超伝導体検出器― SOI技術との融合による遠赤外一光子検出 STJ 開発 ―2015年11月30日 /光量子計測器開発推進室発足会議

武内勇司 (筑波大 数理物質融合科学センター)
on behalf of Neutrino Decay Collaboration
S. H. Kim, K. Takemasa, K. Kiuchi, K. Nagata, K. Kasahara, K. Moriuchi, R. Senzaki,S. Murakami, S. Yagi (U. Tsukuba), S. Matsuura (Kwansei Gakuin U.), H. Ikeda, T.
Wada, K. Nagase (JAXA/ISAS), H. Ishino, A. Kibayashi (Okayama U.), S. Mima(RIKEN), T. Yoshida, R. Hirose, Y. Kato, C. Asano, T. Nakamura (U. Fukui), Y. Kato(Kinki U.), Y. Arai, M. Hazumi, I. Kurachi (KEK), S. Shiki, M. Ukibe, G. Fujii, M.
Ohkubo (AIST), E. Ramberg, J. H. Yoo, M. Kozlovsky, P. Rubinov, D. Sergatskov(FNAL), S. B. Kim (Seoul National U.) and S. Kawahito (Shizuoka U.)
1

Superconducting Tunnel Junction (STJ)• Superconductor / Insulator /Superconductor
Josephson junction device2Superconductor

Insulator

E

300nm

Ns(E)
Superconductor
Superconductor
Insulator

Δ: Superconducting gap energy
接合面を挟んで電位差(|V|<2Δ)を印加.
超伝導体に吸収された光子のエネルギーにより複数のクーパー対が解離(励起)し，生成された準粒子によって，エネルギーに比例したトンネル電流が発生．
• 超伝導ギャップ(Δ)は遠赤外フォトンのエネルギーよりもずっと小さい 
原理的には，遠赤外域一光子を検出可能• ~s 程度の比較的高速なパルス応答(Nbの場合)  光子計数することでS/Nの著しい向上
2

STJ I-V 特性• Sketch of a current-voltage (I-V) curve for STJ
 The Cooper pair tunneling current (DC Josephson current) is seen at V =0, and the quasi-particle tunneling current is seen for |V|>2

2Δ

B field
Leak current
光入射の場合Josephson current is suppressed by magnetic field3

宇宙背景ニュートリノ崩壊探索への応用𝛾𝜈3
𝜈22𝑚32 − 𝑚1,2𝐸𝛾 =2𝑚3dN/d(a.u.)in the 3 rest frame distribution in ν3 → 𝜈2 + 𝛾
Sharp Edge
with 1.9Ksmearing

𝒎𝟑 = 𝟓𝟎 𝐦𝐞𝐕Red Shift effect

Two body decay
10100

50050𝜇𝑚(25meV)

[m]Nb/Al-STJarrayΔ𝜃
8 rows𝜆 = 40 − 80𝜇m
𝜆𝐸𝛾 = 16~31meV
450 columns

Surface brightness I [MJy/sr]

宇宙背景ニュートリノ崩壊探索への応用
Zodiacal Emission

CMB

Telescope parameters• Main mirror– D=15cm, F=1m

• detector

Zodiacal LightISDDGLCB decay

SLIntegrated flux from galaxy counts– 波長0.8m あたり100m x 100m x 8 pixels
視野角 : 8 x 10-8 sr

𝜏 = 1 × 1014 yr𝑠wavelength [m]

• Neutrino decay (𝑚3 = 50 meV, 𝜏𝜈 = 1 × 1014 yrs): 𝑰𝝂 =25kJy/sr– 3.3 x 10-20 W / 8pixels @ =50m• 200sec の測定でこれを検出
 検出器の性能として最低でも NEP < 6.6 × 10−19 𝑊

𝐻𝑧 が必要– 実際には，本物のフォトンによるバックグラウンド(Zodiacal emission)があるので，更に1 order 位低い必要がある (NEP<1 × 10−19 𝑊 𝐻𝑧)
5

産総研 CRAVITY 製 Nb/Al-STJ

M. Ukibe et al., Jpn. J. Appl. Phys. 51, 010115 (2012)
M. Ohkubo et al., IEEE Trans. Appl. Super, 24, 2400208 (2014)

T~0.3Kw/ B field

100nA
500pA/DIV

V10nA
0.2mV/DIV

LeakageI

0.1nA1nA

50m  50m Nb/Al-STJ
100pA
fabricated in CRAVITY at AIST• Ileak~0.2nA を達成

Temperature(K)•

更に小さな junction size のもの
でテスト中

•

リーク電流のショットノイズ由来のNEPは，リーク電流iL=50pA, 超伝導
ギャップエネルギー=0.6meV, トラッピングゲインG=10とすると

0.3

0.4

1.7Δ 2𝑖𝐿
NEP =~4 × 10−19 𝑊
G𝑒

0.5

0.6

0.7𝐻𝑧6STJパルス光応答特性
V0R0=1M
VIS laserthroughoptical fiber

100uV/div
4us/div

STJ

冷凍機

T.Okudaira，2015.1.16

可視光(465nm)レーザーパルス応答性(AIST製
Nb/Al-STJ 100um角)STJ 応答信号時定数: 立下り ~1s, 立上り~2s

VI
V0=V+R0I(もしくは，これより早い)
定電流モードの回路(右上)で測定

0.4mV VSTJ I-V curve w/light w/o light• Nb/Al-STJは，~1s という比較的早い応答速度 光子計数を行えば，実効的なNEPは劇的に改善可能– 但し読出し系の帯域は>1MHzを確保する必要あり

7

STJ キャパシタンス測定

STJ

Current (nA)

0

CSTJ

I=I(V)

40

100mx100m

緑：実測 (f=2kHz)
黒：シミュレーション-40

CSTJ=0.9nF, 1.2nF,1.5nF, 1.8nF

STJ 等価回路

Voltage (mV)

-0.400.4

STJ は，junction size に比例したキャパシタンス• STJのI-V測定からSTJのキャパシタンスを測定• SIS接合面の面積に比例する成分： ~34fF/m2
 20m角のSTJでも 14pF
 低入力インピーダンスの電荷積分型アンプでの読出しが必要
8

100x100m2 Nb/Al-STJ response to 465nm multi-photons

10M

Laser pulse trigger
ChargeShapersensitive +amp.
pre-amp.

2V/DIV

産総研CRAVITY製100x100m2 Nb/Al-STJ

40μs/DIVCSTJ

I=I(V)

STJ
T~350m(3He sorption)

Pulse height dispersion is consistent
with 10-photon detection in STJ

Nb/Al-STJ の低入力インピーダンス電荷積分アンプ読出• 可視光パルス（波長465nm）に対する応答• 室温に置かれた電荷積分型のアンプでの読出– 観測した出力電荷量は，およそ10光子の検出に対応• 一光子検出には，読み出し系のS/N改善が必要
極低温電荷積分型アンプの開発

9Development of SOI-STJ• SOI: Silicon-on-insulator– CMOS in FD-SOI is reported to work at 4K by T. Wada (JAXA), et al.

J Low Temp Phys 167, 602 (2012)• SOI と STJ の融合 (SOI回路一体型 STJ の基礎研究)– STJ layers are fabricated directly on a SOI pre-amplifier board andcooled down together with the STJ• Started test with Nb/Al-STJ on SOI with p-MOS and n-MOS FET640 um

STJ Nb
SOI

metal padVIA

SOI-STJ2 circuit

Dcapacitor

GCS

700 um

STJ lower layer has electricalcontact with SOI circuit through
VIASTJ

FET

10

1mA

1mA/DIVdrain-source current

SOI上にSTJを形成後の特性
1A
1nA

B~150Gauss
1pA

-0.2

0

0.2 0.4 0.6

0.8

gate-source voltage (V)•2mV/DIV

I-V curve of a STJ fabricatedat KEK on a FD-SOI wafer

nMOS-FET in FD-SOI wafer on which a STJ
is fabricated at KEK
Both nMOS and pMOS-FET in FD-SOI wafer on which a STJ isfabricated work fine at temperature down below 1K– 極低温では，スレッショルド電圧のシフト，サブスレッショルド領域のドレイン電流抑制，飽和領域でのドレイン電流の上昇など，特性が変動••

Nb/Al-STJ fabricated at KEK on FD-SOI works fine
We are also developing SOI-STJ where STJ is fabricated at CRAVITY
11

SOI Pre-amplifier development• 遠赤外一光子検出に向け，前段階として近赤外一光子検出に最適化した
電荷積分型アンプをVDEC* が提供するSPICE simulation で設計中
• 極低温での SOI MOSFET の振る舞いをシミュレーションに組み込むた
め KEK や JAXA と共同研究で SPICE用MOSFETパラメータを構築中– 様々なL(チャンネル長：L=0.4~5um)やW(チャンネル幅: W=1~10um)を持ったFETの3Kにおける特性の測定

500M

CSTJ

I=I(V)

1pF

STJ

V=0.4mV+Output

STJ に定バイアスを印加するのと同時にSTJに発生した電荷を積分

0s

50s

100s

室温でのFET パラメータを仮定したSPICE simulation
入力電荷:2fC1.3eV(波長1m)の一光子入射相当* VLSI Design and Education Center(VDEC), the U. Tokyo in collaboration withSynopsys, Inc., Cadence Design Systems, Inc., and Mentor Graphics, Inc.

12まとめ• 遠赤外(50m)の一光子検出が可能な検出器を STJ + SOI の技術を用いて開発中– 光子計数により，実効的にNEP で~ 10−20 𝑊

𝐻𝑧 を目指す• Nb/Al-STJ は，産総研CRAVITY で世界最高水準の低リーク電流のものが得られている(<200pA @ 50m角)– 20μm角，10m角のものもテスト中• SOIに技術を用いた極低温アンプによる読出し回路を開発中– 様々なW/LをもつSOI MOSFETの極低温でのI-V測定SPICE シミュ
レーションにもちいるFETパラメータ抽出– 光子計数の利点を最大限に生かす高速アンプ(帯域>1MHz)– SOI アンプ一体型STJの可能性

13

Backup14

Neutrino• Neutrino has 3 mass generations (1, 2, 3)• Neutrino flavor states (e, , ) are not mass eigenstates𝜈𝑒𝜈1
𝑈𝑒1 𝑈𝑒2 𝑈𝑒3
𝜈𝜇 = 𝑈𝜇1 𝑈𝜇2 𝑈𝜇3 𝜈2𝜈𝜏𝜈3𝑈𝜏1 𝑈𝜏2 𝑈𝜏3 Neutrino flavor oscillates during the flight, and squared mass
22differences (Δ𝑚12, Δ𝑚23) have been measured, but theirabsolute masses are not measured yet!
 Heavier neutrinos (2, 3) are not stableγe, 𝜇, 𝜏─ Neutrino can decay through the loop diagrams– 𝜈3 → 𝜈1,2 + 𝛾– Neutrino mass can be determined from the decay

𝜈1,2𝑊

𝜈3

 However, neutrino lifetime is expected to be very long (muchlonger than the age of universe) We adopt Cosmic neutrino background (CB) as the neutrino
source for neutrino decay search
15

Cosmic neutrino background (C𝜈B)
102𝑛𝛾 = 411/cm3
𝑇𝛾 = 2.73 K

CB (=neutrino decoupling)~1s after
the big bang
14 3𝑇𝜈 =𝑇𝛾 = 1.95K
11𝑝𝜈 = 0.5meV/c

Density (cm-3)

CMB

The universe isfilled with neutrinos.
However, they
have not been
detected yet!

10−73 𝑇𝜈𝑛𝜈 + 𝑛𝜈 =4 𝑇𝛾

3𝑛𝛾 = 110 cm316

Motivation of -decay search in CB• Search for 𝜈3 → 𝜈1,2 + 𝛾 in cosmic neutrino background (C𝜈B)– Search for anomalous magnetic moment of neutrino– Direct detection of C𝜈B2– Determination of neutrino mass: 𝑚3 = 𝑚32 − 𝑚1,22𝐸𝛾• Aiming at a sensitivity to 𝜈 lifetime for 𝜏 𝜈3 = Ο 1017 yr𝑠– Standard Model expectation: 𝜏 = Ο(1043 yr𝑠)
– Experimental lower limit: 𝜏 > Ο(1012 yr𝑠)
– L-R symmetric model (for Dirac neutrino) predicts down to𝜏 = Ο(1017 yr𝑠) for 𝑊𝐿 -𝑊𝑅 mixing angle 𝜁 < 0.02
Magnetic momentterm(need L-R coupling)𝜈𝑗𝐿 𝑖𝜎𝜇𝜈 𝑞𝜈 𝜈𝑖𝑅γ

𝜈𝑗𝐿
𝜈𝑖𝑅

SM: SU(2)Lｘ U(1)Y
𝜈1,2𝐿γ𝑊𝐿𝑒𝐿 , 𝜇𝐿 , 𝜏𝐿
𝜈3𝐿
𝜈3𝑅𝑚𝜈3

𝚪~ 𝟏𝟎𝟒𝟑 𝒚𝒓−𝟏

Suppressed by 𝑚𝜈 and GIM

LRS: SU(2)LｘSU(2)RｘU(1)B-L
γ𝑊1

𝜈1,2𝐿

𝜏𝐿𝑚𝜏
𝜏𝑅

≃ 𝑊𝐿 − 𝜁𝑊𝑅𝚪~ 𝟏𝟎𝟏𝟕 𝒚𝒓

𝜈3𝑅−𝟏

PRL 38,(1977)1252, PRD 17(1978)1395

cos𝜁
𝑊1=𝑊2sin𝜁−sin𝜁cos𝜁

𝑊𝐿𝑊𝑅1026
enhancement toSM

Only suppressed by L-R mixing (𝜁)17Photon Energy (Wavelength) in Neutrino Decay•𝜈22𝑚3

Two body decay

m3=50meVE =24.8meV(𝜆 = 50𝜇𝑚)
m2=8.7meV
m1=1meVE =24meV(𝜆 = 51𝜇𝑚)

E =4.4meV(282𝜇𝑚)

2– Δ𝑚12~ 7.65 × 10−5 𝑒𝑉 2

•From Planck+WP+highL+BAO– ∑𝑚𝑖 < 0.23 eV

 50meV<𝑚3 <87meV𝑬rest=14~24meV (𝝀𝜸 =51~89m)𝜸 distribution in ν3 → 𝜈2 + 𝛾dN/d(a.u.)𝜈3𝐸𝛾 =2𝑚32 − 𝑚1,22Δ𝑚23= |𝑚32 − 𝑚22 | ~ 2.4 × 10−3 𝑒𝑉 2–in the 3 rest frame

𝛾From neutrino oscillation

Sharp Edge
with 1.9K
smearing

10𝒎𝟑 = 𝟓𝟎 𝐦𝐞𝐕Red Shift effect

10050𝜇𝑚(25meV)

500

[m]
18

Surface brightness I [MJy/sr]C𝜈B decay signal and Backgrounds

CIB summary from Matsuura et al.(2011)ZECMBAKARICOBEat λ＝50μm
Zodiacal Emission(ZE)𝐼𝜈 ~ 8 MJy/sr

CIB

ZL

𝜆𝐼𝜆 ~ 0.1-0.5 MJy/sr
ISD
DGLCB decayCB decay

Expected 𝑬𝜸 spectrum

SL𝑚3 = 50meV

wavelength [m]E [meV]

λ＝50 μmE=25 meV𝜏 = 3 × 1012 yr𝑠
𝐼𝜈 ~ 0.8MJy/sr

Excluded by S.H.Kim et. al 2012

𝜏 = 1 × 1014 yr𝑠
𝐼𝜈 ~ 25kJy/sr
19

Proposed rocket experimentwith a diffraction grating and Nb/Al-STJ array combination

200-sec measurement at altitude of 200~300km


Telescope with diameter of 15cm and focal length of 1m
All optics (mirrors, filters, shutters and grating) will be cooled at ~1.8K

At the focal point, diffraction grating covering =40-80m (16-31meV) andarray of Nb/Al-STJ pixels of 50(in wavelength distribution) x 8(in spatialdistribution) are placed
 Each Nb/Al-STJ pixel is used as a single-photon counting detector for FIR

photon in =40−80m (Δ𝜆 = 0.8𝜇𝑚)
 Sensitive area of 100mx100m for each pixel (100rad x 100rad in viewingangle)

Nb/Al-STJarrayΔ𝜃8 rows

𝜆 = 40 − 80𝜇m
𝜆𝐸𝛾 = 16~31meV
50 columns

20Surface brightness I [MJy/sr]

Expected precision in the spectrum measurement
Zodiacal Emission

CMB

Telescope parameters• Main mirror– D=15cm, F=1m

• detector

Zodiacal LightISDDGLCB decay

SL

Integrated flux from galaxy counts

wavelength [m]

• Zodiacal emission ⇒ 343Hz / pixel– sensitive area
100mx100m / pixel– 50 x 8 array
Δ𝜆 =80𝜇𝑚−40𝜇𝑚= 0.8𝜇𝑚50

𝜏 = 1 × 1014 yr𝑠– 200sec measurement: 0.55M events / 8 pixels (at 𝜆 = 50𝜇𝑚)– 0.13% accuracy measurement for each wavelength: 𝜹 𝑰𝝂 =11kJy/sr• Neutrino decay (𝑚3 = 50 meV, 𝜏𝜈 = 1 × 1014 yrs): 𝑰𝝂 =25kJy/sr– 2.3σ away from statistical fluctuation in ZE measurement

 decay with 𝝉𝝂 = 𝟏𝟎𝟏𝟒 yrs is possible to detect, or set lower limit!

21

Sensitivity to neutrino decay

Parameters in the rocket experiment simulation

••
•
•
•

telescope dia.: 15cm
50-column (: 40m – 80 m)  8-row array
Viewing angle per single pixel: 100rad  100rad
Measurement time: 200 sec.
Photon detection efficiency: 100%••Can set lower limit on 3 lifetime at 4-6  1014 yrs if no neutrino decay observed
If 3 lifetime were 2  1014 yrs, the signal significance is at 5 level
22

Energy/Wavelength/Frequency

𝐸𝛾 = 25 meV
𝜈 = 6 THz
𝜆 = 50𝜇𝑚
23

STJ back-tunneling effect• Quasi-particles near the barrier can mediate Cooper pairs, resulting intrue signal gain

– Bi-layer fabricated with superconductors of different gaps Nb>Al to enhancequasi-particle density near the barrier– Nb/Al-STJ Nb(200nm)/Al(10nm)/AlOx/Al(10nm)/Nb(100nm)• Gain: 2～200

PhotonNbAlAlNb24

STJ energy resolution
Statistical fluctuation in number of quasi-particles  energy resolution
Smaller superconducting gap energy Δ yields better energy resolution

𝜎𝐸 =Δ: Superconducting gap energy
F: fano factor
E: Photon energy

1.7Δ 𝐹𝐸
SiNb

AlHf9.23

1.20

0.165

Δ[meV] 1100 1.550

0.172

0.020

Tc[K]
Nb

Well-established as Nb/Al-STJ(back-tunneling gain from Al-layers)

Nq.p.=25meV/1.7Δ=9.5

Poor energy resolution, but asingle-photon detection ispossible

Tc :SC critical temperature
Need ~1/10Tc for practicaloperationHf

Hf-STJ is not established as a practical photon
detector yet

Nq.p.=25meV/1.7Δ=735

2% energy resolution is achievable if Fano factor <0.3for a single-photon25検出器に要求されるNEP
Telescope parameters•

Main mirror: D=15cm, F=1m

• detector– 波長0.8m (=(80m-40m)/50, =c/50m-c/50.8m=94GHz)あたり
100m x 100m x 8 pixels  視野角 : 8 x 10-8 sr• Neutrino decay (𝑚3 = 50 meV, 𝜏𝜈 = 1 × 1014 yrs): 𝑰𝝂 =25kJy/sr @ =50m𝑭 = 𝟐𝟓 𝒌𝑱𝒚 𝒔𝒓 × 𝟖 × 𝟏𝟎−𝟖 𝒔𝒓 × 𝝅 𝟏𝟓𝒄𝒎 𝟐 𝟐 × 𝟗𝟒𝑮𝑯𝒛 = 𝟑. 𝟑 × 𝟏𝟎−𝟐𝟎 𝑾 𝟖𝒑𝒊𝒙

• Zodiacal emission: 𝑰𝝂 =8MJy/sr @ =50m
𝑭𝒁𝑬 = 𝟏. 𝟏 × 𝟏𝟎−𝟏𝟕 𝑾 𝟖𝒑𝒊𝒙

•t 時間でFZE 積分した際の揺らぎ
エネルギーのフォトン数揺らぎ起因：𝜖𝛾 𝐹𝑍𝐸 Δ𝑡 𝜖𝛾 =•𝜖𝛾 𝐹𝑍𝐸 Δ𝑡測定条件，検出器要件を決める不等式
𝑁𝐸𝑃 × 2Δ𝑡 <𝜖𝛾 𝐹𝑍𝐸 Δ𝑡 < 𝐹Δ𝑡 t>40sec (1), t>200sec (2.2) NEP < 1.5 × 10−19 𝑊

𝐻𝑧 for t=200sec with 8 pix
26