to improve the sta-
bility of the glass scintillators via a formulation approach.
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Since observation of Tg in DSC is not a measure of long
term glass stability, each sample was subjected to acceler-
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ated aging conditions; whereby, samples were aged at
temperatures close to their Tg, a technique known to ac-
celerate crystallization by several orders of magnitude.14
We found that mixtures of compounds 4 and 5 were in-
definitely resistant to crystallization, even under these
conditions (Table 1).
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Figure 4. DPA doping concentration vs light yield and
PSD in glass samples of 1.
Figure 2. A 2 gram glass sample of 1 and 2 (90:10 w/w) after
aging at 60 °C for 72 hours.
Triplet Harvesting Dopants
In a larger scale experiment, a formulation of 1 and 2
demonstrated the same resistance towards crystallization
(Figure 2).
Taking further inspiration from the OLED literature
and similar work in radioluminescent plastics,8,19,20 we
experimented with the radioluminescence response of
glasses containing iridium triplet harvesting dopants. This
strategy has been shown to be effective in maximizing the
light yield when incorporated in polymer-based host ma-
terials at Ir3+ concentrations of 0.8-5.5 wt%. Incorporating
Bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)
iridium(III) (FIrpic) in an organic glass provided high
scintillation light yields at Ir3+ concentrations as low as
0.03 wt% (Figure 5). The highest light yields were
achieved at 0.13-0.28 wt% Ir3+, yielding 1.8-2 times the
scintillation light output relative to stilbene. This result is
notable since it corresponds to a light yield that is four
times greater and an Ir3+ doping level six times smaller
than for a corresponding scintillator based on the OLED
host material poly(9-vinylcarbazole).20 The presence of
Ir3+ confers a long fluorescence lifetime on the order of 1
μs, with pulse shapes possessing a very diminished
prompt emission peak. The dominance of delayed emis-
sion precludes the ability to do particle discrimination;
Singlet wavelength shifters
The nature of a glassy medium enables facile tuning of
the fluorescence properties via incorporation of secondary
dopants. Indeed, by casting a glass of 3 with 0.05% of 9,10-
diphenylanthracene (DPA) the Stokes’ shift was increased
to possess less self-absorption than undoped microcrys-
talline 3, indicative of efficient Förster Resonance Energy
Transfer (FRET) (Figure 3). While it was observed that
light yield increases in conjunction with DPA concentra-
tion, after a threshold concentration the particle discrim-
ination ability diminished (Figure 4). On the 200 mg
samples tested, it was determined that the maximum
benefit for light yield and PSD was obtained with a DPA
concentration of 0.05% (w/w), outperforming stilbene in
both respects. A survey of other wavelength shifters emit-
ting in the 400 – 500 nm range demonstrated a similar
relationship between performance and concentration,
with ca. 0.05% being the optimum concentration (vide
infra).
Figure 5. Plot of the relative 137Cs scintillation light yield as a
function of Ir3+ dopant concentration. The glass matrix was a
9:1 mixture of 1:2 and the Ir3+ complex was FIrpic.
Figure 3. Photoluminescence excitation and emission spec-
tra of 3 in the microcrystalline form (red) and glass undergo-
ing FRET with DPA (blue).
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however, the increase in light yield at such a low Ir3+ con-
centration was an interesting discovery in its own right as
one of the most sensitive radioluminescent organic mate-
rials known.2
greater PSD than stilbene. Further analysis of a histogram
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2
3
4
5
6
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plot of the data after pulse-processing revealed some in-
teresting features (Figure 7a, b). There is excellent separa-
tion between the neutron and gamma bands down to low
energy events, a desirable feature not present in organic
liquids and plastics. When the binned histogram data is
analyzed using the FOM definition and plotted as a func-
tion of energy, it is clear that the PSD capability of G5
outperforms stilbene across the energy range (Figure 7c).
Radioluminescence of Organic Glasses: Light
Yield, PSD, Radiation Timing, Neutron Response
Comparison to Benchmark Materials
Using the best formulation from the co-melt and wave-
length shifter concentration studies, we were able to cast
2 gram scintillators containing various dopants (Figure 6)
for comparison to benchmark materials of similar dimen-
sion (Table 2). Apart from concentration, the identity of
the wavelength shifter also had a significant impact of
fluorescence and PSD. To our satisfaction, the fluores-
cence lifetime of singlet emitting doped glasses were fast-
er than stilbene, with the exception of G2.
9
Another often overlooked aspect of scintillator per-
formance is the neutron light yield. Since it is difficult to
subject samples to a calibrated neutron energy source, a
gamma calibrated PSD plot is typically reported using
gamma-ray sources of known energies (i.e. keVee scale
units). Often times a scintillator can be formulated to
possess a higher PSD-FOM, but at the expense of the neu-
tron sensitivity and light yield. To evaluate neutron sensi-
tivity without utilizing a calibrated neutron source, we
simply integrated the counts from the histogrammed data
across the relevant energy region with respect to the PSD
parameter for G5 and compared it to stilbene under iden-
tical conditions (Figure 7d). The larger neutron:gamma
integral for G5 reveals that the high PSD-FOM values for
G5 does not come at the expense of neutron sensitivi-
ty/light yield; in fact, the neutron sensitivity/light yield is
higher than stilbene. Since fast neutrons have the highest
interaction cross-section with hydrogen atoms, the neu-
tron response is dependent on the hydrogen content of a
scintillator. The identical H/C ratios (0.857) for both G5
and stilbene indicate that the improvement in neutron
scintillation efficiency is attributed to the scintillation
properties and not due to a difference in molecular for-
mula.
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Figure 6. Singlet wavelength shifters and Iridium triplet har-
vesting dopant.
Table 2. Comparison of radioluminescence of 2 gram
glass formulation to known standards
Fast scintillation timing is also important for a number
of radiation detection applications including correlated
particle counting and imaging.21 To demonstrate that the
faster fluorescence timing of G5 was duplicated in the
scintillation response, a time correlated single-photon
counting experiment was used to construct pulse shapes
under 137Cs gamma irradiation (See Supporting Infor-
mation). As compared to stilbene and EJ-200 reference
materials, the glass samples possess faster scintillation
rise-time and decay characteristics. This is promising for
applications such as active interrogation that demand fast
counting as well as neutron discrimination.22
Samplea Dopant (w/w) Lifetimec
(ns)
QYd
(Φ)
Light PSDf
Yielde
Stilbeneb
–
–
2.41
1.00
1.00 3.15
0.63 2.57
EJ-301
2.00 (89%) 0.81
16.21 (11%)
EJ-200
G1
–
1.50
1.68
8.09
0.95
1.05
0.74
0.92
1.79
1.50
1.61
0.63 0.66
0.62 2.48
–
G2
0.05% DPA
1.11
1.11
2.53
3.56
G3
0.05% POPOP 1.52
0.05% DPVBi 1.50
G4
0.88 3.06
Scale up and Practical Application
To demonstrate scalability, a casting of G1 was made in
a size and shape that is compatible with many photomul-
tiplier tubes. The resulting sample possessed very good
optical transparency and incurred no damage while han-
dling (Figure 8). This observation was encouraging in that
practical radiation detection with these materials are pos-
sible as a replacement for existing crystalline, plastic, and
liquid scintillators. There is also the potential of casting
into form factors for applications that are not yet realized.
G5
0.05% MSB
0.07% MSB
1% FIrpic
1.45
1.10
1.14
3.73
3.56
G6
G7g
1.53
1.01 μs
0.40
2.00 N/A
aGlass samples composed of 9:1 mixture of 1:2 bSingle Crystal
c
of similar size Fluorescence lifetime measured at emission
d
maximum QY relative to stilbene, measured at the sample
absorption maximum eCs-137 relative light yield (478 keVee)
fPSD-FOM at 478 keVee) g200 mg sample.
From this data, G5 was identified as possessing the
best performance with 10% greater light yield and 18%
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b)
a)
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d)
Figure 7. PSD histograms for a) trans-stilbene crystal and b) G5 glass obtained using an AmBe mixed n/γ-ray source. c) Plot of
the neutron/gamma pulse-shape discrimination figure-of-merit as a function of gamma-ray energy for G5 glass (red) in compari-
son to three reference scintillators. d) Total integral of neutron and gamma-ray counts in the energy range of 75-2100 keVee for
trans-stilbene crystal (black) and G5 glass (red).
■ CONCLUSION
The current work is the first example of organic glasses
as a new class of materials for radiation detection. The
development of a high quantum yield matrix in an easily
processable form factor is the next evolutionary step in
the field of radiation detection and discrimination. The
ease by which additives can be incorporated into the ma-
trix imparts versatility in designing new types of scintilla-
tors. As compared to benchmark materials, improvements
were made across all metrics we measured, including
light yield, pulse-shape discrimination, neutron response
and timing. The combination of these improvements can
lead to radiation detectors with better detection efficien-
cy, which enhance national security and non-proliferation
mission capabilities.
Figure 8. A 2” diameter, ½” thick casting of G1 (19 grams).
To the best of our knowledge, this is also the first ex-
ample of a large (> 1 gram), indefinitely stable small-
molecule organic glass, which was achieved through for-
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mulation of two different scintillating compounds. This (7) Bourne, M. M.; Clarke, S. D.; Adamowicz, N.; Pozzi, S.
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The Supporting Information is available free of charge on the
ACS Publication website at DOI: ###, which includes: exper-
imental procedures for the synthesis and casting of glasses,
photoluminescence spectra, radioluminescence data, DSC
spectra and X-ray crystallographic data.
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AUTHOR INFORMATION
Corresponding Author
*plfeng@sandia.gov
Author Contributions
The manuscript was written through contributions of all
authors.
Funding Sources
This work was supported by the office of NA-22, NNSA, U.S.
Department of Energy. Sandia National Laboratories is a
multi-mission laboratory managed and operated by National
Technology and Engineering Solutions of Sandia, LLC., a
wholly owned subsidiary of Honeywell International, Inc., for
the U.S. Department of Energy’s National Nuclear Security
Administration under contract DE-NA0003525.
(14) a) Xi, H.; Sun, Y.; Yu, L. J. Chem. Phys. 2009, 130,
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
We would like to thank F. Patrick Doty and Mateusz Monter-
ial for lively discussions.
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