[table]capposition=top

# Wavelength-shifter coated polystyrene as an easy-to-build and low-cost
plastic scintillator detector

A. Brignoli  A. Conaboy  V. Dormenev  D. Jimeno,11footnotetext: Corresponding
author. D. Kazlou  H. Lacker  C. Scharf  J. Schmidt  H. G. Zaunick

(October 2022)

###### Abstract

We studied the light yield of a pure polystyrene slide coated with wavelength-
shifter molecules, coupled to a photomultiplier, using $\beta^{-}$ particles
from a 90Sr source, as a possible easy-to-build, low-cost plastic scintillator
detector. Comparison measurements were performed with an uncoated polystyrene
slide as well as with uncoated and coated PMMA slides, the latter which can
only produce Cherenkov light when being traversed by charged particles. The
results with the single (double) coated polystyrene slides show about 4.9
(6.3) times higher detected photon yield compared to the uncoated slide. For
comparison, the light yield of a polystyrene-based extruded plastic
scintillator material doped with PTP and POPOP was measured as well. The
absolute detected light yield motivates future studies for developing easy-to-
build, low-cost polystyrene-based plastic scintillator detectors.

##  1 Introduction

For several years, wavelength-shifting optical modules (WOMs) are being
considered as possible photon detectors with large area and low noise and
costs [2, 1]. They have been originally proposed as water-Cherenkov photon
detectors in the context of the Upgrades of the IceCube neutrino telescope
[2]. In fact, WOMs will be deployed for a seven-string extension of the
IceCube detector planned for the 2022/2023 South Pole deployment season [3].
Following the first proposal [2], they have been also proposed as photon
detectors for the liquid-scintillator based Surround Background Tagger of the
SHiP experiment proposal at the CERN SPS [4, 5], with a proof-of-principle for
such a WOM-based liquid-scintillator detector reported in [6].

A standard WOM is a large-area transparent tube, typically made of PMMA or of
quartz glass, coated from outside222Coating from inside can be performed as
well, which increases the photon absorption probability. with a wavelength-
shifting (WLS) dye, e.g. using a dip-coating technique [2, 1]. Typically,
wavelength-shifting in this context refers to absorption of ultraviolet (UV)
photons by the WLS dye followed by re-emission of photons in the blue range,
although the principle can of course be applied as well to absorption and re-
emission in other wavelength ranges. The absorption process inside the WLS
layer has a high probability, if the WLS layer thickness is sufficiently
large. If the WOM tube is placed inside a medium of much smaller refraction
index, such as air, most of the secondary photons, which are emitted
isotropically from the point of absorption, fulfil the condition of total
reflection when hitting the inner or outer WOM-tube walls. In the absence of
absorption and scattering losses, $74.6\text{\,}\mathrm{\char 37\relax}$ of
the secondary photons are guided to the two ends of the WOM tube [2]. As a
result, the majority of the secondary photons can be guided to one of the two
WOM tube ends where they can be eventually detected by a photomultiplier (PMT)
that only has to cover the WOM-tube diameter or by a ring-array of SiPMs that
only has to cover the WOM tube exit area. For the WOMs considered for the
IceCube extension and the SHiP surround background tagger, the following WLS
dye, here referred as the WLS standard dye, is used:
$77.31\text{\,}\mathrm{\char 37\relax}$ toluene, $22.29\text{\,}\mathrm{\char
37\relax}$ Paraloid B723, and as the two wavelength shifters,
$0.13\text{\,}\mathrm{\char 37\relax}$ bis-MSB and $0.27\text{\,}\mathrm{\char
37\relax}$ p-terphenyl (PTP) [2]. The absorption spectrum of the WLS dye
ranges from about 250 nm up to about 400 nm and its emission spectrum ranges
from about 400 nm up to 500 nm [3]. For the study presented in this paper, an
additional modified dye was used. To produce the modified dye,
$22\text{\,}\mathrm{\char 37\relax}$ of the toluene in the standard dye was
evaporated. This leads to an increase in the viscosity of the dye, which in
turn increases the thickness of a dip-coated dye layer [1].

In this work, we modify the WOM concept in two ways: 1) Instead of a tube
geometry, we consider a planar geometry, e.g. a slide. The WOM principle
concerning the condition of total reflection will be preserved in a planar
geometry and therefore it is expected that many photons can be guided to the
thin-sized side walls of a slide. 2) The WOM base material serves now as the
stand-alone active detector material. Hence UV photons produced by Cherenkov
radiation and, depending on the material, possibly also by scintillation
inside this material are wavelength-shifted and guided to the photon detector.
The idea to use a WLS layer on a transparent material for Cherenkov light
detection was already considered in Ref.[7]. For our planar WOM material, we
consider here in particular pure polystyrene (PS) that can be commercially
purchased, at least for specific thicknesses varying between 1 and
$5\text{\,}\mathrm{mm}$, at quite low cost. Pure polystyrene is not only
producing Cherenkov light but is also scintillating, although with low light
output, increasing the overall light yield. Pure PS has an emission spectrum
from scintillation in the UV range between about 290 nm and 400 nm with an
emission maximum of about $320\text{\,}\mathrm{nm}$ [8], which fits well to
the WLS absorption spectrum. The position of the emission maximum and the
fluorescence yield of polystyrene can vary between about 310 and 340 nm,
depending on the concentration of the styrene monomer inside polystyrene,
which depends on the polymerisation temperature, and whether it is casted or
non-casted polystyrene, as well as on the degree of polymerisation cross-
linking induced by an additional agent [9]. As a result, by dip-coating a PS
slide with the standard WLS dye, it is expected that a part of the UV
scintillation photons can be absorbed inside the WLS dye after a relatively
short travel distance, controlled by the thickness of the slide and the
attenuation length of the PS material, and re-emitted as visible photons that
can be detected by a photosensor (e.g. a PMT or a SiPM). One can not expect
light yields as large as for standard PS-based plastic scintillators doped
with a primary fluorophore and with WLS molecules being mixed into the PS.
However, the advantage of the material and procedure studied in our work is
that one can produce without large efforts planar PS detectors by simply dip-
coating a commercially available, very cheap material that can be cut into the
desired slide geometries. The limitations of commercially available PS
material are geometries and thicknesses. To demonstrate the feasibility of the
concept, we studied with a $\beta^{-}$ source various square-shaped PS slides
of $50\text{\,}\mathrm{mm}$ side length and about $5\text{\,}\mathrm{mm}$
thickness, being the maximum thickness found on the commercial market, and, as
a comparison, also slides made of non-UV-transparent PMMA, which does not
scintillate and acts only as a Cherenkov light radiator when being traversed
by a high-energy charged particle.

For comparison, we studied as well an extruded scintillator material based on
polystyrene with a PTP concentration of 1.5% and a concentration of POPOP of
0.01% produced by the UNIPLAST plant (Vladimir, Russia) [10] that was e. g.
studied as a possible base material for 3 m long scintillator bars equipped
with WLS-fibres and silicon photomultipliers to be used in the muon detector
[11] of the proposed SHiP experiment [5]. Such scintillators can be produced
at moderate costs and generate high light-yield, however with only moderate
attenuation lengths of about 10 to 15 cm in the visible range [11], which are
even smaller than the attenuation length of pure polystyrene.

##  2 Experimental setup and measurements

###  2.1 Sample preparation

We studied the following samples of plastic material slides: UV-non-
transparent PMMA without WLS coating (PMMA), UV-non-transparent PMMA dip-
coated with the standard WLS dye (PMMA-WLS), UV-non-transparent PMMA dip-
coated twice with the modified WLS dye (PMMA-WLS-2) to enhance the WLS layer
thickness, polystyrene without WLS coating (PS), polystyrene dip-coated with
the standard WLS dye (PS-WLS), and polystyrene dip-coated twice with the
modified WLS dye (PS-WLS-2).

All PS slides were cut from a larger, commercially available, $1\times 0.5$
${\rm m}^{2}$ plate of about $5\text{\,}\mathrm{mm}$ thickness 333The PS plate
was purchased for a price of 29,45 Euro., and polished after cutting. The WLS
absorption spectrum covers the range
$250\text{\,}\mathrm{nm}385\text{\,}\mathrm{nm}$, the emission spectrum starts
at $400\text{\,}\mathrm{nm}$ with the maximum at about
$420\text{\,}\mathrm{nm}$ going up to about $500\text{\,}\mathrm{nm}$. The
slides were dipped into the WLS dye for 80 s, withdrawn from the WLS dye at a
speed of $93\text{\,}\mathrm{mm}\text{\,}{\mathrm{s}}^{-1}$ using the dip-
coater ND-DC Dip Coater from the company Nadetech Innovations S.L. also used
in our production of dip-coated WOM tubes, and then dried for at least
$24\text{\,}\mathrm{h}$.

The dip-coating procedure was optimized in dedicated studies to maximize the
absorption probability of PMMA WOM tubes developed for the SHiP Surround
Background Tagger [12]. With respect to the standard WLS dye,
$22\text{\,}\mathrm{\char 37\relax}$ of the toluene from the WLS-2 dye was
partially evaporated before dip-coating to enhance the dye viscosity, since
this helps in producing a larger WLS layer thickness [1]. Coating with the
standard paint results in WLS layers of about only
$5\text{\,}\mathrm{\SIUnitSymbolMicro m}$ thickness, as measured on dip-coated
glass slides using the stylus profiler DektakXT from Bruker Corporation.
Significantly higher thicknesses can be achieved using the modified paint with
the maximal coating speed of 150 mm/min achieved with the dip-coater in use
(ND-DC Dip Coater from Nadetech Innovations S.L.) and coating the slide twice.
For double-coated slides, it is important to avoid a significant immersion
time of the slide in the dye during second coating, as the toluene dissolves
the already existing WLS layer. With this procedure, WLS layer thicknesses on
glass slides between $25\text{\,}\mathrm{\SIUnitSymbolMicro m}$ and
$40\text{\,}\mathrm{\SIUnitSymbolMicro m}$ have been achieved with the
modified WLS dye. Using dip-coated glass slides, also the attenuation length
of the WLS layer has been estimated from transmission measurements, ranging
from $7\text{\,}\mathrm{\SIUnitSymbolMicro m}$ at $320\text{\,}\mathrm{nm}$
down to $4\text{\,}\mathrm{\SIUnitSymbolMicro m}$ at $350\text{\,}\mathrm{nm}$
and up again to $8\text{\,}\mathrm{\SIUnitSymbolMicro m}$ at
$380\text{\,}\mathrm{nm}$.

Determining the WLS layer thickness on a plastic material slide, either by
comparing the weight of the slide before and after dip-coating or by using the
profiler method applied for coated glass slides, does not work since the
toluene dissolves a part of the plastic material surface during the coating
process. Comparisons of dip-coated glass slides with extruded and casted PMMA
slides however show that the transmission behaviour with respect to the WLS
dye layer agree with each other. Therefore, similar WLS layer thicknesses can
be achieved on plastic material as on glass. Since the WLS thicknesses
achieved when coating twice with the modified WLS dye are much larger than the
attenuation lengths, one expects a very high absorption probability of UV
photons in the range between 300 and 400 nm.

To compare the performance with a high-light-yield plastic scintillator, a
slide with the same dimensions was cut out from the extruded plastic
scintillator material produced by UNIPLAST.

###  2.2 Sample characterization

In this sub-section, we describe pre-measurements to characterize the
materials under study: 1. the transmission spectra through the uncoated PS
slide, the uncoated PMMA slide, and the UNIPLAST reference slide were
measured, from which the corresponding light attenuation lengths were
determined; 2. the primary scintillation light yield in pure PS and UNIPLAST
samples were estimated.

####  2.2.1 Transmission measurements

Transmission spectra as a function of wavelength were measured for various
samples with two different transmission spectrophotometers, using A) a
PerkinElmer Lambda 950 spectrophotometer (PE Lambda 950) measuring in the
range between $280\text{\,}\mathrm{nm}$ and $450\text{\,}\mathrm{nm}$, B) a
Hitachi 3200 spectrophotometer (Hitachi 3200) [13] measuring in the range
between $280\text{\,}\mathrm{nm}$ and $900\text{\,}\mathrm{nm}$. The
attenuation length as a function of wavelength was calculated by correcting
the measured transmission $T_{meas}$ for the reflectivity $R$ of light at the
front and back surface of the sample, taking into account multiple
reflections[14]:

$\lambda_{abs}=\frac{d}{\text{ln}\Biggl{(}\cfrac{T^{2}+\sqrt{T^{4}+4R^{2}T_{meas}^{2}}}{2T_{meas}}\Biggr{)}}$
(1)

where $T$ and $R$ are the predicted transmittance and reflectivity for the
case of no absorption in the material calculated as

$R=1-T=\left|\frac{1-n}{1+n}\right|^{2}$ (2)

with $n$ the wavelength-dependent refraction index of the material taken from
[15]. An overview of the studied material samples is provided in Table 1. The
corresponding transmission measurements are summarized in Fig. 1 and the
corresponding attenuation lengths in Fig. 2.

Table 1: List of plastic material samples (pure PS, PMMA, and UNIPLAST extruded scintillator) used for the transmission measurements. The geometrical dimensions of the planar slide geometry are quoted as well as the light path length through the material, and the spectrophotometer used for the transmission measurement. For the UNIPLAST scintillator, two different beam positions on the sample were measured to quantify the systematic uncertainty on the transmission measurement and corresponding attenuation length as a result of the inhomogeneous material transparency. Slide | Dimension | Light path length | Spectro-
---|---|---|---
Material | ${\rm mm}^{3}$ | mm | photometer
PS | $50\times 50\times 4.5$ | 4.5 | PE Lambda 950
PS | $50\times 50\times 4.5$ | 4.5 | Hitachi 3200
PS | $50\times 50\times 4.5$ | 50 | Hitachi 3200
PMMA | $50\times 50\times 5.0$ | 5.0 | PE Lambda 950
UNIPLAST | $35\times 35\times 7$ | 7 | Hitachi 3200
UNIPLAST | $35\times 35\times 7$ | 35 | Hitachi 3200
|  | position 1 |
UNIPLAST | $35\times 35\times 7$ | 35 | Hitachi 3200
|  | position 2 |

At small wavelengths, where in general the attenuation length becomes
significantly smaller than the actual sample thickness, the spectrophotometer
transmission measurements lose sensitivity due to the very small light
intensities. As a result, we only plot the extracted attenuation lengths above
these wavelength values.

The transport of the UV photons, either generated by scintillation or by
Cherenkov radiation, to the WLS coating requires a sufficiently large
transparency (attenuation length) of the plastic material in the UV range
$280\text{\,}\mathrm{nm}400\text{\,}\mathrm{nm}$ to avoid too large absorption
losses before reaching the WLS layer on the slide surface. Secondary photon
emission from the WLS layer in the visible range between
$400\text{\,}\mathrm{nm}$ and $500\text{\,}\mathrm{nm}$, where many types of
photomultipliers or silicon photomultipliers reach also the highest
sensitivities, requires even larger attenuation lengths in this wavelength
range to build detectors of larger volume.

Fig. 1 shows the transmission spectrum through the uncoated, 5 mm thick PMMA
slide (blue line) and Fig. 2 the corresponding attenuation length. The PMMA
material under study becomes only transparent above about
$380\text{\,}\mathrm{nm}$. As a result, only Cherenkov photons within the
range $380\text{\,}\mathrm{nm}400\text{\,}\mathrm{nm}$ can be absorbed by the
WLS layer on the surface of the PMMA slide and therefore a rather small light
yield is to be expected from generation of Cherenkov light by charged
particles traversing this material.

Fig. 1 shows as well the transmission spectra for the pure PS slide (red
dotted and dashed lines) for a light path length through the material of about
$4.5\text{\,}\mathrm{mm}$ measured with both spectrophotometers. The results
obtained with both spectrophotometers are in very good agreement for
wavelengths below $380\text{\,}\mathrm{nm}$.

In contrast to the PMMA material, the PS material shows transparency already
at about $290\text{\,}\mathrm{nm}$ (see Fig. 1) with an attenuation length
close to $10\text{\,}\mathrm{mm}$ at $325\text{\,}\mathrm{nm}$ (see Fig. 2),
where the emission maximum of scintillation light in PS is located. As a
result, much more of the UV photons that are generated in the PS slide can be
absorbed in the WLS compared to the PMMA slide.

Figure 1: Transmission spectra measured with a PerkinElmer Lambda 950
spectrophotometer (PE Lambda 950) and with a Hitachi 3200 spectrophotometer
(Hitachi 3200). In red: transmission spectra for the uncoated pure PS slide
with light path lengths through the material of $4.5\text{\,}\mathrm{mm}$
(dotted and dashed line) and $50\text{\,}\mathrm{mm}$, respectively (solid
line). In blue: transmission spectrum for the uncoated PMMA slide with a light
path length of $5\text{\,}\mathrm{mm}$. In black: transmission spectra for the
UNIPLAST slide with light path lengths of $7\text{\,}\mathrm{mm}$ (solid line)
and $35\text{\,}\mathrm{mm}$ , respectively (dashed line: at sample position
1, dotted line: at sample position 2, as described in the text).  Figure 2:
Attenuation lengths extracted from the transmission spectra presented in Fig.
1 for samples made of polystyrene (PS), PMMA, and UNIPLAST. The meaning of the
line styles are identical to those in Fig. 1. The attenuation lengths are only
plotted in those wavelengths ranges where the spectrophotometers have
sufficient sensitivity to measure transmission values above the noise level.

The optical transmission of the $50\times 50\times 4.5{\rm mm}^{3}$ PS sample
was measured as well with the Hitachi 3200 spectrophotometer for a
$50\text{\,}\mathrm{mm}$ light path length through the material, which allows
to determine the attenuation length of the PS material also with good
precision at higher wavelengths where the attenuation length is much larger
than $4.5\text{\,}\mathrm{mm}$ thickness of the slide. The corresponding
transmission curves measured up to wavelenghts of 650 nm are summarized in
Fig. 1 and the corresponding attenuation lengths in Fig. 2. Wavelengths well
above 500 nm are not relevant for our study because of the emission spectra of
the materials under study and the fact that the R1924A PMT used in our studies
with a $\beta$ source (see section 2.3) has negligible quantum efficiency,
respectively, sensitivity above 650 nm.

Since the transmission results for the $4.5\text{\,}\mathrm{mm}$ PS slide
measured with both spectrophotometers are in (very) good agreement, the
attenuation length results are very similar for both spectrophotometer results
as well. The attenuation length result obtained for the
$50\text{\,}\mathrm{mm}$ light path length agrees with the two results
obtained for $4.5\text{\,}\mathrm{mm}$ light path length in the wavelength
range between $300\text{\,}\mathrm{nm}$ and $360\text{\,}\mathrm{nm}$. Hence,
we consider the results from $50\text{\,}\mathrm{mm}$ thickness valid as well
for larger wavelengths above $360\text{\,}\mathrm{nm}$, where the results for
the $4.5\text{\,}\mathrm{mm}$ thickness are less trustworthy. The attenuation
length results for the $50\text{\,}\mathrm{mm}$ thickness can be considered
trustworthy as long as the extracted attenuation length does not become too
large with respect to the sample thickness, as for example in the region above
$630\text{\,}\mathrm{nm}$. In the WLS emission range
$400\text{\,}\mathrm{nm}500\text{\,}\mathrm{nm}$, the attenuation length
extracted from the $50\text{\,}\mathrm{mm}$ light path length measurement
grows from $20\text{\,}\mathrm{cm}$ to $50\text{\,}\mathrm{cm}$.

The obtained results of the PS attenuation length has consequences for
building a WLS-coated PS plastic scintillator detector:

  1. 1.

As long as the plastic scintillator detector thickness is not much larger than
the attenuation length of about $7\text{\,}\mathrm{mm}$ at
$320\text{\,}\mathrm{nm}$, which corresponds to the emission maximum of
primary scintillation light in pure PS, a large fraction of these primary UV
photons can be absorbed by the wavelength shifter in the thin layer on the
slide surface. Hence, a slide thickness of about $5\text{\,}\mathrm{mm}$ is
well chosen, although the thickness is subject to optimization because the
number of primary scintillation photons produced by a traversing charged
particle increases with the scintillator thickness.

  2. 2.

Due to the attenuation length of the order $10\text{\,}\mathrm{mm}$ in the UV
range, the determination of the primary scintillation light yield requires
setups using rather small PS sample sizes, as described in 2.2.2.

  3. 3.

To build longer strips made of WLS-coated PS plastic scintillator material,
with photon detection taking place at the small cross-section ends of the long
strip, the attenuation length in the visible range ($400\text{\,}\mathrm{nm}$
\- $500\text{\,}\mathrm{nm}$) and in particular close to the WLS emission
maximum of $425\text{\,}\mathrm{nm}$ is relevant. The attenuation length
results for the pure PS material suggest that scintillator strips with
sufficient light-yield output on both strip sides of up to
$50\text{\,}\mathrm{cm}$ might be possible, which will be studied in more
detail in future work.

The transmission through a UNIPLAST slide was measured as well with the
Hitachi 3200 spectrophotometer (see Fig. 1) with the corresponding
attenuation-length results shown in Fig. 2, using a $35\times 35\times 7{\rm
mm}^{3}$ sample, with two different orientations, resulting in light path
lengths through the material of either 7 mm or 35 mm. Since the UNIPLAST
material is a PS-based scintillator, it is assumed that the extruded UNIPLAST
scintillator has the same refraction index as pure polystyrene, when
translating the transmission spectrum into the attenuation length as function
of wavelength (see Fig. 2). This assumption is supported by the fact that the
transmission spectra for UNIPLAST and pure PS agree reasonably well at larger
wavelengths (above about 480 nm) for the sample thickness of 7 mm (UNIPLAST)
respectively 4.5 mm (PS) where the transmission is dominated by the reflection
of light at the front and back side of the slides.

We note that the UNIPLAST scintillator material has a visible non-uniformity
in transparency inside the samples, which generates significant differences in
the measured transmission spectra and hence the corresponding attenuation
lengths when choosing different intersection points for the same light path
length through the sample. Therefore, the transmission-spectrum results for
two different positions (called 1 and 2) of the light beam on the UNIPLAST
slide for the $35\text{\,}\mathrm{mm}$ light path length are presented.

####  2.2.2 Scintillation light yield of pure polystyrene and of the UNIPLAST
reference samples

Using a Hamamatsu R2059-01 [16] reference PMT with a calibrated bialkali
photocathode, the light yield of a UNIPLAST sample with dimensions of
$35\times 35\times 7{\rm mm}^{3}$ was measured at room temperature as a
function of integration time. The scheme of the setup for the light yield
measurement is shown in Figure 3. All sample surfaces were optically polished
and the sample was wrapped inside multiple layers of Teflon film serving as a
diffuse light reflector. Baysilone oil M 300.000 (Momentive Performance
Materials Company) was used as an optical coupling agent to couple the
UNIPLAST scintillator material to the R2059-01 PMT. The position of the PMT’s
single electron peak was used for the light yield value calibration [17, 18].
The response to an 241Am ($E_{\gamma}$ = $59.5\text{\,}\mathrm{keV}$)
$\gamma$-source was measured for two different orientations of the sample on
the R2059-01 PMT entrance window. The source was placed on the sample side
opposite to the side attached to the PMT. A noise contribution is measured
without a sample on the PMT for every integrating gate as a pedestal value and
subtracted from the values measured with the samples and the source installed
on the PMT. The 241Am spectrum is shown in Figure 4 as example. Since we used
a calibrated PMT, one can easily convert the light yield measured in
photoelectrons/MeV into photons/MeV taking into account a quantum efficiency
at the luminescence maximum of the material as QE(420 nm) = 17.7%. The results
of the light yield measurement are summarized in Figure 5. One observes that
the “horizontal” position ($35\times 35{\rm mm}^{2}$ side attached to R2059-01
PMT) results in about 12 % higher value of the light yield in comparison with
the “vertical” position ($7\times 35{\rm mm}^{2}$ side attached to PMT). This
can be explained by the fact that for the “vertical” position the average path
length of scintillating photons is larger. The absolute value of the light
yield reaches about
$10\,000\text{\,}\mathrm{p}\mathrm{h}\mathrm{/}\mathrm{M}\mathrm{eV}$ for a 1
microsecond of integration time.

Figure 3: Scheme of the setup for light yield measurements. A signal from the
R2059-01 PMT is splitted in two. One goes through the passive delay chain to
an input of a CAEN QDC v792N unit, the second goes through a constant fraction
discriminator and forms the integrating gate for the QDC. The data acquisition
is performed with a PC based on the ROOT software.  Figure 4: Photoelectron
yields obtained with a UNIPLAST sample of dimensions $35\times 35\times 7{\rm
mm}^{3}$ coupled to a calibrated reference PMT Hamamatsu R2059-01 using a
241Am ($E_{\gamma}$ = $59.5\text{\,}\mathrm{keV}$) source. The acquisition
time of every spectrum was 600 seconds, the integration time gate was 1000 ns.
Figure 5: Scintillation light yield in photons per MeV energy deposition as a
function of integration time of a $35\times 35\times 7{\rm mm}^{3}$ UNIPLAST
sample, with two different sides coupled to the calibrated R2059-01 PMT as
described in the text.

The light yield of samples of pure polystyrene were characterized as well with
the R2059-01 reference PMT. The samples have dimensions of $50\times 50\times
4.5{\rm mm}^{3}$ and $25\times 25\times 4.5{\rm mm}^{3}$. To minimize the
effect of the photon absorption due to long path lengths, we tested the pure
PS sample with dimensions of $25\times 25\times 4.5{\rm mm}^{3}$ with a 241Am
$\gamma$-source as well as with a 137Cs $\gamma$-source. The measurements were
done at the same conditions as for the UNIPLAST sample and for two
orientations on the R2059-01 PMT: “horizontal” ($25\times 25\,{\rm mm}^{2}$
side attached to the PMT) and “vertical” ($4\times 25{\rm mm}^{2}$ side
attached to the PMT). The results are presented in Figure 6.

Figure 6: Photoelectron yields obtained with a pure polystyrene sample of
dimensions $25\times 25\times 4.5{\rm mm}^{3}$ coupled to a calibrated
reference PMT Hamamatsu R2059-01 using a 137Cs ($E_{\gamma}$ =
$662\text{\,}\mathrm{keV}$) $\gamma$-source as described in the text.
Horizontal position on PMT (blue): $25\times 25{\rm mm}^{2}$ side attached to
the PMT. Vertical position (green): $4.5\times 25{\rm mm}^{2}$ side attached
to the PMT. Photoelectron yield without any source (red) and with a 241Am
($E_{\gamma}$ = $59.5\text{\,}\mathrm{keV}$) $\gamma$-source (black), single
electron peak and a response of the PMT without a sample on the 137Cs source
(violet). For all spectra excepting the last, a threshold cutting the single
electron peak was applied. The acquisition time of every spectrum was 600
seconds, the integration time gate was 1000 ns.

No well-identifiable response was observed with a 241Am $\gamma$-source for
both sample orientations, as shown in Fig. 6. No photo peak of
$662\text{\,}\mathrm{keV}$ $\gamma$ quanta was observed, likely due to the low
density of polystyrene and the relatively small thickness of the sample, but
one can clearly recognize the shift of the Compton edge to lower photoelectron
(PE) yield when changing from the “horizontal” to the “vertical” orientation
of the sample. Qualitatively, this behaviour is to be expected: in vertical
position, the Compton scattering positions are distributed over a length of
$25\text{\,}\mathrm{mm}$ compared to only $4.5\text{\,}\mathrm{mm}$ in the
case of horizontal position. Since the attenuation length in the UV range for
pure PS is rather small (around $10\text{\,}\mathrm{mm}$ at
$320\text{\,}\mathrm{nm}$ wavelength, see Fig. 2), one expects to collect less
photons in vertical position. The position of the Compton edge for the
“horizontal" orientation is located at about 60 photoelectrons and corresponds
to 2/3 of the $\gamma$ energy ($441\text{\,}\mathrm{keV}$ for
$662\text{\,}\mathrm{keV}$ $\gamma$ quanta). Using the calibrated PMT
Hamamatsu R2059-01, we convert the light yield measured in photoelectrons/MeV
into photons/MeV, taking into account a quantum efficiency at the luminescence
maximum of polystyrene as QE(325 nm) = 24.22% and the known energy deposition
of the $441\text{\,}\mathrm{keV}$ electron (corresponding to the Compton edge
in the photoelectron spectrum). From the 60 photoelectrons observed at the
Compton edge when attaching the sample with the $25\times 25{\rm mm}^{2}$ side
to the PMT, one obtains $60/0.2422/0.441=560$ ± (5%) photons/MeV. This light
yield is likely still underestimated due to the fact that the attenuation
length inside pure PS is not much larger than the sample dimensions (see Fig.
2).

###  2.3 Measurements with the $\beta$-source setup

This sub-section decribes the setup to measure the number of photoelectrons
detected with the coated PS and PMMA slides and the UNIPLAST reference slide
coupled always to a Hamamatsu-R1924A PMT[19] and using the same 90Sr
$\beta^{-}$ source. We present first the calibration of the R1924A PMT used in
our dedicated $\beta^{-}$ setup.

####  2.3.1 Calibration of the R1924A PMT with a dedicated laser setup

To quantify the PE yield detected by the R1924A PMT, measurements with a
pulsed laser (PILas [20] delivering pulses with a full width at half maximum
of $54\text{\,}\mathrm{ps}$ at a wavelength of $402.6\text{\,}\mathrm{nm}$)
were performed. The PMT was placed inside a dedicated dark box and the laser
light pulses were directed through a diffusor onto the PMT cathode, not using
the plastic slides. The PMT signals were read out using a 16-channel
WaveCatcher digitizer [21]. The data were collected in runs, consisting of a
set of events defined by the measurement trigger. Data taking for all these
measurements was triggered with the trigger signal from the laser driver. Each
run was then analysed using a C++ code based on the ROOT software framework
[22]. Fig. 7 shows the averaged recorded waveforms normalized to the maximum
amplitude for a laser calibration measurement. The waveforms of each
individual event for these measurements were time-integrated within an
integration window of $16\text{\,}\mathrm{ns}$ width, starting
$4\text{\,}\mathrm{ns}$ before the maximum of the signal. This time window was
chosen to be significantly shorter compared to the measurements with the
plastic slides to minimize the effect from noise. Resulting time-integrated
spectra are shown in Fig. 8.

Figure 7:  The averaged waveform (dotted line), normalized to the maximum
amplitude, for the calibration of the R1924A PMT with the laser setup is
shown, where about 3 photoelectrons on average are detected by the PMT within
$\text{FWHM}=$54\text{\,}\mathrm{ps}$$. The other curves show a comparison of
the averaged waveforms normalized to the maximum amplitude for the uncoated PS
(black dashed line) and PMMA (grey dashed line) slides and the double coated
PS-WLS-2 (black solid line) and the PMMA-WLS-2 (grey solid line) slides, the
latter of which are very similar to the corresponding single coated PS-WLS and
PMMA-WLS slides (not shown).

The gain of the R1924A PMT is determined from a fit of the ideal PMT function
(Eq. 5 of [23]) to the time-integrated spectra, which allows one to translate
the time-integral into number of photoelectrons. Since the gain of the R1924A
PMT is comparable to two times the standard deviation of the PE peaks, it is
difficult to distinguish the individual peaks and to determine the gain. Under
these circumstances, the individual peaks are best distinguishable if the mean
number of PE detected by the R1924A PMT per laser pulse is between 1.5 and 3.0
PE. For the calibration, we have tuned the laser intensity such that we
achieve a mean number of PE in this range and have taken three measurements at
different mean number of PE. The three measurements are fitted simultaneously
with the gain and the standard deviation of the PE peaks as global fit
parameters.

The calibration measurements are shown in Fig. 8 for a PMT bias voltage of
1000 V. The global fit of the laser calibration measurements gives 1 PE =
(46.05 $\pm$ 0.47) mV·ns for a bias voltage of 1000 V and 1 PE = (19.97 $\pm$
0.21) mV·ns for a bias voltage of 900 V.

(a)

(b)

(c)

Figure 8: Spectra of the time-integrated waveforms for the gain calibration
measurements of the R1924A PMT at a bias voltage of $1000\text{\,}\mathrm{V}$
using short laser pulses that are reduced in intensity by attenuators
resulting in a low average number of detected PE in the PMT per pulse (average
number of PE given in the captions, compare Fig. 7 for corresponding waveform
forms). The time-integrated spectra (black lines) are fitted simultaneously
with the fit function described in the text. The gain value extracted from the
global fit (dashed red lines) is used to translate time-integrated spectra
into PE spectra. The individual PE peaks (gray lines) from the global fit are
also shown.

####  2.3.2 Measurements with the dedicated $\beta$-source setup

To ensure a well-defined position of the plastic slide under study, it was
placed inside a black plastic cover of $1.2\text{\,}\mathrm{mm}$ wall
thickness. To enable high-energetic $\beta^{-}$ particles traversing the
plastic material slides, the cover had circular holes of
$16.8\text{\,}\mathrm{mm}$ diameter on the top and bottom side that were
covered with thin black tape to guarantee minimal energy loss together with
light tightness. The slide was then coupled to one of the four
$5\text{\,}\mathrm{mm}$ sides via optical gel (Bayer silicone Baysilone) to
the Hamamatsu R1924A PMT (cathode diameter: $22\text{\,}\mathrm{mm}$, quantum
efficiency QE = $26\text{\,}\mathrm{\char 37\relax}$ at
$420\text{\,}\mathrm{nm}$) [19], as shown in Fig. 9, operated at a voltage of
$1000\text{\,}\mathrm{V}$.

Figure 9: The plastic slide, wrapped in Dupont™ Tyvek® 1073D paper, enclosed
inside a black plastic cover, coupled to the R1924A PMT, which is surrounded
by a metal housing.

To increase the light collection efficiency, the plastic slides are wrapped
with Dupont™ Tyvek® 1073D paper, a diffuse reflector, only leaving out
$10\text{\,}\mathrm{mm}$ of the side in contact with the R1924A PMT to prevent
the optical gel from leaking between the slide and the wrapping.

As for the calibration measurements, the R1924APMT signals were read out using
the same 16-channel WaveCatcher digitizer. The trigger signal was built from
two small circular scintillator slides placed right on top of each other, each
coupled using optical gel (Bayer silicone Baysilone) to a Hamamatsu R5900 PMT
operated at a voltage of $780\text{\,}\mathrm{V}$ and installed inside a box
("trigger box"). The upper scintillator has a diameter of
$20\text{\,}\mathrm{mm}$ diameter, the lower one has a diameter of
$18\text{\,}\mathrm{mm}$, and both of them are $1\text{\,}\mathrm{mm}$ thick.
The trigger box was located $3\text{\,}\mathrm{mm}$ below the slide under
study.

Figure 10: The experimental setup consists of a 90Sr source located above the
plastic slide under study coupled to the R1924A PMT and the trigger box with
the two trigger scintillators, each coupled to a Hamamatsu R5900 PMT, located
inside the trigger box below the plastic slide.

Data were taken with a 90Sr source located $4\text{\,}\mathrm{mm}$ above the
plastic slide, as shown in Fig. 10. Given the thickness of the plastic slides
under study and the other materials to be traversed by the electrons from the
$\beta^{-}$ source to reach the lower trigger scintillator, the $\beta^{-}$
kinetic energy required is at least $2\text{\,}\mathrm{MeV}$. This condition
is fulfilled by a fraction of electrons coming from $\beta^{-}$ decays of the
90Sr daughter isotope 90Y. The trigger rate, within the trigger window of 15
ns, without the 90Sr source was $0.3\text{\,}{\mathrm{s}}^{-1}$, while with
the source it was $36.6\text{\,}{\mathrm{s}}^{-1}$. Fig. 7 shows a comparison
of the averaged R1924A PMT waveforms, summed over the events of a typical run
for the measurements taken with the 90Sr source, between the PS and the PMMA
slides, as well as between the PS-WLS-2 and the PMMA-WLS-2 slides, the latter
of which looks the same as for the single coated slides.

(a)

(b)

Figure 11: Averaged waveforms (same as Fig. 7) for the PS (black lines) and
PMMA (grey lines) slides and the deconvolved PS waveforms (see text, dashed
lines) with a fit (dash-dotted lines) of the exponential decay of the tail.
(a) shows the uncoated slides and (b) the double-coated slides. The
fluorescence decay time constant extracted from the fit is given in the
legend.

The averaged waveforms allow us to conclude on the main light generation
mechanism in the PMMA and PS slides, since Cherenkov light is emitted promptly
along the particle trajectory, while scintillation light is emitted with a
delay governed by the fluorescence decay time. For the uncoated slides, one
observes a longer tail for PS than for PMMA (see Fig. 7), which is expected
for PS since not only Cherenkov photons but also scintillation photons are
produced in PS. For the coated PMMA and the coated PS slides, the longer tail
is even more pronounced for the PS material (see Fig. 7). For pure PS, typical
fluorescence decay times in the range of
$10\text{\,}\mathrm{ns}20\text{\,}\mathrm{ns}$ are reported [9]. This is
consistent with the decay times of
$19.2\text{\,}\mathrm{ns}19.3\text{\,}\mathrm{ns}$ observed in the coated and
uncoated PS slides, determined by fitting an exponential to the falling edge
[9, 24, 25] of the deconvolved averaged waveforms from about
$20\text{\,}\mathrm{ns}$ for the uncoated slide and from
$30\text{\,}\mathrm{ns}$ for the double coated slide until
$70\text{\,}\mathrm{ns}$ after the maximum of the waveform, as shown in Fig.
11. The fit function is exp$(-(t-t_{0})/\tau)$ with the decay time $\tau$ and
an offset $t_{0}$. The deconvolution has been performed using the response of
the PMT and the read-out to a $\text{FWHM}=$54\text{\,}\mathrm{ps}$$ short
laser pulse (see Fig. 7) as the transfer function of the setup. We can assume
the photons of the short laser pulses arrive simultaneously at the PMT and the
visible structure of the resulting pulse, shown in Fig. 7, is the transfer
function of the system including ringing and reflections. The averaged
waveforms are deconvolved with this transfer function and the result is
smoothed with a Gaussian with a standard deviation of
$800\text{\,}\mathrm{ps}$ (see Appendix B of [14]). It should be noted that
the fluorescence decay time of the WLS dye is of the order of
$1.5\text{\,}\mathrm{ns}$ [26] and is difficult to measure with this setup.

To quantify the number of detected photoelectrons, the waveform of each
individual event was time-integrated within an integration window between
$110\text{\,}\mathrm{ns}$ and $200\text{\,}\mathrm{ns}$. Fig. 12 shows the
time-integrated spectra obtained from the individual waveforms for the same
data runs as shown in Fig. 7. In order to estimate the reproducibility of the
optical coupling of the PMT to the plastic slide under study, two measurements
were taken, the second one after decoupling and coupling again the slide.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 12: Spectra of the time-integrated waveforms for the six plastic slides
under study for measurements with the 90Sr $\beta^{-}$ source corresponding to
the data shown in Fig. 7. The curve shows the result of a fit to the measured
spectrum using as a fit function the convolution of a Landau distribution with
a Gaussian function in order to determine the most-probable value of the
distribution, measured with the Hamamatsu R1924A PMT at a bias voltage of 1000
V.  Figure 13: Spectrum of the time-integrated waveforms for the UNIPLAST
slide for a measurement with the 90Sr $\beta^{-}$ source. The curve shows the
result of a fit to the measured spectrum like in Fig. 12, measured with the
Hamamatsu R1924A PMT at a bias voltage of 900 V.

##  3 Results on the detected PE yield

Since the energy loss of a charged particle inside a thin layer of material
follows a Landau distribution and the non-negligible electronic noise follows
a Gaussian distribution, a convolution of a Landau distribution and Gaussian
function was used to fit the time-integrated spectra of the measurements with
the plastic slides (as shown in Fig. 12). To quantify the number of PE
detected by the PMT, we choose the most probable value (MPV) of this fit,
translating it using the equivalence between PE and mV·ns obtained from the
R1924A PMT calibration measurements as described above.

Table 2 presents the PE yield obtained from the MPVs detected for the seven
plastic slides under study. The PE values shown in the table correspond to the
weighted mean of the MPVs of two measurements performed for each slide,
divided by the calibration value. The first quoted uncertainty
($\sigma_{\scalebox{0.5}{MPV}}$) is the uncertainty of the weighted mean,
quantifying the statistical uncertainty in the MPV determination. The second
quoted uncertainty ($\sigma{\scalebox{0.5}{sys}}$) is calculated as the
weighted standard deviation of the two measurements from the MPVs mean,
allowing us to estimate whether the decoupling and coupling again the slide to
the PMT introduces a significant systematic uncertainty. In addition, there is
about a 1% uncertainty for all measurements related to the SPE calibration
value.

Table 2: PE yield obtained from the MPVs detected by the PMT for the seven plastic slides under study, using a 90Sr $\beta$ - source. The number in brackets quotes the thickness of the plastic slide. The quoted uncertainties as described in the text correspond to the statistical uncertainty in the MPV determination ($\sigma_{\scalebox{0.5}{MPV}}$) and to the systematic uncertainty due to the optical coupling ($\sigma_{\scalebox{0.5}{sys}}$), respectively. In addition, there is about a 1% uncertainty related to the calibration value. | PE $\pm\;\sigma_{\scalebox{0.5}{MPV}}\pm\sigma_{sys}$
---|---
PMMA ($5.0\text{\,}\mathrm{mm}$) |  $4.38\pm 0.04\pm 0.01$
PMMA-WLS ($5.00\text{\,}\mathrm{mm}$) |  $5.96\pm 0.03\pm 0.02$
PMMA-WLS-2 ($5.0\text{\,}\mathrm{mm}$) |  $6.92\pm 0.04\pm 0.04$
PS ($4.5\text{\,}\mathrm{mm}$) |  $8.66\pm 0.06\pm 0.28$
PS-WLS ($4.8\text{\,}\mathrm{mm}$) |  $42.10\pm 0.27\pm 0.23$
PS-WLS-2 ($4.7\text{\,}\mathrm{mm}$) |  $54.58\pm 0.28\pm 0.34$
UNIPLAST ($5.1\text{\,}\mathrm{mm}$) |  $506.22\pm 1.66\pm 7.17$

Let us first discuss the results for the PMMA slide. While PMMA does not
scintillate, it can result into radiation of Cherenkov light when being
traversed by a charged particle with a velocity above the speed-of-light
inside PMMA, $c_{PMMA}=1/n_{PMMA}$, with $n_{PMMA}=1.520$ at a wavelength of
$400\text{\,}\mathrm{nm}$ [15].

The number of Cherenkov photons that can be detected by the PMT depends on the
incident angle of the charged particles, since the Cherenkov radiation is
emitted under a defined angle with respect to the particle‘s flight direction.
Both, the Cherenkov angle and the critical angle of total reflection are
defined by the refraction index. The measurement for PMMA-WLS (PMMA-WLS-2)
shows a 1.4 (1.6) times higher PE yield compared to the one for the uncoated
PMMA. In absolute terms, the light yield even for the PMMA-WLS-2 slide with
6.92 PE on average is considered to be of minor interest for a charged-
particle detector applications. A possible increase in light-output might be
achieved for WLS-coated PMMA in case of UV-transparent PMMA plate material,
which however is typically significantly more expensive than UV-non-
transparent PMMA plate material.

The light yield detected for the uncoated PS slide (8.66 PE on average) is
already two times higher than for the uncoated PMMA slide, profiting from the
additional scintillation photons in PS and the higher PS transparency in the
UV range. The measurement for PS-WLS (PS-WLS-2), shows a 4.9 (6.3) times
higher PE yield compared to the one for the uncoated PS slide as result of the
absorption of the scintillation as well as the Cherenkov UV photons in the WLS
layer after a short travelling distance and the subsequent re-emission of
photons in the visible range profiting from the high PS transparency in this
wavelength range. In absolute terms, the detected light yield of about 55
photoelectrons in case of the double-coated PS is sufficiently large to
further study and develop the concept as a detector technology, even if the
light yield is a factor of 9.3 lower than for the UNIPLAST extruded
scintillator that was used for comparison.

##  4 Summary and Outlook

This work is based on the research and development work with wavelength-
shifting optical modules (WOMs), which can e.g. be used as photon detectors
for large-area liquid-scintillator detectors. In this work, we modify the WOM
concept in two ways: 1) Instead of a tube geometry, we consider a planar
geometry. 2) The WOM base material serves not only as a light guide but in
addition as an active detector material, hence UV photons are produced either
by Cherenkov radiation or scintillation inside the material. For this WOM
material, we considered in particular pure polystyrene (PS), because it is
intrinsically scintillating in the UV with sizeable transparency in this
wavelength range and commercially available at relatively low cost. While one
can not achieve the same high light yields as for standard PS-based plastic
scintillators (e.g. the extruded plastic scintillator material produced by
UNIPLAST that was used for comparison), which are doped with a primary
fluorophore and WLS molecules, the advantage of the material and procedure
presented here is that one can produce without large efforts planar
scintillators by simply dip-coating commercially available polystyrene plate
material that can be cut into the desired slide geometries.

We studied the light yield of a square-shaped commercial polystyrene slide of
50 mm side length and $O(5)$ mm thickness coated with wavelength-shifter
molecules (p-terphenyl and bis-MSB), coupled to a photomultiplier, using
$\beta^{-}$ particles from a 90Sr source. Comparison measurements were made
with an uncoated polystyrene slide as well as with uncoated and coated PMMA
slides, the latter two of which can only produce Cherenkov light when being
traversed by charged particles. The light yield produced by minimum ionizing
particles detected with a Hamamatsu R1924A PMT for the single (double) coated
polystyrene slide was about a factor of 4.9 (6.3) higher than the light yield
detected for the uncoated polystyrene slide. The absolute yield measured with
the single (double) coated polystyrene slide was about 42 (55) photoelectrons.
With the same setup and using a very similar geometry, the light yield
obtained with an extruded polystyrene-based plastic scintillator doped with
PTP of 1.5% concentration and POPOP of 0.01% concentration produced by
UNIPLAST was measured to be about 506 photoelectrons.

While the light yields for the coated pure polystyrene slides were much
smaller than for the doped extruded plastic scintillator, they are still
sufficiently large to make this material interesting for application in
particle detection.

On the basis of these promising results, future studies will focus on one hand
on better understanding of the relative contribution of Cherenkov light and
scintillation light in the used polystyrene material, and on the other hand on
testing longer polystyrene strips, typically used for building larger-scale
detectors, in order to quantify the effective attenuation length, the energy
resolution, as well as the time and the spatial resolution of such a detector,
complemented by radiation-hardness studies. The attenuation length of pure PS
material, as the one under study, will likely constrain the length of such
strips to O(1 m) or below.

##  5 Acknowledgements

* We acknowledge the fruitful discussion with all the individuals from JGU Mainz and DESY involved in establishing the WOM as a sensor for IceCube. We thank Emil List-Kratochvil and Andreas Opitz from the Institute of Physics at Humboldt University of Berlin for helpful discussions on increasing the WLS layer thickness in the WLS dip-coating process and for providing access to the transmission spectrophotometer PerkinElmer Lambda 950. We thank Axel Hagedorn from DESY for providing Tyvek paper. We acknowledge the support from BMBF via the High-D consortium.

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