Development of Superconducting Tunnel
Junction Photon Detector using Hafnium
Shin-Hong Kim,
Hyun-Sang Jeong, Kenji Kiuchi, Shinya Kanai, Takashi Onjo,
Ken-ichi Takemasa, Yuji Takeuchi (University of Tsukuba) ,Hirokazu Ikeda, Shuji Matsuura (JAXA/ISAS) ,Hiromi Sato (RIKEN) Masashi Hazumi (KEK) ,Soo-Bong Kim (Seoul National University)
June 11, 2011 at TIPP2011● Motivation● Superconducting Tunnel Junction (STJ) Detector● Status of Hf-STJ Development

Motivation
Search for radiative decay of cosmic background neutrino
z Δm2ij have been measured accurately by neutrino oscillation experiments.
but neutrino mass itself has not been measured. Can we measure it ?
Detection of neutrino decay enables us to measure an independent quantity ofthe difference between squares of neutrino mass. Thus we can obtain neutrinomass itself from these two independent measurements.

ν3W

τ,μν2

γ

2
m32 − m22 Δm23Eγ ==2m32m3z As the neutrino lifetime is very long, we need use cosmic background neutrino
to observe the neutrino decay. To observe this decay of the cosmic backgroundneutrino means a discovery of the cosmic background neutrino predicted bycosmology.

Neutrino Mass Relations
and Expected Photon Energy Spectrum
Decay Photon Energy Spectrum

ν3 → ν2 γ
Eγ = 25meV
20meV
15meV

Neutrino oscillation
m3 = 50meV, m2 = 10 meV,Eγ = 24meVResults from direct measuremenmt(Tritium Decay)m(ν e ) < 2eV

Red shift effectSharp edge with
1.9K smearing⎤dNρc ⎡⎢⎛⎜ Eγrest ⎞⎟ΩM + ΩΛ ⎥=⎟⎜
dEγ dSdΩdt 4πτH 0 Eγ ⎢⎝ Eγ ⎠⎥⎣⎦3−12Eγrest : photon energy in ν 3 rest frame, ρ : ν 3 density , τ : ν 3 lifetime,
H 0 : Hubble constant, Ω M : Matter density(0.76), Ω Λ : cosmological constant(0.24)

Neutrino Decay Lifetime
M. Beg, W. Marciano and M. Rudeman Phys. Rev. D17 (1978) 1395-1401
R. E. Shrock Nucl. Phys. B206 (1982) 359-379
Calculate the neutrino decay width in SU(2)L ｘ SU(2)R ｘ U(1) model
M（WR）=∞ and sin ζ = 0 corresponds to Standard Model.

W 1 = W L cos ζ − W R sin ζW 2 = W L sin ζ + W R cos ζWL and WR are fields withpure V-A and V+A couplings,
respectively, and ζ is a mixing
angle.

Using a lower mass limit M (W R ) > 715GeV/c 2 , a mixing angle limit ζ < 0.013,
and m3 = 50meV,

τ (ν → ν + γ ) = 1.5 ×1017 year
32( 2.1×10 43 year in Standard Model )the CIB and the Decay Photon Energy Spectrumwith Various Energy Resolution
with a telescope of 20cm diameterand 0.1 degree viewing angledN（Eγ）/dEγfor CIB + νdecayCIB
νdecayEγ (eV)● The energy resolution is required to be better than 2% at 25meV.
● NEP (Noise Equivalent Power) is required to be less than 3 x 10-19 WHz-1/2 .
● Expected 5σ observation lifetime is 1.5 x 1017 year with a telescope of 20cm
diameter, 0.1 degree viewing angle and 3 hour running for m3 of 50meV .

CIB Observation Plan ( by JAXA Dr. Matsuura)
2010

Fluctuation（small） AKARIFluctuation（large）
Absolute value

2020

2015

SPICA

CIBER-2MIRIS

CIBER
EXZIT

COB-rocket（案）Foreground radiationFIR-rocket（案）

Search for new source

放射強度

初代天体によるCIRBスペクトル

黄道光（前景放射）

あかり

CIBER

COBE
CMB

CIBER-2

SPICA

EXZIT
可視 近赤外 中赤外背景放射

DGL

系外銀河
第一世代の星?

AKARI

ゆらぎパワー: log l(l+1)Cl /2π [nW/m2/sr]

110

遠赤外

サブミリ波

100 波長 [μm]

角度スケール: logθ[度]
-2
10-1
2
第一世代の星
0-2

系外銀河
CIBER
あかり
CIBER-2
SPICA
EXZIT
12
354
空間周波数（多重極子）: log l6Superconducting Tunnel Junction(STJ) Detector

STJ (Superconducting Tunnel Junction）Detector• Superconductor / Insulator / Superconductor JosephsonJunction

At the superconducting junction,
excited electrons over their energy gap go
through tunnel barrier by a tunnel effect.
By measuring the tunnel current ofelectrons excited by an incident particle,
we measure the energy of the particle.
Excited Electrons

Incident Particle
2Δ

Cooper Pairs

SISSTJ Energy Resolution
STJ Energy Resolution

Band gap energy
σ E = 1.7 Δ( FE )F: Fano factor (= 0.2)
Using Hf as a superconductor, E: Incident particle energy
Δ:σ E / E = 1.7%

at E = 25meV
Tc : Critical TemperatureOperation is done at a temperaturearound 1/10 of Tc

No paper on Hf-STJ test in the world.

Basic Properties of STJ Detector
Nb-STJ current -voltage ( I-V) curve• Leakage current ( Dynamic resistance Rd in |V| < 2Δ/e)• Energy gap Δ• Critical current Ic

Operation Mode

Josephson Current

Slope Rd

Josephson Current is suppressed by a magnetic
field parallel to the insulator planeNb/Al - STJ Response to 5.9keV X rays
Tunnel barrier

ADC output distribution
Up: 5.9keV X ray signal after preamplifier
Down: 5.9keV X ray signal after preamp + shaper
at T=0.4K

Double peak comes fromthat X rays are absorbed
both in the upper layer andthe under layer.

Status of Hf-STJ Development

Hf-STJ Structure
Mask Design5mm

50, 100, 200 μm

Enlarging

HfO2(1‐2nm)

Cross Section

NbHf(250nm)
Si

SiO2

R&D Status(1) Search for the best condition for making a flat Hf layer :various pressures and voltages.
•2.0 Pa，70W (optimized)

Distortion Force σ (GPa)Distortion Force vs. Voltage
δσ∝δ

2.0Pa 70W

Voltage (V)R&D Status(2) Search for the best condition for making the insulator layer(1 – 2 nm thick ) as a tunnel barrier: various pressures andperiods of oxidation.
• 5 Torr，12 minutes Oxidation sample (TEM picture )– Confirmed 1.3nm-thick HfO2 layer
TEM picture

Hf

EDX measurement

O

HfOx

Hf-STJ

Hf５nm•TEM: Transmission Electron Microscope•EDX: Energy Dispersive X-ray Spectroscopy

R&D Status(3) Operation of He3/He4 Dilution Refrigerator.
z We borrowed a He3/He4 Dilution Refrigerator from a group of Low Temperature
Material Science at University of Tsukuba in 2008.
z Achieved 49mK on July 2009.

Lowest: 49mKI-V curve measuredtime

Hf-STJ I-V Curve(Oxidation Condition: 10Torr 60min.)V : 20 μV/div
I : 20 μA/div

T ~ 120 mK
pixel size 200 μm × 200 μm
I C = 24 μA, R d = 1ΩJosephson Current

With a magnetic field (2Gauss)IVz First observation of Josephsoncurrent with Hf-STJ.
z Need to reduce the large leakage
current and the large noise

Josephson Current
Disappeared.

PlansSuperconducting Detector R&Dz2011-2012: Develop a single cell Hf-STJ
and low-temperature electronics( to see the Infrared photon signal).
z2013: Multi-cell Hf-STJ developmentz2011: Hf-MKID ( Microwave Kinetic Inductance Detectors )development

CIB Data Analysis for Neutrino Decay Searchz2011z2015-: Analysis of AKARI CIB data
: FIR-Rocket (2015 ?), SPICA(2018), EXZIT(2020?)

BACKUPMINOS, K2K and Super-K
Experiment Results
MINOS Best Fit
Δm232 = (2.43 ±0.13) x10-3eV2
Super-K Best Fit
sin2(2θ23) > 0.93

Three independent measurements
agree with each other.

Results from Kamland and Solar NeutrinoOscillation Observation

 (b) Solar Global: SNO, SK, Cl, Ga, Borexino
 (c) Solar Global + KamLAND :Δm122 = (7.59 + 0.19/ -0.21) x10-5eV2
θ12 = 34.4 +1.3/ -1.2 degrees

Neutrino Masses and Decay Photon Energy

ν3 →ν 2 + γ
ν 2 → ν1 + γ

ν3W

ν2

τ,μγ

m3=0.050eVγΔm=0.05eV:

m2=0.010eV
m1=0.001eVEγ =0.024eV

Δm=0.009eV:

m3 = Eγ +2

ν3Eγ =0.0045eV

Eγ + m22 ,ν2m32 − m22
Eγ =2m3Eγ

Mass Relations from Neutrino OscillationResults and Neutrino Decay
ν3 → ν2 γ

ν2→ν1 γEγ = 25meV
20meV
15meV

Eγ = 4.5meV
4meV
3meV

Neutrino oscillation
Neutrino oscillation

Results from direct measuremenmt (Tritium Decay)m(ν e ) < 2eV

Decay Photon Energy versus m3

Δm232 = (2.43 ±0.13) x10-3eV2±1σ bandSpectra of Decay Photon Energy after1.9K ν Smearing
Energy distributioin of cosmic background neutrino is a Planck distribution
with a temperature of 1.9K（0.6meV).

γ
ν3ν2Eγ
1.9K(0.6meV)ν 2 → ν1 + γ

ν3 →ν 2 + γ

E γ = 0.006eV

E γ = 0.03eV

σ= 2.8%meanσmean= 0.5%

Red Shift Effect on the Photon Energy SpectraObserved photon energy Eγ is given by Eγ = Eγ rest /(1+z) ，where z is a red
shift and Eγ rest is a photon energy without Doppler shift.

ν 2 → ν1 + γ

ν3 →ν 2 + γ

ν3 Decay Lifetime versus m3
SU(2)L x SU(2)R x U(1) Model

Δm232 = (2.43 ±0.13) x10-3eV2±1σ bandνdI/dν (nW m-2 sr-1)

Lower Limit of Lifetime from the Energy Spectrum Fit to
the CIB measured by COBE and AKARI
χ2= 6.6
τ= (2.8±0.2) x1012 year
= (0.85±0.06) x1020 sec

COBEAKARI

Photon Energy (eV)Lifetime limit vs m3
2.4 x 1012 years.
We need 4 - 5 order
Improvements.

m3 (meV)

Using the CIB at 60, 100（ ApJ, 544, 81, 2000 ), 140, 240 µm ( ApJ, 508, 25, 1998 ), 65, 90,
140, and 160 µm ( arXiv:1002.3674, 2010 ), the photon energy spectrum from neutrino radiative
decay gives a lifetime lower limit of 2.4 x 1012 year at 95% C.L. for m3 = 0.05eV and m2 =0.01eV. ( My calculation )Basic Properties of STJ Detector
By measuring the curve of current -voltage ( I-V curve ), weknow
z Superconducting phase transitionz Josephson junction
Current I
z Leakage currentz Energy gap
Josephson Current

Voltage V

Critical Voltage Vc : 2Δ/e mV
Critical Current Ic : a few ~ a few 100μA

Nb/Al - STJ IV Curve
Tunnel barrier

T = 0.4K

Josephson Current issuppressed by amagnetic field

V: 1mV/div(horizontal)
I: 5 mA/div(vertivcal)

Nb/Al - STJ Response to 5.9keV X rays
by RIKEN group
5.9keV X ray signal

Double peak comesfrom that X rays
are absorbed bothin the upper layer
and the under layer.

Surface Flatness measured with AFM

2.0Pa 70W
No Distortion
RMS 3.5nm
1/4* AFM : Atomic Force Microscopy

0.5Pa 50W
Distortion Force ~1.4GPa
RMS 0.9nm

Hf/Al/AlOx/Hf-STJ I-V Curve
Oxidation Condition: 30Torr 1hour

Leak Current and Ic are high.

Al(20nm)/AlOx

HfwaferV : 10μV/div
I : 20 μA/div

IV

Hf

Josephson Current issuppressed by amagnetic field of2GaussSPICA SENSITIVITY
By Yasuo Doi at SPICAWorkshop Dec 16-17, 2010

1Jy = 10－26Wm-2Hz-1
SPICA (launched in 2018) sensitivity: For photons with a wavelength of 30－200μ,1-hour measurement gives 5σlimit of 4ｘ10－19Wm-2.
Assuming the signal spreads uniformly over FOV 0.1degree (6.0arcmin), 5σlimit is3.2x10-12Wm-2sr-1.
NEP=2ｘ10－19 WHz-1/2 is good enough.

PlansSuperconducting Detector R&Dz2011- start a collaboration with Fermilab Milli-Kelvin Facility group
who will work on the readout electronics at low temperaturearound 1K.