# Deployment of Water-based Liquid Scintillator in the Accelerator Neutrino
Neutron Interaction Experiment

M. Ascencio-Sosa  Z. Bagdasarian  J. Beacom  M. Bergevin  M. Breisch  G.
Caceres Vera  S. Dazeley  S. Doran  E. Drakopoulou  S. Edayath  R. Edwards  J.
Eisch  Y. Feng  V. Fischer  R. Foster  S. Gardiner  S. Gokhale  P. Hackspacher
C. Hagner  J. He  B. Kaiser  F. Krennrich  T. Lachenmaier  F. Lemmons  D.
Maksimovic  M. Malek  J. Martyn,**footnotetext: Corresponding author. A.
Mastbaum  C. McGivern  J. Minock  M. Nieslony  M. O’Flaherty  G. D. Orebi Gann
T. Pershing  L. Pickard  N. Poonthottathil  C. Reyes  B. Richards  R. Rosero
M. C. Sanchez  D. T. Schmid  M. Smy  M. Stender  A. Sutton  R. Svoboda  E.
Tiras  M. Vagins  V. Veeraraghavan  J. Wang  A. Weinstein  M. Wetstein  M.
Wurm  M. Yeh  T. Zhang

###### Abstract

The Accelerator Neutrino Neutron Interaction Experiment (ANNIE) is a 26-ton
water Cherenkov neutrino detector installed on the Booster Neutrino Beam (BNB)
at Fermilab. Its main physics goals are to perform a measurement of the
neutron yield from neutrino-nucleus interactions, as well as a measurement of
the charged-current cross section of muon neutrinos. An equally important
focus is placed on the research and development of new detector technologies
and target media. Specifically water-based liquid scintillator (WbLS) is of
interest as a novel detector medium, as it allows for the simultaneous
detection of scintillation and Cherenkov light. This paper presents the
deployment of a 366 L WbLS vessel in ANNIE in March 2023 and the subsequent
detection of both Cherenkov light and scintillation from the WbLS. This proof-
of-concept allows for the future development of reconstruction and particle
identification algorithms in ANNIE, as well as dedicated analyses, such as the
search for neutral current events and the hadronic scintillation component
within the WbLS volume.

##  1 Introduction

The Accelerator Neutrino Neutron Interaction Experiment (ANNIE) [1, 2] is a
26-ton Gadolinium loaded water Cherenkov detector deployed on the Booster
Neutrino Beam (BNB) at Fermilab. The physics goals of ANNIE focus on, but are
not limited to, the measurement of the final-state neutron multiplicity
produced by $\nu_{\mu}$ interactions with nuclei, as well as the measurement
of the charged-current cross section of $\nu_{\mu}$. Additionally, ANNIE is
also meant to be a test platform for novel detection concepts and therefore
has a strong focus on different detector research and development tasks. These
include the deployment of Large Area Picosecond Photodetectors (LAPPDs) [3, 4]
as a new photodetector technology, as well as Water-based Liquid Scintillator
(WbLS) [5, 6], as a novel detector medium, which is the focus of this paper.

Given the success of monolithic, large scale water Cherenkov and liquid
scintillator detectors in the context of neutrino physics, there is currently
an ongoing interest in the concept of hybrid event detection. The aim is to
combine the advantages of water Cherenkov detectors — large optical
transparency, sensitivity to the particle direction, as well as particle
identification through the Cherenkov ring topology — with the low-threshold
calorimetric information of liquid scintillator detectors. The latter would
provide better energy resolution, additional particle identification
capabilities and allow for the detection of neutral particles or particles
with energies below the Cherenkov threshold. To achieve such a hybrid event
detection it is necessary to differentiate between the Cherenkov and
scintillation component of the events.

For this a number of different approaches have been developed in recent years,
such as spectral photon sorting [7, 8], time separation through fast
photodetectors [9, 10, 11] or the development of novel target materials, such
slow scintillators [12, 13] and WbLS. It is also possible to combine these
separation approaches, such as the use of fast LAPPDs together with a WbLS
medium [14]. WbLS is an admixture of a low percentage of liquid scintillator
in water, featuring high transparency and which is an enabling medium for
hybrid neutrino event detection. Compared to pure liquid scintillator it has a
reduced scintillation yield, which allows for easier identification of the
Cherenkov light. The WbLS hence provides a larger total number of photons
compared to Cherenkov light from pure water. Conceptually similar ideas have
already been used successfully in LSND [15] and in MiniBooNE [16], using a
diluted scintillator and mineral oil as detection media, respectively.

ANNIE is part of a wider experimental effort for the technical demonstration
of the hybrid optical detection concept. This includes bench-top scale
experiments such as CHESS [17, 5] and FlatDot [11] but also other mid-scale
detectors setups: Eos [18], the 1-ton [19] and 30-ton purification
demonstrator at Brookhaven, and NuDot [20]. Furthermore WbLS is explored as a
detection medium for the proposed Theia detector [21]. Several recent
community-planning exercises have been highlighting the importance of the
developments summarized above [20, 22, 23].

The purpose of this paper is to provide the first result on the deployment of
a small WbLS vessel in ANNIE and to present the first detection of accelerator
neutrinos in a WbLS medium. Section 2 gives a short overview of the ANNIE
detector, while section 3 explains the mechanical structure, filling and
deployment of the WbLS vessel inside the ANNIE water tank. Section 4 provides
a description of the WbLS deployed in ANNIE and shows its long term stability.
Section 5 shows a data-on-data comparison of events with and without the WbLS
scintillation. This provides a proof for the successful deployment of WbLS in
ANNIE, corresponding to the detection of both Cherenkov and scintillation
light associated with the accelerator neutrino events. The paper then
concludes in section 6.

##  2 The ANNIE detector

Figure 1: Detector schematic with the WbLS-filled vessel, called SANDI
(Scintillator for ANNIE Neutrino Detection Improvement).

ANNIE consists of three separate main detector elements: (1) the front muon
veto (FMV), (2) the main water Cherenkov detector, and (3) the muon range
detector (MRD), as shown schematically in Fig. 1. The ANNIE hall [1] consists
of three levels (top, middle, and lower). The top is at ground level and is
the entrance to the hall with the middle and lower levels below ground level.
The middle level houses the electronics racks and has access to the lid of the
ANNIE tank.

The FMV, located upstream of the main detector volume, consists of two layers
of scintillating paddles. Each paddle is read out at one end by a 2-inch PMT
coupled to a glass light guide. The primary purpose of the FMV is to reject
events originating from an interaction in the dirt upstream of ANNIE. Only the
existence of an incoming particle is relevant and not the exact position,
therefore both layers are oriented horizontally. The two layers are read out
on opposite ends to mitigate the impact of attenuation losses from a particle
passing through the far end. The FMV was found to have a muon tagging
efficiency of $\left(95.6\pm 1.6\right)\%$ [24]. This efficiency was
determined through the use of the MRD muon track reconstruction, by back-
propagating found muon tracks to the position of the FMV and checking for a
coincidence with a FMV paddle hit.

The MRD is placed downstream of the main detector volume, and like the FMV,
consists of layers of scintillator paddles read out by 2-inch PMTs, arranged
in alternating horizontal and vertical orientations. This provides the
capability to determine the muon direction through the MRD.

The primary detector of ANNIE is a 26-ton, cylindrical, highly pure water
volume loaded with 0.2% of gadolinium sulfate by weight. The interior of the
tank is instrumented with 132 PMTs for 10% photo cathode coverage. Five Large
Area Picosecond Photodetectors (LAPPDs), provided by Incom Inc., will
supplement the PMTs to provide improved timing and spatial resolution. LAPPDs
are $20\text{\,}\mathrm{cm}$$~{}\times~{}$$20\text{\,}\mathrm{cm}$
photodetectors that utilize micro-channel plates (MCPs) to achieve sub-
centimeter spatial resolution and a timing resolution down to the order of
tens of picoseconds [4]. The ANNIE tank has a removable top hatch measuring
about $1\text{\,}\mathrm{m}$ in diameter, which supports four PMTs.
Additionally, the tank lid has several small ports where calibration sources
can be lowered into the water and slots through which LAPPDs can be deployed.

ANNIE is placed $100\text{\,}\mathrm{m}$ downstream of the BNB target. The BNB
is produced by impinging $8\text{\,}\mathrm{GeV}$ protons onto a beryllium
target. The resulting hadrons are focused by a toroidal magnetic horn that is
typically pulsed to focus positive hadrons while defocusing negative ones.
These hadrons, primarily pions, then decay to produce neutrinos. The
associated charged leptons are stopped by an absorber $50\text{\,}\mathrm{m}$
downstream of the target. The BNB produces a flux consisting of 93.6%
$\nu_{\mu}$ and 5.9% $\mathrel{\mathop{\nu}\limits^{\vbox
to-0.60275pt{\kern-2.5pt\hbox{$\scriptstyle\textbf{--}$}\vss}}}_{\mu}$ peaked
around $700\text{\,}\mathrm{MeV}$ [25], and operates at a maximum average rate
of $5\text{\,}\mathrm{Hz}$, delivering about $4\times 10^{12}$ protons on
target (POT) over a $1.6\text{\,}\mathrm{\SIUnitSymbolMicro s}$ spill.

##  3 Deployment of WbLS in ANNIE

WbLS was deployed in ANNIE from March to May of 2023 in a program dubbed SANDI
(Scintillator for ANNIE Neutrino Detection Improvement). The SANDI vessel is a
cylinder made of $2.54\text{\,}\mathrm{cm}$ thick acrylic with an interior
measuring $90\text{\,}\mathrm{cm}$ in height and $72\text{\,}\mathrm{cm}$ in
diameter giving it a $366\text{\,}\mathrm{L}$ capacity. The vessel is held by
a stainless steel frame, which serves as a lifting structure. Electro-polished
SAE 304 stainless steel is used to ensure compatibility with the gadolinium-
loaded water. With the stainless steel structure, the vessel weighs
$\sim$$163\text{\,}\mathrm{kg}$ when empty. After fabrication, the vessel was
leak tested by pressurizing it to $2\text{\,}\mathrm{p}\mathrm{s}\mathrm{i}$
for 24 hours, during which time the vessel maintained this pressure.

To deploy SANDI, a steel support structure was designed, fabricated, and
installed by the Fermilab engineering staff. The structure consists of two
4-inch square steel support tubes with 1/4-inch thick walls. A W8 $\times$ 13
steel I-beam spans the columns to support the SANDI vessel. Connected to the
I-beam is an electrical winch with a teflon-wrapped stainless steel cable. The
winch is on rollers and was used to lift the vessel, maneuver it into
position, and lower it into the water. The vessel is shown in Figure 2 hanging
from the deployment structure.

Figure 2: The SANDI vessel suspended from the deployment structure.

Filling the SANDI vessel took two days to complete. Two days prior to filling,
the SANDI vessel was flushed with nitrogen to reduce the potential for
biological contamination. To account for fluid displacement, Gd-water was
removed from the tank, stored in two drums lined with polyethylene, and later
reused after extracting SANDI. The top hatch of the ANNIE tank, along with its
four PMTs, was removed and stored on the tank lid. The empty SANDI vessel was
then lowered into the center of the tank, just to the point where it became
buoyant. As the vessel was filled, it was lowered in steps to maintain access
while limiting the additional force on the support structure.

Due to the small size of the ANNIE hall, the WbLS barrels were held outside of
the ANNIE hall under a tent. The WbLS was pumped from the barrels through a
0.2-micron filter and into $10\text{\,}\mathrm{L}$ containers. The containers
were then carried down to the lower level where they were emptied through a
funnel connected to the inlet of the SANDI vessel. In addition to the inlet
port, the SANDI vessel was also fitted with a teflon overflow tube. When the
vessel was nearly full, the inlet was capped off with a stainless steel
fitting, and the overflow tube was filled with about $3\text{\,}\mathrm{m}$ of
WbLS to remove the last air pocket. Once the vessel was lowered to its final
location, a temporary PVC lid with a cutout for the suspension cable and
overflow tube was fitted to the ANNIE tank. The overflow tube and temporary
lid were both covered with opaque plastic.

During the deployment period, water samples were extracted from the ANNIE tank
for observation. The samples were analyzed with a UV-vis spectrophotometer to
monitor the absorption spectrum and check for WbLS contamination. The resin
filters of the ANNIE water system were expected to capture WbLS from any small
leaks. Throughout the duration of the deployment, the UV-vis monitoring did
not show any change in the absorption spectrum that would indicate WbLS
contamination.

The extraction of the SANDI vessel proceeded much the same way as the
deployment. However, in this case we pumped directly from the vessel on the
middle level of the ANNIE hall up to the WbLS storage barrels on the top
level. Samples of the extracted WbLS were taken back to Brookhaven National
Laboratory (BNL) for analysis.

##  4 Production and long term stability of the deployed WbLS

The organic component of the WbLS used in SANDI is a mix of modified
polyethylene glycol-based surfactants and a diisopropylnaphthalene (DIN) base
liquid scintillator loaded with a fluor of 2,5-diphenyloxazole (PPO). All raw
components were purified using vacuum distillation and an exchange column
before synthesis. To fill the SANDI tank, approximately 500 L of WbLS were
produced using an in-situ mixing technique [19] with a double-jacketed 90 L
Chemglass reaction vessel at BNL. This WbLS formula was designed to be
compatible with Gadolinium loading, similar to that which was deployed in
CHESS for precision light-yield and time profile measurements [26], and at the
BNL 1000 L testbed for optical transparency, stability, and larger-scale
testing [19]. This formulation differs from the WbLS formulation studied in
[6, 5] in order to provide compatibility with Gadolinium loading, and
therefore the optical parameters are expected to differ.

The reaction vessel for the production of the WbLS has several injection ports
made of polytetrafluoroethylene (PTFE) to prevent any chemical complication of
adding organic materials. All the tubing, filtration system, liners, and the
mixing system were pre-cleaned with ethanol (Ethyl alcohol 190 proof), rinsed
with ultrapure water (resistivity 18.2 M$\Omega\cdot\text{cm}$) and dried with
nitrogen gas. Afterward, it was sealed in an inert environment until use. At
different stages of the synthesis, 1% (mass) of purified organic materials
were introduced through different ports into the reactor where pure water (99%
by mass) was pre-filled and mixed at 100 rpm for four hours. Finally, the WbLS
was filtered through a 0.2-micron PTFE membrane using a 316-stainless steel
filtration housing and stored in 55-gal lined drums for shipment.

Each drum was outfitted with a dual polyethylene liner, each measured 5-micron
in thickness, to serve as a reservoir for liquids. The maximum storage
capacity of each drum was set at 180 L, equivalent to two 90 L batches from
the vessel system, to facilitate handling and prevent overflow. Further,
nitrogen cover gas was introduced to the liquid to ensure its stability during
transport. Three WbLS drums ($\approx$450 L) were shipped to and placed in a
temperature-controlled warehouse at Fermilab in November 2022.

The performance of the WbLS used in SANDI was assessed using a Shimadzu UV-Vis
spectrophotometer with a 10 cm quartz cell for optical transmission and a
Beckman LS6500 coincidence counter using a 137Cs source for a light-yield
measurement at BNL. Figure 3(a) shows the optical stability before and after
the SANDI deployment. The variation in optical transmission is within the
statistic uncertainty (0.0003) of the two baselines. Fig. 3(b) shows the
Compton spectra induced from 137Cs. This represents the light-yield stability
of the WbLS before and after its deployment, as the spectra are the same
within the measurement uncertainty. Together the UV and light-yield data
represent the overall WbLS stability through different processes including
shipment, storage, deployment, and draining over a period of nine months since
production.

(a)

(b)

Figure 3: (a) The WbLS optical transparency before and after the SANDI
deployment. (b) The WbLS light-yield before and after the SANDI deployment,
given by the Compton spectra induced from 137Cs.

##  5 Detection of WbLS scintillation in ANNIE

This section describes the detection of the WbLS scintillation light in ANNIE
from the BNB muon-neutrinos. Two independent analyses are performed to
estimate the effect of the WbLS-filled SANDI vessel on the number of detected
photoelectrons (p.e. or charge) in ANNIE, relative to the Cherenkov light of
pure water. This is done through a data-on-data comparison of throughgoing
muon events and Michel electrons, which is meant to be a principal proof for
the WbLS feasibility in a neutrino beam. The two data sets used in this
section are the pre-SANDI (without WbLS) data, running from December 2022 to
March 2023, and the SANDI data (with WbLS), from March 2023 to May 2023.

###  5.1 Throughgoing muon analysis

Muons can be produced by the interaction of the beam $\nu_{\mu}$ with nuclei.
If such a muon is produced outside of SANDI it can cross the ANNIE detector
from front to back and is called a throughoing muon. These events are well
suited to estimate the impact of the WbLS-filled SANDI vessel on the number of
detected photoelectrons and the additional contribution of the scintillation.
The reason for this is that these muons are minimally ionizing particles which
produce a relatively well defined photoelectron spectrum, depending on the
entrance angle and total track length in ANNIE, smeared by the detector
response.

The principal event selection of neutrino-associated muon events is performed
in a data acquisition window of $\Delta t=2\,\mu\text{s}$ of every incoming
beam trigger. First, a coincidence is required between the water tank cluster
and the MRD cluster within $100$ ns to select events with a muon in the final
state. Here a water tank cluster is defined as at least five tank PMT hits
registered in a pre-defined time window of 50 ns and a MRD cluster is defined
as at least four MRD PMT hits in a time window of 30 ns. The MRD coincidence
guarantees the selection of a muon event and the MRD is sensitive to the muon
track direction. Events are required to have exactly one reconstructed muon
track to simplify the comparison analysis. The MRD track is then extrapolated
back through the ANNIE water tank and events are then selected if the muon
track intersects the SANDI volume. This is expected to provide a large number
of selected events which can produce scintillation light, as they pass the
SANDI volume. A similar cut on the muon tracks has been performed for the
ANNIE data without the SANDI vessel, where muon tracks were required to pass
through the space where SANDI would be. It has to be noted here that the MRD
track angle reconstruction has an estimated uncertainty of 0.17 rad and an
uncertainty on the MRD entry point of $12$ cm in both the X and Y coordinates
[24]. This means that not all selected events have a true intersection with
the SANDI volume. Additionally, a lower limit on the detected number of
photoelectrons in the water tank of $200$ p.e. is selected to minimize the
impact of noise.

These data sets are further split in two, depending on whether the FMV trigger
is coincident with the PMT tank trigger or not. If such a coincidence between
the FMV, the water tank and the MRD occurs then the corresponding event can be
identified as a throughgoing muon. This event selection is well suited for an
estimation of the impact of the additionaly scintillation, as the throughgoing
muons provide a relatively simple event topology. Muons that go through the
entire ANNIE detector provide an energy distribution that is more narrow than
muons that get produced inside the tank, because the spread in track length is
smaller for the throughgoing muons. If the FMV is not triggered in coincidence
with the water tank and MRD, then the event is considered a neutrino
candidate, i.e. the neutrino interacts inside the water tank and the
associated muon is detected by the MRD. These selection cuts provide four data
sets with 1771 pre-SANDI neutrino candidate events, 1181 SANDI neutrino
candidate events, 983 pre-SANDI throughgoing muon events, and 615 SANDI
throughgoing muon events.

In the following, the water tank PMTs are divided into two categories: The
upstream PMTs with a Z-position $<0$ m and the downstream PMTs with a
Z-position $>0$ m, where the Z-axis is defined by the neutrino beam direction
and the origin of the coordinates is the center of the water tank. For
Cherenkov light it is expected that the downstream PMTs should detect a
relatively large number of photoelectrons, as it is emitted in a forward cone
along the muon direction. The upstream PMTs are not able to see direct
Cherenkov light, but they can detect a small fraction of the downstream p.e.
coming from reflections off the ANNIE structures and downstream PMT glass. The
scintillation from the WbLS vessel on the other hand is emitted isotropically.
It is therefore expected that both the upstream and downstream PMTs should
detect a substantial amount of direct scintillation light, while seeing a very
different amount of direct or indirect Cherenkov light. This effect can be
seen for the throughgoing muon data sets in Fig. 4.

(a)

(b)

Figure 4: Distribution of the charge for the selected throughgoing muon events
in units of the number of detected photoelectrons for (a) upstream PMTs and
(b) downstream PMTs. The events are selected if they have an intersection of
the reconstructed muon track of ANNIE with the WbLS vessel volume. The black
distributions show the data of ANNIE with the WbLS vessel and the red
distributions show the data of ANNIE without the WbLS vessel. The latter
distribution is normalized to the statistics of the black distribution of the
ANNIE with WbLS events.

The left Fig. 4(a) shows the measured distribution for the total number of
detected photoelectrons (charge) of all upstream PMTs for the SANDI data
(ANNIE with the WbLS vessel) in black and for the pre-SANDI data (without the
WbLS vessel) in red. The latter distribution is normalized in the plot to the
statistic of the SANDI data for better comparison. It can be seen, that the
pre-SANDI data has a charge distribution with a single peak around $900$ p.e.,
which corresponds to the reflected Cherenkov light of muons traversing the
entire tank. The upstream charge distribution of the SANDI data shows two
peaks at around $900$ p.e. and $1700$ p.e. These two peaks originate from the
non-perfect MRD track reconstruction, which gives rise to two populations of
events: (1) Those events for which the reconstructed muon track has an
intersection with the WbLS vessel, while the true muon track does not
intersect with the WbLS vessel. (2) Those events for which both the
reconstructed and true muon track do have an intersection with the WbLS
vessel. Here a more sophisticated track direction and interaction vertex
reconstruction could provide a better event selection, by using the charge and
time information of the tank photodetectors. Such a technique is under
development. The peak with a lower number of p.e. corresponds to events of the
first type, which did not produce scintillation light, while the peak at the
larger number of p.e. includes both the reflected Cherenkov light and the
direct scintillation light. Therefore Fig. 4(a) provides a qualitative
demonstration of the detection of scintillation from the WbLS. It is
interesting to note that the charge distribution of muons that miss the WbLS
has a peak position that is comparable to that of the distribution for the
pre-SANDI data events. This likely indicates that the reflection of Cherenkov
light from the SANDI vessel structure is relatively small. If the SANDI vessel
structure would have reflected a large fraction of the Cherenkov photons, then
the average upstream charge for the muon events that miss the WbLS would have
been shifted noticeably, compared to the pre-SANDI data muon events. This
observation has been qualitatively confirmed by a simple simulation of the
detector geometry. It indicates that the reflections off the SANDI vessel
structure can only provide a few-percent level effect, relative to the
detected number of downstream photons.

Figure 4(b) shows the downstream charge distribution for ANNIE without the
WbLS vessel in red and for ANNIE with the WbLS vessel in black. Here both
distributions have a peak around $3400$ p.e., where the presence of the WbLS
vessel provides a distribution that is broadened compared to the distribution
for pure Cherenkov light. This can be seen by comparing the root mean square
of the two histograms, which is around 590 p.e. for the pre-SANDI data and
around 710 p.e. for the SANDI data. As expected, the number of detected
photoelectrons for the downstream PMTs is larger than for the upstream PMTs
due to the directionality of the Cherenkov light from the selected muons. The
effect of the broadening for ANNIE with the WbLS vessel can be explained by
the two event populations described previously. Those muon tracks that are
mis-reconstructed such that the true muon track does not intersect the WbLS
vessel do not produce scintillation light. At the same time their Cherenkov
photons can still traverse the WbLS, which has a lower transparency than the
pure water, and therefore these events are shifted to a lower average number
of downstream p.e., compared to the events for ANNIE without the WbLS. Those
events for which the muon tracks do have an intersection with the WbLS vessel
produce both scintillation and Cherenkov light, which shifts the downstream
p.e. distribution to higher values compared to ANNIE without the WbLS. This is
assumed to provide a broader downstream charge distribution for the SANDI
data, as the contribution of the scintillation is not large enough compared to
the direct Cherenkov hits to produce two distinctly separate peaks.

(a)

(b)

Figure 5: The average number of detected upstream photoelectrons plotted
against the intersection of the reconstructed muon track with the X-Y-plane
for $\text{Z}=0$. The black outline shows the position and dimensions of the
WbLS vessel. (a) For throughgoing muon events of ANNIE with the WbLS vessel.
(b) For throughgoing muon events of ANNIE without the WbLS vessel.

A supplementary investigation for the principal detection of the WbLS
scintillation is shown in Fig. 5. Here all throughgoing muons are selected,
not only those which have a reconstructed track that intersects the WbLS
volume. The plots show the number of detected upstream photoelectrons,
averaged for each position bin, plotted against the intersection of the
reconstructed muon track with the X-Y-plane for the center of the water tank
($\text{Z}=0$). The WbLS volume, centered at $\text{X}=0$ m and
$\text{Y}=-0.65$ m, is shown as a black frame. For the SANDI data in the left
Fig. 5(a) the events with the highest average number of detected upstream
photoelectrons correspond largely to the position of the WbLS-filled SANDI
vessel, smeared by the resolution of the track reconstruction. The comparison
with the pre-SANDI data in the right Fig. 5(b) shows the absence of this
feature for the same volume without the WbLS. This shows that the WbLS-filled
SANDI vessel is responsible for the increase in the detected number of
upstream photoelectrons, which meets the expectations of the isotropic WbLS
scintillation.

Figure 4 makes it possible to provide a rough estimate of the increased light
output due to the WbLS, using an analytical fit of the p.e. distributions. For
the SANDI data two bi-Gaussian fits are performed on the upstream and
downstream p.e. distributions. The same events that have a clear scintillation
contribution in the upstream p.e. distribution must also provide scintillation
to the downstream p.e. Therefore the upstream p.e. distribution can be used to
estimate the number of events with and without a scintillation contribution,
which can then be fixed for the fit of the downstream p.e. distribution. The
bi-Gaussian fit on the number of photoelectrons $Q$ is defined as follows:

$f(Q)=C+\frac{A_{\text{W}}}{\sqrt{2\pi
Q}\sigma_{\text{W}}}\exp{\left(-\frac{1}{2}\left(\frac{Q-\mu_{\text{W}}}{\sigma_{\text{W}}\sqrt{Q}}\right)^{2}\right)}+\frac{A_{\text{S}}}{\sqrt{2\pi
Q}\sigma_{\text{S}}}\exp{\left(-\frac{1}{2}\left(\frac{Q-\mu_{\text{S}}}{\sigma_{\text{S}}\sqrt{Q}}\right)^{2}\right)}$
(5.1)

Here $C$ is a constant offset, $A_{\text{W}}$, $A_{\text{S}}$ are the
amplitudes, $\mu_{\text{W}}$, $\mu_{\text{S}}$ are the mean values, and
$\sigma_{\text{W}}$, $\sigma_{\text{S}}$ are the standard deviations for the
two different event populations, where S denotes those events that go through
the WbLS vessel and produce scintillation and W denotes those events that do
not. The additional term of $\sqrt{Q}$ takes into account the impact on the
p.e. distribution smearing due to the Poissonian nature of the number of
photoelectrons. Here the fit parameters of interest are the mean p.e. values
of the two event populations: The number of upstream p.e.
$\mu_{\text{W}}^{\text{U}}=(880\pm 20)$ p.e. and the number of downstream p.e.
$\mu_{\text{W}}^{\text{D}}=(3056\pm 59)$ p.e. for those muon events that do
not cross SANDI and the corresponding values
$\mu_{\text{S}}^{\text{U}}=(1713\pm 34)$ p.e.,
$\mu_{\text{S}}^{\text{D}}=(3637\pm 51)$ p.e. for those muon events that do
cross SANDI and produce scintillation light. The estimated ratio of the muon
tracks that go through SANDI relative to all selected events is $0.44\pm
0.07$, corresponding to about 270 events. The best fit results are also
illustrated in Fig. 6 for the SANDI data. Here the black histogram shows the
data, the red line shows the best fit distribution, and the blue and green
lines show the charge distributions of the fit corresponding to the two event
populations for illustration purposes. The pre-SANDI data is fitted with a
single Gaussian, resulting in $\mu_{\text{pre-SANDI}}^{\text{U}}=(887\pm 9)$
p.e., $\mu_{\text{pre-SANDI}}^{\text{D}}=(3473\pm 25)$ p.e.

(a)

(b)

Figure 6: Illustration of the bi-Gaussian charge fit. The data with the WbLS
vessel is shown in black, the full fit is shown in red and the two event
populations of muons passing through the WbLS vessel or missing it are shown
in green and blue, respectively.

The muons that cross the WbLS vessel are not fully contained inside it and the
above fit values include a large fraction of Cherenkov light from pure water
as well as scintillation and Cherenkov light from the WbLS vessel.
Additionally, the WbLS can absorb Cherenkov light, without re-emitting
scintillation, and the SANDI structure can also have a shadowing effect. It is
possible to use the above p.e. mean values and the MRD reconstructed muon
tracks to correct for the muon tracklength through the WbLS vessel and
estimate the loss of Cherenkov light due to SANDI. For this purpose a simple
toy-MC simulation has been used to estimate the average true track length $L$
of the muons, given the reconstructed MRD tracks and their uncertainties. The
estimated value for the average track length through the entire ANNIE water
tank is $l_{\text{ANNIE}}=(2.951\pm 0.012)\,\text{m}$. A small selection
effect has been observed on this value for the two event populations, where
the muon tracks missing the WbLS vessel give a value of
$l_{\text{W}}=(2.896\pm 0.012)\,\text{m}$ and the events crossing it give
$l_{\text{S}}=(3.019\pm 0.011)\,\text{m}$. The reason for this effect is the
cylindrical shape of the water tank, where muon tracks that miss the WbLS
vessel tend to intersect the tank more often at off-center positions. The
average track length of the muons through the WbLS vessel is
$l_{\text{WbLS}}=(0.597\pm 0.007)\,\text{m}$. The toy-MC has also been used to
estimate the average track length of the Cherenkov photons through the WbLS
vessel, which are emitted along the muon tracks, as $l_{\text{che}}=(0.463\pm
0.010)\,\text{m}$. The average fraction of the Cherenkov photons that pass
through the WbLS vessel is estimated likewise as $r_{\text{che}}=0.123\pm
0.003$.

In the following we define $q_{\text{W}}$ [p.e. / m] as the estimated number
of detected Cherenkov photoelectrons per meter of muon track in pure water,
$q_{\text{S}}$ [p.e. / m] as the estimated number of Cherenkov and
scintillation photoelectrons from the WbLS, and $A_{\text{eff}}$ as the
effective absorption of the Cherenkov light by the SANDI vessel, its steel
holding structure and the WbLS itself. The following equations can be used to
estimate these parameters, assuming that the total number of detected
photoelectrons is simply proportional to the track length of the muon through
the medium, with $q_{\text{W}}$, $q_{\text{S}}$ as the relevant
proportionality constants:

$\begin{split}\mu_{\text{pre-SANDI}}^{\text{U}}+\mu_{\text{pre-
SANDI}}^{\text{D}}&=q_{\text{W}}\cdot l_{\text{ANNIE}}\\\
\mu_{\text{W}}^{\text{U}}+\mu_{\text{W}}^{\text{D}}&=q_{\text{W}}\cdot
l_{\text{W}}\cdot\left(1-A_{\text{eff}}\right)\\\
\mu_{\text{S}}^{\text{U}}+\mu_{\text{S}}^{\text{D}}&=q_{\text{W}}\cdot
l_{\text{S}}\cdot\left(1-A_{\text{eff}}\right)+q_{\text{S}}\cdot
l_{\text{WbLS}}\end{split}$ (5.2)

Solving the above equations results in $q_{\text{W}}=(1395\pm
10)\,\text{p.e./m}$ and $q_{\text{SANDI}}=(2159\pm 154)\,\text{p.e./m}$, where
the errors have been added in quadrature. Therefore, the ratio for the
estimated number of photoelectrons per meter of muon track length from the
WbLS compared to the number of photoelectron from pure water is
$\frac{q_{\text{S}}}{q_{\text{W}}}=1.42\pm 0.13$. Additionally, the equations
allow for an estimation of the total absorption of Cherenkov light. The
absorption parameter $A_{\text{eff}}$ is an effective parameter, which
includes all possible sources of the Cherenkov absorption due to the
deployment of SANDI, such as the acrylic SANDI vessel, its steel holding
structure and the WbLS attenuation of the Cherenkov light. This value is
estimated as $A_{\text{eff}}=0.08\pm 0.03$, meaning that roughly 8% of the
Cherenkov light is missing due to the WbLS-filled SANDI vessel, compared to
the normal configuration of the ANNIE water tank0303.

A number of potential sources of systematic uncertainty have been investigated
for the above calculations. First, the position of SANDI inside the ANNIE
water tank is known only with a certain precision. The SANDI position has been
estimated through the bi-Gaussian fit of Eq. 5.1 and the comparison of the
relative contribution of true scintillation events $A_{\text{S}}/A_{\text{W}}$
for different assumed positions of SANDI. The corresponding SANDI position
uncertainty has been estimated conservatively as $\pm 0.1\,\text{m}$. Re-
performing the calculations again for different assumed SANDI positions
results in a systematic uncertainty of
$\Delta\frac{q_{\text{S}}}{q_{\text{W}}}=\pm 0.16$. Second, the analytical fit
has also been re-performed with some variations, such as leaving out the
sliding standard deviation term $\sqrt{Q}$, without the constant $C$, and
within different fit ranges. The corresponding systematic uncertainty is
estimated as $\Delta\frac{q_{\text{S}}}{q_{\text{W}}}=\pm 0.07$. Adding these
uncertainties in quadrature results in a ratio of
$\frac{q_{\text{S}}}{q_{\text{W}}}=1.42\pm 0.13\text{\,(stat.)}\pm
0.18\text{\,(syst.)}=1.42\pm 0.23\text{\,(stat. + syst.)}$.

It has to be noted here that the above value is a simple measurement of the
expected number of photoelectrons with and without the WbLS-filled SANDI
vessel, given the selected throughgoing muons events. This is not a
measurement of the WbLS scintillation light-yield, whose in-situ measurement
necessitates a dedicated, full Monte Carlo based analysis, which takes into
account the full detector response and geometry, as well as all interactions
of the photons with the different detector media.

###  5.2 Neutrino candidate events

(a)

(b)

Figure 7: Scatter plots of the number of detected photoelectrons for the
downstream PMTs vs. the upstream PMTs for the selected events of ANNIE without
the WbLS vessel in black and for the events of ANNIE with the WbLS vessel in
red. (a) Throughgoing muon events, which required a coincident FMV trigger.
(b) Neutrino candidate events, which required the absence a coincident FMV
trigger.

The next part of this section shows WbLS scintillation light for neutrino
candidate events in ANNIE. These beam neutrino events have been selected in
the same way as the throughgoing muon events, with the difference that now the
FMV is required to have not seen a signal in coincidence with the tank PMTs
and the MRD. This selects muon events that have been produced through neutrino
interactions inside the ANNIE tank. Here again the MRD reconstructed muon
track is required to intersect the WbLS vessel.

Figure 7 shows scatter plots of the number of detected photoelectrons for the
downstream PMTs against the upstream PMTs, where the pre-SANDI data without
the WbLS vessel is shown in black while SANDI data with the WbLS vessel is
shown in red. Throughgoing muon events are shown on the left in Fig. 7(a). The
same features described in Fig. 4 are also visible here. There are again two
populations of events for the SANDI data, where the mis-reconstructed muon
track events without scintillation are hidden in the plot behind the black
points. The events with scintillation show stronger linear dependency between
the upstream and downstream photoelectrons compared to the Cherenkov light
from ANNIE without WbLS, as is expected from the isotropy of scintillation.

Figure 7(b) shows the neutrino candidate events, i.e. neutrino interactions
which produce muons inside the water tank, as selected by the FMV. Below a
downstream photoelectron number of around $1500$ p.e. there is no clear
difference between the pre-SANDI and SANDI data events. As the number of
downstream photoelectrons is proportional to the muon path length through the
water tank this likely means that these muons are produced in neutrino
interactions behind the WbLS vessel. For larger numbers of downstream
photoelectrons the SANDI data shows again two event populations, as described
previously. The events which produce scintillation light have a distribution
which is well distinguishable from the events of ANNIE without the WbLS. This
scatter plot therefore provides a qualitative proof that WbLS produces a
larger amount of light than pure water for accelerator neutrino events in an
operating large scale neutrino detector.

###  5.3 Michel electron analysis

Michel electrons, produced by decaying muons, follow a well known spectrum
that can be used as a standard candle for the comparison of the detected
number of photoelectrons in water and in the WbLS. The total energy spectrum
of Michel electrons in vacuum can be described as follows [27]:

$N(E)=3\left(\frac{E}{E_{\text{max}}}\right)^{2}-2\left(\frac{E}{E_{\text{max}}}\right)^{3},$
(5.3)

where E is the energy of electron and $E_{\text{max}}\approx 53\,$MeV is the
maximal possible electron energy in the decay.

The contribution of anti-muons from the decay of anti-muon neutrinos is
considered negligible for the following selection of Michel electrons. A
coincident event pair is required for the Michel electron selection: A prompt
event caused by a muon, followed by a second event caused by the Michel
electron. The prompt event is selected by several cuts to exclude most of the
background: First, the prompt event is required to be triggered within the 2
${\mu}$s acquisition window and it is required to be the brightest signal
among all the signals within a time window of the corresponding BNB trigger.
Second, a FMV hit is required to be in coincidence with the muon candidate
event to enforce that the muon source is located outside of the ANNIE
detector. Furthermore, a prompt event is required to have a bary center
located in the downstream part of the detector and the event time is required
to have no MRD signal within a 50 ns time window. This cut enforces the
selection of muons stopping in the tank. Finally, a photoelectron range of
[1000, 4000] p.e. is required to further exclude muon candidates that are
falsely tagged as stopping muons to compensate for the MRD inefficiency. In
the case that an event passes all the cuts mentioned above, all subsequent
clusters within a time window of ${\Delta}t$ $\in$ [1000, 6000] ${\mu}$s are
tagged as Michel electron candidate events. A charge balance cut of
$q_{\text{cb}}$ < 0.2 is applied to the Michel electron candidate events to
further reduce background caused by a large, single PMT signal. The charge
balance parameter $q_{\text{cb}}$ is defined using the following equation:

$q_{\text{cb}}=\sqrt{\frac{{\Sigma}Q^{2}_{i}}{Q_{\text{sum}}^{2}}-\frac{1}{N}},$
(5.4)

where $Q_{\text{sum}}$ represents the total number of observed photoelectrons
in the cluster, $Q_{i}$ references the number of photoelectrons that have been
observed by $\text{PMT}_{i}$, and $N$ is the number of PMTs involved in the
cluster. Charge balance values close to 1 indicate that most of the charge has
been seen by a single PMT, indicating that the cluster was probably a noise
event.

(a)

(b)

Figure 8: Histograms for the timing distribution between Michel electron
events and the preceding muon events, which follows the exponential decay rule
for (a) the data without the WbLS vessel and (b) the data with the WbLS
vessel.

A good comparison of the number of photoelectrons produced by Michel electrons
in pure water and in the SANDI vessel requires a pure sample with minimum
background. As a measure of sample purity, the time distribution of the
selected Michel electron events is shown in Fig. 8. Here the pre-SANDI data is
shown on the left and the SANDI data is shown on the right. The timing
spectrum is fitted by a function of an exponential decay plus noise that is
assumed to be a constant:

$A=A_{0}e^{\frac{-t}{\tau}}+C,$ (5.5)

where $A_{0}$ denotes the initial amplitude at $t=0$, $\tau$ is the decay
constant and $C$ denotes the noise. The fitted decay constants are
$\tau=(1.78\pm 0.10)\,{\mu}$s for the SANDI data and $\tau=(1.85\pm
0.09)\,{\mu}$s for the pre-SANDI data. The expected muon lifetime is
calculated using the following equation:

$\displaystyle\frac{1}{\tau_{\text{tot}}}=R_{\text{tot}}$
$\displaystyle=R_{\text{vac}}+R_{\text{oxy}},$ (5.6)

where ${R_{\text{tot}}}$ denotes the total muon decay rate, ${R_{\text{vac}}}$
and ${R_{\text{oxy}}}$ denote the muon decay rate in vacuum and the capture
rate in pure oxygen respectively. $\tau_{\text{tot}}$ is the overall lifetime
of the muon. From the experimental results of [28], the expected muon lifetime
$\tau_{\text{tot}}$ is $(1.788\pm 0.002){\mu}$s, which is well in agreement
with the results of both event selections. Here the effect of muon capture on
hydrogen is ignored.

(a)

(b)

Figure 9: Histograms of the Michel electron charge distribution collected by
PMTs inside the ANNIE tank. (a) The data of ANNIE without the WbLS vessel in
red and for ANNIE with the WbLS in black. The ANNIE without WbLS data is
normalized to the statistics of ANNIE with the WbLS vessel for better
comparison. (b) The fit of the Michel electron data for ANNIE with the WbLS
vessel. This is a combination of events inside and outside of the WbLS vessel,
estimated by the fit and shown in blue and green respectively.

Figure 9(a) shows the charge distribution of the selected Michel electron
events for the pre-SANDI data in red and for the SANDI data in black. It can
be seen, that the selected Michel electrons fall into two categories: Those
electron events for which the parent muon decayed within the SANDI vessel and
those electron samples for which the parent muon decayed outside of the SANDI
vessel. For those Michel electrons that have been generated outside of SANDI
the source of the detected photoelectrons is from Cherenkov radiation, while
for the ones generated inside SANDI, the source is a combination of both
Cherenkov radiation and WbLS scintillation. The latter event category can be
seen in comparison with the pre-SANDI Michel electron charge distribution
above $\sim 600\,$p.e.

The comparison of the mean values of these two event populations of the SANDI
Michel electrons can provide an estimation of the additional number of
photoelectrons provided by the deployment of the WbLS-filled vessel. A
Gaussian smeared Michel spectrum is used to describe the total number of
photoelectrons collected by the PMTs inside ANNIE tank $Q$. The spectrum is
described by the following equation:

$g(Q)=\int_{0}^{E_{m}}A\cdot f(E)\cdot G\left(Q,\mu(E),\sigma\right)dE,$ (5.7)

where $f(E){\propto}N(E)$ is defined in equation (5.3) as the Michel electron
energy spectrum, $G(Q,\mu(E),\sigma)$ is the Gaussian distribution,
$\mu(E)=kE$ is the mean number of photoelectrons as a function of energy and
is naively assumed to be proportional to the energy deposited inside the ANNIE
tank. $\sigma$ is the energy resolution of the detector and is a constant
here.

Equation (5.7) is then used to fit the photoelectron distribution in Fig.
9(b). Here the charge distribution of the SANDI data is a combination of the
two event populations described above, each with a unique mean value for the
number of detected photoelectrons. The parameter of interest is the
proportionality factor $k$ of the photoelectron mean value for the two
distributions: $k_{\text{SANDI}}=(14.44\pm 0.87)$ p.e./MeV and
$k_{\text{Water}}=(8.17\pm 0.14)$ p.e./MeV. The correlation coefficient of
these parameters given by the fit is $0.0073$. The ratio of
$k_{\text{SANDI}}/k_{\text{Water}}=1.77\pm 0.06$ can be used to estimate the
relative increase of the detected number of photoelectrons in ANNIE due to the
WbLS-filled SANDI vessel, compared to pure water. Like with the previous
analysis of throughgoing muons the systematic uncertainty here has been
estimated by varying the analytical fit function; with and without a sliding
standard deviation $\sigma=\sigma_{0}\sqrt{E}$. The corresponding uncertainty
is $\Delta\frac{k_{\text{SANDI}}}{k_{\text{Water}}}=\pm 0.05$, resulting in
$\frac{k_{\text{SANDI}}}{k_{\text{Water}}}=1.77\pm 0.06\text{\,(stat.)}\pm
0.05\text{\,(syst.)}=1.77\pm 0.08\text{\,(stat.+syst.)}$.

##  6 Conclusions

WbLS is a novel target material which allows for the simultaneous detection of
Cherenkov radiation and scintillation. Cherenkov radiation provides
sensitivity to the particle direction and track reconstruction, as well as
allowing for particle identification through the Cherenkov ring topology. The
addition of scintillation improves the energy resolution and allows for the
detection of particles below the Cherenkov threshold. The work described in
the present paper is part of the ongoing technical demonstration and
experimental validation of the hybrid detection concept, which is important
for future, large scale, advanced optical neutrino detectors such as Theia
[21].

Here, we present the first implementation of WbLS in a running neutrino beam,
using a system dubbed SANDI (Scintillator for ANNIE Neutrino Detection
Improvement). SANDI, a cylindrical vessel with a diameter of $72$ cm and $90$
cm height, was successfully filled with WbLS and deployed in the ANNIE water
tank for about two months. Samples of the WbLS have been measured at
Brookhaven National Laboratory before and after the SANDI deployment. No
deterioration of optical transparency or light-yield have been found, which
indicates a long term stability of the WbLS under real deployment and beam
measurement conditions.

The WbLS scintillation signal can be quantified by the comparison of the ANNIE
data with the WbLS vessel and of ANNIE data without the SANDI vessel. The
scintillation has been observed for throughgoing muons, Michel electrons from
muon decays inside ANNIE, as well as beam neutrino interactions inside ANNIE.
Two independent estimations of the increase of the detected number of
photoelectrons due to the deployment of the WbLS vessel have been performed,
one with throughgoing muons and one with Michel electron events. These
analyses have been performed through analytical fits on the distributions of
the detected number of photoelectrons. The analyses results are expressed as a
ratio of the number of detected photoelectrons for events that deposit energy
within the WbLS-filled SANDI vessel and the detected number of photoelectrons
for events outside of the SANDI vessel. These ratios are
$q_{\text{SANDI}}/q_{\text{Water}}=1.42\pm 0.23\text{\,(stat.+syst.)}$ for the
throughgoing muons and $k_{\text{SANDI}}/k_{\text{Water}}=1.77\pm
0.08\text{\,(stat.+syst.)}$ for the Michel electrons.

The observed increase in the detected number of photoelectrons is clearly
caused by the additional scintillation of the WbLS and the order of magnitude
of this detected increase is in agreement with the expectations given by
bench-top experiments [26]. Nonetheless, it has to be noted that above results
cannot be directly interpreted as a measurement of the intrinsic scintillation
light-yield of the WbLS. They rather express the increase in the number of
detected photoelectrons with and without the deployment of the WbLS-filled
SANDI vessel. An in-situ measurement of the intrinsic WbLS light-yield
requires a detailed Monte Carlo model of the full ANNIE detector response that
includes the effects of the SANDI vessel structure and the absorption, re-
emission and scattering of light in the WbLS, as well as the full ANNIE
detector response. The work of producing such a Monte Carlo model is currently
underway.

The results presented here will form the basis for the further development of
hybrid event detection in the GeV energy range in ANNIE. Future reconstruction
and particle identification algorithms will make use of both the Cherenkov
light and the WbLS scintillation. These algorithms are expected to depend on
critical parameters of the WbLS, such as its attenuation length and
scintillation light-yield. Using the upgraded reconstruction techniques, the
planned follow-up analyses on the collected WbLS data regard the detection of
neutron captures on hydrogen, the search for neutral current events, as well
as the investigation of the hadronic scintillation component, all within the
WbLS vessel. Further plans include the re-deployment of SANDI with gadolinium-
loaded WbLS to study the enhanced neutron detection and potentially filling
the entire ANNIE tank with WbLS to permit a better assessment of the
performance of hybrid event reconstruction in a future large-scale experiment.

## Acknowledgments

This work was supported by: (1) the U.S. Department of Energy (DOE) Office of
Science under award number DE-SC0009999 (UC Davis) and award number DE-
SC0015684 (Iowa State University), and by the DOE National Nuclear Security
Administration through the Nuclear Science and Security Consortium under award
number DE-NA0003180 (UC Davis), (2) Deutsche Forschungsgemeinschaft grants
456139317 (Hamburg) and 490717455 (Mainz and Tübingen), (3) National Science
Foundation Grant No. PHY-2310018 (Ohio State), PHY-2047665 (Rutgers), and
OIA-2132223 (South Dakota Mines), (4) UK Research and Innovation FLF
MR/S032843/1 (Warwick), (5) Scientific Research Projects (BAP) of Erciyes
University under the grant numbers of of FBAU-2023-12325, FDS-2021-11525 and
FBA-2022-12207, Work conducted at Lawrence Berkeley National Laboratory was
performed under the auspices of the U.S. Department of Energy under Contract
DE-AC02-05CH11231. Work conducted at Lawrence Livermore National Laboratory
was performed under the auspices of the U.S. Department of Energy by Lawrence
Livermore National Laboratory under contract DE-AC52-07NA27344, release number
LLNL-JRNL-858196. The work conducted at Brookhaven National Laboratory was
supported by the U.S. Department of Energy under contract DE-AC02-98CH10886.
The project was funded by the U.S. Department of Energy, National Nuclear
Security Administration, Office of Defense Nuclear Nonproliferation Research
and Development (DNN R&D). Finally, we gratefully acknowledge all the Fermilab
scientists and staff who supported this work through their technical expertise
and operational assistance at the Booster Neutrino Beam.

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