Physics Procedia最新文献

As the demand for moulds and other tools becomes increasingly specific and complex, an additive manufacturing approach to production is making its way to the industry through laser based consolidation of metal powder particles by a method known as powder bed fusion. This paper concerns a variety of design choices facilitating the development of an experimental powder bed fusion machine tool, capable of manufacturing metal parts with strength matching that of conventional manufactured parts and a complexity surpassing that of subtractive processes. To understand the different mechanisms acting within such an experimental machine tool, a fully open and customizable rig is constructed. Emphasizing modularity in the rig, allows alternation of lasers, scanner systems, optical elements, powder deposition, layer height, temperature, atmosphere, and powder type. Through a custom-made software platform, control of the process is achieved, which extends into a graphical user interface, easing adjustment of process parameters and the job file generation.

We report on calculations of exciton nonlinearity in dielectric nanocomposites. The effect of various parameters on the spectrum of nonlinear increment to the refractive index, such as size and form factor of the nanoparticles shown. Numerical simulations of the optical response of dielectric nanoparticles Al₂O₃ presented.

Monte Carlo is one of the methods to simulate the generation and transport of radiation through matter. The most widely used radiation simulation codes are MCNP and Geant4. The simulation of fission production and transport by MCNP has been thoroughly benchmarked. There is an increasing number of users that prefer using Geant4 due to the flexibility of adding features. However, it has been found that Geant4 does not have the proper fission-production cross sections and does not produce the correct fission products. To achieve accurate results for studies in fissionable material applications, Geant4 was modified to correct these inaccuracies and to add new capabilities. The fission model developed by the Lawrence Livermore National Laboratory was integrated into the neutron-fission modeling package. The photofission simulation capability was enabled using the same neutron-fission library under the assumption that nuclei fission in the same way, independent of the excitation source. The modified fission code provides the correct multiplicity of prompt neutrons and gamma rays, and produces delayed gamma rays and neutrons with time and energy dependencies that are consistent with ENDF/B-VII. The delayed neutrons are now directly produced by a custom package that bypasses the fragment cascade model. The modifications were made for U-235, U-238 and Pu-239 isotopes; however, the new framework allows adding new isotopes easily. The SLAC nuclear data library is used for simulation of isotopes with an atomic number above 92 because it is not available in Geant4. Results of the modified Geant4.10.1 package of neutron-fission and photofission for prompt and delayed radiation are compared with ENDFB-VII and with results produced with the original package.

The paper reports the study of photoluminescence (PL) of disc- and elliptical-shaped SiO2 nanoparticles exited by photons with energy lower than silica bandgap. Differences in the PL spectra are found to be associated with the structure of the nanoparticle excitonic states of optical electrons.

Current methodical developments improve the spatial resolution of neutron imaging facilities. Objects with dimensions down to several microns should be detectable. However, the minimum object size detectable depends not only on the facility hardware like detector resolution or neutron optics, but also on the attenuation contrast. In this paper the relation between illumination time needed, neutron contrast of the objects and their minimal size detectable is derived and an analysis of the minimal dimension of an object can be detected in neutron radiography and tomography is discussed at two examples: zirconium hydride ZrH₂ in Zircaloy-4 as a high contrast system and zirconium nitride ZrN in zirconium oxide ZrO₂ as a low contrast system. It is concluded which minimal sizes of the precipitates can be detected in realistic times.

The idea of a new high transparency device based on Micro Channel Plates (MCP) has been recently presented for monitoring the flux and spatial profile of neutron beams. It consists of the assembly of a very thin aluminum foil with a ⁶Li deposit placed in the beam and a MCP equipped with a phosphor screen readout viewed by a CCD camera. A peculiar feature of this device is that it uses a 90° electrostatic mirror to minimize the perturbation of the neutron beam, i.e. absorption and scattering. It can be used at existing time-of-flight facilities, in particular at the n_TOF facility at CERN, for monitoring the flux and spatial profile of neutron beams in the thermal and epithermal region. In this contribution the first experimental test carried out by using radioactive sources will be presented and the related results discussed.

An integrated assessment method for a nuclear fuel with high decay heat and high radioactivity is required to establish fast reactor system with Trans-Uranium (TRU) fuel containing minor actinides. In addition, a Pu quantitation method with rapidity and accuracy is also necessary in a viewpoint of nuclear security. For these demands, a quantitative evaluation technique for nuclei concentration, thermal property and physical information of such fuel has to be developed. The present study focuses on the non-destructive imaging using pulsed neutrons. Experiments are carried out at Hokkaido University Neutron Source (HUNS) and a gas electron multiplier (GEM) is applied to obtain 2-D information of time-of-flight (TOF). To simulate a nuclear fuel pellet, a sample with equivalent thermal neutron cross-section to the enriched uranium fuel is prepared and the transmitted images of the simulated sample are acquired. Furthermore, a small piece of In, which simulates the Pu spot in the actual fuel, is inserted into the sample and the detectability of the small spot is discussed.

The Michigan Ion Beam Laboratory (MIBL) at the University of Michigan in Ann Arbor, Michigan, USA, plays a significant role in supporting the mission of the U.S. DOE Office of Nuclear Energy. MIBL is a charter laboratory of the NSUF (National Scientific User Facility – US DoE) and hosts users worldwide. The laboratory has evolved from a single accelerator laboratory to a highly versatile facility with three accelerators (3 MV Tandem, a 400 kV Ion Implanter and a 1.7 MV Tandem), seven beam lines and five target chambers that together, provide unique capabilities to capture the extreme environment experienced by materials in reactor systems. This capability now includes simultaneous multiple (dual, triple) ion irradiations, an irradiation accelerated corrosion cell, and soon, in-situ dual beam irradiation in a transmission electron microscope (TEM) for the study of radiation damage coupled with injection of transmutation elements. The two beam lines that will connect to the 300 kV FEI Tecnai G2 F30 microscope are expected to be operational by the end of 2017. Multiple simultaneous ion beam experiments involving light and heavy ions are already in progress. This paper will outline the current equipment and will focus on the new capability of running dual and triple ion beam experiments.

Fossil dental remains are an archive of unique information for paleobiological studies. Computed microtomography based on X-ray microfocus sources (X-μCT) and Synchrotron Radiation (SR-μCT) allow subtle quantification at the micron and sub-micron scale of the meso- and microstructural signature imprinted in the mineralized tissues, such as enamel and dentine, through high-resolution “virtual histology”. Nonetheless, depending on the degree of alterations undergone during fossilization, X-ray analyses of tooth tissues do not always provide distinct imaging contrasts, thus preventing the extraction of essential morphological and anatomical details. We illustrate here by three examples the successful application of neutron microtomography (n-μCT) in cases where X-rays have previously failed to deliver contrasts between dental tissues of fossilized specimen.

The Department of Physics and Astronomy at the University of Kentucky operates a 7-MV CN Van de Graaff accelerator that produces primary beams of protons, deuterons, and helium ions. An in-terminal pulsing and bunching system operates at 1.875 MHz and is capable of providing 1 ns beam bunches at an average current of several microamperes. Nearly all ongoing research programs involve secondary pulsed neutrons produced with gas cells containing deuterium or tritium, as well as with a variety of solid targets. Most experiments are performed at a target station positioned over a deep pit, so as to reduce the background created by backscattered neutrons. Recent experiments will be described; these include: measurements of n-p scattering total cross sections from E_n = 90 to 1800 keV to determine the n-p effective range parameter; the response of the plastic scintillator BC-418 below 1 MeV to low-energy recoil protons; n-p radiative capture cross sections important for our understanding of nucleosynthesis approximately 2 minutes after the occurrence of the Big Bang; γ-ray spectroscopy following inelastic neutron scattering to study nuclear structure relevant to double-β decay and to understand the role of phonon-coupled excitations in weakly deformed nuclei; and measurements of neutron elastic and inelastic scattering cross sections for nuclei that are important for energy production and for our global understanding of the interaction of neutrons with matter.