X-ray CCDs have superior spatial resolution of ~20μm and moderate energy resolution of ~130 eV(FWHM)
at 5.9 keV. On the other hand, the number of pixels assigned to each readout node is generally so large that it
takes several seconds to process a frame data of the entire chip. Relatively low pixel readout rate in order to
keep readout noise low also limits the timing resolution of X-ray CCDs. Although a large number of readout
nodes is essential to improve the timing resolution, size and power consumption of conventional readout circuits
prevent us from being implemented in X-ray CCD camera systems onboard satellites. We are developing an
application specific integrated circuits (ASIC) for multi readout of X-ray CCD signals. The ASIC with the size
of 3mm×3mm has four channels of readout electronics that employs the delta-sigma (ΔΣ) digitization. The
fabrication process is a 0.35μm complimentary metal-oxide semiconductor (CMOS) process provided by Taiwan
Semiconductor Manufacturing Company (TSMC). The equivalent input noise was about 33μV and the power
consumption was about 70mW per chip at the pixel rate of 44 kHz. When we used the X-ray CCD whose
sensitivity was 3 μV/e-, the equivalent noise charge was 10.8e- and the energy resolution was 168 eV(FWHM)
at 5.9 keV. The noise level of our ASIC is comparable to that of the conventional readout systems.
The next Japanese X-ray astronomical satellite mission, NeXT, was proposed to ISAS/JAXA following the Astro-E2 Suzaku satellite which was launched in July 2005. We develop an X-ray CCD camera system, SXI (Soft X-ray Imager), for NeXT. The Hard X-ray Telescope (HXT) onboard NeXT provides imaging capability up to 80 keV, using the multilayer-coated X-ray mirror technology, called "Supermirror", newly developed in Japan. SXI is one of the focal plane detectors of HXT, which covers the soft energy band in the 0.5-12 keV in the baseline and 0.3-25 keV in the goal. We develop p-type CCDs for the baseline of SXI because p-type CCDs have been successfully used for previous X-ray astronomical satellites. We developed a prototype of a p-type CCD for SXI, called "CCD-NeXT1". CCD-NeXT1 is a frame-transfer CCD with two readout nodes. The image area of CCD-NeXT1 consists of 2Kx2K pixels with a pixel size of 12 μm x 12 μm. We evaluated performance of CCD-NeXT1 devices, KG-4 and KG-5. The energy resolution was 141.8±0.6 eV full width at half maximum at 5.9 keV, the readout noise was 4.7±0.2 e-, the horizontal CTI was < 5.1 x 10-7, and the vertical CTI was < 2.4 x 10-7 for KG-5. The performance of KG-4 was more or less the same as that of KG-5. The thickness of the depletion layer was 82±7 μm for KG-4 and 76±6 μm for KG-5. We conclude that our technology for p-type CCDs is sufficient to satisfy the CCD performance for the baseline of SXI.
The NeXT (New X-ray Telescope) satellite to be launched around 2010, has a large effective area in the 0.1-80
keV band with the use of the multilayer super mirror (HXT). As one of the focal plane detectors for NeXT,
we have been developing the Soft X-ray Imager (SXI). SXI consists of charge coupled devices (CCDs). In order
to increase the quantum efficiency (Q.E.) as high as possible, i.e., to detect X-rays collected by HXT as many
as possible, we developed a "fully-depleted and back-illuminated CCD" in the attempt to improve the Q.E.
of soft X-rays by the back-illuminated structure and that of hard X-rays by thickening of a depletion layer.
Thanks to a high-resistivity (over 10kΩ•cm) n-type Si, we have successfully developed Pch CCDs with very thick
depletion layer of over 300 micron, which is 4 times thicker than that of established X-ray MOS CCDs (for example
XIS, EPIC-MOS and ACIS-I). Furthermore, we have already confirmed we can thin a wafer down to 150 micron
independent of its resistivity from the experience of the development of the back supportless CCD. Based on
these successful results, we fabricated a test device of "fully depleted and back-illuminated CCD" with the high
resistivity (10kOhm cm) N-type Si thinned down to 200 micron. The pixel number and size are 512 x 512 and 24
x 24 μm, respectively. For optical blocking, we coated the surface with Al. We evaluated this test device and
confirmed the thickness of depletion layer reaches 200 micron as we expected. In this paper, we present progress in
development of these devices for SXI.
We have developed X-ray charge-coupled devices (CCD) for the next Japanese X-ray astronomical satellite mission, NeXT (Non-thermal energy eXploration Telescope). The hard X-ray telescope(HXT) onboard the NeXT can focus X-rays above 10 keV. Therefore, we need to develop an X-ray CCD for a focal plane detector to cover the 0.3-25 keV band in order to satisfy the capability of the telescope. We newly developed an n-type CCD fabricated on an n-type silicon wafer to expand the X-ray energy range as a focal plane detector of the
HXT. It is possible to have a thick depletion layer of approx. 300μm with an n-type CCD because it is easy to obtain high resistivity with an n-type silicon wafer compared to a p-type silicon wafer. We developed prototypes of n-type CCDs and evaluated their X-ray performance, energy resolution, charge transfer inefficiency(CTI) and the thickness of the depletion layer of two devices, designated Pch15 and Pch-teg. We measured the thickness of the depletion layer of Pch15 to be 290±33μm. For Pch-teg, the energy resolution was 152±3eV full width at half maximum (FWHM) at 5.9 keV and the readout noise was 7.3 e-. The performance of the n-type CCDs was comparable to that of p-type CCDs, and their depletion layer were much thicker than those of p-type CCDs.
KEYWORDS: Modulators, Video, Charge-coupled devices, Signal processing, Analog electronics, Video processing, Imaging spectroscopy, Control systems, Device simulation, X-rays
Delta Sigma digitizers generally have excellent linearity, precision and noise rejection. They are especially well
suited for implementation as integrated circuits. However, they are rarely used for time bounded signals like
CCD pixels. We are developing a CCD video digitizer chip incorporating a novel variant of the Delta Sigma
architecture that is especially well suited for this application. This architecture allows us to incorporate video
filtering and correlated double sampling into the digitizer itself, eliminating the complex analog video processing
usually needed before digitization.
We will present details of a multichannel ASIC design that will achieve spectroscopic precision and linearity
while using much less energy than previous CCD digitizers for technical applications such as imaging X-ray
spectroscopy. The low conversion energy requirement together with the ability to integrate many channels will
enable us to construct fast CCD systems that require no cooling and can handle a much wider range of X-ray
intensity than existing X-ray CCD systems.
We give overview and the current status of the development of the Soft X-ray Imager (SXI) onboard the NeXT
satellite. SXI is an X-ray CCD camera placed at the focal plane detector of the Soft X-ray Telescopes for Imaging
(SXT-I) onboard NeXT. The pixel size and the format of the CCD is 24 x 24μm (IA) and 2048 x 2048 x 2
(IA+FS). Currently, we have been developing two types of CCD as candidates for SXI, in parallel. The one is
front illumination type CCD with moderate thickness of the depletion layer (70 ~ 100μm) as a baseline plan.
The other one is the goal plan, in which we develop back illumination type CCD with a thick depletion layer
(200 ~ 300μm). For the baseline plan, we successfully developed the proto model 'CCD-NeXT1' with the pixel
size of 12μm x 12μm and the CCD size of 24mm x 48mm. The depletion layer of the CCD has reached 75 ~ 85μm.
The goal plan is realized by introduction of a new type of CCD 'P-channel CCD', which collects holes in stead
of electrons in the common 'N-channel CCD'. By processing a test model of P-channel CCD we have confirmed
high quantum efficiency above 10 keV with an equivalent depletion layer of 300μm. A back illumination type
of P-channel CCD with a depletion layer of 200μm with aluminum coating for optical blocking has been also
successfully developed. We have been also developing a thermo-electric cooler (TEC) with the function of the
mechanically support of the CCD wafer without standoff insulators, for the purpose of the reduction of thermal
input to the CCD through the standoff insulators. We have been considering the sensor housing and the onboard
electronics for the CCD clocking, readout and digital processing of the frame date.
The hard x-ray detector (HXD) is one of the three experiments of the Astro-E mission, the fifth Japanese X-ray Satellite devoted to studies of high energy phenomena in the universe in the x-ray to soft gamma-ray region. Prepared for launch at the beginning of 200 via the newly developed M-V launch vehicle of the Institute of Space and Astronomical Science, the Astro-E is to be thrown in to a near-circular orbit of 550 km altitude, with an inclination of 31 degrees. The flight model has been finished assembled this year, and we carried out various tests to verify the performance. We acquired the background spectrum at sea level, and confirmed that our system is operating effectively in reducing the background level. The HXD will observe photons in the energy range of 10-600 keV, and the calculations based on the preflight calibration suggest that the HXD will have the highest sensitivity ever achieved in this energy range. We also verified that our electronic system will maintain its performance against charged particle events expected in orbit.
The Hard x-ray Detector (HXD) is one of three instruments on the fifth Japanese x-ray astronomy satellite, Astro-E, scheduled for launch in 2000. The sensitivity of the Astro-E HXD will be higher by more than one order of magnitude than that of nay previous instrument between 10 keV and several 100 keV. The electronic system is designed to handle many independent data channels from the HXD within the limitation of size and power consumption required in Astro-E. In this paper, we will present the design and the preliminary performance of the processing electronic system.
We have developed the analog electronics of the ASTRO-E hard x-ray detector (HXD). The ASTRO-E is the fifth Japanese x-ray astronomy satellite scheduled for launch in 2000. Three experiments will be on board the satellite, one of which being the HXD. The detector consists of 16 units of well-type phoswich counters with silicon PIN diodes embedded therein, and covers the energy range of 10 approximately 600 keV with photon collecting area of about 350 cm2. The readout circuit for the HXD handles many signal channels (96 channels in total) under the limitation of power consumption and size set by the satellite. To meet the limitations, we have developed two types of bipolar semicustom LSIs. One is the pulse-shape discriminator (PSD-LSI) for phoswich counters and the other is for silicon PIN diodes (PIN-LSI). The PSD-LSI selects clean GSO hits and reduces the off-aperture x rays and internal background of the detector down to 10-5 c/s/cm2keV. One PIN-LSI handles signals from two PIN diodes, each consisting of an amplifier, a peak-hold circuit, and a comparator to trigger the readout system. Test pieces of these LSIs meet the specifications such as power consumptions and linearities. Using PIN-LSI, we could successfully obtain x-ray spectrum from 241Am with a PIN diode.
ASTRO-E is the next Japanese x-ray satellite to be launched in the year 2000. It carries three high-energy astrophysical experiments, including the hard x-ray detector (HXD) which is unique in covering the wide energy band from 10 keV to 700 keV with an extremely low background. The HXD is a compound-eye detector, employing 16 GSO/BGO well-type phoswich scintillation counters together with 64 silicon PIN detectors. The scintillation counters cover an energy range of 40 - 700 keV, while the PIN diodes fill the intermediate energy range from 10 keV to 70 keV with an energy resolution about 3 keV. In this paper, we report on the developments of the large area, thick silicon PIN diodes. In order to achieve a high quantum efficiency up to 70 keV with a high energy resolution, we utilize a double stack of silicon PIN diodes, each 20 by 20 mm2 in size and 2 mm thick. Signals from the two diodes are summed into a single output. Four of these stacks (or eight diodes) are placed inside the deep BGO active-shield well of a phoswich counter, to achieve an extremely low background environment. Thus, the HXD utilizes 64 stacked silicon PIN detectors, achieving a total geometrical collecting area of 256 cm2. We have developed the 2 mm thick silicon PIN diodes which have low leakage current, a low capacitance, and a high breakdown voltage to meet the requirements of our goal. Through various trials in fabricating PIN diodes with different structures, we have found optimal design parameters, such as mask design of the surface p+ layer and the implantation process.
Astro-E is the x-ray satellite to be launched in the year 2000 by Inst. of Space & Astronautical Science. This report deals with the design and expected performance of the hard x-ray detector (HXD), one of the 3 experiments aboard Astro- E. The HXD is a combination of GSO/BGO well-type phoswich counters and silicon PIN diodes: the two combined will cover a wide energy band of 10 - 700 keV. The detector is characterized by its low background of approximately 10-5/s/cm2/keV and its sensitivity higher than any past missions between a few 10 keV and several 100 keV. Combined with the other 2 experiments, a micro-calorimeter array (XRS) and 4 CCD arrays (XIS), both with x-ray mirrors, the mission will cover the soft and hard x-ray range at a highest sensitivity.
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