Nuclear Inst. and Methods in Physics Research, A 959 (2020) 163549

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Nuclear Inst. and Methods in Physics Research, A
journal homepage: www.elsevier.com/locate/nima

Evaluation of gadolinium’s action on water Cherenkov detector systems with
EGADS
Ll. Marti a ,∗, M. Ikeda a , Y. Kato a , Y. Kishimoto a,b , M. Nakahata a,b , Y. Nakajima a,b , Y. Nakano a ,S. Nakayama a,b , Y. Okajima a , A. Orii a , G. Pronost a , H. Sekiya a,b , M. Shiozawa a,b , H. Tanaka a ,K. Ueno a , S. Yamada a , T. Yano a , T. Yokozawa a , M. Murdoch b , J. Schuemann b , M.R. Vagins b,c ,K. Bays c , G. Carminati c , N.J. Griskevich c , W.R. Kropp c , S. Locke c , A. Renshaw c , M.B. Smy c,b ,P. Weatherly c , S. Ito d , H. Ishino d , A. Kibayashi d , Y. Koshio d,b , T. Mori d , M. Sakuda d ,R. Yamaguchi d , P. Fernandez e , L. Labarga e , I. Bandac f , J. Perez f , J. Amey g , R.P. Litchfield g ,A. Sztuc g , Y. Uchida g , W.Y. Ma g , A. Goldsack h,b , C. Simpson h,b , D. Wark h,i,j , L.H.V. Anthony k ,N. McCauley k , A. Pritchard k , F. Di Lodovico l , B. Richards l , A. Cole m , M. Thiesse m ,L. Thompson m , J. Imber n , S.V. Cao o , K. Ito p , Y. Takeuchi q,b , R. Akutsu r , Y. Nishimura r ,K. Okumura r , S. Hirota s , F. Muto t , M. Yokoyama u,b , Y. Suda u , H. Zhang vaKamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo, Kamioka, Gifu 506-1205, Japan
Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, Japanc
Department of Physics and Astronomy, University of California, Irvine, CA 92697-4575, USAd
Department of Physics, Okayama University, Okayama 700-8530, Japane
Department of Theoretical Physics, University Autonoma Madrid, 28049 Madrid, Spainf Laboratorio Subterraneo de Canfranc, 22880 Canfranc Estacion, Huesca, Spaing Department of Physics, Imperial College London, London, SW7 2AZ, United Kingdom
h Department of Physics, Oxford University, Oxford, OX1 3PU, United KingdomiSTFC, Rutherford Appleton Laboratory, Harwell Oxford, United KingdomjDaresbury Laboratory, Warrington, OX11 0QX, United KingdomkDepartment of Physics, University of Liverpool, Liverpool, L69 7ZE, United KingdomlDepartment of Physics, King’s College London, London, WC2R 2LS, United Kingdomm
Department of Physics and Astronomy, University of Sheffield, S3 7RH, Sheffield, United Kingdom
n Ecole Polytechnique, IN2P3-CNRS, Laboratoire Leprince-Ringuet, F-91120 Palaiseau, Franceo High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japanp Department of Physics, Tokai University, Hiratsuka, Kanagawa 259-1292, Japanq
Department of Physics, Kobe University, Kobe, Hyogo 657-8501, Japanr
Research Center for Cosmic Neutrinos, Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, Chiba 277-8582, Japans
Department of Physics, Kyoto University, Kyoto 606-8502, Japant
Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, Aichi 464-8602, Japanu
Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, Japanv Department of Engineering Physics, Tsinghua University, Beijing, 100084, Chinab

ABSTRACT
Used for both proton decay searches and neutrino physics, large water Cherenkov (WC) detectors have been very successful tools in particle physics. They arenotable for their large masses and charged particle detection capabilities. While current WC detectors reconstruct charged particle tracks over a wide energy
range, they cannot efficiently detect neutrons. Gadolinium (Gd) has the largest thermal neutron capture cross section of all stable nuclei and produces an 8 MeV
gamma cascade that can be detected with high efficiency. Because of the many new physics opportunities that neutron tagging with a Gd salt dissolved in water
would open up, a large-scale R&D program called EGADS was established to demonstrate this technique’s feasibility. EGADS features all the components of a
WC detector, chiefly a 200-ton stainless steel water tank furnished with 240 photo-detectors, DAQ, and a water system that removes all impurities from water
while keeping Gd in solution. In this paper we discuss the milestones towards demonstrating the feasibility of this novel technique, and the features of EGADS
in detail.

∗ Corresponding author.

E-mail address: martillu@suketto.icrr.u-tokyo.ac.jp (Ll. Marti).
https://doi.org/10.1016/j.nima.2020.163549
Received 6 August 2019; Received in revised form 27 January 2020; Accepted 27 January 2020
Available online 3 February 20200168-9002/© 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Nuclear Inst. and Methods in Physics Research, A 959 (2020) 163549

1. Introduction
Water Cherenkov (WC) detectors typically contain a large numberof protons, both to study their possible decay as well as to present
a large target for neutrino interactions. These detectors are now awell-established tool for conducting particle physics research. However,neutrons cannot be efficiently detected [1] in WC detectors. Free neutrons in these detectors are first thermalized and then mostly captured
on protons within about 200 μs (neutron capture cross section on free
protons is 0.3 barns while on oxygen it is 0.19 millibarns). The capture
on a proton produces a single 2.2 MeV gamma that is very difficult
to detect because the Compton scattered electron is relatively close to
Cherenkov threshold, and so produces too few photons given the typicalphotocathode coverage in WC detectors. In addition, at these low
energies there are many background processes present, in particular
those produced by radon and spallation.
In 2003, GADZOOKS! was proposed, the idea of enriching WC detectors with a water soluble gadolinium (Gd) salt [2]. Naturally occurring
Gd has the largest cross section for the capture of thermal neutronsof all the naturally occurring elements (∼49,000 barns). The largest
contributions come from the two isotopes 157 Gd and 155 Gd, with about
255,000 and 61,000 barns respectively, and natural abundances of
15.65% and 14.80%. After neutron capture on 157 Gd and 155 Gd, gamma
cascades follow with total energies 7.9 MeV and 8.5 MeV, respectively.
Hereafter, we will collectively refer to these gamma cascades as 8 MeV
gamma cascades.
Gd is insoluble in water but there are Gd compounds that could
be used. Gd nitrate, Gd(NO3 )3 , has been used as a neutron poison in
nuclear reactors but nitrates are mostly opaque in the UVA region [3]which covers a large portion of the effective spectrum. Gd chloride,
GdCl3 , is easily soluble and has good Cherenkov light transparency.
Gd sulfate, Gd2 (SO4 )3 , has a similar solubility and transparency, andin addition it is less reactive than GdCl3 and thus more suitable to be
used in a detector. Therefore, we chose Gd sulfate. Gd sulfate is easier
to dissolve when octahydrated: Gd2 (SO4 )3 ⋅ 8H2 O (8 molecules of water
per Gd atom). Hereafter, we will refer to it as just Gd sulfate and omitthat it is octahydrated.
To achieve 90% of the neutron captures on Gd after dilution,
we need to achieve a concentration of about 0.2% of Gd sulfate bymass, i.e. about 0.1% of dissolved Gd; see Fig. 1. This means wewill need to dissolve about 100 tons of Gd sulfate into the 50 kton
Super-Kamiokande (Super-K, SK) to achieve this goal. With this Gd
concentration, neutrons thermalize and are then captured within about
30 μs (see Fig. 10 in Section 5).

Fig. 1. Fraction of neutrons captured on Gd as a function of its concentration in waterby mass.

and antineutrinos of all species, represent only about 3% of all theevents [4], but are very useful because they point back to the SN. Since
we cannot distinguish electrons from positrons (positron annihilation isentirely invisible in WC detectors), directional elastic scattering eventsare diluted by non-directional IBD events which limits the pointing
accuracy. By efficiently identifying IBD events by the observation ofneutron captures on Gd and removing them from the elastic scattering
sample, the pointing accuracy doubles [6,7]. As a consequence, the areaof the sky in which astronomers would expect to eventually observe the
SN would be reduced by a factor of four.
Among other benefits, a Gd-loaded Super-K would have enhanced
sensitivity to late black hole formation [8] and extend the neutrino
observation of the cooling phase to later times. Also, efficient neutron
detection opens up the possibility to see stellar neutrinos from silicon
fusion in nearby massive stars [9,10] (distance less than 1 kpc and M> 13 M⊙). This is the last phase in the lifetime of a massive star andlasts from a few hours to a few days before the stellar core collapse, so
it serves as a pre-supernova warning.
We expect about two or three SNe per century in our galaxy [11].
The neutrino flux of a single SN far from our galaxy is not large enoughto be detected. However, there have been many SNe in the historyof the universe, creating a copious, ubiquitous and isotropic neutrino
flux: the diffuse SN neutrino background (DSNB) [2]. The predicted
spectra are shown in Fig. 2. In this case, we would not be able to linka given event in our detector with a specific SN. However, if we could
collect enough events we would acquire information about the neutrino
spectrum of an average SN, the history of stellar formation and collapse,
the percentage of optically failed SNe, the universe’s expansion rate,
and would establish the most stringent constrains to neutrino decay.
The DSNB has not yet been observed, though the current best
limits have been set by Super-K [13]. Reactor and atmospheric antineutrinos limit the search below 8 MeV and above 30 MeV, respectively, which defines the search window. However, this analysis islimited by currently irreducible backgrounds in this window. Thesebackgrounds would be greatly reduced by requiring the distinctive coincident prompt/delayed signals arising from efficient neutron tagging
capabilities. In addition, we would be able to lower the current energy
threshold in the analysis. Reactor antineutrinos with energies up to
about 8 MeV would impose an upper limit to observing DSNB fromSNe with redshifts of about 𝑧 = 1. After adding gadolinium sulfate to
Super-K, we expect to record up to six DSNB events per year [2].
Atmospheric neutrinos
Efficient neutron tagging adds more information about the neutrino,
the interaction type and the neutrino energy. To give an idea of the

1.1. Physics
Neutron tagging in WC detectors opens up many new possibilities
because it is a powerful tool to reduce backgrounds. Here we will
discuss some of the physics topics most likely to benefit from efficientneutron tagging in WC detectors: galactic supernova neutrinos, diffuse
supernova neutrino background, atmospheric neutrinos, and proton
decay analyses. In addition, though not considered here, Gd loadingis also expected to enhance both long-baseline and reactor neutrino
studies.
Galactic supernova neutrinos and diffuse supernova neutrino
backgroundA core-collapse supernova (SN) releases about 1046 J. Out of thisvast amount of energy, ∼99% is released in neutrino production. Since
neutrinos interact with matter only weakly, they leave the explodingstar and travel through space without significant attenuation. Neutrino
detectors like Super-K can easily detect galactic supernova explosions(SNe) through inverse beta decay (IBD) events 𝜈 𝑒 + 𝑝 → 𝑛 + 𝑒+ . Becauseof the large cross section and the relatively large positron energies
involved, these events represent about 88% of the total events [4,5].
Elastic scattering events, 𝜈𝑥 + 𝑒− → 𝜈𝑥 + 𝑒− , where 𝜈𝑥 are neutrinos
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Nuclear Inst. and Methods in Physics Research, A 959 (2020) 163549

1.2. EGADS demonstrator
GADZOOKS! (Gadolinium Antineutrino Detector Zealously
Outperforming Old Kamiokande, Super!) was first proposed in late2002, and soon after investigations into its application at Super-K beganat the University of California, Irvine (UCI), and in particular on how
to purify Gd-loaded water. UCI also developed a device to precisely
monitor the water transparency resulting from that water purification
system.
An important milestone was reached when in 2009 the
EGADS (Evaluating Gadolinium’s Action on Detector Systems) project
was funded to test the results from the UCI R&D on a larger scale.
Located in a newly excavated cavern near Super-K (about 1000 m
underground in the Kamioka mine, Hida-city, Gifu, Japan; see Fig. 4),
EGADS initially featured: a 200-ton stainless steel tank, a pre-treatment
system (Gd sulfate dissolving system and its pre-treatment), a water
purification system (see Section 2) and a transparency measurement
device called UDEAL (Underground Device Evaluating Attenuation
Length, see Section 3). Like Super-K, EGADS has a cylindrical shape(see Fig. 5) but has an inner detector only. Calibration ports are alsoavailable at the top. To measure Gd sulfate concentrations, a Hitachi
atomic absorption spectrometer (AAS) of the ZA3000 series was used.
Fig. 2. Energy spectrum of the diffuse supernova neutrino background for several
effective neutrino temperatures [12], T𝑒𝑓 𝑓 . The spectra of reactor and atmosphericantineutrinos are also shown. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

The EGADS project started with five goals:• Demonstrate that the purification system can achieve and maintain good water quality while keeping the Gd concentration constant.
• Show that Gd sulfate has no adverse effects on the Super-K
detector components.
• Demonstrate that the addition of Gd sulfate will not negatively
affect existing Super-K analyses.
• Study how to reduce the now visible neutron background fromspallation, U/Th fission chains in Gd sulfate impurities, ambient
neutrons, etc.
• Prove that Gd can be added/removed in an efficient and economical way.

potential information gain from efficient neutron tagging, a Monte
Carlo (MC) study was done. The MC simulates 500 years of atmospheric neutrinos at Super-K where the flux was taken from [14] andneutrino primary interactions were calculated with NEUT [15]. The
gamma cascade spectrum from neutron captures on Gd was calculatedusing GEANT4 [16] while secondary interactions were simulated with
GEANT3 [17]. Three examples of these simulations are shown in Fig. 3:- Neutrino–antineutrino separation: although the neutron multiplicity increases with energy due to interactions inside the nucleus, theneutron multiplicity is larger for antineutrinos than for neutrinos. See
Fig. 3a.
- Neutral and charged current separation: neutral current (NC) interactions are non-flavor changing interactions and thus carry no flavor
information while charged current (CC) interactions do. In oscillation
analyses, NC is an important contamination source of e-like events. NC
events tend to deposit a larger amount of energy in the target nucleus
than CC interactions. As a consequence, they produce on average larger
numbers of neutrons as shown in Fig. 3b.
- Neutron-corrected neutrino energy: accurate neutrino energy reconstruction is very important in the atmospheric oscillation analysis.
Fig. 3c shows the fraction of non-visible energy as a function of thenumber of tagged neutrons on simulated atmospheric neutrino interactions at Super-K, where E𝜈 is the neutrino energy and E𝑣𝑖𝑠 is thereconstructed energy from charged particles. These neutrinos often
interact with the nuclear media, enhancing neutron production; thecurrent analysis cannot take into account the energy consumed in thisprocess. With efficient neutron tagging this feature can be used toreconstruct the neutrino energy, E𝐺𝑑
𝑟𝑒𝑐 , from the visible energy.
Proton decay
Large WC detectors were first built in the early 1980’s with proton
decay searches inspired by SU(5)-based predictions [18,19] as theirprimary motivation. The main background for proton decay analyses
comes from atmospheric neutrino interactions. As shown in Figs. 3a and3b, atmospheric neutrinos often produce at least one neutron. On theother hand, and taking the canonical mode 𝑝 → 𝑒+ 𝜋 0 as a reference, few
neutrons are expected to be produced in a proton decay. A significant
sensitivity gain for the proton lifetime limit determination when using
neutron tagging is therefore expected.

In this paper, we will show EGADS has achieved these goals.

2. EGADS water purification systems
The Super-K water purification system produces ultrapure waterwith a resistivity very close to 18.2 MΩ-cm, the theoretical maximum.
This very high purity is achieved by using several stages including
𝜇-filters, ultrafilters (UF), UV lamps, reverse osmosis (RO), vacuum
and membrane degasifiers, and resins that remove impurities, dissolved
ions, and kill and remove bacteria [20–22]. This means that if wewere to simply add Gd sulfate to Super-K as it now stands, the current
purification system would naturally remove the Gd along with all theother impurities.
In order to maintain good water quality it is necessary to have awater purification system that removes all impurities, ions included,
except Gd3+ and its anionic partner (Cl− , SO4 2− , etc.), and thus maintain
good water transparency. In EGADS, as in Super-K, water from the topof the detector is circulated through the water purification systems,
cleaned, and then injected again into the detector from the bottom(see Fig. 5a). In EGADS there are three different Gd-tolerant water
purification systems: the Gd sulfate mixing and pre-treatment, the fastrecirculation system, and the band-pass system. These systems usually
undergo an annual maintenance to ensure optimal efficiency removing
impurities. They are described in the next three subsections.
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Nuclear Inst. and Methods in Physics Research, A 959 (2020) 163549

Fig. 3. Monte Carlo study of (a) neutron multiplicity production for neutrinos and antineutrinos; (b) NC, CC DIS (Deep Inelesatic Scattering), and CC non-DIS interactions; (c)energy correction to the visible energy as a function of the neutron multiplicity. (For interpretation of the references to color in this figure legend, the reader is referred to the
web version of this article.)

Fig. 4. Located near Super-K, EGADS initially featured: a 200-ton tank, a pre-treatment system (Gd sulfate dissolving system and its pre-treatment), UDEAL (a transparency
measurement device) and the band-pass water purification system.

Fig. 5. Left (a) schematic view of the EGADS tank, its sampling ports (bottom, center and top at 4990, 3320, and 1660 mm from the tank-top) and inflow and outflow from andto the water purification systems. Right (b) top view of the EGADS tank with its calibration and water sampling ports.

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Nuclear Inst. and Methods in Physics Research, A 959 (2020) 163549

flow inside the EGADS detector is from the bottom (purified water) to
the top; it takes about two days to turn over the entire water volume.
We also found that when using new membranes, it is crucial tothoroughly flush them in advance. For this task we built a dedicatedflushing system with a large buffer tank and DI resin unit. The goal is
to completely remove the preservative liquids that are used when thesemembranes are packaged at their factories, thereby achieving properconditioning of the membranes before usage in our water filtration
systems.

Fig. 6. Pre-treatment schematic view: a 15-ton tank with a stirrer to dissolve the Gd
sulfate, 3 and 0.2 μm filters, TOC and UV lamps, the AE resin, a heat exchanger, andan ultrafilter.

2.3. Fast recirculation system
The fast recirculation system (see Fig. 7) provides increased cleaning
power, albeit not as powerful as the band-pass system since it has
no element to remove cations. It consists of most of those elementsthat cannot remove Gd: TOC, UV, AE resin, and UF. In addition, it
is equipped with a heat exchanger to remove the heat from theseelements. Note that lower temperatures allow easier control of bacterial
growth as well as a higher solubility of Gd sulfate.

2.1. Pre-treatment system
The pre-treatment system consists of a 15-ton polyethylene mixing
tank (including a stirrer to dissolve the Gd sulfate), TOC (Total Organic Compounds) and UV lamps to kill and remove bacteria together
with 𝜇-filters, a special strongly basic anion exchange (AE) resin, aheat exchanger, and an UF. This resin was designed and shown toremove uranium with an efficiency above 99% and with no Gd loss.
A schematic view is shown in Fig. 6.
After filling the 15-ton mixing tank with pure water derived fromthe XMASS experiment’s water purification system [23], Gd sulfate isadded. The built-in stirrer then runs until dissolving is complete. Dueto the limited size of the mixing tank, concentrations higher than 0.2%
are often prepared. Since the band-pass system is optimized to run atconcentrations up to 0.2%, concentrated Gd-loaded water coming fromthe pre-treatment system has to be diluted – typically with water fromthe 200-ton main tank – back down to this concentration. Then, after
going through the band-pass system, Gd-loaded water is injected into
the 200-ton tank from the bottom.

3. Monitoring water transparency
Water quality is a key parameter of any WC detector. It ensures
good water transparency and, as a consequence, that the Cherenkov
light attenuation length is long compared to the detector size. In sucha case, even charged particles at low energies creating few photons
can be efficiently detected. In addition, while usually present at levels
much too small to directly affect water transparency, trace amounts ofradioactive impurities in the water can increase backgrounds via decaysthat mimic true physics signals. Removing such impurities is vital since
they can easily make events within the detector’s fiducial volume.
The more abundant kinds of impurities (dissolved iron, amines, bacteria, etc.) typically reduce the water transparency by increasing lightabsorption and/or scattering, and this has to be monitored regularly.
Thus, a dedicated device was developed to measure water transparency
at UCI. UDEAL (Underground Device Evaluating Attenuation Length)
was built based on this device.
UDEAL consists of three basic components as shown in Fig. 8a: alight injector for seven laser beams and intensity monitor (integrating
sphere, top right), a pipe (8.6 m tall, middle) and an integrating sphereto monitor the transmitted light (below the pipe). The beam injector
consists of seven lasers (3 laser pointers, 3 professional laser diodes
and a nitrogen laser) spanning a range of wavelengths matched to
the Cherenkov spectrum and phototube response function: 337, 375,
405 445, 473, 532, and 595 nm. UDEAL operates only one of theselaser beams at a time, and automatically cycles through all seven ina pre-determined sequence. To reduce beam jitter effects on the lightintensity measurements, integrating spheres randomize the positionand direction of the detected laser light. These custom-build integrating
spheres (the one at the top is a 10-cm diameter sphere while the one
at the bottom is 30 cm) are coated on the inside with a special diffuse
high reflectivity paint. They are read out by 10 × 10 mm UV-enhanced
silicon photodiodes inlaid in the sphere walls. A baffle shields thephotodiodes from seeing the beam spot on the inside of the spherewalls, which ensures that there are at least two diffuse reflections
before a photon can contribute to a signal.
The beam (top left) hits a beam splitter where part of it is directedto the pipe (downwards) and part of it is directed to an integrating
sphere (top right). This first integrating sphere monitors the beam
intensity fluctuations. The pipe is automatically filled with water todifferent heights. For each height, measurements of the beam intensities at the bottom integrating sphere are performed. Hence, there is ameasurement of the intensity difference for each height and wavelength(see Fig. 8b). These measurements are conducted both while fillingand draining the pipe to reduce the impact of several time dependent
systematic effects. While the measurements are taken, the water height

2.2. Band-pass filtration system
The band-pass filtration system (see Fig. 7) is EGADS’ main filtration
system and under usual conditions it runs in parallel with the fastrecirculation system (see Section 2.3). The total flow through both
systems is about 90 l/min, approximately evenly split between the two.
The band-pass system takes water from the top of the 200-ton
EGADS detector. The first important elements are a UV unit and 𝜇filters to kill and trap bacteria before they can enter the system. The
second element is a chiller: Gd sulfate dissolves more readily at lower
temperatures since this process is exothermic. Later elements (pumps,
filters) will increase the water temperature. Thus, to avoid precipitationin key elements, a chiller is needed. The third element is an UF tofurther remove relatively large impurities.
The fourth element is a series of nanofilters (NF). With the proper
pass/reject flow settings these can be used to split the flow between
Gd-less water (well below 1 ppm) and concentrated Gd water from the
NF reject. This last line directly goes into a 0.5 ton collection buffer
tank. The other line contains basically no Gd and hence we proceed toremove all remaining impurities. If any Gd is present in this line, it will
be removed from the system along with all the other impurities throughdeionization (DI) and RO. This produces ultrapure water; about 15
liters/minute undergo this treatment. At the end of it, this line joins theconcentrated Gd water from the NF reject line in the 0.5 ton collection
buffer tank.
From the 0.5 ton collection buffer tank, in addition to similarelements described above, there is a membrane degasifier (to remove
dissolved air), a TOC lamp and finally the AE resin before water is
injected back into the bottom of the EGADS detector. The AE resin was
initially used to remove uranium (see Section 2.1). In addition to this,it was discovered that this resin also improves the water transparency
LL15 value by about 3% (the LL15 metric is defined in Section 3). The
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Fig. 7. Schematic view of the band-pass system and the fast recirculation system (inside dashed line). These systems were built in cooperation with the South Coast Water company.

Fig. 8. On the left (a) schematic view the water transparency monitor device, UDEAL: beam injector and splitter with an integrating sphere to monitor the beam intensity (top),a pipe (water height is adjustable) and integrating sphere to measure the beam intensity at the bottom. Right (b), example of water transparency measurement: light intensity forthe seven beam wavelengths as a function of water height. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of thisarticle.)is kept constant. With these measurements we can then calculate theabsolute light attenuation length in water for each wavelength.
To monitor the water quality, we compared the changes in watertransparency for every wavelength for pure water and water with different Gd sulfate concentrations, as well as with different filtration systems
and setups. To make these comparisons easier we often combine theseven wavelength measurements into a single number. In order todefine this number, we used the Geant3 based Super-K MC to simulate
electrons uniformly distributed throughout the detector volume withrandom directions. The Cherenkov photons were then tracked and thetravel distances recorded as well as the wavelengths of the recorded
photons by every photo-multiplier (PMT). The average photon travel
distance in Super-K was determined to be 15.2 m. Therefore, thepercentage of Cherenkov light left after a travel distance of 15 m (LL15)– what would remain available to activate SK’s phototubes after passing
through the fluid in question – was used as a standard comparison fordifferent Gd sulfate loading concentrations, different filtration setups or
any other change in experimental conditions. For a given wavelength,
the light left at 15 m can be calculated from the attenuation length anda simple exponential.

To determine the Cherenkov LL15, the wavelengths of the recorded
photons by every PMT from the MC were used. The relevant wavelengths are between 300 and 600 nm, the same wavelengths coveredby the UDEAL lasers. With this information, we can determine theaverage fraction of Cherenkov light which is represented by each of the
wavelengths used in UDEAL. The fractions were determined to be: 0.25,
0.25, 0.21, 0.14, 0.11, 0.04, and 0.003 for the UDEAL wavelengths,
337, 375, 405, 445, 473, 532, and 595 nm, respectively. A weighted
sum of all the LL15 for each wavelength is then calculated. This is
the Cherenkov light left after traveling 15 m with PMT efficiencies in∑cluded: Cherenkov LL15 = 𝜆𝑖 𝑊𝑖 e−15∕𝐿𝑖 , where the sum runs over theseven wavelengths (𝜆𝑖 ) and 𝑊𝑖 and 𝐿𝑖 are the fractions of Cherenkov
light and attenuation lengths for a given wavelength, 𝜆𝑖 .
UDEAL was designed to take data automatically every day. Since
the water purification system supplies clean water at the bottom of
the tank and draws from the top, a water transparency gradient is
expected: cleaner at the bottom and more impurities as we move to thetop. Hence, these measurements are done by taking samples from threedifferent ports inside the tank situated at the top, center, and bottom of
the tank, 4990, 3320, and 1660 mm from the tank top respectively, see
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Nuclear Inst. and Methods in Physics Research, A 959 (2020) 163549

Fig. 9. EGADS PMT types and covers map. This is the unrolled, inner view of the cylindrical detector. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

Fig. 5a. One LL15 measurement per day per sampling position covering
all wavelengths is taken.

4.2. Soak tests results
Among all the materials only two showed a measurable effect after
the soak test: the Super-K inner detector (ID) cables and the blackrubber friction pads used to hold the ID PMTs; the rubber showed
the largest effect. Although the effect is stronger with Gd sulfate, theeffect is clearly seen in pure water as well but has been unnoticed
until now after more than 20 years of data taking with Super-K. Thismeans that the current SK water purification system can take care of
the impurities coming from this material through continuous water
circulation and purification. Taking into account the surface/volume
ratio difference between the soak tests and the actual ratio at SuperK and other effects, it was concluded that even this case does not
represent a potential problem. In addition, the high water quality in
EGADS, in which the black rubber is also present (as will be discussed
in Section 5.1), demonstrates this point.

4. Potential impact on Super-K detector components
The 200-ton EGADS main tank is made of the same stainless steel
as the 50-kton Super-K tank, SUS 304. All the other components thatwere later installed and that are in contact with water are also madewith the same materials as were used in Super-K. Examples are PMTs,
acrylic covers and fiber-glass reinforced plastic (FRP) cases to avoid
a PMT implosion chain reaction, cables, shrink tubes for PMT connector protection, screws, etc. In this section, results of soak tests arepresented.
4.1. Soak tests for current Super-K materials
Soak tests of all materials used at Super-K were done in the USA andOkayama University in Japan. The procedure for the soak tests was thefollowing:

5. Running EGADSAs mentioned in Section 1.2, among the goals of the EGADS project
was to demonstrate that the purification system can achieve and maintain good water quality. We proceeded systematically.
We started with pure water in the uninstrumented EGADS 200-ton
stainless steel tank in the first half of 2011. The band-pass system wasable to achieve the goal with ultrapure water.
Next, from mid-2011 to the end of 2012 we loaded 0.2% Gd sulfate
in the 15-ton mixing tank, which is made of plastic. The band-passsystem filtering this Gd-loaded water was also able to achieve andmaintain good water quality.
In 2013 we loaded the still uninstrumented 200-ton tank with 0.2%
Gd sulfate with positive results again.
After the previous successes, in the summer of 2013 we decided to
install 240 PMTs in the EGADS tank. Out of these 240, 224 are the 50cm Super-K ID PMTs [24], while 16 are several types of photosensors
for a Hyper-Kamiokande (HK) R&D project [25,26]. Among the 224 50cm PMTs: 148 PMTs have no cover, 16 PMTs have an FRP housing only,
and 60 PMTs have both an FRP and acrylic cover (same as all ID PMTs
at Super-K). All PMT types, covers, and relative positions are shownin Fig. 9. Similar to Super-K’s ID, the active photocathode coverage isabout 40%. As in the Super-K ID, the remainder is covered by black
polyethylene terephthalate sheets. There is no outer detector (OD) in
EGADS.

• Soak each material both in Gd-loaded water and Super-K purewater at room temperature (around 25 ◦ C, while Super-K temperature water is about 13 ◦ C).
• With an spectrophotometer (JASCO V-550 with wavelengths between 190 and 900 nm and 2 cm cells), measure the transparencyof the solution (typically after about 3 months of soaking).
• Estimate the impact of each material in the attenuation length
at Super-K (taking into account temperature and volume/surface
ratios of the samples as compared to those in Super-K).
The Super-K purification system can achieve and maintain ultrapure
water quality as has been demonstrated after running for many years.
The materials were soaked in both Gd-loaded water and pure Super-K
water: if a given material steadily emanates a measurable amount of
impurities in pure water, it has been established that the Super-K water
purification system can keep up with removing them. Similarly, any potential adverse effect of the material being soaked in Gd sulfate-loaded
water will be reduced via continuous circulation and filtration.
Due to the variability and availability of materials and differencesamong manufacturers, soak tests of all materials to be used in any
future WC detector should be done in ultrapure and Gd-loaded water.
7

Ll. Marti, M. Ikeda, Y. Kato et al.

Nuclear Inst. and Methods in Physics Research, A 959 (2020) 163549
Table 1
Relevant radioactive contamination and typical impurities in untreated Gd sulfate andthe requirements from DSNB and solar neutrino physics goals. Units in this table are
in mBq/Kg.

A magnetic field compensation coil was also activated, reducing theterrestrial magnetic field to less than 0.1 Gauss inside the EGADS tank.
The front-end and read-out electronics and related software, as wellas the trigger system that was installed in EGADS during 2013 werereused Super-K-I/II/III ATMs [27]. The high voltage system is a CAEN
SY1527LC and an AP1932.
The installation of PMTs and their ancillary electronics turned the
EGADS 200-ton tank into a proper detector. As an example, here weshow data taken with an Am/Be source at EGADS in Fig. 10. Fig. 10ashows the time difference between the prompt event and the delayed
neutron capture with an Am/Be source for a Gd sulfate concentration of
0.2%. The fitted neutron capture time here for the data is 29.9 ± 0.3 μ𝑠
and for MC is 30.0 ± 0.8 μ𝑠. Fig. 10b shows the reconstructed energy
from the delayed neutron signals.

ChainPart of thechain

Typical

DSNB

Solar

CompanyA

Company
B

Company
C

238 U

238 U

50
5

<5
–

–
<0.5

<0.04
<0.2

<0.04
<0.2

<0.04
1

10
100

–
–

<0.05
<0.05

0.02
<0.3

0.06
<0.26

0.09
2

32
300

–
–

<30
<30

<0.4
<1.5

<0.3
<1.2

<1.3
<3.1

232

Th

226

Ra

232

Th

228 Th
235

U

235
227

U
Ac/227 Th

All units in mBq/Kg.

5.1. Transparency and Gd concentration
constant around the final target concentration. This means that we
could not detect any Gd loss after two and a half years of running
EGADS at full concentration, which involved passing the entire water
volume through the water systems 650 times.

In 2014 we started loading Gd into the EGADS detector. Because
at this point EGADS mimicked the conditions that will exist in Super-K
once Gd sulfate is loaded (SK-Gd), this was the definitive test to demonstrate the capabilities of the band-pass system and the null effects on
detector components. Water transparency was measured at least daily
and Gd sulfate concentration was monitored with variable frequency(more often while conditions were changing and more sparsely when instable conditions). Both transparency and concentration are measured
from three sampling ports inside EGADS. These are at the bottom,
center and top sections of the detector, see Fig. 5. The results are shownin Fig. 11. The blue band represents the typical LL15 values for SuperK’s ultrapure water over the last decade; these are between 75% and82%, left 𝑦-axis scale. EGADS’ LL15 values as a function of the sampling
date are the upper blue, red, and green lines for the bottom, center, andtop sampling ports, respectively. As described in Section 2.2, the water
purification system takes water from the top and injects it back into
EGADS from the bottom. As a consequence LL15 values are expected
to be slightly larger in the bottom than at the top. The lower blue,
red, and green lines are the measured Gd sulfate concentrations on agiven sampling date for the bottom, center, and top sampling ports,
respectively. The concentration scale can be read on the right and thehorizontal dashed line indicates the final expected value. The verticalhatched areas indicate events for which the conditions changed in thedetector operation; a short description is added.
At the end of November 2014 the first Gd sulfate loading took
place in this final version of the detector. The goal was to reach aconcentration of 0.02% Gd sulfate. The LL15 had a sudden drop below
the blue band at all sampling positions. After that the LL15 recovered
as water was circulated, and the LL15 values returned into the typicalSuper-K ultrapure range. The Gd sulfate concentration increased in all
sampling positions and quickly became homogeneous throughout thedetector.
Three additional Gd sulfate loadings followed: end of January 2015
(0.1% Gd sulfate), middle April 2015 (0.16% Gd sulfate), and endof April 2015 (0.2% Gd sulfate). All loadings demonstrated a similar
pattern to the first one: sudden LL15 drop followed by a rapid recovery
back to the typical Super-K transparency values, while the Gd concentration as measured by the AAS rose homogeneously throughout thedetector to the expected values.
A primary result is that the LL15 values stay within the blue band
if no disruptive event happens. In time-sequential order these events
were: fast recirculation system was turned off for a test to see theimpact of this system in LL15 (after that it was turned back on), bandpass system off (to test its impact in LL15), 2016’s annual water system
maintenance, band-pass and fast recirculation systems off (due to an
unexpected system failure), and 2017’s water system maintenance. In
each case the LL15 values drop but after a short period of stablerunning the values return to the typical Super-K values. Note that theGd sulfate concentration remains, within measurement uncertainties,

5.2. Main lessons from running EGADS
In summary, after full Gd-loading, the water transparency is within
typical Super-K values, which means that Gd sulfate is essentiallytransparent to Cherenkov light. The water filtration system can achieve
and indefinitely maintain good water quality. In addition, the Gd losses
are minimal and we could not detect them after more than two years
operating at full concentration. The Gd sulfate quickly dissolves and ishomogeneously distributed throughout the detector (no stratification).
Although there was no sign of deterioration of any detector component, EGADS was emptied in November 2017 to perform an eye-checkof the inner structure, PMTs, etc. After these two and a half years underconditions designed to be as close as possible to those that will existat SK-Gd, we found no sign of deterioration in any of the detector
components in what constitutes the most realistic possible soak test.
6. Backgrounds in Gd sulfate
The number of free neutrons in the ultrapure Super-K water is very
low. However, when adding Gd sulfate these neutrons will become
visible. Further, impurities in the Gd sulfate may decay. The decay ofthese impurities create more neutrons as well as alpha, gamma and beta
particles that, alone or in coincidence with other events may mimic
signals in low energy analyses. Thus, the radiopurity of Gd sulfate has
to be taken into account.
In Table 1 we show the relevant radioactive chains (first and second
columns) and the typical activities per kilogram of Gd sulfate powder (third column) from Stanford Materials. We studied these chains
and their potential impact on DSNB and solar neutrino analyses by
mimicking their signals once Gd sulfate is mixed. The calculated upper
limits that would be tolerable for our physics goals are indicated in the
fourth and fifth columns for the most restrictive cases for DSNB andsolar analyses, respectively. If no number is given (-) it means that thecorresponding requirement is less restrictive.
We have been collaborating with several chemical companies to
produce a Gd sulfate powder that meets these requirements. They
produce highly radiopure Gd sulfate and we test the radiopurity withlow background germanium detectors at Canfranc (Spain) and Boulby(UK), as well as with ICP/MS measurements at Kamioka (Japan) [28].
The radioactive contamination levels achieved by these companies areshown in the last three columns of Table 1. There is one company that
already meets the requirements within our measurement uncertainties.
There are others that are still working on increasing the radiopurity of
their Gd sulfate powder.
8Ll. Marti, M. Ikeda, Y. Kato et al.

Nuclear Inst. and Methods in Physics Research, A 959 (2020) 163549

Fig. 10. Left (a) delayed neutron capture with an Am/Be source for a Gd sulfate concentration of 0.2%. Right (b) reconstructed energy from the delayed neutron signals. (Forinterpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 11. Upper three lines: Cherenkov light left [%] at 15 m (left 𝑦-axis scale), and lower three lines: Gd sulfate concentration for the three sampling positions in the EGADS
detector (right 𝑦-axis scale). The line colors for the bottom, center and top sampling positions are blue, red and green, respectively. The blue band represents the typical Super-K
ultrapure water LL15 values while the horizontal dashed line represents the Gd sulfate final target concentration. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)

7. Gd removalThis resin has been successfully tested at EGADS. We analyzed thetreated water with an ICP/MS and we could not find any Gd trace.
Taking into account the ICP/MS uncertainties, we determined that the
Gd concentration has been reduced from one part per thousand in thefeedstock to less than 0.5 parts per billion in the waste stream.

To achieve a concentration of 0.2% Gd sulfate in Super-K we willneed to dissolve about 100 tons of Gd sulfate powder. Someday we willneed to empty the tank and remove the dissolved Gd sulfate, at leastat the end of the SK-Gd phase. We have been investigating methods toremove Gd in a quick, as well as cost effective, manner. These methods
range from filter presses and precipitation via pH control to cyclonicseparation or spillover filtration among others.
We found that the most effective and straightforward method todeploy is the use of a cation ion-exchange resin. This resin releases
three Na+ ions for every captured Gd3+ ion. The highly-charged Gd3+
ion is then tightly bound to the resin matrix and cannot accidentallyescape once it has been captured. The only method to retrieve thecaptured Gd is to flush the resin with concentrated acid. Used resin
is inert and stable, and has been designed for safe transport.

8. Summary
As proposed by GADZOOKS!, WC detectors would greatly benefit
from efficient neutron tagging capabilities. Adding Gd while maintaining good water quality will make this possible in large detectors likeSuper-K, and EGADS has demonstrated the feasibility of this technique.
Gd sulfate is essentially transparent to Cherenkov light and dissolves
easily and homogeneously; Gd sulfate can be produced to the required
levels of radio-purity needed for our studies; finally, it can be removed
in an efficient and economical way whenever needed.
9Ll. Marti, M. Ikeda, Y. Kato et al.

Nuclear Inst. and Methods in Physics Research, A 959 (2020) 163549

The success of EGADS showing the feasibility of this technique was
key to the decision made by the Super-Kamiokande collaboration torefurbish the Super-K detector in 2018 as a first step to load Gd sulfate
and benefit from effective neutron tagging capabilities.

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Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared toinfluence the work reported in this paper.
Acknowledgments
We gratefully acknowledge the cooperation of the Kamioka Mining and Smelting Company. This work was supported by the JSPS
KAKENHI Grant Numbers JP21224004, JP26000003, JP24103004 andJP17H06365. Funding support was provided by Kavli IPMU (WPI),
the University of Tokyo and the US Department of Energy. We thankthe ‘‘Consorcio Laboratorio Subterraneo de Canfranc’’ (Spain) and the
Boulby Underground Research Laboratory and in particular the staffof the BUGS facility (UK) for supporting the low-background materials screening work. Some of us have been supported by funds fromthe European Union H2020-MSCA-RISE-GA872549-SK2HK, the Spanish
Ministry of Science and Innovation (grant PGC2018-099388-B-100),
the Science and Technology Facilities Council (STFC) and GridPP, UK,and the European Union’s H2020-MSCA-RISE-2018 JENNIFER2 grant
agreement no. 822070.
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