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We have developed a completely new type of general-purpose CCD data acquisition system which enables one to drive any type of CCD using any type of clocking mode. A CCD driver system widely used before consisted of an analog multiplexer (MPX), a digital-to-analog converter (DAC), and an operational amplifier. A DAC is used to determine high and low voltage levels and the MPX selects each voltage level using a TTL clock. In this kind of driver board, it is difficult to reduce the noise caused by a short of high and low level in MPX and also to select many kinds of different voltage levels.Recent developments in semiconductor IC enable us to use a very fast sampling (∼ 10MHz) DAC with low cost. We thus develop the new driver system using a fast DAC in order to determine both the voltage level of the clock and the clocking timing. We use FPGA (Field Programmable Gate Array) to control the DAC. We have constructed the data acquisition system and found that the CCD functions well with our new system. The energy resolution of Mn Kα has a full-width at half-maximum of ≃ 150 eV and the readout noise of our system is ≃ 8 e − .

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We have investigated the radiation damage effects on a CCD to be employed in the Japanese X-ray astronomy mission including the Monitor of All-sky X-ray Image (MAXI) onboard the International Space Station (ISS). Since low energy protons release their energy mainly at the charge transfer channel, resulting a decrease of the charge transfer efficiency, we thus focused on the low energy protons in our experiments. A 171 keV to 3.91 MeV proton beam was irradiated to a given device. We measured the degradation of the charge transfer inefficiency (CTI) as a function of incremental fluence. A 292 keV proton beam degraded the CTI most seriously. Taking into account the proton energy dependence of the CTI, we confirmed that the transfer channel has the lowest radiation tolerance. We have also developed the different device architectures to reduce the radiation damage in orbit. Among them, the "notch" CCD, in which the buried channel implant concentration is increased, resulting in a deeper potential well than outside, has three times higher radiation tolerance than that of the normal CCD. We then estimated the charge transfer inefficiency of the CCD in the orbit of ISS, considering the proton energy spectrum. The CTI value is estimated to be 1.1 × 10 −5 per each transfer after two years of mission life in the worse case analysis if the highest radiation-tolerant device is employed. This value is well within the acceptable limit and we have confirmed the high radiation-tolerance of CCDs for the MAXI mission.

We have investigated the radiation damage effects on a CCD to be employed in the Japanese X-ray astronomy mission including the Monitor of All-sky X-ray Image (MAXI) onboard the International Space Station (ISS). The X-ray CCD camera, ACIS, onboard Chandra have been seriously damaged by low energy protons having energy of ∼150 keV since low energy protons release their energy mainly at the charge transfer channel, resulting a decrease of the charge transfer efficiency. We thus focused on the low energy protons in our experiments. A 171 keV to 3.91 MeV proton beam was irradiated to a given device. We measured the degradation of the charge transfer inefficiency (CTI) and dark current as a function of incremental fluence. A 292 keV proton beam degraded the CTI most seriously. Taking into account the proton energy dependence of the CTI, we confirmed that the transfer channel has a lowest radiation tolerance. On the other hand, dark current increased after proton irradiation for all energies except 171 keV. We have also developed the different device architectures to reduce the radiation damage in orbit. We then investigated the spatial distribution of the low energy protons in the orbit of the ISS. We found that their density has a peak around l ∼ 20 • and b ∼ −55 • independent of the attitude. The peak value is roughly two orders of magnitude larger than that at the South Atlantic Anomaly. Taking into account the new anomaly and orbit of the ISS, we estimated the charge transfer inefficiency of MAXI CCDs to be 1.1 × 10 −5 per each transfer after two years of mission life in the worse case analysis if the highest radiation-tolerant device is employed. This value is well within the requirement and we have confirmed the high radiation-tolerance of MAXI CCDs.