Masaharu Muramatsu

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.

Microdischarges of AC-coupled silicon strip sensors

Abstract Microdischarge at the edges of strips in AC-coupled silicon strip sensors has been investigated. A steep increase in the leakage current (breakdown) and a sudden onset of burst noise were observed at a low reverse-bias voltage when the bias potential was across the AC-coupling capacitor. This can be explained by the occurrence of microdischarges along the edges of strips. These discharges have been confirmed by observing IR light emission. A calculation of the field strength at the strip edge suggests that a fringe field of the external electrode generates the microdischarge at the strip edge when the bias voltage is 50–80 V. This is consistent with our observations. We discuss a design for AC-coupled sensors that eliminates this discharge problem.

Double-sided microstrip sensor for the barrel of the SDC silicon tracker

Abstract A full-size prototype microstrip sensor for the silicon tracker of the SDC detector to be used at the Superconducting Super Collider has been fabricated at Hamamatsu Photonics. The sensor is double-sided, using an AC-coupled readout with 50 μm pitch strips. The sensor size is 3.4 × 6.0 cm 2 . Polycrystalline silicon is used as a bias feeding resistor on both surfaces. Each ohmic strip is isolated by a p + blocking line. The detailed requirements for the silicon tracker and the corresponding specifications as well as how to achieve them are discussed. The static performances of this prototype sensor are presented.

Recent results of the fully-depleted back-illuminated CCD developed by Hamamatsu

We have been developing fully-depleted CCDs fabricated on N-type silicon wafer in collaboration with HAMAMATSU Photonics K.K.We have made several wafer runs to optimize the basic characteristics of the devices such as the charge transfer efficiency (CTE), the full-well capacity and the amplifier gain, followed by the optimization of the backside treatment to improve quantum efficiency (QE) in blue wavelengths. The optimization process is successfully completed, and Hamamatsu recently started to deliver the 2k × 4k (15 μm pixel) four-side buttable devices for acceptance evaluation at the National Astronomical Observatory of Japan. Based on the measured QE in the X-ray, the depletion depth reaches 200 μm with CTE as good as >0.999995 for serial and parallel directions and with readout noise of < 5 e- for 130 kHz readout. The size of charge diffusion is estimated to be < 7.5 μm (one sigma) for pinhole image at wavelength of 450 nm. The device flatness is < 15-20 μm, and the dark current is a few e-/hour/pixel at -100°C and ~ 20 e-/hour/pixel at -80°C.

Development of thick back-illuminated CCD to improve quantum efficiency in optical longer wavelength using high-resistivity n-type silicon

Quantum Efficiency (QE) of CCDs decreases at λ >~ 0.7 μm since photons penetrate a depletion layer of CCD. If one makes the layer thicker, the QE will be largely improved. In collaboration with HAMAMATSU Photonics, we have been developing the thicker CCDs which are implemented on the high resistivity n-type silicon wafers. We have made several wafer runs to optimize the basic characteristics of the devices such as charge transfer efficiency (CTE), full-well and node sensitivities of the amplifiers. The results obtained so far mostly satisfied the specifications imposed by astronomical observations. We also attempted to build back-side illuminated devices to realize high QE in wider wavelength. The test devices shows that the QE exceeds 60% at 1 μm, which is roughly 5 ~ 6 times improvement over ordinary CCDs. We will present the current status of the projects.

Characterization and performance of hyper Suprime-Cam CCD

Hyper Suprime-Cam (HSC) is a second-generation wide field imaging camera for Subaru telescope with 10 times wider field of view (FOV) compared with Suprime-Cam (SC) currently being used. HSC makes the survey speed considerably faster than SC, while maintaining the high image quality of SC. The 1.5 degrees in diameter FOV is covered with 116 of 2K × 4K fully depleted back-illuminated CCDs with 15 μm pixels developed by HAMAMATSU Photonics K. K. and National Astronomical Observatory of Japan (NAOJ). The CCDs for HSC are designed to have higher quantum efficiency than those for SC in a wider range in the visible wavelengths, especially in the blue region. We at NAOJ have started acceptance inspection of the CCDs being delivered from HAMAMATSU. We used the X-ray source of 55Fe and the LED to measure charge transfer efficiency, readout noise, linearity, and full-well capacity of 33 CCDs. In addition, we measured the quantum efficiency of 7 CCDs. We confirmed all the CCDs have good performances and quality. In this paper, we report the results from the acceptance inspection and characterization of these CCDs.

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

Evaluation of the fully-depleted back-illuminated CCD for Subaru Suprime-Cam

In order to improve the quantum efficiency (QE) at longer wavelength, we have developed fully-depleted backilluminated CCDs in collaboration with Hamamatsu Photonics K.K (HPK). Recently, HPK delivered 10 CCDs for Subaru Prime Focus Camera (Suprime-Cam). These CCDs are made on N-type, high resistivity silicon wafers. Each CCD has a 200 μm thick depletion layer. The CCD format is four-side buttable, 2k × 4k, 15 μm square pixels with 4 low noise output amplifiers. The characteristics of the CCDs have been tested in the laboratory before they are installed into Suprime-Cam dewar. These CCDs have excellent performance; readout noise < 5 e-, dark current < 2 e-/hour/pixel, parallel and serial charge transfer efficiency (CTE) > 0.999995, and full-well ~ 180,000 e-. The QE of λ = 1 μm was 40 % at -100°C. All CCDs have good cosmetics. Surface flatness is ~ 25 μm peak to value (P-V). The specification was acceptable. We are also developing CCDs for Hype Suprime-Cam (HSC), the next generation instrument for Subaru Telescope. HPK optimized back side process and has developed blue enhanced CCDs for HSC.

Development of p-Channel Charge-Coupled Device for NeXT, the Next Japanese X-ray Astronomical Satellite Mission

We are developing an X-ray charge-coupled device (CCD) for the next Japanese X-ray astronomical satellite mission, NeXT (New X-ray Telescope/Nonthermal energy eXploration Telescope). We developed a trial product of the p-channel CCD fabricated on an n-type silicon wafer. It is possible to have a thick depletion layer of ~300 µm with a p-channel CCD because it is easy to obtain high resistivity using an n-type silicon wafer compared with a p-type silicon wafer. We evaluated the performance of the p-channel CCD. The imaging area of the CCD consists of 512×512 pixels with a pixel size of 24×24 µm2. The horizontal charge transfer inefficiency (CTI) of the CCD can be improved by reducing the operating temperature and increasing the readout frequency. We obtained the best horizontal CTI of (0.98±0.09)×10-5 with an energy resolution of (202±6) eV full width at half maximum (FWHM) for 5.9 keV X-rays and a readout noise of 18 e- (rms) when the CCD was operated at a temperature of -110 °C and a readout frequency of 67 kHz. We measured the thickness of the depletion layer to be (290±33) µm from the detection efficiency of the 22.4 and 24.9 keV emission lines from 109Cd.

Development of the x-ray CCD for SXI on board ASTRO-H

We report on the development of the X-ray CCD for the soft X-ray imager (SXI) onboard ASTRO-H. SXI CCDs are P-channel, back-illuminated type manufactured by Hamamatsu Photonics K. K. Experiments with prototype CCD for the SXI shows the device has a depletion layer as thick as 200μm, high efficiency for hard X-rays. By irradiating soft X-rays to the prototype CCD for the SXI. At the same time, we found a significant low energy tail in the soft X-ray response of the SXI prototype CCD. We thus made several small size CCD chips with different treatment in processing the surface layers. CCDs with one of the surface layers treatment show a low energy tail of which intensity is one order of magnitude smaller than that of the original SXI prototype CCD for 0.5keV X-ray incidence. The same treatment will be applied to the flight model CCDs of the SXI. We also performed experiments to inject charge with the SXI prototype CCD, which is needed to mitigate the radiation damage in the orbit. We investigated the operation conditions of the charge injection. Using the potential equilibration method, charges are injected in each column homogeneously, though the amount of the charge must be larger than 20ke-.