We are developing imaging Cadmium Telluride (CdTe) pixel detectors optimized for astrophysical hard X-ray
applications. Our hybrid detector consist of a CdTe crystal 1mm thick and 2cm × 2cm in area with segmented
anode contacts directly bonded to a custom low-noise application specific integrated circuit (ASIC). The CdTe
sensor, fabricated by ACRORAD (Okinawa, Japan), has Schottky blocking contacts on a 605 micron pitch in a
32 × 32 array, providing low leakage current and enabling readout of the anode side. The detector is bonded
using epoxy-gold stud interconnects to a custom low noise, low power ASIC circuit developed by Caltech's
Space Radiation Laboratory. We have achieved very good energy resolution over a wide energy range (0.62keV
FWHM @ 60keV, 10.8keV FWHM @ 662keV). We observe polarization effects at room temperature, but they
are suppressed if we operate the detector at or below 0°C degree. These detectors have potential application for
future missions such as the International X-ray Observatory (IXO).
We have developed a Compton camera with a double-sided silicon strip detector (DSSD) for hard X-ray and
gamma-ray observation. Using a DSSD as a scatter detector of the Compton camera, we achieved high angular
resolution of 3.4° at 511 keV. Through the imaging of various samples such as two-dimentional array sources and
a diffuse source, the wide field-of-view (~ 100°) and the high spatial resolution (at least 20 mm at a distance of
60 mm from the DSSD) of the camera were confirmed. Furthermore, using the List-Mode Maximum-Likelihood
Expectation-Maximization method, the camera can resolve an interval of 3 mm at a distance of 30 mm from the
DSSD.
The Hard X-ray Imager (HXI) is one of three focal plane detectors on board the NeXT (New exploration X-ray
Telescope) mission, which is scheduled to be launched in 2013. By use of the hybrid structure composed of
double-sided silicon strip detectors and a cadmium telluride strip detector, it fully covers the energy range of
photons collected with the hard X-ray telescope up to 80 keV with a high quantum efficiency. High spatial
resolutions of 400 micron pitch and energy resolutions of 1-2 keV (FWMH) are at the same time achieved with
low noise front-end ASICs. In addition, thick BGO active shields compactly surrounding the main detection
part, as a heritage of the successful performance of the Hard X-ray Detector (HXD) on board Suzaku satellite,
enable to achive an extremely high background reduction for the cosmic-ray particle background and in-orbit
activation. The current status of hardware development including the design requirement, expected performance,
and technical readinesses of key technologies are summarized.
We developed Schottky CdTe detectors using Al as an anode electrode and measured their performances. We
first fabricated monolithic detectors with four different thicknesses of 0.5, 0.75, 1.0, and 2.0 mm. An Al anode
electrode was implemented with a guard-ring structure. For the 0.5 mm thick CdTe detector, an energy resolution
of 1.2 keV (FWHM) at 122 keV was achieved at a temperature of −20 °C and a bias voltage of 400 V. Using
the same technology, we next developed 8 × 8 pixel CdTe detectors, again with the four different thicknesses.
The Al anode electrode was pixelated and the Pt cathode was made as a single plate. Signals from all pixels
were successfully obtained and an energy resolution of 1.3 keV and 1.9 keV (FWHM) for 59.5 keV and 122 keV
gamma-rays, was achieved at a temperature of −20 °C and a bias voltage of 400 V using the 0.5 mm thick CdTe
detector. The energy resolution was nearly the same in each pixel.
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