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Laser-based compact MeV X-ray sources are useful for a variety of applications such as radiography and active interrogation of nuclear materials. MeV X rays are typically generated by impinging the intense laser onto ~mm-thick high-Z foil. Here, we have characterized such a MeV X-ray source from 120 TW (80 J, 650 fs) laser interaction with a 1 mm-thick tantalum foil. Our measurements show X-ray temperature of 2.5 MeV, flux of 3 × 1012 photons/sr/shot, beam divergence of ~0.1 sr, conversion efficiency of ~1%, that is, ~1 J of MeV X rays out of 80 J incident laser, and source size of 80 m. Our measurement also shows that MeV X-ray yield and temperature is largely insensitive to nanosecond laser contrasts up to 10−5. Also, preliminary measurements of similar MeV X-ray source using a double-foil scheme, where the laser-driven hot electrons from a thin foil undergoing relativistic transparency impinging onto a second high-Z converter foil separated by 50–400 m, show MeV X-ray yield more than an order of magnitude lower compared with the single-foil results.
Public stigma alters attitudes towards people with mental illness, and is a particular concern for young people since most mental health problems occur in adolescence and young adulthood. However, little is known about the long-term effects of repeated filmed social contact (FSC) on reducing mental health-related stigma among young adults in the general population, compared with self-instructional Internet search (INS) and control interventions.
This study is a parallel-group randomised controlled trial over 12 months conducted in Tokyo, Japan. A total of 259 university students (male n = 150, mean age = 20.0 years, s.d. = 1.2) were recruited from 20 colleges and universities between November 2013 and July 2014, without being provided information about the mental health-related survey or trial. Participants were assigned to one of three groups before completion of the baseline survey (FSC/INS/control = 89/83/87). The FSC group received a computer-based 30-min social contact film with general mental health education and five follow-up web-based FSCs at 2-month intervals. The INS group undertook a 30-min search for mental health-related information with five follow-up web-based reminders for self-instructional searches at 2-month intervals. The control group played PC games and had no follow-up intervention. The main outcome measures were the future (intended behaviour) domain of the Reported and Intended Behaviour Scale at 12 months after the intervention. Analysis was conducted in September 2015.
At the 12-month follow-up, 218 participants completed the survey (84.1%, 75:70:73). The FSC group showed the greatest change at the 12-month follow-up (FSC: mean change 2.11 [95% CI 1.49, 2.73], INS: 1.04 [0.29, 1.80], control: 0.71 [0.09, 1.33]; FSC v. INS p = 0.037, FSC v. controls p = 0.004). No adverse events were reported during the follow-up period.
FSC was more successful in reducing stigma at 12 months after intervention than INS or control interventions. FSC could be used to reduce stigma in educational lectures and anti-stigma campaigns targeted at young people.
This study is registered at UMIN-CTR (No. UMIN000012239).
In 2013, an unusual increase of paratyphoid fever cases in travellers returning from Cambodia was reported in Japan. From December 2012 to September 2013, 18 cases of Salmonella Paratyphi A infection were identified. Microbiological analyses revealed that most isolates had the same clonal identity, although the epidemiological link between these cases remains unclear. It was inferred that the outbreak was caused by a common and persistent source in Cambodia that was likely to have continued during 2014. The information of surveillance and laboratory data from cases arising in travellers from countries with limited surveillance systems should be timely shared with the country of origin.
Over 30 funerary bundles were excavated in 2005 from a large chamber tomb at the prehispanic religious center of Pachacamac on the central coast of Peru. The largest and most elaborate bundle was found in the innermost part of the tomb, tightly surrounded by other bundles. We hypothesized that this bundle contained the deceased leader of a social group whose members collectively cared for their ancestor's bundle (for example, by rewrapping it) and continued to use the tomb to inter deceased individuals from subsequent generations. We tested this hypothesis by dating samples from different layers of the wrapping materials and soft tissue from the bodies and conducting a Bayesian analysis of the resultant dates. We determined carbon and nitrogen isotope ratios in the diet of the interred individuals to correct for marine reservoir effects. Our findings suggest that (1) rewrapping did not occur; (2) the tomb was used for over 500 years starting at cal A.D. 1000; and (3) existing bundles were reshuffled each time new bundles were introduced. Overall, diverse lines of evidence indicate that the tomb had a complex use history and contained individuals with diverse geographical and social origins. This challenges conventional thinking about the social and chronological significance of coexisting bundles in large tombs.
Genotypes are associated with the natural course of hepatitis C virus (HCV) infection and response to antiviral therapy for HCV. HCV genotype 1b has been the dominant genotype in Japan, where the prevention of HCV transmission through blood transfusion or nosocomial infection has been established since 1990. The distribution of HCV genotype was investigated based on patient's birth year in 5515 HCV-infected Japanese individuals at three institutions from different areas of Japan. At all three institutions, the proportion of HCV genotype 1b decreased and was <50% in individuals born after 1970. By contrast, the percentage of HCV genotype 2b increased in subsequent birth cohorts after 1920–1929. Significant changes in HCV genotype distribution were observed across Japan regardless of area.
Submicron-sized hydroxyapatite (HAp) particles were loaded on the titanium plate by anodic oxidation over spark discharge voltage in HAp-containing bath. The biocompatibility and bioactivity were drastically improved by HAp deposition on the anodic oxide films. The affinity with living bone was the following order: anodized titanium plate with HAp > anodized titanium plate ≫ titanium. Bone-bonding strength between HAp-loaded titanium plate and living bone after 4 weeks' implant was much higher than that of anodized titanium plate.
Thirty-period In0.28Ga0.72As(100Å)/GaAs (100Å) multiple quantum wells (MQWs) have been grown by molecular beam epitaxy on (lOO)-oriented GaAs substrate with a 0.5μm-thick InyGa1-yAs (0≤y≤0.28) buffer layer interposed between the QW layer and the GaAs substrate. The MQWs with y close to the average InAs mole fraction of the QW layer exhibited good crystalline quality. It indicates that the strain is well-balanced between the GaAs and In0.28Ga0.72As layer. A significant change in their photoluminescence spectra has been observed when annealed above 750δC by means of rapid thermal annealing, implying a structural disorder in the QW“region.
Bonding phenomena with clean and smooth surfaces at room temperature (293 K) in ultrahigh vacuum has been studied. The metal surfaces, finished to required smoothness in nm level, were bombarded by Ar ions to remove surface contaminants. Then, two cleaned surfaces were put together. Interaction of silver and copper was examined by SEM, AES, and TEM. In order to observe the fundamental phenomena, bonding was attempted between a silver disk with a flat surface and a copper sphere 80 pm in diameter. A bonding area of about 40 μm2 was confirmed under slight load at 293 K. Strong bonds were achieved and the fracture took place almost completely in silver bulk. Furthermore, it was found that the bonding area obtained by the process was much larger than the one derived from conventional plastic theory.
Nanocrystalline/microcrystalline thin films prepared at relatively low temperatures by plasma-enhanced chemical vapor deposition (PECVD), in particular hydrogenated microcrystalline Si films (μc-Si:H), have attracted an increasing attention not only as potential materials for thin film solar cells, but also as active layers in thin film transistor arrays for flat panel displays. This paper reviews recent progress in the investigation of these materials; preparation methods, structural and optical properties, and electronic transports. Emphasis is placed on the understanding of the growth mechanism of μc-Si:H films as well as the microscopic characterization of the film structure.
The luminescence properties of silicon nanoparticles have been studied as a function of the excitation light intensity, the temporal Nature of the excitation source, and of sample temperature. The excitation intensity dependence of the luminescence was found to depend strongly on the temporal Nature of the excitation source. Under high intensity excitation from a pulsed 355 nm source, the photoluminescence (PL) intensity saturates and the peak PL wavelength shifts to the blue at room temperature. This behavior persists at reduced temperature. In contrast, under high intensity excitation using a cw 488 nm source at room temperature, the PL intensity saturates but does not shift in wavelength. At reduced temperatures, there is no saturation of luminescence intensity with high intensity cw excitation. These differences indicate that photogenerated carrier recombination occurs via different pathways depending on the temporal profile of the excitation, with cw excited samples following the expected Auger pathway while pulsed samples exhibit a state filling mechanism. Auger models for the pulsed behavior are found to be inconsistent with the experimental data. The temperature dependence of the PL from a pulsed excited sample for a constant excitation intensity was also monitored. The variation of the peak emission wavelength was found to be similar in magnitude to that observed for amorphous silicon, suggesting that structural disorder may play a role in the luminescence. The change in emission intensity was fairly weak, indicating enhanced carrier confinement, as would be expected in a quantum confined system.
The Si/Si1-xGex quantum well system is attractive for high temperature thermoelectric applications and for demonstration of proof-of-principle for enhanced thermoelectric figure of merit Z, since the interfaces and carrier densities can be well controlled in this system. We report theoretical calculations for Z in this system, based on which Si/Si1-xGex quantum-well structures were grown by molecular-beam epitaxy. Thermoelectric and other transport measurements were made, indicating that an increase in Z over bulk values is possible through quantum confinement effects in the Si/Si1-xGex quantum-well structures.
The relationship between the oxidation states and PL properties and the effects of H/O termination exchange on the PL properties of PS have been investigated using synchrotron radiation photoemission spectroscopy (SR-PES), Auger electron spectroscopy (AES), Fourier transform infrared (FTIR), and photoluminescence (PL) techniques. The energy band gap, the peak energy and FWHM of the PL spectrum are almost unchanged by the oxidation process and by the H/O termination exchange. After the oxidation, the PL peak intensity decreased, suggesting the creation of nonradiative centers. In the H/O termination exchange experiment, the PL peak intensity decreased by more than 65% upon annealing. However, it recovered the initial PL intensity by oxygen exposure. These results suggest that the surface termination itself functions to eliminate the nonradiative centers without depending on the termination species of hydrogen or oxygen, and that the skeletal structure of PS crystallites is important in the PL mechanisms.
LEDs based on silicon-rich silicon oxide (SRSO) have been fabricated utilizing thermal oxidation of electrochemically etched c-Si and standard microelectronic processes including LPCVD, photolithography, ion implantation and metallization. The bipolar devices consist of heavily-doped polycrystalline Si, a transition layer made of oxidized mesoporous Si, an active SRSO layer (doped or intrinsic) and a crystalline Si substrate. The LED's electrical properties exhibit a low interface state density which is explained by the partial filling of the micropores within the transition layer by polycrystalline Si. The dominant carrier transport in SRSO is due to thermally-assisted tunneling at low fields and electric field-assisted (Fowler-Nordheim) tunneling at high fields. The electroluminescence (EL) is stable and the EL modulation is limited by the carrier transition time (not by the carrier lifetime) which explains the high modulation speed (≥ 10 MHz). We have fabricated a prototype alphanumeric 7-segment display with an isolated version of the LEDs and standard microelectronic processing techniques.
We have studied the photoluminescence (PL) in oxidized porous silicon (PSi), prepared from anodized crystalline Si followed by annealing at temperatures ranging from 700 to 1000°C. It has been found that two PL bands with spectral peaks at 1.6 e V (near-IR band) and near 2 eV (red band) exist with a strong dependence on preparation (annealing) conditions. Recent experimental results show a correlation between the intensity of the near-IR band and the level of leakage current in the diode-like structure. The suppression of the near-IR emission results in improved carrier transport, and the enhancement of the red band emission maximizes the electroluminescence (EL) efficiency. The PL study of thermally oxidized PSi indicates different recombination mechanisms. The red PL band is associated with a mechanism similar to band-tail-recombination within the quasi-bandgap of Si nanograins, whereas the near infra-red PL is associated with recombination via defect centers. These mechanisms will be discussed.
We present the temperature and frequency dependence of the current-voltage (I-V) characteristics of Al barriers deposited on porous Si grown on p-type Si substrates. These barriers exhibit a rectifying behaviour when the temperature is higher than 250 K. The I-V characteristics can be understood by a conduction in porous Si taking place via free electrons thermally excited from Pb centers associated with the existence of a SiO2-Si interface and via hopping between these Pb centers.
In order to investigate the fundamentals of electron migration in nanostructured metal-oxide semiconductors, the transient photocurrent response of dye-sensitized porous nanocrystalline TiO2 is studied. The time-resolved photocurrent response at light steps or pulses shows a faster transient upon increasing the light intensity. Intensity-modulated photocurrent spectroscopy (IMPS) reveals that the transient photocurrent is dominated by two time constants, i.e. the geometrical one and a characteristic time related to electron trapping. A theoretical model is derived in which the occupation dynamics of a single electron trap is considered using Shockley-Read-Hall kinetics. The geometrical RC time of the electrode is also included. Excellent agreement between this model and the measured IMPS spectra is obtained.
Silicon-rich silicon oxide (SRSO) films were prepared by thermal oxidation (700°C-950°C) of electrochemically etched crystalline silicon (c-Si). The annealing-oxidation conditions are responsible for the chemical and structural modification of SRSO as well as for the intrinsic light-emission in the visible and near infra-red spectral regions (2.0–1.8 eV, 1.6 eV and 1.1 eV). The extrinsic photoluminescence (PL) is produced by doping (via electroplating or ion implantation) with rare-earth (R-E) ions (Nd at 1.06 μm, Er at 1.5 μm) and chalcogens (S at ∼1.6 μm). The impurities can be localized within the Si grains (S), in the SiO matrix (Nd, Er) or at the Si-SiO interface (Er). The Er-related PL in SRSO was studied in detail: the maximum PL external quantum efficiency (EQE) of 0.01–0.1% was found in samples annealed at 900°C in diluted oxygen (∼ 10% in N2). The integrated PL temperature dependence is weak from 12K to 300K. Light emitting diodes (LEDs) with an active layer made of an intrinsic and doped SRSO are manufactured and studied: room temperature electroluminescence (EL) from the visible to 1.6 μmhas been demonstrated.