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Improving light output and coincidence time resolution of scintillating crystals using nanoimprinted photonic crystal slabs

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Scintillating crystals are used in numerous applications of ionizing radiation detectors. In time of flight positron emission tomography (TOF-PET) for example, both energy- and coincidence time resolution (CTR) are important characteristics that could significantly benefit if more light from scintillators, otherwise trapped, could be collected by the photodetector. A novel and promising method to extract more efficiently the light produced in crystal scintillators with high index of refraction is to introduce a thin nanopatterned photonic layer on the readout surface. In this paper, we describe the patterning process of a photonic crystal layer made of TiO with 390 nm diameter ”pillars” in a square lattice with a periodicity of 580 nm and a structure thickness of 300 nm on one side of a 10x10x10 mm LYSO cube. The production process used was nanoimprint lithography. A substantial increase in light yield of 50% has been measured in good agreement with our simulations. An interesting result from these measurements is that the improvement in light output is independent of whether the crystal is read out from its photonically patterned side or from the one opposite to it. For all cases studied, the energy resolution improved by a factor of 1.1. On the other hand, the CTR, being very threshold dependent, is unlike the light yield not subject to a constant improvement. It turns out that, at low thresholds, the gain (improvement) in CTR is limited to 1.2, and then rapidly increases to a value of more than 2 at higher thresholds. This is mainly explained by an additionally induced light transfer time spread of the photonic pattern. Several configurations with and without Teflon wrapping were investigated.
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Nuclear Inst. and Methods in Physics Research, A 940 (2019) 254–261
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A
journal homepage: www.elsevier.com/locate/nima
Improving light output and coincidence time resolution of scintillating
crystals using nanoimprinted photonic crystal slabs
Rosalinde Hendrika Pots a,b,, Matteo Salomoni a,c, Stefan Gundacker a,c, Silvia Zanettini d,
Valentin Gâté d,e, Elise Usureau e, Daniel Turover d,e, Paul Lecoq a, Etiennette Auffray a
aCERN, CH-1211, Geneva 23, Switzerland
bRWTH Aachen, Templergraben 55, 52062 Aachen, Germany
cUniversità degli studi di Milano Bicocca, Piazza dell’Ateneo Nuovo 1, 20126 Milano, Italy
dSILSEF SAS, 382 rue Louis Rustin, Archamps Technopole, F74160 Archamps, France
eNAPA Technologies SAS, 382 rue Louis Rustin, Archamps Technopole, F74160 Archamps, France
ARTICLE INFO
Keywords:
Scintillators
Photonic crystals
Coincidence time resolution
Light yield
Nanoimprint lithography
Fast timing detector
ABSTRACT
Scintillating crystals are used in numerous applications of ionizing radiation detectors. In time of flight positron
emission tomography (TOF-PET) for example, both energy and coincidence time resolution (CTR) are important
characteristics that could significantly benefit if more light from scintillators, otherwise trapped, could be
collected by the photodetector. A novel and promising method to extract more efficiently the light produced
in crystal scintillators with high index of refraction is to introduce a thin nanopatterned photonic layer on the
readout surface. In this paper, we describe the patterning process of a photonic crystal layer made of TiO𝟐with
390 nm diameter "pillars" in a square lattice with a periodicity of 580 nm and a structure thickness of 300 nm
on one side of a 10x10x10 mm 𝟑LYSO cube. The production process used was nanoimprint lithography. A
substantial increase in light yield of 50% has been measured in good agreement with our simulations. An
interesting result from these measurements is that the improvement in light output is independent of whether
the crystal is read out from its photonically patterned side or from the one opposite to it. For all cases studied,
the energy resolution improved by a factor of 1.1. On the other hand, the CTR, being very threshold dependent,
is unlike the light yield not subject to a constant improvement. It turns out that, at low thresholds, the gain
(improvement) in CTR is limited to 1.2, and then rapidly increases to a value of more than 2 at higher
thresholds. This is mainly explained by an additionally induced light transfer time spread of the photonic
pattern. Several configurations with and without Teflon wrapping were investigated.
1. Introduction
Scintillating crystals are widely used for the detection of ionizing
particles in various applications, e.g. in high energy physics calorime-
try, medical detectors, and homeland security.
An important characteristic of scintillators is their energy resolution.
In positron emission tomography (PET) applications, where scintillators
are used to detect two 511 keV gammas from electron-positron anni-
hilation, the energy resolution enables to filter out scattered and other
background events having energies other than the 511 keV photoelec-
tric events. High energy resolution (𝐸𝑟𝑒𝑠) increases the signal to noise
ratio and hence the detector sensitivity. The statistical contribution
to the energy resolution 𝐸𝑟𝑒𝑠 depends on the collected light in the
following way:
𝐸𝑟𝑒𝑠 1
𝐿𝑌𝑐𝑜𝑙𝑙
Corresponding author at: CERN, CH-1211, Geneva 23, Switzerland.
E-mail address: rosalinde.hendrika.pots@cern.ch (R.H. Pots).
where 𝐿𝑌 𝑐𝑜𝑙𝑙 denotes the measured light yield.
Furthermore, for time-of-flight PET (TOF-PET) systems the coinci-
dence time resolution (CTR) also plays an important role. High CTR
is sought to reduce noise hits along the line of response and thereby
further improve the signal to noise-ratio. Similar to energy resolution,
the CTR depends on the measured light yield (LY):
𝐶𝑇 𝑅 1
𝐿𝑌𝑐𝑜𝑙𝑙
Therefore, both the energy resolution and the CTR can be improved if
more light is collected by the photodetector. One suitable way in this
direction, e.g., is to select specific scintillators with a high intrinsic light
yield or to wrap the scintillator with reflectors or diffusing materials
such as Vikuiti [1] or Teflon. Also using optical coupling between the
scintillator and photodetector helps improving light collection signifi-
cantly. Nonetheless, using as an example a 2 ×2×20 mm3LYSO crystal
https://doi.org/10.1016/j.nima.2019.06.026
Received 8 April 2019; Received in revised form 9 June 2019; Accepted 12 June 2019
Available online 17 June 2019
0168-9002/©2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
R.H. Pots, M. Salomoni, S. Gundacker et al. Nuclear Inst. and Methods in Physics Research, A 940 (2019) 254–261
Fig. 1. Processing steps in nano-imprint lithography: (1) LYSO scintillator with subsequent layers of TiO2, aluminum, and a resist deposited on its surface. (2) A stamp independently
fabricated beforehand with the desired pattern imprints the pattern into the resist. (3) The pattern in the resist is transferred to the aluminum through wet-etching. (4) The imprinted
aluminum layer has the pattern of the resist and is now used as a hard mask for the dry-etching process. (5) Dry-etching of the TiO2transfers the Al-pattern to the TiO2. (6) The
hard mask is removed and the TiO2is imprinted on the LYSO crystal with its final pattern.
wrapped with Teflon and mounted onto a PMT with optical coupling
grease with index of refraction of 1.42 only 50% of the light produced
in the crystal is extracted [2,3].
For specific applications, like PET and high energy calorimetry, scin-
tillators are required to have high density so as to absorb a maximum
of energy of the traversing ionizing particles. This generally results in
a high refractive index (𝑛= 1.82 for LYSO) making light extraction
from such scintillators difficult. If the medium, e.g. air, between the
photodetector and the crystal has a lower refractive index, the interface
between them will cause a significant amount of light to be trapped
inside the crystal. Furthermore, the entry windows of photodetectors
have a typical refractive index of the order of 𝑛= 1.5. This aggravates
the mismatch in the involved indices even when applying an optical
coupling between the scintillator and the photodetector. Therefore,
there will always be a critical angle 𝜃𝑐that defines an extraction cone
where every light outside of this cone will be internally reflected at the
interface of the materials with different refractive indices.
A promising means to extract part of the (otherwise lost) light
from outside of the extraction cone is to introduce a photonic crystal
slab onto the readout surface of the scintillator. A photonic crystal
slab is a thin layer of dielectric material imprinted on the scintillator
with a periodic nanostructure where the periodicity is of the order
of the wavelength of the light. If this structure is properly designed,
it has the potential to significantly enhance light extraction through
the diffraction of light impinging on the crystal’s readout surface. In
this way, light from higher than 0th order diffraction modes can be
extracted beyond the extraction cone [4,5].
2. Produced sample
We have designed and produced a photonic crystal layer on the
readout surface of a 10 ×10 ×10 mm3LYSO:Ce cube to increase the
amount of light to be extracted from this crystal. The cube used in
this study was produced by Crystal Photonic, Inc. (CPI), with all six
faces polished. On the bulk crystal, a TiO2layer was imprinted with a
nanopattern by SILSEF and NAPA Technologies [6], using nanoimprint
lithography as shown in Fig. 1. TiO2has a refractive index as high as
2.4 and is transparent to light emitted by LYSO:Ce at 420 nm. These
are the two important features of any candidate material for photonic
crystals [5]. The production method used for our slab is described in
detail by the following six steps (see also Fig. 1:)
First, a 300 nm layer of TiO2is sputtered on one of the surfaces,
usually denoted as the exit window of the crystal. Thereafter a
layer of aluminum (Al) is deposited on the TiO2coat, and then a
resist applied on top of these (step 1 in Fig. 1).
This is the layer onto which the desired pattern will then be
imprinted via the nanoimprint lithographic process; it is a unique
method where the pattern is imprinted into the resist layer with a
so-called stamp (step 2 of Fig. 1) replicated from a master mold.
The master mold itself is produced beforehand using electron
beam lithography.
Fig. 2a-b. Illustrations of the photonic crystal pattern with pillars in a square lattice
(a). Photo of the nanoimprinted surface of the LYSO:Ce cube showing the typical
iridescent diffraction effects of photonic layers (b). Note, that the photonic pattern
does not extend over the entire surface of the cube.
After having imprinted the resist, the pattern is transferred to the
aluminum layer via wet-etching (step 3) where the aluminum
only serves as a hard mask for the dry-etching of the TiO2(step
5), which will then produce the final, patterned layer on the
scintillator (step 6). For our sample the chosen pattern consists
of pillars arranged in a square lattice on top of the scintillator, as
illustrated in Fig. 2a.
After the production of the photonic crystal on the bulk LYSO scintil-
lator, the crystal is first visually inspected to assess how much of the
surface is covered with the pattern, and to check for inhomogeneities
visible by eye. Due to diffraction, the photonic crystal layer exhibits an
iridescent shine on the scintillator surface, as seen in Fig. 2b.
To examine the fabricated pattern on the scintillator more closely,
the photonic crystal slab was visualized with a scanning electron micro-
scope (SEM). Since the sample is nonconductive and hence subject to
electrostatic charging during the imaging process the resulting images
are not perfectly sharp. By imaging the sample from the top, the
periodicity and diameter of the pattern could be evaluated, but also
possible defects in the shape of the structures and inhomogeneities in
the pattern spotted. When the crystal is tilted one can also estimate the
thickness of the nanostructure close to the edges of the crystal.
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R.H. Pots, M. Salomoni, S. Gundacker et al. Nuclear Inst. and Methods in Physics Research, A 940 (2019) 254–261
Fig. 3a-c. SEM images: made from top of sample with 4k magnification (a); top view SEM image with 20k magnification (b); SEM image of sample tilted by 70 degrees with 20k
magnification (c).
Figs. 3a and 3b show SEM images recorded of the sample, all seen
from top-down. The pattern shows regular periodicity and exhibits
almost no defects. The diameter of the pillars on the pattern was
measured to be 390 nm and the periodicity of the pattern to be 580 nm.
Fig. 3c gives an image of the sample when tilted by 70 degrees. After
inspection of multiple images, we come to an average pillar height of
180 nm. Since the original TiO2layer was 300 nm thick this would then
give rise to the assumption that the TiO2layer between the pillars was
not entirely etched away, i.e. all the way down to the bare scintillator
surface. This could have been caused by too short an exposure time
during the etching process (step 4 in Fig. 1), therefore possibly leaving
a residual TiO2layer of 120 nm.
3. Simulations
3.1. Simulation framework
A simulation framework was set up to predict the increase in the
amount of light extracted from the scintillator with a photonic crystal
slab on the scintillator’s readout surface compared to a bare scintillator.
This scheme consists of Geant4 simulating the macroscopic part of our
system, and CAMFR modeling the nano-patterned photonic crystal slab.
Geant4 is a free toolkit for the simulation of the passage of particles
through matter [7]. CAMFR is a so-called ‘‘Maxwell solver’’, based on
eigenmode expansion [5,8].
With Geant4 we simulate the light production in the LYSO cube due
to radiation being converted inside the crystal and determine the tra-
jectories of the produced scintillation photons in the cube, potentially
including reflective wrapping. The LYSO cube is modeled with a surface
roughness of 𝜎𝛼= 1.7, where the meaning of 𝜎𝛼is described in [9],
except for the edges [3,10] simulated with a different 𝜎𝛼of 57. From
this Geant4 simulation we extract the angular distribution of the light
impinging on the scintillator’s readout surface from the interior of the
crystal.
In CAMFR we define the shape of the photonic crystal. CAMFR then
calculates the behavior of this pattern on the incident light using as
input the internal angular light distribution produced before by Geant4
and, in this way, determines how much of the light is extracted and
how much of it reflected. It is important to note that CAMFR is an
analytical tool that simulates a pattern without defects. It is impossible
to simulate the effect of non-periodic defects and therefore estimate
their relevance.
3.2. Results
We have simulated the pattern obtained from the SEM on our
sample, in other words the pillars of 300 nm height and a diameter of
390 nm in a square lattice with 580 nm periodicity. The transmission
of the 420 nm light in this photonic crystal slab is shown in Fig. 4.
Fig. 4. Simulation of light transmission at the LYSO crystal–air interface, with and
without a 300 nm thick photonic crystal layer as described above. The red-shaded
area indicates light internally reflected by the photonic crystal, coming from the inside
of the extraction cone that otherwise would have exited the crystal in the case of
no photonic pattern. The green-shaded area indicates extracted light from outside of
the extraction cone, i.e. light that would have been internally reflected and thus lost
without the photonic nanopattern . (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
The red-shaded area in the graph of Fig. 4 shows that the photonic
nanopattern reflects a fraction of the light coming from the inside of
the extraction cone that otherwise would have been extracted with no
photonic pattern on the crystal. On the other hand, the green-shaded
area in Fig. 4 denotes that part of the light that lies outside of the
extraction cone, i.e. light that would have been reflected internally
and hence lost without the photonic slab, and now being extracted
because of this layer. Furthermore, light still not being extracted by
the photonic crystal is understood to be internally reflected by the
photonic crystal in a diffracted manner and therefore under angles
different from the incident angle. As angles of the reflected light from
outside of the extraction cone change, a significant fraction of this light
is reflected inside the extraction cone and can therefore be extracted
from the opposite side of the cube at the non-patterned crystal face.
This provides an additional benefit in light yield when one reads out
the crystal from the face opposite to the patterned surface.
Simulations were run for the following cases: a cubic 10 ×10 ×10
mm3LYSO crystal, coupled to air, with and without a photonic crystal
slab, and also with and without Teflon wrapping as a comparison.
Figs. 5a and 5b, respectively, show their effect on light extraction. In
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R.H. Pots, M. Salomoni, S. Gundacker et al. Nuclear Inst. and Methods in Physics Research, A 940 (2019) 254–261
Fig. 5a-b. Simulation of light transmission at the crystal–air interface, in the case of
a 10 ×10 ×10 mm3LYSO cube equipped with and without a photonic layer for the
two cases that the scintillator is unwrapped (left) and wrapped with Teflon (right). For
both cases, the crystal is coupled via air at the photodetector interface.
Table 1
Simulated gain in light yield at first incidence for a 10×10×10 mm3
LYSO cube with a photonic crystal of 300 nm thickness minus a
residual TiO2layer, 390 nm in diameter and with a periodicity of
580 nm.
Residual TiO2Layer [nm]
0 120
LY Gain without Teflon: 1.51 1.59
LY Gain with Teflon: 1.25 1.35
the case where there is no wrapping (Fig. 5a), we calculate a light
gain of 1.51 due to the photonic crystal layer at first incidence. In the
case, however, where the scintillator cube is wrapped with Teflon the
benefit from the photonic layer is reduced resulting in a gain of only
1.25 at first incidence. This difference is attributed to the two different
internal angular light distributions (due to different light reflection
from the side walls and the back of the crystal) from the two separate
configurations studied.
Further simulations were made to understand the effect of a possible
residual TiO2layer estimated to be 120 nm thick, i.e. a remnant layer
from a possibly incomplete etching process, as observed in the SEM
image in Fig. 3c and discussed above. The effect of a residual TiO2
thickness of 120 nm was simulated and is shown in Table 1. In these
simulations we have assumed the total thickness of the TiO2layer prior
to etching to be 300 nm. Absorption by the residual TiO2, however, was
not considered, since it is highly transparent to 420 nm light. Simulated
gain in light yield at first incidence for a 10 ×10 ×10 mm3LYSO
cube with a photonic crystal of 300 nm thickness minus a residual TiO2
layer, 390 nm in diameter and with a periodicity of 580 nm.
From this we infer that the presence of the residual TiO2layer does
not necessarily lead to a degradation in light output; it may even have
a beneficial effect on the light yield. Further studies are needed though
to corroborate this assumption.
4. Measurements
4.1. Characterization methods
4.1.1. Light yield
The light yield is measured by exciting the scintillating crystal with
a137Cs gamma source. The generated light is collected by a photo-
multiplier (Hamamatsu R2059) mounted, without optical coupling, to
one face of the crystal. The PMT signal is digitized, and an energy
spectrum produced. The position of the photopeak is then equivalent
to the number of collected photons. The ratio between light measured
with an un-patterned and patterned crystal defines the gain in light
yield due to the introduced pattern.
4.1.2. CTR
The test bench for the CTR measurements consists of two scin-
tillators facing each other in a back-to-back arrangement and being
excited by two correlated and colinear gammas (511 keV) from a 22Na
source. As one of the crystals is used as a standard or reference crystal
with its own intrinsic time resolution determined from an independent
CTR measurement prior to our test series, the time resolution of the
crystals under investigation can be derived from the deconvolution of
the reference time resolution and the jointly measured CTR. Both the
reference crystal and the crystal under test are coupled to a SiPM; in
our case, the crystal under test is coupled to the SiPM with an air
gap, from where the signal is split (a) for time stamping with a high
frequency amplifier (1.5 GHz bandwidth) [11] and (b) for an inde-
pendent pulse height measurement with a low-noise analog operational
amplifier [11], geared to obtain the energy of the photoelectric peak.
The signals are digitized by a LeCroy DDA 735Zi oscilloscope. After
event selection constraining data to the photopeak (511 keV events),
the joint CTR is derived from the FWHM of the Gaussian fit of the
correlated time stamp (time delay) histogram. In order to measure the
light signal from the 10 ×10 ×10 mm3LYSO:Ce cube, we used a
Hamamatsu S13360 SiPM with
6×6 mm2size having 50x50 μm2single photon avalanche diodes.
This means that not the whole surface area was coupled to the SiPM and
only the central light was measured, resulting in a deterioration of the
CTR. Nevertheless, this does not compromise the validity of comparison
studies.
4.2. Results from light yield and energy resolution measurements
LY was investigated and compared for two wrapping scenarios,
i.e. without and with Teflon wrapping of the crystals, and respectively
three and two configurations each:
1. Without Teflon wrapping (three configurations)
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R.H. Pots, M. Salomoni, S. Gundacker et al. Nuclear Inst. and Methods in Physics Research, A 940 (2019) 254–261
Fig. 6. Coincidence time resolution obtained from a nanoimprinted LYSO cube without
Teflon wrapping or optical coupling mounted on a SiPM, compared to a reference
or un-patterned crystal: two crystal orientations w.r.t. the SiPM window were used:
patterned-face-to-SiPM (red squares), and opposite-face-to-SiPM (blue triangles). The
CTR is measured with a 3% accuracy. Data for the reference crystal are shown as
yellow dots. For all three configurations, the first data point at a threshold of 2 mV
is in the electronic noise floor of the readout and thus leads to very high CTR values
(not shown in the plot).
Fig. 7. Coincidence time resolution values of a nanoimprinted LYSO cube with Teflon
wrapping (but no optical coupling) compared to a reference, un-patterned LYSO cube.
The CTR is measured with a 3% accuracy. Measurements were made with a SiPM and
high frequency readout. For both crystals, the first data point at a threshold of 2 mV
is in the noise floor of the electronic readout.
a. Non-patterned reference crystal mounted to PMT;
b. Patterned crystal with patterned face mounted to PMT;
c. Patterned crystal with opposite face mounted to PMT.
2. With Teflon wrapping (two configurations)
a. Non-patterned reference crystal mounted to PMT;
b. Patterned crystal with patterned face mounted to PMT.
c. Measurements with the opposite face mounted to the
SiPM were not performed in order not to damage the
photonic pattern with the Teflon wrapping.
The results are shown in Table 2.
Fig. 8. Ratio of the CTRs obtained for the patterned and un-patterned crystal without
Teflon wrapping at the same detector threshold. The CTR is measured with a 3%
accuracy, leading to an accuracy of 4% for the measured ratio. This demonstrates
that anywhere, other than near the noise threshold, the photonic crystal has superior
performance, when it effectively improves CTR by more than a factor of two at highest
thresholds.
Fig. 9. Ratio of the CTRs obtained for the patterned crystal with Teflon-wrapping (in
only one mounting position) and the reference crystal. The CTR is measured with a 3%
accuracy, leading to an accuracy of 4% for the measured ratio. Data are taken without
optical coupling at thresholds of 2 mV to avoid noise saturation.
4.2.1. PMT measurements without Teflon wrapping
For the case of no wrapping, the patterned crystal improves LY and
energy resolution by a factor of 1.5 and 1.1 respectively. This is in good
agreement with the simulations assuming 0 nm residual TiO2layer. It is
interesting to see that a nearly identical gain in light yield is achieved
when the crystal is read out from the untreated side, opposite to the
patterned surface. This indeed is also expected from the simulations as
explained in Section 3.2. The gain in energy resolution is in line with
what one would expect on purely statistical grounds Eq. (1), taking into
account the error on the measurement.
4.2.2. PMT measurements with Teflon wrapping
When the crystals, reference and patterned ones, are wrapped in
Teflon, the relative gain in light yield drops to 1.4. This is slightly
higher than expected from the simulations and could be an indication
of the presence of the residual TiO2layer presumed in one of our
simulation schemes, in which case the light yield would match the
simulations perfectly. However, if the residual layer of 120 nm indeed
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Table 2
Comparison of simulated and measured LY and energy resolution and their improvements (gain). Both LY and energy are measured with a 5% accuracy, leading to an accuracy
of 7% for the measured gain.
Simulated
Gain with
0 nm
residual TiO2
Simulated
Gain with
120 nm
residual TiO2
Measured LY
with PMT
[Ph/MeV]
(×103)
Measured
Energy-Resolution
with PMT [%]
Measured
Gain in LY
with PMT
Measured Gain
in Energy
Resolution with
PMT
Expected Gain in
Energy
Resolution from
LY
Reference crystal
without wrapping
– – 4.4 19 – –
PhC facing detector
without wrapping
1.5 1.6 6.5 16 1.5 1.1 1.2
PhC from opposite side
without wrapping
6.5 17 1.5 1.1 1.2
Reference crystal
with wrapping
– – 13 11 – –
PhC facing detector
with wrapping
1.3 1.4 19 9.4 1.4 1.2 1.2
exists, the crystal measurements without wrapping should also match
the corresponding simulations, which is not the case. Another contribu-
tion to the slightly higher measured values with Teflon wrapping could
be due to a difference in how the Teflon affects the directionality of the
light in the simulations compared to the actual behavior. The energy
resolution improves in a configuration with Teflon wrapping i.e. by a
factor of 1.2, in line with photostatistics.
Comparison of simulated and measured LY and energy resolution
and their improvements (gain). Both LY and energy are measured with
a 5% accuracy, leading to an accuracy of 7% for the measured gain.
4.3. Results from coincidence time resolution (CTR) measurements
Similar to our foregoing LY measurements, the CTR was investi-
gated and compared for five different configurations as described in
Section 4.2, however, using SiPMs instead of PMTs.
Over a large threshold range, i.e. 2–115 mV, and the above configu-
rations a series of coincidence time resolution (CTR) measurements was
made using the high frequency readout for time stamping as explained
in Section 4.1.2. The results of these runs are shown in Fig. 6 (scenario
1) and Fig. 7 (scenario 2), where the CTR is plotted against the applied
threshold.
From Fig. 6 (unwrapped scenario) we notice that the highest co-
incidence time resolution (i.e. lowest CTR value) is obtained for the
photonic crystals of 390 ps FWHM with the patterned surface read out
by the SiPM, and 375 ps when read out from the opposite crystal face
(values taken from the fits in Fig. 6). In this scenario, the reference
crystal achieves a CTR of 450 ps FWHM only. This translates into a
CTR-gain of 1.2 at lowest thresholds increasing systematically towards
higher threshold values (see also Fig. 8).
On the other hand, Fig. 7 (wrapped scenario) clearly shows that
wrapped scintillators, as expected [12], provide higher time resolution
than non-wrapped crystals. Yet, the photonic crystal still has a superior
CTR than its reference counterpart, i.e. achieving 300 ps FWHM versus
317 ps FWHM (values taken from the fits in Fig. 7) constituting a factor
of about 1.1 improvement as compared to the non-patterned crystal at
low thresholds, and increasing steadily at higher thresholds (Fig. 9).
The measurements above also show that the CTR is very sensitive
to threshold changes, though less pronounced for the photonic crystals
compared to their untreated references. The same holds for those
scintillators that are wrapped in Teflon in contrast to the unwrapped
ones. This correlation is better visualized in Figs. 8 and 9corresponding
to unwrapped and wrapped crystals, respectively, where the CTR ratio
of reference and photonic crystals is shown as a function of threshold.
In Tables 3 and 4, corresponding to the two scenarios of unwrapped
and wrapped scintillators, we list some specific values for the gain
in CTR at given thresholds and compare this with the CTR gain to
be expected from LY measurements considering photostatistics only,
i.e. taking the square root of the light yield gain. It can be seen that
the resulting CTR improvements (gain) correlate (within the statistical
uncertainties) with the light yield gain for low thresholds, i.e. 1.22
(=1.5) versus a LY gain of 1.5 in the un-wrapped case. On the
other hand, for the wrapped case, the expectation in CTR improvement
considering pure photostatistics is higher, i.e. 1.4 =1.18 as compared
to the measured value of about 1.1.
The results demonstrate that the nanoimprinted scintillator transfers
light more efficiently than an un-treated crystal. Additionally, and in
line with our findings for LY and energy resolution, it again makes no
difference from which side, front face or reversed, the crystal is read
out.
The improvement in CTR over that of the reference device becomes
rather high when raising thresholds to >10 mV, notwithstanding its
excellent values also at lower thresholds. It is also worth noting that
the CTR of the patterned crystal is much less sensitive to threshold
changes than the reference crystal. This is important for highly inte-
grated systems, where tradeoffs in the electronic bandwidth and power
consumption do not allow to operate the detectors at lowest thresholds
possible.
The high dependence of the CTR gain on the leading-edge threshold,
i.e. low gain for low thresholds and high gain for high thresholds,
can be explained by the change of light transfer modes in the pho-
tonic crystal in contrast to its non-patterned counterpart. In order to
investigate this behavior, we conducted very preliminary Monte-Carlo
simulations and found that, if an additional photon transfer time spread
due to the presence of the photonic layer is included, the modeled CTR
improvement versus the leading-edge detection threshold approaches
that of the measurements. The additional time smearing in the photonic
crystal arises from the fact that about 50% of the direct photons are
reflected back into the crystal whereas delayed photons that normally
are not collected by the photodetector can now, under the influence of
the nanopattern, reach the SiPM. This behavior can be understood by
looking at Fig. 4, where larger angles for photons exiting the crystal also
mean a longer travel path and therefore a larger delay time. Additional
photons extracted by the photonic crystal at larger angles thus come at
later times and do contribute to the signal formation at higher leading-
edge thresholds and therefore improve the CTR at higher thresholds. On
the other hand, the slightly reduced number of photons arriving very
early at the photodetector lowers the CTR gain at lower thresholds.
In other words, the photonic pattern in this particular case transfers
early arriving photons to later times, nevertheless, increasing the total
amount of photons extracted. Hence, also at earlier times the number
of photons is higher than in the non-patterned crystal and therefore
improves the photostatistics leading to an overall improved CTR. In
this sense the photonic pattern changes the weight of the diffractive
modes. Depending on the application and the scintillator geometry this
behavior varies, and it is even thinkable to use this feature of photonic
crystals to optimize the time structure of detected photons in special
cases.
259
R.H. Pots, M. Salomoni, S. Gundacker et al. Nuclear Inst. and Methods in Physics Research, A 940 (2019) 254–261
Table 3
List of CTR measurements and their gain compared to expected values derived from LY measurements: All measurements are made without
Teflon wrapping. The values are taken from the fits in Fig. 6. The CTR is measured with a 3% accuracy, leading to an accuracy of 4% for the
measured gain in CTR. Considering the error in the measured gain in LY, the expected gain in CTR from the measured LY as an accuracy of
about 4%.
Crystal face
being read out:
Best measured
CTR FWHM [ps]
CTR Improvement (Gain)
@ best CTR @ 10 mV
Threshold
@ 100 mV
Threshold
Expected from LY
measured w/ PMT
Reference crystal 450
Photonic 390 1.2 1.3 1.9 1.22
Opposite 375 1.2 1.3 2.0 1.22
Table 4
List of CTR measurements and their gain compared to expected values derived from LY measurements: All measurements are
made with Teflon wrapping. The values are taken from the fits in Fig. 7. The CTR is measured with a 3% accuracy, leading
to an accuracy of 4% for the measured gain in CTR. Considering the error in the measured gain in LY, the expected gain in
CTR from the measured LY as an accuracy of about 4%.
Crystal face
being read out:
Best measured
CTR FWHM [ps]
CTR Improvement (Gain)
@ best CTR @ 10 mV
Threshold
@ 100 mV
Threshold
Expected from LY
measured w/ PMT
Reference crystal 317 – – – –
Photonic 300 1.1 1.1 1.3 1.18
5. Summary and discussion
We have successfully produced a photonic crystal slab, manufac-
tured via nanoimprint lithography and made of TiO2on top of a
10 ×10 ×10 mm3LYSO:Ce cube. The produced pattern is of high
quality, where the imprinted structures have the desired shape of pillars
with fine-grained periodicity. We have simulated the produced pattern
and also the effect of a possible residual TiO2layer left over from
the etching process. From these simulations we can conclude that the
residual layer does not necessarily have a detrimental effect and hence
decrease the gain in light yield.
The photonic crystal delivers a significant increase in light yield,
both when extracted from the patterned surface or from the face oppo-
site to it. In the case that no wrapping of the crystal is used, the total
gain in light yield is 1.5 and the corresponding improvement in energy
resolution 1.1, irrespective of the two adjacent exit faces, patterned
or un-patterned, being read out. This gain in light yield agrees with
our predictions from the simulations. The gain in energy resolution,
however, is slightly lower than expected from the equivalent gain in
LY on arguments that only photostatistics is taken into account. This
might be due to inhomogeneities in the nanopattern of the photonic
crystal.
Time resolution seems to particularly benefit from photonic pattern-
ing, especially for bare (un-wrapped) crystals and at higher detection
thresholds. In that case, gains in CTR ranging from 1.2 at low threshold
to more than a factor of 2 at higher thresholds have been observed.
Particularly, CTR improvements at highest time resolutions obtained
near the detection threshold are well in line with our expectations from
photostatistics and confirmed by the corresponding LY measurements.
Still further work is needed to identify and factorize all influences, other
than statistical ones, on the time resolution, especially for data at higher
thresholds.
In the case where the tested crystals are wrapped in Teflon tape, a
method traditionally used to increase their light yield, the ‘‘photonic’’
effect and its benefit on the time resolution become less pronounced
than observed with bare crystals. In terms of LY and energy resolution
we have observed an improvement of 1.4 and 1.1, respectively, owing
to the photonic pattern, where the gain in energy resolution is slightly
lower than expected from pure photostatistics. The obtained gains in
CTR are, as we had expected from our simulations, more moderate
accordingly, i.e. 1.1 at lowest threshold and 1.3 at higher thresholds.
In conclusion, it is shown that photonic imprinting of scintillators,
in particular with the chosen process and its resulting high-quality
pattern, can significantly improve light yield, energy and time reso-
lution in scintillator-based detection systems. While the effect is still
modest as long as wrapped scintillators are used in conjunction with
detectors operating at very low detection thresholds, the potential of
this technique is far from being exhausted, hence giving new incentives
for further investigations on the basis of novel and more elaborate
patterns and their production methods. Those efforts could then include
a comparison with different crystal surface states, such as de-polishing
or micro-structuring of the crystal face. There is still room for im-
provement and optimization of suitable pattern types and shapes, in
conjunction with different types of wrapping and optical coupling for
the crystals.
Acknowledgments
This research has been carried out in the framework of the Crystal
Clear Collaboration. This work was supported by the Eurostars Eu-
reka project No. 8974 (TURBOPET), ERC Advanced Grant No. 338953
(TICAL), ERC Proof of Concept Grant No. 680552 (Ultima), the Wolf-
gang Gentner Program of the German Federal Ministry of Education
(grant No. 05E15CHA), CERN Knowledge Transfer Fund and collabo-
ration CERN-Haute Savoie.
The authors are grateful to Thomas Meyer for his profound assis-
tance and help in writing and editing this article.
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261
... 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][20][21][22] (self-assembly and e-beam, and nanoimprint), GYGAG and SrI 2 [23] (nanoimprint), CsI(Na) [24,25], and BGO [26] (self-assembly). A more comprehensive description of the PhC physics is reported in [17,18]. ...
... However, the PhC structures have been mostly optimized and tested using air coupling 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 suitable 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, 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 [17][18][19][20][21][22]26]. ...
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The influence of the geometry of the scintillators is presented in this paper. We focus on the effect of narrowing down the section of crystals that have a given length. The light output of a set of crystals with very similar scintillating properties but different geometries measured with several coupling/wrapping configurations is provided. We observe that crystals shaped in thin rods have a lower light output as compared to bulk or sliced crystals. The effect of unpolishing the crystal faces is also investigated, and it is shown that highest light outputs are not necessarily obtained with crystals having all faces polished. Simulation results based on a realistic model of the crystal that implements light scattering on the crystal edges are in agreement with the experimental data. Fine-tuning of this model would allow us to further explore the details of light propagation in scintillators and would be highly valuable to fast timing detection and highly granular detectors.
Recent developments in Geant4. Nuclear Instruments and Methods in Physics Research Section A: 344
  • J Allison
J. Allison et al. Recent developments in Geant4. Nuclear Instruments and Methods in Physics Research Section A: 344