Nuclear Instruments and Methods in Physics Research A 867 (2017) 108–114

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A
journal homepage: www.elsevier.com/locate/nima

Measurement of radon concentration in super-Kamiokande's buffer
gas
Y. Nakano a,n, H. Sekiya b,c, S. Tasaka b, Y. Takeuchi a,c, R.A. Wendell d,c, M. Matsubara e,
M. Nakahata b,ca

Department of Physics, Kobe University, Kobe, Hyogo 657-8501, Japan
Kamioka Observatory, Institute for Cosmic Ray Research, The University of Tokyo, Gifu 506-1205, Japanc
Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, University of Tokyo, Kashiwa,
Chiba 277-8583, Japand
Department of Physics, Kyoto University, Kyoto 606-8502, Japane
Information and Multimedia Center, Gifu University, Gifu 501-1193, Japanb

art ic l e i nf o

a b s t r a c t

Article history:
Received 20 December 2016
Received in revised form
21 April 2017
Accepted 22 April 2017
Available online 24 April 2017

To precisely measure radon concentrations in puriﬁed air supplied to the Super-Kamiokande detector as
a buffer gas, we have developed a highly sensitive radon detector with an intrinsic background as low as
0.33 ± 0.07 mBq/m3. In this article, we discuss the construction and calibration of this detector as well as
results of its application to the measurement and monitoring of the buffer gas layer above Super-Kamiokande. In March 2013, the chilled activated charcoal system used to remove radon in the input buffer
gas was upgraded. After this improvement, a dramatic reduction in the radon concentration of the supply
gas down to 0.08 ± 0.07 mBq/m3. Additionally, the Rn concentration of the in-situ buffer gas has beenmeasured 28.8 ± 1.7 mBq/m3 using the new radon detector. Based on these measurements we havedetermined that the dominant source of Rn in the buffer gas arises from contamination from the SuperKamiokande tank itself.
& 2017 Elsevier B.V. All rights reserved.

Keywords:
Radon
Super-Kamiokande
Solar neutrino
Electrostatic collection

1. Introduction
Super-Kamiokande (Super-K) is a water Cherenkov detector
containing 50,000 t of ultra-pure water and 11,129 20-in-diameterphotomultiplier tubes that observe the light produced by charged
particles traversing the detector's inner volume [1,2]. To study theinteractions of neutrinos produced in the atmosphere, solar interior, and cosmos, the detector is situated 1000 m underground(2700 m water equivalent) in Gifu prefecture, Japan. With thecosmic ray background largely suppressed at this depth, Super-K isable to observe the recoil electron produced by 8B solar neutrinos(Eν ≤ 15 MeV) scattering elastically with electrons in the detectors
water. As these recoiling electrons preserve the direction of theirparent neutrinos, their measurements played a critical role in establishing neutrino oscillations as the solution to the “solar neutrino problem” [3]. While our understanding of solar neutrinos has
improved dramatically since then [4,5], the predicted distortion of
the energy spectrum induced by the effects of solar matter onnCorresponding author. Present address: Kamioka Observatory, Institute for
Cosmic Ray Research, The University of Tokyo, Gifu 506-1205, Japan.
E-mail address: ynakano@km.icrr.u-tokyo.ac.jp (Y. Nakano).

http://dx.doi.org/10.1016/j.nima.2017.04.037
0168-9002/& 2017 Elsevier B.V. All rights reserved.

their oscillation, the so-called Mikheyev-Smirnov-Wolfenstein(MSW) mechanism [6,7], is yet to be conﬁrmed completely. Although current data is well described by the predictions of theMSW model at low energies, where so-called vacuum oscillations
are dominant [8–15] as well as high energies, where matter effects
dominate [16–24], the shape of the electron neutrino survival
probability in the transition region between these two energy
regimes has not been determined yet. At present several alternative models have been proposed, such as sterile neutrino models[25,26], mass varying neutrinos [27], and non-standard interactions [28,29], all of which explain the current experimental data
but provide different predictions in the transition region. In order
to different between these and the standard MSW effect, precision
solar neutrino measurements at intermediate energies are needed.
At the same time, Super-K's underground location makes it
subject to similar-energy backgrounds produced by radioactive
elements in the host mine's air, water supply, and surroundingrock. One of major backgrounds in the kinetic energy regionE ≤ 6 MeV is electron events produced by radon (222Rn). Thesebackgrounds are partially mitigated by continuously circulating
and purifying the detector water and by supplying a layer of
puriﬁed air between the top of the steel structure deﬁning the

Y. Nakano et al. / Nuclear Instruments and Methods in Physics Research A 867 (2017) 108–114

detector and the tank water as a buffer gas [1]. Although recent
upgrades to both of these systems [2] and the Super-K front end
electronics [30] have resulted in an analysis energy threshold of
3.5 MeV (electron kinetic energy) [19], in order to observe the
MSW spectrum distortion, the threshold must be reduced further.
Currently, the threshold is limited by a few mBq/m3 of Rn estimated to exist in the 4.0–6.0 MeV energy region of the analysis[31,32]. Reducing the threshold requires that this concentration be
reduced to less than 0.1 mBq/m3 and motivates careful analysis ofpotential radon sources. Dissolution of Rn into the detector waterfrom the buffer gas layer is one such source. In this article, we
discuss studies of the Rn concentration in the detector buffer gas
layer using a new detector designed for precise measurements.
This paper is organized as follows. In Section 2, the designs of
the new 80 L Rn detector and its data acquisition system are described. In addition, the detection method and typical pulse heightspectra are discussed. In Section 3, the developed calibration setup
and the calibration results are described. In Section 4, the detector's intrinsic background level is brieﬂy discussed. In Section 5,
a Rn monitoring system to measure the Rn concentration in thebuffer gas as well as an improved Rn-reduced air system in SuperK are described. Then, the results of measurements taken from
December 1st , 2012 to April 9th , 2014 are presented. Finally, thestudy is concluded and its future prospects are discussed in Section 6.

2. Ultra sensitive 80 L Rn detector
While previous Rn measurements at Super-K used detectors ofsize 70 L [33] and 700 L [34], limitations in their measurement
capabilities led to the development of an 80 L detector in thecurrent study [35]. Like its predecessors, the Rn concentration of
the sample air ﬁlling the detector volume is measured by electrostatically trapping 222Rn decay products, namely, 210Po(5.30 MeV), 214Po (7.69 MeV), and 218Po (6.00 MeV), and then, a
PIN photodiode is used to observe their subsequent α decays. With
both the highest α energy of the three daughters and a high efﬁciency for electrostatic collection, 214Po is used as the primary
signal for this study. The sample's Rn concentration is proportional
to the number of observed α decays from this daughter.
2.1. Structure of the new radon detector
The detector consists of an 80 L stainless steel vacuum vessel
with a PIN photodiode connected to a ceramic feedthroughmounted on the top ﬂange of the vessel. The photodiode is offset
from the bottom of the feedthrough by 5 cm. A high-voltage divider and ampliﬁer circuitry rests atop this ﬂange and is connectedto the external end of the feedthrough. In this conﬁguration, thep-layer of the PIN photodiode is biased at −1.9 kV (discussed laterin Section 3.3), while the vessel is held at ground. Fig. 1 shows the
structure of the detector.
Measurement limitations suffered by predecessor detectors
due to materials used in their construction have been largely
overcome in designing the new detector. For instance, Vitons
O-rings, which were used as gaskets, have been replaced in thecurrent design with metal gaskets because Vitons is known toemanate Rn. Similarly, acrylic ﬂanges used in earlier designs havebeen replaced with knife-edge ﬂanges with metal gaskets (CFﬂanges) to reduce the lower limit of the vessel's vacuum from
10 kPa to <1.0 × 10−5 Pa . To further reduce backgrounds intrinsic
to the detector itself, the inner surface of the stainless vessel has
been electropolished to a roughness of less than 0.8 mm. It should
be noted that while it is advantageous to improve the performance

109

Fig. 1. The structure of the 80 L Rn detector.

the present detector's 700 L predecessor, electropolishing its larger
surface area and ﬁtting it with a CF ﬂange with a 100 cm are bothcost and labor intensive. For this reason, the present work seeks toestablish the capabilities of these upgrades with the smaller 80 L
detector.
In the current design, the PIN photodiode has been upgraded toa HAMAMATSU-S3204-09 with dimensions 18 mm  18 mm,
whose window has been removed to reduce energy loss as α
particles strike the photodiode. Rn emanation from the high-voltage feedthrough has additionally been reduced by switching its
ceramic to high-purity alumina (Al2O3).
2.2. Electronics circuit and data acquisition system
The readout electronics for the 80 L Rn detector are similar to
those used in earlier detectors [34]. However, the previous charge
sensitive preampliﬁer, CLEAR PULSE CS513-1, has been replacedwith the CS515-1 model. Working with CLEAR-PULSE Co. Ltd, a
new data acquisition system has been developed, which not only
records signals from the Rn detector, but also monitors several
environmental parameters which affect the Rn detection efﬁciency, such as the temperature, dew point, and ﬂow rate of thesample air. The data logger (CP 80278) consists of one CPU board(CP 1233), two shaping ampliﬁer boards (CP 4485), two pulse
height analyzer boards (CP 1234), and two analog-to-digital converter (ADC) boards (CP 1235).
2.3. Radon spectrum and event selection
An integrated pulse height spectrum from a background run
performed with pure air (c.f. Section 4) is shown in Fig. 2. There
are ﬁve peaks in the spectrum originating from either 222Rn or
220
Rn daughters. Here, 210Po, 214Po and 218Po are the decay products of 222Rn while 212Po and 216Po are the decay products of220
Rn. In the measurement presented below the 214Po signal(7.69 MeV ) is used to calculate 222Rn concentrations because there
are no other α sources with energies near its peak value; theclosest being the 6.78 MeV α from 216Po. In addition, 214Po is
known to have a higher collection efﬁciency than 218Po [33–35].
The 214Po signal region in the pulse height spectrum is determined using the following criteria. In order to prevent contamination by 216Po events due to the resolution of the PIN photodiode, a lower bound on the signal region is deﬁned as ﬁve ADC
channel above the peak position of the 216Po signal. Loss of higher

110

Y. Nakano et al. / Nuclear Instruments and Methods in Physics Research A 867 (2017) 108–114

Fig. 3. Schematic diagram of the calibration system. Arrows indicate the directionof air ﬂow.
222

220

Fig. 2. Typical energy spectrum from
Rn andRn daughters. The region between ADC channels 158–178 and shown by the red dashed lines deﬁnes the 214Po
signal region used to calculate the 222Rn concentrations for this particular detector.
(For interpretation of the references to color in this ﬁgure legend, the reader isreferred to the web version of this article.)energy 214Po signals is prevented using an upper bound, chosen to
be ﬁve ADC channels above the 214Po peak position. These cuts
provide near 100% selection efﬁciency for the signal with minimal
backgrounds from the tail of the 216Po spectrum. However, we
revised the selection criterion in order to accumulate more 214Po
signals. Contamination from the latter represents a negligible
source of measurement uncertainty in comparison to other systematic errors discussed below. We note that in the previous
publication [35], the range of ADC channels deﬁning the signal
region was determined using other criteria and resulted in a 214Po
detection efﬁciency of only 90%. For the spectrum shown in Fig. 2,
the signal region for 214Po α decay is taken to be ADC channels
158–178, which corresponds to energies between 7.03 and7.92 MeV.

3. Detector calibration
3.1. Calibration factor
In order to calculate the 222Rn concentration from the observed
Po count rate, the 80 L Rn detector must be calibrated. A calibration factor (CF) is deﬁned as follows:214

CF [(counts/day)/(mBq/m3)] =measured 214Po signal rate.
222
Rn concentration(1)

Here, the numerator is the measured 214Po signal rate on the PIN
photodiode in units of counts/day , while the denominator is the222
Rn concentration to be determined in the unit of mBq/m3. The
CF forms the core of the following measurements, and its initialvalue is calibrated using a Rn source of well-measured activity as
described below.
3.2. Calibration setup for 80 L Rn detector
The calibration system constructed at Kamioka observatory is
shown in Fig. 3. Using this calibration system, both the high-voltage and humidity dependence of the CF were measured (see also[35]).
The system consists of the 80 L Rn detector, a cold bath tocontrol the dew point in the target air, an air circulation pump, a
dew-point meter (VAISALA, DMT340), a mass ﬂow controller(HORIBA STEC Z512), a reference radon monitor (ionizationchamber, SAPHYMO Alpha GUARD PQ2000), and a 222Rn source(PYLON RNC). During calibrations 226Ra with an activity of 199 Bq±4% is used to maintain radioactive equilibrium within the entire
system. Two stainless 0.5 in. U-shaped pipings are used as heatexchangers to control the system's dew-point. They are placed inan ethanol bath with temperature control provided by a refrigerator (THOMAS,TRL-107SLC). The temperature and dew pointof the air through the entire system are monitored by the dew
point meter. To suppress emanation of 222Rn from the piping, allpiping used in the system is made of electropolished stainless steel(SUS EP tube). All connections use VCRs gaskets to prevent air
leakage and contamination from environmental air. Commerciallyavailable G1-grade high-purity air (impurity concentration<0.1 ppm ) is used to minimize background 222Rn in the calibration
ﬁll air itself. The ﬁll air is continuously circulated at 1 atm with a
ﬂow rate of 0.8 L/min controlled by the mass ﬂow controller. It
should be noted that the system's pump is only capable of circulating air at this ﬂow rate.
3.3. High-voltage dependence of the calibration factor
As an electric ﬁeld is formed between the PIN photodiode andthe stainless steel vessel by the supplied high voltage, the electrostatic collection efﬁciency directly depends on the suppliedhigh voltage. Accordingly, the detector calibration was conductedwith various supply voltages to test this dependence. First, theentire system was evacuated down to 1.0 × 10−5 Pa without thereference Rn monitor and without the 226Ra source. Then, thesystem was ﬁlled with G1-grade air at atmospheric pressure andthe valves in front of and behind the 226Ra source were opened
and air circulation was started. Finally, after the closed systemreached radioactive equilibrium, the Rn concentration measured
by the reference Rn monitor and the number of 214Po counts in the
80 L Rn detector were compared in order to determine the CF. The
supplied high voltage was varied from −0.2 to −2.0 kV during thiscalibration. Fig. 4 shows the applied high-voltage dependence of
the CF. During calibration, the dew point was maintained at a value
measured to be 0.0021 g/m3.
Although the CF is not saturated at −2.0 kV , this voltage waschosen for other calibrations and measurements because it is thehighest that can be applied. However, the circuit's high-voltage
divider also supplies a reverse bias to the PIN photodiode [34],which increases as the supplied high voltage increases. The bias
voltage is −0.1 kV , the limit of the photodiode's tolerance, at a
supply voltage of −2.0 kV . This conﬁguration therefore provides−1.9 kV for the generation of the detector's electrostatic ﬁeld.
3.4. Humidity dependence of the calibration factor
Because the positively charged 222Rn daughter nuclei are captured and neutralized by water in air, the detection efﬁciency of
the Rn detector also depends on the absolute humidity [36].

Y. Nakano et al. / Nuclear Instruments and Methods in Physics Research A 867 (2017) 108–114

111

Table 1
Summary of systematic uncertainties.

Fig. 4. High-voltage dependence of the CF for the electrostatic collection method ofpure air. These data were obtained at an absolute humidity of 0.0021 g/m3. The bars
show both statistical and systematic errors.

SourceMeasured value Assigned standard
uncertainty

Reference Rn monitor

2.2 kBq/m3±0. 1 kBq/m3

Dew point meter
Estimation of the wholevolume−75 °C
85 L±(2 ∼ 3) °C
72 L

Subtracting background0.33 mBq/m3±0.07 mBq/m3

GUARD monitor and found them to agree within ±5% . As a result
this value is used for the uncertainty of the reference monitor in
the present analysis. In addition the dew-point meter has an uncertainty of ±2% and the uncertainty arising from the estimation of
the total volume in the calibration setup due to, for instance, theuncertainty in the volume of its ﬂexible tubing, is taken to be ±2% .
Based on these uncertainties, the total systematic uncertainty istaken to be ±5.7% for each CF measurement. The uncertainty forthe calibration is summarized in Table 1.

4. Intrinsic background levels
After these calibrations, the background level intrinsic to the
80 L Rn detector itself was evaluated. Data were taken over
156 days with no input Rn source. During this time period theaverage absolute humidity was 0.078 g/m3 and the observed
0.74 ± 0.07 (statistical uncertainty only) counts/day for the 214Po
decay corresponds toFig. 5. Absolute humidity dependence of the CF (red). Calibration data for the 80 L
Rn detector were taken at −2.0 kV . The bars show both statistical and systematic
errors. The solid line (red) shows the best-ﬁt function. The dashed line (black)shows the old result, which was obtained by the old 70 L Rn detector at −1.5 kV[33]. (For interpretation of the references to color in this ﬁgure legend, the reader isreferred to the web version of this article.)

Therefore, calibrations to determine the humidity dependence of
the CF were conducted using the same setup and settings used forthe high-voltage dependence calibration. After the system reachedradioactive equilibrium, the dew-point temperature was controlled by the refrigerator in order to vary the absolute humidity
throughout the entire system. Fig. 5 shows the absolute humidity
dependence of the 80 L Rn detector's CF for air, which is seen to
decrease at higher absolute humidities as expected. While the CF
for earlier 70 L detectors was found to be constant above 1.5 g/m3,
the current detector demonstrates a humidity dependence even
above this value. Relative to the previous design, the current detector's larger CF at absolute humidities above 10−1 g/m3 is a consequence of the higher vacuum its CF ﬂanges provide. The data arefound to be best described by an empirical function [37]CF = 2.25 − 0.29 AH , where AH is the absolute humidity.
3.5. Uncertainty budget for the detector calibration
Several sources of systematic uncertainty are present in thecalibration of the CF. The reference monitor has an uncertainty of±5% . According to the manufacturer's documentation, the uncertainty of the Alpha GUARD calibration is ±3% [38]. However, inorder to conﬁrm this uncertainty, we compared the expected Rnconcentration of the measurement system in radioactive equilibrium with a calibration source with that measured by the Alpha

Cdetector,BG = 0.33 ± 0.07 mBq/m3.

(2)

This intrinsic background level is lower than that of the previous
70 L detectors ( 2.4 ± 1.3 count/day ) [33] by a factor of three. The
characteristic detection limit of the detector is estimated as
Lc = 1.64 × σBG/CF [ mBq/m3], where σBG is the uncertainty of the
background rate [34], which results in 0.54 mBq/m3 for a single
day measurement. The energy spectrum from the measurement isshown in Fig. 2 and is discussed further in later Sections.

5. Measurement of the Rn concentration in the Super-K buffer
gas layer
5.1. Introduction
In the Super-K tank, there is a buffer gas layer between thesurface of the water and the top of the tank [1] to prevent the tankseal from breaking due to the motion of its water during anearthquake. The height of the buffer gas layer is 60 cm and its total
volume is 753.6 m3.
Rn-reduced air system [34] supplies puriﬁed air for the Super-K
buffer gas layer, injecting it into the tank from an inlet at(x, y ) = ( + 18. 382 m, −4. 242 m) in the detector coordinates. Thissystem provides a gas ﬂow rate of 11 m3/h and maintains thepressure of the buffer gas layer at þ0.3 kPa relative to atmosphericpressure. Since the top of the Super-K tank is exposed to the domeof its experimental hall, where the Rn concentration is40 − 100 Bq/m3, continuous purging of the buffer gas in thismanner is necessary to suppress Rn emanation into the buffer
layer.

112

Y. Nakano et al. / Nuclear Instruments and Methods in Physics Research A 867 (2017) 108–114

Fig. 6. Schematic of the Rn concentration monitoring system for the Super-K buffergas.

5.2. Rn-reduced air system improvements
The Super-K Rn-reduced air system uses 50 L of chilled activated charcoal to remove Rn from throughput air. Since it is
known that the Rn-trapping efﬁciency of activated charcoal increases signiﬁcantly below temperatures around −60 °C [39], thesystem was upgraded in March 2013 to bring its cooling power
down from −40 °C [34] in the previous design, to below thisthreshold. In order to do so, the system's coolant was changed to
3 MNOVECTM7100 and its refrigerator upgraded.

Fig. 7. Rn concentration in the input air (light-green) and output air (red) of theSuper-K detector's buffer layer. The data set is from December 1st , 2012 to April 9th ,2014. Data from the shaded regions were used for the analysis of concentrations
before and after the improvement of the Rn-reduced air system. Error bars show
both statistical and systematic errors. (For interpretation of the references to color
in this ﬁgure legend, the reader is referred to the web version of this article.)

5.3. The monitoring system
An upgraded measurement system used for the measurementsof the buffer gas Rn concentration is shown in Fig. 6. Using thissystem, the Rn concentration both before and after the activatedcharcoal cooling system upgrade was measured. Two 80 L Rn detectors were set on the top of the Super-K tank, one to measure theRn concentration of the gas supplied to the tank (input air), andthe other to measure gas sampled from the buffer layer (output
air). The output air was sampled from the sampling port at(x, y ) = ( − 14.494 m, + 12.019 m), which is ∼23 m away from thebuffer gas inlet. Both the input and output air were sampled using
circulation pumps with ﬂow rates set by the mass ﬂow controllers
to 0.8 L/min .
Because the absolute humidity of the output air is above 5 g/m3,
a point not measured during the CF calibration, a thermoelectric
dehumidiﬁer (KELK DH-209C) was installed just before the Rn
detector to reduce the humidity, as shown in Fig. 6. The absolute
humidity after the installing the dehumidiﬁer was ~3. 5 g/m3.
5.4. Results
Monitoring of the Rn concentration in the buffer gas began on
December 1st , 2012. The results are summarized in Fig. 7 with theabsolute humidity of the sampled air during the same period
shown in Fig. 8. Gaps in the data are due to intermittent problems, including a laboratory-wide power failure, network connection issues, and problems with the data acquisition system.
During the period from May 31st to July 11th , 2013 failure of an
automatic valve in the Rn-reduced air system resulted in thebuffer gas accidentally bypassing the chilled activated charcoal
system. This in turn caused the Rn concentrations in both the
input and the output air to increase during this period. On September 16th , 2013, a power failure occurred as a result of athunder storm, and the activated charcoal was not subject tocooling for more than four hours. After we resumed running the

Fig. 8. Absolute humidity from the input (light-green) and output (red) December
1st , 2012 to April 9th , 2014. Data from the shaded regions were used for the analysis
of concentrations before and after the improvement of the Rn-reduced air system.
(For interpretation of the references to color in this ﬁgure legend, the reader isreferred to the web version of this article.)

system, the absolute humidity of the input air became higher
than before but, fortunately, the Rn concentration did not change.
In addition, on November 9th , 2013, a scheduled laboratory-wide
power outage cut power to the Rn-reduced air circulation system.
When the system was restarted, unpuriﬁed air was accidentally
supplied to the buffer layer, causing the Rn concentration in the
output air to increase for a few days.
Over the 90 days prior to the improvement of the Rn-reduced
air system the input and output air Rn concentrations were
measured to be,

CInput,before =3.5 ± 0.3 mBq/m3 andCOutput,before = 28.9 ± 1.7 mBq/m3,
respectively. These values were derived after subtracting Cdetector,BG .
It should be noted that the Rn concentration in the output air was
found to be much higher than that of the input air both before andafter the improvement, which conﬁrms that the buffer gas layer iscontaminated with Rn. That is, a Rn source exists inside the SuperK tank. Although the origin of this source has not yet beenY. Nakano et al. / Nuclear Instruments and Methods in Physics Research A 867 (2017) 108–114

Fig. 9. Spectrum measurements using a single 80 L Rn detector. The gray spectrum
corresponds to the background measurement, while the light-green (red) corresponds to the input air measurement before (after) the air system improvement. The
214
Po signal region for this detector is deﬁned as counts between ADC channels 158
and 178, corresponding to energies between 7.03 and 7.92 MeV, based on theprocedure described in Section 2.3. (For interpretation of the references to color inthis ﬁgure legend, the reader is referred to the web version of this article.)

identiﬁed, the most likely candidate is emanation from the detector structure.
Data from December 1st , 2013 to April 9th , 2014 were used toevaluate the Rn concentrations after the improvement of the Rnreduced air system, During this period the operation of the Rnreduced air system and the data acquisition were stable. The results for measurements taken 131 days after the system improvement are,CInput,after = 0.08 ± 0.07 mBq/m3 andCOutput,after = 28.8 ± 1.7 mBq/m3,
after background subtraction. Energy spectra from measurementsof the input air are shown in Fig. 9. It can be seen that the Rndaughters in the supplied buffer gas have been reduced by a factor
of four following the air system upgrade. Therefore, it can beconcluded that the new −60 °C chilled activated charcoal works
efﬁciently to reduce Rn and the product gas has become effectively
free of Rn. On the other hand, the Rn concentration of the output
air after the improvement has not changed. This indicates that
contamination from the Super-K tank is the dominant source of Rn
in the buffer gas.
It should be noted that the count rate of 210Po in the background spectrum is larger than the other measurements because it
was performed last. The increase is likely due to positively charged
210
Pb from the 222Rn decay chain accumulating on the surface of
the PIN photodiode over the measurement history of the detector.
Since it decays with a 22-year half-life to produce the observed
210
Po, the increased count rate thus indicates that 222Rn daughtersremain on the photodiode once they have been collected.

6. Conclusion and future prospects
In order to measure the Rn concentration in the buffer gas used inSuper-Kamiokande, a new 80 L Rn detector has been developed andcalibrated onsite at Kamioka observatory using pure air. Typical calibration factors for the detector are 2.25 ± 0.13 CPD/(mBq/m3) at
0.002 g/m3 and 1.66 ± 0.14 CPD/(mBq/m3) at 4.1 g/m3. Compared toprevious designs the current detector has an extremely low intrinsic
background, measured to be 0.33 ± 0.07 mBq/m3. Using this detector, the Rn concentration of the buffer gas in the Super-K tank has
been measured precisely. Following improvements to the Rn-

113

reduced air system, the Rn concentration of the supplied buffer gas
was reduced to 0.08 ± 0.07 mBq/m3. However, the concentration in
the buffer gas layer itself has been measured to be
28.8 ± 1.7 mBq/m3 suggesting the existence of Rn sources in the top
region of the Super-K tank.
As a next step, the Rn concentration in the Super-K puriﬁed
water is being measured in order to evaluate the contribution of
background events originating from Rn daughters to solar neutrino analysis. For this purpose, a system based on the new 80 L
detector and capable of measuring Rn concentrations in puriﬁed
water at the level of ∼0.1 mBq/m3 has been developed [32]. Detailsof the system design and measurements of the Rn concentration in
the Super-K tank water will be reported in a subsequent publication. Several other applications using the 80 L Rn detector are
ongoing, including Rn concentration measurements in several
novel gases [35] and Rn emanation measurements from solidmaterials for detector material screening [40].

Acknowledgments
The authors would like to thank the Super-Kamiokande collaboration for their help in conducting this study. We gratefully
acknowledge the cooperation of the Kamioka Mining and Smelting
Company. Y. N thanks M. Kanazawa for assembling both the calibration and the measurement setups. This work is partially supported by MEXT KAKENHI Grant Number JP26104008 and theinter-university research program at ICRR.

References[1] Y. Fukuda, et al., The Super-Kamiokande detector, Nucl. Instrum. Methods
Phys. Res. Sect. A 501 (2003) 418–462, http://dx.doi.org/10.1016/S0168-9002(03)00425-X.
[2] K. Abe, et al., Calibration of the Super-Kamiokande, Nucl. Instrum. Methods
Phys. Res. Sect. A 737 (2014) 253–272, http://dx.doi.org/10.1016/j.
nima.2013.11.081.
[3] J.N. Bahcall, The solar neutrino problem, Nucl. Instrum. Methods 110 (1973)
381–384, http://dx.doi.org/10.1016/0029-554X(73)90715-5.
[4] J. Hosaka, et al., Solar neutrino measurements in Super-Kamiokande-I, Phys.
Rev. D 73 (2006) 112001, http://dx.doi.org/10.1103/PhysRevD.73.112001.
[5] Q.R. Ahmad, et al., Measurement of the rate of νe + d → p + p + e− interactions
produced by 8B solar neutrinos at the Sudbury Neutrino Observatory, Phys.
Rev. Lett. 87 (2001) 071301, http://dx.doi.org/10.1103/PhysRevLett.87.071301.
[6] S.P. Mikheyev, A.Y. Smirnov, Sov. J. Nucl. Phys. 42 (1985) 913–917.
[7] L. Wolfenstein, Neutrino oscillations in matter, Phys. Rev. D 17 (1978) 2369,
http://dx.doi.org/10.1103/PhysRevD.17.2369.
[8] B.T. Cleveland, et al., Measurement of the solar electron neutrino ﬂux with theHomestake Chlorine Detector, Astrophys. J. 496 (1998) 505–526, http://dx.doi.
org/10.1086/305343.
[9] J.N. Abdurashitov, et al., Measurement of the solar neutrino capture rate withgallium metal. III Results for the 2002–2007 data-taking period, Phys. Rev. C 80
(2009) 015807, http://dx.doi.org/10.1103/PhysRevC.80.015807.
[10] M. Altmann, et al., Complete results for ﬁve years of GNO solar neutrino observations, Phys. Lett. B 616 (2005) 174–190, http://dx.doi.org/10.1016/j.
physletb.2005.04.068.
[11] G. Bellini, et al., Measurement of the solar 8B neutrino rate with a liquid
scintillator target and 3 MeV energy threshold in the Borexino detector, Phys.
Rev. D. 82 (2010) 033006, http://dx.doi.org/10.1103/PhysRevD.82.033006.
[12] G. Bellini, et al., First evidence of pep solar neutrinos by direct detection in
Borexino, Phys. Rev. Lett. 108 (2012) 051302, http://dx.doi.org/10.1103/
PhysRevLett.108.051302.
[13] G. Bellini, et al., Final results of Borexino Phase-I on low-energy solar neutrino
spectroscopy, Phys. Rev. D 89 (2014) 112007, http://dx.doi.org/10.1103/
PhysRevD.89.112007.
[14] G. Bellini, et al., Neutrinos from the primary proton-proton fusion process in
the Sun, Nature 512 (2014) 383–386, http://dx.doi.org/10.1038/nature13702.
[15] A. Gando, et al., Be7 solar neutrino measurement with KamLAND, Phys. Rev. C
92 (2015) 055808, http://dx.doi.org/10.1103/PhysRevC.92.055808.
[16] J. Hosaka, et al., Solar neutrino measurements in Super-Kamiokande-I, Phys.
Rev. D 73 (2006) 112001, http://dx.doi.org/10.1103/PhysRevD.73.112001.
[17] J.P. Cravens, et al., Solar neutrino measurements in Super-Kamiokande-II, Phys.
Rev. D 78 (2008) 032002, http://dx.doi.org/10.1103/PhysRevD.78.032002.
[18] K. Abe, et al., Solar neutrino results in Super-Kamiokande-III, Phys. Rev. D 83

114

Y. Nakano et al. / Nuclear Instruments and Methods in Physics Research A 867 (2017) 108–114(1983) 052010, http://dx.doi.org/10.1103/PhysRevD.83.052010.
[19] K. Abe, et al., Solar neutrino measurements in Super-Kamiokande-IV, Phys.
Rev. D 94 (2016) 052010, http://dx.doi.org/10.1103/PhysRevD.94.052010.
[20] Q.R. Ahmad, et al., Measurement of the rate of νe. d → p. p. e− interactions
produced by B8 solar neutrinos at the Sudbury Neutrino Observatory, Phys.
Rev. Lett. 87 (2001) 071301, http://dx.doi.org/10.1103/PhysRevLett.87.071301.
[21] Q.R. Ahmad, et al., Direct evidence for neutrino ﬂavor transformation fromneutral-current interactions in the Sudbury Neutrino Observatory, Phys. Rev.
Lett. 89 (2002) 011301, http://dx.doi.org/10.1103/PhysRevLett.89.011301.
[22] B. Aharmim, et al., Electron energy spectra, ﬂuxes, and day-night asymmetriesof 8B solar neutrinos from measurements with NaCl dissolved in the heavywater detector at the Sudbury Neutrino Observatory, Phys. Rev. C. 72 (2005)
055502, http://dx.doi.org/10.1103/PhysRevC.72.055502.
[23] B. Aharmim, et al., Measurement of the νe and total 8B solar neutrino ﬂuxes
with the Sudbury Neutrino Observatory phase-III data set, Phys. Rev. C. 87(2013) 015502, http://dx.doi.org/10.1103/PhysRevC.87.015502.
[24] S. Abe, et al., Measurement of the 8B solar neutrino ﬂux with the KamLAND
liquid scintillator detector, Phys. Rev. C 84 (2011) 035804, http://dx.doi.org/
10.1103/PhysRevC.84.035804.
[25] P.C. de Holanda, A. Yu, Smirnov, Homestake result, sterile neutrinos, and low
energy solar neutrino experiments, Phys. Rev. D 69 (2004) 113002, http://dx.
doi.org/10.1103/PhysRevD.69.113002.
[26] P.C. de Holanda, A. Yu, Smirnov, Solar neutrino spectrum, sterile neutrinos,
and additional radiation in the Universe, Phys. Rev. D 83 (2011) 113011, http://dx.doi.org/10.1103/PhysRevD.83.113011.
[27] V. Barger, Patrick Huber, Danny Marfatia, Solar mass-varying neutrino oscillations, Phys. Rev. Lett. 95 (2005) 211802, http://dx.doi.org/10.1103/
PhysRevLett.95.211802.
[28] A. Friedland, et al., Solar neutrinos as probes of neutrino-matter interactions,Phys. Lett. B 594 (2004) 347–354, http://dx.doi.org/10.1016/j.
physletb.2004.05.047.
[29] O.G. Miranda, et al., Are solar neutrino oscillations robust? J. High. Energy
Phys. 10 (2006) 008.

[30] S. Yamada, et al., Commissioning of the new electronics and online system for
the Super-Kamiokande experiment, IEEE Trans. Nucl. Sci. 57 (2010) 428–432(doi:10.1109/NSSMIC.2008.4774675).
[31] Y. Takeuchi, et al., Measurement of radon concentrations at Super-Kamiokande, Phys. Lett. B 452 (1999) 418–424, http://dx.doi.org/10.1016/S0370-2693(99)00311-1.
[32] Y. Nakano, 8B solar neutrino spectrum measurement using Super-Kamiokande
IV, (Ph.D. thesis), University of Tokyo (2016), available at 〈http://www-sk.icrr.
u-tokyo.ac.jp/sk/publications/index-e.html〉(accessed 20.12.16).
[33] Y. Takeuchi, et al., Development of high sensitivity radon detectors, Nucl. Instrum. Methods Phys. Res. Sect. A 421 (1999) 334–341, http://dx.doi.org/
10.1016/S0168-9002(98)01204-2.
[34] C. Mitsuda, et al., Development of super-high sensitivity radon detector for theSuper-Kamiokande detector, Nucl. Instrum. Methods Phys. Res. Sect. A 497(2003) 414–428, http://dx.doi.org/10.1016/S0168-9002(02)01923-X.
[35] K. Hosokawa, et al., Development of a high-sensitivity 80 L radon detector for
puriﬁed gases, Progress. Theor. Exp. Phys. 033H01 (2015, https://doi.org/
10.1093/ptep/ptv018).
[36] T. Iida, et al., An electrostatic integrating 222Rn monitor with cellulose nitrateﬁlm for environmental monitoring, Health Phys. 54 (1998) 139–148, http://dx.
doi.org/10.1093/ptep/ptv018.
[37] K.D. Chu, P.K. Hopke, Neutralization kinetics for polonium-218, Environ. Sci.
Technol. 22 (1988) 711–717, http://dx.doi.org/10.1021/es00171a016.
[38] Genitron Instruments GmbH, FACTORY CALIBRATION AT GENITRON INSTRUMENTS FACILITIES (Calibration Procedure of the AlphaGUARD Radon Monitor)
Version: 03/2006.
[39] M. Shimo, et al., Experimental study of charcoal adsorptive technique formeasurement of radon in air, J. At. Energy Soc. Jpn. 25 (1983) 562–570, http://dx.doi.org/10.3327/jaesj.25.562.
[40] H. Sekiya, Quest for the lowest-energy neutrinos in Super-Kamiokande,
in: Proceedings of the Fifth Low Radioactivity Techniques Workshop (LRT2015), Seattle, WA, USA, 2015. (AIP Conference Proceedings 1672 (2015)
080001, http://dx.doi.org/10.1063/1.4927996).