Development of far-infrared single-photon spectrometers
based on superconducting tunnel junction for search for thecosmic background neutrino decay

Sep 28-30, 2015 / Epochal Tsukuba

Yuji Takeuchi (Univ. of Tsukuba)
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

Contents• Introduction to neutrino decay search– Proposed rocket experiment– Prospects for the neutrino decay search• Candidates for far-infrared single-photon
spectrometer– Nb/Al-STJ with diffraction grating– Hf-STJ• Summary2

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 mass22
differences (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 search3Cosmic neutrino background (C𝜈B)
102𝑛𝛾 = 411/cm3
𝑇𝛾 = 2.73 K

CB (=neutrino decoupling)~1s after
the big bang
14 3𝑇𝜈 =𝑇𝛾 = 1.95K11𝑝𝜈 = 0.5meV/c

Density (cm-3)

CMB

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

10−73 𝑇𝜈𝑛𝜈 + 𝑛 𝜈 =4 𝑇𝛾

3𝑛𝛾 = 110 cm34

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 𝑞 𝜈 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-Lγ
𝑊1

𝜈1,2𝐿

𝜏𝐿𝑚𝜏𝜏𝑅≃ 𝑊𝐿 − 𝜁𝑊𝑅𝚪~ 𝟏𝟎𝟏𝟕 𝒚𝒓

𝜈3𝑅−𝟏

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

𝑊1cos𝜁
=𝑊2sin𝜁−sin𝜁cos𝜁

𝑊𝐿𝑊𝑅1026
enhancement toSM

Only suppressed by L-R mixing (𝜁)5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 + 𝛾
dN/d(a.u.)𝜈3𝐸𝛾 =2𝑚32 − 𝑚1,22Δ𝑚23= |𝑚32 − 𝑚22 | ~ 2.4 × 10−3 𝑒𝑉 2–in the 3 rest frame𝛾

From neutrino oscillation

Sharp Edge
with 1.9Ksmearing

10𝒎𝟑 = 𝟓𝟎 𝐦𝐞𝐕Red Shift effect

10050𝜇𝑚(25meV)

500

[m]
6

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

CIB summary from Matsuura et al.(2011)

ZECMB
AKARI
COBE

at λ＝50μm
Zodiacal Emission(ZE)𝐼𝜈 ~ 8 MJy/srCIB

ZL

𝜆𝐼𝜆 ~ 0.1-0.5 MJy/sr
ISDDGLC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/sr7Detector requirements for neutrino decay search• High-precision photon energy spectrum around 𝜆=50𝜇m– Photon-by-photon spectroscopy in the far-infrared region
with better than 2% resolution for 𝐸𝛾 = 25meV (𝜆=50𝜇m )• Lower dark noise in the detector• Identify the sharp edge in the spectrum from the neutrino decay– Rocket- and/or satellite-borne telescope with this detector.
• A ground-based experiment is impossible.

• Superconducting Tunneling Junction (STJ) detectors– Array of 50 Nb/Al-STJ pixels with diffraction grating covering 𝜆 = 40 − 80𝜇m
• For a rocket experiment aiming at O(1014 yrs) in a 200-sec measurement
Improve the current experimental lower limit for 𝝉(𝝂𝟑 ) by 2 orders– STJ using Hafnium: Hf-STJ for a satellite experiment• Δ = 20𝜇eV : Superconducting gap energy for Hafnium• 𝑁q.p. = 25meV 1.7Δ = 735 for 25meV photon: Δ𝐸 𝐸 < 2% is achievable8Superconducting Tunnel Junction (STJ) Detector• Superconductor / Insulator /Superconductor
Josephson junction device2Superconductor
Insulator

E

300nm

Ns(E)
Superconductor
Superconductor
Insulator

Δ: Superconducting gap energy
A bias voltage (|V|<2Δ) is applied across the junction.
A photon absorbed in the superconductor breaks Cooper pairs and createstunneling current of quasi-particles proportional to the deposited photon energy.
• Much lower gap energy (Δ) than FIR photon  Can detect FIR photon• Fast response (~s)  Suitable for single-photon detection
9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
SiNbAl

Hf

9.23

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 <0.3
for a single-photon
10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 1mAll 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-STJ
arrayΔ𝜃8 rows

𝜆 = 40 − 80𝜇m
𝜆𝐸𝛾 = 16~31meV50 columns

11Surface brightness I [MJy/sr]

Expected precision in the spectrum measurement
Zodiacal Emission

CMB

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

• detector

Zodiacal LightISD

DGLC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!

12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radMeasurement 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
13

Status of Nb/Al-STJ photon detector development
Requirements for Nb/Al-STJ• Single-photon detection for E=25meV (=50m)• Detection efficiency: ~1• Dark count rate < 30Hz  leak current < 0.1nA• Sensitive area: 100m  100m per pixel

100nA
10nA

50m  50m Nb/Al-STJ
fabricated in CRAVITY at AISTLeakage

2.9mm

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

0.1nA1nA• Ileak<1nA achieved at AIST 100pA•Will test STJs with a smaller
junction size

Temperature(K)
0.3

0.4

0.5

0.6

0.7

14

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

10M

465nm laser
through opticalfiber

2V/DIV

100x100m2 Nb/Al-STJ fabricated at CRAVITY

40μs/DIV

Chargesensitive shaper
pre-amp. amp.

STJ

T~350m(3He sorption)

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

A response from Nb/Al-STJs to NIR-VIS photons at single-photon level wasobserved with a charge-sensitive amplifier at the room temperature• Response time of STJ: O(1μs)
Due to the readout noise, a FIR single-photon detection is not achieved yet

Need ultra-low noise readout system for STJ signal
 Considering a cryogenic pre-amplifier close to STJ

15

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)• A development of SOI-STJ for our application– 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 FET
640 umSTJ Nb
SOI

metal pad

VIA

SOI-STJ2 circuit

Dcapacitor

GCS

700 um

STJ lower layer has electricalcontact with SOI circuit through
VIA

STJ

FET

16

1mA

1mA/DIVdrain-source current

FD-SOI on which STJ is fabricated
1A

2mV/DIV

1nA

B~150Gauss
1pA-0.2

00.2 0.4 0.6

0.8

gate-source voltage (V)nMOS-FET in FD-SOI wafer on which a STJis fabricated at KEK

I-V curve of a STJ fabricatedat KEK on a FD-SOI wafer• Both nMOS and pMOS-FET in FD-SOI wafer on which a STJ isfabricated work fine at temperature down below 1K• Nb/Al-STJ fabricated at KEK on FD-SOI works fine
• We are also developing SOI-STJ where STJ is fabricated at CRAVITY
17

Pre-amplifier development• A charge-sensitive preamplifier in SOI for STJ
readout is underdevelopment using theSPICE simulation
supported by VDEC*• For the circuit design,
parameterization of
MOSFET Id-Vg(Vd) for the
SPICE simulation is inprogress with KEK andJAXA.

STJ+Output

V=-0.4mV
STJ readout circuit to apply aconstant voltage on STJ as well as
integrate the tunnel current from
response to a photon.

* VLSI Design and Education Center(VDEC), the U. Tokyo in collaboration withSynopsys, Inc., Cadence Design Systems, Inc., and Mentor Graphics, Inc.

18

Hf-STJ development• We succeeded in observation of Josephsoncurrent by Hf-HfOx-Hf layer in 2010
S.H.Kim et. al, TIPP2011

A sample in 2012
B=0 Gauss

B=10 Gauss

Hf(250nm)
Hf(350nm)
Si wafer
HfOx：20Torr,1hour
anodic oxidation：
45nm

200×200μm2
T=80~177mK

Ic=60μA
Ileak=50A@Vbias=10V
Rd=0.2Ω

However, to use this as a detector, much improvement in leakcurrent is required. (𝐼leak is required to be at pA level or less)19Hf-STJ Response to DC-like VIS light
Laser ON

Laser pulse: 465nm, 100kHz

~10μA

I

Laser OFFV
50μA/DIV

10uA/100kHz=6.2×108e/pulse20μV/DIV

We observed an increase of tunnel current in Hf-STJ response to visible light20

Summary• We propose an experiment to search for neutrino radiative
decay in cosmic neutrino background.
• Requirements for the detector is an ability of photon-byphoton spectroscopy with better than 2% energy resolutionat =50m (𝐸𝛾 = 25 meV) with low dark count rate (<30Hz)• Nb/Al-STJ array with a diffractive grating and Hf-STJ are
considered for the experiment.
– Nb/Al-STJ fabricated at CRAVITY meets our requirements.
– FD-SOI readout for STJ signal is under development.
– Hf-STJ development is in progress, yet need much improvement.

• Improvement of the neutrino lifetime lower limit up toO(1014yrs) is feasible for 200-sec measurement in a rocketborne experiment with the detector.
21

Backup22

Energy/Wavelength/Frequency

𝐸𝛾 = 25 meV
𝜈 = 6 THz
𝜆 = 50𝜇𝑚
23

STJ I-V curve• 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

Josephson current is suppressed by magnetic field
24

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: 2～200PhotonNb

AlAlNb25