The SK-Gd Experiment
A New Experimental Phaseto Search for Supernova Relic Neutrinos
Okayama University1

Shintaro ItoMy Research History• Apr. 2011. The PIENU Experiment at TRIUMF in Canada.
- Precise measurement of the pion branching ratio <0.1%.
Phys. Rev. Lett. 115 071801 (2015), ﬁnal result is coming soon.
Nucl. Instrum. Methods Phys. Res., Sect. A 609, 102 (2015).

- Another new physics; heavy neutrinos, neutral bosons, etc.
Phys. Rev. D 97, 072012 (2018), Phys. Lett. B 798 (2019)134980,Phys. Rev. D 101, 052014 (2020), Phys. Rev. D 102, 012001 (2020).
KAKENHI Grant-in-Aid for Scientiﬁc Research (C)• Apr. 2016. The Super-Kamiokande Experiment- Preparations for the new experiment “SK-Gd”.
- Neutrino analyses.
✓Atmospheric neutrinos: Astrophysical neutrinos.
➡ Solar Physics Journal (arXiv:1909.10715)✓Solar neutrinos: Detector calibration, systematic studies.
• Jul. 2020. New experimental phase “SK-Gd” !!!!!
2

My Research History• Apr. 2011. The PIENU Experiment at TRIUMF in Canada.
- Precise measurement of the pion branching ratio <0.1%.
Phys. Rev. Lett. 115 071801 (2015), ﬁnal result is coming soon.
Nucl. Instrum. Methods Phys. Res., Sect. A 609, 102 (2015).

- Another new physics; heavy neutrinos, neutral bosons, etc.
Phys. Rev. D 97, 072012 (2018), Phys. Lett. B 798 (2019)134980,
Phys. Rev. D 101, 052014 (2020), Phys. Rev. D 102, 012001 (2020).
KAKENHI Grant-in-Aid for Scientiﬁc Research (C)• Apr. 2016. The Super-Kamiokande Experiment- Preparations for the new experiment “SK-Gd”.
- Neutrino analyses.
✓Atmospheric neutrinos: Astrophysical neutrinos.
➡ Solar Physics Journal (arXiv:1909.10715)✓Solar neutrinos: Detector calibration, systematic studies.
• Jul. 2020. New experimental phase “SK-Gd” !!!!!
3

Super-Kamiokande (SK)Japan~1km

41.4 m• ~1km underground the Mt.-Ikenoyama
Cherenkov light

50kton of pure water Cherenkov detector.

• Inner detector (ID): 11,126 20-inch PMTs
Charged particle
Neutrino➡ 22.5 kton of ﬁducial volume.

• Outer detector (OD): 1,885 8-inch PMTs
➡ For veto of incoming cosmic rays

Particle ID using Cherenkov ringʼs pattern,
opening angle, and showering-type.
4

History of Super-Kamiokande Experiment• Since April 1996, the SK experiment has been started toaccumulate neutrinos from various sources and proton decay data.
rνalo
S~3.5 MeVsNRS

20~100not
Pro cayde~1 GeV

Atmospheric νTeV
High Energy

Low Energy• The SK experiment has several experimental phases.
➡SK-I~SK-V: pure water phase• SK-Gd has started by loading 0.02% Gd2(SO4)3 into SK
to discover Supernova relic neutrinos (SRNs).
199620022018

2006 2008

Now!!!

SK-I

SK-II

SK-III

SK-IV

40% photo coverage

20%

40%

40%

40%

New electronics “QBee” andbetter water puriﬁcation system.

Gd has been dissolved(0.01% Gd, 50% n-tag)

Accident
Installed 5,000 PMTs

5SK-V

SK-Gd

Tank open workSupernova Relic Neutrinos (SRN)• Neutrinos emitted from all past core-collapse supernovae.
• Many models have been constructed to predict the ﬂux
and shape of spectrum, but not observed yet.

• Many astro and particle physics implications.
Φ=∫[ν emission] ⊗ [Star formation] ⊗ [Universe expansion]

6

Search for SRNs in SK• The sensitivity of SRNs is limited by the background; decay-eevents due to the atmospheric muon neutrinos.

• Need to identify invisible muon events.
Phys. Rev. D 85, 052007 (2012).
➡Neutron tagging to identify neutrinos and anti-neutrinos.
Signal region of SRNsµ-likee-likeMulti-γ likeνμp
μ−Invisible muoneCherenkov angle

7−(Te = 0 ∼ 52.3 MeV)Neutron Tagging in SK• Neutron tagging by Gadolinium (Gd). ν̅e- Background rejection.
- Observation of ν̄e SRNs.

npGde+γ⌫ e + p ! e+ +n| + x Gd ! x+1 Gd + (s) (8 MeV)|+H!D+(2.2 MeV)

8 MeV
ΔT~30µs, Vertices within 50cm

~18% of n-tag

Final goal

Minba imiza
ckgro tion
un ofds

Initial loading(Now!!!)

Naka

zatoeta

8l.

The SK-Gd Experimentadoliniumproject• The Super-Kamiokande Gadolinium (SK-Gd) experiment aims toCaptures on Gd

detectbySRNs
by tagging
dissolving
into the SKBeacomtank. and Vagins PRL93,171
p events
neutronwithGdGadolinium.
solubility,2(SO4)3・8H2O is used for.
➡Forfor
sectionthermalGd
neutron(48.89kb)
0.1% Gd (0.2% in Gd2(100%turedemits
3-4
γs4in
total2O
8 MeV
tonsof Gd)3・8H
were initially dissolved
into SK2(SO• 14 Gdgives~90% efficiency
most➡
of50%
BG from
RIseﬃciency
and the SK(0.01%trigger thresholdn-tagGd)
for n capture
capture efficiency at 0.1% loading
80% Gd)• Final goal: 140 tons, 90% n-tag eﬃciency (0.1%
was selected to dissolve→0.2% loading
To dissolve Gd2(SO4)3・8H2O, there were many R&D.
•it corresponds to 100 tons of loading60%
1.Low radio-impurity Gd2(SO4)3・8H2O.
Gd2(SO4)3SK tank.
2.Water leakage ﬁxing fromthe40%

20%

0%0.0001%0.001%
90.01% 0.1

Gd in Water

Why We Need Low Radio-Impurity
Gd2(SO4)3・8H2O ???

10Why We Need Low Radio-Impurity
Gd2(SO4)3・8H2O ???
• For SRNs searches, no neutron background.
• 238U: spontaneous ﬁssion➡~10 MeV γ-ray + neutron➡Mimics n-tag signal.
γ-rayNeutrons
238U

Spontaneous ﬁssion

11Uranium chain

Why We Need Low Radio-Impurity
Gd2(SO4)3・8H2O ???
• In SK-Gd, solar neutrino measurement will also be continued.
• 8B solar neutrinos, energy region 3.5~17 MeV.

12

Why We Need Low Radio-Impurity
Gd2(SO4)3・8H2O ???
• In SK-Gd, solar neutrino measurement will also be continued.
• 8B solar neutrinos, energy region 3.5~17 MeV.
• e.g. 208Tl β-decay (in 232Th chain)

Background tosolar neutrino analysis

3.5 MeV

Reconstructed energy spectrum of 208Tl β-decay.
13How We Developed Low Radio-Impurity Gd2(SO4)3·8H2O???

• Table shows the requirements for SK-Gd.
➡We contacted several companies to produce very clean Gd.
➡Measurement of radio-impurity and feedback to companies• I have developed very sensitive methods to measure
low radio-impurities in Gd2(SO4)3・8H2O.
- Long lifetime (238U (4.5e9 y), 232Th (1.4e10 y)): ICP-MS
S. Ito et al. Prog. Theor. Exp. Phys. 2017, 11 113H01- Short lifetime (226Ra (1.6e3 y)): Ge detector
Accepted in PTEP (S. Ito et al., arXiv: 2006.09664)
RadioImpurity

238U

232Th

226Ra

Commercial

50(4ppb)100(25ppb)

5(0.14ppt)

SK-Gd
requirement<5(0.4ppb)<0.05(13ppt)<0.5(0.014ppt)

14

Unit: [mBq/kg](ppt=10-12 g/g)

14 tons of Gd2(SO4)3・8H2O• Iʼve measured radio-impurities in many Gd2(SO4)3·8H2O samples
from several companies, and achieved the requirements.

• We ordered mass production (14 tons for “real” Gd2(SO4)3·8H2O
loaded to the SK tank) to the company.

• Iʼve measured all real Gd products to check radio-impurities.
➡238U, 232Th, and 226Ra are less than our requirements

14 tons of Gd2(SO4)3·8H2O(500 kg/bag)

15

Water Leakage Fixing• The SK tank had to be refurbished to ﬁx water leakage
before loading Gd2(SO3)4·8H2O.

• The SK experiment was stoped on 2018/May/31
and the tank was opened.

- Fixing water leakage: ~1 ton/day➡goal <0.03 ton/day- Replacement of broken PMTs.
- Cleaning works.
2018

June

July

AugustSeptember

OctoberNovember

DecemberWaterLevel

May 31

Water drain

16

Empty

Water reﬁllLeakage Monitor ̶Water Level̶• Water circulation system was stopped from 2019. Jan. 31
to Feb. 7 to monitor the water level.
➡ No any signiﬁcant water leakage.
➡ <0.017 ton/day (Goal: 0.03 ton/day)• Water level is still being monitored, and Gd2(SO4)3·8H2O wasintroduced into SK in 2020 Jul., but no any leakage is observed.

17

Leakage Monitor ̶ICP-MS̶• Itʼs important to monitor concentration of Gd around SK.
• I have-introduced another ICP-MS to monitor Gd.
setup clean room.
developed another chemical extraction procedure.
➡ S. Ito et al., PTEP 2019 6 063H03➡ Monitor of Gd is being continued since 2020 June,currently no any Gd excess is observed.

18ICP-MS and clean room

Current Status of SK-Gd• Dissolving Gd2(SO4)3·8H2O was started from July 14th.
• SK water circulation system➡Supply to the bottom and return from the top.
• 0.02% Gd2(SO4)3 is supplied to the bottom.
• ~33 days to achieve “full" 0.02% Gd2(SO4)3.
• For uniformed concentration of Gd in the tank, recirculation
with higher ﬂow rate (~120 t/h) than usual data taking isbeing performed.
Jul. 14
Out: pure water

Jul. 22

Aug. 1

Out: pure water

Out: pure water

Pure water

0.02% Gd2(SO4)3
0.02% Gd2(SO4)3

In: 0.02% Gd2(SO4)3

Out: pure water

Pure water

Pure water

In: 0.02% Gd2(SO4)3

Aug. 17

19

In: 0.02% Gd2(SO4)3

0.02% Gd2(SO4)3
but not uniformed

Current Status of SK-Gd
̶Neutron Tagging Events̶
• To check the neutron tagging events, the calibration data weretaken on 22 July 2020: Gd at Z=-10m.

• Am/Be source + BGO crystal.
241

Am →237 Np + α
Prompt signal
Delayed signal captured by Gd
9Be + α →12 C + γ(4.4 MeV) + n• 4.4 γ-ray produces scintillation light: prompt trigger signal.
• The calibration data were taken at Z=-12, 0, and +12 m.

Z=12 m

Z=0 m

Z=-12 m20

Current Status of SK-Gd
̶Neutron Tagging Events̶

Normalized tothe number of prompt signals

n + H+ → D + γ (2.2 MeV)

p0*exp(-p1*t) + p2
Time [µs]Expected time constant: ~130 us for 0.01% Gd (~50 us for 0.1% Gd)

Neutron tagging events by Gd can be seen!!!
New physics with neutron tagging in SK-Gd is coming soon!!!
21

Summary of SK-Gd• The SK-Gd experiment has been started since this July!!!
➡ Loading Gd2(SO4)3·8H2O for neutron tagging.

• Many R&D before loading.
- Low radio-impurity Gd2(SO4)3·8H2O✓ICP-MS (PTEP 2017 11 113H01)✓Ge detector (PTEP 2018 9 091H01, arXiv: 2006.09664)- Water leakage ﬁxing✓Tank open work.
✓Gd monitoring (PTEP 2019 6 063H03)• 0.02% Gd2(SO4)3 is currently dissolved in SK and the signals
by Gd capture can clearly be seen.

• More calibration data will be taken and measurement will becontinued 1 year, then the concentration of Gd will be increased.
➡Grant-in-Aid for Scientiﬁc Research on Innovation Area.
22

Analysis of SK Neutrino Data

23

Particle ID in SK• The particle ﬂavor can be identiﬁed using the ring patterns.
- Muon: Minimal scattering so clear rings with sharp edges.
- Electron: The shapes of the rings are “fuzzy” due to multiple
scatterings and electro magnetic showers.

μ+24Solar Flare Neutrinos• A solar ﬂare is an explosive event at the surface of the sun.
➡Through a process of magnetic reconnection, a solar ﬂarereleases energy 1026~1032 erg.

• To study the proton acceleration process in the solar ﬂare,the neutrino events were searched for in SK.

• Flux highly depends on the model, but Fargion predicted“solar ﬂare neutrino” can be observed ~1 events/ﬂare.
Earthν νννν νν
ν
ν

Sunν
ν

SK25 Fargion and Moscato, 2003 Chin. J. Astron. Astrophys. 3, 75•

SolarFlareNeutrinos
Production process is the same with the atmospheric neutrinos.
➡~10 events/day, major background.
➡Determination of the search time window using theinformation of satellites to minimize backgrounds• Solar satellites such as GOES, RHESSI, and GEOTAIL.
➡Accepted in Solar Physics Journal, arXiv: 1909.10715• All solar ﬂares in SK-I to SK-IV were studied (23 ﬂares)➡Two candidate events, but consistent with background.

GOES soft X-ray.
3σ for the time window.
26

Solar Flare Neutrinos Candidate27

SK Calibration ̶DT̶•DT(deuteron-tritium) generator

2H + 3H → n + He ⇨ 16O(n,p) → 16N ⇨ γ-rays are isotropically emitted- Energy scale- Position dependence.
- Zenith angle dependence.

ZYX28

SK Calibration ̶LINAC̶
• To determine the energy scale, electron LINAC is directory
inserted into the SK tank.

• Single monoenergetic electrons with 5~16 MeV are ejected.
LINAC

DTG

En=14.2 MeV

e-Ee=5~16 MeV29