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Light Extraction Enhancement Techniques for Inorganic Scintillators

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Scintillators play a key role in the detection chain of several applications which rely on the use of ionizing radiation, and it is often mandatory to extract and detect the generated scintillation light as efficiently as possible. Typical inorganic scintillators do however feature a high index of refraction, which impacts light extraction efficiency in a negative way. Furthermore, several applications such as preclinical Positron Emission Tomography (PET) rely on pixelated scintillators with small pitch. In this case, applying reflectors on the crystal pixel surface, as done conventionally, can have a dramatic impact of the packing fraction and thus the overall system sensitivity. This paper presents a study on light extraction techniques, as well as combinations thereof, for two of the most used inorganic scintillators (LYSO and BGO). Novel approaches, employing Distributed Bragg Reflectors (DBRs), metal coatings, and a modified Photonic Crystal (PhC) structure, are described in detail and compared with commonly used techniques. The nanostructure of the PhC is surrounded by a hybrid organic/inorganic silica sol-gel buffer layer which ensures robustness while maintaining its performance unchanged. We observed in particular a maximum light gain of about 41% on light extraction and 21% on energy resolution for BGO, a scintillator which has gained interest in the recent past due to its prompt Cherenkov component and lower cost.
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crystals
Article
Light Extraction Enhancement Techniques for
Inorganic Scintillators
Francesco Gramuglia 1,* , Simone Frasca 1, Emanuele Ripiccini 1, Esteban Venialgo 1, Valentin Gâté 2,
Hind Kadiri 2,3, Nicolas Descharmes 4, Daniel Turover 2, Edoardo Charbon 1and Claudio Bruschini 1


Citation: Gramuglia, F.; Frasca, S.;
Ripiccini, E.; Venialgo, E.; Gâté, V.;
Kadiri, H.; Descharmes, N.;
Turover, D.; Charbon, E.; Bruschini, C.
Light Extraction Enhancement
Techniques for Inorganic Scintillators.
Crystals 2021,11, 362. https://
doi.org/10.3390/cryst11040362
Academic Editor: Shujun Zhang
Received: 3 March 2021
Accepted: 25 March 2021
Published: 30 March 2021
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4.0/).
1Advanced Quantum Architecture Lab (AQUA), École Polytechnique Fédérale de Lausanne (EPFL),
2002 Neuchâtel, Switzerland; simone.frasca@epfl.ch (S.F.); emanuele.ripiccini@epfl.ch (E.R.);
esteban.veni@gmail.com (E.V.); edoardo.charbon@epfl.ch (E.C.); claudio.bruschini@epfl.ch (C.B.)
2NAPA-Technologies, 74160 Archamps, France; valentin.gate@napa-technologies.com (V.G.);
hind.kadiri@utt.fr (H.K.); daniel.turover@napa-technologies.com (D.T.)
3Laboratoire Lumière, Nanomatériaux et Nanotechnologie, CNRS ERL 7004, Université de Technologie de
Troyes, 12 rue Marie Curie, 10004 Troyes CEDEX, France
4
Photovoltaics and Thin Films Electronics Laboratories (PV-LAB), École Polytechnique Fédérale de Lausanne
(EPFL), 2002 Neuchâtel, Switzerland; nicolas.descharmes@epfl.ch
*Correspondence: francesco.gramuglia@epfl.ch
Abstract:
Scintillators play a key role in the detection chain of several applications which rely on the
use of ionizing radiation, and it is often mandatory to extract and detect the generated scintillation
light as efficiently as possible. Typical inorganic scintillators do however feature a high index
of refraction, which impacts light extraction efficiency in a negative way. Furthermore, several
applications such as preclinical Positron Emission Tomography (PET) rely on pixelated scintillators
with small pitch. In this case, applying reflectors on the crystal pixel surface, as done conventionally,
can have a dramatic impact of the packing fraction and thus the overall system sensitivity. This
paper presents a study on light extraction techniques, as well as combinations thereof, for two of the
most used inorganic scintillators (LYSO and BGO). Novel approaches, employing Distributed Bragg
Reflectors (DBRs), metal coatings, and a modified Photonic Crystal (PhC) structure, are described in
detail and compared with commonly used techniques. The nanostructure of the PhC is surrounded
by a hybrid organic/inorganic silica sol-gel buffer layer which ensures robustness while maintaining
its performance unchanged. We observed in particular a maximum light gain of about 41% on light
extraction and 21% on energy resolution for BGO, a scintillator which has gained interest in the recent
past due to its prompt Cherenkov component and lower cost.
Keywords:
coating; light extraction; nanostructure; optical interface; packing fraction; PET; photonic
crystals; radioactive source; scintillators; thin films
1. Introduction
Scintillating materials are commonly used in high energy physics and medical ap-
plications because of their capability to downconvert high energy radiation into optical
photons. Scintillators are usually instrumented with a photosensor and coupled to its
sensitive surface. Originally, Photomultiplier Tubes (PMTs) were the most used photodetec-
tors to read out the scintillation light, but nowadays silicon-based devices such as Silicon
Photomultipliers (SiPMs) have moved into a prominent position. This trend is due to their
more compact structure, lower supply voltage, and robustness to magnetic fields. The
light sensitivity of both PMTs and SiPMs do usually not exceed 30% at short wavelengths
(e.g., 300 nm)
in commercial devices [
1
], and peaks at around 50–60% in the visible for
SiPMs [
2
]. The amount of light generated during the scintillation process in inorganic
scintillators is in the range 8–60 k photons/MeV (e.g., the LYSO light yield is around
30 k photons/MeV) [
3
,
4
]. However, in standard configurations only a small percentage
Crystals 2021,11, 362. https://doi.org/10.3390/cryst11040362 https://www.mdpi.com/journal/crystals
Crystals 2021,11, 362 2 of 15
of this light reaches the photodetector. Extracting as much light as possible from the
crystal becomes thus crucial, given that both energy and time resolution depend strongly
on the amount of detected light. Indeed, extracting more light enables a more accurate
estimation of the energy deposited in the crystal by the incoming radiation, which in turn
allows a better discrimination of events that underwent Compton scattering. Moreover,
the Coincidence Resolving Time (CRT), which represents the Full-width at Half-maximum
(FWHM) of the distribution of the time difference between two events in coincidence, has
been demonstrated to be proportional to the inverse square root of the amount of detected
photons [5,6].
Several phenomena limit the amount of light that can be extracted from the crystal and
then detected by a photodetector. These include, at the scintillator-photodetector interface
(Figure 1a–c), the actual light collection efficiency, which determines the amount of light
that reaches the active area of the photodetector, as well as total internal reflection and
Fresnel losses. In addition, light absorption in the scintillator itself affects the number
of photons that are lost along their path to the output surface of the crystal, with the
absorption length reducing at shorter wavelengths [3].
Figure 1.
Main sources of light losses at the scintillator (light blue)-photodetector (D) interface: (
a
) Light collection
efficiency (
L2
, with Lbeing the distance between the light source and the photodector); (
b
) total internal reflection
(TIRangle =sin(ncrys tal /nair )); and (c) Fresnel losses (∝ ∆(n)2).
Moreover, if not covered by any material, the scintillator can let light escape through
its lateral surfaces, thus losing a significant amount of optical photons (Figure 2, left). A
solution to overcome this problem is to confine the scintillation light inside the scintillator
and redirect the optical photons toward the output surface. One possibility is to apply a
specular reflector to the lateral surfaces (Figure 2, right). However, in such a scenario most
of the light generated during scintillation cannot be extracted and remains trapped inside
the crystal until it gets absorbed. This phenomenon is due to the relatively large difference
of refractive index between the scintillator (e.g., 1.8 for LYSO and 2.15 for BGO at the
peak emission), the air (n= 1) between crystal and detector, and the photosensor window
(typically 1.4–1.5 for glass) coupled on its surface. To mitigate this effect, an optimization
of the optical coupling is possible, e.g., by means of refractive index matching compounds
such as greases or waxes; the latter can increase the critical angle of total internal reflection,
allowing the extraction of more light (Figure 2, right). Another possibility is represented
by the use of diffusors on the scintillator surfaces, which redirect the impinging photons
after each reflection (Figure 3left). This results in a decrease of the total internal reflection
probability at the output surface, with a higher chance of the scintillation photons being
extracted at the first interaction such an indirect redirection scheme does however not
prevent internal absorption from occurring. A more sophisticated approach has also been
proposed, which relies on the redirection within the escape cone of the photons which
impinge on the lateral surfaces, by means of diffraction gratings whose periodicity has
been specifically designed [7]. This approach has not yet been implemented.
Crystals 2021,11, 362 3 of 15
γ
Eγ = 511 keV
Optical coupling material
Photodetector
Photon extracted from the crystal
Photon extracted using refractive index matching materials
Photon trapped into the crystal
Reective Surface
Figure 2.
Simplified ionizing radiation detection scheme. (
Left
) The scintillator, attached to a photodetector, is irradiated
with gamma rays. The interaction between the ionizing particles and the crystal structure generates optical photons
isotropically. The latter can reach directly the output surface and be extracted from the crystal (blue arrow). Some photons
will however be refracted at the other crystal surfaces and eventually escape from the crystal (red arrow). Other photons
will undergo total internal reflection and be trapped within the crystal (purple arrow). (
Right
) Only a limited portion of
photons is going to be extracted when employing specular reflective surfaces (violet structures). The situation improves by
optimizing the optical coupling.
Incident Light Diffuse Reflection
Multiple layers of teflon
Diffusive Surface
Air-Crystal interface
Figure 3.
Typical scintillator wrapping examples. (
Left
): Teflon (diffusive) wrapping vs. (
Right
): ESR (specular) wrapping.
(
Top row
): Schematic representation, (
Bottom row
): Actual crystals attached to the window of a Photomultiplier Tube (PMT).
In this work we analyze, combine, and compare several conventional as well as
novel light extraction techniques, especially targeted for Positron Emission Tomography
(PET) applications, applied to two conventional inorganic scintillators, namely LYSO and
BGO. While LYSO is the scintillator of reference in Time-of-Flight PET (ToF-PET), BGO
has seen renewed interest in the recent past mostly due to the possibility of detecting the
small but prompt Cherenkov emission component thanks to impressive progress in SiPM
performance [
8
]. BGO features a higher attenuation coefficient and photoelectric fraction
than LYSO as well as lower cost, but suffers from lower light yield and a slower scintillation
component. However, it is highly transparent down to 310 nm and has a high refractive
index of about 2.15 at the peak emission wavelength (480 nm), which are expected to make
it a good Cherenkov radiator.
Crystals 2021,11, 362 4 of 15
The novel techniques, which we investigated include the use of Photonic Crystals
(PhCs) nanoimprinted on the crystal output surface, as well as Distributed Bragg Reflectors
(DBRs) and metal coatings applied to the other crysal surfaces. We studied in particular a
modified photonic crystal structure, designed to increase its structural robustness against
mechanical stress while ensuring performance gains under realistic conditions similar
to those often reported with air-coupled scenarios. The DBRs were simulated prior to
fabrication and two different structures were compared, namely a conventional mirror
centered on the BGO scintillation spectrum, and one whose reflectivity extends to the NUV,
coupled to a silver mirror. The experimental set-up consisted of sets of
10 ×10 ×10 mm3
crystal samples coupled to a PMT, interfaced to a Multi-Channel Analyzer (MCA), to
ensure reproducibility and ease of use. An analysis of the results and a final discussion
close the paper.
2. Light Extraction Techniques: State-of-the-Art
2.1. Conventional Light Extraction Enhancement Techniques
In current radiation detectors based on scintillators, one of the most commonly used
techniques to overcome the lateral light losses consists in the use of Teflon wrapping
(Figure 3left)
. This material is cheap and easily available. It is typically applied on five
of the six surfaces of the crystal and acts as a light diffuser, whereby its properties also
depend on the thickness of the applied layer [
9
,
10
]. A detailed study of the influence of
Teflon wrapping on scintillators can be found in [11,12].
The adoption of this solution, using a sufficiently thick layer of Teflon (larger than
100
µ
m) on the sidewalls to ensure enough reflectivity (>80%) [
9
,
10
], can dramatically
improve the light extraction efficiency of the system, even if it appears to be unsuitable
for applications in which the crystal is pixelated in small needles. This is for example the
case for preclinical PET scanners [
13
], which typically rely on small pitch (sub-millimeter)
scintillators to increase the spatial resolution. Indeed, as shown in Figure 4, the efficiency of
the system is quickly reduced when the crystal size becomes comparable to the size of the
coating due to the reduced packing fraction (ratio of crystal cross-section without coating
to cross-section with coating).
Figure 4.
Coating thickness impact on the packing fraction. The coating thickness becomes non-
negligible when reducing the crystal size (L), thus affecting the overall efficiency of the system.
This geometric effect can be mitigated by changing the scattering behavior and resort-
ing to other materials such as the Enhanced Specular Reflector (ESR) [
14
], which features a
typical thickness of 65
µ
m; its analysis is reported in [
15
]. The ESR is a film of dielectric
material (Figure 3right) with high reflectivity over a very large spectrum, and represents
Crystals 2021,11, 362 5 of 15
the current gold standard when reflective coatings are applied on scintillator surfaces.
However, its reflectivity drops at 400 nm, limiting its use in those cases when the emission
spectrum of scintillators is shifted toward the blue-Near Ultraviolet (NUV) range. Figure 3
illustrates a visual comparison between the two aforementioned reflectors.
However, as discussed in the Introduction, the use of reflectors alone is not sufficient.
Indeed, very few systems are air-coupled—almost all practical PET implementations rely
on an optimization of the optical interface between crystal and detector to mitigate the light
losses at the photosensor interface. This is typically achieved by applying a refractive index
matching material to smooth the refractive index (n) transition and increase the critical
angle, thereby reducing the amount of light that undergoes total internal reflection and
Fresnel losses. Several solutions are available with different refractive indices depending
on the target scintillator. The compounds being used (especially during prototyping and
research) are often in the form of waxes or greases because they are practical to employ and
guarantee a good degree of system stability. These compounds provide high transmittance
also at a short wavelength and a refractive index usually comprised between 1.4 and 1.6.
2.2. Novel Light Extraction Enhancement Techniques
Recently, the use of nanostructures applied to the output surface of the scintillator, com-
monly called Photonic Crystals (PhC), has been investigated for PET applications [1618].
Photonic crystals are periodic structures of hundreds of nanometers in size directly built
on the output surface of the scintillator. Their principle of operation relies on scattering the
light photons at the interfaces of materials with a different refractive index. The scattered
waves interact constructively or destructively (depending on the structure) with each other.
The coating acts as a diffraction grating and leads to higher extraction efficiency, de facto
overcoming the limitations of total internal reflection described by Snell’s law [17]. Initial
developments have relied on e-beam lithography, which is very accurate but usually slow
and very costly, moving then to nanoimprint with the potential of large area coverage,
process simplification, and cost reduction; self-assembly techniques have also been em-
ployed more recently. These structures rely on a high refractive index contrast between
the nanostructure and the surrounding medium, leading to interesting optical properties
especially if applied on the surface of high refractive index material. Indeed, PhCs have
shown to be capable of increasing the amount of light extracted from inorganic scintillators,
albeit to varying degrees and under often quite different experimental conditions, such
as for LSO [
17
], LYSO [
19
22
] (self-assembly and e-beam, and nanoimprint), GYGAG and
SrI2[23]
(nanoimprint), CsI(Na) [
24
,
25
], and BGO [
26
] (self-assembly). A more comprehen-
sive description of the PhC physics is reported in [
17
,
18
]. Their use has also been suggested
by our group in combination with microlensed SiPMs [
27
], whereby the two components
would work in synergy, with the PhCs reducing the angular spread of the scintillation light,
thereby increasing the efficiency of the microlenses.
However, the PhC structures have been mostly optimized and tested using air cou-
pling with the photodetector optical window [
21
], in some cases with techniques that do
not scale to large patterning areas. In addition, their structure is very fragile and not suit-
able to be used easily and reliably in standard configurations, in which a refractive index
matching compound is employed between the crystal and sensor optical window. Indeed,
the shear stress created during the coupling process can easily damage the nanostructure,
thereby compromising its performance and measurement repeatability. Moreover, the
PhC architecture, as mentioned, is very sensitive to the refractive index contrast, and the
use of optical grease can compromise the effectiveness of the solution [
17
]. To overcome
this issue, in this study we elaborated a nanoimprint-based solution that uses a hybrid
organic/inorganic (O/I) silica sol-gel buffer layer (n= 1.46) which encapsulates the nanos-
tructure. This approach ensures structural robustness and maintains the PhC performance
unvaried (Figure 5).
Crystals 2021,11, 362 6 of 15
Inorganic Scintillator
Photodetector Optical Window
Optical grease
Photonic Crystal
Photodetector
Reflective Coating
TixOy
SiOx
Optical grease n=1.4
n=1.4
n=2.1
Optical window n=1.5
Scintillator n>1.8
TiO2
SiO2
Figure 5.
Proposed photonic crystal solution: The scintillator is covered by reflective material and the novel Photonic Crystal
(PhC) structure is implemented on the output surface. The PhC is composed of titanium oxide nanocones surrounded by a
buffer layer of silicon oxide with a refractive index of about 1.4. The solution allows a stable coupling with the photodetector
by means of a refractive index matching material, such as grease.
In addition to the core work on improved photonic crystals, the use of thin film
coatings, such as DBR [
28
30
] and metal coatings (Al and Ag), was explored to overcome
the aforementioned limitations on the packing fraction. A DBR (Figure 6) is a very thin
(order of 1
µ
m) periodic structure formed by alternating quarter wavelength stacks of
dielectric layers. The reflectivity spectral width depends on the refractive index contrast of
the materials used and is tunable. A DBR typically contains a large number of layers with a
high refractive index contrast. It can be used to achieve nearly total reflection within a range
of wavelengths, and it is thus employed as a reflector in waveguides and optical fibers,
presenting extremely low losses compared to ordinary metallic mirrors [
31
]. Each interface
between the two materials contributes a Fresnel reflection. At the design wavelength,
the optical path length difference between reflections from subsequent interfaces is half
the wavelength, in addition, the reflection coefficient amplitudes for the interfaces have
alternating signs. Therefore, all reflected components interfere constructively, which results
in a strong reflection.
n
H
nH
nH
nH
nH
nL
nL
nL
nL
Incident
Reected light
Transmitted light
nH=High n material
nL=Low n material
lL
lH
light
Figure 6.
Distributed Bragg Reflector (DBR) scheme.
Left
: Schematic overview. The DBR structure is composed of a periodic
alternation of layers with different index of refraction (
nH
and
nL
). The number of layers and the difference in refractive
index between the layers have an impact on the mirror performance.
Right
: SEM cross-section of a DBR implemented in
the EPFL’s clean rooms at the Center of MicroNanotechnology (CMI). The layer stack is clearly visible. Scale bar: 200 nm.
The reflectivity achieved is determined by the number of layer pairs and by the refrac-
tive index contrast between the layer materials. However, because of their stratification, the
reflectivity of the DBR turns out to be highly angular dependent. Indeed, the reflectivity
Crystals 2021,11, 362 7 of 15
changes significantly for incidence angles above 30
°
. It is worth mentioning here the
possibility of using the so-called perfect or omnidirectionnal mirrors which reflect light
whatever the angle of incidence and polarization, assuming that the average refractive
index of the DBR is higher than the one of the scintillating crystal [
32
]. This approach has
however not yet been used for scintillating crystals. Finally, it is worth noting that DBRs
can be combined with metal layers to enhance the reflectivity spectrum especially at long
wavelengths, while at the same time compensating the loss of reflectance below 450 nm
typical of metal mirrors.
3. Materials and Methods
3.1. Coating Fabrication
In this work, PhCs were produced by Nanoimprint Lithography (Figure 7Left). The
patterns are reproduced on a resist using “soft stamps”, replicated from master molds made
by different lithography techniques, such as e-beam, photo, laser interference, or colloidal
lithography. In the present case, the master mold was made by colloidal lithography
followed by Reactive Ion Etching (RIE) [
33
]. The soft stamp is then placed in contact with
the resist and put under pressure. The nano-imprint process was used here with a TiO
2
sol-gel resist, which was deposited by spincoating at 5500 rpm on the scintillator surface
before being patterned via nanoimprint [
34
38
]. The patterned layer is then annealed at a
temperature optimized to match the required height and refractive index (n= 2.15). The
pattern has a periodicity of 1000 nm and features cones of 560 nm in height and 300 nm in
basal diameter as shown in Figure 8. The pattern has been encapsulated in a layer of silica
sol-gel with refractive index of 1.46, matching that of the used optical grease. The hybrid
organic/inorganic encapsulation layer was deposited via spray coating using an air brush
spray gun with a 0.5-mm nozzle. The coating was first dried at room temperature, then
annealed at 150
°
C during 30 min to ensure its mechanical properties. This encapsulation
procedure also allows to avoid the formation of air gaps, which could compromise the
overall optical performance, between the bottom of the patterns and the grease if the latter
was applied directly on top of the PhC. A schematic view of the implemented structure
has already been shown in Figure 5. Modifications can be introduced, if required, to the
pitch of the periodic structures as well as their shapes. This allows structural optimizations,
which can be combined with different refractive index materials and wavelength ranges
tuned to the applications at hand.
Figure 7.
Simplified nanoimprinting process steps for pattern implementation (
left
) (for details see text). Atomic Force
Microscope (AFM) image of a small area of the implemented pattern (right) [39].
Crystals 2021,11, 362 8 of 15
200 nm
Figure 8.
SEM photonic crystal nanostructure images before the implementation of the buffer layer.
Left
: Top view
(scale bar: 200 nm), Right: Side view (scale bar: 2000 nm).
Two different DBR structures were implemented in this work (Figure 9). The first
structure is shown on the left of Figure 9and consists of a conventional mirror fabricated
with an alternation of 13 layer pairs of
SiO2
and
SiNx
(81.2 nm and 60.9 nm in thickness
respectively, for a central wavelength of 475 nm). This mirror was designed to align its
reflectivity spectrum with the emission spectrum of the BGO crystal [
30
,
40
]. This coating
was fabricated by means of a Plasma-enhanced Chemical Vapor Deposition (PECVD)
process on a glass substrate. The reflectivity of this mirror was measured for several samples
to validate the uniformity and reproducibility. All samples showed a very similar behavior,
which is also in agreement with the simulation results. The simulations were performed
using a custom Python routing based on the CAMFr (Cavity Modeling Framework) [
41
]
libraries. The second mirror, shown on the right of Figure 9, was designed using an
alternation of
Ta2O5
and
SiO2
layers (61 nm and 39 nm in thickness respectively, for a
central wavelength of 360 nm). The design of this mirror aimed at extending the reflectivity
also in the region where conventional reflectors (e.g., ESR) start to be lossy [
42
]. Another
motivation for this choice is that several inorganic scintillators, such as LYSO:Ce, show an
emission spectrum shifted toward the blue [
40
]. The design center wavelength was thus
selected to be 360 nm and the reflectivity spectrum of such a DBR is shown in Figure 9, right
(blue line). This NUV-DBR, when coupled to a silver mirror, creates an enhanced reflective
surface that ranges from 330 nm all the way to the infrared with a reflectivity in excess of
90% (Figure 9, right, green line). In this case as well, the mirror has been fabricated on a
glass substrate but using RF sputtering (Alliance-Concept DP650) and characterized by
mean of a spectroscopic reflectometer.
3.2. Experimental Setup
A set of ten 10
×
10
×
10 mm
3
crystal samples (Epic-Crystal) with polished surfaces
was used. The scintillator was coupled in all cases to the photodetector using optical grease.
A bare crystal without any coating represented the simplest configuration (ID = 1). Two
additional configurations relied on the addition of ESR or Teflon applied to the side walls
(ID = 3 and ID = 4), whereas in one further case a DBR was applied to the top surface only
(ID = 2). Three configurations employed a PhC on a bare crystal (ID = 5), as well as together
with Teflon wrapping on all sides or in combination with a DBR (ID = 6 and ID = 7), again
only on the top surface. The DBRs used for the crystals are those shown in
Figure 9
, the left
one for BGO and the right one for LYSO. The DBRs, deposited on glass substrates, were
air-coupled in both cases. For each experiment, a crystal was placed with the exit surface
coupled to a PMT (ET Enterprises Electron Tubes 9266KB) with optical grease (
n=
1.4), to
measure the output light yield. The choice of a PMT was motivated by its high linearity
with respect to the detected light, ease of use, and measurement reproducibility; the latter
was also enhanced by the use of a micropositioner to control the optical glue layer down to
a thickness of around 100
µ
m. Each sample was irradiated with a gamma source (
22
Na)
Crystals 2021,11, 362 9 of 15
of 4 MBq to induce scintillation inside the material. The output signal from the PMT was
injected in an Ortec Digibase MCA, which provided the (uncalibrated) energy spectrum.
Figure 9.
Reflectivity spectra of the DBRs implemented on glass substrates.
Left
: A conventional DBR designed with a
center wavelength of 480 nm, to match the emission spectrum of BGO.
Right
: A combination of DBR (centered at 360 nm)
and silver coating to extend the spectrum toward the red.
Top row, left
: Simulation results vs. measurements of different
samples,
Top row, right
: Measurement results of the complete structure as well as its components. Bottom row: Schematic
representation of the respective DBR architectures.
Each sample was tested for all the configurations before and after the implementation
of the PhC on the output surface to verify the relative performance improvement. In
addition, all the crystals were taken from the same batch and measured in the standard
configuration. This procedure was needed to avoid evaluation errors due to possible light
yield variations between crystals (especially from different batches) and to ensure that the
PhC fabrication process did neither damage the crystals nor affect their performance.
The measurement was performed by coupling the crystal sample to the PMT inside
a light tight box to avoid background noise given by environment light. The radioactive
source was then placed on a custom-designed stand to keep a constant distance from the
sample. The MCA was used to read the PMT photocurrent pulses, digitize them, and create
a histogram. The measurement was performed for enough time, typically a few minutes,
to allow the accumulation of a sufficiently high count number in the energy spectrum
histogram and reduce the errors in the statistical analysis to a negligible level.
3.3. Data Analysis
The data acquired with the MCA was analyzed with a custom software in order to
extract the energy resolution value and the number of ADC channels corresponding to the
511 keV and 1275 keV photopeaks. The spectrum was calibrated by utilizing the two
22
Na
photopeaks. The final energy resolution was evaluated on the calibrated spectrum as the
FWHM at the first photopeak, calculated using a Gaussian fit, divided by 511 keV. This
value is independent of different systematic errors and variations between different samples
of the same material. The comparison of the (calibrated) energy spectra resulting from
different configurations did then allow us to measure the performance of each configuration
in terms of variability, light gain, and energy resolution.
Crystals 2021,11, 362 10 of 15
4. Results
The results obtained with the setup described in the previous section are summarized
in Table 1. The table shows the measured light gain and energy resolution gain for seven
crystal configurations, for both BGO and LYSO, taking as reference a standard configuration
featuring a bare crystal wrapped in Teflon coupled with optical grease (ID = 4).
The use of a PhC pattern (Figure 8) showed a light extraction improvement of
41%
and
10% for BGO and LYSO respectively, measured with respect to the best configuration
without PhC (i.e., Teflon wrapping and optical grease). Concerning the corresponding
energy resolution, the improvement was of
21% for BGO and
4% for LYSO. Figure 10
shows the energy spectra obtained using BGO (Top row) and LYSO (Bottom row) crystals;
the use of a PhC did clearly improve in a significant way the overall amount of light in
output of the scintillator. Moreover, a closer look at the BGO Sample 2 (Figure 10, Top left)
curve illustrates how the spectrum becomes more detailed (the K-shell peak starts to be
visible), indicating as well an improvement in energy resolution.
Figure 10.
(
Top row, left
): Examples of energy spectra obtained with BGO samples in different configurations.
Ref1
:
Reference crystal with Teflon wrapping, air-coupled;
Sample 1
: Crystal wrapped with Teflon with PhC on the output
surface, air-coupled;
Ref 2
: Reference crystal with Teflon wrapping and optical grease;
Sample 2
: Crystal with Teflon
wrapping, PhC on the output surface and optical grease. (
Top row, right
): Corresponding calibration curves derived from
the position of the two photopeaks (511 keV and 1275 keV) to eliminate any offset. The slope of the curves provides a
visualization of the light gain improvement. Bottom row: Same results for LYSO.
The use of the previously described DBRs on the top surface of a bare crystal showed
a modest improvement compatible with what was previously reported in [
30
]. This is
mostly due to the light that escapes from the lateral surfaces and will be discussed in more
detail in the next section. Conventional solutions based on ESR or Teflon (3rd and 4th
configurations in Table 1) featured quite similar performance and in line with expectations.
Crystals 2021,11, 362 11 of 15
The silica sol-gel structure was extensively tested: It featured a resistance to cracking
on bending higher than 1 T; an impact resistance of 18 J; a surface pencil hardness larger
than 2 H, and an abrasion resistance (ISO3160) with no modification after 6 h (EN 13523).
Table 1.
Summary of all experimental results on 10
×
10
×
10 mm
3
BGO and LYSO crystals in seven configurations.
Reference: A standard configuration featuring a bare crystal wrapped in Teflon (ID = 4). The scintillator was coupled in all
cases to the photodetector using optical grease. The light gain and the energy resolution have been calculated on the first
peak of the spectrum (511 keV).
Comparison of Experimental Results
ID Crystal Configuration Light Gain Energy Resolution (%) Energy Resolution
Improvement
1 BGO Bare crystal & Opt. Grease 0.55 20.8 ±0.48 0.74
2 BGO
Bare crystal & Opt. Grease & DBR (top)
0.64 19.3 ±0.26 0.80
3 BGO ESR & Opt. Grease 0.98 15.6 ±0.29 0.99
4 BGO Teflon & Opt. Grease 1.00 15.4 ±0.19 1.00
5 BGO PhC Pattern & Opt. Grease 0.80 17.2 ±0.58 0.90
6 BGO PhC Pattern, Teflon & Opt. Grease 1.41 12.7 ±0.36 1.21
7 BGO
PhC Pattern & Opt. Grease & DBR (top)
0.88 16.4 ±0.57 0.94
1 LYSO Bare crystal & Opt. Grease 0.74 12.2 ±0.32 0.85
2 LYSO
Bare crystal & Opt. Grease & DBR (top)
0.79 11.8 ±0.36 0.88
3 LYSO ESR & Opt. Grease 1.00 10.4 ±0.12 1.00
4 LYSO Teflon & Opt. Grease 1.00 10.4 ±0.15 1.00
5 LYSO PhC Pattern & Opt. Grease 0.85 11.4 ±0.33 0.91
6 LYSO PhC Pattern, Teflon & Opt. Grease 1.10 10.0 ±0.24 1.04
7 LYSO
PhC Pattern & Opt. Grease & DBR (top)
0.86 11.3 ±0.34 0.92
5. Discussion
The most important result achieved by this study was the significant light extraction
enhancement when a modified nanoimprinted photonic crystal, enhanced with a sol-gel
protection (hybrid organic/inorganic silica sol-gel buffer layer) to increase its robustness,
was applied on the output surface of an inorganic scintillator. The best result was obtained
by encapsulating the crystal with reflectors (e.g., Teflon) on the side surfaces to redirect the
light toward the output. The PhC efficiency clearly improved with the growing scintillator
refractive index. A quite substantial improvement in the performance of BGO has indeed
been obtained, whereas its use on LYSO crystals showed a milder gain. This is not sur-
prising per se, given the large difference of refractive index between the two scintillators
(2.15 for BGO vs. 1.8 for LYSO), and the fact that the simple use of a refractive index
matching material is already providing good optical coupling for LYSO. This explanation
is also accredited by similar performance trends for both LYSO and BGO in experiments
featuring PhCs deposited on crystals which are air-coupled to photodetectors. Indeed, from
measurements performed on our air-coupled samples with Teflon wrapping we observed
an improvement of
25% and
15% in energy resolution and
50% and
30% in light
gain for LYSO and BGO, respectively. In this case, the gain provided by the PhC coating,
without employing optical coupling compounds, was in line with what reported in the
literature [1722,26].
Concerning the use of DBR mirrors, only a modest result (in particular considering
the energy resolution) was obtained when using a single mirror applied on the top surface
of the two kinds of crystals. This is mainly due to the fact that in this configuration (2nd
in Table 1), the lateral surfaces of the crystal are uncovered, and a significant part of
the scintillation light gets lost. The light gain does slightly increase when air-coupling
the mirrors on five of the six surfaces, although it is still below our expectations. The
main reasons lies in the technological hurdles which need to be overcome for the direct
deposition of the reflective coating on the surface of the scintillators. The corresponding
process is indeed quite complex, suffering from edge effects or generating a sufficient
Crystals 2021,11, 362 12 of 15
amount of heat to compromise the crystals’ performance. As a result, the performance
of the coating (especially DBRs) changes close to the edges of the sample. Moreover,
we observed a degradation of performance when a plasma-based process was used; the
underlying reasons are currently being investigated.
6. Conclusions and Outlook
Different light extraction enhancement techniques for two inorganic scintillators (LYSO
and BGO) were analyzed and compared. While LYSO still represents the scintillator of
reference in PET applications, we chose to investigate BGO as well due to renewed interest,
mostly thanks to its small but prompt Cherenkov emission component as well as lower cost.
Novel approaches using thin film coatings and nanoimprinted nanostructures (PhCs) were
proposed (Figure 5) and characterized by means of a set of 1 cm
3
cubic crystal samples
coupled with grease to the window of a PMT. Each sample was irradiated with a gamma
source and tested for all the configurations before and after the implementation of the PhC.
Concerning the PhC, we added a silica buffer layer which encapsulates the nanostructures,
thereby ensuring structural robustness and optimal coupling to the photodetector optical
window, contrary to most previous air-coupled implementations, while ensuring unvaried
PhC performance. The use of the PhC on the output surface showed a significant improve-
ment in light extraction and energy resolution (see Table 1), in particular for BGO. This
could possibly lead to reconsider the use of this scintillator in the development of future
TOF-PET modules.
In addition to the core work on improved photonic crystals, we did also investigate the
use of thin film coatings. Such reflective very thin structures (order of 1
µ
m), once applied
to the sides of the scintillators, can allow a significant reduction of the dead space between
crystals, and a corresponding improvement of the packing fraction (which in turn translates
into an improvement of the overall detection efficiency of the system). This is of particular
relevance when implementing tiles of miniaturized (sub-millimiter) crystal needles where
the thickness of the crystal coating has a high impact, and can also contribute to suppression
of optical crosstalk between neighboring needles. Several DBRs were simulated prior to
fabrication and two different structures compared, both deposited on a glass substrate.
One structure was optimized for BGO and another, whose reflectivity extended to the NUV,
was coupled to a silver mirror, resulting in a combined very broad reflectivity range. Other
wavelength ranges can be implemented if needed, thereby exploiting the full tunability
potential of DBRs. The measured reflectivity spectra of the aforementioned DBRs agreed
well with the simulated data, but the experimental results obtained when using them
air-coupled to the crystals were somewhat below expectations. This was mostly due to
several technological hurdles which still need to be overcome for the direct deposition
of the reflective coating on the surface of the scintillators, and which we are currently
exploring, together with the direct deposition of metal films.
We did indeed investigate the possibility of directly depositing metals on the crystal
surface, either by metal evaporation, or by Physical Vapor Deposition (PVD). The former
works at room temperature, i.e., without involving a plasma entering in contact with the
crystal surface, and was implemented with success for the deposition of silver, although
we encountered adhesion issues. The latter was tested with aluminum, resulting in no
apparent damage to the surface of the crystal, with an adhesion better than the one of silver,
also without resorting to adhesion layers. The study of these additional treatments is still
under development.
The next steps concerning the PhC will consist in extending the experimental charac-
terization to crystal needles as well as measuring the possible impact of the PhC on the
spatial reconstruction on one side, and the temporal on the other in terms of timing per-
formance. We expect an improvement of CRT as also suggested in [
18
,
21
,
43
,
44
]. Another
interesting, though challenging perspective is represented by the simulation of the light
interaction with the entire scintillator-photodetector interface and the scintillating crystal
itself, to optimize the output gain. This will require various simulation tools and more
Crystals 2021,11, 362 13 of 15
specifically, multiscale modeling. Such tools are currently available and will be tested in
the near future [45].
Author Contributions:
Conceptualization, F.G., N.D., C.B.; methodology, F.G., E.R., E.V., C.B.;
software, E.R.; validation, F.G., E.R.; data analysis, E.R.; investigation, F.G., E.R., V.G., S.F.; optical
simulations, S.F., N.D.; sample fabrication, H.K., V.G., D.T., S.F., N.D.; setup implementation, F.G., E.R.,
E.V.; sample characterization, F.G., E.R., S.F.; data curation, E.R.; writing—original draft preparation,
F.G., E.R., S.F.; writing—review and editing, F.G., E.R., S.F., D.T., E.C., C.B.; visualization, F.G., E.R.,
S.F.; supervision, D.T., E.C., C.B.; funding acquisition, E.C., C.B. All authors have read and agreed to
the published version of the manuscript.
Funding:
This research was supported, in part, by the Swiss National Science Foundation under
grant 200021-169465 and Sinergia CRSII5-177165.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments:
The authors would like to thank Loic Le Cunff and Gilles Lerondel for fruitful
discussions and for suggesting the use of self-assembled structures for light extraction. H.K. would
also like to acknowledge the use of the Nano’mat platform and the guidance of Gilles Lerondel for the
fabrication of the silicon mold using colloidal self-assembly combined with dry etching techniques.
EPFL also gratefully acknowledges the generous support of the Swiss National Science Foundation
and of EPFL’s CMi (Center of MicroNanoTechnology) staff.
Conflicts of Interest: The authors declare no conflict of interest.
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Photonic crystals coated on the surface of scintillators can be used to improve the light extraction efficiency by partially eliminating the total internal reflection. However, the traditional self-assembly technique is not applicable to the hygroscopic scintillators. In the present investigation, we have proposed an efficient method to prepare the photonic crystals on the surface of CsI(Na) hygroscopic scintillators by a combination of the self-assemble of polystyrene (PS) microspheres and the subsequent dry-transfer procedure. For obtaining optimal parameters of photonic crystals, the light output of the CsI(Na) sample is enhanced by 43.2% compared to the reference sample without photonic crystals under the excitation of alpha particles from 241Am source. The energy resolution is improved from 11.2% to 7.8%. This technique based on the dry-transfer procedure has a promising prospect in the preparation of photonic crystals for hygroscopic scintillators.
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We have demonstrated that the scintillation light output of CsI(Na) crystals can be enhanced by photonic crystals consisting of arrays of nanospheres. The photonic crystals on the surfaces of the CsI(Na) crystals were prepared by a dry-transfer technology which is a suitable method for hygroscopic scintillators. Under the excitation of X-ray, compared with a reference sample, a 2.79-fold wavelength-integrated enhancement has been obtained for the optimal photonic crystal parameters. The increased light output could improve the performance of scintillator-based detectors.
Article
A (Lu,Y)2SiO5:Ce (LYSO) crystal, as a heavy inorganic scintillator, is currently in high demand for various applications in the fields of particle detection. However, its high refractive index (n = 1.83) gives restriction on the measurements of rare events or weak particle flow, where high energy resolution is urgently required for detectors based on the crystal. Utilizing the electron beam lithography technique and the ion beam lithography method, we have successfully prepared a 2.0 × 2.0 mm² large area two-dimensional photonic crystal (PhC) structure on the LYSO crystal surface. Compared with the plain reference sample, the optical measurements show a 53% enhancement of light extraction for the LYSO nanostructured surface, and the resulted improvement of energy resolution (full width at half maximum) is measured to be 43.8% by gaussian fittings to the energy spectra excited by the ²⁴¹Am α source. With the advantage of high-resolution patterning, high thermal stability, and firm stickiness on the substrate, the present prescription of the PhC fabrication is still favorable for some special fields (e.g., homeland security and space exploration) though the writing process is extremely time consuming and expensive to use.
Article
Utilizing the self-assembly method, we have successfully prepared a two-dimensional photonic crystal (PhC) of monolayer SiO2 spheres onto the Bismuth Germinate (Bi4Ge3O12, BGO) crystal and therefore supply another photonic crystal structure with the advantages of low-cost and large-area preparation to improve the light extraction efficiency of scintillator. A light extraction enhancement ratio up to 145.3% as compared the BGO coated by the PhC with the plain reference sample is observed and the underlying mechanism of scintillator light extraction is discussed in detail via a rigorous coupled-wave analysis calculation. The calculated transmission spectra and electric-field distribution exhibit the electric-field eigenmodes confined in SiO2 spheres resonating by the way of whispering gallery modes (WGMs) and becoming leaky modes due to the diffraction effect. Since almost no SiO2 nanospheres over the polystyrene spheres are immersed in preparation, we have demonstrated that the introduction of PhC structures using the SiO2 nanospheres onto the scintillator surface is an effective way to solve the light-trapping problem and therefore the design is very useful to be applied to the particle detection of radiation field.
Article
In this paper, we have investigated the light extraction of scintillator using by the photonic crystal consisting of an array of monolayer polystyrene nanospheres and a conformal layer. The effect of the conformal layer on the extraction efficiency was systematically analyzed. It is found that the conformal layer will strongly affect the optical properties of the photonic crystal. The coupling efficiency can be controlled by increasing mode number through varying the thickness and refractive index of the conformal layer. When the mode number increases, the multi-extraction process plays an important role in the ultimate extraction efficiency. Thus, at this situation, the extraction efficiency is more favorable for a thin scintillator. This conclusion is also applicable to other solid-state light-emitting devices.