National Institutes for Quantum and Radiological Science and Technology
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Experimental evaluation of a reed valve air intake system for microwave rocket in ground-static tests
No full textMicrowave Rocket, a next-generation space transportation system, generates thrust by utilizing millimeter-wave energy beamed from the ground. The thruster operates cyclically, alternating between thrust generation ? where air is compressed and exhausted via millimeter-wave-supported detonation ? and an air-refilling process. Although reed valve systems have been studied for air intake, their effect on thrust performance has not been experimentally verified. This study experimentally evaluates the propulsion characteristics of a thruster equipped with a six-stage reed valve air intake system (36 valves in total, length 500 mm, radius 28 mm) under ground testing. A Gaussian beam with a frequency of 170 GHz, an output power of 550 kW, and a pulse width of 1.2 ms was repeatedly applied at pulse detonation engine (PDE) cycle frequencies ranging from 50 to 250 Hz, and the resulting thrust impulse was measured. Results show that approximately 60% of the first-cycle impulse was sustained up to 150 Hz, beyond which the partial filling effect led to a decrease in impulse. The time-averaged thrust, defined as the product of impulse and PDE cycle frequency, reached a peak of 8.3 kN per square meter at 206 Hz due to the trade-off between impulse degradation and frequency increase. Furthermore, modifications to the reed valve design, specifically reducing stiffness and fundamental frequency, led to up to 80% impulse recovery and enhanced thrust performance at lower PDE cycle frequencies.journal articl
In vivo deep brain microscopy at submicrometer resolution with refractive index-matched prism interfaces.
No full textMammalian brain contains huge amount of neurons working together to process information and regulate brain functions, and thus is thick. Various optical methods have been developed for investigating the deep brain dynamics, but the previous methods have technical issues and limitations in invasiveness, spatial resolution, and/or field of view. To address this, we developed a low-invasive in vivo deep-brain imaging method using an implantable and optically-advanced microprism interface with a refractive index matched to the brain and water, demonstrating enhanced brightness and sub-micron resolution in deep-brain areas with wide field of view.journal articl
Importance of track density for radiolytic reactions: Changes in yields of OH radical molecular probe under Bragg peak energy protons.
No full textThe aim of the present study is to elucidate the mechanisms of sparing effects of tissues observed under ultra-high-dose rate (UHDR:?>?40 Gy/s) irradiations. To do so, under Bragg peak energy protons, we evaluate changes in OH radicals, that efficiently react with DNA and proteins, consequently, they govern the indirect action, by varying the spatial distribution of proton tracks, while maintaining absorbed dose rate. By applying beam with 2 μm?×?2 μm (“micro” beam), yields of OH radicals decrease by ??38?±?7%, compared to that with 100 μm?×?100 μm (“macro beam). The present results show that radical?radical reactions between neighboring tracks occur by UHDR irradiations even around the Bragg peak energy. Additionally, radical?radical reactions, leading the reduction of yields of OH radicals, are one of the mechanisms of sparing effects seen by UHDR irradiations.journal articl
Helical and antiparallel structures from truncated long-range interactions in water and dipolar spins
No full textIt is known that dipolar systems such as water produce highly ordered layered structures when long-range dipolar interactions are truncated at a finite distance. However, many parts of this long-range phenomenon remain unsolved. Herein, we fundamentally explored this phenomenon using Monte Carlo simulations for dipolar spins and molecular dynamics simulations for water. The results show that the width of the layers does not depend on the system size but is determined by the cutoff length. At the same time, layer formation occurred even without periodic boundary conditions. These results suggest that layer formation is a consequence of cutoff rather than the periodic boundary conditions. We also found that water has a helical dipole distribution, whereas dipolar spin has an antiparallel distribution, with these two forms being energetically competing. Our findings on the relationship between long-range interactions and the resulting structures are expected to contribute to future explorations of structures with similar ordering in magnetic, dielectric, and soft materials.journal articl
The iron and nickel spectra measured with CALET on the international space station
No full textRecent direct measurements of the energy spectra of the charged cosmic ray have revealed unexpected spectral features, most notably the onset of a progressive hardening at few hundreds of GeV/n not only of proton and He spectra but also observable for heavier nuclei. Thus, the study of the spectra behavior of heavy elements may shed light on understanding propagation and acceleration phenomena in our Galaxy. In particular, Fe and Ni provide favorable conditions for observations thanks to the low background contamination from spallation of higher mass elements they are affected by. The CALorimetric Electron Telescope, CALET, has been measuring high-energy cosmic rays on the International Space Station since October 2015. The instrument consists of two layers of segmented plastic scintillators, a 3 radiation length thick tungsten-scintillating fiber imaging calorimeter and a 27 radiation length thick PWO calorimeter. It identifies the charge of individual elements up to Ni and beyond and it measures the energy of cosmic-ray nuclei providing a direct measurement of their spectra. In this contribution, the iron and nickel spectra, resulted after 5 years of data acquisition, are presented in the energy range between 10 and 2000 GeV/n and between 8.8 and 240 GeV/n, respectively. The analysis procedure and the assessment of systematic errors are detailed, in addition to the ratio between the two fluxes. Both spectra show similar shape and energy dependence.journal articl
Development and implementation of mesh ripple filter for multi-ion therapy
No full text[Purpose] In scanned charged-particle therapy with heavy ions, e.g. He, C, O, and Ne ions, the Bragg peak width is broadened by a ripple filter to reduce the number of energy layers. A conventional ripple filter has a ridge-and-groove structure with approximately 1 mm intervals. The ridge-and-groove structure may cause lateral dose inhomogeneity at a patient surface, especially for heavy ions like Ne ions due to their low scattering power. The purpose of this study was to develop and verify a multi-layered mesh ripple filter (mRiFi), which broadens the Bragg peak width without surface dose inhomogeneity, for clinical trials of the multi-ion therapy with He-, C-, O-, and Ne-ion beams. [Methods] A mRiFi was fabricated by stacking mesh sheets at random positions and angles. The random microstructure broadens the Bragg peak width without surface dose inhomogeneity. The width of the broadened Bragg peak is determined by the wire material, wire diameter, wire interval, and the number of mesh sheets. Aluminum alloy was selected as a wire material with low atomic number for suppressing the lateral beam spread. To effectively broaden the Bragg peak width, the wire diameter and wire interval were determined to be 0.29 mm and 1.27 mm, respectively. The number of mesh sheets were determined to be 25 to achieve clinically required Bragg peak width of ~1.5 mm. To evaluate the developed mRiFi, we conducted experiments using He-, C- O- and Ne-ion beams. The integrated depth doses (IDDs) of the ion beams with various energies broadened by the mRiFi were measured using an in-house parallel plane ionization chamber with a diameter of 15 cm in water. The in-air beam sizes of the ion beams were measured using a fluorescence screen and a camera. The measured beam data were then modeled and registered to the TPS. The spread-out Bragg peak (SOBP) plans of the four ion beams were made with the TPS and delivered to the water tank. The dose distributions measured with a Markus chamber and pinpoint ionization chambers were compared with the plan distributions. [Results] The Bragg peaks of pristine He-, C-, O-, and Ne-ion beams were broadened by the developed mRiFi into a normal distribution shape with the standard deviations σ of 1.45, 1.47, 1.44, and 1.43 mm, respectively. For all SOBP plans, measured dose distributions were consistent with the planned dose distributions within 2%. [Conclusion] We developed and verified a mRiFi, which broadens the Bragg peak width of scanned charged-particle beams without surface dose inhomogeneity. The mRiFi was implemented in the clinical irradiation system at the QST and has been used in ongoing clinical trials of multi-ion therapy with He-, C-, O-, and Ne-ion beams.第129回日本医学物理学会学術大会conference presentatio
Excited-State Annihilation of Positronium in Cryogenic Silica Aerogel for In-Material Laser Cooling
No full text本研究では、ポジトロニウム（Ps）のボース・アインシュタイン凝縮（BEC）を実現に向けた、ナノ多孔質材料中での熱化と紫外（UV）レーザーによるドップラー冷却実験を行っている。室温および8 Kの異なる温度条件下で保持したシリカエアロゲルを用い、2P状態に励起されたPsのレーザー誘起消滅を調べた。243 nmでの共鳴励起によりガンマ線放出は大きく増強される現象が観測されるが、低温条件では消滅信号が室温に比べて約50％低下した。さらに、Ps生成からレーザー励起までの遅延時間を長くすると、消滅確率は一層低下した。これらの結果は、Psの熱化が2P-Psの消滅に影響を与えることを示している。また、ナノ多孔質媒体における表面誘起2P状態崩壊モデルを支持し、励起状態Psが表面相互作用を高感度で検出するプローブとなることを示唆する。現在、凝縮に向けたPsダイナミクス最適化のため、細孔構造、表面化学、温度効果の系統的評価を進めている。conference pape
原型炉の磁性体第一壁内の磁化ベクトル計算に向けたアルゴリズム検討
No full text原型炉設計活動において、磁化した第一壁の影響評価は設計条件を定めるために重要である。影響評価の数値計算においては第一壁をメッシュ化し、各メッシュにおける磁化ベクトルを定め、体積積分を行う。この際、磁化ベクトルは外部磁場に依存するが、正確な磁化ベクトルの決定にはメッシュ間の相互作用も考慮に入れねばならない。これはメッシュ数分の連立１次方程式を解くに等しく、多大な計算コストがかかる。本研究では計算コスト低減に向けたアルゴリズムを検討中であり、年会においてはその成果について発表する。第42回プラズマ・核融合学会年会conference poste
Misfit accommodation in a single interface atomic layer at a highly lattice-mismatched InN/GaN
No full textHeterostructures of covalent semiconductors provide an invaluable platform for synthesizing the distinct properties of materials, leading to unprecedented functions in electronic and optoelectronic devices. The main challenge has been to forge high-quality interfaces of the heterostructures that guarantee the designed properties. To date, high-quality interfaces can be attained in heterostructures with a lattice mismatch of less than a few percent. For highly lattice-mismatched interfaces, such as InN/GaN (0001) (11.1% mismatch), interfacial structures remain unknown. Here, we investigate the atomic structure of the InN/GaN interface using atomic resolution transmission electron microscopy and large-scale density-functional calculations. Our findings show that an interface structure without any misfit dislocations are formed, where the InN single monolayer at the interface accommodates the entire misfit. We argue that the mechanism underlying the formation of this interface monolayer is the flexibility of the group III ? nitrogen bond network.journal articl
A neuroimaging dataset during sequential color qualia similarity judgments with and without reports
No full text最近の神経科学の研究は意識についての理解を進展させているが、意識の特定の質的側面である「クオリア」と特定の脳領域やネットワークとの関連は依然として明確ではない。従来の方法では参加者からの言葉による説明に依存しており、これが神経画像研究での課題となっている。これに対処するため、私たちのグループはクオリアの徹底的な構造的および関係的比較に依存する新しい「クオリア構造」パラダイムを導入した。この研究では、35人の参加者から、各試行で9つの色クオリアのうち2つについての関係的類似性判断を捉えた初めてのfMRIデータセットを提示する。このデータセットは、報告の有無の影響を評価するために、試行の半分に「無報告」条件も含まれている。また、スキャナー外で行われた色差別能力評価を各参加者に対して実施した。私たちのデータは色クオリアに関連する脳機能についての貴重な洞察を提供し、意識の神経基盤の理解を深めることに寄与する。journal articl