1. R&D of STJ Detector We also looked at the response of Nb/Al-STJ (4μm 2 ) to the visible light (456nm) at 1.9K, and found that a single photon peak is separated from pedestal by 1σ. 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. As we aim at 5σ separation of one photon signal from pedestal, we need some low noise preamplifier working around 1K such as SOI. Feasibility of Photon Detection from Cosmic Background Neutrino Search for Cosmic Background Neutrino Decay Shin-Hong Kim (University of Tsukuba) For Neutrino Decay collaboration S. H. Kim, Y. Takeuchi, K. Nagata, K. Kasahara, T. Okudaira (Univ. of Tsukuba), H. Ikeda, S. Matsuura, T. Wada (JAXA/ISAS), H. Ishino, A. Kibayashi (Okayama Univ.), S. Mima (RIKEN), T. Yoshida, S. Kobayashi, K. Origasa (Fukui Univ.), Y. Kato (Kinki Univ.), M. Hazumi, Y. Arai (KEK), E. Ramberg, J. H. Yoo, M. Kozlovsky, P. Rubinov, D. Sergatskov (FNAL), S. B. Kim (Seoul National Univ.) SPICA Conference, Jun. 18-21, 2013 at University of Tokyo SOI-STJ We have processed Nb/Al-STJ on a SOI wafer, and confirmed that both Nb/Al-STJ detector and SOI MOSFET worked normally at 700mK. In the next step, we will look at the response signal of SOI-STJ to photons. Introduction Only neutrino mass is unknown in the present elementary particles. To determine the neutrino masses is an important key issue to reveal the mass origin as well as the study of Higgs characteristics. Δm 2 ij have been measured accurately by various neutrino oscillation experiments. but neutrino mass itself has not been measured. Detection of neutrino decay enables us to measure an independent quantity. Thus we can obtain neutrino mass itself from these two independent measurements. 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 background neutrino means a discovery of the cosmic background neutrino predicted by cosmology. The current lifetime limit is about 3 x 10 12 year obtained from COBE and AKARI CIB data. Cosmic Infrared Background Neutrino Decay (τ ＝ 1.5 x 10 17 year ） Sharp edge with 1.9K smearing and detector resolution of 0% to 5% 5σ observation sensitivity Search Region: λ ＝ 35 ～ 250μm （ E γ ＝ 35 ～ 5meV ） In Rocket experiment, λ ＝ 40 ～ 80μm （ E γ ＝ 31 ～ 15meV ） dN/dE γ d 2 N γ /dE γ 2 for energy resolutions of 0~3% Rate/50pixel-spectrometer = 1.4 kHz ( 28Hz/pixel) Measurements for 200 s → 280 k events /50pixel-spectrometer. Using 8 x 50pixel- spectrometer, σ/N=0.066% 2σ = 0.13% x 0.5μW/m 2 /sr = 0.65nW/m 2 /sr (1.3% times present limit 50nW/m 2 /sr ） Zodiacal LightZodiacal Emission Galactic dust emission Wavelength[  m] Integrated flux from galaxy counts galaxy evolution model CIB measurements(  AKARI,  COBE) Astrophys. J. 737 (2011) 2 Red shift effect 6.7  Nb/Al-STJ We aim at detecting a single infrared photon for the neutrino decay search experiment. We measured the response of Nb/Al-STJ (100μ x 100μ) to the infrared (1.31μm) light at 1.9K. Time spread at FWHM is 1μsec. The number of photon was estimated to be 93±11 from the spread of the signal charge distribution. Square is 2.9 mm on a side. 200nA /DIV. leak current of Nb/Al-STJ is about 6nA at 0.5mV. pedestal Signal charge distribution 50μV/DIV 0.8μs/DIV Signal charge distribution Source Drain Gate STJ 2mV /DIV. After applying150 Gauss to STJ. Signal shape (1.31μ, N(photon)=93±11)