Nuclear Inst. and Methods in Physics Research, A 977 (2020) 164297

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

Measurement of the radon concentration in purified water in the
Super-Kamiokande IV detector
Y. Nakano a ,∗, T. Hokama b , M. Matsubara c , M. Miwa d , M. Nakahata e,f , T. Nakamura g ,H. Sekiya e,f , Y. Takeuchi a,f , S. Tasaka e , R.A. Wendell h,fa Department of Physics, Graduate School of Science, Kobe University, Kobe, Hyogo 657-8501, Japanb Nuclear Emergency Assistance and Training Center, Japan Atomic Energy Agency, Ibaraki 311-1206, Japanc

Head Office for Information and Management, Gifu University, Gifu 501-1193, Japan
Division of Radioisotope Experiment, Life Science Research Center, Gifu University, Gifu 501-1193, Japane
Kamioka Observatory, Institute for Cosmic Ray Research, The University of Tokyo, Gifu 506-1205, Japanf
Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo,
Kashiwa, Chiba 277-8583, Japang Department of Physics, Faculty of Education, Gifu University, Gifu 501-1193, Japanh Department of Physics, Kyoto University, Kyoto, Kyoto 606-8502, Japand

ARTICLEINFO

Keywords:
Super-Kamiokande
Radon
Solar neutrino
Charcoal
Radon extraction column

ABSTRACTThe radioactive noble gas radon can be a serious background source in the underground particle physics
experiments studying processes that deposit energy comparable to its decay products. Low energy solar
neutrino measurements at Super-Kamiokande suffer from these backgrounds and therefore require precise
characterization of the radon concentration in the detector’s ultra-pure water. For this purpose, we havedeveloped a measurement system consisting of a radon extraction column, a charcoal trap, and a radon
detector. In this article we discuss the design, calibration, and performance of the radon extraction column. We
also describe the design of the measurement system and evaluate its performance, including its background.
Using this system we measured the radon concentration in Super-Kamiokande’s water between May 2014 andOctober 2015. The measured radon concentration in the supply lines of the water circulation system was
1.74 ± 0.14 mBq∕m3 and in the return line was 9.06 ± 0.58 mBq∕m3 . Water sampled from the center regionof the detector itself had a concentration of < 0.23 mBq∕m3 (95% C.L.) and water sampled from the bottom
region of the detector had a concentration of 2.63 ± 0.22 mBq∕m3 .

1. Introduction
The framework of three-flavor neutrino oscillations [1,2] is increasingly well understood. However, there remain unknown quantities,
including the absolute scale of the neutrino mass states and whether CP
symmetry is violated in neutrino mixing. Hints of oscillations amongsolar neutrinos were first obtained from the difference between thesolar-neutrino fluxes measured with the elastic-scattering channel at
Super-Kamiokande (Super-K) and the charged-current channel at theSudbury Neutrino Observatory (SNO) in 2001 [3,4]. Though only
electron neutrinos are produced in the core of the Sun, this result
demonstrated the existence of other neutrino components in the solar
neutrino flux. Solar neutrino oscillations were subsequently confirmed
by including neutral-current measurements from SNO [5]. Precise solar
neutrino oscillation measurements require large statistics and SuperK provided such measurements with its large detector volume [6–9].
Its large size further benefits Super-K’s efforts to test for the presenceof solar and terrestrial matter effects in the solar neutrino oscillationsas predicted by Mikheyev, Smirnov and Wolfenstein [10,11], the socalled MSW effect. Although the MSW effect is enough to explain
the current solar neutrino data, direct evidence for it has not yet
been obtained. At Super-K, it manifests as a distortion in the energy
spectrum of recoiling electrons produced by solar neutrino interactions
in water. Measurements of the electron recoil energy spectrum and theasymmetry of the solar neutrino interaction rate during the day andduring the night [12] allow Super-K to directly probe the matter effects
in the Sun and Earth, respectively.
Moreover, this distortion is expected to increase at lower energies (the so-called ‘‘upturn’’) and therefore requires an energy thresholdthat is as low as possible. At Super-K, the energy threshold is limitedby radioactive backgrounds, most of which arise from radon (Rn)contamination in the detector water [13]. In order to achieve a suitably
low background for solar neutrino analysis at the lowest energies, theRn emanation from all components of the Super-K detector must be∗ Corresponding author.

E-mail address: ynakano@phys.sci.kobe-u.ac.jp (Y. Nakano).
https://doi.org/10.1016/j.nima.2020.164297
Received 9 October 2019; Received in revised form 13 May 2020; Accepted 23 June 2020
Available online 26 June 20200168-9002/© 2020 Elsevier B.V. All rights reserved.

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Nuclear Inst. and Methods in Physics Research, A 977 (2020) 164297

Fig. 1. Schematic diagram of the SK-IV water system. Purified water is supplied through inlets at the bottom and drained from outlets in both the bottom and top regions asshown in details in Fig. 2.

background due to electrons from the 𝛽 decay of 214 Bi (Rn daughter).
The 𝑄-value of this decay is 𝐸𝛽max = 3.27 MeV. In order to understandthis background and its contribution to the analysis of solar neutrinos,
precise measurements of the Rn concentrations in the water of SK-IV
detector and in the outputs of the system are necessary.

reduced. For this purpose, we have developed a new measurement
system to monitor the Rn concentration in purified water. Herein we
present details of the system as well as results of in-situ measurements
at Super-K.
This paper is organized as follows. In Section 2 we describe theSuper-K detector and its water system as well as summarize the historyof Rn studies at Super-K. Section 3 presents a new method of extracting
Rn from water and subsequently describes the design and data analysis
of a system to measure the extracted Rn. We discuss backgroundlevels and systematic uncertainties accompanying this measurementin Section 4 before presenting measurement results in Section 5. In
Section 6, we discuss several possible Rn sources in the Super-K tank.
Finally in Section 7, we conclude this study and outline prospects forthe future. Note that we use the term Rn to refer specifically to 222 Rn
in this paper, unless otherwise stated.

2.1. Water purification system
The original water purification system for Super-K has been described in Ref. [38]. During the first phase of Super-K (SK-I), it consistedof mechanical 1 μm filters, a heat exchanger (HE1), mixed-bed deionization resins (Ion Exchanger, IE), ultraviolet (UV) sterilizers, avacuum degasifier (VD), a high-quality IE (Cartridge Polisher, CP), andultrafilters (UF). Their ordering here is the same as that of their appearance in the recirculation process. Over the intervening 20 years, the system has been continuously upgraded to reduce impurities and improve
control over the detector environment. For example, a reverse-osmosis
membrane (RO) was added to the recirculation line during the second
phase of operations (SK-II), and an additional heat exchanger (HE3)
was installed during the third phase (SK-III). Furthermore, at the beginning of SK-IV an upgraded heat exchanger (HE4) was added to system
in order to further control the input water temperature and thereby
suppress convection in the tank water, which causes Rn and otherimpurities to mix into the fiducial region of the detector (details below).
Afterwards the water temperature is controlled at 13.06 ◦ C with anaccuracy of 0.01 ◦ C [16,39]. The total circulation rate during SK-IV was
60 ton/h, which doubles that of SK-III. Fig. 1 shows the configurationof the system at the end of SK-IV.

2. The Super-Kamiokande detector
Super-K is a water Cherenkov detector containing 50 000 tonnes ofhighly purified water that is viewed by 11 129 20-in. photomultipliertubes (PMTs) [14]. It is located roughly 1000 m underground (2700 m
water equivalent) inside the Ikenoyama mountain in Gifu prefecture,
Japan. We define the local coordinate system of the detector as (𝑥, 𝑦, 𝑧),where (𝑥, 𝑦) represents the plane of the cylinder as viewed from above
and 𝑧 represents the height within the detector tank [14]. The origin isplaced at the center of the tank.
After the installation of new front end electronics in 2008 [15], thefourth phase of Super-K (SK-IV) started and was concluded 10 years
later in May 2018. With improvements in the water circulation system,
calibration methods [16], and event selection, the detector’s energy
threshold was lowered to 3.5 MeV in terms of electron recoil kinetic energy [9]. These upgrades have also allowed for precision measurementsof the electron recoil energy spectrum and the Super-K data together
with data from SNO provide the strongest constraint on the solar electron neutrino survival probability [9,17]. However, experimental datafrom other solar neutrino experiments, including radiochemical experiments [18–20], water Cherenkov experiments [21–24], and liquid
scintillator experiments [25–31] have not yet demonstrated the upturnof the recoil energy spectrum expected from the MSW effect. This null
observation has motivated several theoretical explanations including
models with sterile neutrinos [32–34], mass-varying neutrinos [35],and non-standard interactions [36,37].
At energies below 5 MeV Super-K has observed an excess of events
close to the detector structure near the bottom and barrel regions of
the detector [9]. However, due to the limited energy resolution of thedetector [13], this energy region overlaps with that of the primary

Water is supplied to Super-K through inlets at the bottom region of
the inner detector (ID), which is shown in Fig. 2. The inlets extend up
to 𝑧 = −16.5 m in the tank, which is 40 cm below the bottom edge ofthe fiducial volume used for analysis [9]. The ID is separated opticallyfrom the outer detector (OD) by a Tyvek sheet [16], but the water inthe tank still flows from ID to OD. As shown in Fig. 2, water is drained
through outlets placed at the top region of the tank and at the bottom
region of the OD for recirculation and purification. The outlets placedin the OD drain water from its barrel region (‘‘OD barrel’’).
There are twelve inlets placed at the bottom of ID and four outlets
placed in OD. As a result of this configuration and the precise temperature control enabled by HE4, the water tank above 𝑧 = −11 m
experiences laminar flow, and therefore lower Rn backgrounds, whilethat below this level undergoes convection [16].
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Nuclear Inst. and Methods in Physics Research, A 977 (2020) 164297

Fig. 2. Left: Schematic view of the water circulation paths in the Super-K tank. The solid line shows the Super-K water tank, and the dashed line shows the inner detector. The
arrows show the direction of water flows. Middle and Right: The locations of the water inlets and outlets at the bottom (top) of the Super-K tank. The 𝑥 and 𝑦 coordinate axes are
drawn as defined in Ref. [14]. The middle figure illustrates their locations in the bottom region, and the right figure illustrates their locations in the top region. The solid circles
describe the Super-K water tank, while the dashed circles outline the inner detector in the tank. The outlets placed in the OD region drain the water from the ‘‘OD barrel’’ region.

2.2. History of Rn studies at Super-K
Several techniques for evaluating the Rn and radium (226 Ra) concentrations in water have been developed for underground experiments [40–47]. In particular, a Rn assay system for the Super-K water
was developed during SK-I [48]. The sensitivity of that detector is
0.7 mBq∕m3 for a single-day measurement and is limited by statisticalfluctuations in the background count rate. Using that system, the Rnconcentration in the supply water was measured to be 0.4±0.2 mBq∕m3
in 2001, while that in the tank water itself was <2.0 mBq∕m3 [14]. This
was the last of such measurements prior to the results discussed below.
In order to understand the Rn concentration sufficiently, we requiresensitivity to the Rn concentration in purified water at the 0.1 mBq∕m3
level.
3. Experimental setup
In order to measure ultra-low levels of Rn in water, Rn must first be
extracted into air so that daughters of Rn (especially, 218 Po and 214 Po)
can be electrostatically collected in air because they are tend to havea positive charge [49]. This technique requires efficient Rn extraction
as well as trapping to allow enough atoms to be collected during ameasurement. To accomplish this we have developed a new water–
air extraction column and have expanded on the 80 L electrostatic
detection system detailed in [50,51] by introducing a chilled charcoal
trap to enhance the Rn collection. The accumulated Rn is released andtransferred to a detector, whose sensitivity is typically 0.5 mBq∕m3 in
air for a single-day measurement [50]. Finally, an overall sensitivity of∼0.1 mBq∕m3 in water has been achieved as determined by fits to theshape of the Rn decay curve. This represents an order of magnitudeimprovement over the previous systems [38,48].

Fig. 3. Schematic diagram of the Rn extraction system. The top (bottom) part of thesystem is the extraction column (water buffer tank). Numerical values give lengths inmillimeters (mm), while solid (open) arrows show the flow of sampled water (purge
air).

Degassed water passing through the extraction column is collectedin the buffer tank. The tank is made of transparent polyvinyl chloride (PVC), so the water level can be monitored. In order to prevent
gasses from the external environment from entering into the extraction
column, the tank is divided into three layers (1st, 2nd and 3rd layers
shown in Fig. 3) and Rn-free air is supplied to the innermost 3rdlayer during operations. The degassed water is finally drained fromthe pipe of water outlet (the 3rd layer in Fig. 3). This design allows
the extraction column to maintain both the water level and the innerpressure at the 1st layer.
Urethane gaskets have been used between the extraction column
and water tank here since this material was found to emanate lessRn [39,44] though EPDM (ethylene propylene diene monomer) or butyl
gaskets are commonly used to connect pipes in such systems.
Sampled water enters through the water inlet at the top and at thesame time purge air enters the extraction column through the purge
air inlet. When the sampled water falls down through the extraction
column and strikes its wings, the water is turned into mist allowing thedissolved Rn to escape into the air and be transported out of the system
via the upper air outlet.
The total amount of Rn in the water and in the air during thisprocess is conserved before and after the extraction, as expressed by

3.1. Column for radon extraction
Fig. 3 shows a schematic diagram of the extraction column, whichmixes flowing water with purge gas (air) to extract Rn. The extraction
column consists of 12 spiral wing units and is equipped with inlets forboth the purge air and the sampled water as well as outlets for Rndegassed water and purge air. A buffer tank is located below the column
to store Rn-degassed water.
The spiral wing unit1 is a combination of six right-turn units and six
left-turn units, which are welded to each other alternately. The insideof a wing unit has four welded wings that are directed downward witha height, inner diameter, and outer diameter of 60.0 mm, 41.6 mm and48.6 mm, respectively. Each wing contains several holes to improve its
Rn extraction efficiency. The surfaces of the wings and holes have been
electro-polished in order to reduce emanation of Rn from their surfaces.
1
The unit itself is constructed with a MU-reactorTH , a product of the MU
Co., Ltd. http://www.mu-company.com/en_index.html.

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Nuclear Inst. and Methods in Physics Research, A 977 (2020) 164297
Table 1
Systematic uncertainties in the calibration of the extraction efficiency. The second
column shows typical values measured during the calibration procedure described in
the text.
Systematic uncertainty

Typical value

Estimated uncertainty

Ionization chamber
70 L Rn detector [38]Liquid scintillation counter
Air leaks in sampling vials
Water flow rate
Air flow rate

8–12 Bq/L
0.01 Bq/L
7–9 Bq/L or 2–3 Bq/L–
3.58 L/min or 4.0 L/min
2.0 L/min

±5.0%
±6.8%
±10.0%
±7.5%
±2.0%
±2.0%

Table 2
Summary of the measured extraction efficiencies with a constant air flow rate of
𝐹𝑎 = 2.0 [L/min]. The first and the second terms are defined in Eq. (2).

the following equation:(1)

Here 𝐶𝑤,0 (𝐶𝑎,0 ) is the Rn concentration of the sampled water (purgeair) before extracting in units of Bq∕L, 𝐶𝑤 (𝐶𝑎 ) is that of degassed
water (purge air) after extracting, and 𝐹𝑤 (𝐹𝑎 ) is the flow rate of thewater (air) through the system in units of L∕min.
In order to evaluate the Rn extraction efficiency, we rewrite Eq. (1)as follows:
𝐶𝑤,0 𝐹𝑤 = 𝐶𝑤 𝐹𝑤 + (𝐶𝑎 − 𝐶𝑎,0 )𝐹𝑎 ,𝐶𝑎 − 𝐶𝑎,0𝐹𝐶× 𝑎.
1= 𝑤 +
𝐶𝑤,0
𝐶𝑤,0
𝐹𝑤

𝐹𝑎 [L/min]

First term

Second term

Total

4.00
3.58

2.0
2.0

0.33 ± 0.02
0.30 ± 0.02

0.64 ± 0.04
0.73 ± 0.03

0.97 ± 0.05
1.03 ± 0.04

ionization chamber. Using the mass-flow controller, the water flow rate
was maintained at either 𝐹𝑤 = 4.0 L∕min or 𝐹𝑤 = 3.58 L∕min; which areflow rates used in the Super-K measurements described in Section 5.
Calibrations were performed as follows. First, tap water and purge
air are supplied to the extraction column. After filling the buffer tankwith the degassed water, we controlled their flow rates to maintaina constant water level in the PVC vessel (1st layer in Fig. 3). We
then sampled the tap water and the Rn-degassed water at the sametime using shake flasks. Note that when we found air bubbles on
the inner surface of the shake flasks they were carefully removed inorder to avoid Rn exchange between the sampling water and the airbubbles. During the extraction process, the Rn concentrations in thepurge air and output air were monitored using the 70 L Rn detector
and the ionization chamber. Finally, the Rn extraction efficiency wasdetermined according to Eq. (1).
The sources of systematic uncertainties in the calibration measurements are summarized in Table 1 and are primarily based on
estimations from the technical specifications of the measurement devices. Measurements of the same sampling vials used in calibration
found fluctuations in the Rn concentration of ±7.5% [54], which havebeen attributed to potential air leaks. If we assume that air leaks
occurred during our measurements, the Rn concentration in the water
may decrease, resulting in an erroneously low measured value. This
would lead to an erroneously high extraction efficiency. In order tocompensate for such problems, we take the value of these fluctuations
as a systematic uncertainty. Additional systematic uncertainties aretaken on the water flow rate stability, ±2.0%, and the stability of the
air flow rate, which is also ±2.0%.
In total, we performed 6 (11) calibrations with the water flow rate
set to 𝐹𝑤 = 4.0 L/min (𝐹𝑤 = 3.58 L/min), as shown in Fig. 5. The
measured extraction efficiencies are summarized in Table 2. Combining
the first term with the second term in Eq. (2), we obtain the value of thesum to be 0.97±0.05 (1.03±0.04) for 𝐹𝑤 = 4.0 L/min (𝐹𝑤 = 3.58 L/min).
Both values are consistent with 1.0. This result demonstrates that the
total radioactivity before and after extraction is conserved to within themeasurement uncertainty.
To understand the stability of the extraction efficiency, we performed additional calibrations at a constant water flow of 𝐹𝑤 =4.0 L∕min (𝐹𝑤 = 3.58 L/min) while changing the air flow from 1.6 L∕minto 4.4 L∕min (from 1.65 to 2.35 L∕min). The ratio of the air and waterflow rates varies between 0.4 and 1.1 (between 0.45 and 0.65). Fig. 6
shows the dependence of the extraction efficiency on this ratio. Thereis no correlation as summarized in Table 3.

Fig. 4. Schematic diagram of the calibration setup. Thin arrows show the direction of
the air flow, while wide arrows show the direction of the water flow.

𝐶𝑤,0 𝐹𝑤 + 𝐶𝑎,0 𝐹𝑎 = 𝐶𝑤 𝐹𝑤 + 𝐶𝑎 𝐹𝑎 .

𝐹𝑤 [L/min](2)

The second term on the right side in Eq. (2) can be regarded as
the Rn extraction efficiency of the system. Note that the total amountof extracted Rn depends on the flow rates of the sampled water andthe purge air. Accordingly, the factor (𝐹𝑎 ∕𝐹𝑤 ) in the second term isrequired to normalize the extraction efficiency by taking into accountthe total volume of water and air used in the Rn extraction (mixing)
process. It can be determined by measuring the Rn concentrations
in both the sampled water and the purge air before and after the
extraction process.
To determine the Rn extraction efficiency, we built a calibration system at Gifu University as shown in Fig. 4. The system consists of a 70 L
Rn detector [38], an air mass-flow controller (HORIBA STEC Z512), twopressure gauges (Naganokeiki Co. Ltd. ZT67), a water mass-flow controller (TOFCO Corp. FLC620), an electrical dehumidifier (KELK DH209C), an air pump, and an ionization chamber (OHKURA ELECTRIC
Co. Ltd. RD1210B).
Purge air for the calibration was taken from environment air at
Gifu University and had a typical Rn concentration of ∼0.01 Bq∕L as
measured by the 70 L Rn detector. After the extraction, the concentration in the outflow air was measured with the ionization chamber. Tap
water from the university was used as the Rn source. Its Rn content
was measured with a liquid scintillator counter (LSC) system (Tri-Crab
2900TR produced by PerkinElmer Inc.), as is standard for evaluationof hot spring water [52–54], and was found to be 5–7 Bq∕L. The
pressure inside the extraction column was monitored by two pressuregauges, located at the inlet and the outlet of the extraction column,
because the Rn extraction efficiency may depend on this quantity. The
electric dehumidifier was installed just after the extraction column toremove water vapor from the output air before sending it into the

3.2. Experimental setup and a method for radon concentration measurements
Charcoal efficiently absorbs various impurities [55] and is widelyused to trap Rn from several gases [14,40,50,56–59]. It has almost 100%
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Nuclear Inst. and Methods in Physics Research, A 977 (2020) 164297

Fig. 5. Efficiencies of the Rn extraction from the calibration measurements. The left figure shows the results for the water flow rate of 4 L/min and the right figure shows resultsfor 3.58 L/min. Blue squares (red upward-pointing triangles) show the first (second) term defined in Eq. (2), and black downward-pointing triangles show their sum. Shaded bands
show the total uncertainties for each term.

Fig. 6. Dependence of the Rn extraction efficiency on the air flow rate. The water flow was fixed at 4.0 L/min for the left figure and 3.58 L/min for the right figure, while theair-flow rate was changed from 1.6 L/min to 4.4 L/min (from 1.65 L/min to 2.35 L/min). In the right panel, the third value is calculated by taking average of 11 measurements
shown in Fig. 5. Other values are calculated by using the result from one measurement. Colored markers have the same definitions as in Fig. 5.
Table 3Air flow rate dependence of the extraction efficiency. The first and the second terms
are defined in Eq. (2).
𝐹𝑤 [L/min]𝐹𝑎 [L/min]

First term

Second term

Total

4.00
3.58

1.6–4.4
1.65–2.35

0.31 ± 0.01
0.31 ± 0.01

0.67 ± 0.02
0.73 ± 0.03

0.98 ± 0.02
1.04 ± 0.03

A water sample can be supplied from the Super-K tank, from its
pure water supply line, or from the return line to the water purification system. We used commercially-available G1-grade high-purity
air (impurity <0.1 ppm) as the purge gas to minimize intrinsic Rnbackgrounds. It is important to remove water from the air before boththe charcoal trap and the Rn detector, as their efficiencies depend on
the humidity [61]. The electrical dehumidifier and three copper wool
traps are used for this purpose. Each copper wool trap is a 3∕4 in. Ushaped pipe filled with 12.5 g of 𝜑 80 μm copper wool (Nippon Steel
Wool Cp., Ltd.). We placed these in an ethanol bath kept below −80 ◦ C.
Note that Rn is not captured by the cooper wool.2 Further, we installed
0.4 μm mesh filters (Pall Corp., CNF1004USG6) before and after the Rntrap to prevent pieces of charcoal from escaping into the measurement
system. We used 1∕2 in. nylon tubes (NITAA, MOORE N2) to sample thewater. Other system components are made of electro-polished stainlesssteel (NISSHO Astec Co., Ltd., MGS-EP SUS316L) and all joints areconnected by VCR® gaskets to minimize Rn emanation and possible
air leaks, both of which can affect backgrounds in the measurement.
There are three steps to measure the Rn concentration in water.
First, Rn is extracted from the water using the extraction column
described before and is concentrated in the chilled charcoal trap (concentration process). In the next step, the Rn gas is released from thetrapping efficiency below −60 ◦ C and the trapped Rn can be removed
with ∼100% efficiency by heating the charcoal upto +120 ◦ C [60]. In
order to take an advantage these properties, we designed a simple
trap using a charcoal-filled 1∕2 in. U-shaped electro-polished stainlesssteel pipe. The trap was filled with 12.5 g of charcoal (DIASORB G4-8,
produced by Calgon Carbon Japan KK). This charcoal has also beenused in previous studies [59]. When trapping Rn, the U-shaped pipe
is placed in a refrigerated ethanol bath. To release Rn from the trap,
the U-shaped pipe is removed from the bath and heated with a bandheater.
Fig. 7 shows a schematic diagram of the entire Rn measurement
system. It consists of a water pump (Iwaki Co. Ltd., MDG-R15T100),a water mass-flow controller (TOFLO Corp., FLC620), a temperature
sensor (TOFLO Corp., CF-SCMT, PTM-23), the Rn extraction systemdescribed in Section 3, two pressure gauges (Naganokeiki Co. Ltd.,
ZT67), an electrical dehumidifier (KELK, DH-209C), three copper wool
traps for further water removal, the charcoal trap, a dew-point meter (VAISALA, DMT340), an air mass-flow controller (HORIBA STEC,
Z512), an air-circulation pump, and the 80 L Rn detectors [50,51]. We
used three Rn detectors to conduct three measurements in parallel andthey are identical.

2
We evaluated the capture rate of Rn on the copper wool with anothercalibration set up. We confirmed that the Rn concentration did not decrease
when the pure air was circulated through the cooled copper trap in the closed
set up. Thereby, the capture of Rn on the copper wool is negligible in theanalysis described in Section 3.3.

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Nuclear Inst. and Methods in Physics Research, A 977 (2020) 164297

Fig. 7. Schematic diagram of the setup for the Rn concentration process. The thin arrows show the direction of the purge air flow, while thick arrows show the direction of thesampled (and degassed) water.

trap and transferred to the Rn detector (transfer process). Finally, theRn concentration is measured with the 80 L detector (measurement
process).
Before any measurement, the entire system except for the waterlines, the extraction column, and the charcoal trap is first evacuateddown to <1.0 × 10−4 Pa. The air leak rate of the system was measured
to be less than 10−10 Pa m3 ∕s using a helium leak detector (HELIOT
712D2, produced by ULVAC Equipment Sales Inc). The trap is thenheated to +200 ◦ C for about one hour to completely remove any
residual Rn. Afterwards the system is filled to atmospheric pressure
with commercially-available G1-grade high purity air (impurity concentration <0.1 ppm) and the trap is cooled with a refrigerator. During
the concentration process, we set the water sampling rate to 3.58 L∕minor 4.0 L∕min and set the flow rate of the purge air to 2.0 L∕min.
After the water level and the air pressure in the extraction column
stabilize, valves before and after the charcoal trap were opened, as
shown in Fig. 7. Sampling and concentration periods varied from 0.5 h
to 18 h, depending upon the expected Rn concentration of a given
measurement.
After the concentration process, the valves before and after the
charcoal trap were closed and the water sampling was stopped. During
the transfer process, we heated the trap to +200 ◦ C and then opened
the valves again to supply pure air at 1.0 L∕min in order to fill the Rn
detector with the accumulated Rn. The entrance valve was closed after
the Rn detector reached atmospheric pressure. After these procedures
measurements were started and performed typically for 20 days in orderto determine the shape of the Rn decay curve.

curve. Fig. 8 shows an example of the measured concentration as afunction of time. The data is fitted with the following function:
𝐶(𝑡) = 𝐴𝑒−𝜆𝑡 + 𝐵detector (1 − 𝑒−𝜆𝑡 ),where 𝑡 [day], 𝐶(𝑡) [mBq∕m3 ], 𝜆 [day−1 ], 𝐴 [mBq∕m3 ], and 𝐵(3)
detector[mBq∕m3 ] are the elapsed time since the start of the measurement, theRn decay constant (𝜆 = ln 2∕{3.82 day} = 0.181 day−1 ), the Rn concentration at time 𝑡, the initial Rn concentration, and the background due tothe activity of 226 Rn in the Rn detector, respectively. These parameters
are listed in Table 4. Note that in a previous publication [50] we found𝐵detector to be 0.33 ± 0.07 mBq∕m3 for a particular 80 L detector. Afterthe actual measurements described in Section 5, we have evaluatedthe background of three Rn detectors used in the measurement system.
The backgrounds were measured by closing the Rn detector. We used
about 89 days data taken from November 1st 2015 to January 29th
2016. Each backgrounds are 1.24 ± 0.10 mBq∕m3 , 0.27 ± 0.04 mBq∕m3 ,and 0.23 ± 0.04 mBq∕m3 , respectively. The first detector has the larger
background than that of others. This is because this detector was
reused from the previous Rn detector by replacing its top flange and by
performing the electropolish to its inner surface. Due to such additional
production processes, it was slightly contaminated.
In the fitting process of the analysis, the parameter of the detector
background, 𝐵detector , is fitted by limiting its value within ±3𝜎 of thestatistical uncertainty of each average background rate listed in Table 4.
After obtaining 𝐴 from the fit, we derived the Rn concentrationin the air. We obtained the initial total radioactivity in the detector
as 𝐴 × 𝑉det , where 𝑉det = 0.080 m3 is the total volume of the Rn
detector. On the other hand, the total radioactivity in the purge air after
extracting is given by 𝐶PAD 𝑉purging , where 𝐶PAD is the Rn concentrationin the purge air after the extraction and 𝑉purging = 𝐹𝑎 × 𝑡con is the total
volume of the purge air that passes through the charcoal trap during
the concentration process (𝐹𝑎 is the flow rate of the purge air). Since

3.3. Analysis method
Since Rn decays with a constant half-life its initial concentration at
the start of the measurement can be derived from the measured decay
6

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Nuclear Inst. and Methods in Physics Research, A 977 (2020) 164297

Table 4
Summary of the parameters used to calculate the Rn concentration in water.
Parameter
𝑡
𝐶(𝑡)
𝐴
𝜆
𝐵detector

Value

Unit

Definitionln 2∕3.82

day
mBq∕m3mBq∕m3day−1

Elapsed time since the start of measurement
Rn concentration measured by the Rn detector at time 𝑡
Rn concentration at the start of measurement
222 Rn decay constantmBq∕m3Background of 80 L Rn detector No. 1
Background of 80 L Rn detector No. 2
Background of 80 L Rn detector No. 3

h
h
L/min
mBq∕m3m3
m3

Duration of the concentration process
Combined duration of the concentration and transfer processes
Purge air (sampled water) flow rate
Rn concentration in purge air after degassing
Volume of the 80 L Rn detector
Purge air volume
Correction factor due to Rn decay during concentration andtransfer processes

1.24 ± 0.10
0.24 ± 0.04
0.27 ± 0.04

𝑡con
𝑡total𝐹𝑎 (𝐹𝑤 )𝐶PAD
𝑉det
𝑉purge𝛽corr

0.080
𝐹𝑎 × 𝑡conexp(−𝜆𝑡total )𝜀mixing𝜀trap
𝜀rel

See Table 2
0.99 ± 0.01
0.99 ± 0.01

Extraction column Rn extraction efficiency
Charcoal trap trapping efficiency
Charcoal trap release efficiency

the intrinsic background of the Rn detector itself, defined as 𝐵detectorin Eq. (3). Possible background sources are (A) residual Rn in the
G1-grade pure air, (B) Rn emanation from the inner surface of the
air flow line, including the electrical dehumidifier and copper wool
traps, (C) emanation from the inner surface of the extraction column,
including the PVC acrylic vessel, (D) emanation from the charcoal, and(E) emanation from the components of the water sampling line, such as
the nylon tube and the water pump. The total background in the system
is given by 𝐵system = 𝐵air−line + 𝐵column + 𝐵water−line , where 𝐵air−line is the
backgrounds (A), (B) and (D), 𝐵column is the background from (C), and𝐵water−line is that from (E).
4.1. Backgrounds from the air lines
In order to evaluate 𝐵air−line , we performed measurements after
bypassing the extraction column. The valves before and after the extraction column were closed and air was supplied directly to thedehumidifier, copper wool traps, and the charcoal trap during the concentration process. We then performed the transfer and measurement
processes and obtained 𝐵air−line from

Fig. 8. Measurement results. Super-K detector supply water Rn concentration (red),that of its return water (pink), and water from its center (black). The horizontal axis
shows the elapsed time and the vertical axis shows the Rn concentration as measured
with the 80 L Rn detector. The fitted curves are defined by Eq. (3).

the initial total radioactivity 𝐴𝑉det should be the same as 𝐶PAD 𝑉purge ,we obtain the Rn concentration in the purge air after degassing as
𝐶PAD = 𝐴𝑉det ∕𝑉purge . Here, the efficiencies of Rn trapping (𝜀trap ) andrelease (𝜀rel ) are assumed to be 0.99 ± 0.01, since the former (latter)reaches almost 100% when the trap is cooled down to −60 ◦ C (baked
at more than +120 ◦ C) [60], as mentioned above. Then, we derived
the Rn concentration in the sampled water by dividing 𝐶PAD by theRn extraction efficiency. Since the concentration and transfer processes
take hours, Rn decay during the entire measurement must also be
considered. We used the correction term 𝛽corr = exp(−𝜆𝑡total ) for thisanalysis, where 𝑡total is the total time required for both the concentration and transfer processes. Finally, we obtain the Rn concentration of
the sampled water (𝐶sample ) from
𝐶sample =𝐹𝑎𝑉𝐴
1× det ××,
𝛽corr 𝑉purge 𝜀mixing 𝐹𝑤 𝜀trap 𝜀rel

𝐵air−line =𝑉𝐴
1× det ×,𝛽corr 𝑉purge 𝜀trap 𝜀rel

(5)

where 𝐴, 𝛽corr , 𝑉det , 𝑉purge , 𝜀rel and 𝜀trap are listed in Table 4.
We performed four measurements for different durations of theconcentration process (19, 21.5, 22, and 24 h). During the concentration
process of these measurements, we cooled the charcoal trap down to−70 ◦ C as usual. After this process, we baked the charcoal trap torelease the absorbed Rn and transferred the accumulated Rn to the
Rn detector as usual. These measurements give the total backgroundcaused by intrinsic background from purified air, Rn emanation fromthe charcoal, and from the system itself. Using Eq. (5) we estimatedthe backgrounds from the air line to be 𝐵air−line = 0.013 ± 0.002 mBq∕h,which is also listed in Table 5. Since this background is proportional tosome extent to the total duration of the concentration process (𝑡con ),this estimated background can be normalized by the total amountof air used and can be expressed as 𝐵air−line = 0.11 ± 0.02 mBq∕m3 .
Accordingly, the intrinsic Rn concentration in the G1-grade pure air
is below this level.

(4)

where the parameters used in this equation are listed in Table 4.
4. Backgrounds and systematic uncertainties

4.2. Backgrounds from the extraction column and water lines
Although all materials and system components were carefully selected, a Rn background still exists due to air leaks and emanationfrom contaminants on the inner surfaces of the setup. To evaluate thisbackground we performed several dedicated measurements. Note that
this background, which we define as 𝐵system , is to be distinguished fromIn order to determine 𝐵mixer + 𝐵water−line , a closed-loop running [40,44], where Rn-degassed water is sent to the extraction column repeatedly, is used. From Eq. (2), the Rn concentration in the degassed water
after one extraction cycle is given by 𝐶𝑤 = 𝐶𝑤,0 (1 − 𝜀mixing ) = 𝐶𝑤,0 𝑝,
7

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Nuclear Inst. and Methods in Physics Research, A 977 (2020) 164297

Table 5
Summary of background evaluated in Section 4. As described in the main text, the total
background is proportional to the total duration of measurement processes. Therefore,
the hourly background is listed in the second column. In addition to this, the Rnconcentration of background is also calculated by normalizing total amount of usedair (sampled water) as listed in the third column.
BackgroundmBq∕hmBq∕m3𝐵air−line
𝐵column + 𝐵water−line

0.013 ± 0.002
0.143 ± 0.027

0.11 ± 0.01
0.67 ± 0.12

Table 6
Summary of the systematic uncertainties for the measurement system.
SourceSystematic uncertainty

Rn extraction efficiency (𝜀mixing )80 L Rn detector calibration
Difference among three 80 L Rn detectors
Water flow rate (𝐹𝑤 )Air flow rate (𝐹𝑎 )Rn trapping efficiency (𝜀trap )Rn release efficiency (𝜀rel )Table 1
±5.7% [50]±10.0% [50,51]
±2.0%
±2.0%
±1.0%
±1.0%

Fig. 9. Measured Rn concentrations in various Super-K water samples as a function oftime. The black crosses, red squares, green upward-pointing triangles, blue downwardpointing triangles, and pink circles show results for the center region, the supply water,water sampled from the surface of the tank water, the bottom region, and the return
water, respectively.

where 𝐶𝑤 , 𝐶𝑤,0 , and 𝜀mixing are as defined in Section 3.3. Here 𝑝 is
(1 − 𝜀mixing ). The Rn concentration in the sampling water just before
the extraction column is
𝐶𝑤,0 = 𝐶sample + 𝐵water−line ,the supply water were done using a sampling port located after the laststage of the water circulation system (after HE4 in Fig. 1). Using datataken from June 2014 to October 2015, the Rn concentration in the
supply water was found to be stable at 𝐶Supply = 1.74 ± 0.14 mBq∕m3 asshown in Fig. 9.

(6)where 𝐶sample is the concentration in the sampled water. The concentration in the Rn-degassed water is then𝐶degassed = 𝑝(𝐶𝑤,0 + 𝐵column ) = 𝑝(𝐶sample + 𝐵water−line + 𝐵column ).

5.2. ID bottom(7)

Since water is supplied to the Super-K tank through ID via inlets
at its bottom (Fig. 2), measurements of the Rn concentration there
should track those of the supply water itself. Water is sampled from thisregion by inserting a 1/2-in. nylon tube from the top of the detectorat (𝑥, 𝑦, 𝑧) = (+0.353, −0.707) m and lowering it to 𝑧 = −12.000 m.
This location is also used to calibrate the detector near its energy
threshold [16,62] and therefore Rn backgrounds in this area are ofparticular interest. We performed several measurements as shown in
Fig. 9 and found 𝐶ID−bottom = 2.63 ± 0.22 mBq∕m3 . Note that this is ahigher concentration than that of the supply water, the implications ofwhich are discussed in the next section.

After performing the extracting processes 𝑛 times (i.e., after looping
through the system 𝑛 times), the Rn concentration in the degassed
water (𝐶𝑛−looped ) can be written as
𝐶𝑛−looped = 𝑝𝑛 𝐶sample +1 − 𝑝𝑛(𝐵+ 𝐵column ).
1 − 𝑝 water−line(8)

Therefore, after a large number of mixings, the concentration,
𝐶closed−loop , reaches an equilibrium that is determined by the background and the extraction efficiency:
𝐶closed−loop =1(𝐵+ 𝐵column ).
1 − 𝑝 water−line(9)5.3. Center of the Super-K tank

In order to prepare water for the closed-loop running, we conducted
this operation for more than 6 h (𝑛 > 50) and measured the Rnconcentration following the method described in the previous section.
The result is 𝐶closed−loop = 0.196 ± 0.028 mBq∕h for a water flow rate
𝐹𝑤 = 3.58 L/min. Thus, we obtain the background as 𝐵water−line +𝐵column = 0.143 ± 0.027 mBq∕h as listed in Table 5. We use this value
in the measurements below. Moreover, the estimated background isnormalized by the total amount of sampled water and expressed as
𝐵water−line +𝐵column = 0.67±0.12 mBq∕m3 , which corresponds to the total
background when 1 m3 of water is sampled (about 5 h for 3.58 L/min,bout 4 h for 4.0 L/min.). Since the inner surface of the extraction
column has been electro-polished, the Rn emanation rate from thispart of the system is expected to be low. In contrast, we expect thebackground from the PVC vessel to be relatively high and it may be themain background source. Systematic uncertainties for measurementswith this system are summarized in Table 6.

Typically the center region of the Super-K detector is the most radiopure, since it is far from the detector walls, water inputs and returns.
As a result, it is the best candidate for studying very low energy solar
neutrino interactions and consequently it is essential to understandbackgrounds from any residual Rn daughters therein. Accordingly,
water was sampled at the same (𝑥, 𝑦) location as discussed above, but
at 𝑧 = +0.400 m in order to study the center of the detector. The results
are shown in the thick black line of Fig. 9. As expected, the Rn concentration in this region is quite low, often consistent with zero within
the measurement errors. The measurement shows a negative central
value, which has been attributed to fluctuations of the background.
We note that in these cases the upper limit on the concentration is
𝐶Center < 0.23 mBq∕m3 (95% C.L.).
5.4. Return water

5. Radon concentration measurements
The SK-IV water circulation system pumps water out of the tank for
re-purification via outlets placed at the top and bottom of the detector(Fig. 2). This return water is a mixture of water from the OD surface,
the OD barrel (i.e. the OD outlet shown in the right panel of Fig. 2), theID top, and the OD bottom. The corresponding flow rates are listed inTable 7. Since a significant fraction of the water is from the OD, which
is surrounded by Rn sources, including the tank lining and cavern

5.1. Supply water
Since Rn contamination in the Super-K supply water can be a potentially dangerous source of backgrounds, monitoring its concentration
is essential to understand those backgrounds and the stability of thedetector’s response to them. Measurements of the Rn concentration in8

Y. Nakano, T. Hokama, M. Matsubara et al.

Nuclear Inst. and Methods in Physics Research, A 977 (2020) 164297

Fig. 10. Relationship between the extracted Rn concentration and the total amount of sampled water. The vertical axis shows the extracted Rn concentration after correcting forthe time factor 𝛽corr and the lower (upper) horizontal axis shows the total amount of sampled water (total duration of the concentration process, 𝑡con in Table 4). The left figure
shows results for water sampled from the tank, as well as for the closed-loop water, at a flow rate 𝐹𝑤 = 3.58 L/min. Similarly, results for the supply and return lines at a flowrate of 𝐹𝑤 = 4.0 L/min are shown in the right figure.
Table 8
Summary of the measurement results obtained from the methods of Eqs. (4) and (10),as well as the 𝜒 2 per degree of freedom and corresponding 𝑝-value from the latter.

Table 7
Super-K water flow. As shown in Fig. 2, the return water is a mixture of the waterfrom the OD surface, ID+OD top, and OD bottom regions.
Position

Water flow rate [ton∕h]

OD surface
ID+OD top
OD bottom

12
36
12

Return60

rock, it is important to measure the return water’s Rn concentration
to identify potential background sources in the ID.
Return water was sampled from a port located just after the pump
to send water from the tank back to the circulation system (returnpump in Fig. 1). The measured Rn concentration was 𝐶Return = 9.06 ±0.58 mBq∕m3 . It is clear that water that has passed through PMTs andthe detector structure has higher concentration than that of the supply
water. Comparison of the return and supply concentrations indicatesthat the water circulation system’s total Rn removal efficiency is 0.81 ±
0.08.

Eq. (10)[mBq∕m3 ]𝜒 2 ∕d.o.f𝑝-value

SupplyID bottom
CenterReturn1.74 ± 0.14
2.63 ± 0.22
<0.23 (95% C.L.)
9.06 ± 0.58

1.64 ± 0.50
2.63 ± 0.30
<0.26 (95% C.L.)
8.71 ± 1.96

5.053∕7
11.68∕540.76∕13
0.37∕5

0.653
0.039
0.010
0.996

(1 − 𝑝)𝐶closed−loop= 𝐵air−line + 𝐵column

0.67 ± 0.12

0.67 ± 0.10––

Table 9 shows a comparison of the Rn concentrations in the supply
and return water measured in this study as well as previous measurements [13,38,48]. The Rn concentrations in the supply water andreturn water during SK-IV are higher than those measured during SKI. However, the Rn reduction efficiency of the former (0.81 ± 0.08)
is comparable to that of the latter (∼0.80). Accordingly, the higher
Rn concentration in the SK-IV return water resulted in the higher
concentration in the supply water.
In the previous study, the Rn concentrations in the tank were
estimated using a Rn injection calibration method [13]. Table 10shows a comparison of Rn concentrations in the tank water. For theregion between ±0 m and +10 m an upper limit of <1.4 mBq∕m3
was obtained during SK-I. Further, the estimated Rn concentration
at 𝑧 = −6 m (𝑧 = −11 m) was 3.0 mBq∕m3 (5.0 mBq∕m3 ). On theother hand, the measured Rn concentrations with system used in thismanuscript are <0.23 mBq∕m3 in the center region (𝑧 = +0.4 m)
and 2.63 ± 0.22 mBq∕m3 in the bottom region (𝑧 = −12 m). Basedon those results, the measurement technique presented in the article
has improved the measurement sensitivity to the Rn concentration in
purified water comparing with the past techniques.
The results are summarized in Table 10 and suggest that the Rnconcentrations in the center region and the bottom region, which areused for solar neutrino analysis, are clearly lower in SK-IV. Therefore,
we conclude that we have successfully reduced the Rn in the bothregions by optimizing both the supply water temperature and thewater circulation rate. On the other hand, this optimization results inhigher Rn concentrations in the supply water and in the return water,
indicating that the dominant sources of Rn are in the Super-K tank itself
and not in the water system or buffer gas (see below).
Further discussion related to Rn sources in the water tank and theobserved background rate caused by Rn daughters in the Super-K solar
neutrino analysis will be presented in a subsequent publication.

As shown in Fig. 9, the Rn concentrations in various water samples
have been stable to within their measurement uncertainties. Assumingthat the concentrations are constant on the time scale of several hours,
we can test the validity of the measurement as follows. If the total
amount of Rn is proportional to the total amount of sampled water,
the accumulated Rn (𝐴∕𝛽corr ) should be a linear function of the total
sampled water volume (integrated flow). The slope of this function
corresponds to the Rn concentration in the sampled water. Fig. 10shows the accumulated Rn concentration as a function of the total
amount of sampled water and displays the expected linear relationship.
By fitting the slope, the Rn concentrations in the sampled water canbe derived from the following equation, independent of Eq. (4):𝑉det × 𝑠𝑙𝑜𝑝𝑒.
𝜀trap 𝜀rel 𝜀mixing

Eq. (4)[mBq∕m3 ]5.6. Comparison with past measurements

5.5. Measurement verification

𝐶sampled =Sampled water(10)

The Rn concentrations obtained with this method are summarized inTable 8 and compared with those from Eq. (4) after subtracting thebackgrounds described in Section 4. The table also lists the 𝜒 2 values
and corresponding 𝑝-values from the fits. Regarding the latter, wenote that with the exception of the center region, which has sufferedfluctuations in the background, each fit is compatible with the hypothesized linear relationship. Further, the consistency of results acrossmeasurement methods indicates that the observed Rn concentrations in
the Super-K water are accurate at the mBq∕m3 level.
9Y. Nakano, T. Hokama, M. Matsubara et al.

Nuclear Inst. and Methods in Physics Research, A 977 (2020) 164297

Table 9
Summary of the measured Rn concentration in supply water and return water.
Sampling water

Phase

Result [mBq∕m3 ]MethodSupplySupplySupplyBeginning of SK-I
End of SK-I
SK-IV (This study)<3.2
0.4 ± 0.2
1.74 ± 0.14

70 L Rn detector w/ plastic ball [13,38]
700 L Rn detector w/ membrane [14,48]
80 L Rn detector w/ extraction column

ReturnReturnReturnBeginning of SK-I
End of SK-I
SK-IV (This study)

<5.0
<2.0
9.06 ± 0.58

70 L Rn detector w/ plastic ball [13,38]
700 L Rn detector w/ membrane [14,48]
80 L Rn detector w/ extraction column

Table 10
Summary of the measured Rn concentration in the tank water.
Detector position

Phase

Result [mBq∕m3 ]MethodTank water0 ≤ 𝑧 ≤ 10 m
𝑧 = −6 m
𝑧 = −11 m

Beginning of SK-I
Beginning of SK-I
Beginning of SK-I
Beginning of SK-I

<5.7
<1.4
3.0
5.0

70 L Rn detector w/ plastic ball [13,38]
Rn injection calibration [13]Rn injection calibration [13]Rn injection calibration [13]𝑧 = +0.4 m
𝑧 = −12 m

SK-IV (This study)
SK-IV (This study)<0.23
2.63 ± 0.22

80 L Rn detector w/ extraction column
80 L Rn detector w/ extraction column

6. Other OD measurements

values of the water flow, we additionally found the efficiency has no
dependence on the air flow rate.
Combining the extraction column with an existing 80 L Rn detector
we have constructed a new Rn measurement system for the Super-K
water. Using this system, we measured the Rn concentration at several
places in the Super-K tank and the water system with a backgroundof 0.143 ± 0.027 mBq∕h, which corresponds to 0.67 ± 0.12 mBq∕m3 when
1 m3 of water is sampled. During the period from June 2014 to October
2015, the Rn concentrations were stable at 1.74 ± 0.14 mBq∕m3 inthe supply water, <0.23 mBq∕m3 (95% C.L.) in the ID center, 2.63 ±0.22 mBq∕m3 at the bottom of the ID, and 9.06 ± 0.58 mBq∕m3 in the
return water. Comparing the supply water with the return water, weconclude that the dominant Rn sources are in the Super-K tank rather
than in the water system or buffer gas.
The method developed in this study will enable other solar neutrino
detectors, such as SK-Gd and Hyper-Kamiokande [65], to monitor theRn concentrations in their purified water in detail.

The bottom region of OD may also have a large Rn concentration because dust from the detector volume may settle and accumulate there.
Although measurements of its water may provide hints at possible Rnsources in the tank, it is impossible to directly sample water from thisregion.
However, the Rn concentration in the OD bottom water can be indirectly estimated since part of the return water is taken from this region(see Table 7). The estimation proceeds via the following equation:
𝐶OD−bottom=()
𝐶return 𝐹return − 𝐶OD−surface 𝐹OD−surface + 𝐶ID−top 𝐹ID−top + 𝐶OD−barrel 𝐹OD−barrel
𝐹OD−bottom

,

(11)

where variables beginning with 𝐶 represent the Rn concentrations ofeach water sample, and similarly, those with 𝐹 represent the flow
rates. In order to obtain 𝐶OD−barrel for this estimate, we sampled waterfrom the barrel region of the OD barrel by inserting a sampling tube
at (𝑥, 𝑦, 𝑧) = (+17.321, −3.535, +17.000) m on November 6, 2014 andmeasured the Rn concentration of 3.48 ± 0.60 mBq∕m3 . Although thewater tank in the top region of ID has not been measured directly,
we estimate its Rn concentration to be equal to that of the ID center,i.e., 𝐶ID−top ∼𝐶center , based on Super-K low energy background data [9,63,64]. Using these measurements and assumptions, we estimate theconcentration in the OD bottom to be 𝐶OD−bottom = 33.97±3.30 mBq∕m3 .
Note that this region is expected to be the least radio-pure region in the
Super-K tank.
In the Super-K tank there is a buffer gas layer between the surfaceof the OD water and the top of the tank [14]. In previous publications [50], we have concluded that Rn contamination from the Super-K
tank itself is the dominant source of Rn in this buffer gas. This suggeststhat Rn in the buffer gas, whose concentration was measured to be
28.8 ± 1.7 mBq∕m3 , dissolves into the OD top water. In order to confirm
this Rn contamination directly, we measured the Rn concentration at
depth of 20 cm below the surface of the tank water by inserting anylon tube into the calibration hole at (𝑥, 𝑦) = (−0.950, −1.064) m. The
resulting Rn concentration was 2.51 ± 0.47 mBq∕m3 , indicating thatthe surface water is not the main source of Rn in the buffer gas andsuggesting further the detector structure is the primary Rn source.

CRediT authorship contribution statement
Y. Nakano: Writing - original draft, Investigation, Validation, Formal analysis, Visualization. T. Hokama: Investigation. M. Matsubara:
Software. M. Miwa: Investigation. M. Nakahata: Supervision. T. Nakamura: Investigation. H. Sekiya: Resources, Investigation, Writing review & editing. Y. Takeuchi: Methodology. S. Tasaka: Conceptualization, Methodology. R.A. Wendell: Writing - review & editing.
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
The authors would like to thank the Super-Kamiokande collaboration for their help in conducting this study. Especially, we thankM. Miura who supported the scheduling to open the tank. We acknowledge the cooperation of the Kamioka Mining and Smelting Company. We would like to thank the reviewers for their thoughtful comments and careful review, which helped improve our manuscript. Y. N
thanks M. Kanazawa for helping the work to assemble the measurement
system at the experimental area of the Super-K. In addition, Y. N
thanks N. Nozawa, T. Onoue, Y. Tamori, T. Higashi and T. Ushimaru
for transporting air bottles. Y. N thanks K. Watanabe and H. Nagao
who supported the calibration works. Y. N also thanks M. Ikeda,

7. Conclusion and future prospects
We have developed a new technique for measuring ultra-low Rnconcentrations in purified water. For this purpose, we developed andcalibrated an extraction column to extract Rn from water with extraction efficiencies of 0.64 ± 0.03 for a water flow rate of 𝐹𝑤 = 4.0 L∕minand 0.73 ± 0.04 for 𝐹𝑤 = 3.58 L∕min when 𝐹𝑎 = 2.0 L∕min. For fixed10

Y. Nakano, T. Hokama, M. Matsubara et al.

Nuclear Inst. and Methods in Physics Research, A 977 (2020) 164297

T. Yano, G. Pronost, S. Ito, K. Iwamoto, Y. Suda, A. Orii, R. Akutsu,
D. Fukuda, C. Xu, K. Ito and Y. Sonoda who supported the water
sampling work. This work is partially supported by the inter-university
research program at ICRR, Japan. This work is partially supported by
MEXT KAKENHI Grant Number 26104008, 17K17880, and 18H05536.

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