渡辺 伸

We describe recent progress on the use of Schottky CdTe diode detectors for spectrometry. The low leakage current of the CdTe diode allows us to apply a much higher bias voltage than was possible with previous CdTe detectors. For a relatively thin detector of 0.5-1 mm thickness, the high bias voltage results in a high electric field in the device. Both the improved charge-collection efficiency and the low-leakage current lead to an energy resolution of better than 600 eV full-width at half-maximum at 60 keV for a 2 × 2 mm2 device without any charge-loss correction electronics. Large-area detectors with dimensions of 21 × 21 mm2 are now available with an energy resolution of ∼ 2.8 keV. Long-term stability can be easily attained for relatively thin (< 1 mm) detectors if they are cooled or operated under a high bias voltage.

We describe a stacked detector made of thin cadmium telluride (CdTe) diode detectors. By using a thin CdTe device, we can overcome the charge loss problem due to the small mobility and short lifetime of holes in CdTe or cadmium zinc telluride (CdZnTe) detectors. However, a CdTe detector with a thickness of more than 5 mm is needed for adequate detection efficiency for gamma-rays of several hundred keV. Good energy resolution and good peak detection efficiency are difficult to obtain using such a thick CdTe detector. The stacked detector enabled us to realize a detector with both high-energy resolution and good efficiencies for gamma rays up to several hundred keV. In order to verify this concept, we constructed a prototype made of ten layers of a 0.5-mm-thick CdTe diode detectors with a surface area of 21.5 mm × 21.5 mm. With this, we have achieved 5.3-keV and 7.9-keV energy resolution [full width at half maximum (FWHM)] at 356 keV and 662 keV, respectively, at the temperature of -20°C.

In order to characterize CdTe/CdZnTe detectors in a planar configuration, we have developed a new method to extract μτ products. In this method, we prepare an analytic spectral model based on the charge transport properties in the device, which is intended to be used in fitting calculation. The low mobility-lifetime (μτ) products of carriers in CdTe/CdZnTe detectors produce a position dependency in the charge induction efficiency. The model takes the induction efficiency and interaction positions of photons into account. Since the model is parameterized by μτ products, it can extract μτ products. Here, we demonstrate how the model works based on the results from 2-mm-thick HPB CdZnTe and THM CdTe detectors.

We describe recent progress on the use of Schottky CdTe diode detectors for spectrometry. The low leakage current of the CdTe diode allows us to apply a much higher bias voltage than was possible with previous CdTe detectors. For a relatively thin detector of 0.5-1 mm thick, the high bias voltage results in a high electric field in the device. Both the improved charge collection efficiency and the low-leakage current lead to an energy resolution of better than 600 eV FWHM at 60 keV for a 2x2 mm(2) device without any charge-loss correction electronics. Large area detectors with dimensions of 21x21 mm(2) are now available with an energy resolution of similar to2.8 keV Long term stability can be easily attained for relatively thin (&lt; 1 nun) detectors, if they are cooled or operated under a high bias voltage.

In order to characterize CdTe/CdZnTe detectors in a planar configuration, we have developed a new spectral model based on the charge transportation properties in the device. The low mobility-lifetime (mur) products of carriers in CdTe/CdZnTe detectors produce a position dependency in the charge induction efficiency. The model takes the induction efficiency and interaction positions of photons into account. Since the model is parameterized by mutau products, it can also be used as a new method to extract mutau products. Here, we demonstrate how the model works based on the results from 2 mm thick HPB CdZnTe.

We describe recent progress on the use of Schottky CdTe diode detectors for spectrometry. The low leakage current of the CdTe diode allows us to apply a much higher bias voltage than was possible with previous CdTe detectors. For a relatively thin detector of 0.5-1 mm thick, the high bias voltage results in a high electric field in the device. Both the improved charge collection efficiency and the low-leakage current lead to an energy resolution of better than 600 eV FWHM at 60 keV for a 2×2 mm2 device without any charge-loss correction electronics. Large area detectors with dimensions of 21×21 mm2 are now available with an energy resolution of ∼2.8 keV. Long term stability can be easily attained for relatively thin (< 1 mm) detectors, if they are cooled or operated under a high bias voltage.

We describe a stacked detector made of thin CdTe diode detectors. By using a thin CdTe device, we can overcome the charge loss problem due to the small mobility and short lifetime of holes in CdTe or CdZnTe detectors. However, a CdTe detector with a thickness of more than 5 mm is needed for adequate detection efficiency for gamma-rays of several hundred keV. Good energy resolution and good peak detection efficiency are difficult to obtain using such a thick CdTe detector. The stacked detector enabled us to realize a detector with both high energy resolution (∼ 1%) and good efficiencies for gamma-rays up to several hundred keV. In this paper, we report the advantage of CdTe thin detectors and the performance of CdTe stacked detectors made of ten layers of a 0.5 mm thick CdTe diode detectors with a surface area of 21.5 mm × 21.5 mm.

In order to characterize CdTe/CdZnTe detectors in a planar configuration, we have developed a new spectral model based on the charge transportation properties in the device. The low mobility-lifetime (μτ) products of carriers in CdTe/CdZnTe detectors produce a position dependency in the charge induction efficiency. The model takes the induction efficiency and interaction positions of photons into account. Since the model is parameterized by μτ products, it can also be used as a new method to extract μτ products. Here, we demonstrate how the model works based on the results from 2 mm thick HPB CdZnTe.

We report the timing properties of CdTe and CdZnTe detectors in planar configuration. By utilizing 241Am doped scintilator, we have developed a new method to evaluate the timing performance of the semiconductor detector. We confirm that the slow mobility and short life time of holes significantly degrades the timing performance. To achieve high carrier speed, either by applying a high electric field or by selecting only electrons, is very important for obtaining a detector with fast, ∼ nsec timing capability. To select only the electron events, we adopt the pulse height selection with a fast-slow shaping amplifier. In conjunction with a newly developed CdTe diode, we obtain a superior performance of 5.8 nsec. We also discuss the application for Positron Emission Tomography with 511 keV gamma-gamma coincidence method, and found that a geometrical arrangement in which electrodes are parallel to the incident γ-rays gives about 3 time better timing response than is available when the electrodes are perpendicular to the γ-ray beam.

We are developing a large array detector composed of 1024 Individual CdTe diodes. The each detector has the dimensions of 1.2mm × 5.0mm and a thickness of 1.2 mm. An edge-on geometry is used for the injection of gamma-rays. With this geometry, the distance between the two electrodes can be kept small, and we can therefore apply the high electric field which is necessary to achieve the high energy resolution (by reducing the low energy tail) and also to sustain the long-term stability of the CdTe diode. Signals from each detector element are fed into newly developed low noise ASICs. We use 32 chips for the readout of 1024 elements. In this paper, we will report basic characteristics of individual detectors and overall performance of the gamma-camera. Design of the readout electronics system is also described.

A 0.5-mm-thick Cadmium Telluride (CdTe) semiconductor pixel sensor with 1024 pixels has been bump-bonded onto a two-dimensional (2-D) single photon counting pixel read out chip (MPEC 2.1) using a special gold-stud technique. The pixel size is 200 × 200 μm2, the active area is 6.4 × 6.4 mm2. The successful operation of this high-Z imaging pixel device is demonstrated. Noise and threshold dispersion as well as the imaging performance are reported.

Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) have been regarded as promising semiconductor materials for hard X-ray and Gamma-ray detection. The high atomic number of the materials (Z_{Cd} =48, Z_{Te} =52) gives a high quantum efficiency in comparison with Si. The large band-gap energy (Eg ~ 1.5 eV) allows us to operate the detector at room temperature. However, a considerable amount of charge loss in these detectors produces a reduced energy resolution. This problem arises due to the low mobility and short lifetime of holes. Recently, significant improvements have been achieved to improve the spectral properties based on the advances in the production of crystals and in the design of electrodes. In this overview talk, we summarize (1) advantages and disadvantages of CdTe and CdZnTe semiconductor detectors and (2) technique for improving energy resolution and photopeak efficiencies. Applications of these imaging detectors in future hard X-ray and Gamma-ray astronomy missions are briefly discussed.

We report the current status of the ASCA Medium Sensitivity Survey (AMSS; or the GIS catalog project). The latest results from the optical identification program for a hard-band selected sample are presented.

The ASTRO-E Hard X-ray Detector utilized GSO/BGO well-type phoswich counters in compound-eye configuration [1], to achieve an extremely low background level of a few ×10-5 counts s-1cm-2keV-1. The GSO scintillators installed in the BGO active shield wells observes 30-600 keV photons, while silicon PIN diodes of 2 mm thick placed in front of each GSO crystal covers 10 - 60 keV photons with energy resolution of ∼3.5 keV FWHM. The design goals both of low background and high energy resolution in the hard X-ray bands were confirmed to be achieved through the preflight calibration experiments.

The ASTRO-E Hard X-ray Detector utilizes GSO/BGO well-type phoswich counters in compound-eye configuration, to achieve an extremely low background level of about a few times 10-5 counts s-1 cm-2 keV-1. The GSO scintillators placed at the bottom of the BGO well observe photons in the energy range 30-600 keV. To cover the lower energy range of 10-60 keV, silicon PIN diodes of 2 mm in thickness and 21.5 × 21.5mm2 in size were newly developed, and placed in front of the GSO scintillators. The PIN diode exhibits complex spectral responses, including subpeak and low energy tail components. To examine the origin of these components, we measured spatially-resolved response of the PIN diode, and confirmed that the subpeak and the low energy tail are related to the electrode structures and electric fields in the PIN diode, respectively.

Using a high quality Cadmium Telluride (CdTe) wafer, we formed a Schottky junction and operated the detector as a diode (CdTe diode). The low leakage current of the CdTe diode allows us to apply a much higher bias voltage than was possible with the previous CdTe detectors. For a relatively thin detector of ≈ 0.5 mm thick, the high bias voltage results in a high electric field in the device. Both the improved charge collection efficiency and the low-leakage current lead to an energy resolution of 1.1 keV FWHM at 60 keV for a 2×2 mm2 device and 2 keV for a 10×10 mm2 device at 5°C without any charge-loss correction electronics. For astrophysical applications, we have developed a an initial prototype CdTe pixel detector based on the CdTe diode. The detector has 400 pixels with a pixel size of 625 × 625 μ2. Each pixel is gold-stud bonded to a fanout board and routed to a front end ASIC to measure pulse height information for each γ-ray photon.