Search

Amorphous-selenium (a-Se) based photodetectors are promising candidates for imaging devices, due to their high spatial resolution and response speed, as well as extremely high sensitivity enhanced by an internal carrier multiplication. In addition, a-Se is reported to show sensitivity against wide variety of wavelengths, including visible, UV and X-ray, where a-Se based flat-panel X-ray detector was proposed. In order to develop an ultra high-sensitivity photodetector with a wide detectable wavelength range, a photodetector was fabricated using a-Se photoconductor and a nitrogen-doped diamond cold cathode. In the study, a prototype photodetector has been developed, and its response to visible and ultraviolet light are characterized.

Accepted: 2006-11-20, 資料番号: SA1000049000

著者人数: 12名, 資料番号: SA1000348000

We report our development of CdTe pixel detectors for Si/CdTe semiconductor Compton cameras. We have constructed a prototype of a Si/CdTe Compton camera, consisting of six layered double-sided silicon strip detectors and three CdTe pixel detectors. By using this prototype, we have demonstrated the concept of the Si/CdTe Compton cameras. We have successfully obtained Compton reconstructed images for 80–662 keV gamma-rays. The achieved angular resolution is about 4 degrees for 511 keV gamma-rays. The energy resolution is 14 keV (FWHM) at 511 keV. In order to improve the performance of the Compton camera, we have evaluated all CdTe pixel detectors we have constructed. We found that the I – V curve is helpful to select good detectors.

We describe the recent progress on the use of Schottky CdTe diode detectors for gamma-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 eciency and the low leakage current lead to a good energy resolution even at room temperature. Largearea CdTe diode detectors with dimensions of 21:5 £ 21:5 mm 2 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 gamma-rays.

Amorphous selenium is one of the most promising candidates as photoconducting material for future imaging devices. It can exhibit ultra-high photosensitivity by using avalanche multiplication inside the solid. However, the requirement of high vacuum by the electron emitters prevents amorphous selenium from being a realistic imaging device in general. Diamond, with its outstanding properties of negative electron affinity and high chemical stability, can be an ultimate emitter with low extraction field and low vacuum operation, especially for imaging devices. This report presents the first successful operation of an amorphous selenium photodetector with low operational vacuum and extraction field using a diamond cold cathode.

The hard X-ray detector (HXD) onboard Suzaku covers an energy range of 8-700 keV, and thus in combination with the CCD camera (XIS) gives us an opportunity of wide-band X-ray observations of celestial sources with a good sensitivity over the 0.3-700 keV range. All of 64 Si-PIN photo diodes, 16 GSO/BGO phoswich scintillators, and 20 anti-coincidence BGO scintillators in the HXD are working well since the Suzaku launch on July 2005. The rejection of background events is confirmed to be as effective as expected, and accordingly the HXD achieved the lowest background level of the previously or currently operational missions sensitive in the comparable energy range. The energy and angular responses and timing have been continuously calibrated by the data from the Crab nebula, X-ray pulsars, and other sources, and at present several % accuracy is obtained. Even though the HXD does not perform simultaneous background observations, it detected weak sources with a flux as low as ~0.5 mCrab; stars, X-ray binaries, supernova remnants, active galactic nuclei, and galaxy clusters. Extensive studies of background subtraction enables us to study weaker sources.

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

We have been developing a semiconductor Compton telescope to explore the universe in the energy band from several tens of keV to a few MeV. We use a Si strip and CdTe pixel detector for the Compton telescope to cover an energy range from 60 keV. For energies above several hundred keV, the higher efficiency of CdTe semiconductor in comparison with Si is expected to play an important role as an absorber and a scatterer. In order to demonstrate the spectral and imaging capability of a CdTe-based Compton Telescope, we have 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 equation from 122 keV to 662 keV. The energy resolution (FWHM) of reconstructed spectra is 7.3 keV at 511 keV and 3.1 keV at 122 keV, respectively. The angular resolution obtained at 511 keV is measured to be 12.2 degree (FWHM)., Comment: 11 pages, 13 figures, presented at SPIE conference HARD X-RAY AND GAMMA-RAY DETECTOR PHYSICS VII", San Diego, 2005. To be published in Proc. SPIE 2005

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 /spl times/ 2 mm/sup 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 /spl deg/C . 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 /spl deg/C with a bias of 800 V. Detailed study of the characteristics of these devices working as a gamma-ray detector is presented.

The Hard X-ray Detector (HXD-II), one of instruments onboard the Astro-E2 satellite to be launched in February 2005, is in the final stage of its development. The HXD-II probes the universe in the energy range of 10-600 keV with a sensitivity by an order of magnitude better than those of previous missions. The assembly of the HXD-II completed in January 2004, followed by a series of pre-launch qualification tests. As a result, the design goals of the HXD-II have been met. These include; a background level of 5 x 10-6 counts/s/keV/cm2 at 200 keV for GSO and 1 x 10-5 counts/s/keV/cm2 at 30 keV for PIN; energy resolutions of 2.9 keV (PIN diode, at 59.5 keV) and 10% (GSO scintillator, at 662 keV); and low energy thresholds of 10 keV for PIN diodes and 30 keV for GSO scintillators. The measured background predicts a continuum sensitivity of a few x 10-6 photons/s/keV/cm2. Anti-Counter units surrounding the HXD-II provide 50 keV-5 MeV information on gamma-ray bursts and bright X-ray transients.

We are developing a Compton telescope based on high resolution Si and CdTe imaging devices in order to obtain a high sensitivity astrophysical observation in sub-MeV gamma-ray region. In this paper, recent results from the prototype Si/CdTe semiconductor Compton telescope are reported. The Compton telescope consists of a double-sided Si strip detector (DSSD) and CdTe pixel detectors, combined with low noise analog LSI, VA32TA. With this detector, we obtained Compton reconstructed images and spectra from line gamma-rays ranging from 81 keV up to 356 keV. The energy resolution is 3.8 keV and 7.9 keV at 122 keV and 356 keV, respectively, and the angular resolution is 9.9 degrees and 5.7 degrees at 122 keV and 356 keV, respectively., 12 pages, 14 figures, submitted to SPIE conference proceedings vol. 5501, "High-Energy Detectors in Astronomy", Glasgow UK, 6/21-6/24 2004

Photoconductivity measurements of amorphous selenium (a-Se) and arsenic selenide (a-As2Se3) were carried out. Samples where annealed from 333–358 K at 5 K step for 5 minutes and the photoconductivity was measured after each annealing step. It was found that the dark current increases permanently for a-Se and the photoconductivity ratio increased drastically after 338 K, but drops after further annealing. Amorphous-As2Se3 however, has no noticeable change in the dark current neither in the photoconductivity ratio. Arsenic is introduced into a-Se to prevent degradation, which causes loss of image quality, but the volume must be controlled in order to maintain high photoconductivity.

The cause of slow response, which has been observed in amorphous selenium (a-Se) photodetector with nitrogen-doped chemical vapor deposition diamond, is discussed. The effect of high resistance and capacitance of the electric circuit in the photodetector was investigated by using cathodes with different thicknesses. Furthermore, the emission current from the diamond cathode was measured independently to examine its response to transient anode voltage. The results suggest that the surface potential of a-Se shows transient and unstable behavior, and causes an unstable extraction voltage for the diamond cathode. Hence, the slow response is not due to the diamond cold cathode, but unstable surface potential of a-Se. In order to obtain a faster response of the photodetector, a triode structured photodetector should be developed.

The first successful operation of a vacuum tube photodetector with an amorphous selenium photoconductor and a diamond cold cathode in triode-structure is presented. By using triode-structure, the detector can achieve a wide dynamic range of observable illumination intensity and faster photo-response compared to diode-structure.

In this paper, the combination of diamond electron emitter and a-Se photoconductor, diode and triode-structure photodetectors have been fabricated and their photo-response are reported.

著者人数：13名, 資料番号: SA1002232000