Hirohisa Nagata

The Infrared Astronomical Mission AKARI

AKARI, the first Japanese satellite dedicated to infrared astronomy, was launched on 2006 February 21, and started observations in May of the same year. AKARI has a 68.5 cm cooled telescope, together with two focal-plane instruments, which survey the sky in six wavelength bands from mid- to far-infrared. The instruments also have a capability for imaging and spectroscopy in the wavelength range 2-180 mu m in the pointed observation mode, occasionally inserted into a continuous survey operation. The in-orbit cryogen lifetime is expected to be one and a half years. The All-Sky Survey will cover more than 90% of the whole sky with a higher spatial resolution and a wider wavelength coverage than that of the previous IRAS all-sky survey. Point-source catalogues of the All-Sky Survey will be released to the astronomical community. Pointed observations will be used for deep surveys of selected sky areas and systematic observations of important astronomical targets. These will become an additional future heritage of this mission.

The Far-Infrared Surveyor (FIS) for AKARI

The Far-Infrared Surveyor (FIS) is one of two focal-plane instruments on the AKARI satellite. FIS has four photometric bands at 65, 90, 140, and 160 mu m, and uses two kinds of array detectors. The FIS arrays and optics are designed to sweep the sky with high spatial resolution and redundancy. The actual scan width is more than eight arcminutes, and the pixel pitch matches the diffraction limit of the telescope. Derived point-spread functions (PSFs) from observations of asteroids are similar to those given by the optical model. Significant excesses, however, are clearly seen around tails of the PSFs, whose contributions are about 30% of the total power. All FIS functions are operating well in orbit, and the performance meets the laboratory characterizations, except for the two longer wavelength bands, which are not performing as well as characterized. Furthermore, the FIS has a spectroscopic capability using a Fourier transform spectrometer (FTS). Because the FTS takes advantage of the optics and detectors of the photometer, it can simultaneously make a spectral map. This paper summarizes the in-flight technical and operational performance of the FIS.

Cryogenic capacitive transimpedance amplifier for astronomical infrared detectors

We have developed a new capacitive transimpedance amplifier (CTIA) that can be operated at 2 K, and have good performance as readout circuits of astronomical far-infrared array detectors. The circuit design of the present CTIA consists of silicon p-MOSFETs and other passive elements. The process is a standard Bi-CMOS process with 0.5 /spl mu/m design rule. The open-loop gain of the CTIA is more than 300, resulting in good integration performance. The output voltage swing of the CTIA was 270 mV. The power consumption for each CTIA is less than 10 /spl mu/W. The noise at the output showed a 1/f noise spectrum of 4 /spl mu/V//spl radic/Hz at 1 Hz. The performance of this CTIA nearly fulfills the requirements for the far-infrared array detectors onboard ASTRO-F, Japanese infrared astronomical satellite to be launched in 2005.

Progress on GaAs cryogenic readout circuits for SISCAM

We are developing cryogenic readout circuits for the array of superconducting tunneling junctions (STJs) at submillimeter wavelength SISCAM (Superconductive Imaging Submillimeter-wave CAMera). A current conceptual design of SISCAM will employ a direct hybrid array system just like CMOS image sensors widely used at optical and infrared wavelength. Because of relatively large impedance of the STJ fabricated by RIKEN (~10 MΩ in a dark condition), it requires readout preamplifier with low current noise. Therefore, it is not suitable for the STJ to use a readout system by Superconductive Quantum Interferences Devices as for Transition Edge Sensor. Instead, we selected capacitive transimpedance amplifier (CTIA) using a SONY n-type GaAs Junction Field Effect Transistor (JFET). However, the CTIA has not been used as the readout of the STJ. Therefore, we measured the photocurrent of the STJ by the CTIA with Silicon JFETs and by transimpedance amplifier (TIA), which is a conventional readout for the STJ, in the same bias condition, and confirmed both results are in good agreement. Additionally, we report development of readout integrated circuits with GaAs JFETs. In order to design the CTIA circuit with the GaAs JFETs, we fabricated the independent GaAs JFETs and matched pairs of them. We measured electrical characteristics of these GaAs JFETs at the cryogenic temperatures less than 4.2 K. We demonstrated performance of an operational amplifier fabricated with the GaAs JFETs measuring a differential amplifier with the dual GaAs JFET, and additionally estimate amplifier gain, offset voltage, and power consumption of the CTIA by the circuit simulation using the PSPICE. In consequence, the expected performance fulfills the requirements for the readout amplifier of the STJs except for the noise performance.

CONSTRAINTS ON THE INTRACLUSTER DUST EMISSION IN THE COMA CLUSTER OF GALAXIES

We have undertaken a search for the infrared emission from the intracluster dust in the Coma cluster of galaxies by the Multiband Imaging Photometer for Spitzer. Our observations yield the deepest mid and far-infrared images of a galaxy cluster ever achieved. In each of the three bands, we have not detected a signature of the central excess component in contrast to the previous report on the detection by Infrared Space Observatory (ISO). We still find that the brightness ratio between 70 μm and 160 μm shows a marginal sign of the central excess, in qualitative agreement with the ISO result. Our analysis suggests that the excess ratio is more likely due to faint infrared sources lying on fluctuating cirrus foreground. Our observations yield the 2σ upper limits on the excess emission within 100 kpc of the cluster center as 5 × 10–3 MJy sr–1, 6 × 10–2 MJy sr–1, and 7 × 10–2 MJy sr–1, at 24, 70, and 160 μm, respectively. These values are in agreement with those found in other galaxy clusters and suggest that dust is deficient near the cluster center by more than 3 orders of magnitude compared with the interstellar medium.

Design of wide-field submillimeter-wave camera using SIS photon detectors

SIS photon detectors are niobium-based superconducting direct detectors for submillimeter-wave that show superior performance when compared with bolometric detectors for ground-based observations. We present the design and development of the SIS photon detectors together with optical and cryogenic components for wide field continuum observation system on Atacama Submillimeter Telescope Experiment (ASTE). Using antenna coupled distributed junctions, SIS photon detectors give wide band response in a 650-GHz atmospheric window as well as high current sensitivity, shot noise limited operation, fast response and high dynamic range. Optical noise equivalent power (NEP) was measured to be 1.6x10-16 W/Hz0.5 that is less than the background photon fluctuation limit for ground based submillimeter-wave observations. Fabrication of focal plane array with 9 detector pixels is underway to install in ASTE. Readout electronics with Si-JFETs operating at about 100 K will be used for this array. Development of readout electronics for larger array is based on GaAs-JFETs operating at 0.3 K. For the purpose of installing 100 element array of SIS photon detectors, we have developed remotely operable low-vibration cryostat, which now cools bolometers for 350, 450, 850-µm observations down to 0.34 K. GM-type 4-K cooler and He3/He4 sorption cooler is used, which can be remotely recycled to keep detectors at 0.34 K. Since we have large optical window for this cryostat, sapphire cryogenic window is used to block infrared radiation. The sapphire window is ante-reflection coated with SiO2 by chemical vapor deposition (CVD). The transmittance of the cryogenic window at 650 GHz is more than 95%.

Performance of SIS photon detectors for superconductive imaging submillimeter-wave camera (SISCAM)

High sensitivity submillimeter-wave focal plane array using SIS photon detector is being developed, which we call SISCAM, the superconductive imaging submillimeter-wave camera. In the course of the detector evaluations, we have measured performance of the SIS photon detectors under various operating conditions. Advantages of the SIS photon detectors are explained by the nature of antenna coupled quantum detectors. Their input coupling can be designed to have band-pass characteristics owing to the distributed junction design. This reduces requirements for infrared blocking filters and enhances optical efficiency. The detector performance is evaluated under background loading and they show background limited performance. Measurement at 4 K shows the SIS photon detector operates under shot noise limit of thermal leakage current and its NEP is 1x10-14 W/Hz0.5, that is better than bolometers at 4.2 K, whereas the same detector has NEP of 10-16 W/Hz0.5 at 0.3 K. Dynamic range of SIS photon detectors is estimated to be higher than 109, which surpass the dynamic range achievable with TES bolometers. Nine-element array of SIS photon detector, SISCAM-9, is developed and their performance is evaluated in a submillimeter-wave telescope. With a development of integrated electronics with GaAs-JFET charge integrating readout circuit, the SIS photon detector will be an ideal imaging array in submillimeter-wave region. Due to its large dynamic range and shot noise limited performance under various operating condition, SIS photon detectors can be used for various astronomical instrumentations as well as for other fields of terahertz technologies.

The Cryogenic Digital Readout Module with GaAs JFET ICs

In general, it is known that n‐type GaAs JFETs offer good cryogenic performance. This makes them suitable for use with high impedance cryogenic detectors. We designed several kinds of integrated circuits (ICs) with n‐type GaAs JFETs and demonstrated their performance at cryogenic temperatures. Our AC‐coupled capacitive trans‐impedance amplifiers (CTIA) showed good performance. In particular, the input offset voltages were less than +/−0.5 mV. The voltage hold time of the sample‐and‐holds was more than 0.1 seconds. The maximum clock speed of the NAND type shift‐registers was 100 kHz or greater. Power dissipations of each circuit are 0.1–3 μW per one unit. These properties express the possibility to realize the multi‐channel cryogenic digital readout system. We have fabricated 16‐channel AC coupled CTIAs, 32:1 multiplexers with sample‐and‐holds, 32‐channel shift‐registers, and 32‐channel voltage distributors. We describe plans for using this technology in the near future to build a 32‐channel cryogenic digi...

The cryogenic readout system with GaAs JFETs for multi-pixel cameras

Our purpose is to realize a multi-pixel sub-millimeter/terahertz camera with the superconductor - insulator - superconductor photon detectors. These detectors must be cooled below 1 K. Since these detectors have high impedance, signal amplifiers of each pixel must be setting aside of them for precise signal readout. Therefore, it is desirable that the readout system work well even in cryogenic temperature. We selected the n-type GaAs JFETs as cryogenic circuit elements. From our previous studies, the n-type GaAs JFETs have good cryogenic properties even when those power dissipations are low. We have designed several kinds of integration circuits (ICs) and demonstrated their performance at cryogenic temperature. Contents of ICs are following; AC coupled trans-impedance amplifiers, voltage distributors for suppressing input offset voltage of AC coupled CTIAs, multiplexers with sample-and holds, and shift-registers for controlling multiplex timing. The power dissipation of each circuit is 0.5 to 3 micro watts per channel. We also have designed and manufactured 32-channel multi-chip-modules with these ICs. These modules can make 32- channel input photo current signals into one or two serial output voltage signal(s). Size of these is 40mm x 30mm x 2mm and estimated total power dissipation is around 400 micro watts.

Development of a cryogenic GaAs AC-coupled CTIA readout for far-infrared and submillimeter detectors

We have been developing cryogenic readout integrated circuits (ROICs) for sensitive detectors at far-infrared and submillimeter wavelengths: The ROICs are constructed from SONY GaAs-JFETs, which have excellent performance even at less than 1 K. In addition, it is suitable device for ultra low background applications because of the extremely low gate leakage current. In the spring of 2008, we have designed and fabricated 4-ch AC-coupled capacitive transimpedance amplifiers and several basic digital circuits giving multiplex function for 32-element SIS photon detector array. The expected performance of the amplifier is as follows; open loop gain of >2000, power consumption <1.5 μW, and input referred noise ~ 1 μV/√Hz@1Hz. A summary of this 2008s experimental production and initial test results are presented in this paper.