Koei Yamamoto

Hyper Suprime-Cam

Hyper Suprime-Cam (HSC) is an 870 Mega pixel prime focus camera for the 8.2 m Subaru telescope. The wide field corrector delivers sharp image of 0.25 arc-sec FWHM in r-band over the entire 1.5 degree (in diameter) field of view. The collimation of the camera with respect to the optical axis of the primary mirror is realized by hexapod actuators whose mechanical accuracy is few microns. As a result, we expect to have seeing limited image most of the time. Expected median seeing is 0.67 arc-sec FWHM in i-band. The sensor is a p-ch fully depleted CCD of 200 micron thickness (2048 x 4096 15 μm square pixel) and we employ 116 of them to pave the 50 cm focal plane. Minimum interval between exposures is roughly 30 seconds including reading out arrays, transferring data to the control computer and saving them to the hard drive. HSC uniquely features the combination of large primary mirror, wide field of view, sharp image and high sensitivity especially in red. This enables accurate shape measurement of faint galaxies which is critical for planned weak lensing survey to probe the nature of dark energy. The system is being assembled now and will see the first light in August 2012.

Electron bombardment CCD tube

For low light level imaging application, a proximity focused electron bombardment CCD (EB-CCD) tube has been developed. In the tube, electrons emitted from the multi-alkali (S-20) photocathode in response to incident light are accelerated by the electric field and bombarded the specially processed CCD which is sensitive to electrons. The electron bombardment gain is 600 at applied voltage of -8kV to the photocathode. Single photon counting operation is possible, because the gain is larger than the readout noise and the dark noise of the CCD. The spatial resolution is better than 360 TV lines, which is the theoretical limit of the full frame transfer CCD (FFT-CCD) of 512 by 512 pixels. No major degradation of either the photocathode sensitivity or the incorporated CCD was observed during the operation for a few tens hours. The life of the EB-CCD tube is being under evaluation. Keywords: Electron tube, Photocathode, Image intensifier, Electron-bombardment, CCD, Low light level imaging

A Two-Dimensional Photon-Counting Tube

Publisher Summary A photomultiplier tube has been used as a detector for a photoelectron-counting device by making use of its high quantum efficiency and large electron gain coupled with low dark counts. There are demands for a two-dimensional photoelectron-counting imaging in many fields. At very low light levels, a device should incorporate electron multiplication and signal output preserving the input spatial information. To meet these specifications, a two-dimensional photoelectron-counting tube has been developed that has an inverter-type image intensifier structure with a silicon position sensitive device (Si-PSD) instead of a phosphor screen. For applications in the visible wavelength region, a multialkali photocathode was used. This chapter describes the two-dimensional photon-counting tube.

High resolution large formatted CMOS flat panel sensors for X-ray

This paper describes world largest monolithic CMOS flat panel sensor (FPS) and high speed amplifier with columnar CsI scintillator directly deposited onto the active area. These elements have been employed as a compact digital camera module for dynamic X-ray imaging will be used in digital mammography and scientific image measurement. The FPS has a diagonal size of 11 inches and 4,416 /spl times/ 3,520 pixel array (15,544,320 pixels), which features a high resolution of 10 lp/mm with single pixel drive. The key material of the FPS is monocrystalline silicon optimized for use in digital radiography. The sensor utilizes a direct deposited scintillator layer made from high resolution, high luminance CsI crystals grown into a needle structure onto a large formatted photodiode array for indirect detection of X-ray images. The CsI scintillation spectrum well matches the spectral response range of the photodiode. The CsI has the advantages of high sensitivity and high resolution compared to GOS phosphors screen. These image sensors are manufactured in a 0.15/spl mu/ 12 inches CMOS process allowing a high fill factor of 76% for 50/spl mu/ pixel. Only one chip is taken from one 12 inches wafer in order to realize a seamless active area. The monolithic amplifier blocks have 4,416 channels of charge amplifiers with internal CDS (correlated double sampling) circuit has an optimal design yielding a high gain of 0.26 /spl mu/V per electron and a data transfer speed of 17.8 Mbytes per second in sufficient low noise. This paper discusses the system design, dynamic range, resolution, detective quantum efficiency (DQE) and imaging performance using ACR mammography phantom.

Utilization of MOS gate structure for capacitive charge division readout of silicon strip detector

Abstract We have developed a new method to supply bias voltage to the floating strips of a silicon strip detector by utilizing MOS gate structure. The voltage of the floating strips is controlled by the voltage of the MOS gate.

Electro-optic KTN deflector stabilized with 405-nm light irradiation for wavelength-swept light source

We have developed a highly stable electro-optic KTa1-xNbxO3 (KTN) deflector by enhancing electron transportation through KTN crystal. The amount of current is increased with 405-nm light irradiation to rapidly generate a stable refractive-index change, which induces deflection. The deflection angle is set at 160 mrad within tens of seconds and is kept at that angle for 3,000 hours. The developed deflector has been applied to a wavelength-swept light source to measure the thickness of Si wafers with a 3.6-mm optical length. The precision of 0.1-μm has been continuously achieved corresponding to the stability of the KTN deflector.

Stable wavelength-swept light source designed for industrial applications using KTN beam-scanning technology

Using light-beam scanning technology based on a potassium tantalate niobate (KTa1-xNbxO3, KTN) single crystal, we constructed a wavelength-swept light source for industrial applications. The KTN crystal is placed in an external cavity as an electro-optic deflector for wavelength scanning without any mechanical operation. Cavity arrangement and mechanism elements are specially designed for long-term stability and environmental robustness. In addition, we updated the handling of the KTN crystal. We used a pair of thermistors for accurate temperature monitoring, and weakly irradiated the crystal with a 405-nm light during operation to achieve drift suppression. We selected a moderate repetition rate of 20 kHz to suit the practical application. The output of the light source was 6.2 mW in average power, 1314.5 nm in central wavelength, and 83.3 nm in bandwidth. The interference fringes of the light enable us to specify the thickness of a wafer sample by the peak positions of the point spread functions. We measured the thickness of a silicon wafer as 3651 μm in the optical path length using a reference quartz plate. The distribution of the obtained values is about 0.1 μm (standard deviation). We experimentally confirmed that this property persists continuously at least over 153 days. Our light source has a remarkable feature: extremely low timing jitter of the sweep. Thus, we can easily reduce the noise level by averaging several fringes, if necessary.

Development of new MPPC with higher NIR sensitivity and wider dynamic range

The Multi-Pixel Photon Counter (MPPC), which is also called a silicon photomultiplier (SiPM)1,2, is one promising candidate for automotive light detection and ranging (LIDAR)3. Due to high internal gain around 106, photon counting is possible and satisfies long range measurement. Compared to photo diodes (PDs) and avalanche photo diodes (APDs), read-out circuits for MPPCs are very simple because no external amplifier is needed. Conventional MPPCs have been developed for targeting blue scintillation light around 400 nm for positron emission tomography (PET) and high energy physics experiments. In this paper we report new near-infrared (NIR)-enhanced MPPCs whose development targets include 905 nm laser light for automotive LIDAR systems. Conventional MPPCs have a p-on-n structure and show 2% photon detection efficiency (PDE) at 905 nm. Our newly developed n-on-p MPPC achieved 7% PDE without greatly changing the impurity concentration profile of the depletion layer. This n-on-p MPPC has been released as an NIRenhanced MPPC: S13720-1325CS. For further improvement of NIR sensitivity, we tried several silicon wafers and process conditions of p-n junction profiles. Even though dark noise and the voltage range have to be modified, the latest sample shows 11% PDE, suggesting potential for further sensitivity improvement.

Radiation damage of the x-ray CCD

We studied the proton damage effects of the x-ray CCD. We have measured x-ray CCD performances after the irradiation of energies at 2 and 9.5 MeV, and confirmed clear degradation of charge transfer efficiency (CTE) and the energy resolution. To recover degraded CTE and the energy resolution, we tried the charge injection technique, and found the improvement of CTI and the energy resolution to be one-quarter and one-third, respectively. We also estimated the energy level of the deep trap, which causes the quantization of the dark current from the radiation-damaged pixels. The trap energy level is about 0.57 eV, or near the center of forbidden band.

Development of silicon hybrid SPAD 1D arrays for lidar and spectrometer applications

The MPPC, a sort of silicon photomultiplier, has good photon-counting ability and timing accuracy. Recently, a new type of MPPC that has excellent sensitivity in the green region or near-infrared, wide dynamic range, higher photon detection efficiency, and various format configurations (single channel and 1D array) was developed. To utilize its advantageous performance, dedicated readout electronics is required. For LIDAR applications, the red-enhanced MPPC can be used, and the functional for the readout system are estimating the number of photons and recording the precise time-of-arrival. For these requirements, a time-over-threshold circuit that can recognize the incoming energy down to 1 photon and a time-to-digital converter that can record time-of-arrival with 312ps resolution were integrated onto a single die. We have demonstrated that the system has the capability to measure distance with centimeter accuracy. For situations that require higher dynamic range, a high-speed comparator and counter array configuration can be provided. For weak-light-level applications like spectroscopy, a configuration consisting of a SPAD 1D array, active quenching circuit and gate function can be used. We will propose a 1D hybrid SPAD that is the optimal combination for various applications.