Soft X-ray Imager (SXI) Onboard ASTRO-H

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1 Soft X-ray Imager (SXI) Onboard ASTRO-H Hiroshi Tsunemi a,kiyoshihayashida a,takeshigotsuru b,tadayasudotani c, Junko S. Hiraga d, Naohisa Anabuki a, Aya Bamba c,isamuhatsukade g,takayoshikohmura e,koji Mori g, Hiroshi Murakami f, Hiroshi Nakajima a,masanobuozaki c, Hiroyuki Uchida a,makoto Yamauchi g, and the ASTRO-H SXI team a Department of Earth and Space Science, Graduate School of Science, Osaka University, Machikaneyama, 1-1, Toyonaka, Osaka, , Japan; b Department of Physics, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto, , Japan: c Institute of Space and Aeronautical Science, 3-1-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa, , Japan; d Research Center for the Early Universe Graduate School of Science, the University of Tokyo, Hongo, Bunkyo-ku, Tokyo, , Japan; e Physics Department, Kogakuin University, , Nakano, Hachioji, Tokyo, , Japan; f Department of Physics, Faculty of Science, Rikkyo University, , Nishi-Ikebukuro, Toshima-ku, Tokyo, , Japan; g Department of Applied Physics, Faculty of Engineering, University of Miyazaki, 1-1 Gakuen Kibana-dai Nishi, Miyazaki , Japan ABSTRACT We are designing an X-ray CCD camera (SXI) for ASTRO-H, including many new items. We have developed the CCD, CCD-NeXT4, that is a P-channel type CCD. It has a thick depletion layer of 200µm with an imaging area of mm square. Since it is back-illuminated, it has a good low energy response and is robust against the impact of micro-meteorites. We will employ 4 chips to cover the area more than 60 mm square. A mechanical rather than peltier cooler will be employed so that we can cool the CCD to 120 C. We will also introduce an analog ASIC that is placed very close to the CCD. It performs well, having a similar noise level to that assembled by using individual parts used on SUZAKU. We also employ a modulated X-ray source (MXS), that improves the accuracy of the calibration. The SXI will have one of the largest SΩ among various satellites. Keywords: X-ray CCD, Back-illumination, Charge injection, Optical blocking layer, ASIC, ASTRO-H 1. INTRODUCTION X-ray charge-coupled devices (CCDs) are now standard detectors in X-ray astronomy satellites. When combined with optics, they have good spatial resolution and moderate energy resolution. However, there are also some less desirable characteristics in X-ray CCDs, namely size, time resolution, pile-up, radiation damage and detection efficiency at high energy. X-ray use requires a low-noise read-out, a thick depletion layer, thin absorption layer, large format and radiation hardness. The ASCA satellite 29 employed X-ray photon counting CCDs (SIS) for the first time. The following satellites have X-ray CCDs: Chandra ACIS, 5 XMM-Newton EPIC, 26, 35 SUZAKU XIS, 12 Swift 6 and MAXI SSC. 34 ASCA, SUZAKU 17 and MAXI 16 are put into low earth orbit (LEO) where an active cooling system is required. A peltier cooler was employed, reaching 60 C in ASCA and MAXI 34 and 90 CinSUZAKU. 12 Chandra and Newton were put into deep eccentric orbit where the CCD could be passively cooled to 120 or less. Judging from the CCD performance against the radiation damage, a working temperature of 120 is considered desirable. Further author information: (Send correspondence to H. T.) H.T.: tsunemi@ess.sci.osaka-u.ac.jp, Tel: (+81)

2 Therefore, we must seek a system to cool the CCD to the desirable temperature even in LEO. Furthermore, we learned that radiation damage by low energy protons and micro-meteorites is very dangerous to the CCDs and must take it into account for future CCD design. Since the CCD is made of silicon, it has poor detection efficiency at high energy, particularly above 10 kev. All the CCD cameras onboard satellites function mainly below 10 kev. When the CCD chip is built in a large format, either in a single chip or in multiple chips, the number of pixels increases, resulting in reduced time resolution. A low noise read-out requires a relatively slow read-out speed. The number of analog electronics chains must be increased in order to improve the time resolution or avoid the pile-up. Due to the limited mass, space and power, we must develop an ASIC for the CCD data process that fits the space use. We are now planning the ASTRO-H mission27 that will be launched in FY2013. It will carry an X-ray micro-calorimeter (SXS18 ), a hard X-ray imager (HXI11 ), a soft gamma-ray detector (SGD28 ), X-ray telescopes (SXT1 ) and an X-ray CCD camera (SXI: reported here). The SXS has excellent energy resolution below 10 kev (about 6 ev at 6 kev) with a relatively small field of view (FOV). The HXI will have good efficiency at high energy X-ray (5 80 kev) with a narrow FOV. The SXI will have a large FOV (38" square) with a wide energy band ( kev). We will report here the development status of the SXI for ASTRO-H. 2. SXI: OVERVIEW We have developed X-ray CCD cameras for ASCA, SUZAKU and MAXI and are designing the SXI based on the experiences obtained to date. Fig. 1 shows an outlook of the SXI. The length and width are 490 mm 570 mm. The SXI base plate is about 100 mm above the base plate of the satellite. The height of the SXI is about 700 mm, including the baffle. The SXI must be thermally isolated from the base plate of the satellite. We employ heat pipes connected to the radiator outside the satellite. Since it is difficult to cool the CCD below 90 C using the peltier cooler at LEO, we decided to employ a mechanical cooler to safely reach a temperature of around 120 C. The mechanical cooler has been employed in the SUZAKU, AKARI and SMILES space missions, and shows high reliability in space. Although it has enough cooling power, we will install two sets for redundancy. Since the mechanical cooler will run on constant input power, precise temperature control will be achieved via electric heaters. The SUZAKU XIS12 employs a backilluminated (BI) device that can cover the energy range down to 0.3 kev. Furthermore, the XIS performs superbly at low energy, which demonstrates the validity of the BI. It also shows radiation hardness against proton damage since the charge transfer channel is much deeper than that of the front-illuminated (FI) defigure 1. Outlook of the SXI except the PE and DE electronics boxes. vice. Some satellites suffer impacts from micro-meteorites that become a practical problem. The result of the BI on the XIS also indicates that the former is more robust against the impact of the micro-meteorite than the FI. Therefore, we decide to employ the BI rather than the FI. We have developed CCDs32 for X-ray use in collaboration with Hamamatsu Photonics K.K. The HAYABUSA mission used 5 FI CCDs with a depletion layer of about 20 µm. The MAXI SSC10 employs 32 FI CCDs with a

3 depletion layer of about 75 µm. A new type of P-channel CCD has become available with a thick depletion layer of 200 µm. It was confirmed in optical measurement 8, 9 and also in X-ray measurement. 13 The thick depletion layer of the CCD not only expands the detection efficiency at high energy but increases the production efficiency of the BI device. Accordingly, we have designed the CCD-NeXT4 that is a frame transfer type CCD with an imaging area of mm square. The code name of the CCD represents the history of the development. In a photon count mode of the CCD, a low noise analog chain is required, which makes the electronics large and massive. Careful tuning is also needed. An analog ASIC can ease these constraints. The first ASIC for space use CCD is developed for the pn-ccd on board Newton. 3 We have been developing a readout ASIC for X-ray 15, 22 CCDs. We employed a Σ modulator 4 to obtain moderate speed and accuracy. After fabricating several ASICs, we obtained an analog ASIC with good performance. Various environmental tests are needed for a final decision. 20 Calibration in orbit is important even if considerable pre-flight calibration is performed. We will employ a modulated X-ray source (MXS) that is also employed in the SXS on ASTRO-H, which enables us to illuminate the entire CCDs with X-rays whenever needed. A radioactive source will be installed for redundancy. Careful study of the background using the GEANT4 gives us a reasonable design for the camera body. Taking into account many requirements and environmental conditions, we have designed the SXI given in the following sections. 3. X-RAY CCD AND FPA There are three minimum requirements on the SXI that come from the science goal of the ASTRO-H mission. One is that the FOV of the SXI should exceed 18. This corresponds to an effective area of 30 mm, taking into account the focal length of the X-ray telescope (SXT-I) for the SXI. The second is that the SXI should have enough angular resolution to eliminate the contribution of point sources. The angular resolution is limited by the imaging capability of the SXT-I that is about 1.7.Wecaneasilyachievethisrequirementsincetheplatescaleis 1.6 mm/arcmin with a CCD pixel size of 24 µm square. The third is that the SXI should cover an energy range up to 12 kev so that we can have enough overlap in the energy band with that of the HXI. This requirement reveals an effective area of 360 cm 2 at 6 kev, which can be achieved by having a depletion layer of 60 µm. Table 1. Specification of the CCD-PchNeXT4 CCD configuration Frame Transfer IA effective pixels IA effective area 30.72mm 30.72mm Silicon chip size mm mm Silicon thickness 200µm Pixel size 24µm (H) 24µm (V)(IA) 24-22µm (H) 16µm (V)(SA) V-clock phase 2 phase and 2 line H-clock phase 2 phase and 4 line Charge injection Vertical input Surface coat Al-Polyimide-Al Since ASTRO-H is to be an observatory, we must ensure the best possible performance for the SXI and hence set secondary goals other than the minimum requirements. The background level must be as low as that of the SUZAKU XIS. Furthermore, we must have a response at low energy 19 at a level equivalent to the XIS. We set our secondary goal of an effective area at 1 kev to be 300 cm 2 so that we can perform a well-balanced observation with the SXS. Since CCDs will be degraded by charged particles, we must employ certain countermeasures. One is a mechanical cooler to reach a low temperature of 120 C. The other is a charge injection (CI) method. 7 The CI is introduced to SUZAKU in orbit 25 for the first time and offsets the degradation to some extent. 2, 21, 36 We also confirmed its validity to the CCD fabricated in Hamamatsu. 30 The CCD-NeXT4 is also equipped with a notch structure that improves the radiation hardness by a factor of three. 31 There are some other requirements: contamination, optical photons, stray light, alignment, calibration and working temperature. Taking these requirements into account, we determine the SXI configuration.

4 Table 1 and Fig. 2 indicate the specification and outlook of the CCD-NeXT4 designed for the SXI. It is a Pchannel CCD with thick depletion layer of 200µm.33 It is a fully depleted device with a BI configuration. There is a surface coat (Optical Blocking Layer, OBL) consisting of Aluminum-Polyimide-Aluminum that prevents visible light and EUV from entering the CCD, which makes it possible to eliminate the optical blocking filter (OBF). To evaluate the performance of polyimide that cuts out EUV, we measured the EUV transmission of the OBL at energy levels ranging between ev by using the synchrotron facility and confirmed the consistency with the model (3% at 41 ev).39 The imaging area of the CCD-NeXT4 is mm square with a pixel size of 24µm square. There are 4 readout nodes in a single chip. We design it such as to render 2 out of 4 nodes selectable to read-out the entire imaging area for redundancy. We will employ 4 tightly aligned CCDs to ensure an effective imaging area of 60 mm square. Figure 2. Design of the CCD-NeXT4. The imaging area is mm square. Figure 3. Structure of the FPA. Two 1ST coolers are connected to the CCD plate through flexure. Fig. 3 shows a close up view of the focal plane assembly (FPA) where we see the CCD plate housing 4 chips. The CCDs will be cooled by using two mechanical coolers, single stage sterling coolers (1STs), for redundancy. The CCD plate is supported by 6 insulator poles under which two print circuit boards (PCBs) are installed. The CCD outputs are sent though a flexible cable to the PCB, which converts the signals from analog to digital. 4. THERMAL DESIGN In ASCA, SUZAKU and MAXI, we employed a combination of the peltier devices and a radiator to cool the CCD. We can cool the CCD to around 60 C in both ASCA and MAXI. In SUZAKU, we designed the system to cool down as far as possible to around 90 C. Cooling further in LEO is difficult. However, we require the working temperature of the X-ray CCDs to be 120 C. This value comes from the condition of the radiation hardness and thermal noise. At the beginning of the mission when the radiation damage is minor, the working temperature can be higher, which will reduce the contamination effect. We designed the FPA such that it can accommodate 4 CCD-NeXT4 chips and calculated the heat input to the FPA with the condition that the FPA is 120 C and the surrounding is +30 C. The heat input through a flex print cable (FPC) is 907 mw. The heat input through radiation onto the cold part is 622 mw and that through thermal conduction of poles is 335 mw, while other heat load sources also exist. In total we estimate a maximum heat load of 1973 mw.

5 We selected a mechanical cooler, 1ST, which is now considered a space-proven system through experience with previous satellites. The 1ST can cool to around 220 C or lower, while we require it to cool the FPA to around 130 C so that we can safely cool the CCD to 120 C. We confirmed through laboratory experiments that the 1ST can accept a heat load of up to 4 W with 30 W input power at 130 C. Therefore, the performance of the mechanical cooler in the SXI is limited not by heat input but by the available input power. Since it is critical to keep the CCD temperature low, we employ two 1STs for redundancy. In cases of failure, where one 1ST does not work, it will be an extra heat load of 0.5 W. If the 1ST runs at 130 C, we confirm that the CCD will be 120 C. Therefore, we have enough margin to cool the CCD. 5. ELECTRONICS Fig. 4 shows a block diagram of the SXI. Analog signals from CCDs are sent to ASICs inside the SXI-body where they are digitalized. Subsequently, they are sent to the SXI-FE (front end electronics) that is next to the SXI-body on the base plate of the satellite. After the SXI-FE, SXI-PE and SXI-DE are on the side panel of the satellite. We mainly follow the space wire interface. Figure 4. Block diagram of the SXI. We have two 1STs to cool the FPA, the driver electronics of which are not included in the SXI-PE/DE. We also have a calibration source (MXS), whose driver electronics are also excluded from the latter. These are not shown in the block diagram in this figure.

6 5.1 CCD Driver and Video Processor One set of the analog and digital electronics is dedicated to the drive and receive data from one CCD chip. Therefore, there are four independent sets with the exception of the PSU (Power Supply Unit). There are two PSUs, each supplying two sets of electronics. There is a separate box (SXI-S-FE) next to the SXI-S-BODY, containing Driver, HK (House Keeping), and TCE (Thermal Control Electronics) boards. The CCD driver circuitry consists of DACs, Op-amp buffers, voltage regulators, analog switches and some bias voltage generators, as are common with X-ray CCD camera systems in SUZAKU XIS. 8-bit DACs are used to select 256 levels within the range -10 to +10 V. The video board, which is installed in the SXI-body, includes ASIC chips (see 5.1.1), FPGA, and other interface ICs. CCD video signals from each readout node are processed by four video processors, whereupon we calculate the average of the four digitized signals to improve the readout noise level. We adopted this method, not only for implementing the redundancy but also for detecting single-bit error in the video processor ASIC Figure 5. Signal processing diagram of the SXI ASIC. We can set the preamplifier gain by using 5-bit DAC. We set an offset to the signal level in order to utilize the input signal range of Σ modulators, which convert the analog signal to 155-bit stream per pixel. odd and even modulators work in turn to improve the readout rate. The SXI is equipped with an analog ASIC for the front-end electronics.14, 24 The 3 mm square bare chip has four channels that process the CCD video signals simultaneously.24 As shown in Fig. 5, each channel is composed of a preamplifier, a 5-bit DAC, and two Σ modulators.4 It has been fabricated through the Taiwan Semiconductor Manufacturing Company (TSMC) in a 0.35 µm CMOS process. Fig. 6 shows an actual mask design. There are sixteen readout nodes in the SXI CCDs and we implement the majority voting logic in the video processor as mentioned above. Therefore, we install 16 ASIC chips (64 channels) in the video board. As shown in Fig. 7, the test results of the front-end electronics demonstrated that it works properly with a low input noise of <30 µv at the pixel rate of 80kHz. The power consumption is sufficiently low at Figure 6. Mask design of the ASIC in a 150mW per chip. The dynamic range of input signals is ±20mV, which 3 mm square chip. covers the effective energy range of the SXI CCD, while the integrated non-linearity (INL) of 0.2 % satisfies the same performance as the conventional CCD detectors in orbit. The radiation tolerance against total ionizing dose (TID) effect and single event latch-up (SEL) have also been investigated. Irradiation tests using 60 Co γ-rays and proton beam showed that the ASIC has sufficient tolerance against TID up to 200 and 167 krad respectively, although the gain degraded after 30 krad in the γ-ray

7 test (Fig. 8). The absorbed dose of 30 krad corresponds to 27-year operation in the planned in the LEO, which thoroughly exceeds the expected operating duration. Furthermore, the gain is recovered by annealing for several hundred hours at room temperature, meaning the degradation level in orbit is not serious and can be offset by referring to the result of the onboard calibration source. We have recently fabricated ASIC chips made of an epitaxial wafer and used them to perform the SEL test utilizing Xe ions of 6 MeV/u that corresponds to the LET of 57.9 MeV cm2 /mg. Since we observed no latch-up through this test, the cross section of the SEL, σsel, was derived < cm2 /(Ion ASIC). We are planning to perform radiation tests other than the SEL test by using epitaxial chips. Figure 7. Power consumption, equivalent input noise, and INL as a function of the pixel rate. We only show the results of one out of four channels for simplicity in the latter two parameters. Figure 8. The gain in the unit of ch/mv is plotted as a function of the absorbed dose. There are two modulators in each channel, where we only show one of them (even modulators) for simplicity. 6. CALIBRATION AND CONTAMINATION 6.1 Calibration CCD chips will be damaged in space due to the radiation of charged particles, resulting in an increase in the charge transfer inefficiency (CTI). This CTI increase degrades the CCD performance with regard to gain and energy resolution. We should carry out calibration in orbit to properly process the observed data. In ASCA, the degree of degradation of the CCD differs from column to column.40 We need to correct the CTI for each column. In SUZAKU, the CCD on the XIS has a charge injection gate via which we can apply a certain charge for each column to measure the CTI.37 This method improves the calibration accuracy whereas the charge injection method is also subject to radiation damage. We conclude it best to ensure uniform irradiation of the fluorescence X-rays onto the CCD. We are planning to have two kinds of on-board calibration sources. One of which is the MXS that irradiates the entire area of the CCDs intermittently. The other Figure 9. The MXS location source is the radioactive source of 55 Fe that continuously irradiates the corner pixels for the SXI. The MXS is mounted at the lower panel of the CCDs. of the satellite. The MXS for the SXI is placed on the lower panel of the satellite as shown in Fig. 9. Since the X-ray intensity of the MXS can be controlled by the LED flux, we can achieve a rapid response.38 We will turn the MXS on only during non-observing periods, such as the Earth occultation of celestial objects. The X-ray intensity of the MXS is planned to be several hundred counts/sec per CCD chip in the current configuration, meaning we can obtain sufficient X-ray events to calibrate the gain and

8 energy resolution for each column without reducing the observation time. X-rays from the MXS indicate the fluorescent line of the electron target. The candidate materials of the electron target are Cu (8.05 kev) and Ti (4.51 kev), while the MXS is also employed in the SXS. Therefore, the control electronics and PSU are identical to those for the SXS. The configuration and intensity of 55 Fe are almost identical to the calibration source of the XIS on board SUZAKU. 12 By using radioactive sources, we can continuously monitor the CCD gain. 6.2 Contamination control It is essential to avoid contamination to the sensor to preserve effective sensitivity at a low energy band, especially below 1 kev. We note the importance of the contamination control very well through past experiences, e.g. SUZAKU XIS that suffered from the contamination of 100 µg cm 2. To minimize the impact on low-energy sensitivity, the amount of contamination on the sensor is expected to be less than a few µg cm 2 that corresponds to a 10% reduction in sensitivity at 0.4 kev. This is our goal for the contamination control. In the case of ASTRO-H, the majority of bus components are installed inside the satellite. Because the telescopes are also located inside the satellite, the SXI sensor, working at temperatures as low as 120 C, is inevitably exposed to outgas from various components. All the outgas from the entire satellite is estimated to be in the order of 100 g. We will suffer considerably from contamination unless an appropriate countermeasure is applied. It is noteworthy that the X-ray mirrors are also significant contaminant sources, meaning the isolation of the telescope system (the X-ray mirror and the focal plane detector) from the bus components is insufficient to prevent contamination from the SXI sensor. Therefore, we plan to install a room temperature filter in front of the CCD so that the coldest part is isolated from other components. The amount of the contamination is heavily dependent on the surface temperature, and we can reduce the effect of the contaminants by introducing a warmer surface. There are some points we must consider regarding the filter. To avoid reduced sensitivity caused by the filter itself, it must be made very thin. We plan to use a 0.2µm polyimide film with a support mesh, similar to the thermal shield of the X-ray telescope (XRT). Temperature control of the filter is also important: if the filter is cooled, contaminants from the outgas will condense on it. We will install a heater to warm it up. We must also prevent contamination from the PCBs inside the SXI-body since the cold surface of the CCD and the warm PCB are in the same volume. We thus separate the inside of the SXI camera body into the cold part including the CCD chip and the electric circuit board part, to avoid self-contamination. Figure 10. Effective areas are plotted as a function of energy for various detectors on ASTRO-H. Figure 11. SΩ and on-axis effective area are plotted for various satellites.

9 7. BACKGROUND AND EXPECTED PERFORMANCE In Fig. 10, we show the effective areas for detectors on ASTRO-H as a function of energy. The SGD covers energy above 50 kev up to 600 kev. The HXI covers the energy range between 5 and 80 kev while the SXI will cover the energy range between 0.4 to 12 kev. We can safely cover a wide energy range with enough energy overlap between the detectors. The SXS will have good energy resolution whose narrow FOV can be offset by the SXI. Fig. 11 shows SΩ and the on-axis effective area for various satellites. Since the SXI has one telescope, the effective area is relatively small. However, we have a large detector area so that the SΩ becomes one of the largest in various satellites. 8. SUMMARY We have reviewed the SXI that contains new ideas and methods not previously employed, which will improve the performance of the X-ray CCD. However, there are many new pre-launch concerns that must be solved. The CCD temperature can be lowered to 120 C using a mechanical cooler, which will strengthen the radiation hardness and low noise performance. However, this can also be a cause of contamination that must be carefully treated. The analog ASIC will surely reduce the complexity of the electronics. The next step for the SXI is to perform a test with full configuration. ACKNOWLEDGMENTS The SXI is supported not only by the authors but by many other people. Many graduate students contributed and will continue to contribute to the development of various parts of the SXI, as do many company people: Mitsubishi Heavy Industry, Hamamatsu Photonics, Noqsi Aerospace and Sumitomo Heavy Industry. The authors would like to express their thanks to all who contribute to the SXI. REFERENCES [1] H. Awaki, et al., Current status of the Hard X-ray Telescope onboard ASTRO-H, in Space Telescopes and Instru-mentation 2010: Ultraviolet to Gamma Ray, Monique Arnaud, Stephen S. Murray and Tadayuki Takahashi, eds., Proceedings of SPIE, , thisissue,(2010) [2] M. W. Bautz; B. J. LaMarr; E. D. Miller; S. E. Kissel; G. Y. Prigozhin; et al., Mitigating CCD radiation damage with charge injection: first flight results from Suzaku, Proceedings of SPIE, 6686, 66860Q,(2007) [3] H. Bräuninger, R. Danner, D. Hauff, P. Lechner, G. Lutz, N. Meidinger, E. Pinotti, C. Reppin, L. Strüder, J. Trümper, et al., First results with the pn-ccd detector system for the XMM satellite mission, Nucl. Instrum. and Meth., A326, pp ,(1993) [4] J. P. Doty et al., An ASIC for Delta Sigma Digitization of Technical CCD Video in High Energy, Optical, and Infrared Detectors for Astronomy II, D. A. Dorn and A. D. Holland, eds., Proceedings of SPIE, 6276, pp , (2006) [5] G. P. Garmire, M. W. Bautz, P. G. Ford, J. A. Nousek, and G. R. Ricker, Advanced CCD imaging spectrometer (ACIS)instrument on the Chandra X-ray Observatory, in X-Ray and Gamma-Ray Telescopes and Instruments for Astronomy, J. E. Truemper and H. D. Tananbaum, eds., Proceedings of SPIE, 4851, pp , (2003) [6] N. Gehrels, et al., The Swift Gamma-Ray Burst Mission, Astrophysical Journal, 611, pp ,(2004) [7] K. C. Gendreau, G. Y. Prigozhin, R. K. Huang, M. W. Bautz, A technique to measure trap characteristics in CCD s using X-rays, Electron Devices, IEEE Transactions, 42, pp ,(1995) [8] Y. Kamata, S.Miyazaki, M.Muramatsu, H. Suzuki, K.Miyaguchi, T. G. Tsuru, S. Takagi & E. Miyata, Development of thick back-illuminated CCD to improve quantum efficiency in optical longer wavelength using high-resistivity n-type silicon, Proceedings of SPIE, 5499, pp.210,(2004) [9] Y. Kamata, S. Miyazaki, H. Nakaya, E. Miyata, H. Tsunemi, T. G. Tsuru, S. Takagi, K. Miyaguchi, M. Muramatsu & H. Suzuki, Recent results of the fully depleted back-illuminated CCD developed by Hamamatsu, Proceedings of SPIE, 6276, pp.52,(2006)

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