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著者人数: 14名, 資料番号: SA1004306000

The development of high-resolution, UV-enhanced avalanche photodiode (APD) arrays usable in high-resolution PET imaging is underway. These APD arrays were specifically designed as photosensors capable of direct coupling with pixelized Pr-doped Lu3Al5O12 (Pr:LuAG) scintillators. An excellent quantum efficiency (QE) of 55% was achieved at the peak emission of Pr:LuAG (310 nm), namely, a substantial improvement from the QE ≤ 5 % as measured with the conventional Hamamatsu reverse-type APDs (S8664 series). Each APD device has 8×8 (TYP1) and 12×12 (TYP2) pixel structures with active areas of 3×3 mm2 and 2×2 mm2 in each pixel, respectively. A gain uniformity of ±8% and low dark noise of ≤ 2 nA / pixel have been achieved, measured at +25 °C. We also report on the large size single crystal growth of improved Pr:LuAG scintillators and the preliminary performance test of the same. An energy resolution of 4.2% (FWHM) was obtained for 662 keV gamma-rays for 10×10×10 mm3 crystal, measured with a PMT employing a super-bialkali photocathode. We made a test module consisting of a UV-enhanced APD-array (either TYP1 or TYP2) optically coupled with an 8×8 (or 12×12) pixel Pr:LuAG matrix. The linearity between the output signals and incident gamma-ray energy of TYP1 and TYP2 gamma-ray detectors were only 0.27 and 0.33%, as measured at +25 °C for various gamma-ray sources, respectively. Energy resolutions of 7.0±0.2% (FWHM) and 9.0±0.6% (FWHM) were, respectively, obtained for TYP1 and TYP2 detector arrays for 662 keV gamma-rays. The uniformity of the pulse height distributions was also measured at less than 8% for both detectors. Finally, we measured the coincidence timing resolution of these gamma-ray detectors and obtained 4.0±0.1 ns (FWHM) for the 511 keV annihilation quanta from a 22Na source. These results suggest that UV-enhanced APD-arrays coupled with Pr:LuAG scintillators could be a promising device for future application in nuclear medicine.

著者人数: 12名, Accepted: 2011-02-14, 資料番号: SA1003128000

The avalanche photodiode (APD) is a high-performance and compact light sensor recently applied in various fields of experimental physics. Among several types of APDs, the reach-through APD offers an advantage in direct X-ray detection, thanks to its thick depletion layer ( ⩾ 100 μ m ) in front of the amplification region. This type of APD is also sensitive to weak scintillation light from gamma-ray scintillators with a high quantum efficiency of ∼ 80 % (at λ ≃ 500 nm ). In this paper, we propose a novel design of a compact X-ray-to-gamma-ray detector widely applicable between 1 keV and several hundreds of keV. The prototype consists of a reach-through APD (transmission type) optically coupled with a cubic CsI(Tl) crystal 4 × 4 × 4 mm 3 in size. By applying the pulse shape discrimination technique to the APD output, we successfully discriminated the X-ray signals directly detected within the APD (1–40 keV), and gamma-ray signals absorbed in a CsI(Tl) scintillator (10–800 keV) located immediately behind the APD. Optimum FWHM energy resolutions of 15.1 ± 0.2 % , 6.6 ± 0.4 % , and 7.6 ± 0.1 % were obtained for 5.9 keV X-rays, 32 keV X-rays, and 662 keV gamma rays, respectively, measured at + 20 ∘ C . This stacked configuration is viable for various future applications in space science and nuclear medicine.

The performance of the large area reach-through avalanche photodiode (APD), manufactured by Hamamatsu Photonics, K.K. as a high resolution X-ray detector is presented. The mentioned APD has an area of 3 mm ∅ , a fast time response for signal carrier collection and its thick depletion layer of 130 μ m shows a potential to be used as an effective X-ray absorber below 20 keV. Having a capacitance of ∼ 10 pF and a low dark current of 5 nA for a gain of 15, at room temperature, this APD had demonstrated one of the best energy resolutions within this kind of devices: 6.4% (FWHM) for 5.9 keV photons with a minimum detectable energy of 0.3 keV, measured at - 20 ∘ C . The experiments for the timing property were made in a synchrotron beam facility using an 8 keV X-ray beam; the reached count rate was above 10 8 counts/s, corresponding to a very short dead time of 4.5 ns/pulse. In order to test the radiation hardness of the APD, the device was irradiated at a Ring Cyclotron Facility with a 53.5 MeV proton beam. The total dose was of 11.3 krad and no fatal damage was found in the APD, although the dark current of the APD had shown an increase of one order of magnitude. Finally, the obtained results allow us to affirm that the reach-through APD has the potential to become an excellent X-ray detector, especially in the space mission application.

Avalanche photodiodes (APDs) are promising light sensor for various fields of experimental physics. It has been argued, however, that variation of APD gain with temperature could be a serious problem preventing APDs from replacing traditional photomultiplier tubes (PMTs) in some applications. Here we develop an active gain-control system to keep the APD gain stable under moderate temperature variations. As a performance demonstration of the proposed system, we have tested the response of a scintillation photon detector consisting of a 5x5 mm^2 reverse-type APD optically coupled with a CsI(Tl) crystal. We show that the APD gain was successfully controlled under a temperature variation of DT = 20deg, within a time-cycle of 6000 sec. The best FWHM energy resolution of 6.1+-0.2 % was obtained for 662 keV gamma-rays, and the energy threshold was as low as 6.5 keV, by integrating data from +20deg - 0deg cycles. The corresponding values for -20deg - 0deg cycles were 6.9+-0.2 % and 5.2 keV, respectively. These results are comparable, or only slightly worse than that obtained at a fixed temperature. Our results suggest new potential uses for APDs in various space researches and nuclear physics. As examples, we briefly introduce the NeXT and Cute-1.7 satellite missions that will carry the APDs as scientific instruments for the first time., 14 pages, 10 figures, accepted by NIM-A

The avalanche photodiodes (APDs) have attracted considerable attention in various field of experimental physics, but their uses are still limited in only a few experiments, possibly due to their small surface areas. Here, we report the development of the large-area ( ∼ 20 mm square) APDs, for future applications to high-resolution X-rays and γ -rays detection. We have made two prototypes of reverse-type APDs based on different concepts, one consists of a 2 × 2 array of 10 × 10 mm 2 pixels (APD1) and the other is a monolithic pixel of 19 × 19 mm 2 size (APD2) to achieve a large effective area. By comparing the dark current and gain characteristics at room temperature ( + 20 ∘ C ) and lightly cooled environment ( - 20 ∘ C ), we quantitatively discussed the origin of predominant noise source at different temperatures. As a performance demonstration of newly developed APDs, we made a scintillation γ -ray detector consisting of a 20 × 20 × 5 mm 3 CsI(Tl) crystal and a 2 cm-square APD. The best FWHM energy resolution of 5.5 ± 0.2 % were obtained for 662 keV γ -rays at room temperature. Similarly, the best FWHM energy resolution of 8.5 ± 0.2 % were obtained for 122 keV γ -rays at lightly cooled environment. We showed that the minimum detectable energy for scintillation light was 15 keV at 20 ∘ C and less than 5 keV at - 20 ∘ C .

We have studied the performance of large area avalanche photodiodes (APDs) recently developed by Hamamatsu Photonics K.K, in high-resolution X-rays and Gamma-rays detections. We show that reach-through APD can be an excellent soft X-ray detector operating at room temperature or moderately cooled environment. We obtain the best energy resolution ever achieved with APDs, 6.4 % for 5.9 keV X-rays, and obtain the energy threshold as low as 0.5 keV measured at -20deg. Thanks to its fast timing response, signal carriers in the APD device are collected within a short time interval of 1.9 nsec (FWHM). This type of APDs can therefore be used as a low-energy, high-counting particle monitor onboard the forthcoming Pico-satellite Cute1.7. As a scintillation photon detector, reverse-type APDs have a good advantage of reducing the dark noise significantly. The best FWHM energy resolutions of 9.4+-0.3 % and 4.9+-0.2 % were obtained for 59.5 keV and 662 keV Gamma-rays, respectively, as measured with a CsI(Tl) crystal. Combination of APDs with various other scintillators (BGO, GSO, and YAP) also showed better results than that obtained with a photomultiplier tube (PMT). These results suggest that APD could be a promising device for replacing traditional PMT usage in some applications. In particular 2-dim APD array, which we present in this paper, will be a promising device for a wide-band X-ray and Gamma-ray imaging detector in future space research and nuclear medicine., Comment: 11 pages, 7 figures, accepted by NIM-A

We have carried out study of a large area ( 10 × 10 mm 2 ) , reverse-type avalanche photodiode (APD) recently developed by Hamamatsu photonics. It has low dark current of 3 nA at room temperature, and the gain stability was almost the same as prototypical APDs reported in our previous paper. We studied the performance as a γ -ray detector with four scintillators, CsI(Tl), BGO, GSO(Ce), and YAP(Ce) crystals. For example we obtained the best energy resolution of 4.9 ± 0.2 % (FWHM) for 662 keV γ -rays, as measured with a 10 × 10 × 10 mm 3 CsI(Tl) crystal. The minimum detectable energy was as low as 10 keV at 20 ∘ C and 3.1 keV at - 20 ∘ C . Thanks to its large effective area, this APD can effectively read out photons from larger size scintillators. When coupling to a 300 × 48 mm 2 BGO plate of 3 mm thickness, an FWHM energy resolution of 20.9 ± 0.2 % was obtained for 662 keV γ -rays, with the minimum detectable energy of about 60 keV at - 15 ∘ C . These results suggest that our prototype APD can be a promising device for various applications replacing traditional PMTs such as use in space for Japan's future X-ray astronomy mission NeXT.

We report on the performance of large-area (5×5 mm 2 ) avalanche photodiodes (APD) produced by Hamamatsu Photonics, as a low-energy X-rays and γ-rays scintillation detector. Hamamatsu APD has a reverse structure and works at relatively low bias voltage of 300– 350 V . The leakage current is 1.2 nA at room temperature (25°C) and decreases to 10 pA at −20°C for an avalanche gain of 50. The best FWHM energy resolutions of 9.4±0.3% and 7.4±0.3% were obtained for 59.5 keV γ-rays from 241 Am and 122 keV γ-rays from 57 Co sources, respectively, as measured with a 5×5×5 mm 3 cubic CsI(Tl) crystal. We show that the minimum detectable energy for the scintillation light is remarkably low; 4.6 keV at room temperature (20°C) and 1.1 keV at −20°C. 5.9 keV X-rays from 55 Fe were clearly resolved at −20°C with an FWHM resolution of 32.9±0.3%. These results suggest that Hamamatsu APD can be a promising device for future applications in low-energy scintillation detection.

We report on the development of two versatile, high spatial resolution gamma-ray imagers for medical imaging. One is a compact gamma-ray camera, the other is a tweezers type coincidence imaging system. These applications consisting of a large-area monolithic Multi-Pixel Photon Counter (MPPC) and submilIimeter pixelized scintillator matrices. The MPPC array has 4 × 4 channels with a three-side buttable, very compact package. Each channel has a photosensitive area of 3 × 3 mm2 and 3600 Geiger mode avalanche photodiodes (APD). For a typical operational gain of 7.5 × 105 at + 20 degrees, gain fluctuation over the entire MPPC device is only ± 5.6%, and dark count rates (as measured at the 1 p.e. level) amount to ≤ 400 kcps per channel. We particularly selected Ce-doped (Lu,Y)2(SiO4)O (Ce:LYSO) and a brand-new scintillator, Ce-doped Gd3Al2Ga3O12 (Ce:GAGG) due to their high light yield and density. To improve the spatial resolution, these scintilla tors were fabricated to 22 × 22 or 15 × 15 matrices of 0.5 × 0.5 mm2 pixels. These scintillator matrices were coupled to the MPPC array with an acrylic light guide with 1 mm thick, and signals were read out using the charge division resistor network, which compiles signals into four position-encoded analog outputs. The spatial resolution of 1.2 mm was achieved with the compact gamma-ray camera using collimated 57Co source, and a radiography image of a bearing was successfully obtained. On the other hand, the spatial resolution of 1.1 mm was achieved with the coincidence imaging system using a 22Na source. Furthermore the experimental measurements for a PET scanner was performed, and the spatial resolution of 0.91 mm was achieved. These results suggest that the gamma-ray imagers has excellent potential for their uses as a high spatial medical imaging, and also be promising for positron emission tomography (PET).

Cute-1.7+APD II is the third pico-satellite developed by students at the Tokyo Institute of Technology. One of the primary goals of the mission is to validate the use of avalanche photodiodes (APDs) as a radiation detector for the first time in a space experiment. The satellite was successfully launched by an ISRO PSLV-C9 rocket in Apr 2008 and has since been in operation for more than 20 months. Cute-1.7+APD II carries two reversetype APDs to monitor the distribution of low energy particles down to 9.2 keV trapped in a Low Earth Orbit (LEO), including South Atlantic Anomaly (SAA) as well as aurora bands. We present the design parameters and various preflight tests of the APDs prior to launch, particularly, the high counting response and active gain control system for the Cute-1.7+APD II mission. Examples of electron/proton distribution, obtained in continuous 12-hour observations, will be presented to demonstrate the initial flight performance of the APDs in orbit.

[1] The Cute-1.7+APD II, 10 × 15 × 20 cm3 in size and 5 kg in mass, is the third picosatellite developed by students at the Tokyo Institute of Technology. One of the primary goals of the Cute-1.7+APD II mission is to validate the use of avalanche photodiodes (APDs) as a radiation detector for the first time in a space experiment. While the mission itself is immature compared to the forefront satellites of space plasma physics, use of APDs offers various possibilities regarding a brand-new electron energy analyzer for medium-energy electrons and ions (1–100 keV), as well as a high-performance light sensor for the future X-ray astronomy missions. The satellite was successfully launched by ISRO PSLV-C9 rocket on 28 April 2008 and has since been in operation for more than a year. The Cute-1.7+APD II carries two reverse-type APDs to monitor the distribution of low-energy particles (mainly electrons and protons) down to 9.2 keV trapped in a low Earth orbit (LEO), including the South Atlantic Anomaly (SAA) as well as aurora bands. We present the design parameters and various preflight tests of the APDs prior to launch, particularly, the high counting response and active gain control system for the Cute-1.7+APD II mission. Examples of electron/proton distribution, obtained in continuous 12 h observations, will be presented to demonstrate the initial flight performance of the APDs in orbit.

It is quite important to obtain wide band spectra of high energy astrophysical phenomena at the same time in order to probe emission processes or structures. Especially observations of transient objects, such as gamma‐ray bursts, expect detectors with wide energy band coverage for the sake of an efficient spectroscopy within limited time windows. An avalanche photo diode (APD) is a compact photon sensor with an internal gain of ∼100. We have developed an X‐ray/gamma‐ray detector using a reach‐through APD (5×5 mm2) optically coupled with a conventional CsI(Tl) scintillator, which covers typically from 1 keV to 1 MeV. Further, we developed a 1‐dimensional array of the 8/16 APDs (net 16×20) mm2) for the purpose of an imaging photon detector combined with coded masks, to be applied in future missions. We present the current status and performances of our hybrid detector using the 8‐channel array.

We report on the development of versatile APD-based PET modules with time-of-flight capability. The module consists of a LYSO matrix optically coupled with a position-sensitive avalanche photodiode (APD) array, and front-end circuits (FEC) directly connected to the rear-end of the APD package. Each APD device has a monolithic 16×16 (or 8×8) pixel structure with an active area of 1.0 (or 4.0) mm2 for each pixel. Time resolutions of 155 ps and 214 ps (FWHM) were obtained for 1.0 mm2 and 4.0 mm2 APD pixels, respectively, measured by the direct detection of 10 keV X-rays. The FEC carries two identical analog ASICs specifically designed for the APDs in TSMC 0.35 μm CMOS technology. Each ASIC consists of 32-channel charge-sensitive amplifiers, band-pass filters, differentiators, pulse-height and timing discriminators, and two-channel time-to-amplitude converters. The noise characteristic of the ASIC, mounted in a low temperature co-fired ceramics (LTCC) package, is 560 +30 e/pF with an electric timing resolution of 484 ps (rms). The overall dimension of the module (including APD-array, LYSO matrix and FEC) is 30×30×80 mm3. The variation of signal amplitude was less than 20% among all pixels. The average energy resolutions of 11.7 ± 0.7 % and 13.7 ± 1.1 % were obtained for 662 keV gamma-rays, measured with 8×8 and 16×16 arrays, respectively. An attainable spatial resolution is < 0.8mm (FWHM) for 16×16 array in a reconstructed image. These results suggest the APD-based PET module can be a promising device for future applications, especially for high resolution MRI- and TOF-PET.

It is quite important to obtain wide band spectra of gamma‐ray bursts (GRBs) at the same time in order to probe the emission processes or the structure of GRBs. An avalanche photo diode (APD) is a compact photon sensor with an internal gain of 100. We have developed an X‐ray/gamma‐ray detector using a back‐illuminated reach‐through APD (5×5 mm2) optically coupled with a conventional CsI(Tl) scintillator, which covers typically from 1 keV to 1 MeV. Further, we developed a 1‐dimensional array of the 8/16 APDs (net 16×20 mm2) for the purpose of an imaging photon detector to be used in future GRB missions. Here we present the current status and performance of our hybrid detector.

We report on the performance of the most recent avalanche photodiodes produced by Hamamatsu Photonics, as low-energy X-rays and γ-rays detectors. APDs share good features of both photo diodes and PMTs, as they are very compact, produce an internal gain of 10-100, and have a high quantum efficiency close to 100% in the visible right. Until very recently, however, APDs were limited to very small surfaces, and were mainly used as a digital device for light communication. We have developed large area (up to 10x10 mm2) APDs which can be used in the physics experiments. The best energy resolution of 6.4% (FWHM) was obtained in direct detection of 5.9 keV X-rays. The FWHM results of 9.4% and 4.9% were obtained for 59.5 keV and 662 keV γ-rays respectively, as measured with the CsI(Tl) crystal. The minimum detectable energy for the scintillation light was as low as 1 keV at lightly cooled environment (-20°C). Note that our results are the best records ever achieved with APDs. Various applications of APDs are presented for future space research and nuclear medicine. In particular 2-dimensional APD arrays will be a promising device for a wide-band X-ray and γ-ray imaging detector.

In this paper, modeling of the unsteady behavior of various vehicle tunnel ventilation systems by means of bond graphs is researched. In the case of a longitudinal ventilation system, modeling of jet fans, saccardo nozzle, vertical supply/exhaust shafts, and electrostatic precipitator (EP) are investigated. Besides, modeling of a partial transverse ventilation system is also discussed. The modeling of traffic ventilation force which influences exhaust pollutant concentration and longitudinal ventilation velocity is developed.

A composite microporous membrane made of poly(N-benzyl-4-vinylpyridinium chloride) that showed strong affinity with bacterial cells was prepared as a filter material for removing airborne bacteria. Thickness, pore diameter and porosity of the membrane were 0·72 mm, 14·5 μm and 63%, respectively. Electron micrographic analysis revealed that the membrane consisted of a very large number of connected beads of 1·4 μm in diameter made of the pyridinium-type polymer. Filtration using the membrane was performed easily at low flow rates with insignificant pressure drop across the membrane. Filtration at 63·7 cm/sec gave 99·98% and 99·996% removal (3·7 and 4·4 log10-unit reduction in concentration) of Escherichia coli and Pseudomonas aeruginosa, respectively. Staphylococcus aureus was not detected in filtrates. Since pores of the membrane were much larger than these bacteria, the efficient removal was best explained in terms of the affinity of the polymer with bacterial cells.

著者人数: 11名, Accepted: 2013-05-02, 資料番号: SA1004632000

We conducted a study to examine the performance of the multi-pixel photon counter(MPPC). The MPPC is a novel type of semiconductor photodetector consisting of multiple avalanche photodiode (APD) pixels operated in Geiger mode. Whereas the MPPC offers a great advantage in signal multiplication comparable to that achieved by the photomultiplier tube (PMT), the detection of weak scintillation light signals is difficult due to the severe contamination of dark counts. In this study, we first compared the energy resolutions and energy thresholds of a 3 ? 3 mm2 MPPC with those of a 3 ? 3 mm2 APD as scintillation detectors. The MPPC and APD were optically coupled with 5 ? 5 ? 5 mm3 scintillation crystals of BGO, Tl:CsI, Pr:LuAG, and YAG. It turned out that the APD had better energy resolutions for 662 keV gamma-rays, while the MPPC had lower energy thresholds as measured using a test pulse. Despite the low energy thresholds, it is difficult for the MPPC to detect low energy gamma-rays due to the contamination of dark counts. Secondly, we applied a coincidence technique to discriminate weak gamma-ray signals from dark counts by using scintillation detectors that consisted of a 2 ? 2 MPPC-array optically coupled with 10 ? 10 ? 10 mm3 crystals of GSO, BGO, and Pr:LuAG. With this technique, we demonstrated that dark counts achieved a rejection efficiency of more than 99.8%. As a result, 22.2 keV gamma-rays were successfully detected with a GSO scintillator as measured at +20?C.

We report the development of a monolithic MPPC array, which consists of 3 × 3 mm2 elements arranged as a 4 × 4 array manufactured by Hamamatsu applicable to next generation PET scanners. We show that the MPPC is suitable for time of flight PET applications by simple measurement using coincident back-to-back 511 keV gamma rays. We demonstrated that the MPPC has much better timing resolution of ~ 600 ps than the APD. We coupled the monolithic MPPC array with the Ce:LYSO and Pr:LuAG scintillator matrices as gamma-ray detectors. The energy resolutions were evaluated as ~ 14% with 662 keV gamma-rays and the Ce:LYSO achieved the best. We also used a resistor network readout circuit with some optimization. The averaged positional resolution is estimated as ~ 0.27 mm in both x and y directions, while the energy resolution of each pixel was 9.9% for 662 keV gamma rays. Finally we applied the GHz class fast sampling waveform acquisition system to improve performance, and demonstrated efficient noise reduction by the clear detection of 22 keV gamma rays.

The performance of a large-area, monolithic Hamamatsu multi-pixel photon counter (MPPC) was tested consisting of a 2 ×2 array of 3 ×3 mm 2 pixels. MPPC is a novel type of semiconductor photodetector comprising multiple avalanche photodiode (APD) pixels operated in Geiger mode. Despite its great advantage of signal multiplication comparable to that achieved with the photomultiplier tube (PMT), the detection of weak scintillation light signals is quite difficult due to the severe contamination of dark counts, which typically amounts to ≃1 Mcps/3 ×3 mm 2 at room temperature. In this study, a coincidence technique was applied for scintillation detectors to improve the detection efficiency for low energy gamma-rays. The detector consisted of a 10 ×10 ×10 mm 3 crystals of GSO, BGO, and Pr:LuAG optically coupled with the 2 ×2 MPPC-array. With this technique, we demonstrated that the contamination of dark counts was reduced with a rejection efficiency of more than 99.8%. As a result, 22.2 keV gamma-rays were succ...

SUMMARYUnwoven cloth coated with 32 mg/g of a copolymer ofN-benzyl-4-vinyl-pyridinium chloride and styrene was found to be effective in removing micro-organisms from water. In experiments demonstrating removal of Escherichia coli by filtration through ten sheets of the unwoven cloth, the rate of removal was 99·99% at a filtration rate of 2·6 cm/h, and remained at 99% even at a high filtration rate of 300 cm/h and a low influent concentration of the bacterial cells such as 10 cells/ml. The rate of removal tended to increase with a decrease in the influent bacterial concentration. Seven other bacteria and two yeasts were effectively removed by filtration through the unwoven cloth. Filtration through the unwoven cloth was also effective in removing spores of fungi from water but was not very effective in removing bacteriophage T4 from aqueous solution.

The dynamic characteristics of a spherical spiral groove bearing have been analysed using a narrow groove hypothesis for the cases of incompressible fluid film and isothermal gas film. The usefulness of the proposed method is demonstrated by a comparison of results with those obtained using established methods. The method is especially efficient for optimization design and other analysis problems for which other available calculation methods require significant computational effort. The influence of bearing parameters and compressibility of the fluid on dynamic performance and stability are presented.