∎ 11institutetext:  INFN Laboratori Nazionali del Gran Sasso, Assergi (AQ)
67100, Italyemail 22institutetext: Department of Physics, Carleton University,
Ottawa, ON K1S 5B6, Canada 33institutetext: AstroCeNT, Nicolaus Copernicus
Astronomical Center of the Polish Academy of Sciences, 00-614 Warsaw, Poland
44institutetext: INFN Bologna, Bologna 40126, Italy 55institutetext: Gran
Sasso Science Institute, L’Aquila 67100, Italy 66institutetext: Physics
Department, Princeton University, Princeton, NJ 08544, USA 77institutetext:
Fondazione Bruno Kessler, Povo 38123, Italy 88institutetext: Trento Institute
for Fundamental Physics and Applications, Povo 38123, Italy 99institutetext:
Department of Physics, Royal Holloway University of London, Egham TW20 0EX, UK
1010institutetext: Physik Department, Technische Universität München, Munich
80333, Germany 1111institutetext: Biological and Chemical Research Centre,
Faculty of Chemistry, University of Warsaw, Żwirki i Wigury 101, 02-089
Warsaw, Poland 1212institutetext: School of Physics and Astronomy, University
of Edinburgh, Edinburgh EH8 9YL, UK 1313institutetext: Physics and Astronomy
Department, University of California, Los Angeles, CA 90095, USA
currentNikhefCurrently at Nikhef and the University of Amsterdam, Science
Park, 1098XG Amsterdam, Netherlands emailCorresponding address:
<EMAIL_ADDRESS>
# Direct comparison of PEN and TPB wavelength shifters in a liquid argon
detector

M. G. BoulayLNGS, Carleton  V. CamilloLNGS  N. CanciLNGS  S.
ChoudharyAstroCeNT  L. ConsiglioLNGS  A. FlamminiBOINFN  C. GalbiatiGSSI,
Princeton  C. GhianoLNGS  A. GolaFBK, TIFPA  S. HorikawaGSSI  P. KachruGSSI
I. KochanekLNGS  K. KondoLNGS  G. KorgaRHUL, LNGS  M. KuźniakAstroCeNT  M.
KuźwaAstroCeNT  A. LeonhardtTUM  T. ŁęckiCNBCh  A. MazziFBK, TIFPA  A.
MoharanaGSSI  G. NieradkaAstroCeNT  G. PaternosterFBK, TIFPA  T. R.
PollmannTUM,currentNikhef  A. RazetoLNGS  D. SabloneLNGS  T.
SworobowiczAstroCeNT  A. M. SzelcEdinburgh  C. TürkoğluAstroCeNT  H. WangUCLA

###### Abstract

A large number of particle detectors employ liquid argon as their target
material owing to its high scintillation yield and its ability to drift
ionization charge over large distances. Scintillation light from argon is
peaked at $128\text{\,}\mathrm{nm}\text{/}$ and a wavelength shifter is
required for its efficient detection. In this work, we directly compare the
light yield achieved in two identical liquid argon chambers, one of which is
equipped with PolyEthylene Naphthalate (PEN) and the other with TetraPhenyl
Butadiene (TPB) wavelength shifter. Both chambers are lined with enhanced
specular reflectors and instrumented with SiPMs with a coverage fraction of
approximately 1%, which represents a geometry comparable to the future large
scale detectors. We measured the light yield of the PEN chamber to be
39.4$\pm$0.4(stat)$\pm$1.9(syst)% of the yield of the TPB chamber. Using a
Monte Carlo simulation this result is used to extract the wavelength shifting
efficiency of PEN relative to TPB equal to $47.2\pm 5.7\text{\,}\mathrm{\char
37\relax}\text{/}$. This result paves the way for the use of easily available
PEN foils as a wavelength shifter, which can substantially simplify the
construction of future liquid argon detectors.

###### Keywords:

Liquid Argon Chambers Wavelength Shifters TPB PEN Light Yield SiPM

††journal: Eur. Phys. J. C

##  1 Introduction

Liquid argon (LAr) based detectors are used in direct dark matter searches,
experiments investigating properties of neutrinos and in other applications.
LAr acts as an efficient scintillator, however its emission peak is in the
vacuum ultraviolet (VUV) at $128\text{\,}\mathrm{nm}\text{/}$. Due to lack of
photo-sensors highly efficient at VUV wavelengths, the scintillation light is
typically wavelength shifted (WLS) to the visible range by dedicated films or
coatings applied on the detector walls WlsReview .

Production of vacuum evaporated coatings of Tetra-Phenyl Butadiene (TPB)
Francini , which is the most commonly used WLS in LAr detectors, requires high
vacuum conditions and a process that is not trivial to scale up to hundreds of
square meters of surface area, needed for the upcoming generation of
experiments. PolyEthylene Naphthalate (PEN), a polymeric wavelength shifting
film available in large formats, has been recently proposed as a scalable and
inexpensive alternative to TPB for large LAr-based detectors KuzniakPEN .
Earlier literature-based estimates as well as measurements of wavelength
shifting efficiency (WLSE) of PEN relative to that of TPB have suffered from
large systematic uncertainty caused by: (1) temperature and excitation-
wavelength dependence of the fluorescence process, different in both materials
KuzniakPEN , (2) significant sample-to-sample and grade-to-grade variation
ProtoDPspread  and (3) geometrical effects SotoOton .

In this work we directly compare the light yield (LY) obtained in two
identical LAr detector chambers, using PEN or TPB as WLS. This approach allows
to reduce the systematic uncertainty on the WLSE of PEN and provides a robust
performance test in conditions matching the intended application. For the
first time, the geometry used for such comparison is fully representative of
large LAr detectors, particularly the planned veto detector of the
DarkSide-20k experiment, where photo-sensors cover only a small fraction of
the surface ESPPU , and the rest is lined with wavelength-shifting reflectors.

##  2 Setup

Figure 1: Schematic of 2PAC (see text for more details). Sectional view shows
the location of photo-detectors and the arrangement of foils: ESR (yellow) and
PEN (blue) in the left chamber, TPB-coated ESR (cyan) in the right chamber.

A dedicated detector setup has been manufactured for this test, dubbed 2PAC (2
Parallel Argon Chambers), consisting of two cylindrical chambers, with the
endcaps instrumented with SiPMs and supplied with spacer rings. The chambers
are made out of aluminum and contain a volume of LAr with
$47.6\text{\,}\mathrm{mm}\text{/}$ diameter and
$81\text{\,}\mathrm{mm}\text{/}$ length.

The fraction of inner surface covered with photo-sensors ($F$) is
approximately $1\text{\,}\mathrm{\char 37\relax}\text{/}$, while the rest is
lined with the Enhanced Specular Reflector (ESR) Vikuiti  from 3M, which is a
$98\text{\,}\mathrm{\char 37\relax}\text{/}$ reflective polymeric multilayer
mirror, commonly used in LAr detectors due to its superior reflectivity to
visible light. In one chamber, the ESR is coated with a
$3\text{\,}\mathrm{\SIUnitSymbolMicro m}\text{/}$ layer of vacuum-evaporated
TPB. The other chamber instead contains a
$25\text{\,}\mathrm{\SIUnitSymbolMicro m}\text{/}$ PEN film (Teonex
Q51)111This grade of PEN has been selected as the most promising candidate in
a campaign of ex-situ measurements, to be described in a separate publication.
attached in front of the ESR with up to 1 mm of LAr gap between both films,
maintained by spacer rings at the endcaps. Both chambers are fully immersed in
a LAr bath and are held with a frame attached to threaded rods suspended from
the lid of the cryostat, as shown in Figure 1.

The small SiPM coverage forces wavelength shifted photons to be reflected,
passing each time through WLS, on average $1/F$ times, before they reach a
photo-detector. This makes the overall light yield very sensitive not only to
WLSE, but also to effective reflectivity of the ESR/WLS, including the
attenuation in WLS itself.

The motivation behind having a dual chamber system was the possibility to
characterize both configurations together in the same LAr bath and with the
same radioactive source, thus reducing the systematic uncertainty from varying
run conditions.

###  2.1 Cryogenics

Figure 2: The STAR (SiPM Testbench in Argon) cryogenic system.

The measurement is carried out inside a cryogenic test setup (STAR), which
provides a clean liquid argon environment and enables detector tests with the
argon scintillation light. Figure 2 shows the whole cryogenic system, which
consists of a cryostat and an argon re-circulation circuit.

The cryostat is a vacuum insulated double jacket dewar with an inner diameter
of $240\text{\,}\mathrm{mm}\text{/}$ and a height of
$930\text{\,}\mathrm{mm}\text{/}$ and a CF250 top flange from CryoFAB. It is
equipped with an argon gas condenser cooled by a Cryomech PT60 single-stage
pulse tube cryo-cooler capable of delivering $60\text{\,}\mathrm{W}\text{/}$
corresponding to a theoretical liquefaction rate of
$6\text{\,}\mathrm{s}\mathrm{l}\mathrm{/}\mathrm{min}$.

A liquid nitrogen powered heat exchanger pre-cools the input argon gas for the
condenser from room temperature to about $100\text{\,}\mathrm{K}\text{/}$. The
result is an increased liquefaction speed to
$15\text{\,}\mathrm{s}\mathrm{l}\mathrm{/}\mathrm{min}$ with a consumption of
about $40\text{\,}\mathrm{s}\mathrm{l}\mathrm{/}\mathrm{min}$ of nitrogen gas.

An inner cylindrical container (⌀$190\text{\,}\mathrm{mm}\text{/}$ by
$300\text{\,}\mathrm{mm}\text{/}$) limits the volume of liquid argon necessary
to operate the system, significantly reducing the filling time to around
$8\text{\,}\mathrm{h}\text{/}$. Several PT100 RTDs are installed to monitor
the level of liquid argon.

The argon gas is purified through a SAES PS4-MT3 getter saes , which
eliminates impurity in argon gas to sub-ppb level. The argon gas flow is
promoted by a circulation pump (Metal Bellows MB-111) and regulated by a
Sierra SmartTrack 100 gas flow controller. The nominal flow in steady state is
$5\text{\,}\mathrm{s}\mathrm{l}\mathrm{/}\mathrm{min}$ and the operational
pressure of Ar inside the cryostat is about
$+50\text{\,}\mathrm{mbarg}\text{/}$.

The system allows the injection of isotopes of [83m]Kr into the re-circulation
line, as a calibration source with an activity of few tens of becquerels. Such
a source is used in several dark matter detectors ds50first  since it is not
filtered by the getter, has a half life of $1.83$ hours (after which it
becomes inert) and provides a q-value of $41.5\text{\,}\mathrm{keV}\text{/}$.

###  2.2 Readout

Figure 3: The photo-detectors lodge a $1\text{\,}{\mathrm{cm}}^{2}\text{/}$
SiPM with a low noise cryogenic amplifier in a single
$15\text{\times}30\text{\,}{\mathrm{mm}}^{2}\text{/}$ board.

Photons are detected by cryogenic photo-detectors, designed to incorporate a
$1\text{\,}{\mathrm{cm}}^{2}\text{/}$ SiPM and a low noise pre-amplifier in a
compact board, Figure 3. The units implement the circuit discussed in cryo-pre
, with a gain of $10.7\text{\,}\mathrm{kV}\text{/}\mathrm{A}$ (halved by the
$50\text{\,}\mathrm{\SIUnitSymbolOhm}\text{/}$ back termination), an output
voltage noise of $350\text{\,}\mathrm{\SIUnitSymbolMicro V}\text{/}$ (RMS-AC)
for a bandwidth of $36\text{\,}\mathrm{MHz}\text{/}$ (at
$-3\text{\,}\mathrm{dB}\text{/}$). The circuit board is realized in Arlon 55NT
to match the coefficient of thermal expansion of silicon iza-bari .

The signals are acquired by a CAEN V1720 unit capable of digitizing $8$
channels at $250\text{\,}\mathrm{M}\mathrm{S}\mathrm{/}\mathrm{s}$ with
$12\text{\,}\mathrm{b}\mathrm{i}\mathrm{t}\mathrm{s}$. A discrete trigger
system, based on NIM modules, implements a requirement for a majority of two
channels observing at least ten photo-electrons.

The photo-detectors were individually qualified in liquid nitrogen before the
installation on the setup.

###  2.3 SiPM performances

The photo-detectors are based on SiPMs of the FBK NUV-HD-Cryo family
$7.9\text{\times}11.7\text{\,}\mathrm{mm}\text{/}$ with a single photon
avalanche diode cell size of $30\text{\,}\mathrm{\SIUnitSymbolMicro
m}\text{/}$ and a quenching resistor of
$5\text{\,}\mathrm{M\SIUnitSymbolOhm}\text{/}$ (at
$86\text{\,}\mathrm{K}\text{/}$).

We studied the gain of the SiPM (defined as the integral of the generated
current) in the range
$29\text{\,}\mathrm{V}\text{/}38\text{\,}\mathrm{V}\text{/}$. The gain scales
linearly with a cell capacitance of $65\pm 2\text{\,}\mathrm{fF}\text{/}$,
well in agreement with the expectations from the RLC bridge measurements. The
breakdown, obtained from the intercept of the gain versus bias, is
$V_{bd}^{C}$=$26.8\pm 0.1\text{\,}\mathrm{V}\text{/}$. The peak amplitude of
the amplified signal corresponds to $1.5\text{\,}\mathrm{mV}\text{/}$ per volt
of bias above the break-down, $V_{bd}^{A}$=$27.5\pm
0.1\text{\,}\mathrm{V}\text{/}$, at $77\text{\,}\mathrm{K}\text{/}$.

Figure 4: Correlated noises for the NUV-HD-Cryo SiPM at
$77\text{\,}\mathrm{K}\text{/}$ as excess of photo-electrons measured in a
gate of $7\text{\,}\mathrm{\SIUnitSymbolMicro s}\text{/}$ for single photo-
electron events. Lines are drawn to guide the eyes.

SiPM-based photo-detectors are affected by the presence of detection noise:
the dark rate (DCR), the cross-talk (iCT) and the after-pulse (AP) cryo-nuv .
All these elements may potentially affect the signal shape and the
reconstructed number of photo-electrons. It is therefore important to quantify
them. Measurements were performed in a dedicated setup with the same photo-
detectors operating in liquid nitrogen to avoid the intrinsic scintillation of
argon. Relevant properties of the SiPMs remain stable over the temperature
variation between liquid argon and liquid nitrogen cryo-nuv ; cryo-rgb .

The DCR drastically drops below $100\text{\,}\mathrm{K}\text{/}$: we observe a
rate of $1\text{\,}\mathrm{cps}\text{/}$ per SiPM at the maximum over-voltage.
Therefore the contribution to argon scintillation (that lasts a few
microseconds) can be safely neglected. With such a low DCR rate, acquiring
enough statistics for the iCT analysis in dark is impractical: therefore we
used a short laser pulse to illuminate the SiPM. The iCT is calculated by
scaling per each over-voltage the average number of acquired photo-electrons
by the expected laser occupancy, measured on the empty events. Our data are
approximated by the geometric chain cross-talk introduced by Vinogradov
vinogradov-analytical  that underestimates the iCT at higher over-voltage by a
maximum of $10\text{\,}\mathrm{\char 37\relax}\text{/}$. The AP is calculated
by measuring the relative charge of single photo-electron events in a gate of
$7\text{\,}\mathrm{\SIUnitSymbolMicro s}\text{/}$ after the primary pulse.
This definition matches the analysis of the scintillation events which
integrates the normalized waveforms. The results of these measurements are
shown in Figure 4 for both iCT and AP in the over-voltage range of
$2.5\text{\,}\mathrm{V}\text{/}10.5\text{\,}\mathrm{V}\text{/}$.

##  3 Analysis

Figure 5: [241]Am data (red) and environment data (blue) in the detectors with
TPB and PEN, acquired at an over-voltage of $6.5\text{\,}\mathrm{V}\text{/}$.

An [241]Am gamma-ray source was used to perform the energy calibration of the
system. As a cross-check in Run 2 an additional calibration with a [83m]Kr
source was performed. Calibrating the detectors with two sources allowed us to
confirm the linearity of the energy scale. The [241]Am source was fixed on the
outer surface of the Dewar, at a location corresponding to the center of the
detector system. The $59.5\text{\,}\mathrm{keV}\text{/}$ gammas penetrate the
liquid argon buffer outside the 2PAC detectors (reaching the active volumes)
and exhibit a high cross-section for the full absorption peak. The [83m]Kr is
diffused uniformly inside the detector Kr-mixing-paper  and decays with two
transitions with a half-life of $154\text{\,}\mathrm{ns}\text{/}$.

In the analysis, we integrate the normalized waveforms for a gate of
$7\text{\,}\mathrm{\SIUnitSymbolMicro s}\text{/}$. In such a window
$99\text{\,}\mathrm{\char 37\relax}\text{/}$ of the scintillation light is
detected (including the ballistic deficit due to the finite recharge time of
the SiPMs). Absorption and re-emission by the WLS and delays introduced by the
optical paths inside the detectors have no effect on this timescale. The
normalization procedure includes removal of the baseline, calculated in the
pre-trigger, and scaling by the gain of the photo-detectors. The SiPM
correlated noises are accounted for statistically by dividing the integrals by
the factor $(1+\textrm{AP})(1+\textrm{iCT})$, where the values of AP and the
iCT are indicated in Figure 4.

Data were acquired without the [241]Am and [83m]Kr sources to subtract the
environmental background and isolate better the full absorption peaks, which
are then fitted with a Gaussian model, as shown in Figure 5. From the fit we
extract the mean number of photo-electrons for the sum of both photo-detectors
in each chamber. The light yield is given by the ratio of the detected photo-
electrons and the energy deposited in the medium. The light yield values
obtained with both sources were found to be consistent with each other.

Figure 6: Light yield of the chambers equipped with PEN and TPB (top panel)
and their ratio (bottom panel) for two experiments runs.

One of the photo-detectors (bottom of TPB chamber) suffered from reduced light
collection efficiency due to window misalignment. The size of the effect was
evaluated with a Monte Carlo (see later) based on as-built geometry and
factored into the reported light yield as a correction for the raw charge
spectrum of the interested photo-detector. This correction increases the LY of
the TPB chamber by $6\text{\,}\mathrm{\char 37\relax}\text{/}$.

To check the PDE consistency of all four SiPMs (which were extracted from the
same silicon wafer) a second run was acquired with top photo-detectors of the
two chambers swapped. The top/total asymmetry was at the level of $50\pm
1\text{\,}\mathrm{\char 37\relax}\text{/}$ for both chambers and in both runs,
proving the homogeneity of the SiPMs (and the goodness of the correction for
the misaligned window).

The results are shown in Figure 6: for the second run the light yield is lower
by about $15\text{\,}\mathrm{\char 37\relax}\text{/}$. This can be explained
by the different purity of the argon in the system, e.g. a sub-ppm level of
Nitrogen contamination Acciarri_2010 . Monitoring of the LAr purity and
triplet lifetime sufficiently sensitive to confirm such level of contamination
exceeds the current capabilities of the setup.

##  4 Simulation

As wavelength shifted photons reflect from the walls many times before
reaching photosensors, the final LY strongly depends not only on WLSE, but
also on the reflectivity of all inner surfaces. In order to decouple both
effects and evaluate the PEN WLSE, a Monte Carlo model of 2PAC was implemented
in Geant4, taking into account its geometry and optical properties of all
relevant materials: liquid argon, ESR, TPB, PEN and SiPMs, as well as the
geometry and materials of the surrounding cryostat. The Monte Carlo results
were validated against a simplified analytic model for estimation of the light
yield of scintillation detectors Segreto_2012 .

Figure 7: Hemispherical reflectivity measured at 7∘ angle of incidence with a
spectrophotometer equipped with an integrating sphere for: ESR, PEN air-
coupled to ESR, TPB evaporated on ESR, TPB evaporated on ESR corrected for the
spurious fluorescence component based on Francini , and SiPMs (see legend).

The reflectivity spectra used in the simulation for ESR, TPB-coated ESR, PEN
air-coupled to ESR and SiPMs have been measured using a Shimadzu UV-3600
spectrophotometer equipped with a 15 cm diameter integrating sphere accessory
(LISR-3100), collecting the light reflected into the entire hemisphere, see
Fig. 7. The measurements were performed relative to a BaSO4 reference, with
$98.0\pm 0.5\text{\,}\mathrm{\char 37\relax}\text{/}$ absolute reflectivity in
the visible range.

Literature values for other parameters have been used, namely the emission
spectra of PEN Mary:1997  and TPB Francini  at 87 K. Absorption of VUV light
in LAr and of visible light in LAr and in TPB was considered negligible.

A range of values for the TPB WLSE is present in the literature WlsReview : in
the simulation it was fixed at $100\text{\,}\mathrm{\char 37\relax}\text{/}$,
since the relative performance of both WLS was of main interest. In the
relative measurement presented here, factors identically affecting the
absolute LY for both configurations cancel out, particularly the LAr
scintillation yield and the SiPM PDE scaling.

The PDE for SiPMs, taken at $6\text{\,}\mathrm{V}\text{/}$ of over-voltage,
was measured at room temperature by FBK, similar to the published result ds20k
, and its values in LAr are not known. For the parameters described above, the
simulation predicts a light yield of $2.6\pm
0.5\text{\,}\mathrm{p}\mathrm{e}\mathrm{/}\mathrm{k}\mathrm{eV}$ for the TPB
chamber, consistent with the corresponding measurement, interpolated to
$2.5\pm 0.05\text{\,}\mathrm{p}\mathrm{e}\mathrm{/}\mathrm{k}\mathrm{eV}$. The
scaling of the PDE cancels out in the determination of the relative PEN WLSE
(as discussed above), however the change of shape of the PDE curve with
respect to the room temperature data could bias the result. In the Monte Carlo
we simulated a shift of PDE by $20\text{\,}\mathrm{nm}\text{/}$ towards lower
wavelengths as a result of the increased extinction coefficient of visible
light in silicon at cryogenic temperature dash-newman . This shift produces an
effect of about $2\text{\,}\mathrm{\char 37\relax}\text{/}$ that is negligible
in respect to the other uncertainties.

Input | Allowed range | $\delta_{syst}/\frac{LY_{PEN}}{LY_{TPB}}$
---|---|---
$R$ calibration |  0.980$\pm$0.005 | 0.07
(reference) |  |
$R$ correction |  0.994$\pm$0.006 | 0.08
(slits and bolts) |  |
$R$ in LAr |  1.000$\pm$0.0035 | 0.05
TPB $R$ spectrum | extrapolate | 0.02
(spurious component | below 415 nm |
removal) |  |
SiPM reflectivity |  0.171$\pm$0.017 (PEN) | 0.01
|  0.167$\pm$0.017 (TPB) | 0.01
PDE curve shape | shift by –20 nm | 0.02
$T$ dependence |  |
Total |  | 0.12
Table 1: Systematic uncertainties on key inputs to the Monte Carlo simulation
and their relative effect on the final LY ratio of PEN and TPB configurations.
See text for more details.

Table 1 lists the systematic uncertainties on input parameters for the Monte
Carlo and their effect on the simulated LY ratio of PEN and TPB
configurations. The dominant factors are the uncertainty on the absolute
calibration of the reflectivity measurements, and on the losses due the
fraction of 2PAC surface not covered with the ESR film.

The SiPM reflectivity is non-trivial to model due to the anti-reflective
coating. We used our own measurement, shown in Fig. 7 and estimated the
maximum variation of reflectivity between air and LAr for bare silicon. The
final curve used in the model was the mean of the air and LAr-corrected
values, with a systematic error assigned to cover both extremities.

To evaluate the reflectivity change of PEN/ESR, TPB/ESR and ESR in LAr with
respect to ex-situ measurement in air, the optical stack model of ESR used in
Ref. LOIGNONHOULE201762  was extended to accommodate an additional film layer,
a separating gap, and the LAr medium. The effect due to the change of the
medium averaged over the respective WLS emission spectrum is small and was
included as a systematic.

In the integrating sphere measurement, the reflectivity curve of TPB-coated
ESR below $415\text{\,}\mathrm{nm}\text{/}$ is affected by a spurious
wavelength shifted component, therefore we extrapolated our data as described
in (Francini, , Figure 18 therein). Similar effect for PEN takes place only
below 380 nm where the overall reflectivity is already low and has therefore
negligible impact on the result.

PEN bulk absorption length for Teonex Q51 films has not been measured
directly, however the integrating sphere reflectivity measurement of PEN/ESR
used in the simulation (blue dashed line in Fig. 7) combines reflection,
refraction and absorption effects. To avoid accounting for them twice, in the
simulations the refractive index of PEN was set equal to that of LAr and the
absorption in PEN was disabled. As a cross-check, a separate simulation was
performed following the microscopic rather than the effective approach, where
instead the literature refractive index and absorption length of PEN were
used, together with the reflectivity of bare ESR (black dotted line in Fig.
7). The attenuation length varies about 1 cm near the peak region (425 nm to
475 nm), as reported for injection moulded PEN tiles Efremenko_2019 ; for the
cross-check a constant 1 cm value was set in the simulation. Both results
agree to 1%, and varying the absorption length by 1 cm changes the result
approximately by the size of the overall systematic uncertainty. The Monte
Carlo results are robust against sub-mm non-uniformities in the size of the
LAr gap between PEN and ESR.

Finally, we consider the recently reported effect of TPB ‘emanation’ from
evaporatively coated ESR foils, potentially biasing the relative measurement
through presence of non-zero concentration of TPB dissolved or suspended in
LAr inside of the PEN chamber. In Ref. Asaadi_2019  2 ppb molar fraction of
TPB in LAr was observed after 24-72 h soak of a 103 cm2 foil in 119 cc of LAr.
In 2PAC VUV photons travel on average 2 cm prior to reaching PEN. Assuming
conservatively that similar concentration is reached in 2PAC (157 cm2 of the
foil in 8.5 l of LAr) within 40-140 h of data taking and applying the TPB
extinction coefficient at 200 nm of 2$\times$104 (M$\cdot$cm)-1
WallaceWilliams_1994 , one arrives at $<$0.6% contribution from VUV light
converted in TPB. Therefore, the effect is small in comparison with other
systematic uncertainties and neglected in the analysis.

In order to simulate LY for both configurations, monoenergetic 59.5 keV gammas
were generated from a point on the outer cryostat wall, corresponding to the
actual geometry of the experimental setup. In the simulation gammas are
propagated through the materials of the cryostat, and then scintillation
photons induced by energy deposits in LAr are tracked, taking into account
wavelength shifting, attenuation, reflection and detection processes. A
Gaussian fit to the full absorption peak in the photoelectron spectrum was
used to evaluate the simulated LY, similarly as in the analysis of the
detector data.

##  5 Results

Figure 6 reports the ratio of the light yields of the two chambers. Weighted
average of such ratio, for all over-voltages and runs, results $39.4\pm
0.4\textrm{(stat)}\pm 1.9\textrm{(sys)}\text{\,}\mathrm{\char
37\relax}\text{/}$, where the systematic uncertainty includes contributions
from the residual top/bottom asymmetry and the difference between the runs.
The ratio is affected by the reduced LY of the second run by less than
$2\text{\,}\mathrm{\char 37\relax}\text{/}$: the quoted value is the average
ratio from both runs.

We used the MC simulation, which accounts for the geometry and optics of the
system, to evaluate the relative WLSE of PEN with respect to TPB from the
observed LY. The systematic uncertainties on the input parameters have been
studied and summarized in Table 1. Systematic uncertainty on the result from
all contributions have been added in quadrature and propagated into the final
result, derived as

$\frac{WLSE_{PEN}}{WLSE_{TPB}}=\left(\frac{LY^{exp.}_{PEN}}{LY^{exp.}_{TPB}}\right)/\left(\frac{LY^{sim.}_{PEN}}{LY^{sim.}_{TPB}}\right).$
(1)

The resulting value for WLSE of PEN relative to TPB for the
$25\text{\,}\mathrm{\SIUnitSymbolMicro m}\text{/}$ thick Teonex Q51 foil is
47.2$\pm$5.7%, where the uncertainty is dominated by the systematic error.
This is a significantly higher result than the previously projected/measured
results for other PEN grades, particularly $34.0\pm 1.1\text{\,}\mathrm{\char
37\relax}\text{/}$ for $125\text{\,}\mathrm{\SIUnitSymbolMicro m}\text{/}$
thick Teonex Q53 Abraham:2021otn  or $38.0\pm 7\text{\,}\mathrm{\char
37\relax}\text{/}$ for $125\text{\,}\mathrm{\SIUnitSymbolMicro m}\text{/}$
thick Teonex Q83 KuzniakPEN .

##  6 Conclusions

Scalability and simplicity of PEN motivates its use as an alternative WLS in
large LAr detectors. Benchmarking its performance against commonly used TPB is
challenging due to temperature and excitation wavelength dependence as well as
non-trivial geometrical effects. The 2PAC detector, built to emulate in small
scale the key features of a large LAr detector (walls lined with WLS reflector
and SiPM coverage fraction of approximately 1%) allowed, for the first time,
for a meaningful comparison of both materials in conditions equivalent to the
end-goal application: in LAr and with the 4$\pi$ light collection.

The ratio of LYs obtained in 2PAC chambers is a robust result which can be
used to model the yield of larger LAr detectors equipped with SiPMs and with
reflector and SiPM coverage fractions comparable with 2PAC. Employing a Monte
Carlo simulation we extracted the value of PEN WLSE relative to TPB, which
exceeded that of those recently reported in the literature for other PEN
grades.

We note that Teonex Q51 has no planarizer coating (contrary to Q65
MacDonald2007 ), and does not undergo high temperature annealing step during
production (which is the case for Q81, Q83 and Q65 grades, in order to reduce
their shrinkage properties), which may affect the film crystallinity. Finally,
lower film thickness, in comparison to $125\text{\,}\mathrm{\SIUnitSymbolMicro
m}\text{/}$ used previously, reduces losses from self-absorption. Detailed
results of a survey of available PEN grades will be published separately.

The PEN WLSE result is sufficient for many applications and can now be
reliably used to simulate arbitrary detector configurations. Given that such a
result was obtained with a technical grade of PEN, which was industrially
mass-produced and not optimized for wavelength shifting of LAr scintillation
light, it is compelling to further develop PEN and similar polymeric materials
for applications as WLS in the future very large detectors, such as ARGO or
DUNE, where WLS surfaces between hundreds and thousands of square meters will
be needed.

## Acknowledgements

This work was supported from the European Union’s Horizon 2020 research and
innovation programme under grant agreement No 952480 (DarkWave project). The
2PAC detector construction was carried out with the use of CEZAMAT (Warsaw)
cleanroom infrastructures financed by the European Regional Development Fund;
we are grateful to Maciej Trzaskowski of CEZAMAT for support. We are grateful
to Prof. Magdalena Skompska for access to the spectrophotometer, which was
purchased by CNBCh (University of Warsaw) from the project co-financed by EU
from the European Regional Development Fund. We acknowledge support from the
Istituto Nazionale di Fisica Nucleare (Italy) and Laboratori Nazionali del
Gran Sasso (Italy) of INFN, from NSF (US, Grant PHY-1314507 for Princeton
University), from the Royal Society UK and the Science and Technology
Facilities Council (STFC), part of the United Kingdom Research and Innovation,
and from the International Research Agenda Programme AstroCeNT (MAB/2018/7)
funded by the Foundation for Polish Science (FNP) from the European Regional
Development Fund. Fruitful discussions with Gemma Testera are gratefully
acknowledged.

## References

  * (1) M. Kuźniak A.M. Szelc, Instruments 5(1) (2021).  DOI 10.3390/instruments5010004
  * (2) R. Francini, R.M. Montereali, E. Nichelatti et al., J. Instrum. 8(09), P09006 (2013).  DOI 10.1088/1748-0221/8/09/p09006
  * (3) M. Kuźniak, B. Broerman, T. Pollmann G.R. Araujo, Eur. Phys. J. C 79(4), 291 (2019).  DOI 10.1140/epjc/s10052-019-6810-8
  * (4) R. Dorrill et al.  Performance studies of PEN and TPB as UV wavelength shifting coatings in liquid argon (2020).  Presented at Neutrino 2020
  * (5) J. Soto-Otón.  Measurements of the polyethylene naphthalate performance as a wavelength shifter in ProtoDUNE-DP (2020).  DOI 10.5281/zenodo.4252545.  Presented at Neutrino 2020
  * (6) The Global Argon Dark Matter Collaboration.  Future dark matter searches with low-radioactivity argon (2018).  Input to the European Particle Physics Strategy Update 2018-2020, https://indico.cern.ch/event/765096/contributions/3295671
  * (7) Application Guidelines ESR Family (2018).  http://multimedia.3m.com/mws/media/1389248O/application-guide-for-esr.pdf
  * (8) PS4-MT3/15-R/N specifications (2002).  www.saespuregas.com/Library/documents/s110-243_rev__528.pdf
  * (9) P. Agnes, T. Alexander, A. Alton et al., Physics Letters B 743, 456 (2015).  DOI 10.1016/j.physletb.2015.03.012
  * (10) M. D’Incecco, C. Galbiati, G.K. Giovanetti et al., IEEE Trans. Nucl. Sci. 65(4), 1005 (2018).  DOI 10.1109/TNS.2018.2799325
  * (11) I. Kochanek, Nucl. Instrum. Methods Phys. Res. A 980, 164487 (2020).  DOI 10.1016/j.nima.2020.164487
  * (12) F. Acerbi, S. Davini, A. Ferri et al., IEEE Trans. Elec. Dev. 64(2), 521 (2017).  DOI 10.1109/TED.2016.2641586
  * (13) C.E. Aalseth, F. Acerbi, P. Agnes et al., JINST 12(09), P09030 (2017).  DOI 10.1088/1748-0221/12/09/P09030
  * (14) S. Vinogradov, Nucl. Instrum. Methods Phys. Res. A 695, 247 (2012).  DOI 10.1016/j.nima.2011.11.086
  * (15) D.S. Akerib, S. Alsum, H.M. Araújo et al., Phys. Rev. D 96, 112009 (2017).  DOI 10.1103/PhysRevD.96.112009
  * (16) R. Acciarri et al., JINST 5(06), P06003 (2010).  DOI 10.1088/1748-0221/5/06/p06003
  * (17) E. Segreto, JINST 7(05), P05008 (2012).  DOI 10.1088/1748-0221/7/05/p05008
  * (18) D. Mary, M. Albertini C. Laurent, J. Phys. D Appl. Phys. 30(2), 171 (1997).  DOI 10.1088/0022-3727/30/2/004
  * (19) C.E. Aalseth et al., Eur. Phys. J. Plus 133(3) (2018).  DOI 10.1140/epjp/i2018-11973-4
  * (20) W.C. Dash R. Newman, Phys. Rev. 99, 1151 (1955).  DOI 10.1103/PhysRev.99.1151
  * (21) F. Loignon-Houle, C.M. Pepin, S.A. Charlebois R. Lecomte, Nucl. Instrum. Methods Phys. Res. A 851, 62 (2017).  DOI 10.1016/j.nima.2017.01.051
  * (22) Y. Efremenko, L. Fajt, M. Febbraro et al., JINST 14(07), P07006 (2019).  DOI 10.1088/1748-0221/14/07/p07006
  * (23) J. Asaadi, B. Jones, A. Tripathi et al., JINST 14(02), P02021 (2019).  DOI 10.1088/1748-0221/14/02/p02021
  * (24) S.E. Wallace-Williams, B.J. Schwartz, S. Moeller et al., The Journal of Physical Chemistry 98(1), 60 (1994).  DOI 10.1021/j100052a011
  * (25) Y. Abraham et al., (2021).  arXiv:2103.03232
  * (26) W.A. MacDonald, M.K. Looney, D. MacKerron et al., Journal of the Society for Information Display 15(12), 1075 (2007).  DOI 10.1889/1.2825093