K. Nakazawa - Academia.edu (original) (raw)

Papers by K. Nakazawa

IEEE Transactions on Nuclear Science, 2009

We developed CdTe double-sided strip detectors (DSDs or cross strip detectors) and evaluated thei... more We developed CdTe double-sided strip detectors (DSDs or cross strip detectors) and evaluated their spectral and imaging performance for hard X-rays and gamma-rays. Though the double-sided strip configuration is suitable for imagers with a fine position resolution and a large detection area, CdTe diode DSDs with indium (In) anodes have yet to be realized due to the difficulty posed by the segmented In anodes. CdTe diode devices with aluminum (Al) anodes were recently established, followed by a CdTe device in which the Al anodes could be segmented into strips. We developed CdTe double-sided strip devices having Pt cathode strips and Al anode strips, and assembled prototype CdTe DSDs. These prototypes have a strip pitch of 400 µm. Signals from the strips are processed with analog ASICs (application specific integrated circuits). We have successfully performed gamma-ray imaging spectroscopy with a position resolution of 400 µm. Energy resolution of 1.8 keV (FWHM: full width at half maximum) was obtained at 59.54 keV. Moreover, the possibility of improved spectral performance by utilizing the energy information of both side strips was demonstrated. We designed and fabricated a new analog ASIC, VA32TA6, for the readout of semiconductor detectors, which is also suitable for DSDs. A new feature of the ASIC is its internal ADC function. We confirmed this function and good noise performance that reaches an equivalent noise charge of 110 e − under the condition of 3-4 pF input capacitance.

The Astrophysical Journal, 2009

The Astrophysical Journal, 2010

A3667 is the archetype of a merging cluster with radio relics. The northwest (NW) radio relic is ... more A3667 is the archetype of a merging cluster with radio relics. The northwest (NW) radio relic is the brightest cluster relic or halo known and is believed to be due to a strong merger shock. We have observed the NW relic for ∼40 ks of net XMM-Newton time. We observe a global decline of temperature across the relic from 6 to 1 keV, similar to the Suzaku results. Our new observations reveal a sharp change of both temperature and surface brightness near the position of the relic. The increased X-ray emission on the relic can be equivalently well described by either a thermal or nonthermal spectral model. The parameters of the thermal model are consistent with a Mach number M ∼ 2 shock and a shock speed of ∼1200 km s −1. The energy content of the relativistic particles in the radio relic can be explained if they are (re)-accelerated by the shock with an efficiency of ∼0.2%. Comparing the limit on the inverse Compton X-ray emission with the measured radio synchrotron emission, we set a lower limit to the magnetic field in the relic of 3 μG. If the emission from the relic is nonthermal, this lower limit is in fact the required magnetic field.

Monthly Notices of the Royal Astronomical Society, 2000

We measured metal abundances of the intracluster medium in the central regions of 34 nearby clust... more We measured metal abundances of the intracluster medium in the central regions of 34 nearby clusters of galaxies, using ASCA data. Clusters that have a sharp X-ray emission centred on a cD galaxy are commonly found to exhibit a central increment in the Fe abundance, which is more pronounced in lower temperature clusters; 1(0.1±0.2) solar at kT. 5 keVY compared with 1(0.2±0.3) solar at 1X5 , kT , 4 keVX These central excess metals are thought to be ejected from cD galaxies. Several low-temperature cD type clusters also show significant Si abundance increase by 1(0.1±0.2) solar at the central region. Compared with the Si-rich abundances observed in the outer regions of rich clusters, the Si to Fe abundance ratio of central excess metals tends to be near the solar ratio, implying that type Ia products from cD galaxies are dominant for the central excess metals. On the other hand, some other clusters do not show the central Fe abundance increase. As these clusters tend to contain two or three central giant galaxies, it is suggested that galaxy interactions have removed the central abundance increase.

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2013

ABSTRACT Astro-H is the sixth Japanese X-ray space observatory which will be launched in 2014. Tw... more ABSTRACT Astro-H is the sixth Japanese X-ray space observatory which will be launched in 2014. Two of onboard instruments of Astro-H, Hard X-ray Imager and Soft Gamma-ray Detector are surrounded by many number of large Bismuth Germanate (Bi4Ge3O12; BGO) scintillators. Optimum readout system of scintillation lights from these BGOs are essential to reduce the background signals and achieve high performance for main detectors because most of gamma-rays from out of field-of-view of main detectors or radio-isotopes produced inside them due to activation can be eliminated by anti-coincidence technique using BGO signals. We apply Avalanche Photo Diode (APD) for light sensor of these BGO detectors since their compactness and high quantum efficiency make it easy to design such large number of BGO detector system. For signal processing from APDs, digital filter and other trigger logics on the Field-Programmable Gate Array (FPGA) is used instead of discrete analog circuits due to limitation of circuit implementation area on spacecraft. For efficient observations, we have to achieve as low threshold of anti-coincidence signal as possible by utilizing the digital filtering. In addition, such anti-coincident signals should be sent to the main detector within 5μs to make it in time to veto the A–D conversion. Considering this requirement and constraint from logic size of FPGA, we adopt two types of filter, 8 delay taps filter with only 2 bit precision coefficient and 16 delay taps filter with 8 bit precision coefficient. The data after former simple filter provides anti-coincidence signal quickly in orbit, and the latter filter is used for detail analysis after the data is down-linked.

IEEE Transactions on Nuclear Science, 2004

Compton telescopes based on semiconductor technologies are being developed to explore the gamma-r... more Compton telescopes based on semiconductor technologies are being developed to explore the gamma-ray universe in an energy band 0.1-20 MeV, which is not well covered by the present or near-future gamma-ray telescopes. The key feature of such Compton telescopes is the high energy resolution that is crucial for high angular resolution and high background rejection capability. The energy resolution around 1 keV is required to approach physical limit of the angular resolution due to Doppler broadening. We have developed a low noise front-end ASIC (Application-Specific Integrated Circuit), VA32TA, to realize this goal for the readout of Double-sided Silicon Strip Detector (DSSD) and Cadmium Telluride (CdTe) pixel detector which are essential elements of the semiconductor Compton telescope. We report on the design and test results of the VA32TA. We have reached an energy resolution of 1.3 keV (FWHM) for 60 keV and 122 keV at 0 • C with a DSSD and 1.7 keV (FWHM) with a CdTe detector. Index terms-Analog integrated circuits, Gamma-ray detectors, Compton Camera, Silicon radiation detectors, Cadmium Telluride.

IEEE Transactions on Nuclear Science, 2005

We are developing low noise analog ASICs for hard X-ray and gamma-ray astronomy. They are based o... more We are developing low noise analog ASICs for hard X-ray and gamma-ray astronomy. They are based on a sub-micron CMOS process. Our design includes a finely-adjustable high-resistance circuit, which, for example, enables us to realize a pole-zero-cancellation in the analog chain. A two-dimensional ASIC with a fast read-out scheme and a one-dimensional ASIC with an improved analog design are developed and verified. With the latter ASIC connected with a CdTe detector, we have obtained a radioactive source spectrum with an energy resolution of 4.6 keV (FWHM) at 59.5 keV.

IEEE Transactions on Nuclear Science, 2009

We developed CdTe double-sided strip detectors (DSDs or cross strip detectors) and evaluated thei... more We developed CdTe double-sided strip detectors (DSDs or cross strip detectors) and evaluated their spectral and imaging performance for hard X-rays and gamma-rays. Though the double-sided strip configuration is suitable for imagers with a fine position resolution and a large detection area, CdTe diode DSDs with indium (In) anodes have yet to be realized due to the difficulty posed by the segmented In anodes. CdTe diode devices with aluminum (Al) anodes were recently established, followed by a CdTe device in which the Al anodes could be segmented into strips. We developed CdTe double-sided strip devices having Pt cathode strips and Al anode strips, and assembled prototype CdTe DSDs. These prototypes have a strip pitch of 400 µm. Signals from the strips are processed with analog ASICs (application specific integrated circuits). We have successfully performed gamma-ray imaging spectroscopy with a position resolution of 400 µm. Energy resolution of 1.8 keV (FWHM: full width at half maximum) was obtained at 59.54 keV. Moreover, the possibility of improved spectral performance by utilizing the energy information of both side strips was demonstrated. We designed and fabricated a new analog ASIC, VA32TA6, for the readout of semiconductor detectors, which is also suitable for DSDs. A new feature of the ASIC is its internal ADC function. We confirmed this function and good noise performance that reaches an equivalent noise charge of 110 e − under the condition of 3-4 pF input capacitance.

The Astrophysical Journal, 2009

The Astrophysical Journal, 2010

A3667 is the archetype of a merging cluster with radio relics. The northwest (NW) radio relic is ... more A3667 is the archetype of a merging cluster with radio relics. The northwest (NW) radio relic is the brightest cluster relic or halo known and is believed to be due to a strong merger shock. We have observed the NW relic for ∼40 ks of net XMM-Newton time. We observe a global decline of temperature across the relic from 6 to 1 keV, similar to the Suzaku results. Our new observations reveal a sharp change of both temperature and surface brightness near the position of the relic. The increased X-ray emission on the relic can be equivalently well described by either a thermal or nonthermal spectral model. The parameters of the thermal model are consistent with a Mach number M ∼ 2 shock and a shock speed of ∼1200 km s −1. The energy content of the relativistic particles in the radio relic can be explained if they are (re)-accelerated by the shock with an efficiency of ∼0.2%. Comparing the limit on the inverse Compton X-ray emission with the measured radio synchrotron emission, we set a lower limit to the magnetic field in the relic of 3 μG. If the emission from the relic is nonthermal, this lower limit is in fact the required magnetic field.

Monthly Notices of the Royal Astronomical Society, 2000

We measured metal abundances of the intracluster medium in the central regions of 34 nearby clust... more We measured metal abundances of the intracluster medium in the central regions of 34 nearby clusters of galaxies, using ASCA data. Clusters that have a sharp X-ray emission centred on a cD galaxy are commonly found to exhibit a central increment in the Fe abundance, which is more pronounced in lower temperature clusters; 1(0.1±0.2) solar at kT. 5 keVY compared with 1(0.2±0.3) solar at 1X5 , kT , 4 keVX These central excess metals are thought to be ejected from cD galaxies. Several low-temperature cD type clusters also show significant Si abundance increase by 1(0.1±0.2) solar at the central region. Compared with the Si-rich abundances observed in the outer regions of rich clusters, the Si to Fe abundance ratio of central excess metals tends to be near the solar ratio, implying that type Ia products from cD galaxies are dominant for the central excess metals. On the other hand, some other clusters do not show the central Fe abundance increase. As these clusters tend to contain two or three central giant galaxies, it is suggested that galaxy interactions have removed the central abundance increase.

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2013

ABSTRACT Astro-H is the sixth Japanese X-ray space observatory which will be launched in 2014. Tw... more ABSTRACT Astro-H is the sixth Japanese X-ray space observatory which will be launched in 2014. Two of onboard instruments of Astro-H, Hard X-ray Imager and Soft Gamma-ray Detector are surrounded by many number of large Bismuth Germanate (Bi4Ge3O12; BGO) scintillators. Optimum readout system of scintillation lights from these BGOs are essential to reduce the background signals and achieve high performance for main detectors because most of gamma-rays from out of field-of-view of main detectors or radio-isotopes produced inside them due to activation can be eliminated by anti-coincidence technique using BGO signals. We apply Avalanche Photo Diode (APD) for light sensor of these BGO detectors since their compactness and high quantum efficiency make it easy to design such large number of BGO detector system. For signal processing from APDs, digital filter and other trigger logics on the Field-Programmable Gate Array (FPGA) is used instead of discrete analog circuits due to limitation of circuit implementation area on spacecraft. For efficient observations, we have to achieve as low threshold of anti-coincidence signal as possible by utilizing the digital filtering. In addition, such anti-coincident signals should be sent to the main detector within 5μs to make it in time to veto the A–D conversion. Considering this requirement and constraint from logic size of FPGA, we adopt two types of filter, 8 delay taps filter with only 2 bit precision coefficient and 16 delay taps filter with 8 bit precision coefficient. The data after former simple filter provides anti-coincidence signal quickly in orbit, and the latter filter is used for detail analysis after the data is down-linked.

IEEE Transactions on Nuclear Science, 2004

Compton telescopes based on semiconductor technologies are being developed to explore the gamma-r... more Compton telescopes based on semiconductor technologies are being developed to explore the gamma-ray universe in an energy band 0.1-20 MeV, which is not well covered by the present or near-future gamma-ray telescopes. The key feature of such Compton telescopes is the high energy resolution that is crucial for high angular resolution and high background rejection capability. The energy resolution around 1 keV is required to approach physical limit of the angular resolution due to Doppler broadening. We have developed a low noise front-end ASIC (Application-Specific Integrated Circuit), VA32TA, to realize this goal for the readout of Double-sided Silicon Strip Detector (DSSD) and Cadmium Telluride (CdTe) pixel detector which are essential elements of the semiconductor Compton telescope. We report on the design and test results of the VA32TA. We have reached an energy resolution of 1.3 keV (FWHM) for 60 keV and 122 keV at 0 • C with a DSSD and 1.7 keV (FWHM) with a CdTe detector. Index terms-Analog integrated circuits, Gamma-ray detectors, Compton Camera, Silicon radiation detectors, Cadmium Telluride.

IEEE Transactions on Nuclear Science, 2005

We are developing low noise analog ASICs for hard X-ray and gamma-ray astronomy. They are based o... more We are developing low noise analog ASICs for hard X-ray and gamma-ray astronomy. They are based on a sub-micron CMOS process. Our design includes a finely-adjustable high-resistance circuit, which, for example, enables us to realize a pole-zero-cancellation in the analog chain. A two-dimensional ASIC with a fast read-out scheme and a one-dimensional ASIC with an improved analog design are developed and verified. With the latter ASIC connected with a CdTe detector, we have obtained a radioactive source spectrum with an energy resolution of 4.6 keV (FWHM) at 59.5 keV.