Comprehensive Understanding of Hillocks and Ion Tracks in Ceramics Irradiated with Swift Heavy Ions

Amorphizable ceramics (LiNbO3, ZrSiO4, and Gd3Ga5O12) were irradiated with 200 MeV Au ions at an oblique incidence angle, and the as-irradiated samples were observed by transmission electron microscopy (TEM). Ion tracks in amorphizable ceramics are confirmed to be homogenous along the ion paths. Magnified TEM images show the formation of bell-shaped hillocks. The ion track diameter and hillock diameter are similar for all the amorphizable ceramics, while there is a tendency for the hillocks to be slightly bigger than the ion tracks. For SrTiO3 (STO) and 0.5 wt% niobium-doped STO (Nb-STO), whose hillock formation has not been fully explored, 200 MeV Au ion irradiation and TEM observation were also performed. The ion track diameters in these materials are found to be markedly smaller than the hillock diameters. The ion tracks in these materials exhibit inhomogeneity, which is similar to that reported for non-amorphizable ceramics. On the other hand, the hillocks appear to be amorphous, and the amorphous feature is in contrast to the crystalline feature of hillocks observed in non-amorphizable ceramics. No marked difference is recognized between the nanostructures in STO and those in Nb-STO. The material dependence of the nanostructure formation is explained in terms of the intricate recrystallization process

Comprehensive Understanding of Hillocks and Ion Tracks in Ceramics Irradiated with Swift Heavy Ions

Amorphizable ceramics (LiNbO3, ZrSiO4, and Gd3Ga5O12) were irradiated with 200 MeV Au ions at an oblique incidence angle, and the as-irradiated samples were observed by transmission electron microscopy (TEM). Ion tracks in amorphizable ceramics are confirmed to be homogenous along the ion paths. Magnified TEM images show the formation of bell-shaped hillocks. The ion track diameter and hillock diameter are similar for all the amorphizable ceramics, while there is a tendency for the hillocks to be slightly bigger than the ion tracks. For SrTiO3 (STO) and 0.5 wt% niobium-doped STO (Nb-STO), whose hillock formation has not been fully explored, 200 MeV Au ion irradiation and TEM observation were also performed. The ion track diameters in these materials are found to be markedly smaller than the hillock diameters. The ion tracks in these materials exhibit inhomogeneity, which is similar to that reported for non-amorphizable ceramics. On the other hand, the hillocks appear to be amorphous, and the amorphous feature is in contrast to the crystalline feature of hillocks observed in non-amorphizable ceramics. No marked difference is recognized between the nanostructures in STO and those in Nb-STO. The material dependence of the nanostructure formation is explained in terms of the intricate recrystallization process

Mathematical Modeling of Severe Acute Respiratory Syndrome Nosocomial Transmission in Japan: The Dynamics of Incident Cases and Prevalent Cases

An outbreak of Severe Acute Respiratory Syndrome (SARS) occurred in Hong Kong in late February 2003, resulting in 8,096 cumulative cases with 774 deaths. The outbreak was amplified by nosocomial transmission in many hospitals. Using mathematical modeling, we simulated the number of new incident and prevalent cases of SARS after one infected person was admitted to a hospital (index case). The simulation was tested stochastically using the SEIR model based on previously reported Gamma distributions. We estimated the duration time until 10 beds in negative pressure rooms in Chiyoda-ku, one of the 23 wards in Tokyo, were fully occupied with SARS-infected patients. We determined the impact of an increasing number of days on the number of prevalent cases until the index case was isolated. The prevalent cases increase exponentially along with the increase of the non-isolation period of the index case, and all the beds were fully occupied if the index case was not isolated until more than 6 days. However even 2 days non-isolation period of the index case could fill up all the beds when 16% of secondary infections are transmitted outside the hospital. There is a possibility that an epidemic will occur with the isolation of the index case even at early days if the infection is transmitted outside the hospital. The simulation results revealed that it was important to recognize and isolate SARS patients as early as possible and also to prevent the transmission spreading outside the hospital to control an epidemic

Analysis of Ion-Irradiation Induced Lattice Expansion and Ferromagnetic State in CeO2 by Using Poisson Distribution Function

The lattice constant and the magnetic state of CeO2 are modified by the irradiation with 200 MeV Xe ions. Under the assumption that these modifications are induced in the narrow one-dimensional region (the ion track) along the ion beam path, the dependence of the lattice constant and the saturation magnetization of CeO2 on the Xe ion fluence was analyzed by using the Poisson distribution function. The analysis reveals that the lattice constant inside the ion track, which is larger than that outside the ion track, is not affected by the overlapping of the ion track. On the other hand, the ferromagnetic state is induced inside the ion track, but the analysis shows that this ferromagnetic state is gradually destroyed due to the overlapping of the ion track. The present result implies that the Poisson distribution function is useful for describing the effect of the ion track overlapping on the ion irradiation-induced ferromagnetic state in CeO2

Nanopore Formation in CeO₂ Single Crystal by Ion Irradiation: A Molecular Dynamics Study

The nanopore formation process that occurs by supplying a thermal spike to single crystal CeO2 has been simulated using a molecular dynamics method. As the initial condition, high thermal energy was supplied to the atoms in a nano-cylinder placed at the center of a fluorite structure. A nanopore was generated abruptly at around 0.3 ps after the irradiation, grew to its maximum size at 0.5 ps, shrank during the time to 1.0 ps, and finally equilibrated. The nanopore size increased with increasing effective stopping power gSe (i.e., the thermal energy deposited per unit length in the specimen), but it became saturated when gSe was 0.8 keV/nm or more. This finding will provide useful information for precise control of the size of nanopores. Our simulation confirmed nanopore formation found in the actual experiment, irradiation of CeO2 with swift heavy ions, but could not reproduce crystalline hillock formation just above the nanopores

FE-SEM observation of chains of nanohillocks in SrTiO3 and Nb-doped SrTiO3 surfaces irradiated with swift heavy ions

高速重イオン (SHI) がセラミックスに真上から入射すると、SHI 一つに対してヒロック (ナノメートルサイズの隆起物) が一つ表面に形成される。一方で近年、SHI がチタン酸ストロンチウム (SrTiO3) や酸化チタン (TiO2)の表面をかするように入射した場合、表面にはイオンの飛跡に沿って連続的に複数個のヒロックが形成されると報告がある。これらは原子間力顕微鏡 (AFM) を用いて観察されており、観察結果には AFM のプローブ寸法由来の測定誤差を含んでいる。本研究では、ヒロックのサイズより十分小さい分解能 (1.5 nm) を有し、非接触で観察可能な電界放出形走査電子顕微鏡 (FE-SEM) を用いて連続ヒロックを観察する手法を検討した。本発表では連続ヒロックのFE-SEM観察結果とともに、新しい連続ヒロックの形状変化メカニズムを提案する

Mathematical Modeling of Severe Acute Respiratory Syndrome Nosocomial Transmission in Japan: The Dynamics of Incident Cases and Prevalent Cases

An outbreak of Severe Acute Respiratory Syndrome (SARS) occurred in Hong Kong in late February 2003, resulting in 8,096 cumulative cases with 774 deaths. The outbreak was amplified by nosocomial transmission in many hospitals. Using mathematical modeling, we simulated the number of new incident and prevalent cases of SARS after one infected person was admitted to a hospital (index case). The simulation was tested stochastically using the SEIR model based on previously reported Gamma distributions. We estimated the duration time until 10 beds in negative pressure rooms in Chiyoda-ku, one of the 23 wards in Tokyo, were fully occupied with SARS-infected patients. We determined the impact of an increasing number of days on the number of prevalent cases until the index case was isolated. The prevalent cases increase exponentially along with the increase of the non-isolation period of the index case, and all the beds were fully occupied if the index case was not isolated until more than 6 days. However even 2 days non-isolation period of the index case could fill up all the beds when 16% of secondary infections are transmitted outside the hospital. There is a possibility that an epidemic will occur with the isolation of the index case even at early days if the infection is transmitted outside the hospital. The simulation results revealed that it was important to recognize and isolate SARS patients as early as possible and also to prevent the transmission spreading outside the hospital to control an epidemic

Effects of Energetic Carbon Cluster Ion Irradiation on Lattice Structures of EuBa2Cu3O7-x Oxide Superconductor

C-axis oriented EuBa2Cu3O7-x oxide films 100 nm thick were irradiated with 0.5 MeV C 10 monoatomic ions, 2 MeV C4 cluster ions and 4 MeV C8 cluster ions at room temperature. Before 11 and after the irradiation, the x -ray diffraction (XRD) measurement was performed using Cu-Kα x-12 ray. The c-axis lattice constant increases almost linearly as a function of numbers of irradiating car-13 bon ions, but it does rarely depend on the cluster size. Cluster size effects are observed in the XRD 14 peak intensity and the XRD peak width. With increasing the cluster size, the decrease in peak in-15 tensity becomes more remarkable and the peak width increases. The experimental result implies 16 that the cluster ions with a larger size provide a more localized energy deposition in a sample, and 17 cause larger and more inhomogeneous lattice disordering. As such a local and large lattice disor-18 dering acts as pinning centers for quantum vortex, energetic carbon cluster ion irradiation will be 19 effective for the increment in the critical current of EuBa2Cu3O7-x superconductors