Research ReportSuper-Kamiokande Prepares for Gadolinium
Loading － Tank Open Work Following 12
Years of Continuous Operation
Shigetaka Moriyama
Kavli IPMU Principal Investigator

Hiroyuki Sekiya
Associate Professor, Institute for Cosmic Ray Research, The University of Tokyo, and Kavli IPMU Scientist

Mark R. Vagins
Kavli IPMU Principal Investigator

Introduction
Located in the mine at Kamioka-cho, Hida-city,
Gifu prefecture, Super-Kamiokande (Super-K, SK)
started taking data in 1996. Using the data collectedduring its first two years of operation, ICRR Professor
and Kavli IPMU PI Takaaki Kajita was awarded theNobel Prize in physics in 2015 for the discovery of
atmospheric neutrino oscillations [1]. Super-K isstill producing high level results such as neutrino
observations from outer space and accelerators [2],as well as studies on proton decay [3].
Following the successful completion of the R&D
phase of the EGADS project [4], Super-Kamiokande
is now being prepared for one of Kavli IPMU＇
s flagship experimental initiatives: the enrichmentof Super-K＇s 50,000 tons of pure water with 100
tons of gadolinium (Gd) sulfate [5-7] to enhance its
detection performance for thermal neutrons andthe electron antineutrinos which produce them.
This future phase of operation has been officiallydesignated SK-Gd.
As shown in Figure 1, the in-tank work began on

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May 31, 2018, after 12 years of continuous detector
operation. There are four main tasks:
1) Fix a small water leak in the Super-K tank.
2) Replace the photomultiplier tubes that have failed(a few hundred out of 13,000) since the previous intank refurbishment in 2006.
3) Clean any rust and other dirt that has accumulated
in the detector since its original completion in 1996.
4) Install additional water piping to increase thetotal water flow for increased water purification,and to enable better control of the flow direction in
the tank.
In the middle of October 2018, we started to refill
the detector with pure water. Now (in December2018) we are finalizing the in-tank work–which
has required a total of thousands of person-daysof effort–to prepare the detector for the next
phase of its long and productive life. In this article
we will describe the eventful work that has takenplace during the last half year and some of the newphysics which awaits Super-K once Gd-loading is areality.

Kavli IPMU News No. 44 December 2018

Figure 1: Working inside the Super-Kamiokande tank on August 18, 2018.

Physics targets of SK-Gd
The SK-Gd program is SK＇s upgrade plan thatenables more efficient detection of electron
antineutrinos through the delayed coincidence of
signals of positrons and neutrons. Reines and Cowan
first discovered the neutrino using this technique
at a nuclear reactor [8]. They were using cadmium
as a neutron capture nucleus, while we are using
the much less reactive gadolinium just as recent
nuclear reactor neutrino experiments employ it
mixed into liquid scintillator [9]. At ~49,000 barns
the thermal neutron absorption cross section ofnaturally occurring Gd is the largest of any stableelement; after absorption a cascade of gamma
rays are emitted with total energy of ~8 MeV, well
above SK＇s energy threshold. Therefore, Gd-doping
makes it possible to detect neutrons even in a water
Cherenkov detector (Figure 2).
The ultimate goal of SK-Gd is the world＇s
first detection of neutrinos from the all thepast supernova explosions. These are known
interchangeably as the Supernova Relic Neutrino[SRN] or the Diffuse Supernova Neutrino Background[DSNB]. In the universe there are 1022-23 stars. Stars
with masses of more than eight times our sun are
believed to end their lives in supernova explosions:these as-yet unobserved neutrinos must suffuse
the universe. Observing the DSNB would leadto a deeper understanding of the energetics ofsupernova explosions, since 99% of the energy
produced by the supernova explosion is released byneutrinos. Also, since the DSNB is the accumulationof neutrinos emitted from all of the supernova
explosions that have occurred since the beginning of
the universe, we can use their properties to explore
the history of massive stars as well as the source ofheavy elements.
Figure 3 shows the range of expected energy
distributions of the DSNB. Naturally the exact shapeis model dependent, but the intensity is roughly
several/cm2/sec. Therefore it is estimated that

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Research
ReportFigure 2: Principle of the delayedcoincidence method with gadolinium
for antineutrino detection.

verification of the DSNB flux will be possible with asignificance level greater than three sigma within 10
years of observation.
For supernova explosions occurring in our galaxy,
positive identification of electron antineutrinos
via neutron tagging will improve the accuracy of
directional determination to the supernova by
allowing those neutrino events that do not preserve
the directional information to be subtracted away.
It has also been suggested that neutron detection
could provide significant early warning–up to a
week in advance–of very nearby explosions of verylarge stars [11]. In addition, the enrichment of SK＇s
water with gadolinium is expected to enhance
the sensitivity of virtually all of the measurements
currently performed in SK , such as the Tokaito-Kamioka long baseline neutrino oscillation
experiment (T2K) and solar neutrino studies, as wellas proton decay searches.

Open Tank work
Figure 4 shows the structure of the Super-K tank.
Inside a cylindrical water tank, there is a stainlesssteel structure indicated by two parallel dashedlines supporting photomultiplier tubes (PMTs). The
structure is two meters distant from the outer10

wall, bottom, and top of the tank. There are 11,129
inner detector (ID) PMTs and 1,885 outer detector(OD) PMTs mounted on the structure. On the innersurface of the support structure, spaces between
PMTs are covered with black-colored PET sheets tosuppress reflection of Cherenkov light. On the outersurface of the structure, white Tyvek sheets are used
to enhance light-collection efficiency of OD PMTs.
Since the sheets are loosely connected by staplers
and pins, the water levels inside and outside are
identical.
A so-called“floating floor”was used to workon the walls of the detector. During that phase of
the refurbishment effort we drained two meters ofwater from the tank every three days, and worked
to seal potential water leakage points on the outerwall as well as replace those failed PMTs which we
could reach from the floor. This idea was originallyprovided by the University of Tokyo Professor
Masatoshi Koshiba during the Kamiokande era;
it is very beneficial in terms of cost savings whencompared with assembling and working fromstandard scaffolding.
The Super-K tank is 40 m in diameter and 42 m
in height and contains 50,000 tons of pure water.
Its sides form an icosagon comprised of 400 pieces
of four mm-thick stainless steel plates, each six

Kavli IPMU News No. 44 December 2018

Figure 3: Expected energy spectra for the DSNB
and other coincident events (reactor neutrinos andatmospheric neutrinos) [10].

meters wide and two meters tall. The bottom also
consists of an assembly of stainless plates. The total
submerged surface area is more than 6,000 m2 andthe total length of welding lines is more than 6.2 km.
More than 3, 200 bolts penetrate its sides. Given thecomplexity and size of the Super-K tank, therefore,
it is not so surprising that there has always been amodest water leak; we observed about one ton ofwater leaking each day, corresponding to 700 cc per
minute. This is not serious as long as we use only
pure water. Although there is no official regulation
regarding Gd-loaded water, it is necessary to stop
the SK water leak to avoid release even in a case ofa large earthquake. This is the first goal indicated in
the introduction of this article.
To achieve a reliable leak fix, Super-K colleagues
have developed a special sealant material. It needed
to be soft, flexible, and able to stand an external
stretching force. It had to meet requirements
regarding small elution into water, low naturalradioactivity, long-time stability during water
exposure, and sufficient attachment strength tostainless steel plates. Another entire article could
easily be filled describing the long and tirelessefforts to realize this special sealant material anddevelopment of its primer.
Figure 5 shows the actual sealant work. Workers

put thick tape along both side of the welding line
to hold the sealant material on the line. They usedtrowels and paintbrushes to spread the sealantmaterial over the weld with the same thickness as
the surrounding tape. A double layer of sealant was
required to allow for unexpected gaps or pinholes in
one of the two layers.
There weren＇t any serious issues during the actual
SK tank sealant work by company people since a
mockup test had been done in advance in a highhumidity environment . However, because of the cold
temperature (~13 C) of the tank wall, stainless plates,
and the solid rock behind them, we experienced
condensation on the inner walls due to the warmer
environment arising from the workers and lightingused inside the water tank. In the worst case thecondensed water made it difficult to apply the thicktape. By trying various methods of ventilation andairflow, we finally overcame this problem.
Since the sealant material is produced by a special
procedure, its viscosity tends to get lower as timegoes by. Adequate viscosity was required for highquality sealing work on the wall, avoiding drippingof the material. Because of this, careful arrangementof the detailed schedule of production, transfer, andconsumption of the sealant was necessary.
Every time we drained the water, a team of 16

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Research
ReportFigure 4: Structure of the Super-K tank. We drained two metersof water every three days in order to seal potential waterleakage points as well as replace failed PMTs.

members cleaned up the wall, while 10 company
employees sealed the welding line. We ultimately
repeated this cycle 20 times.
The dirty wall and structures inside the Super-K
tank required a large amount of human power
to clean up. Since Super-K had been operated forthe last 20 years, it was hard to know the situationin advance. In particular the top and outermost
structures were covered by a heavy residue of
exhaust gas and dust left over from the work in
1994 - 1995 during the detector construction. As
for the cleaning tools, it was dangerous to choose
arbitrary ones since most of them leave other typesof dust which may then degrade the water target
after restarting operation of Super-K . The waterteam tried, investigated, combined, and developed
many kinds of cleaning tools; we found the cleaning
itself was an important research topic.
Ultimately we maintained excellent quality ofsealant attachment and removed a large amount ofradioactivity; at least 100 Bq of radium was removed
during the cleanup according to a measurement by
a germanium detector. As the radon gas emitted
from this radium is comparable to that arising fromthe PMTs themselves, the in-tank shiftworkers
contributed to lowering the background for futuresolar neutrino observations.

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As for the PMT replacement, we were required
to work at full speed as soon as the tank was first
opened since we could not delay the sealant work.
In particular, US colleagues who were responsible
to the OD PMT replacement performed admirably
and completed all their jobs on time. ID PMTs
are much heavier than the smaller OD tubes andtheir replacement requires a great deal of effort,
especially as these inner tubes are encapsulated inanti-implosion covers made of FRP and acrylic toavoid a repeat of the chain reaction implosion that
happened in 2001. Every time we replaced PMTs
the tank needed to be closed and made dark inside
so that we could test the newly-installed PMTs. The
group in charge of DAQ and electronics then needed
to come in early and complete the tests before the 8a.m. start of that day＇s in-tank work.
We aimed to remove most potential sourcesof rust during the cleaning work. Though we aresupposed to have only stainless steel parts inside
Super-K , we sometimes found plain iron bolts andnuts covered by rust. We also found a clamp madeof iron that had been immersed for 12 years inside
the tank. Scratches on the stainless steel tankwall and support structure also had rusted during
the long exposure to pure water. As much of thisrust was removed as possible. Because rust was

Kavli IPMU News No. 44 December 2018

Figure 5: Sealant work on the SK tank wall being
conducted from the floating floor. An outerdetector PMT can be seen at the right side.

a large concern before opening the tank, many
young researchers investigated various tools andcountermeasures from well before the work. We
worked very carefully in order to achieve better
water transparency and stable operations even after
the loading of gadolinium sulfate.
Tyvek sheet installation on the side wall wasanother big job. The non-PMT area of the innerdetector is covered by black sheets so that we canreconstruct events based on arrival time of photons,
but the outer detector is covered by reflective white
Tyvek sheets so that we can achieve higher detection
efficiency for entering muons. One hundred twenty
40-meter long, one-meter wide sheets were pulled
up using gondolas and adjacent Tyvek sheets wereconnected by staples. After the completion ofsealing work on the bottom, we placed Tyvek sheets
over the bottom stainless steel plates as well. A
young Japanese researcher and US leaders worked
together and completed the bottom Tyveking in avery short time. This cooperative work was necessary
to avoid any damage of the recently-applied sealant.
The total shift count this summer was 2,683.
Every day of the week except Sundays, 40 to 50
members worked on Super-K until the middle ofSeptember. These members include not only shiftmembers, experts, managers, leaders, and company
people. Shift members came from Super-K, T2K,
and the Hyper-Kamiokande“proto-collaboration”,

involving researchers from all around the world. We
appreciate the many volunteers who did not belong
to any of these collaborations, but chose to help us
anyway.
One of the challenges we faced involved the OD
gondola due to its limited capacity. Though at least
30 members needed to enter the outer detector
every morning, the single available gondola and its
300-kg capacity caused a long queue of workers.
When we needed to work close to the bottom, it
took 20 minutes for one round trip. It was like a
puzzle to stagger the start time and end time of shiftgroups and car assignments to avoid interferencewith company people.
Since the first priority is always safety for workers,
we made constant efforts to provide sufficient safety
education, utilizing e-learning before their visits,
and guidance just before staring their work. Efforts
to improve the working environment also included
listening to workers, and discussion at the daily
morning meeting. We realized it was not sufficient
to simply tell people not to cause any accidents;
sharing information concerning potential dangers
was also important.

Water systems and the future
In addition to the tank cleaning, leak sealing,
and PMT replacement work, the SK water system

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Research
ReportFigure 6: Improvement of plumbing for better water flow inside the tank.

including in-tank piping was totally redesigned thissummer. The water purification system was originallydesigned with a supply and circulation capacity of
30 m3/h, and the piping inside the tank was madewithout separate lines for the inner region and theouter region. During the previous SK reconstruction
periods in 2001 and 2005 this interior piping was notsubstantially improved, even though the circulation
rate was raised to 60 m3/h from the middle of SK
Phase III (SK-III). This time, as shown in Figure 6, we
decided to improve the water flow in the tank byseparating not only the pipes of the inner volume
and the outer volume, but also the pipes on theside, top, and bottom of the outer volume so that
all the flow rates can be adjusted independently. Up
until now the effective detector volume for solar
neutrino analysis has been limited by the inflow ofbackground radon, but this plumbing improvement
makes it possible to control and optimize the flowof water in the tank . The findings obtained throughthis upgraded system are also being applied to thedesign of Hyper-Kamiokande.
Incidentally, when the water level in the tank was
lowered down to the surface of the bottom PMTs,
we got a surprise. The 12 water inlets at the bottom
should all be made of low background acrylic pipes,
but we found that three of them near the centerof the tank actually were made of conventional

14transparent polyvinyl chloride (PVC) tubes. In theanalysis of SK-IV (which began in 2008), there
are three event“hot spots”corresponding to the
PVC pipe positions. Prior to this discovery various
hypotheses have been discussed; perhaps the flowbalance had been destroyed or some broken PMT
glass which is a radon source became clogged in
the plumbing. Now we know the answer: PVC
containing lots of radioactive impurities was used. It
seems that the three pipes broke during the accident
in 2001 and were then quickly restored usingmaterial in hand at the time of the emergency repair
work in 2002, but this was forgotten by the timeof the full reconstruction work in 2005. We again
realized the importance of recording, reporting,seeing, and checking.
Mixing gadolinium sulfate with ultrapure water
easily produces a high purity gadolinium sulfate
aqueous solution. However, since the SK water
purification system is designed to remove all
impurities from water, if gadolinium sulfate isdissolved and the system is operated as it is, thedesirable ions will be quickly removed along withall the undesirable ones. Consequently, we had todevelop a brand new purification system to keep
only water, gadolinium ions and sulfate ions, and to
remove all other impurities.
Details cannot be described here, but for the

Kavli IPMU News No. 44 December 2018

last few years small prototype systems have been
evaluated and demonstrated. Based on these tests,
a new system for SK-Gd was constructed in 2016.
Commissioning this system has been conducted inparallel with the in-tank work. The new flow rate
will be 120m3/h, which is double the rate in SK-IV:
another reason for the water pipes in the tank tohave been upgraded.
Previous water fills have taken place at a rateof 30 m3/h using the original SK water purification
system; this took 2.5 months to fill the tank. As thewater which had been fed to the tank was nevercirculated and repurified until after the tank became
full, it took another two months or so after the tank
was full before the water quality improved to thepoint that useful data could be collected. This time,
the new SK-Gd water system is to be temporarily
operated as an ordinary water purification/
recirculating system by installing conventional ion
exchange resins made for producing ultrapure
water into the modules in the system. This
special operation should make it possible to startobservations as soon as the tank is filled. Then, theresins in the new system will be replaced with theones safe for use with a gadolinium sulfate solution.
The new system will be operated with pure water atfirst; the start of SK-Gd, i.e., the introduction of thegadolinium sulfate, is planned during the 2019 fiscal
year taking into account the running schedule of theT2K experiment .

Toward the next step
During the tank open period, we had to stop shift
work three times since the access road to Super-K
was closed due to heavy rain or typhoon. A more
serious but related issue is the danger of damaging
the Super-K tank itself due to an extended power
failure when the tank is not full. Since Super-K
is located below the water level in the mine, we
continuously drain the water surrounding theSuper-K tank through the spiral tunnel providing
access to the bottom of the tank . If the powerline for these pumps is cut due to bad weather
and the tank is partially empty, it may result in
the destruction of the Super-K tank from risingexternal water pressure. To cope with this situation,
we prepared an independent power line, a power
generator, and a method of gaining emergency
entrance into the mine. Though we had alreadyfilled up most of the water by December 2018, we
still hope that no power failure due to heavy snow
happens. After some final in-tank work in December
and January 2019, we will close the tank, and thencan finally start a new phase of operation, Super-K-V.
[1] The Super-Kamiokande Collaboration, Phys. Rev.
Lett. 81, 1562 (1998).
[2] The Super-Kamiokande Collaboration, Astrophys.
J. Lett. 857, L4 (2018); The T2K Collaboration,
Phys. Rev. D 96, 092006 (2017).
[3] The Super-Kamiokande Collaboration, Phys. Rev.
D 96, 012003 (2017).
, Its R&D Mission Completed,[4] M. R . Vagins“
EGADS Evolves,”Kavli IPMU News, No. 43,
September 2018, pages 10-13.
, GADZOOKS!
[5] J. F. Beacom and M. R . Vagins“
Anti-neutrino spectroscopy with large water
Cherenkov detectors,”Phys. Rev. Lett. 93, 171101(2004) [hep-ph/0309300].
, Kavli IPMU＇s Neutrino[6] M. R . Vagins“
Forecast: Mostly Sunny, with a Good Chanceof Supernovas,”Kavli IPMU News, No. 19,
September 2012, pages 4-9.
, Super-Kamiokande−Report of the[7] H. Sekiya“Present Status of the Gadolinium Project,”(inJapanese), ICRR News, 96 (2016).
[8] C. L. Cowan, F. Reines et al., Science 124 (1956)
103-104.
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[10] S. Horiuchi et al., Phys. Rev. D 79 (2009) 083013.
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Research
Report