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Register a new userRadiation hardness of a p-channel notch CCD developed for the X-ray CCD camera onboard the XRISM satellite
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We report the radiation hardness of a p-channel CCD developed for the X-ray CCD camera onboard the XRISM satellite. This CCD has basically the same characteristics as the one used in the previous Hitomi satellite, but newly employs a notch structure of potential for signal charges by increasing the implant concentration in the channel. The new device was exposed up to approximately $7.9 times 10^{10} mathrm{~protons~cm^{-2}}$ at 100 MeV. The charge transfer inefficiency was estimated as a function of proton fluence with an ${}^{55} mathrm{Fe}$ source. A device without the notch structure was also examined for comparison. The result shows that the notch device has a significantly higher radiation hardness than those without the notch structure including the device adopted for Hitomi. This proves that the new CCD is radiation tolerant for space applications with a sufficient margin.
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We report on a proton radiation damage experiment on P-channel CCD newly developed for an X-ray CCD camera onboard the Astro-H satellite. The device was exposed up to 10^9 protons cm^{-2} at 6.7 MeV. The charge transfer inefficiency (CTI) was measured as a function of radiation dose. In comparison with the CTI currently measured in the CCD camera onboard the Suzaku satellite for 6 years, we confirmed that the new type of P-channel CCD is radiation tolerant enough for space use. We also confirmed that a charge-injection technique and lowering the operating temperature efficiently work to reduce the CTI for our device. A comparison with other P-channel CCD experiments is also discussed.
NeXT (New X-ray Telescope) is the next Japanese X-ray astronomical satellite mission after the Suzaku satellite. NeXT aims to perform wide band imaging spectroscopy. Due to the successful development of a multilayer coated mirror, called a supermirror, NeXT can focus X-rays in the energy range from 0.1 keV up to 80 keV. To cover this wide energy range, we are in the process of developing a hybrid X-ray camera, Wideband X-ray Imager (WXI) as a focal plane detector of the supermirror. The WXI consists of X-ray CCDs (SXI) and CdTe pixelized detectors (HXI), which cover the lower and higher X-ray energy bands of 0.1-80 keV, respectively. The X-ray CCDs of the SXI are stacked above the CdTe pixelized detectors of the HXI. The X-ray CCDs of the SXI detect soft X-rays below $sim$ 10 keV and allow hard X-rays pass into the CdTe detectors of the HXI without loss. Thus, we have been developing a back-supportless CCD with a thick depletion layer, a thinned silicon wafer, and a back-supportless structure. In this paper, we report the development and performances of an evaluation model of CCD for the SXI, CCD-NeXT1. We successfully fabricated two types of CCD-NeXT1, unthinned CCDs with 625-um thick wafer and 150-um thick thinned CCDs. By omitting the polishing process when making the thinned CCDs, we confirmed that the polishing process does not impact the X-ray performance. In addition, we did not find significant differences in the X-ray performance between the two types of CCDs. The energy resolution and readout noise are $sim$ 140 eV (FWHM) at 5.9 keV and $sim$5 electrons (RMS), respectively. The estimated thickness of the depletion layer is $sim$80 um. The performances almost satisfy the requirements of the baseline plan of the SXI.
We present experimental studies on the charge transfer inefficiency (CTI) of charge-coupled device (CCD) developed for the soft X-ray imaging telescope, Xtend, aboard the XRISM satellite. The CCD is equipped with a charge injection (CI) capability, in which sacrificial charge is periodically injected to fill the charge traps. By evaluating the re-emission of the trapped charge observed behind the CI rows, we find that there are at least three trap populations with different time constants. The traps with the shortest time constant, which is equivalent to a transfer time of approximately one pixel, are mainly responsible for the trailing charge of an X-ray event seen in the following pixel. A comparison of the trailing charge in two clocking modes reveals that the CTI depends not only on the transfer time but also on the area, namely the imaging or storage area. We construct a new CTI model with taking into account with both transfer-time and area dependence. This model reproduces the data obtained in both clocking modes consistently. We also examine apparent flux dependence of the CTI observed without the CI technique. The higher incident X-ray flux is, the lower the CTI value becomes. It is due to a sacrificial charge effect by another X-ray photon. This effect is found to be negligible when the CI technique is used.
We present the single event effect (SEE) tolerance of a mixed-signal application-specific integrated circuit (ASIC) developed for a charge-coupled device camera onboard a future X-ray astronomical mission. We adopted proton and heavy ion beams at HIMAC/NIRS in Japan. The particles with high linear energy transfer (LET) of 57.9 MeV cm^{2}/mg is used to measure the single event latch-up (SEL) tolerance, which results in a sufficiently low cross-section of sigma_{SEL} < 4.2x10^{-11} cm^{2}/(IonxASIC). The single event upset (SEU) tolerance is estimated with various kinds of species with wide range of energy. Taking into account that a part of the protons creates recoiled heavy ions that has higher LET than that of the incident protons, we derived the probability of SEU event as a function of LET. Then the SEE event rate in a low-earth orbit is estimated considering a simulation result of LET spectrum. SEL rate is below once per 49 years, which satisfies the required latch-up tolerance. The upper limit of the SEU rate is derived to be 1.3x10^{-3}events/sec. Although the SEU events cannot be distinguished from the signals of X-ray photons from astronomical objects, the derived SEU rate is below 1.3% of expected non-X-ray background rate of the detector and hence these events should not be a major component of the instrumental background.
We have been developing P-channel Charge-Coupled Devices (CCDs) for the upcoming X-ray Astronomy Satellite XRISM, planned to be launched in 2021. While the basic design of the CCD camera (Soft X-ray Imager: SXI) is almost the same as that of the lost Hitomi (ASTRO-H) observatory, we are planning to reduce the light leakages that is one of the largest problems recognized in Hitomi data. We adopted a double-layer optical blocking layer on the XRISM CCDs and also added an extra aluminum layer on the backside of them. We develop a newly designed test sample CCD and irradiate it with optical light to evaluate the optical blocking performance. As a result, light leakages are effectively reduced compared with that of the Hitomi CCDs. We thus conclude that the issue is solved by the new design and that the XRISM CCDs satisfy the mission requirement for the SXI.
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