arXiv subjects

Ayaki Takeda

Publications and source records attributed to Ayaki Takeda.

At least 19 recordsLinked to original sources

arXiv2025

A detailed study on spectroscopic performance of SOI pixel detector with a pinned depleted diode structure for X-ray astronomy

We have been developing silicon-on-insulator (SOI) pixel detectors with a pinned depleted diode (PDD) structure, named "XRPIX", for X-ray astronomy. In our previous study, we successfully optimized the design of the PDD structure, achieving both the suppression of large leakage current and satisfactory X-ray spectroscopic performance. Here, we report a detailed study on the X-ray spectroscopic performance of the XRPIX with the optimized PDD structure. The data were obtained at $-60^\circ\mathrm{C}$ with the "event-driven readout mode", in which only a triggering pixel and its surroundings are read out. The energy resolutions in full width at half maximum at 6.4 keV are $178\pm1$ eV and $291\pm1$ eV for single-pixel and all-pixel event spectra, respectively. The all-pixel events include charge-sharing pixel events as well as the single-pixel events. These values are the best achieved in the history of our development. We argue that the gain non-linearity in the low energy side due to excessive charge injection to the charge-sensitive amplifier is a major factor to limit the current spectroscopic performance. Optimization of the amount of the charge injection is expected to lead to further improvement in the spectroscopic performance of XRPIX, especially for the all-pixel event spectrum.

astro-ph.IM↗

arXiv2024

Feasibility study of upper atmosphere density measurement on the ISS by observations of the CXB transmitted through the Earth rim

Measurements of the upper atmosphere at ~100 km are important to investigate climate change, space weather forecasting, and the interaction between the Sun and the Earth. Atmospheric occultations of cosmic X-ray sources are an effective technique to measure the neutral density in the upper atmosphere. We are developing the instrument SUIM dedicated to continuous observations of atmospheric occultations. SUIM will be mounted on a platform on the exterior of the International Space Station for six months and pointed at the Earth's rim to observe atmospheric absorption of the cosmic X-ray background (CXB). In this paper, we conducted a feasibility study of SUIM by estimating the CXB statistics and the fraction of the non-X-ray background (NXB) in the observed data. The estimated CXB statistics are enough to evaluate the atmospheric absorption of CXB for every 15 km of altitude. On the other hand, the NXB will be dominant in the X-ray spectra of SUIM. Assuming that the NXB per detection area of SUIM is comparable to that of the soft X-ray Imager onboard Hitomi, the NXB level will be much higher than the CXB one and account for ~80% of the total SUIM spectra.

astro-ph.IM↗

arXiv2024

SUIM project: measuring the upper atmosphere from the ISS by observations of the CXB transmitted through the Earth rim

The upper atmosphere at the altitude of 60-110 km, the mesosphere and lower thermosphere (MLT), has the least observational data of all atmospheres due to the difficulties of in-situ observations. Previous studies demonstrated that atmospheric occultation of cosmic X-ray sources is an effective technique to investigate the MLT. Aiming to measure the atmospheric density of the MLT continuously, we are developing an X-ray camera, "Soipix for observing Upper atmosphere as Iss experiment Mission (SUIM)", dedicated to atmospheric observations. SUIM will be installed on the exposed area of the International Space Station (ISS) and face the ram direction of the ISS to point toward the Earth rim. Observing the cosmic X-ray background (CXB) transmitted through the atmosphere, we will measure the absorption column density via spectroscopy and thus obtain the density of the upper atmosphere. The X-ray camera is composed of a slit collimator and two X-ray SOI-CMOS pixel sensors (SOIPIX), and will stand on its own and make observations, controlled by a CPU-embedded FPGA "Zynq". We plan to install the SUIM payload on the ISS in 2025 during the solar maximum. In this paper, we report the overview and the development status of this project.

astro-ph.IM↗

arXiv2024

Evaluation of the X-ray SOI pixel detector with the on-chip ADC

XRPIX is the monolithic X-ray SOI (silicon-on-insulator) pixel detector, which has a time resolution better than 10 $\rmμ$s as well as a high detection efficiency for X-rays above 10 keV. XRPIX is planned to be installed on future X-ray satellites. To mount on satellites, it is essential that the ADC (analog-to-digital converter) be implemented on the detector because such peripheral circuits must be as compact as possible to achieve a large imaging area in the limited space in satellites. Thus, we developed a new XRPIX device with the on-chip ADC, and evaluated its performances. As the results, the integral non-linearity was evaluated to be 6 LSB (least significant bit), equivalent to 36 eV. The differential non-linearity was less than 0.7 LSB, and input noise from the on-chip ADC was 5~$\rm{e^{-}}$. Also, we evaluated end-to-end performance including the sensor part as well as the on-chip ADC. As the results, energy resolution at 5.9 keV was 294 $\rm{\pm}$ 4 eV in full-width at half maximum for the best pixel.

astro-ph.IM↗

arXiv2024

Design study and spectroscopic performance of SOI pixel detector with a pinned depleted diode structure for X-ray astronomy

We have been developing silicon-on-insulator (SOI) pixel detectors with a pinned depleted diode (PDD) structure, named "XRPIX", for X-ray astronomy. The PDD structure is formed in a thick p-type substrate, to which high negative voltage is applied to make it fully depleted. A pinned p-well is introduced at the backside of the insulator layer to reduce a dark current generation at the Si-SiO$_{2}$ interface and to fix the back-gate voltage of the SOI transistors. An n-well is further introduced between the p-well and the substrate to make a potential barrier between them and suppress a leakage current. An optimization study on the n-well dopant concentration is necessary because a higher dopant concentration could result in a higher potential barrier but also in a larger sense-node capacitance leading to a lower spectroscopic performance, and vice versa. Based on a device simulation, we fabricated five candidate chips having different n-well dopant concentrations. We successfully found out the best n-well design, which suppressed a large leakage current and showed satisfactory X-ray spectroscopic performance. Too low and too high n-well dopant concentration chips showed a large leakage current and degraded X-ray spectroscopic performance, respectively. We also found that the dependency of X-ray spectroscopic performance on the n-well dopant concentration can be largely explained by the difference in sense-node capacitance.

astro-ph.IM↗

arXiv2023

Radiation-Induced Degradation Mechanism of X-ray SOI Pixel Sensors with Pinned Depleted Diode Structure

The X-ray Silicon-On-Insulator (SOI) pixel sensor named XRPIX has been developed for the future X-ray astronomical satellite FORCE. XRPIX is capable of a wide-band X-ray imaging spectroscopy from below 1 keV to a few tens of keV with a good timing resolution of a few tens of $μ$s. However, it had a major issue with its radiation tolerance to the total ionizing dose (TID) effect because of its thick buried oxide layer due to the SOI structure. Although new device structures introducing pinned depleted diodes dramatically improved radiation tolerance, it remained unknown how radiation effects degrade the sensor performance. Thus, this paper reports the results of a study of the degradation mechanism of XRPIX due to radiation using device simulations. In particular, mechanisms of increases in dark current and readout noise are investigated by simulation, taking into account the positive charge accumulation in the oxide layer and the increase in the surface recombination velocity at the interface between the sensor layer and the oxide layer. As a result, it is found that the depletion of the buried p-well at the interface increases the dark current, and that the increase in the sense-node capacitance increases the readout noise.

astro-ph.IM↗

arXiv2023

A broadband X-ray imaging spectroscopy in the 2030s: the FORCE mission

In this multi-messenger astronomy era, all the observational probes are improving their sensitivities and overall performance. The Focusing on Relativistic universe and Cosmic Evolution (FORCE) mission, the product of a JAXA/NASA collaboration, will reach a 10 times higher sensitivity in the hard X-ray band ($E >$ 10~keV) in comparison with any previous hard X-ray missions, and provide simultaneous soft X-ray coverage. FORCE aims to be launched in the early 2030s, providing a perfect hard X-ray complement to the ESA flagship mission Athena. FORCE will be the most powerful X-ray probe for discovering obscured/hidden black holes and studying high energy particle acceleration in our Universe and will address how relativistic processes in the universe are realized and how these affect cosmic evolution. FORCE, which will operate over 1--79 keV, is equipped with two identical pairs of supermirrors and wideband X-ray imagers. The mirror and imager are connected by a high mechanical stiffness extensible optical bench with alignment monitor systems with a focal length of 12~m. A light-weight silicon mirror with multi-layer coating realizes a high angular resolution of $<15''$ in half-power diameter in the broad bandpass. The imager is a hybrid of a brand-new SOI-CMOS silicon-pixel detector and a CdTe detector responsible for the softer and harder energy bands, respectively. FORCE will play an essential role in the multi-messenger astronomy in the 2030s with its broadband X-ray sensitivity.

astro-ph.HE↗

arXiv2022

Single Event Tolerance of X-ray SOI Pixel Sensors

We evaluate the single event tolerance of the X-ray silicon-on-insulator (SOI) pixel sensor named XRPIX, developed for the future X-ray astronomical satellite FORCE. In this work, we measure the cross-section of single event upset (SEU) of the shift register on XRPIX by irradiating heavy ion beams with linear energy transfer (LET) ranging from 0.022 MeV/(mg/cm2) to 68 MeV/(mg/cm2). From the SEU cross-section curve, the saturation cross-section and threshold LET are successfully obtained to be $3.4^{+2.9}_{-0.9}\times 10^{-10}~{\rm cm^2/bit}$ and $7.3^{+1.9}_{-3.5}~{\rm MeV/(mg/cm^2)}$, respectively. Using these values, the SEU rate in orbit is estimated to be $\lesssim$ 0.1 event/year primarily due to the secondary particles induced by cosmic-ray protons. This SEU rate of the shift register on XRPIX is negligible in the FORCE orbit.

astro-ph.IM↗

arXiv2022

Precision beam telescope based on SOI pixel sensor technology for electrons in the energy range of sub-GeV to GeV

We developed a beam telescope system comprising five layers of 300-$μ$m-thick INTPIX4NA monolithic pixel sensors with each pixel size of 17 $μ$m square. The sensors were fabricated using silicon-on-insulator (SOI) technology. The signal-to-noise ratio of 140--230 is realized at a bias voltage of 20~V. The tracker system was tested using a positron beam of 200--822 MeV/c, and various tracking methods are examined to optimize spatial precision achievable at these energies. The best tracking precision including the precision of the sensor under test itself is 11.04 $\pm$ 0.10 $μ$m for 822-MeV/c positrons for an equidistant sensor spacing of 32 mm. The achieved precision results combined with the intrinsic spatial resolution value obtained for a similar system using 120 GeV protons are used to estimate the tracking performance of electrons in the GeV energy range; a tracking precision of 2.22 $μ$m is evaluated for 5-GeV electrons. The method to estimate the tracking performance is verified using a Geant4-based simulation. The developed high precision tracker system enables to map the detailed performance of the sensors with pixel sizes of $\mathcal{O}$(10 $μ$m), therefore will be a powerful system for development of devices targeting precision position resolutions.

physics.ins-det↗

arXiv2022

X-ray Radiation Damage Effects on Double-SOI Pixel Detectors for the Future Astronomical Satellite "FORCE"

We have been developing the monolithic active pixel detector "XRPIX" onboard the future X-ray astronomical satellite "FORCE". XRPIX is composed of CMOS pixel circuits, SiO2 insulator, and Si sensor by utilizing the silicon-on-insulator (SOI) technology. When the semiconductor detector is operated in orbit, it suffers from radiation damage due to X-rays emitted from the celestial objects as well as cosmic rays. From previous studies, positive charges trapped in the SiO2 insulator are known to cause the degradation of the detector performance. To improve the radiation hardness, we developed XRPIX equipped with Double-SOI (D-SOI) structure, introducing an additional silicon layer in the SiO2 insulator. This structure is aimed at compensating for the effect of the trapped positive charges. Although the radiation hardness to cosmic rays of the D-SOI detectors has been evaluated, the radiation effect due to the X-ray irradiation has not been evaluated. Then, we conduct an X-ray irradiation experiment using an X-ray generator with a total dose of 10 krad at the SiO2 insulator, equivalent to 7 years in orbit. As a result of this experiment, the energy resolution in full-width half maximum for the 5.9 keV X-ray degrades by 17.8 $\pm$ 2.8% and the dark current increases by 89 $\pm$ 13%. We also investigate the physical mechanism of the increase in the dark current due to X-ray irradiation using TCAD simulation. It is found that the increase in the dark current can be explained by the increase in the interface state density at the Si/SiO2 interface.

astro-ph.IM↗

arXiv2021

Proton radiation hardness of X-ray SOI pixel sensors with pinned depleted diode structure

X-ray SOI pixel sensors, "XRPIX", are being developed for the next-generation X-ray astronomical satellite, "FORCE". The XRPIX are fabricated with the SOI technology, which makes it possible to integrate a high-resistivity Si sensor and a low-resistivity Si CMOS circuit. The CMOS circuit in each pixel is equipped with a trigger function, allowing us to read out outputs only from the pixels with X-ray signals at the timing of X-ray detection. This function thus realizes high throughput and high time resolution, which enables to employ anti-coincidence technique for background rejection. A new series of XRPIX named XRPIX6E developed with a pinned depleted diode (PDD) structure improves spectral performance by suppressing the interference between the sensor and circuit layers. When semiconductor X-ray sensors are used in space, their spectral performance is generally degraded owing to the radiation damage caused by high-energy protons. Therefore, before using an XRPIX in space, it is necessary to evaluate the extent of degradation of its spectral performance by radiation damage. Thus, we performed a proton irradiation experiment for XRPIX6E for the first time at HIMAC in the NIRS. We irradiated XRPIX6E with high-energy protons with a total dose of up to 40 krad, equivalent to 400 years of irradiation in orbit. The 40-krad irradiation degraded the energy resolution of XRPIX6E by 25 $\pm$ 3%, yielding an energy resolution of 260.1 $\pm$ 5.6 eV at the full width half maximum for 5.9 keV X-rays. However, the value satisfies the requirement for FORCE, 300 eV at 6 keV, even after the irradiation. It was also found that the PDD XRPIX has enhanced radiation hardness compared to previous XRPIX devices. In addition, we investigated the degradation of the energy resolution; it was shown that the degradation would be due to increasing energy-independent components, e.g., readout noise.

astro-ph.IM↗

arXiv2020

Low-Energy X-ray Performance of SOI Pixel Sensors for Astronomy, "XRPIX"

We have been developing a new type of X-ray pixel sensors, "XRPIX", allowing us to perform imaging spectroscopy in the wide energy band of 1-20 keV for the future Japanese X-ray satellite "FORCE". The XRPIX devices are fabricated with complementary metal-oxide-semiconductor silicon-on-insulator technology, and have the "Event-Driven readout mode", in which only a hit event is read out by using hit information from a trigger output function equipped with each pixel. This paper reports on the low-energy X-ray performance of the "XRPIX6E" device with a Pinned Depleted Diode (PDD) structure. The PDD structure especially reduces the readout noise, and hence is expected to largely improve the quantum efficiencies for low-energy X-rays. While F-K X-rays at 0.68 keV and Al-K X-rays at 1.5 keV are successfully detected in the "Frame readout mode", in which all pixels are read out serially without using the trigger output function, the device is able to detect Al-K X-rays, but not F-K X-rays in the Event-Driven readout mode. Non-uniformity is observed in the counts maps of Al-K X-rays in the Event-Driven readout mode, which is due to region-to-region variation of the pedestal voltages at the input to the comparator circuit. The lowest available threshold energy is 1.1 keV for a small region in the device where the non-uniformity is minimized. The noise of the charge sensitive amplifier at the sense node and the noise related to the trigger output function are ~$18~e^-$ (rms) and ~$13~e^-$ (rms), respectively.

astro-ph.IM↗

arXiv2020

Radiation Damage Effects on Double-SOI Pixel Sensors for X-ray Astronomy

The X-ray SOI pixel sensor onboard the FORCE satellite will be placed in the low earth orbit and will consequently suffer from the radiation effects mainly caused by geomagnetically trapped cosmic-ray protons. Based on previous studies on the effects of radiation on SOI pixel sensors, the positive charges trapped in the oxide layer significantly affect the performance of the sensor. To improve the radiation hardness of the SOI pixel sensors, we introduced a double-SOI (D-SOI) structure containing an additional middle Si layer in the oxide layer. The negative potential applied on the middle Si layer compensates for the radiation effects, due to the trapped positive charges. Although the radiation hardness of the D-SOI pixel sensors for applications in high-energy accelerators has been evaluated, radiation effects for astronomical application in the D-SOI sensors has not been evaluated thus far. To evaluate the radiation effects of the D-SOI sensor, we perform an irradiation experiment using a 6-MeV proton beam with a total dose of ~ 5 krad, corresponding to a few tens of years of in-orbit operation. This experiment indicates an improvement in the radiation hardness of the X- ray D-SOI devices. On using an irradiation of 5 krad on the D-SOI device, the energy resolution in the full-width half maximum for the 5.9-keV X-ray increases by 7 $\pm$ 2%, and the chip output gain decreases by 0.35 $\pm$ 0.09%. The physical mechanism of the gain degradation is also investigated; it is found that the gain degradation is caused by an increase in the parasitic capacitance due to the enlarged buried n-well.

physics.ins-det↗

arXiv2020

Development of the detector simulation framework for the Wideband Hybrid X-ray Imager onboard FORCE

FORCE is a Japan-US space-based astronomy mission for an X-ray imaging spectroscopy in an energy range of 1--80 keV. The Wideband Hybrid X-ray Imager (WHXI), which is the main focal plane detector, will use a hybrid semiconductor imager stack composed of silicon and cadmium telluride (CdTe). The silicon imager will be a certain type of the silicon-on-insulator (SOI) pixel sensor, named the X-ray pixel (XRPIX) series. Since the sensor has a small pixel size (30--36 $μ$m) and a thick sensitive region (300--500 $μ$m), understanding the detector response is not trivial and is important in order to optimize the camera design and to evaluate the scientific capabilities. We have developed a framework to simulate observations of celestial sources with semiconductor sensors. Our simulation framework was tested and validated by comparing our simulation results to laboratory measurements using the XRPIX 6H sensor. The simulator well reproduced the measurement results with reasonable physical parameters of the sensor including an electric field structure, a Coulomb repulsion effect on the carrier diffusion, and arrangement of the degraded regions. This framework is also applicable to future XRPIX updates including the one which will be part of the WHXI, as well as various types of semiconductor sensors.

astro-ph.IM↗

arXiv2019

Radiation hardness of a p-channel notch CCD developed for the X-ray CCD camera onboard the XRISM satellite

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.

astro-ph.IM↗

arXiv2019

Measurement of Charge Cloud Size in X-ray SOI Pixel Sensors

We report on a measurement of the size of charge clouds produced by X-ray photons in X-ray SOI (Silicon-On-Insulator) pixel sensor named XRPIX. We carry out a beam scanning experiment of XRPIX using a monochromatic X-ray beam at 5.0 keV collimated to $\sim 10$ $μ$m with a 4-$μ$m$ϕ$ pinhole, and obtain the spatial distribution of single-pixel events at a sub-pixel scale. The standard deviation of charge clouds of 5.0 keV X-ray is estimated to be $σ_{\rm cloud} = 4.30 \pm 0.07$ $μ$m. Compared to the detector response simulation, the estimated charge cloud size is well explained by a combination of photoelectron range, thermal diffusion, and Coulomb repulsion. Moreover, by analyzing the fraction of multi-pixel events in various energies, we find that the energy dependence of the charge cloud size is also consistent with the simulation.

astro-ph.IM↗

arXiv2019

Subpixel Response of SOI Pixel Sensor for X-ray Astronomy with Pinned Depleted Diode: First Result from Mesh Experiment

We have been developing a monolithic active pixel sensor, ``XRPIX``, for the Japan led future X-ray astronomy mission ``FORCE`` observing the X-ray sky in the energy band of 1-80 keV with angular resolution of better than 15``. XRPIX is an upper part of a stack of two sensors of an imager system onboard FORCE, and covers the X-ray energy band lower than 20 keV. The XRPIX device consists of a fully depleted high-resistivity silicon sensor layer for X-ray detection, a low resistivity silicon layer for CMOS readout circuit, and a buried oxide layer in between, which is fabricated with 0.2 $μ$ m CMOS silicon-on-insulator (SOI) technology. Each pixel has a trigger circuit with which we can achieve a 10 $μ$ s time resolution, a few orders of magnitude higher than that with X-ray astronomy CCDs. We recently introduced a new type of a device structure, a pinned depleted diode (PDD), in the XRPIX device, and succeeded in improving the spectral performance, especially in a readout mode using the trigger function. In this paper, we apply a mesh experiment to the XRPIX devices for the first time in order to study the spectral response of the PDD device at the subpixel resolution. We confirmed that the PDD structure solves the significant degradation of the charge collection efficiency at the pixel boundaries and in the region under the pixel circuits, which is found in the single SOI structure, the conventional type of the device structure. On the other hand, the spectral line profiles are skewed with low energy tails and the line peaks slightly shift near the pixel boundaries, which contribute to a degradation of the energy resolution.

astro-ph.IM↗

arXiv2019

Evaluation of Kyoto's Event-Driven X-ray Astronomical SOI Pixel Sensor with a Large Imaging Area

We have been developing monolithic active pixel sensors, named ``XRPIX'', based on the silicon-on-insulator (SOI) pixel technology for future X-ray astronomy satellites. XRPIX has the function of event trigger and hit address outputs. This function allows us to read out analog signals only of hit pixels on trigger timing, which is referred to as the event-driven readout mode. Recently, we processed ``XRPIX5b'' with the largest imaging area of 21.9~mm $\times$ 13.8~mm in the XRPIX series. X-ray spectra are successfully obtained from all the pixels, and the readout noise is 46~e$^-$~(rms) in the frame readout mode. The gain variation was measured to be 1.2\%~(FWHM) among the pixels. We successfully obtain the X-ray image in the event-driven readout mode.

astro-ph.IM↗