Optimization of the Guide Design of MIRACLES, the Neutron Time-of-Flight Backscattering Spectrometer at the European Spallation Source

To boost the science case of MIRACLES, the time-of-flight backscattering spectrometer at the European Spallation Source (ESS), an optimized neutron guide system, is proposed. This systematic study resulted in an enhancement in the transport of cold neutrons, compared with the previous conceptual design, with wavelengths ranging from λ = 2 Å to 20 Å along the 162.5-m distance from source to sample. This maintained the undisturbed main focus of the instrument, viz, to carry out quasielastic and inelastic neutron scattering (QENS and INS) experiments on a large dynamic range and for both energy-gain and energy-loss sides. To improve the collection of cold neutrons from the source and direct them to the sample position, the vertical geometry was adjusted to an adapted version of a ballistic elliptical profile. Its horizontal geometry was conceived to: (i) keep the high-resolution performance of the instrument, and (ii) minimize the background originating from fast and thermal neutrons. To comply with the first requirement, a narrow guide section at the pulse shaping chopper position has been implemented. To fulfil the second, a curved guide segment has been chosen to suppress neutrons with wavelengths λ < 2 Å. Subsequent tailoring of the phase space provided an efficient transport of cold neutrons along the beamline to reach a 3 × 3 cm2 sample. Finally, additional calculations were performed to present a potential upgrade, with the exchange of the final segment, to focus on samples of approximately 1 × 1 cm2; the proposal anticipates a flux increase of 70% in this 1 cm2 sample area.

Ensemble Negatively-Charged Nitrogen-Vacancy Centers in Type-Ib Diamond Created by High Fluence Electron Beam Irradiation

Electron beam irradiation into type-Ib diamond is known as a good method for the creation of high concentration negatively-charged nitrogen-vacancy (NV−) centers by which highly sensitive quantum sensors can be fabricated. In order to understand the creation mechanism of NV− centers, we study the behavior of substitutional isolated nitrogen (P1 centers) and NV− centers in type-Ib diamond, with an initial P1 concentration of 40–80 ppm by electron beam irradiation up to 8.0 × 1018 electrons/cm2. P1 concentration and NV− concentration were measured using electron spin resonance and photoluminescence measurements. P1 center count decreases with increasing irradiation fluence up to 8.0 × 1018 electrons/cm2. The rate of decrease in P1 is slightly lower at irradiation fluence above 4.0 × 1018 electrons/cm2 especially for samples of low initial P1 concentration. Comparing concentration of P1 centers with that of NV− centers, it suggests that a part of P1 centers plays a role in the formation of other defects. The usefulness of electron beam irradiation to type-Ib diamonds was confirmed by the resultant conversion efficiency from P1 to NV− center around 12–19%.

Modifications of Metallic and Inorganic Materials by Using Ion/Electron Beams

Welcome to the Special Issue of Quantum Beam Science entitled “Modifications of Metallic and Inorganic Materials by Using Ion/Electron Beams” [...]

Laser Peening Analysis of Aluminum 5083: A Finite Element Study

In this research, a finite element (FE) technique was used to predict the residual stresses in laser-peened aluminum 5083 at different power densities. A dynamic pressure profile was used to create the pressure wave in an explicit model, and the stress results were extracted once the solution was stabilized. It is shown that as power density increases from 0.5 to 4 GW/cm2, the induced residual stresses develop monotonically deeper from 0.42 to 1.40 mm. However, with an increase in the power density, the maximum magnitude of the sub-surface stresses increases only up to a certain threshold (1 GW/cm2 for aluminum 5083). Above this threshold, a complex interaction of the elastic and plastic waves occurring at peak pressures above ≈2.5 Hugoniot Elastic Limit (HEL) results in decreased surface stresses. The FE results are corroborated with physical experiments and observations.

Measurement of Neutron Dose Equivalent within and Outside of a LINAC Treatment Vault Using a Neutron Survey Meter

This work concerns neutron doses associated with the use of a Siemens Primus M5497 electron accelerator, which is operated in the photon mode at 15 MV. The conditions offer a situation within which a fraction of the bremsstrahlung emission energies exceed the photoneutron threshold. For different field sizes, an investigation has been made of neutron dose equivalent values at various measurement locations, including: (i) At the treatment table, at a source-surface distance of 100 cm; (ii) at the level of the floor directly adjacent to the treatment table; and (iii) in the control room and patient waiting area. The evaluated neutron dose equivalent was found to range from 0.0001 to 8.6 mSv/h, notably with the greatest value at the level of the floor directly adjacent to the treatment couch (8.6 mSv/h) exceeding the greatest value on the treatment table (5.5 mSv/h). Low values ranging from unobservable to between 0.0001 to 0.0002 mSv/h neutron dose were recorded around the control room and patient waiting area. For measurements on the floor, the study showed the dose equivalent to be greatest with the jaws closed. These data, most particularly concerning neutron distribution within the treatment room, are of great importance in making steps towards improving patient safety via the provision of protective measures.

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.

X-ray Imaging-Based Void Fraction Measurement in Saturated Flow Boiling Experiments with Seawater Coolant

Recent studies have shown that the presence of dissolved salts in water can exhibit peculiar flow boiling and two-phase flow regimes. Two-phase flow and convective flow boiling are typically characterized with the help of void fraction measurements. To quantitatively improve our understanding of two-phase flow and boiling phenomenon with seawater coolant, void fraction data are needed, which can not be obtained from optical imaging. In this paper, we present experimental void fraction measurements of saturated flow boiling of tap water and seawater using X-ray radiography. X-rays with a maximum energy level of 40 KeV were used for imaging the exit region of the heated test section. At lower heat flux levels, the two phase flow in seawater was bubbly and homogeneous in nature, resulting in higher void fractions as compared to tap water. With an increase in heat flux, the flow regime was similar to slug flow, and void fraction measurements approached similarity with tap water. The predicted pressure drop using the measured void faction shows good agreement with the measured total pressure drop across the test section, demonstrating the validity of the measurement process.

Modification of SiO2, ZnO, Fe2O3 and TiN Films by Electronic Excitation under High Energy Ion Impact

It has been known that the modification of non-metallic solid materials (oxides, nitrides, etc.), e.g., the formation of tracks, sputtering representing atomic displacement near the surface and lattice disordering are induced by electronic excitation under high-energy ion impact. We have investigated lattice disordering by the X-ray diffraction (XRD) of SiO2, ZnO, Fe2O3 and TiN films and have also measured the sputtering yields of TiN for a comparison of lattice disordering with sputtering. We find that both the degradation of the XRD intensity per unit ion fluence and the sputtering yields follow the power-law of the electronic stopping power and that these exponents are larger than unity. The exponents for the XRD degradation and sputtering are found to be comparable. These results imply that similar mechanisms are responsible for the lattice disordering and electronic sputtering. A mechanism of electron–lattice coupling, i.e., the energy transfer from the electronic system into the lattice, is discussed based on a crude estimation of atomic displacement due to Coulomb repulsion during the short neutralization time (~fs) in the ionized region. The bandgap scheme or exciton model is examined.

Analysis of Cr(VI) Bioremediation by Citrobacter freundii Using Synchrotron Soft X-ray Scanning Transmission X-ray Microscopy

Scanning transmission X-ray microscopy (STXM) was utilized for analysing the bioremediation of Cr(VI) by Citrobacter freundii, a species of gram-negative bacteria. The biosorption and bioreduction processes were analysed by the chemical mapping of cells biosorbed at different concentrations of Cr(VI). STXM spectromicroscopy images were recorded at O K-edge and Cr L-edge. A thorough analysis of the X-ray absorption features corresponding to different oxidation states of Cr in the biosorbed cell indicated the coexistence of Cr(III) and Cr(VI) at higher concentrations. This signifies the presence of partially reduced Cr(VI) in addition to biosorbed Cr(VI). In addition, the Cr(III) signal is intense compared with Cr(VI) at different regions of the cell indicating excess of reduced Cr. Speciation of adsorbed Cr was analysed for the spectral features of biosorbed cell and comparison with Cr standards. Analysis of absorption onset, L3/L2 ratio and absorption fine structure concludes that adsorbed Cr is predominantly present as Cr(III) hydroxide or oxyhydroxide. The evolution of absorption features in the duration of biosorption process was also studied. These time lapse studies depict the gradual decrement in Cr(VI) signal as biosorption proceeds. A strong evidence of interaction of Cr with the cell material was also observed. The obtained results provide insights into the biosorption process and chemical speciation of Cr on the cells.

Effect of Irradiation on Corrosion Behavior of 316L Steel in Lead-Bismuth Eutectic with Different Oxygen Concentrations

In an accelerator-driven system (ADS), the beam window material of the spallation neutron target is heavily irradiated under severe conditions, in which the radiation damage and corrosion co-occur because of high-energy neutron and/or proton irradiation in the lead–bismuth flow. The materials used in ADSs must be compatible with the liquid metal (lead–bismuth eutectic (LBE)) to prevent issues such as liquid metal embrittlement (LME) and liquid metal corrosion (LMC). This study considers the LMC behavior after ion irradiation of 316L austenitic steel for self-ion irradiations followed by the corrosion tests in LBE with critical oxygen concentration. The 316L samples were irradiated by 10.5 MeV-Fe3+ ions at a temperature of 450 °C, up to 50 displacements per atom (dpa). After the corrosion test performed at 450 °C in LBE with low oxygen concentration, a surface of the nonirradiated area was not oxidized but appeared with locally corrosive morphology, Ni depletion, whereas an iron/chromium oxide layer fully covered the irradiated area. In the case of the corrosion surface with high oxygen concentration in LBE, the surface of the nonirradiated area was covered by an iron oxide layer only, whereas the irradiated area was covered by the duplex layers comprising iron and iron/chromium oxides. It is suggested that irradiation can enhance the oxide layer formation because of the enhancement of Fe and/or oxygen diffusion induced by the radiation defects in 316L steel.