IWSSD2014

November 8, 2014

Shin-Hong Kim (University of Tsukuba)
for Neutrino Decay collaboration
Neutrino Decay Collaboration
S.H. Kim, Y. Takeuchi, K. Takemasa, K. Kiuchi, K. Nagata, K. Kasahara, T. Okudaira, T. Ichimura,
M. Kanamaru, K.Moriuchi, R. Senzaki (University of Tsukuba), H. Ikeda, S. Matsuura, T. Wada
(JAXA/ISAS), H. Ishino, H. Kibayashi (Okayama University), S. Mima (RIKEN), T. Yoshida,
K. Orikasa, R. Hirose (University of Fukui), Y. Kato (Kinki University), M. Hazumi, Y. Arai (KEK),E. Ramberg, J.H. Yoo, M. Kozlovsky, P. Ruvinov, D. Sergatskov (Fermilab),
S.B. Kim (Seoul National University)● Introduction
Motivation
Proposal on Search for Cosmic Background Neutrino Decay
Preparatory Rocket experiment● R&D of Superconducting Tunnel Junction (STJ) Detector1 Only neutrino mass is unknown in elementary particles. Detection of neutrino decay
enables us to measure an independent quantity of m2 measured by neutrino
oscillation experiments. Thus we can obtain neutrino mass itself
from these two independent measurements.
3W,

2   
322m 32  m 22 m 23E 
2m 32m 32Using m 23
 ( 2.43  0.09)  10  3 eV2E  10～25 meV at  3 rest frame.
(Far - Infrared region   50～125  ) As the neutrino lifetime is very long, we need use cosmic background neutrino toobserve the neutrino decay. To observe this decay of the cosmic backgroundneutrino means a discovery of the cosmic background neutrino predicted bycosmology.
2 Left-Right symmetric model predicts the neutrino lifetime larger than 1017 year
while the standard model predicts 2 x 1043 year.
Measured neutrino lifetime limit τ > 3 x 1012 year.

3Surface brightness I [MJy/sr]

Rocket experiment coverage (λ＝40～80μm)
CMB

Zodiacal Emission

Zodiacal LightAKARI COBE

CIB
measurements( AKARI, COBE)High Energy
Cutoff (25meV)
Galaxy evolution
modelInterstellar
DGL
DustSL
Integrated flux from galaxy counts

wavelength [m]E [meV]By measuring the energy
spectrum of the Zodiacal
Emission with the CνB
decay continuously, we cansee the CνB decay signal asa high energy cutoff.

Plan: 5minutes data acquisition at 200 km height in 2017 in earliest.
Improve the current limit of lifetime τ(ν3) by two orders of magnitude ( ~1014years).
»Superconducting Tunneling Junction (STJ) detectors in development
˃ Array of 50 Nb/Al-STJ pixels with diffraction grating covering 𝝀 = 𝟒𝟎 − 𝟖𝟎𝝁𝒎

JAXA Rocket CIBExperiment（Feb 2, 1992）

Satellite experiment after 2020 →sensitivity of τ(ν3) ~1017year
5˃ STJ using Hafnium: Hf-STJ for satellite experiment ( S. H. Kim et al. JPSJ 81,024101 (2012)）
 Δ = 20𝜇eV : Superconducting gap energy for Hafnium
 𝑁q.p. = 25meV 1.7Δ = 735 for 25meV photon: Δ𝐸 𝐸 < 2% if Fano-factor is less than 0.3

» Superconductor / Insulator / Superconductor Josephson Junction
At the superconducting junction, quasi-particles overtheir energy gap go through tunnel barrier by a tunnel
effect. By measuring the tunnel current of quasiparticles excited by an incident particle, we measure
the energy of the particle.
Quasi-particlescurrent-voltage (I-V) curvefor STJ

Incident
Particle
2Δ

Josephson Current
Critical current Ic

Cooper Pairs
SEnergy gap ΔIS
6Leakage current( Dynamic resistance Rd in |V| < 2Δ/e)

Nb/Al-STJ
Goal: detection of a single far-infrared photon in the energy range of 15 – 30 meV(λ= 40 -80μm) for the rocket experiment for neutrino decay search.
Signal of Nb/Al-STJ (100 x 100μm2) toinfrared (1.31μm) light at 1.9K.
Time spread at FWHM is 1μsec.
The number of photon : 93±11( from the spread ofthe signal charge distribution ).
the response of Nb/Al-STJ (4μm2 ) to the visible
light (465nm) at 1.9K.
a single photon peak is separated from pedestal by1σ.
The signal charge distribution (Red histogram) is
fitted by four Gaussians of 0, 1, 2 and 3 photon
peaks. Single photon peak has a mean of 0.4fC
and σ of 0.4fC.

Signal charge
distribution7

We need the preamplifier operated at extremely low temperature to detect asingle far-infrared photon (λ= 40 -80μm) .
●Operation at ultra-low temperature• Requirement for leakage current ofNb/Al-STJ is below 100pA.
We reduce it by using smaller one.
−Temperature Dependence of Leakage
Current with Nb/Al-STJ(100x100μm2 )100pA@800mK

𝜟𝒌𝒃𝑻

Thermal excitation(∝ 𝑻𝒆)
We need to make cooler 800mK.
1. Operation below 800mK
3He sorption refrigerator is our candidate●To 100pA

Low power consumption• Typical cooling power of our refrigerator is 400μW.
2. Power consumption of the amplifier should be as low as possible.

●Response speed• The integration time of charge is 2-4μs.
3. Amplification gain should be large enough up to 1MHz.
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FD-SOI (Silicon-On-Insulator) device was proved tooperate at 4K by a JAXA/KEK group (AIPC 1185,286289(200 FD-SOI 9)). It has the following
characteristics:
low-power consumption, high speed, easy large scale
integration and suppression of charge-up by highmobility carrier due to thin depletion layer(～50nm).

To improve the signal-to-noise ratio andto make multi-pixel device easily, we madea SOI-STJ detector where we processed
Nb/Al-STJ on a SOI transistor board.

FD-SOI -MOSFET～50nm

We confirmed that both Nb/Al-STJ
detector and SOI MOSFET workednormally at 750mK.

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We measured the I-V curve of the Nb/Al-STJ (50 x50μm2 junction) processed on the SOI wafer
at 700mK with a dilution refrigerator.

 I-V curve of Josephson Junction
1 mA /DIV.
2 mV /DIV.

Ic～200μA
B=35 G

1 mA /DIV.
2 mV /DIV.

 Leak current @ 0.5mV is 6nA. It isclose to our best record of normal
Nb/Al-STJ(100um x 100um) 10nA.

10 nA /DIV.
500μV /DIV.

 Signal of Nb/Al-STJ in
SOI-STJ for 465nm laserpulse.

50μA /DIV.
2mV /DIV.

 Quality Factor(Rdynamic/Rnormal)

On Si wafer : 5 x 105
On SOI wafer : 3 x 105

10Performance of SOIFET in SOI-STJ detector● Temperature dependence

I-V curves at various temperatures.
SOIFET can be operated at 100mK.

● Power consumption ( at 680mK )Bias voltage of SOIFET in saturation
region (red line) : 0.5 V
Current (Ids)of FET in saturation region
at Vgs = 0.8V : 0.09 μA
Power consumption = 0.5 V ×0.09 μA= 45 nW/FETfor W/L=1.42μm/0.42μm
11Future Plan of SOI-STJ R&D
We are updating the SOI-STJ design for theamplification of the Nb/Al-STJ signal.
1.
2.
3.

Replace the resistance to SOIFET that we use
as a current source.
Use the feedback between the drain and thegate to apply a stable bias voltage
Add the follower to reduce the output
impedance.

Designed the ratio (W/L) to set the operationpower consumption below 120μW.
AIST group joined us on the SOI-STJ R&D and isprocessing the SOI on the STJ3 board made by LAPIS.

We will measure the response of this new
SOI-STJ to laser light soon.
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Hf-STJ
Goal: Measure energy of a single far-infrared photon for neutrino decay searchexperiment within 2% energy resolution.

I-V curve of Hf-STJ (200x200m2 )• T~80mK, Ic=60μA, Rd=0.2Ω

B=0 Gauss

Hf-STJ (100x100m2) shows smaller leakage
current than Hf-STJ (200x200m2) which wehave established to work as a STJ in 2011.

The work to reduce a large leakage current ofHf-STJ is underway.

B=10 Gauss

I-V curve of Hf-STJ (100x100m2 )• T~40mK, Ic=10μA, Rd=0.6Ω

B=0 Gauss

B=10 Gauss
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Hf-STJ Response to DC visible light
I-V Curve with and withoutvisible laser light (λ=465nm f=100kHz)
Hf(250nm)
Hf(350nm)

Red；with light
Yellow：without light

Si wafer
HfOx：20Torr,1hour

100×100μm2
T=39~53mK
Rd=0.6Ω

1.5nm

ΔI =10μA
at 20 μV

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V : 20μV/div
We are testing smaller Hf-STJ’s to decrease the leakage current.

1. We are developing STJ-based detectors to detect a single
far-infrared photon in energy range between 15 and 30meV tosearch for the cosmic background neutrino decay with arocket or satellite experiment.
2. The SOI-STJ detector where Nb/Al-STJ’s were processedon a SOIFET board is being developed. Both SOIFET and STJ
are working well in the SOI-STJ detector below 800mK.
3. Hf-STJ response to the visible light was observed.
Improvement to reduce the leakage current is underway.
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