The PIKACHU experiment– Challenge to discover 2νββ of 160Gd -2023 Dec. 21st (Thu)
Takashi Iida (U. of Tsukuba)
For the PIKACHU collaboration

NME2023 workshop @RCNP Osaka

Contents
１．Double beta decay of 160Gd
２．The PIKACHU experiment

GAGG scintillator
Measurement at Kamioka
Purification of the crystals
Background study with GEANT4

３．Summary and prospect2

Double beta decay of 160Gd• Gadolinium (Gd) is a rare earth
element lanthanide with atomic
number 64 and atomic weight is157.3.
•The 160Gd isotope is one of thedouble beta-decay candidate
nuclei.
 Q-value : 1730 keV
 Natural Abundance : 21.8%

Table of Isotope

Carter Hall, XXXVIII SLAC
Summer Institute (2010)

160Gd

Both 0νββ, 2νββ are undiscovered3

NME of 160Gd 2νββ•Two theoretical models predict 2νββ half-lives whose
predictions differ by about an order of magnitude.
21 yr [1] (pseudo-SU (3) model)
𝑻𝑻𝟐𝟐𝝂𝝂
～6×10
𝟏𝟏/𝟐𝟐
20𝑻𝑻𝟐𝟐𝝂𝝂
𝟏𝟏/𝟐𝟐 ～ 8×10 yr [2] (QRPA model)[1] J. G. Hirsch et al., Phys.Rev.
C 66, 015502 (2002)[2] N. Hinohara et al., Phys.
Rev. C 105, 044314 (2022)
4

Previous research in Ukraine• The world's best ββ search for 160Gd is an experiment in Ukraine[3] using a 2-inch GSO scintillator.
• Alpha radiation from U/Th series impurities in the crystal appears
to have been a serious background (BG).
• If the size and BG could be improved and the sensitivity increasedby more than an order of magnitude, 2νββ could be found.
World best 160Gd ββ search
Detector

GSO scintillator

160Gd mass

100g

Exp. period~2 yr

Main BGs

Internal α/β of U/Th
External γ from PMT0νββ limit※> 2.3×1021 yr2νββ limit※> 2.1×1019 yr[3] F.A.Danevich et al., Nucl. Phys. A, Vol. 694, Iss. 1–2, 2001, Pages 375-391※ 90% C.L.

5

The PIKACHU experiment
Pure Inorganic scintillator experiment inKAmioka for CHallenging Underground sciences
 Double beta decay experiment 160Gd using Ce:Gd3Ga2Al3O12 （GAGG）
GAGG crystal grown
at IMR, Tohoku
6.5 cmφ×14.5 cmL• The aim is to increase the sensitivity by morethan an order of magnitude over previousexperiments using GSO and to discover 2νββ.
• Large GAGG crystals can be fabricated bycollaborators at the Institute for Materials
Research, Tohoku University.
PIKACHU comprises six
Japanese research institutes
and 15 collaborators!
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Our strategy• Large crystal： One GAGG crystal includes 3-4 times more 160Gd than GSO.
• High LY： Six times higher light yield enable us better energy resolution.
• PSD： α and β can be completely separated by PSD。
• Low BG tech.： Low radioactivity PMT for DM search etc.
 It is then necessary to remove radioactive impurities of the U/Th series
inside the crystals.
Ukraine
PIKACHUGAGG averaged pulse（α/β）

AlphaBetaDetector

GSO scintillator

GAGG scintillator

Amount of160Gd

100 g

700 g （2 crysltals）

LY

10,000 ph./MeV

60,000 ph./MeV

Exp. period

2 years

2 years?

BG level

Refer [3]1/10 by PSD

T1/2 limit

T0ν > 2.3×1021 y
T2ν > 2.1×1019 y2νββ discovery
7Measurement in Kamioka 2021
• The detector was made by combining
GAGG, PMT and a light guide.
• It was installed in the Pb/Cu shield inthe Kamioka underground laboratory ofTohoku University near KamLAND.
• BG survey was carried out in 2021.
Date： 2021 7/4～7/6
DAQ period ： 12 hours
 Shield : Pb 15cm ＆Cu 5cm thick GAGG : 6.5 cmφ×14.5 cmL (3.2 kg) Waveform digitizer （1.25ns/bin） Threshold： ~200 keV
8β-Spectrum comparison with GSO
Summary of BG level• Beta events are selected by PSD.
• BG reduced by moving to Underground.
1000 keV• Our BG level is 20 time higher than in
the previous research in Ukraine.
1730 keV2615 keV

VS Tsukuba

VS Ukraine

1/3×10

1/2×201/30×10Previous research
＠Ukraine

Tsukuba
Kamioka
Unit of Y-axis is same as right figure
9Impurity measurement of raw materials• From the BG measurement inKamioka, conventional GAGG
crystals contain high U/Th-series
radioactive impurities.
• The amounts of U/Th impurities
in the main raw materials used toproduce GAGG crystals wereinvestigated using a Ge detector.
1.
2.
3.
4.

Gd2O3 material

Ge detector＠Tsukuba

Gadolinium oxide
Gallium oxide
Aluminium oxide
Cerium oxide
10Strategy to develop high purity GAGG
The following materials are used for growing large-sized
GAGG crystals.
1.
2.
3.
4.

Gadolinium oxide（Gd2O3）
Gallium oxide（Ga2O3）
Aluminum oxide（Al2O3）
Cerium oxide（CeO2）

3.8 kg
2.0 kg
750 g
15 g• Pure Gd2O3 was made in cooperation with Nippon Yttrium (NYC), a company thatmakes Gd sulphate for SK-Gd. Purification by resin was carried out (¥50,000/kg).
• Al2O3 is difficult to purify because it is insoluble in acid; three types of high-purity
Al2O3 are purchased; they are measured with a Ge detector and the one with thelowest impurity concentration is selected for use in the crystals.
• Ga2O3 and CeO2 were also measured with a Ge detector, but these raw materials
were sufficiently high purity, as described later.

Three types of Al2O311

Measurement of Gd2O3 powder••

Several raw materials were measured with a Ge detector toinvestigate the impurities.
From left to right: original raw material, high-purity productfrom the same company, purified by NYC.

Unit is [mBq/kg]

 The purification process reduced the 238U impurity bymore than two orders of magnitude!!

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Al2O3 Material investigation•••[mBq/kg]

The original raw materials are highly impure in the 238U series.
The amount of 238U impurities was reduced as a result of trying
dilute nitric acid cleaning, but not enough.
Al2O3 raw materials from two new companies were purchasedand compared, and it was found that Company B had the lowest
impurity content.
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Ga2O3＆CeO2 material investigation
Ga2O3[mBq/kg]（pure）CeO2

 Both Ga2O3 and CeO2
were originally havelow impurityconcentrations.
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Summary of material impurities[mBq/kg]15

High purity GAGG crystal for PIKACHU••

Using high purity raw materials obtained by purification andselection, GAGG crystals of 2-inch size were grown at Tohoku
University.
The crystals were cut and polished, and the detector wasfabricated by winding a reflective sheet and coupling it witha PMT and light guide.

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Energy calibration
Sourceγ Energy

Cs-137662 keVNa-22

511 keVCo-60• Data acquisition with three gamma-ray
sources• Fit γ-ray peak (exp + gaus)
Co-60

1173 keV
1333 keV

σ = 3.7 % @662keV

α quenching factor in GAGG
• Alpha rays emit less light per MeV (quenching) than beta rays.
• As the data are quenched, the energy dependence of thequenching coefficient was investigated for comparison withGEANT4 simulations.
Alpha ray spectrum
Gd152

U238 U234

Th232 chain

PSD capability of high purity GAGG
• Pulse shape discrimination method (PSD) is
Important for removing BGs.

Pb shield in Tsukuba• Data from BG measurements carried out in Tsukuba, were usedto evaluate the PSD performance of high purity GAGG.
Ratio = (integral of first 130 ns)/(integral of all region)
Ratio

Pulse Height [V]130 nsec

1.43MeV
α崩壊信号

Time[ns]✓Completely distinguishable above 300 keV !!

MeVMeasurement in Kamioka 2023••

A BG survey of high-purity GAGG crystals was carried out in alow-BG environment 1000 m underground in Kamioka.
BG comparison with previous studies. Feasibility study forsensitivity update.
Date： 2023 6/12～6/14
DAQ period ： 18 hours
 Shield : Pb 15cm ＆Cu 5cm thick GAGG : 5.4 cmφ×5.2 cmL (0.8 kg) Waveform digitizer （4 ns/bin） Threshold： ~150 keV20

BG level compared with old crystal••

The radioactive BG levels inside the crystals wereestimated from the α-ray rates.
Assumed that alpha rays are not externally caused.

Old crystal

New crystal

The energy resolutionis also improved bychanging the crystal
composition.
 BG reduction of approximately
one order of magnitude
compared to the old crystals!
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BG modelling with GEANT4• Still remaining BGs should be quantitatively understood.
• So, an evaluation of BG in the data was attempted using GEANT4.
• Assuming radiative equilibrium below the long-lived nucleus, a U/Th-derived
BG model was developed.
238U-chain
234U&230Th

235U-chain

Upper
Lower

UpperMiddle

コンプトン効果？
Lower

232Th-chainUnderstanding α-ray BG by fitting• Fitting α spectrum with the simulated BG a spectrum.
→ Successful identification of impurity nuclides!
Fit region
Gd152

Fitting result

Chisq/ndf=534.17/226

Unit : mBq/kg

Old GAGG
HP GAGG
Gd2O3 mater.

238U upper

238U middle

235U lower

232Th

911±10125±2< 16.316.5±3.5< 0.3< 0..4373.5±15.3
3.2±0.7-64.3±3.0
2.2±0.2
1.7 ±0.4

β-ray BG in high purity GAGG
✓Data: Only beta events were extracted from Kamioka data using PSD.
✓Simulation: two β-ray BG models

U/Th decay series in crystal ⇒ Fixed impurity content from α-ray fitting results
40K of PMT origins ⇒ Spectra generated by Geant4 and fitted

EventQ value

Fitting result

Chi sq/ ndf =475.7/98

238U upper

40K✓The β-ray spectrum was successfully reproduced by Geant4!!
⇒ The dominant BG is upstream of 238U around the Q value (1.73 MeV)
✓40K (1.46 MeV) is beginning to appear in the spectrum.
⇒ Consideration required if purification progresses
24Sensitivity comparison with previous study
GSOLarge GAGG ×2

Crystal mass

635 g

6.40 kg

160Gd included

103.6 g709.9 g

BG in Q±1.5σ

73,673 eve/kg/yr

176,669 eve/kg/yr

Assuming the same measurement time, 𝑺𝑺𝑺𝑺𝑺𝑺/ 𝑩𝑩𝑩𝑩
was compared with previous studies.
Sensitivity improve approximately 1.4 times that ofthe previous study is expected Experiment will start next year with large, high-purity GAGG crystals!
 A further reduction of BG by more than one order of magnitude isneeded to search for 2νββ of 160Gd.
 In parallel with the experiment, further purification will be explored.

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Further purification of GAGG
• The crystal raw materials are of higher purity than the crystals, whichsuggests that U/Th impurities were introduced during the crystal
growing process.
• The insulator made from ZrO2 is contaminated by U/Th impurities.
• New crystals are currently being grown, while devising ways to prevent
contamination from insulation.

26Future prospect• Suppose that one more order of magnitude of crystal
purification is successfully achieved in the next crystal.
• Two large crystals will be grown and experiment will startin Kamioka in 2024.
• After about two years of measurements, a 4~5 times
increase in sensitivity will be achieved.
• In parallel, further purification and an increase in thenumber of crystals are aimed at making the sensitivity 10
times higher than in the Ukrainian experiments.

I want to find a 160Gd 2νββ in about 5 years!
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Summary• Double beta decay search (PIKACHU) experiment for 160Gd.
• Aiming to increase sensitivity by one order of magnitude overprevious studies, to discover 2νββ.
 Development of high-purity GAGG crystals!! BG~1/10
is achieved.
 Sensitivity is equivalent to or higher than that of theprevious study .
New collaborator is always welcome!!
 tiida@hep.px.tsukuba.ac.jp
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