三谷 烈史

The Energization and Radiation in Geospace (ERG) project for solar cycle 24 will explore how relativistic electrons in the radiation belts are generated during space stout's. This geospace exploration project consists of three research teams: the ERG satellite observation team, the ground-based network observation team, and the integrated data analysis/simulation team. Satellite observation will provide in situ measurements of features such as the plasma distribution function, electric and magnetic fields, and plasma waves, whereas remote sensing by ground-based observations using, for example, HF radars, magnetometers, optical instruments, and radio wave receivers will provide the global state of the geospace. Various kinds of data will be integrated and compared with numerical simulations for quantitative understanding. Such a synergetic approach is essential for comprehensive understanding of relativistic electron generation/loss processes through cross-energy and cross-regional coupling in which different plasma populations and regions are dynamically coupled with each other. In addition, the ERG satellite will utilize a new and innovative measurement technique for wave-particle interactions that can directly measure the energy exchange process between particles and plasma waves. In this paper, we briefly review some of the profound problems regarding relativistic electron accelerations and losses that will be solved by the ERG project, and we provide an overview of the project.

We report the performance of an Avalanche Photodiode (APD) that has a large-area (1 cm(2)) and a thick depletion layer (similar to 30 mu m) in application to electron measurements. It is shown that the dead layer is sufficiently thin for the detection of electrons at energies less than 10 keV; the lowest measurable energy is similar to 5 keV. We also confirmed that the energy resolution is not significantly deteriorated by the non-uniformity of the thickness of the dead layer or the internal gain. The energy resolution at the lower energy range (<10 keV) is limited by the electric noise. Dependence of the electric noise on the detector size is also discussed. Furthermore, the effective multiplication profile in the normal direction is inferred from the experimental data.

SPRITE-SAT is a micro satellite in the size of 50 cm cube and weighing 45-kg, designed and developed by Tohoku University. Its mission objective is to conduct scientific observation of atmospheric luminous emissions called "sprites" and terrestrial Gamma-ray flushes. Both are recently discovered phenomena and their mechanisms are still under the veil. SPRITE-SAT was developed to achieve significant observations to determine clear models of these mysterious phenomena. On January 23rd, 2009, SPRITE-SAT was successfully launched by JAXA's H-IIA rocket as a piggyback payload of Greenhouse Gas Observation Satellite (GOSAT). The spacecraft is now in a sun-synchronous polar orbit with 670 km altitude from the Earth's surface. This paper describes a general overview of the spacecraft and its mission.

The combination of an electrostatic analyzer and a silicon strip solid detector is proposed for measuring the energy and charge state of medium energy (10-200 keV/q) ions in space. Based on laboratory experiments, it is shown that a single-sided silicon strip detector (SSSD) has low-noise levels that provide sufficient energy resolution for charge state measurements of medium energy ions. It is also demonstrated that energy loss at a dead-layer is a critical factor for the energy resolution and measurement energy threshold; the dead-layer is measured to be similar to 370 mm. A single-sided silicon strip detector with a thin dead-layer is important for the development of medium energy ion measurements into next generation satellite-borne missions. (C) 2009 Elsevier B.V. All rights reserved.

We have developed a new energy/mass spectrometer for medium energy range (similar to 10-200 keV/q) ion measurements in the Earth's magnetosphere and interplanetary space. The wide field-of-view (similar to 360 degrees fan) enables acquisition of 3-D distribution functions for all the major ions, by utilizing spacecraft spin motions. The g-factor is much larger than the previous ion mass spectrometers in the medium energy range. The mass analysis unit that measures ion time-of-flights is well designed to realize a lightweight and simple signal processing. Laboratory experiments with a test model show that the performance of mass spectroscopy agrees with numerical simulations. Medium energy ion mass spectrometer with this new design will surely be useful for upcoming space missions in the inner magnetosphere, reconnection regions, and other energetic plasma structures/phenomena in space.

The in-orbit performance and calibration of the Hard X-ray Detector (HXD) on board the X-ray astronomy satellite Suzaku are described. Its basic performances, including a wide energy bandpass of 10-600keV, energy resolutions of similar to 4 keV (FWHM) at 40 keV and similar to 11% at 511 keV, and a high background rejection efficiency, have been confirmed by extensive in-orbit calibrations. The long-term gains of PIN-Si diodes have been stable within 1% for half a year, and those of scintillators have decreased by 5-20%. The residual non-X-ray background of the HXD is the lowest among past non-imaging hard X-ray instruments in energy ranges of 15-70 and 150-500 keV. We provide accurate calibrations of energy responses, angular responses, timing accuracy of the HXD, and relative normalizations to the X-ray CCD cameras using multiple observations of the Crab Nebula.

The Hard X-ray Detector (HXD) on board Suzaku covers a wide energy range from 10 keV to 600 keV by the combination of silicon PIN diodes and GSO scintillators. The HXD is designed to achieve an extremely low in-orbit background based on a combination of new techniques, including the concept of a well-type active shield counter. With an effective area of 142 cm(2) at 20 keV and 273 cm(2) at 150 keV, the background level at sea level reached similar to 1 X 10(-5) cts s(-1) cm(-2) keV(-1) at 30 keV for the PIN diodes, and similar to 2 X 10(-5) cts s(-1) cm(-2) keV(-1) at 100 keV, and similar to 7 X 10(-6) cts s(-1) cm(-2) keV(-1) at 200 keV for the phoswich counter. Tight active shielding of the HXD results in a large array of guard counters surrounding the main detector parts. These anti-coincidence counters, made of similar to 4 cm thick BGO crystals, have a large effective area for sub-MeV to MeV gamma-rays. They work as an excellent gamma-ray burst monitor with limited angular resolution (similar to 5 degrees). The on-board signal-processing system and the data transmitted to the ground are also described.

We are developing a semiconductor Compton telescope to explore the universe in the energy band from several tens of keV to a few MeV. A detector material of combined Si strip and CdTe pixel is used to cover the energy range around 60 keV. For energies above several hundred keV, in contrast, the higher detection efficiency of CdTe semiconductor in comparison with Si is expected to play an important role as both an absorber and a scatterer. In order to demonstrate the spectral and imaging capability of a CdTe-based Compton camera, we developed a Compton telescope consisting of a stack of CdTe pixel detectors as a small scale prototype. With this prototype, we succeeded in reconstructing images and spectra by solving the Compton kinematics within the energy band from 122 to 662 keV. The energy resolution (FWHM) of reconstructed spectra is 7.3 keV at 511 keV. The angular resolution obtained at 511 keV is measured to be 12.2 (FWHM). (c) 2006 Published by Elsevier B.V.

The soft gamma-ray detector (SGD) onboard the Japanese future high energy astrophysics mission (NeXT) is a Compton telescope with narrow field of view, which utilizes Compton kinematics to enhance its background rejection capabilities. It is realized as a hybrid semiconductor gamma-ray detector which consists of silicon and cadmium telluride (CdTe) detectors. It can detect photons in a wide energy band (0.05-1 MeV) at a background level of 5 x 10(-7) counts/s/cm(2) /keV; the silicon layers are required to improve the performance at a lower energy band (< 0.3 MeV). Excellent energy resolution is the key feature of the SGD, allowing it to achieve both high angular resolution and good background rejection capability. An additional capability of the SGD, its ability to measure gamma-ray polarization, opens up a new window to study properties of astronomical objects. We will present the development of key technologies to realize the SGD: high quality CdTe, low noise front-end application-specific integrated circuit, and bump bonding technology. Energy resolutions of 1.7 keV (full-width at half-maximum) for CdTe pixel detectors and 1.1 keV for Si strip detectors have been measured. We also present the validation of Monte Carlo simulation used to evaluate the performance of the SGD.

The hard X-ray detector (HXD-II) is one of the three scientific instruments onboard Japanese X-ray astronomy satellite Astro-E2 scheduled to be launched in 2005. This mission is very unique in a point of having a lower background than any other past missions in the 10-600 keV range. In the HXD-II, the large and thick BGO crystals are used as active shields for particle and gamma-ray background to the main detector. They have a wide field of view of similar to 2 pi and a large effective area of 400 cm(2) even at 1 MeV. Hence, the BGO shields have been developed as a wide-band all-sky monitor (WAM) with a broadband coverage of 50-5000 keV. In this paper, overall design and performance of the HXD-II/WAM based on the results of preflight calibration tests carried out in June 2004 are described. By irradiating various radio isotopes with the WAM flight model, we verified that it had comparable capabilities with other gamma-ray burst detectors.

We are developing a Compton camera based on Si and CdTe semiconductor imaging devices with high energy resolution. In this paper, results from the most recent prototype are reported. The Compton camera consists of six layered double-sided Si Strip detectors and CdTe pixel detectors, which are read out with low noise analog ASICs. VA32TAs. We obtained Compton reconstructed images and spectra of line gamma-rays from 122 keV to 662 keV The energy resolution is 9.1 keV and 14 keV at 356 keV and 511 keno respectively.

Employing Fourier-synthesis optics and one-dimensional position-sensitive detectors, we are developing a novel hard X-ray imager which can work in the similar to 10 keV to similar to 200 keV range either as a telescope or a microscope. As the detection part of our imager, we have developed a strip detector made of Schottky CdTe diode, with its cathode divided into 64 channels of 150 mu m pitch. Electrodes of all channels are gold-stud bonded to a fanout board, and connected to low noise analog ASIC. We read out signals from all channels simultaneously. As the grid optics elements, one-dimensional "modulation collimator" grids of 1 mm thick tungsten have been manufactured, with 10 grid pitches ranging from 0.2 mm to 2 mm with harmonic ratios. Combining the CdTe strip detector and the grids, we have verified hard X-ray imaging performance of this system. Using an Am-241 source, we have successfully obtained an image in the 10-70 keV range with a spatial resolution of similar to 0.5 mm.

Double-sided silicon strip detectors (DSSD) provide a very promising technology for constructing a Compton Camera, which is expected to provide a high-sensitivity soft gamma-ray observation in the 0.1-20 MeV energy range. The merits of DSSD are the high-energy resolution, high scattering efficiency, low radio-activation in the orbit, moderate radiation hardness, smaller Doppler broadening, large size, and stable performance. A key feature for optimal performance is the low noise level of the DSSD and the attached frontend electronics. We minimized the noise by optimization of the electrode geometry of the DSSD. We have thus obtained an energy resolution of 1.3 keV (FWHM) for 60 and 122 keV at -10 degrees C. It was found that the detection efficiency for gamma-rays was uniform over the DSSD and the signal charge split between neighboring strips was not significant. We also confirmed that the Compton imaging by two DSSDs achieved a good angular resolution close to the Doppler-broadening limit. (c) 2005 Elsevier B.V. All rights reserved.

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 (E-g = 1.5 eV) allows to operate the detector at room temperature. Based on recent achievements in high-resolution CdTe detectors, in the technology of ASICs and in bump-bonding, we have proposed the novel hard X-ray and gamma-ray detectors for the NeXT mission in Japan. The high-energy response of the super mirror onboard NeXT will enable us to perform the first sensitive imaging observations up to 80keV. The focal plane detector, which combines a fully depleted X-ray CCD and a pixellated CdTe detector, will provide spectra and images in the wide energy range from 0.5 to 80keV. In the soft gamma-ray band up to similar to 1 MeV, a narrow field-of-view Compton gamma-ray telescope utilizing several tens of layers of thin Si or CdTe detector will provide precise spectra with much higher sensitivity than present instruments. The continuum sensitivity will reach several x 10(-8) photons(-1) keV(-1) cm(-1) in the hard X-ray region and a few X 10(-7) photons(-1) keV(-1) cm(-2) in the soft gamma-ray region. (c) 2005 Elsevier B.V. All rights reserved.

A Compton camera is the most promising approach for gamma-ray detection in the energy region from several hundred kiloelectronvolts to megaelectronvolts, especially for application in high energy astrophysics. In order to obtain good angular resolution, semiconductor detectors such as silicon, germanium and cadmium telluride(CdTe) have several advantages over scintillation detectors, which have been used so far. Based on the recent advances of high resolution CdTe and silicon imaging detectors, we are working on a Si/CdTe Compton camera. We have developed 64-pixel CdTe detectors with a pixel size of 2 mm x 2 mm and double-sided Si strip detectors(DSSDs) with a position resolution of 800 mum. As a prototype Si/CdTe Compton camera, we use a DSSD as a scatterer and two CdTe pixel detectors as an absorber. In order to verify its performance, we irradiate the camera with 100% linearly polarized 170 keV gamma-rays and demonstrate the system works properly as a Compton camera. The resolution of the reconstructed scattering angle is 22degrees(full-width at half-maximum). Measurement of polarization is also reported. The polarimetric modulation factor is obtained to be 43 %, which is consistent with the prediction of Monte Carlo simulations.

Recent results from the Schottky CdTe diode detectors employing a guard-ring (GR) electrode are reported. A cathode electrode, made of platinum, was separated into an active electrode(s) and a surrounding GR. Typical leakage current of a device with active area of 2 x 2 mm(2) and 0.5 mm thickness surrounded by a GR, is 7 and 20 pA at a bias of 100 and 500 V, respectively, operated at 20 degreesC. Spectral resolution of this device is 0.93 and 1.2 keV (FWHM) at 59.5 and 122 keV, respectively, operated at 20 degreesC with a bias of 800 V. Detailed study of the characteristics of these devices working as a gamma-ray detector is presented.

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 [full-width at half-maximum (FWHM)] for 60 and 122 keV at 0degrees C with a DSSD and 1.7 keV (FWHM) with a CdTe detector.

We describe the recent progress on the use of Schottky Cadmium telluride (CdTe) diode detectors for γ-ray spectroscopy. The extremely low leakage current of the newly developed CdTe diode allows us to apply a much higher bias voltage in comparison with the previous CdTe detectors. Both the improved charge-collection efficiency and the low leakage current lead to a good energy resolution even at room temperature. Large-area CdTe diode detectors with dimensions of 21.5 × 21.5 mm2 are now available. By stacking 40 layers of these large-area devices, we have achieved a good energy resolution of several keV (FWHM) for MeV γ-rays. © 2003 Elsevier B.V. All rights reserved.

We are developing a medium energy particle detector in the high-count rate environment in order to solve the acceleration mechanism and the process of high-energy particle on the collision less plasma shock region in the space. The key observation is to measure the maximum energy of accelerated particles correctly. It is important to reduce the pile up events in order to measure the correct maximum energy. The new developed detector for high-count rate environment consists of the Double-sided Silicon Strip Detector (DSSD) and the readout LSI chips VA32TA made by IDEA. The performance of the new DSSD system for charged particles was tested using proton beams with energy of 6 MeV from the medium energy accelerator in HIMAC of the National Institute for Radiological Science. It is result that the new DSSD system works well and has high performance to detect not only x-rays but also charged particles. It is certain that this new DSSD system can measure correct energy of incident particles in high-count rate environment using the accelerator.

Employing Fourier-synthesis optics and one-dimensional position-sensitive detectors, we are developing a novel hard X-ray imager which can work in the similar to 10 keV to similar to 200 keV range either as a telescope or a microscope. As the detection part of our imager, we have developed a strip detector made of Schottky CdTe diode, with its cathode divided into 64 channels of 150 mu m pitch. Electrodes of all channels are gold-stud bonded to a fanout board, and connected to low noise analog ASIC. We read out signals from all channels simultaneously. As the grid optics elements, one-dimensional modulation collimator grids of 1 mm thick tungsten have been manufactured, with 10 grid pitches ranging from 0.2 mm to 2 nun with harmonic ratios. Combining the CdTe strip detector and the modulation collimators, we have verified hard X-ray imaging performance of this system. Specifically, by observing an Am-241 source, we have successfully obtained an image in the 10-70 keV range.

The Soft Gamma-ray Detector (SGD) on board NeXT (Japanese future high energy astrophysics mission) is a Compton telescope with narrow field of view (FOV), which utilizes Compton kinematics to enhance its background rejection capabilities. It is realized as a hybrid semiconductor gamma-ray detector which consists of silicon and CdTe (Cadmium Telluride) detectors. It can detect photons in a wide energy hand (0.05-1 MeV) at a background level of 5 x 10(-7) counts/s/cm(2) /keV; the silicon layers are required to improve the performance at a lower energy band (< 0.3 MeV). Excellent energy resolution is the key feature of the SGD, allowing to achieve both high angular resolution and good background rejection capability. An additional capability of the SGD, its ability to measure gamma-ray polarization opens up a new window to study properties of astronomical objects. We will present the development of key technologies to realize the SGD; high quality CdTe, low noise front-end ASIC and bump bonding technology. Energy resolutions of 1.7 keV (FWHM) for CdTe pixel detectors and 1.1 keV for Si strip detectors have been measured. We also present the validation of MC simulation used to evaluate the performance of the SGD.

Recent results from the CdTe Schottky diode detectors employing a guard-ring electrode are reported. Cathode electrode, made of Pt, was separated into an active electrode(s) and a surrounding guard-ring. Typical leakage current of a device with an active area of 2 x 2 mm(2) and 0.5 mm thickness surrounded by a guard-ring, is 7 pA and 20 pA at a bias of 100 V and 500 V, respectively, operated at 20 degreesC. Spectral resolution of this device is 0.93 keV and 1.2 keV (FWHM) at 59.5 keV and 122 keV, respectively, operated at 20 degreesC with a bias of 800 V. Detailed study of the characteristics of these devices working as a gammaray detector is presented.

We are developing a Compton camera based on Si and CdTe semiconductor imaging devices with high energy resolution. In this paper, results from the most recent prototype are reported. The Compton camera consists of six stacked double-sided Si Strip detectors and CdTe pixel detectors, which are read out with low noise analog ASICs, VA32TAs. We obtained Compton reconstructed images and spectra of line gamma-rays from 80 keV to 662 keV. The energy resolution (FWHM) is 10 keV and 16 keV at 356 keV and 511 keV, respectively.

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, 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 0degreesC with a DSSD and 1.7 keV (FWHM) with a CdTe detector.

To bring a breakthrough in the observation of the gamma-ray universe, we are working on the development of Semiconductor Multi-Compton Telescope(SMCT). In the SMCT, all the energy, positional and timing resolution must be high to ensure high sensitivity. The imaging device based on the high resolution CdTe diode and Si, such as CdTe pixel detectors and Double-sided Si Strip Detectors (DSSDs), are promising candidates for the components of the SMCT. Here we report the results of a CdTe pixel detector connected with a low noise analog ASIC. We obtained an energy resolution of 2.5 keV(FWHM) at 122 keV, with a positional resolution of 2 mm. Performance of the first prototype Compton Camera using a DSSD and two CdTe pixel detectors is also described. We irradiated 100% linearly polarised 170 keV gamma-ray line to the system and obtained the polarimetric modulation factor of 43%.

The current status of Schottky CdTe diode detectors, especially in view of their space application for hard X-ray and gamma-ray astronomy, are reported. For practical use in space science, a large-area CdTe diode with a size of 21.5 x 21.5 mm(2) and a thickness of 0.5 mm was developed. A good energy resolution, 2.8 keV (FWHM) at -20degreesC, and high homogeneity to within 0.2% over the detector were achieved for the spectral performance. This device has successfully passed a series of tests required for its use in space, in view of utilizing Japanese M-V rockets. The tests include the mechanical environment test, vacuum test, long run for weeks and proton-beam radiation. Initial results from a 2 x 2 segmented electrode large-area device with a guard-ring are also presented. (C) 2003 Elsevier B.V. All rights reserved.

Cadmium Telluride (CdTe), with its high photon absorption efficiency, has been regarded as a promising semiconductor material for the next generation X/gamma-ray detectors. In order to apply this device to astrophysics, it is essential to investigate the radiation hardness and background properties induced by cosmic-ray protons in orbit. We irradiated Schottky CdTe diodes and a CdTe block with a beam of mono-energetic (150 MeV) protons. The induced activation in CdTe was measured externally with a germanium detector, and internally with the irradiated CdTe diode itself. We successfully identified most of radioactive isotopes induced mainly via (p, xn) reactions, and confirmed that the activation background level of CdTe diode is sufficiently low in orbit. We compared energy resolution and leakage current before and after the irradiation and also monitored the signals from a calibration source during the irradiation. There have been no significant degradation. CdTe diodes are tolerant enough to radioactivity in low earth orbit.

We are developing a-large array detector composed of 1024 individual CdTe diodes. Each detector has the dimensions of 1.2 nun x 5.0 mm and a thickness of 1.2 nun. An edge-on geometry is used for the injection of gamma-rays, to obtain a cross-section thickness of 5 mm. 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 a 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 report the basic characteristics of the individual detectors and the overall performance of the gamma-camera. Design of the readout electronics system is also described.

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 x 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 degreesC.

In order to characterize CdTe/CdZnTe detectors in a planar configuration, we have developed a new method to extract mutau 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 (mutau) 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 extract mutau 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 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 x 2 mm(2) device without any charge-loss correction electronics. Large-area detectors with dimensions of 21 x 21 mm2 are now available with an energy resolution of similar to2.8 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 gammarays 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 (similar to 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 x 21.5 mm.

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 (< 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.