###### Abstract

Wavelength shifters and their applications for liquid argon detectors have
been a subject of extensive R&D over the past decade. This work reviews the
most recent results in this field. We compare the optical properties and usage
details together with the associated challenges for various wavelength
shifting solutions. We discuss the current status and potential future R&D
directions for the main classes of wavelength shifters.

###### keywords:

Wavelength shifters; Fluorescence; Polyethylene naphthalate; PEN; Tetraphenyl
butadiene; TPB; Liquid argon detectors; Noble liquid detectors; VUV detection

xx 1 5 Received: date; Accepted: date; Published: date Wavelength shifters for
applications in liquid argon detectors Marcin Kuźniak 1,∗ and Andrzej M. Szelc
2 Marcin Kuźniak and Andrzej M. Szelc Correspondence<EMAIL_ADDRESS>Tel.: +48-22-120-1823 29.40.Mc and 33.50.Dq and 95.35.+d and 14.60.Pq

##  1 Introduction

Liquid argon (LAr) is used as a scintillator in a number of fundamental
physics experiments. These include direct dark matter searches (WArP Acciarri
and et al (2010), DEAP Amaudruz, P.-A and et al (2019), MiniCLEAN Rielage and
et al (2015), ArDM Boccone and et al (2009), DarkSide Aalseth and et al
(2018)), prototype and full-scale neutrino oscillation experiments (ICARUS
Arneodo and et al (1997), MicroBooNE Acciarri and et al (2017), LArIAT Szelc
(2013), SBND Szelc (2016), ProtoDUNE Pietropaolo (2017), DUNE Abi and et al
(2020)), experiments searching for neutrinoless double beta decay
(0$\nu\beta\beta$-decay) (GERDA Agostini et al. (2017), LEGEND Zsigmond
(2020)) and measuring the coherent elastic neutrino-nucleon scattering
(CENNS-10 Tayloe (2018)). Other applications of LAr-based detectors include
medical physics (PET) 3dp (2018) and nuclear or homeland security Nikkel et
al. (2012); Erlandson and et al (2019). For more details on detectors based on
liquefied noble gasses see Aprile et al. (2006) or one of the more recent
review papers Chepel and Araújo (2013); Marchionni (2013).

The peak wavelength of the LAr scintillation light is 128 nm, i.e. in vacuum
ultraviolet (VUV), where the efficiencies of currently available VUV-optimized
photosensors are typically fairly low, reaching at most 15% Gallina and et al
(2019) (for the most recent review of the VUV photon detector technology see
also Zheng et al. (2020)). Therefore, an efficient wavelength shifter (WLS), a
material which absorbs the VUV light and isotropically re-emits it at a longer
wavelength, is needed to enable its detection with standard blue-sensitive
photosensors: conventional photomultiplier tubes (PMT), silicon
photomultipliers (SiPM) or avalanche photodiodes (APD). Most wavelength
shifting compounds considered typically have very high re-emission
efficiencies and fluorescence lifetimes comparable with LAr scintillation
timing, leading to an overall increase in light collection in LAr detectors.

With the next generation very large detectors and novel application ideas on
the horizon, the past decade has seen extensive R&D in the field of wavelength
shifters for LAr applications. One of the main directions of these efforts is
to develop WLS applicable to detectors ranging between hundreds of tons, for
the Argo dark matter search The Global Argon Dark Matter Collaboration (2018),
and tens of kilotons for the DUNE long-baseline neutrino experiment. These
detectors will have surface areas of the inner detector walls ranging between
hundreds and thousands of square meters, that may need to be covered with an
efficient and stable WLS.

This scaling-up is non-trivial for the WLS dominantly used in the field until
now, e.g. the organic 1,1,4,4-tetraphenyl-1,3-butadiene (TPB). The vacuum
deposition method commonly used to deposit TPB films is very difficult to
scale because of the associated high vacuum requirements, costs and labor. In
consequence, a number of alternative deposition techniques and WLS have been
proposed and investigated, among them polyethylene naphthalate (PEN) polymeric
films. As an independent effort, WLS tailored to particular background
mitigation purposes or optimized for PET or 0$\nu\beta\beta$-decay
applications have been studied and developed.

This paper provides an overview of WLS currently used or considered for LAr
applications, as well as covers practical aspects of deploying them in the
detectors, e.g. in a form of thin coatings. It also highlights the remaining
open questions about TPB properties (including its absolute conversion
efficiency), which are still debated in the literature, as well as collecting
and comparing data on its alternatives. Detailed description of the energy
transfer mechanisms for each of the materials discussed is beyond the scope of
this paper; it is, nevertheless, referenced, when available.

As specific requirements of each application can determine a different WLS as
the optimal choice, there is no single universal WLS ideal for all
applications. The purpose of this review is to provide essential data to allow
to make an informed WLS selection for a specific application, or to guide
defining further R&D goals needed to make such a choice.

##  2 Fundamentals

###  2.1 Basic requirements

A WLS suitable for experimental applications in liquid argon should have the
following properties:

  * •

high absorption cross-section for the short wavelength light (e.g. 128 nm in
LAr),

  * •

high photoluminescence quantum yield (PLQY), i.e. the ratio between the number
of re-emitted photons to the number of absorbed photons,

  * •

high Stokes shift, i.e. low overlap between absorption and emission spectra
resulting in the WLS being transparent to its own emission wavelength.

From a practical point of view, 128 nm light is efficiently absorbed with an
absorption length comparable to its wavelength in a vast majority of
materials, which makes the first requirement trivially met (notable exceptions
include noble gasses and liquids and specialized fluoropolymers).

The PLQY and the Stokes shift requirements are therefore the most critical.
Specifically, the WLS performance at the LAr 128 nm excitation wavelength and
at 87 K temperature is essential for use in LAr detectors. This is non-
trivial, as the PLQY and Stokes shift can depend on both of the wavelength and
temperature. The emission spectrum should ideally have a peak in blue or green
in order to match the most efficient photosensors available on the market.

Important for dark matter and 0$\nu\beta\beta$-decay detectors, and less so
for other applications, is the degree of radiopurity of WLS materials and
technical feasibility of implementing them without introducing radioactive
isotopes into the detector, which could potentially lead to dangerous
backgrounds.

Last but not least, long term stability in cryogenic conditions is a necessary
requirement for all LAr applications.

###  2.2 Conversion mechanisms

Figure 1 depicts the relation between main processes which play the role in
wavelength shifting: absorption, internal conversion, fluorescence,
phosphorescence, intersystem crossing.

Figure 1: Jablonski diagram, explaining photophysical processes in molecular
systems. Empty arrows denote non-radiative transitions: vibrational
relaxation, internal conversion (IC) and intersystem crossing (ISC), see
Valeur and Berberan-Santos (2012).

Absorption of a photon brings a molecule from the fundamental electronic
state, $S_{0}$, to one of the vibrational levels of the first singlet
electronic state, $S_{1}$, (timescale: 10-15 s), which is then followed by a
(non-radiative) vibrational relaxation towards the lowest vibrational level of
S1 (10-12 \- 10-11 s). Fluorescence is the emission associated with the
relaxation $S_{1}\rightarrow S_{0}$ (10-10 \- 10-7 s). Due to the energy loss
in the excited state due to vibrational relaxation, the fluorescence spectrum
is shifted towards higher wavelengths (lower energy) than the absorption
spectrum. The Stokes shift is then the gap between the maximum of the first
absorption band and the maximum of fluorescence.

Naively, the fluorescence emission wavelength should always be longer than
that of absorption, however at room temperature some molecules may be in a
vibrational level higher than the lowest level of $S_{0}$, which could lead to
partial overlap of the energy and emission spectra.

The higher energy singlet states such as $S_{2}$ relax to $S_{1}$ via non-
radiative processes. Radiative transitions predominantly take place from the
lowest excited states of $S_{1}$ to the ground state $S_{0}$ or one of its
excited vibrational sub-levels. In the case of phosphorescence, the process is
similar except it orginates from the first electronic triplet state, $T_{1}$.
The internal conversion process typically competes with the radiative
transition (fluorescence) but has much lower probability. The transition
$S_{1}\rightarrow S_{0}$ is classified as spin-allowed, and is very quick
taking on the order of $\sim$10-9 s. On the other hand, the timescale of the
$T_{1}\rightarrow S_{0}$ transition falls between $\mu$s and ms because the
process is spin-forbidden.

##  3 Wavelength shifter types and properties

There are several WLS used in LAr experiments. In this section we discuss
their basic properties and compare them.

Table 1 summarizes available experimental data and practical information on
the properties of the most representative WLS examples, described in more
detail in the rest of this Section. In a number of cases, and particularly for
the PLQY parameter for some WLS, significant discrepancies in the published
results are present. There are multiple possible reasons for this, which are
not always clearly identifiable, including: (1) aging effects or degradation
of either the WLS (e.g. TPB) sample or the reference sample, (2) geometrical
effects due to inconsistent optical diffusivity of the WLS and reference
samples, (3) coating quality (pinholes) and deposition technique details, and
(4) uncertainties regarding the intensity of the impinging light.

Table 1: Fundamental properties of common WLS materials used in LAr detectors:
peak emission wavelength ($\lambda_{em}$), PLQY, re-emission lifetime
($\tau$), refractive index ($n$), vapour pressure ($p_{sat}$), and approximate
sublimation temperature ($T_{m}$).

Wavelength shifter |  $\lambda_{em}$ [nm] | PLQY @ 128 nm |  $\tau$ [ns] | n | psat [mbar] |  $T_{m}$ [∘C] | Comment
---|---|---|---|---|---|---|---
TPB | 430 | 0.6 Benson et al. (2018)–2 Graybill et al. (2020) | 2 | 1.7 | 10-11 | 204 |
p-Terphenyl | 350 | 0.82 Kumar and Datta (1979) | 1 | 1.65 |  | 213 | PLQY @ 254 nm
bis-MSB | 440 | 0.75–1 Gehman (2013); Araujo (2019) | 1.5 | 1.7 |  | 180 | PLQY rel. to TPB
pyrene | 470 | 0.64 Birks et al. (1966) | 155 | 1.8 | 6$\cdot$10-6 | 150 | PLQY @ 260 nm
PEN | 420 | 0.4–0.8 Kuźniak, M et al. (2019) | 20 | 1.75 | – | 270 | PLQY rel. to TPB

###  3.1 Organic wavelength shifters

The luminescence of organic compounds is essentially based on localized
$\pi$-electron systems (i.e. single or conjugated aromatic rings) within
individual organic molecules.

Organic molecules with well-developed $\pi$-conjugated systems tend to show
intense electronic absorption in the UV to visible wavelength regions because
of a transition from the bonding $\pi$* orbital into the anti-bonding $\pi$*
orbital (i.e. a transition from $S_{0}$ to $S_{1}$ state) Yen et al. (2006).

The most common class of organic WLS, used in LAr detector applications, are
conjugated aromatic hydrocarbons and their derivatives, see chemical
structures in Figure 2 and the emission spectra in Figure 3. Generally, their
absorption as well as the emission maxima, shift to longer wavelengths with
increasing number of the conjugated aromatic rings.

Figure 2: Chemical structures of wavelength shifters relevant for liquid argon
detectors: (a) 1,1,4,4-tetraphenyl-1,3-butadiene (TPB), (b) pyrene, (c)
p-terphenyl, (d) 1,4-Bis(2-methylstyryl)benzene (bis-MSB), (e) polyethylene
naphthalate (PEN).

.

Although not discussed further in the text, for completeness we highlight
other major classes of organic WLS:

  * •

Heterocycles ($\pi$-conjugated molecules containing heteroatoms), e.g.
coumarine or PPO, are another common class of fluors, however with lower
photostability in comparison to aromatic hydrocarbons; and no significant
history in the context of LAr detectors.

  * •

Nanostructured organosilicon luminophores (NOL) Borshchev and et al. (2019)
are a class of compounds in which aromatic hydrocarbon or heterocycle-based
molecules are connected through silicon atoms, which permits excitation energy
transfer via the Förster mechanism (FRET). NOLs can be designed and optimized
for particular properties, including high PLQY at VUV excitation. While
currently not used in LAr detectors, their applications in liquid xenon have
been explored Akimov et al. (2012, 2017).

In general, the quantum yields of organic WLS fluorescence are strongly
dependent on the temperature because of its larger effects on thermal non-
radiative decay processes. At LAr temperature quantum yields tend to be high
as many rotational and vibrational motions of pendant groups are frozen.

Figure 3: Emission spectra of wavelength shifters relevant for liquid argon
detectors, area normalized, and quantum efficiency curves of photosensors.
Spectra measured at room temperature are shown (red, dashed lines) and, when
available, also at 87 K (blue solid lines). (a)
1,1,4,4-tetraphenyl-1,3-butadiene (TPB) Francini et al. (2013), (b) pyrene
Birks et al. (1966), (c) p-terphenyl Akgun et al. (2008), (d)
1,4-Bis(2-methylstyryl)benzene (bis-MSB) Gehman (2013), (e) polyethylene
naphthalate (PEN) Mary et al. (1997). (f) Quantum efficiency curves at room
temperature for FBK NUV-HD-SF SiPMs at 10 V overvoltage (black triangles)
Aalseth and et al (2018) and Hamamatsu R11065 PMTs (green solid line) Acciarri
and et al (2012).

####  3.1.1 TPB

Because of its robustness and blue emission spectrum conveniently matching the
typical PMT photocathode response, TPB is the most commonly used WLS in LAr
applications. It was first identified as a WLS with high PLQY (improving by a
factor of three with respect to sodium salicylate commonly used at that time)
already in 1967 Brunet et al. (1963); Mai and Drouin (1971), and subsequently
proposed as a candidate for commercial and research purposes (UV astronomy)
Burton and Powell (1973). Together with p-terphenyl it was then recognized as
a useful WLS for dark matter detectors based on xenon Lally et al. (1996).
McKinsey et al. McKinsey et al. (1997) reported results for TPB and several
other fluors, comparing their PLQY under exposure to gaseous argon
scintillation light for coatings deposited using all major techniques used
until today (see Section 4). In particular, for evaporated coatings a broad
efficiency maximum was reported for thickness in the 0.2-0.3 mg/cm2 range.

An extensive study of the TPB WLS properties as a function of the excitation
wavelength, temperature and coating type has been published by Francini et al.
Francini et al. (2013). For coatings evaporated on specular reflector
substrates, Vikuiti Enhanced Specular Reflector (ESR) by 3M Vik (2018); Weber
et al. (2000), the vibronic emission spectrum structures of TPB, unresolved at
room temperature, emerged at cryogenic temperatures. At least four distinct
structures were visible at 87 K, which were attributed to the stretching mode
vibration of trans-butadiene. This effect was later independently confirmed
Corning et al. (2020) and is typical for pure organic films. It is not present
in either liquid or solid (e.g. polymeric matrix) solutions, where the
randomness of the medium around the TPB molecule leads to broadening of the
vibronic structures into a single, unresolved band. At 87 K the 128 nm
excitation PLQY increased by about 10% with respect to the room temperature
conditions Francini et al. (2013).

TPB can form four known polymorphs Girlando et al. (2010); Bacchi et al.
(2014) depending on its deposition or growth conditions (temperature,
pressure, solvent), as confirmed on vacuum evaporated coatings Broerman et al.
(2017). Due to different spatial packing these polymorphs exhibit slightly
different solid properties and may also subtly differ in the vibronic
structure. Therefore, the exact conditions of the coating production process
have impact on the properties of the final WLS film.

Microcrystaline structure of TPB coatings leads to significant scattering of
visible light. The mean scattering length for a vacuum evaporated coating was
measured to be in the range 2–3 $\mu$m Stolp et al. (2016).

Despite the importance of this parameter and the widespread use of TPB, its
absolute PLQY remains controversial. While early TPB measurements used sodium
salicylate as a reference, an absolute measurement of PLQY as a function of
excitation wavelength down to 120 nm was first attempted in Ref. Gehman et al.
(2011), which reported a constant shape of the emission spectrum regardless of
the wavelength and a PLQY of 1.2 at 128 nm. However, in a subsequent paper
Benson et al. (2018) released several years later, that result was attributed
to miscalibration of the photodiode used. A re-analysis of that original
experiment was presented based on (1) the corrected photodiode calibration and
(2) a geometric acceptance evaluated with Geant4/RAT Monte Carlo including the
full optical model of the coating and of the setup. Both the old re-analyzed
data and data from a new series of measurements indicated significantly lower
PLQY values of approximately 0.6. As a validation of the optical model used,
experimental data on coating response intensity vs. coating thickness were
reproduced with the simulation. Finally, Graybill et al. Graybill et al.
(2020) has recently reported the TPB effective conversion efficiency of
approximately 2 at the LAr scintillation wavelength, indicating also
significant aging effects that can lead to efficiency decreasing by up to a
factor of three. Clearly, several factors are at play in this puzzle,
including: aging effects (discussed in more detail in Section 5), poorly known
and controlled absolute PLQY of reference samples used in the historic
measurements, and non-trivial light propagation effects caused by scattering
in TPB itself and at the optical interfaces with the medium and with the
substrate, which are notoriously difficult to model and account for.

Hints of multiphoton emission from TPB are possibly linked with the issue of
delayed fluorescence of TPB. The dominant fluorescence time constant of TPB
for evaporated coatings is short: $\sim$1.7 ns Flournoy et al. (1994).
Additional longer time constants, attributed to triplet excited states, have
been first reported in alpha induced TPB scintillation Pollmann et al. (2011);
Veloce et al. (2016). These time constants formed a long non-exponential and
temperature dependent emission tail, which was approximated and parametrized
with a sum of exponentials, with time constants reaching 40 $\mu$s. Segreto
Segreto (2015); Segreto et al. (2016) reported the presence of similar delayed
emission also in the VUV-induced TPB fluorescence, in a measurement using
gaseous Ar with quenched triplet emission as the prompt light source, and
pointed out that the LAr scintillation photon energy (9.7 eV) likely exceeds
the ionization potential of TPB. Electrons released in the process would have
enough energy to excite both singlet and triplet states of surrounding
molecules. Excited triplet states decay via internal conversion to the lowest
lying triplet state $T_{1}$, which is long-lived. These states interact
through the triplet-triplet interaction, $T_{1}+T_{1}\rightarrow S_{1}+S_{0}$,
and at the end the delayed scintillation photon is produced by the de-
excitation of the resulting $S_{1}$ state. Sanford et al. Stanford et al.
(2018) followed up with a measurement of TPB response in LAr to alphas, betas
and 128 nm photons, using a longer 2 ms acquisition window. The data revealed
even longer $\sim$ms scale delayed emission and generally were well described
with the Voltz-Laustriat Voltz, R. and Laustriat, G. (1968) model, developed
to describe the triplet-triplet interaction dynamics in scintillators. Such
TPB time response model, combined with a LAr scintillation model, which
included a LAr physics-driven intermediate component, and the instrumental
response, was then used to successfully fit the LAr pulse shapes in the
DEAP-3600 detector Adhikari, P. and et al. (2020).

Summarizing the above results, there is convincing evidence for ionization of
TPB with 128 nm photons, which leads to the excitation of both singlet and
triplet TPB states, with the triplet excitation responsible for the delayed
TPB emission. Naively, this could lead to the PLQY of $\sim$2, as observed by
Graybill et al. Graybill et al. (2020), however with half of the photons
delayed and therefore not useful for modes of detection requiring fast
conversion (needed for PSD). At the same time, inhibitors, such as dissolved
or trapped oxygen, could affect the triplet, $T_{1}$, TPB states. e.g. causing
quenching Voltz, R. and Laustriat, G. (1968),(Stanford, 2017, Section A.1.3),
further complicating the overall picture. This highlights the need for further
tests, in particular for time resolved measurements of various types of
coatings.

Recapitalizing, TPB remains the most robust WLS of choice for LAr detectors,
and has been characterized in the greatest level of detail, although some of
its fundamental properties are still debated in the literature and deserve
further research. Looking forward, the main challenge is with the scale-up for
applications in the future very large detectors, where both efficient
production of many hundreds of square meters of coated surfaces, and long term
stability, on the timescale of a decade or more, are crucial.

####  3.1.2 Polyethylene naphthalate

PEN is chemically a cousin of the well known polyethylene terephthalate (PET
or Mylar). Because of its low oxygen permeability and convenient mechanical,
electrical and optical properties, it is industrially mass produced for
applications in e.g. food packaging or electronics. Although it is also one of
the components of the Vikuiti ESR, specular reflector commonly used in LAr
detectors, and its fluorescence properties have been known for a long time
Ouchi et al. (2007), it was only proposed as a scintillator in 2011 Nakamura
et al. (2011), and then, recently, as a WLS particularly well suited for large
scale LAr detectors Kuźniak, M et al. (2019).

The broad deep blue (430 nm) emission band from PEN originates from excimer
formation between naphthalene-dicarboxylate units which results in a
1($\pi$,$\pi$*) fluorescence transition. Therefore, similar to pyrene and
contrary to other discussed organic WLS, it exhibits no vibrational peak
structure upon cooldown.

As discussed in Kuźniak, M et al. (2019), early literature suggested a
significant increase of PEN PLQY at 128 nm excitation wavelengths and 87 K,
compared to room temperature and near UV excitation, leading to extrapolated
values consistent with TPB in LAr detector conditions. More recent ex-situ
measurements performed at room temperature relative to TPB indicate PLQY in
the range of 0.3-0.6 w.r.t. TPB Kuźniak, M et al. (2019); Efremenko et al.
(2020). Similar results were obtained in the ProtoDUNE-DP detector where the
PEN foils were installed in LAr as WLS in front of a fraction of PMTs Soto-
Otón (2020). In the context of a significant spread of PLQY results for TPB
discussed in Section 3.1.1, it is difficult to interpret these results in
absolute terms until measurements against a well calibrated and stable
reference will be made. Similarly, R&D work on PEN scintillation performed in
the context of LEGEND, has shown that the scintillation yield of commercially
available PEN is lower by a factor of three Abt et al. (2020) when compared to
the original results by Nakamura et al. Nakamura et al. (2011); a significant
spread of WLS yields for various PEN samples has also been recently reported
Dorril and et al (2020). Although the origin of this apparent discrepancy
remains unclear, it might be caused by aging effects (similar as in the case
of TPB), differences in the production process, or by the different
composition of additives between various PEN grades. It is possible that a
tailored and optimized PEN production process could lead to an improved yield,
comparable to TPB.

The fluorescence lifetime of PEN is approximately 20 ns Janecek (2012) at room
temperature, based on measurements on Vikuiti ESR. Qualitatively, given that
no significant distortion of the LAr scintillation time constants was observed
in other time-resolved measurements using either ESR Baudis et al. (2015) or
PEN Soto-Otón (2020), the PEN lifetime at 87 K should be of similar value. No
subdominant longer time constants have been reported so far in the
photoluminescence spectrum. There is, however, evidence of delayed
scintillation emission induced by irradiation with alpha particles (Wang,
J.-J, 2017, Section 7.3).

The main advantage of PEN is the ease of use and low cost. It appears that
even commercially produced off the shelf grades of PEN perform sufficiently
well for use in certain types of LAr detectors. PEN is available in large
format rolls or sheets and can be easily used as a liner for large LAr tanks.
The maximum PLQY achievable with PEN, assuming optimized production process
and appropriate storage and handling conditions, remains an open question.

####  3.1.3 p-Terphenyl

P-terphenyl (pTP) was one of the first WLS studied for applications in noble
liquid detectors McKinsey et al. (1997). Its PLQY was found to be comparable
with TPB, however the broad emission band extending below 350 nm was less
convenient for use with common photosensors. Also, lower concentrations of pTP
than TPB were achievable in polystyrene (PS) matrix without crystallization
effects.

Otherwise, many phenomena known for TPB also apply for pTP, including the
evidence for delayed alpha induced scintillation, see Xu (2013); Xu et al.
(2017).

Recently, pTP has found an important application in ARAPUCA modules, which are
under development for DUNE, see Section 6.1.4 for details.

pTP is listed as very toxic to aquatic life.

####  3.1.4 bis-MSB

1,4-Bis(2-methylstyryl)benzene (bis-MSB) is a WLS used in large organic liquid
scintillator experiments, where it is directly dissolved in the liquid
scintillator mixture. Because of its quantum efficiency close to unity Feng et
al. (2015) at near UV wavelengths, fast re-emission time constant and cost
effectiveness, it has also been considered an interesting candidate for the
LAr long baseline neutrino program.

References Gehman (2013); Baptista and Mufson (2013) describe relative 128 nm
conversion efficiency measurements of polymeric films doped with bis-MSB and
TPB, showing comparable yields. A more recent measurement showed consistent
PLQY at 128 nm for evaporated coatings of both materials (Araujo, 2019,
Appendix A2).

One disadvantage of bis-MSB is its toxicity, significantly higher than for
other discussed WLS materials, and its environmental impact on aquatic life.

####  3.1.5 Pyrene

Birks et al. Birks et al. (1966) described the properties and fluorescence
mechanism of pyrene in detail. The fluorescence spectrum has features
attributed to excimer (at 470 nm) and monomer emission (below 400 nm).
Although the absorption spectrum of pyrene is characteristic of the monomer,
the emission spectrum is consistent with the expectation for excimer emission.
The crystal structure of pyrene is dimeric in form – the elementary unit of
the crystal lattice is a pair of parallel pyrene molecules. Monomers initially
excited by the incident light can rapidly ($<$1 ns) convert into excimer
excitons with $>$100 ns lifetime, where the excitation physically manifests as
reduced distance between both molecules.

The temperature dependence of pyrene fluorescence results in a $\sim~{}$50%
efficiency and time constant increase at 87 K compared to room temperature
Birks et al. (1966).

Due to high vapour pressure, pyrene coatings are unstable in vacuum, and
cannot be deposited with vacuum evaporation. Pyrene can be used mixed with
polymers, as the movement and diffusion of large fluor molecules in a
polymeric matrix is very strongly constrained, as they essentially become
trapped between tangled polymeric chains, which leads to some of the lowest
diffusion constants ever measured (e.g. D=9.4$\cdot$10-19 cm2/s for pyrene in
PS Procházka (2016)). The ratio of excimer to monomer emission increases with
pyrene concentration in the polymer matrix.

Because of its long time constant, pyrene can be used for background
mitigation purposes, currently under study with potential application in the
context of the DEAP-3600 detector, see Section 6.2.

Pyrene is listed as very toxic to aquatic life with long lasting effects.

###  3.2 Inorganic wavelength shifters

In solid inorganic fluors luminescence is determined by their lattice
structures, as opposed to the molecular structures in organic WLS, which has a
few consequences. Firstly, the luminescence of inorganic materials is altered
or disappears altogether when the crystals melt or decompose. In particular it
is very different for nanoparticles (or quantum dots) and chemically identical
macroscopic bulk materials. Secondly, the lattice structure is much more
resistant to aging and UV damage effects than organic WLS molecules.

Elementary gases can also be used as WLS; one of the first wavelength shifters
used with noble gas scintillators was a gas admixture of nitrogen Aprile et
al. (2006), although it also acted as a quencher, which reduced the overall
scintillation yield. Currently, vibrant R&D is underway on employing xenon as
a distributed WLS for LAr detectors.

####  3.2.1 Nanoparticles

Application of inorganic nanoparticles in LAr detectors has been proposed
recently Sahi et al. (2018). Nanoparticle-based WLS coatings deposited
directly on photosensors could be used instead of traditional organic WLS. A
consequence of the small size of particles is the increase of the energy band
gap as well as formation of discrete energy levels at the edges of the band
gap. The absorption maximum is shifted to higher energies (from visible to UV
wavelengths).

One of the main advantages of inorganic WLS is the durability and resistance
to aging effects that, as discussed later, is a common issue for organic WLS
materials. In addition, the emission wavelength can be tuned to match the
optimal sensitivity of an existing photodetector, since the energy gap is
highly dependent on the nanoparticle size.

Ref. Sahi et al. (2018) lists ZnS:Mn,Eu, ZnS:Mn, Cu-Cy, CdTe and LaF3:Ce
nanoparticles as showing the most potential for LAr applications. The most
significant photosensor QE enhancement is observed in the 225–375 nm range,
with emission maxima ranging between 330 and 600 nm, however also with fairly
long re-emission time constants (32 ns for CdTe, significantly more for the
rest).

Another study Datta et al. (2020) explored the potential of perovskite
(CsPbBr3) quantum dots and showed up to threefold conversion efficiency
enhancement when compared to a TPB-coated sensor, with $<$1 ns lifetime
emission at $\sim$510 nm, and expected even several times higher PLQY at
cryogenic temperatures, which is very promising.

Nanoparticle coatings were deposited from solution through casting, dip or
spray-coating, which are techniques scalable to very large surface areas.
Adequate radiopurity of such coatings would have to be demonstrated for
application in dark matter and 0$\nu\beta\beta$-decay detectors.

####  3.2.2 Xenon

R&D on LAr doped with small admixtures of xenon tested in TPB-coated
detectors, hinted that such mixtures could deliver better energy resolution,
faster decay time (potentially useful for TOF-PET and medical applications),
increased Rayleigh scattering length (helping event position reconstruction)
and improved Pulse Shape Discrimination (PSD) Peiffer et al. (2008); Wahl et
al. (2014).

The process of the triplet (long lived) excitation transfer from Ar to Xe
occurs through collisions and Xe dimers de-excite through emission at 175 nm,
essentially without any efficiency loss. This wavelength is significantly more
accessible to conventional photosensors than 128 nm. Akimov et al Akimov et
al. (2019) showed that LAr doped with even higher concentrations of Xe, and
without TPB, has a better PSD efficiency than pure LAr or Xe-doped LAr with
TPB. That implied that the singlet (short lived) Ar excitation is transferred
to Xe as well, and a mechanism assuming emission of 128 nm photon,
subsequently absorbed by Xe, was proposed.

A detailed energy transfer model, together with experimental evidence showing
that Xe is a faster WLS than TPB, is available in Ref. Galbiati et al. (2020).
The WLS character of Xe indicated by the above brings additional benefits for
the event position reconstruction: (1) WLS is now distributed in LAr with
conversion happening at the event location, more direct information is
available through hit patterns, and (2) timing is faster, which enables more
precise time-of-flight analysis.

PSD performance on a larger data sample, long term stability of Xe-doped LAr
mixtures, including risks and implications of potential aggregation processes,
as well as potential Xe concentration gradients over very large detector
volumes are subject to further studies.

##  4 Coating production techniques

###  4.1 Vacuum evaporation

In the past vacuum evaporation has been most commonly used for WLS deposition,
of TPB in particular. It is based on elevating the temperature of the WLS in
powdered or crystalline form up to the sublimation point in 10-5 mbar or
better vacuum. Resistive or electron gun heating is typically applied,
targeting relatively low 150-250∘C temperatures, which are sufficient for
sublimation of organic WLS molecules. In vacuum the mean free path of
evaporated molecules is sufficiently long to allow them to reach the substrate
and stick to its surface.

A typical process geometry involves a crucible and a flat substrate sheet or
plate located 10-50 cm away. Since molecules essentially follow straight
trajectories, the distance from the crucible allows to control the large scale
uniformity of the coating, see e.g. Pollmann et al. (2011) and references
therein.

Controlled temperature or rotating substrate is sometimes used to improve the
small scale uniformity of coatings, however satisfactory results have also
been obtained on acrylic and glass substrates without these features. More
complicated substrate geometries are also possible to coat with custom
designed crucibles, e.g.:

  * •

on an inner surface of an acrylic tube, with a resistive NiCr mesh wrapped
into a long narrow filament, containing pockets of TPB Amaudruz, P.-A and et
al (2015); Aalseth and et al (2020)

  * •

on long optical fibers, with a movable crucible Csáthy et al. (2016),

  * •

on long rectangular 120$\times$25 cm2 diffuse reflector sheets, with an array
of 13 crucibles Boccone and et al (2009),

  * •

on the inner surface of an 85 cm radius sphere (9 m2 surface area),cryst with
a spherical crucible suspended in the center Broerman et al. (2017).

Largest campaigns of TPB evaporation on rectangular and planar ESR foils were
performed for WArP Francini et al. (2013) and SBND Szelc (2016) detectors. The
vacuum evaporation technique becomes increasingly challenging for coating
large surface areas, mainly due to the high vacuum requirement, and a duty
cycle dominated by the pump down periods of the evaporation chamber needed to
obtain the desired quality of vacuum.

###  4.2 Solvent coating

Solvent coating is an attempt to avoid scale-up challenges related to vacuum
evaporation. The discussed WLS materials (with the exception of PEN) are
soluble in common organic solvents, including toluene and dichloromethane,
although typically concentrations of only up to several percent are
achievable. The solution is then applied to the substrate using one of many
available techniques, such as brushing, dipping, spraying/airbrushing (e.g.
with ethyl ether as a solvent McKinsey et al. (1997)) or spin coating.
Proportionally large amounts of organic, especially chlorinated, solvents
necessary for such processes is a complication on its own due to significant
toxicity and environmental impact of such chemicals.

A typical challenge for this class of techniques is achieving uniform and
reproducible coatings, which requires substantial effort needed for optimizing
and automating the process. Another issue is the crystallization of the WLS
once the solvent evaporates, which can result in relatively large crystals,
sometimes poorly attached to the substrate, and a non-negligible fraction of
the substrate left uncoated. This can directly translate into reduced WLS
efficiency and lead to cracking or poor stability of the film when immersed in
LAr.

Nevertheless, the process can be optimized for a given substrate and lead to a
robust product, as demonstrated in R&D effort for GERDA Baudis et al. (2015),
where a Tetratex substrate coating with a solution of TPB in dichloromethane
was developed, with the WLS efficiency about 2 times lower in comparison to
evaporated coatings. A porous character of Tetratex was a possible factor
allowing the solution to saturate the substrate and, after drying, leave a
strongly attached TPB film. Films of 0.9 mg/cm2 thickness were stable for
long-term operation in liquid argon and insensitive to exposure to ambient
humidity and oxygen.

A demonstration of the spin-coating technique with toluene used as the solvent
Yang et al. (2020), resulted in approximately 200 nm thick TPB coating
(uniform, although with pinholes visible with SEM) showing 0.25 efficiency
relative to evaporated TPB coating.

###  4.3 Doped polymeric matrix

The motivation behind embedding WLS in polymer matrix is twofold: similarly to
the solvent method, simplification of the coating production process and
stabilizing the WLS against long term aging or degradation effects. The choice
of the host polymer has consequences; if the polymer matrix, e.g. polymethyl
methacrylate (PMMA), has no aromatic fragments, its repeating unit cannot be
transferred to an excited state with a lifetime sufficient for further energy
transfer. PS, on the other hand, can take part and mediate in the energy
transfer between WLS molecules (Bower, 2002, Section 3.3).

For LAr WLS applications the coatings typically made are solid solutions, with
WLS molecules not chemically bound to polymeric chains. In the vast majority
of literature reports polymer based WLS coatings (usually TPB in PS),
deposited with a variety of techniques (brushing, spraying, dipping) led to
coatings with the conversion efficiency 2-3 times lower than that of
evaporated coatings Mavrokoridis (2011); Baptista et al. (2014). The process
typically involves dissolving through stirring or shaking finely powdered
polymer in the solvent, optionally at elevated temperature, in concentration
of the order of 1% and a similar amount of TPB. Admixture of ethanol reduces
the surface tension of the mixture and improves the wettability during e.g.
dip-coating Moss et al. (2016).

Excellent results have been achieved with spin-coated TPB/PS films. PS grades
with long chains and O(100k) molecular weight allowed one to dope larger
concentrations of TPB, up to 0.8 TPB/PS mass ratio, without or with only
limited aggregation or crystallization effects. Optimum spin-coated
TPB/polymer films should use high TPB mass ratio suspensions and 100 nm film
thickness Graybill et al. (2020), and can reach up to twice the efficiency of
evaporated TPB coatings, however with caveats discussed in Section 5.

As another successful example, nylon foil "minishrouds" coated with 30–40% TPB
in PS were developed by the GERDA collaboration in order to improve light
collection and in the vicinity of the Ge detectors, and mitigate Ar-42/K-42
backgrounds Lubashevskiy, A et al. (2018). The final concentration was a
compromise between the mechanical stability and the intensity of emitted
light. The coating was applied by brushing both sides of transparent nylon
foils with the dichloromethane based solution and, typically, 0.3 mg/cm2 of
dry film mass deposited. Various tests were performed in order to understand
the usability of such a foil in GERDA. No signs of deterioration, flexibility
loss or significant difference between spectra collected before and after up
to several months exposure to air and to cryogen were observed.

###  4.4 Lamination

PEN, available in large format polymeric foils, is suitable for lamination
with reflective or photosensitive substrates Kuźniak, M et al. (2019) or other
mechanical attachment schemes. Runs have been performed using the CERN FLIC 50
l prototype TPC, with the DUNE-like resistive cathode (1) covered with an ESR
foil evaporated with TPB and (2) coated with PEN, with no obvious issues
observed (Abi and et al, 2020, Section 5.16.2.1).

##  5 Stability and aging

###  5.1 TPB

Mechanical stability of WLS films has been a point of concern and a subject of
multiple studies. Whether coated in a pure or a doped polymeric form, the WLS
layer has to withstand temperature cycling, the transient thermal stresses
during the detector cool-down as well as deliver reasonably stable performance
over the duration of the experiment, which typically means several years.

Extensive R&D campaigns were undertaken by several groups to study and find
the right match of the deposition method and the substrate providing
satisfactory stability and durability, see Boccone and et al (2009); Baudis et
al. (2015); Broerman et al. (2017). For vacuum evaporated coatings it has been
determined that on the commonly used substrates, such as quartz, ITO-coated
quartz, acrylic, or ESR foils, stable and robust coatings are achieved as long
as the film growth rate during the deposition process is kept very low, i.e.
approximately at $\sim$1 Å/s. The resulting coatings are possible to scratch,
but generally are resistant to gentle wiping, manipulation and thermal
cycling. As determined from microscope imaging, rich crystalline structure of
the coatings appears to be resistant even to abrupt liquid nitrogen dunk
tests, as long as they are performed in dry atmosphere Broerman et al. (2017).
Not all substrates behave in the same way, and there are anecdotal reports of
reduced resistance to thermal cycling of TPB deposited on ITO-coated plastics.

It has been found that in liquid xenon after 20 h of immersion the surface
density of evaporated TPB films decreased by 1.6 $\mu$g/cm2, although without
any morphologic or chemical surface changes detected with SEM micrographs or
XPS. This can be interpreted as a slow dissolution process where the TPB
molecules remain intact Sanguino et al. (2016). Recently, a similar
phenomenon, although significantly slower than in xenon, has been observed in
LAr Asaadi et al. (2019). Evidence has been found for TPB concentration in LAr
gradually increasing over a 50 h soak time, up to tens of parts per billion in
LAr by mass, for both evaporated TPB and TPB/PS coating samples present in the
system. The nature of this effect is a subject of further studies.

Independently from the above effects, aging effects related with environmental
and, primarily, UV exposure of TPB are known. Already McKinsey et al. McKinsey
et al. (1997) reported that exposure to the ordinary fluorescent lighting in
laboratory air for 24 h resulted in a twofold yield reduction, with similar
accounts confirmed in more detail elsewhere Acciarri et al. (2013); Chiu et
al. (2012). In particular, the degrading effect of the fluorescence lighting
was confirmed, but found to be mitigated through the use of amber filters. No
difference in degradation rate between storage in ambient air vs. inert dry
gas was observed. This leads to a clear recommendation for storage of WLS
films in dark conditions and limiting the exposure to (unfiltered) lighting
during handling.

Yahlali et al. Yahlali et al. (2017) demonstrated, however, that the
conversion efficiency loss after exposure to VUV light comparable with that
integrated over a $\sim$5 year lifetime of a typical experiment does not to
exceed 15%, which is consistent with the results of e.g. GERDA, DEAP-1,
DEAP-3600 or DarkSide-50. Also, a thick TPB coating (1.6 $\mu$m) had stronger
resistance to VUV radiation damage than a thin coating (130 nm).

The nature of the UV damage to TPB has been studied in detail Jones et al.
(2013), and was attributed to formation of benzophenone, which has known UV
blocking properties, and which accumulates in the surface layer of the coating
under exposure to UV light, and over time leads to yellowing of the coatings,
caused by its further degradation products. The performance and durability
difference between the commercially available standard and the scintillation
TPB grades was also attributed to the presence of benzophenone.

As reported by Graybill et al. (2020) and other sources, immersion in the
polymeric matrix protects TPB molecules from UV induced degradation effects.
However, given that TPB emanation in LAr has been reported also from TPB/PS
foils Asaadi et al. (2019), long term stability of such very thin ($\sim$100
nm thick) spin-coated films deserves further studies in-situ.

###  5.2 PEN

Mechanical robustness of PEN, in a form of a polymeric film, in the LAr
detector environment is implied by the widespread and successful use of ESR
foils for this application, as PEN is one of the two components of ESR
Kuźniak, M et al. (2019).

Some commercially available grades of PEN films are subjected to high
temperature treatment during production (annealing), which can lead to growth
of cyclic oligomers on the surface Macdonald et al. (2007). The cyclic
oligomer crystals can be removed by washing the film with methylethylketone
(or with sanding). Their presence on significant fraction of the surface could
conceivably affect the film WLS efficiency at VUV wavelengths.

There are no known accounts of humidity or storage in ambient air affecting
the fluorescence or scintillation properties of PEN; in fact, this material is
highly impermeable to both air and water. However, UV-induced degradation
effects are known. Up to an order of magnitude deterioration of PLQY for
excitation wavelengths in the 260–380 nm range as well as a shift of the peak
emission wavelength towards higher wavelengths after 5 h of exposure to light
from a 30 W broadband UVA lamp has been reported Mary et al. (2001).
Qualitatively, the magnitude of this effect appears comparable with the case
of TPB, with measured scintillation and fluorescence yields up to 3 times
lower than the best ones reported in the literature. Similar to the case of
TPB, there is a significant spread of PLQY values reported in the literature
(as discussed in Section 3.1.2), which might be caused by inconsistent levels
of UV degradation in tested samples. (As an additional complication, in most
cases PEN yield characterization has been performed relative to TPB coatings,
which introduces additional dependence on the TPB reference history and aging
effects).

While the UV degradation effects deserve further studies, there is good
motivation for storage of PEN films in dark conditions, similar to TPB.

###  5.3 p-Terphenyl

Similar to the case of TPB, there is evidence of liquid xenon dissolving pTP
coatings, evaporated on PTFE reflectors and the entrance window of a
photomultiplier tube Bolozdynya et al. (2008).

###  5.4 Pyrene

Pyrene is particularly susceptible to oxidation, and therefore has to be
applied, handled and stored in oxygen free conditions. Oxidation leads to
yellowing and reduction of both the PLQY and the fluorescence lifetime. The
effect is attributed to the formation of a surface layer of pyrenequinone
Birks et al. (1966).

##  6 Examples of Application

###  6.1 Scintillation light collection

WLS materials are used in LAr experiments both as a transmissive coating on,
or in front of, the photon detectors, and as reflective coating on passive
surfaces to maximize light collection.

Different types of experiments use different general arrangements of WLS and
photon detectors. Dark matter detectors generally opt for high photosensor
coverage and augment them with WLS reflectors covering the walls as well as
the gaps between photosensors. The importance of this reflector coverage grows
particularly as collaborations move towards the next generation experiments
with surface areas exceeding hundreds of m2. We note that the the overall
light yield ($LY$) is strongly dependent on their reflectivity ($R$), because
of the large number of wall reflections before encountering a photosensor:
$LY\sim PLQY\cdot R^{n}$, where $n$ is the inverse of the photosensor coverage
fraction (e.g., for 20% photosensor coverage, n=5). This underlines the
importance of using reflectors with optimized conversion efficiency and
reflectivity. In the case of neutrino detectors the photosensors are usually
placed only behind the transparent anode planes with sparse photosensor
coverage. As this configuration leads to reduced light yield and non-
uniformities in neutrino detector response Garcia-Gamez et al. (2020), WLS
reflectors partly covering the walls or cathode have been used by SBND Szelc
(2016) and LArIAT Szelc (2013) experiments. Another possible configuration is
to distribute the WLS throughout the liquid argon volume. This solution
requires finding a WLS that uniformly mixes with the liquid argon, e.g. by
being soluble. An example is xenon doping of LAr, described in Sec. 3.2.2,
which results in a large fraction of the scintillation light being emitted at
the 178 nm characteristic for xenon.

Large-scale neutrino detectors use the liquid argon scintillation light as a
trigger (MicroBooNE, SBND) to determine the interaction time, $t_{0}$ needed
to reconstruct the absolute position in the drift coordinate, to remove out-
of-beam cosmic events, tag decays as well as improve timing and energy
reconstruction. For these applications, low detection efficiency on the order
of 10-3 is sufficient Szelc (2016); Sorel (2014).

The enhanced collection efficiency obtained in dark matter experiment
geometries leads to an increased light yield and, in consequence, a lower
energy threshold. The energy threshold is a parameter particularly important
for dark matter detectors, as the expected spectrum of nuclear recoils induced
by elastic scattering of weakly interacting massive particles (WIMPs)
increases steeply towards low energies. Therefore the sensitivity of WIMP
detectors strongly and non-linearly depends on the energy threshold and
maximizing the PLQY and minimizing optical losses is of crucial importance. In
the DEAP-3600 detector Amaudruz, P.-A and et al (2019) as much as 16% of the
original number of scintillation photons are registered as photoelectrons;
increasing this number even further is mandatory for the next generation
detectors which will not be attainable without efficient use of WLS.

There is a broad range of WLS applications in detectors where the LY
requirements fall somewhere in-between the accelerator neutrino and dark
matter search experiments. These less stringent light yield requirements of,
e.g., 0$\nu\beta\beta$-decay detectors combined with the technical challenges
of the vacuum evaporation technique motivated R&D on alternative coating
methods. These passive elements are often combined with sparse arrays of
photosensors (as in the LAr veto for the DarkSide-20k detector The Global
Argon Dark Matter Collaboration (2018), for which PEN is considered a
candidate WLS) and/or optimized light collection schemes allowing to at least
partially recover the response uniformity, e.g. by collecting light from a
large area in the detector and guiding it to a limited number of photosensors
(as in GERDA or DUNE).

As collaborations approach the construction of future generation large-scale
detectors, such as Argo The Global Argon Dark Matter Collaboration (2018),
developing a scalable WLS solution allowing to maintain high LY and response
uniformity is crucially important and is currently a very active R&D topic,
which has strong synergies with the large-scale neutrino experiments,
particularly, DUNE Abi and et al (2020).

The WLS applications in LAr detectors can be split into passive elements of
the photon detection systems, such as WLS coated reflective elements, simple
coated photon-sensors, photon-sensors with two-stage WLS and photon traps. The
WLS coatings used can be produced in a variety of ways, as discussed in
Section 4. Their thickness is usually optimized for the specific geometry or
place of application, i.e. typically thinner coatings are used in front on
optical windows or photosensors and thicker ones on reflectors. For example
for the most commonly used evaporated TPB coatings the thickness recommended
for the transmission geometry was 0.1-0.3 mg/cm2 Benson et al. (2018), while
thicker coatings yield better results on diffuse or specular reflectors
Boccone and et al (2009).

####  6.1.1 WLS-coated passive elements

Vacuum evaporated TPB coatings on passive detector elements have been used for
the applications where light collection has been most critical, namely for
dark matter detectors such as WArP Acciarri and et al (2010), MiniCLEAN
Rielage and et al (2015), ArDM Boccone and et al (2009), DEAP-1 Amaudruz, P.-A
and et al (2015) and DarkSide-50 Agnes and et al (2015). The coatings are
typically evaporated ex-situ directly on the reflective material, and then
used to line the walls of the LAr chamber. Following successful tests in the
LArIAT test beam experiment Szelc (2013), where the walls and subsequently the
cathode were lined with TPB coated ESR foils, the SBND neutrino detector
Machado et al. (2019) will install TPB-coated ESR foils on the detector
cathode which required the largest production of such foils to date: roughly
40 m2 of TPB coated foils were produced. Because of only partial coverage, the
resulting light yields are relatively modest compared to dark matter
detectors. A production of a similar scale process is planned for the
DarkSide-20k TPC. PEN or evaporatively TPB-coated ESR foils are also one of
the alternative options considered for the future DUNE modules, see (Abi and
et al, 2020, Section 5.16.2.1) and Szelc et al. (2020).

In the DEAP-3600 detector, the evaporation on the 9 m2 inner surface of the
spherical acrylic vessel was performed in-situ, after deploying the
evaporation crucible in the center, pumping down the detector chamber, and
effectively turning the inner detector volume into a large spherical
evaporation chamber Broerman et al. (2017).

In the LAr veto tank of the GERDA detector ESR reflector foils, dip-coated in
a toluene based solution of TPB and PS, were used as a liner. After
significant R&D effort these were replaced with a film of pure TPB ($\sim$0.9
mg/cm2 thick) deposited on Tetratex diffuse reflector sheets (PTFE based) via
dip coating in a solution of TPB in dichloromethane Baudis et al. (2015). The
TPB/Tetratex reflector gives approximately two times more light than the TPB-
doped PS on ESR. Additionally, TPB/PS-coated Lubashevskiy, A et al. (2018)
nylon foil ”minishrouds” improving light collection in the vicinity of the Ge
detectors were developed.

####  6.1.2 WLS-coated photon detectors

Most liquid argon experiments will opt to coat their photon detectors in WLS,
either via direct coating with TPB using evaporative coatings (ArDM, SBND and
ICARUS Spanu et al. (2018)), dip-coatings e.g. TPB combined with polysterene
(WArP, LArIAT), by coating a window before the actual PMT (DEAP-1, MiniCLEAN,
DarkSide-50) or by using a WLS front plate (MicroBooNE Acciarri and et al
(2017), ProtodDUNE-DP Dorril and et al (2020)). In the last case, because of
rather low resulting light yield, the scintillation light is primarily used
for triggering and identifying high-energy cosmic-ray tracks. In DEAP-3600 the
PMTs were not coated with WLS instead relying on the surface of the inner
chamber to convert the light which was then transported to the PMTs via light
guides.

In large scale detectors the photosensor coverage can be increased by using
light collectors, e.g. based on deploying light guides coated with TPB-doped
PS Bugel et al. (2011). The configuration which was developed and deployed in
ProtoDUNE-SP Abi and et al (2020), uses 207.4 cm long PMMA light guides, with
attenuation length on the order of 2 m Moss et al. (2016). Other solutions
combine different WLS compounds to enhance the collection efficiency and will
be discussed next.

####  6.1.3 Two-stage WLS

A more complex approach involves two stages of wavelength shifting. Firstly,
VUV photons are converted to blue wavelengths using e.g. a TPB-based coating.
These blue photons are then wavelength shifted again in a polymeric substrate
doped with a commercial grade green fluor (e.g. in the form of fibers, plates
or light guides), with the absorption spectrum well matched with the emission
spectrum of TPB. Due to a more favorable difference of refractive indices
between the light guide material and LAr, green light at $\sim$490 nm is more
efficiently trapped in the light guide through total internal reflection and
benefits from a higher attenuation length in the base material.

An example of such an implementation is the so-called double-shift lightguide
bars designed for DUNE and ProtoDUNE-SP. They consist of an outer acrylic
plate spray-coated with TPB dissolved in dichloromethane. The blue photons are
then absorbed and wavelength-shifted by commercial Eljen EJ-280 plates which
are coupled with SiPM detectors Howard et al. (2018); Howard (2018), yielding
a photon detection efficiency of between 0.2% and 0.48%. This design was one
of the technologies deployed and tested during the ProtoDUNE-SP run Abi and et
al (2020).

A concept using a “curtain” (50% coverage) of evaporatively TPB-coated green
wavelength shifting fibers (Saint-Gobain BCF-91A), additionally improving the
yield and response uniformity of TPB-coated reflectors covering the outer
walls of the cryostat, has been developed for GERDA Phase II Csáthy et al.
(2016); Agostini et al. (2017) and is planned for LEGEND Zsigmond (2020).

A bundle of Kuraray fibers, was installed around the acrylic neck of the
DEAP-3600 detector (i.e. outside the LAr volume, contrary to the cases
discussed above) to improve light collection of TPB converted light from the
region of the detector where light collection was reduced due to a gap in the
PMT coverage Amaudruz, P.-A and et al (2019).

####  6.1.4 Photon traps

Another concept is to use dichroic filters combined with WLS to trap light and
effectively enlarge the effective surface area instrumented with SiPM sensors.
This concept is used in the recently developed ARAPUCA Bazetto et al. (2020),
X-ARAPUCA Segreto et al. (2020) and ArCLight Auger et al. (2018) photon
detectors.

The ARAPUCA concept, originally developed for DUNE and installed and tested
during the ProtoDUNE-SP run Abi and et al (2020) uses a box with one or two
faces containing a dichroic filter. The filter is coated with pTP on the
outside which converts the light to a wavelength at which the filter-window is
transparent. Once the photons cross the filter they encounter a second layer
of WLS, in this case TPB, which shifts them to blue, making them reflect off
of the filter and effectively trapping them inside of the reflective box until
they can find the SiPM detector coupled to it. During the ProtoDUNE-SP run the
ARAPUCA were found to achieve a detection efficiency of close to 2%. The
X-ARAPUCA concept improves on the ARAPUCA design by adding a WLS light-guide
inside the box which couples to the SiPMs. Simulations and preliminary
measurements lead to expect a detection efficiency of about 3.5% for this
design. The X-ARAPUCA is the baseline DUNE photon detector as described in the
Technical Design Report (Abi and et al, 2020, Section 5). They will also be
installed in SBND, with half of the detectors coated with pTP and half left
un-coated to be sensitive only to the light wavelength-shifted and reflected
off the cathode.

The ArCLight detectors use a similar concept to the X-ARAPUCA detectors and
have been developed for the ArgonCube detector Auger et al. (2019), where they
are set to be installed in the fieldcage walls. The ArCLight detectors consist
of an Eljen EJ-280 WLS plate covered on the back and sides with highly
reflective dielectric foils. The front face is a dichroic filter coated with
TPB on the outside. Similarly to the X-ARAPUCA design the photons are first
shifted to blue, where they pass the filter and are subsequently shifted to
green by the WLS plate which then performs as a lightguide transporting the
photons to the coupled SiPMs. The ArCLight detectors have been found to have a
detection efficiency of up to 2.2% Auger et al. (2018).

###  6.2 Impact of WLS on dark matter background mitigation

####  6.2.1 WLS effects in Pulse Shape Discrimination

LAr-based dark matter experiments use pulse shape discrimination (PSD) Boulay
and Hime (2006) to select nuclear recoils associated with potential WIMP
signals and to reject electron recoils typically associated with backgrounds.
The difference in ionization density of the two classes of events results in
the scintillation signals from the former being dominated by the scintillation
from the LAr singlet with a lifetime of $\sim$7 ns Hitachi et al. (1983) and
those resulting from electron-recoils to be dominated by the slow, LAr
triplet, light with a lifetime of $\sim$1.5 $\mu s$. This method has been
shown to reject electron-recoil backgrounds at a level of 1:1010 Amaudruz,
P.-A and et al (2016).

The efficiency of the PSD methods depends on the light yield of the detector.
The higher the light yield, the more efficient the background discrimination
can be at lower recoil energies. Therefore all avenues using WLS to improve
scintillation light collection described in the previous section, trivially
result in improved background mitigation prospects.

One should note that the WLS time constants can affect the observed
distribution of the detected light. In order for the PSD methods to be
effective it is therefore important for the WLS dominant time constant to
preserve the original timing structure of the LAr scintillation to a
sufficient degree, i.e. it should not be much higher than the LAr singlet
lifetime ($\sim$7 ns). This condition is fulfilled by all WLS materials
discussed in the review except for pyrene, which will be discussed later in
this section.

We note however, that the TPB emission time Segreto (2015) combined with LAr
scintillation microphysics Adhikari, P. and et al. (2020) have been suggested
as a culprit for the so-called intermediate time component with a lifetime of
$\sim$40 ns observed in a few measurements Acciarri and et al (2010).

####  6.2.2 Alpha Background Removal Using WLS

Apart from gamma/beta backgrounds mitigated with LAr PSD, alpha activity is
another potential source of WIMP-like events in, primarily, single-phase dark
matter detectors, such as DEAP-3600. Radioactive daughters of Rn-222 are
particularly problematic, as gaseous radon present in the ambient air can
diffuse into the surface layer of materials, leaving behind its radioactive
decay daughters, including long lived Pb-210 and alpha decaying Po-210.
Topologies of such Po-210 alpha decays in the vicinity of the detector
surface, in which most of the alpha energy is deposited in the inactive
substrate, or in which only the recoiling nucleus deposits energy in LAr, can
lead to apparent-low energy events contaminating the region of interest for
dark matter searches Amaudruz, P.-A and et al (2015). In dual-phase detectors,
such as DarkSide-50, surface events are easier to identify, but can still lead
to background events through pile-up with other classes of events Agnes and et
al (2018).

Delayed emission from alpha scintillation in TPB and pTP, which is more
abundant than for VUV photon conversion, has been researched as a potential
handle to reject such events, in addition to e.g position reconstruction and
fiducialization. First evidence for such emission, and its low-temperature
parameters, together with a simple PSD parameter designed to reject such
events has been published Pollmann et al. (2011); Veloce et al. (2016).
Further studies in LAr conditions led to an the estimate that the surface
background could be suppressed by a factor of $\sim$10 (100) for TPB (pTP) at
50% (90%) nuclear recoil acceptance Xu et al. (2017) and that for the case of
TPB the projected PSD factor could be as high as 103, when using milliseconds-
long integration windows Stanford et al. (2018); Xu (2013). In practice, the
above mechanism has found so far only limited use in dark matter search
analyses Wang (2014). Typically, applying cuts based on position
reconstruction is sufficient to reject these surface events. Also,
instrumental effects, especially PMT afterpulsing, reduce signal acceptance of
cuts based on alpha PSD parameters, unless the acquisition window
significantly exceeds the $\sim$10 $\mu$s, typically used in single-phase LAr
experiments, which leads to a significant increase in dead time.

To maximize the efficiency of surface event PSD rejection in a 10 $\mu$s time
window, employing a layer of scintillator with a $>\sim$300 ns emission time
constant between the conventional WLS coating, e.g. TPB based and a passive
transparent detector wall material (e.g. acrylic Boulay and Kuźniak (2020);
Gallacher and Boulay (2020)) has been proposed. This configuration would allow
to achieve up to 108 suppression factor using PSD. The scintillator layer
should be sufficiently thick to stop alpha decays originating in the substrate
(>$\sim$50 $\mu$m), see Figure 4(a). This concept could be implemented using
e.g. a commercially available slow scintillator material, e.g. Bicron BC-444.

In addition to background events induced by degraded alphas on the detector
surfaces, shadowed alpha decays are another class of related backgrounds, as
shown in Figure 4(b). VUV LAr scintillation light from alpha decays occurring
in peripheral and shadowed regions of detectors (slits, gaps, inlets/outlets)
is mostly absorbed by peripheral surfaces without the WLS coating. In
consequence, out of 2$\cdot$105 initial VUV photons generated by a Po-210
alpha, depending on the solid angle, only small fraction will be detected,
leading to a continuum in the apparent energy spectrum extending all the way
down to the low energy dark matter search window.

Coating such problematic blind-spots with a slow time constant WLS,
substantially alters the timing of such pathological events enabling the use
of PSD to solve this problem. Care must be taken to only use such material in
the shadowed regions of the detector, so that it would not significantly
interfere with timing of events from the main volume of the detector, spoiling
the PSD against beta/gamma events.

Because of its long $\sim$115 ns singlet fluorescence lifetime, pyrene has
been identified as a promising candidate for such application. It is primarily
coated as a dopant in a PMMA or other polymeric matrix Clark et al. (2016),
given its incompatibility with vacuum depositions. At 87 K the time constant
of the pyrene excimer fluorescence further increases to approximately 155 ns,
which provides a strong PSD handle against events dominantly wavelength
shifted in such coating. A polymer based pyrene coating is under investigation
for robustness in assembly and operation in the neck of the DEAP-3600
detector, in order to mitigate the alpha backgrounds, which currently limit
the sensitivity of the experiment Ajaj and et al (2019).

In summary, fluorescence timing properties of WLS materials play a very
important role in mitigating alpha backgrounds in dark matter searches, and
can be employed in a variety of scenarios, useful also in the next generation
experiments.

Figure 4: WLS-based background mitigation schemes: (a) WLS-coated slow
scintillator on a passive PMMA detector wall. Alpha decays will either deposit
all of their energy in an active region (LAr, WLS or the scintillator) and be
discriminated based on high deposited energy, or will deposit only a fraction
of their energy in the slow scintillator, in which case they can be
discriminated against based on its long decay time Boulay and Kuźniak (2020).
(b) Slow WLS coating. Alpha decays occurring in a slit in the detector wall
can induce a fake low-energy background event, as most of the VUV
scintillation is absorbed before entering the main volume (left). In presence
of the slow WLS in the slit (right), the event will be dominated by visible
photons converted in the slow WLS and could be removed with PSD.

.

##  7 Summary and future directions

Liquid argon scintillation light is a crucial component of a wide range of
detectors - from those searching for low energy nuclear recoils indicating
potential dark matter interactions to those detecting GeV-scale accelerator
neutrinos. This scintillation light can only be feasibly detected with the
help of WLS compounds, which are therefore a crucial element of these
detectors. The last decade has brought huge progress in understanding the
behaviour, performance and stability of the most commonly used WLS compounds,
as well as developing new methods of applying them to detect scintillation
light.

In parallel there has been exciting development both in identifying new WLS
compounds and solutions, as well as innovative applications of known WLS, that
provide better or new functionality. However, significant room for further
research and development still remains in the field. Future large-scale
detectors, such as Argo and DUNE, would require WLS coating of extremely large
surfaces. This would be difficult to achieve on a reasonable timescale with
the current evaporation technology used for TPB coatings. However, the spin-
coating technique could provide a solution, should the high PLQY be confirmed.

PEN, the main alternative to large-scale TPB coatings, which should be
relatively easy to produce for large areas, is currently found to be less
performant than TPB in terms of PLQY, and would need R&D to improve that to be
used in future dark matter detectors. In addition, all WLS solutions installed
in next generation LAr detectors will need to have their long-term stability
established at levels much higher than was needed until now. New directions
include ideas such as doping the large LAr detectors with xenon, which shifts
the light to wavelengths where it is significantly simpler to detect, e.g. not
requiring an external pTP coating on the X-ARAPUCA detectors as well as
completely new ideas, such as quantum dots.

The importance of R&D on WLS compounds in noble liquid detectors has been
recognized by the recently published report on "Basic Research Needs for High
Energy Physics Detector Research & Development" which lists increased light
collection as one of the key elements of Priority Research Direction (PRD) nr
5. The report recognizes the need to collect light "over many tens to hundreds
of square meters" doe (2020). It also mentions developing "high-efficiency
wavelength-shifting techniques" as an important area of research in PRD 5 and
PRD 24, listing several different avenues of proceeding towards achieving this
goal.

All of the above-mentioned R&D areas would benefit greatly from a
multidisciplinary approach providing results that would be applicable to very
different research areas such as searches for dark matter and
0$\nu\beta\beta$-decay, as well as accelerator neutrino measurements,
simultaneously. The LIDINE (LIght Detection In Noble Elements) conference
series provides an excellent venue to exchange these kind of ideas and cross-
pollinate between the different research fields. Indeed many of the solutions
listed in this work were first proposed at one of the conferences from this
series. One key need that would greatly help multi-disciplinarity is for
facilities that could provide a ’complete’ characterization of various WLS
solutions. One of the first tasks of such a facility would likely be to
resolve the outstanding TPB PLQY puzzle which would require a cryogenic setup,
capable of time resolved measurements and an integrating sphere providing
absolute calibration.

In this decade, as several extremely large LAr experiments will be constructed
and start taking data, the importance of WLS and their applications in LAr-
based detectors will only increase.

writing–original draft preparation and editing, M.K; writing–sections on
examples of application and future directions, review and editing, A.M.S and
M.K. This work was supported by the International Research Agenda Programme
AstroCeNT (MAB/2018/7) funded by the Foundation for Polish Science (FNP) from
the European Regional Development Fund. This project has received funding from
the European Union’s Horizon 2020 research and innovation programme under
grant agreement No 962480. It was also partially funded by the Science and
Technology Facilities Council (STFC), part of the United Kingdom Research and
Innovation; and by the Royal Society UK awards: RGF\EA\180209 and UF140089.

The authors declare no conflict of interest.

The following abbreviations are used in this manuscript:

0$\nu\beta\beta$-decay | Neutrinoless double beta decay
---|---
bis-MSB | 1,4-Bis(2-methylstyryl)benzene
ESR | Enhanced specular reflector
FRET | Förster resonance energy transfer
LAr | Liquid argon
NOL | Nanostructured organosilicone luminophore
PEN | Polyethylene naphthalate
PLQY | Photoluminescence quantum yield
PMMA | Poly(methyl methacrylate)
PMT | Photomultiplier tube
pTP | p-Terphenyl
PS | Polystyrene
PSD | Pulse shape discrimination
SiPM | Silicon photomultiplier
TPB | 1,1,4,4-Tetraphenyl-1,3-butadiene
WLS | Wavelength shifter

no

References

yes

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