半減期１０の２７乗年以上の二重Β崩壊観測を実現するＴＬー２０８背景事象除去の検証「第七回極低放射能技術」研究会

2021年3月24日
Grant-in-Aid for Scientific Research on Innovative Areas
19H05093 and 20H05241
Scientific Research (C) 18K03664

Miyagi University of Education

Y. Fukuda, Narengerile, A. Obata,Y. Kamei, D. Anzai

Kamioka Observatory, ICRR, Univ. of Tokyo S. Moriyama, K. Hiraide
Fukui University I. Ogawa
Tokyo University of Science T. Gunji, S. Tsukada, R. Hayami
Institute for Materials Research, Tohoku University S. Kurosawa

Conceptual design of ZICOS detector
Phys.Rev.Lett. 117 (2016) 082503

0𝜈𝜈
NEMO3 : 𝑇𝑇1/2> 9.1 × 1021 yrs

96Zr : 45kg (nat.) → 865kg(50% enrich)→1/20 BG

0𝜈𝜈
𝑇𝑇1/2
> 4 × 1025 yrs → 2 × 1026 yrs → ~1 ×1027yrs

半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除2021年3月24日

2

Discrimination of signal and BG
0νββ event
e-Reconstructed vertex byescintillation light

β decay
2.6MeV γ
Reconstructed vertex
by Cherenkov light
Balloon or
surface ofdetector
半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除去の検証

e-2021年3月24日

3Topological info : averaged angle
Average directione- = Σ didi θi

Averagedangle= Σ θi
Nhit

Average angle with respect to averaged direction
for single electron seems to have a peak at 48
degree which is almost same as Cherenkov angle.
半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除去の検証

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4BG reduction using topological information
Simulated by EGS5

Simulated by EGS5

78%
remainVertex position
Double beta decay event sample
Simulated by EGS5

93%
cut

Topological information fromPMT position which received
PMT hit pattern of 208Tl BG and Cherenkov lights could be
used for reduction of 208Tl BG.
0νββ signal
Vertex position

半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除去の検証

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5Strategy to realize BG reduction using
Cherenkov lights
1. Selection of PMT which receives Cherenkov
lights among Scintillation detection.
• Pulse shape discrimination (done)
2. Confirm topology of Cherenkov lights• Directional difference of event selection (done)• Direct measurement of topological information(averaged angle)
3. Demonstrate BG reduction using beta-gamma
sources with topological information (averagedangle) of Cherenkov lights
半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除去の検証2021年3月24日

6Pulse shape of Cherenkov and scintillation• Pulse shape of 90Sr using
H2431-50 measured by
V1751 with DES mode(2GS/s)• Decay time of scintillation :4.57ns and 8.38ns• Rise time of scintillation :1.45ns• Rise time of Cherenkov :0.75nsUse the charge ratio Qtime/Qtotal. Here, Qtimeis FADC
value in each time, and Qtotalis sum of FADC value
between 55ns and 80ns.
半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除去の検証

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7Charge ratio in rise time using ZICOS LS•••There is difference of
shape between t=57ns
and 58ns
Charge ratio looks
depend on the energy
For t>58.5ns, all shapes
were almost same.
Cherenkov looks dominant
between 57ns and 58ns.
Template waveform of
scintillation between 57.0ns
and 58.0ns for 57Co.

半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除去の検証

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8χ2distribution using 57Co template• Most of backgrounds
have lower χ2 than 1.0• Most of backgrounds
have lower energy than
Cherenkov threshold,then only scintillation
was seen.

It seems to events with
Cherenkov lights shouldhave large χ2 value.
半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除去の検証

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9Measured by Compton edge event and BG sample

χ2 ＜0.1(scintillation like)
403/3970 = 10.4±0.5% forCompton edge event

1772/3606=49.1±1.4% for
BG sample
Inefficiency value between
Compton edge and BG
sample differs with 2.86σ.
Topology of Cherenkov
lights for ~1MeV e- wasstrongly indicated.

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10Measurement of topological information(averaged angle) using HUNI-ZICOS

26

半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除去の検証

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11Hemisphere flask and PMT mounting jig for
HUNI-ZICOS Extension sharpening for hemisphere flask.
 Some extensions for PMT hole.
半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除去の検証

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12Mounting PMTs on jig for hemisphere flask

 Total 26 H3164-12 PMTs were used for HUNI-ZICOS.
 No PMT at center of hexagon location.
 Light shield was necessary due to jig material Nylon.
半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除去の検証

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13Setting hemisphere flask to jig and locate
on supporting stand
La-GPS HUNI-ZICOS was putted on flask clip and the chimney waspinched by clamp.
 Assuming vertex position to be center of truncatedicosahedron jig (not hemisphere flask).
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14Cable connection to FADC and HV
LeCroy 1182 ADC

Need V1751 8ch

Problem : MCX cables wereconnected only half channel
due to thick socket!

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15Measurement of averaged angle using 90Sr

1PMT self trigger

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16Measurement of averaged angle using 90Sr
self trigger
by PMT#1

半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除去の検証

self trigger
by PMT#1

Observed
averaged angle
for selectedevents(Qtotal>1000
and Nhit≧5)
seems to beclustered andthe peak value
agree with anexpected value(47 degree)
within statisticalerror.
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17Measurement of averaged angle using 90Sr

Same clusters were found for other PMTs. Thosepeak values also agree with an expected value.
On the other hands, no cluster was found for sideedge PMT. This PMT should not receive Cherenkov
lights due to collimated direction of incident electron
by Pb sheet.
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Electron with fixed direction and fixedenergy using 88Y gamma source• Compton scatteringwith fixed direction
generate fixed
direction and fixedenergy electron.
• Compton angle 100
deg corresponds to352keV scattered γ
and 1.484MeV
electron.
• Cherenkov angle is47 degree.
半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除去の検証

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19Setup for La-GPS scintillator•••

La-GPS threshold around 250keV.
Trigger coincidence between LaGPS PMT and HUNI-ZICOS
PMTs which connected to V1751.
Event selection for 1182 ADC
count between 2400 and 2600 as
scattered γ.
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Trigger logic for data taking using 88Y

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Measurement of averaged angle using 88Y••••

半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除去の検証

Scattering γ from 88Y
was clearlyobserved around
ADC count 2500.
Event selection
Qtotal>300 andNhit≧3 was applied.
Observed averaged
angle distribution
was spread. The
mean value wasalmost 45 deg.
Need both statistics
and some systematicevaluations using Pb
collimation or MC.
2021年3月24日

22Summary••
•
•
••
•
•

Most serious background 208Tl beta decay for ZICOS could
be removed by topology of Cherenkov lights.
PMTs received Cherenkov lights could be selected by PSD.
Topology of Cherenkov lights was directly indicated by~1MeV electron event selection using PSD.（基盤C目標達成）
Topological information (averaged angle) of Cherenkov lights
was measured by HUNI-ZICOS using actual ~1MeV electron.
Averaged angle using 90Sr was measured and it seems toagree with an expected angel 47deg within statistical error.
Averaged angle distribution using fixed direction and fixedenergy using88Y was spread. The mean value was 45deg.
Need both more statistics and some systematic evaluations
using Pb block collimation or MC. （他方の新学術の目標達成）
We almost confirmed the topology of Cherenkov light emitted
from ~1MeV electron. Next step is to demonstrate βγ BGreduction technique using UNI-ZICOS. （本新学術の目標）
半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除去の検証

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23Demonstration of 208Tl BG reduction• Direct measurement using βγ events by UNI-ZICOS

半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除去の検証

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24Backup slides

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Neutrinoless double beta decay[T1/20ν]-1 =G0ν(E0,Z)|M0ν|2<mββ>2 /me2
T1/2~a(Mt/∆E・B)1/2

a: abundance

M: target masst: measuring time ∆E: energy resolution B: BG rate

Requirement : Low BG, Large target mass, Good E-resolution
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ZICOS experiment for neutrinoless double
beta decay using 96Zr
Liquid Scintillator：(1) 10 wt.% Zr(iprac)4 loaded in
Liquid Scintillator(2) 3~4% at 3.35MeV of energy
resolution with 64% photo
coverage and long attenuation
length.

3.5m

Pure water surrounding innerdetector in order to veto muons andexternal backgrounds.
Inner detector with ~64% photo
coverage 20” PMT including 1.7ton
Zirconium loaded 113 tons LS in
fiducial volume. (Total vol. : 180tons)
半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除去の検証

10m10mPurpose:① Direct measurement of 0νββ② Confirm parameter of nuclear
matrix element model
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Liquid Scintillator solving Zr(iPrac)4Zr(CH3COCHCOOCH(CH3)2)4= Zr(iPrac)4mw : 663.87

Zr(iprac)4 2242mg, PPO
999mg and POPOP 10mg
solved in 20mL Anisole> 70g/L of Zirconium could be solved in anisole.
半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除去の検証

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Qtotal distribution for γ sources•

60Co (1.17MeV/1.33MeV)

Compton edge： 1.04MeV
137Cs (662keV)
Compton edge： 478keV
133Ba (356keV)
Compton edge： 207keV
57Co (122keV)
Under Cherenkov threshold(169kev)•••

Pulse shape with charge
ratio in each FADC time.
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Measured by fixed energy fixed direction events

χ2>0.1：22events/28events=78.6%

BG events=3evets
半減期10の２乗年以降の二重β崩壊観測を実現するTl-208背景事象除去の検証

Fixed energy : 835keV
ADC ch~3400cn
If the events with χ2>0.1
should have Cherenkov
lights, is this inefficiency
21.4±9.6% correct?
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3/8” photomultiplier H3164-12(R1635)

 Sensitivity：４００K
 Dynode type : Line focus/8dynode Applied voltage：125０V
 Gain：1.0×106 Dark current: 50nA
 Time characteristics：0.5ns(TTS) 0.8ns（rise time）
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Check V1742 sampling waveform• Sampling frequency ofV1742 was set as
5GS/s (fastest mode)• Sampling frequency ofV1751 was set as
1GS/s due to 8chread.(No DES mode)• Waveform of same
Cherenkov pulse wascompletely same shape.
• Detailed pulse structure
analysis is available
using V1742.
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Flash ADC V1742 and PMT HV system

Both CAEN HV AG7030SN
and FADC V1742 32ch
(5Gs/s!) were checked andready for the measurement.
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Decay scheme of 208Tｌ

The vertex position reconstructedby scintillation might be within
fiducial volume due to gammas.
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Measurement of T2ν1/2 for 96Zr using ZICOS-I• First physics program to measure T2ν1/2 for 96Zr

・20cm diameter flask
using Ultra-pure quartz
and 30 low ＢＧ 2” PMT
R3378-50 (R2083)
・Synthesis Zr(iPrac)4300g which corresponds
to 96Zr isotope1g
・According to NEMO-3
result, expect 200 2νββ
events/year
・Location: Kamioka mine
Stay tuned!

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Property of Cherenkov light Refractive index of anisole : n=1.518
 Cherenkov angle is determined bycosθ= 1/nβ Assuming 1.65MeV electron, then
β=0.972 and Cherenkov angel θ=47.3
degree are expected.
 Cherenkov light should be measured.
(400nm – 600nm : 100 photon/MeV )c.f. Light yield of Scintillation : ~12000photon/MeV

Cherenkov light = 1~2% of scintillation light
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