Measurement of thermal neutron cross-section and resonance integral for the Yb(n,γ)Yb reaction by the cadmium ratio method

The thermal-neutron cross-section and the resonance integral for the ¹⁷⁴Yb(n,γ)¹⁷⁵Yb reaction were measured by the activation method using a ⁵⁵Mn monitor as single comparator. Analytical grade MnO₂ and Yb₂O₃ powder samples with and without a cylindrical 1 mm Cd shield box were irradiated in an isotropic neutron field obtained from three ²⁴¹Am-Be neutron sources. The gamma-ray spectra from the activated samples were measured with a calibrated n-type high-purity Ge detector. The experimental results were corrected for the correction factors calculated for thermal and epithermal neutron self-shielding effects, epithermal neutron spectrum shape and gamma-ray self attenuation. Thus, the thermal neutron cross-section for the ¹⁷⁴Yb(n,γ)¹⁷⁵Yb reaction is found to be 126.5 ± 6.6 b, relative to that of the ⁵⁵Mn monitor. The resonance integral value for the ¹⁷⁴Yb(n,γ)¹⁷⁵Yb reaction is found to be 59.6 ± 8.5 b, at cadmium cut-off energy of a 0.55 eV. Using the measured cadmium ratios of ⁵⁵Mn and ¹⁷⁴Yb, the result for resonance integral of the ¹⁷⁴Yb(n,γ)¹⁷⁵Yb reaction has also been obtained relative to the reference value of the ⁵⁵Mn monitor. The present results for the ¹⁷⁴Yb(n,γ)¹⁷⁵Yb reaction agree well only with the recent experimental ones obtained by Kafala et al. [1] and De Corte and Simonits [2] within uncertainty limits. However, the previously reported experimental data for the thermal neutron cross-section for this reaction are distributed between 24 and 141 b, and similarly the experimental values for the resonance integral value also show a large scatter in the range of 30-69 b.     

Measurement of thermal neutron cross-sections and resonance integrals for Hf(n,γ)Hf and Hf(n,γ)Hf reactions at the Pohang neutron facility

The thermal-neutron cross-sections and the resonance integrals for the ¹⁷⁹Hf(n,γ)^180mHf and the ¹⁸⁰Hf(n,γ)¹⁸¹Hf reactions have been measured by the activation method. The high purity Hf and Au metallic foils within and without a Cd shield case were irradiated in a neutron field of the Pohang neutron facility. The gamma-ray spectra from the activated foils were measured with a calibrated p-type high-purity Ge detector.In the experimental procedure, the thermal neutron cross-sections, σ₀, and resonance integrals, I₀, for the ¹⁷⁹Hf(n,γ)^180mHf and the ¹⁸⁰Hf(n,γ)¹⁸¹Hf reactions have been determined relative to the reference values of the ¹⁹⁷Au(n,γ)¹⁹⁸Au reaction, with σ₀ = 98.65 ± 0.09 barn and I₀ = 1550 ± 28 barn. In order to improve the accuracy of the experimental results, the interfering reactions and necessary correction factors were taken into account in the determinations. The obtained thermal neutron cross-sections and resonance integrals were σ₀ = 0.424 ± 0.018 barn and I₀ = 6.35 ± 0.45 barn for the ¹⁷⁹Hf(n,γ)^180mHf reaction, and σ₀ = 12.87 ± 0.52 barn and I₀ = 32.91 ± 2.38 barn for the ¹⁸⁰Hf(n,γ)¹⁸¹Hf reaction. The present results are in good agreement with recent measurements.     

Non-Poisson counting statistics of a hybrid G-M counter dead time model

The counting statistics of a G-M counter with a considerable dead time event rate deviates from Poisson statistics. Important characteristics such as observed counting rates as a function true counting rates, variances and interval distributions were analyzed for three dead time models, non-paralyzable, paralyzable and hybrid, with the help of GMSIM, a Monte Carlo dead time effect simulator. The simulation results showed good agreements with the models in observed counting rates and variances. It was found through GMSIM simulations that the interval distribution for the hybrid model showed three distinctive regions, a complete cutoff region for the duration of the total dead time, a degraded exponential and an enhanced exponential regions. By measuring the cutoff and the duration of degraded exponential from the pulse interval distribution, it is possible to evaluate the two dead times in the hybrid model.     

Low energy elastic scattering of positrons by CO: An application of continued fractions and Schwinger variational iterative methods

Iterative Schwinger variational methods and the method of continued fractions, widely used for electron-molecule scattering, are applied for the first time to investigate positron-molecule interactions. Specifically, integral and differential cross sections for elastic positron scattering by CO in the (0.5-20) eV energy range are calculated and reported. In our calculation, a static plus correlation-polarization potential is used to represent the collisional dynamics. Our calculated results are in general agreement with the theoretical and experimental data available in the literature.     

On relationship between surface effective mass and incident angle of oncoming particle in hyperthermal gas-surface scattering

For the purpose of analyzing the interactive mechanisms between hyperthermal particles and a solid surface, a formal expression describing the dependency of surface effective mass upon the incident angle of the oncoming particle is presented. The formulation is deduced based on the interrelationships among the surface effective mass, the distance of impulse propagation and the non-conservative tangential momentum of the oncoming particle. Following the deduction, some reasoning is attempted toward the viewpoint adopted frequently by the exiting works where the mass is assumed to be constant.     

Electronic excitations induced modifications of structural and optical properties of ZnO-porous silicon nanocomposites

The influence of swift heavy ion (SHI) irradiation on structural and photoluminescence (PL) properties of ZnO nanocrystallites deposited into porous silicon (PS) templates by the sol-gel process was studied. The ZnO/PS nanocomposites were irradiated using 120 MeV Au ions at different fluences varying from 1 × 10¹² to 1 × 10¹³ ions/cm². The intensity of the X-ray diffraction peaks is suppressed at the high fluence, without evolution of any new peak. The PL emission from PS around 700 nm is found to decrease with increase in ion fluence, while the PL emission from deep level defects of ZnO nanocrystallites is increased with ion fluence. At the highest fluence, the observation of drastic increase in PL emission due to donor/acceptor defects in the region 400-600 nm and suppressions of XRD peaks could be attributed to the defects induced structural modifications of ZnO nanocrystallites.     

Microstructure characterization of oxidation of aluminized coating prepared by a combined process

Alumina layer is a good candidate for the tritium penetration barrier that is important in the control of tritium losses due to permeation through structural materials used in high-temperature gas-cooled reactors and in fusion reactors. This paper describes the microstructure of the oxide film of the tritium penetration barrier formed on 316L stainless steel, which was prepared by a combined process, namely, aluminizing and oxidizing treatments using a double glow plasma technology. Microstructure and phase structure of the coatings investigated were examined by scanning electronic microscope (SEM), X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM), respectively. The chemical composition and the chemical states of Al, O elements in the oxidation film were identified by X-ray photoelectron spectroscopy (XPS). After aluminization, the typical microstructure of the coating mainly consisted of an outer high aluminum-containing intermetallic compound layer (Fe₂Al₅ and FeAl) and intermediate ferritic stainless steel (α Fe(Al))layer followed by the austenitic substrate. After the combined process, an oxide layer that consisted of Al₂O₃ and spinel FeAl₂O₄ had been successfully formed on the aluminizing coating surface, with an amorphous outmost surface and an underlying subsurface nanocrystalline structure.     

Measurement of nitrogen depth profile in steel

The characterization of nitridated steel samples, in special the depth profile of nitrogen, aims to help improving the quality of the surface and to increase the durability of the steel pieces. In this work we used ERDA and NRA to determinate the profile of nitrogen in different sets of stainless steel samples. An incident beam of ³⁵Cl of 56 MeV was used for ERDA analysis of a first set of samples. Results indicated an homogeneous distribution for most of the identified elements, with atomic nitrogen concentrations around 2% in the analyzed depth range (0.2 μm) and the presence of thin films on the surface (about 50 × 10¹⁵ at/cm²), one of C and the other of iron oxide. In a second set of samples, 4.43 MeV gamma rays produced from ¹⁵N(H,αγ)¹²C reaction, using an external proton beam of 1.3 MeV, were used to quantify nitrogen concentration. N concentrations of about 0.47% were obtained comparing the gamma production rate of the samples with a referenced material (Stainless steel CRM298 - 0.236% of N in mass) irradiated in the same conditions. Also PIXE analyses were done on both sets of samples in order to identify main elements in the matrix.     

Observations of liquid lithium uptake in a porous molybdenum foam

Recent experiments with liquid lithium in magnetically confined plasmas have demonstrated improved plasma performance. These results have led researchers working on the National Spherical Torus Experiment to consider using a porous molybdenum foam and liquid lithium composite as a future liquid lithium divertor. In order to better understand the properties of this composite material, a small experimental apparatus was constructed at the University of Illinois to test lithium wetting uptake into the porous material. We report here results of the wetting behavior of a porous molybdenum foam with liquid lithium. Based on these observations, a simple model was used to estimate the thermal properties of the lithium infused porous material. Finally, the results of water-bath cleaning tests of the porous metal after lithium exposure are shown.     

Temperature dependence of the chemical sputtering of amorphous hydrogenated carbon films by hydrogen

The temperature dependence of chemical erosion and chemical sputtering of amorphous hydrogenated carbon films due to exposure to hydrogen atoms (H⁰) alone and combined exposure to argon ions and H⁰ was measured in the temperature range from 110 to 950 K. The chemical erosion yield for H⁰ alone is below the detection limit for temperatures below about 340 K. It increases strongly with increasing temperature, goes through a maximum around 650–700 K and decreases again for higher temperatures. Combined exposure to Ar⁺ and H⁰ results in substantial chemical sputtering yields in the temperature range below 340 K. In this range the yield does not depend on temperature, but it increases with energy from about 1 (eroded carbon atoms per impinging Ar⁺ ion) to about 4 if the ion energy is increased from 50 to 800 eV. For temperatures above 340 K the measured erosion rates show the same temperature dependence as for the H⁰-only case, but they are higher than for H⁰-only. The difference between the Ar⁺ and H⁰ and the H⁰-only cases increases monotonically with increasing ion energy.     

Near-edge elastic photon scattering in amorphous systems

The structure of valence and unoccupied electron orbitals and the neighbouring electron density distribution of atoms and ions in amorphous systems can be examined through use of resonance in the elastic photon scattering-cross-section in the vicinity of core atomic orbital energies. So-called anomalous X-ray scattering (AXS) is a mode of analysis that offers similar information to that of EXAFS but can be obtained concurrently with diffraction mode imaging. Of interest is whether the dilute-ion aqueous system provides an environment suitable for testing independent particle approximation (IPA) predictions. With the aqueous environment as the reference system for calibrating relative cross-sections, particular challenges include photons scattered by the medium being subsequently absorbed by the ion, limiting the thickness of the attenuating medium and motivating use of bright synchrotron photon sources where tunable X-rays are obtained at sub-eV resolution using a Si 111 monochromator. Measured scattering intensities and fluorescent yields were compared and shown to agree qualitatively with Monte Carlo calculations utilising amplitudes calculated from modified form-factors with anomalous scatter factors at a resolution of several eV determined from the Dirac-Slater exchange potential. Experimentally determined form-factors for pure water were used to calibrate fluorescent yield and elastic scattering intensities for measurement of the energy dependent variation of these quantities near edge and XRF imaging of the Zn concentration in wax mounted, formalin fixed, breast tumour samples. Results indicate the distribution of Zn at higher resolution than sampling dimensions used in previous studies. Shifts in the position and profile of K-edge absorption and elastic scattering features in aqeuous Zn, Zn doped sol-gel glass and Zn in tissue are shown to reflect changes in the atomic charge state and environment and offer support for the presence of non-nutrient Zn bearing components at elevated concentrations in tumour, of which the matrix-metalloprotein MMP-II is a likely contender.     

Modification of poly(methyl methacrylate) by keV Ar deposition

Classical molecular dynamics simulations are used to examine 1 keV Ar atom bombardment on the surface of poly(methyl methacrylate) (PMMA), which induces sputtering and chemical modifications to the surface. The simulations are carried out at various surface temperatures that range from 200 to 600 K. The results indicate that different fragments of PMMA, as characterized by their mass, are preferentially sputtered from the surface at the various temperatures considered. In addition, the simulations predict that small fragments are produced by the high energy deposition process. However, larger sized fragments are generated when the surface temperature is close to the glass transition temperature of PMMA. The atomic-scale processes by which these occur are elucidated by the simulations.     

A method to measure the validity of trajectory simulation by removing the boundary-dependent coherent scattering contribution

A method is shown for measuring the validity of trajectory simulation, as compared to exact quantum calculation. The object is to calculate the differential cluster cross section obtained by multiple elastic scattering of a particle in a cluster of N randomly distributed point scatterers. After discussing the application of the analytic Foldy theory to the calculation of the coherent part of the cluster cross section, i.e. the diffraction pattern that is associated with the finite size of the cluster, it is shown that, for N up to ∼10³, this coherent part can be removed by a simple, exact procedure, allowing the trajectory simulation to be compared with the remaining incoherent part. A scalar measure of the relative error in the trajectory differential cluster cross section is introduced. In the examples shown, this relative error is negligible at a low density of scatterers, but increases substantially with increasing density. The relevance for low-energy electron scattering is discussed.     

Low energy positron scattering from helium

A new experiment has been developed for high resolution studies of positron scattering from atoms and molecules. Based on the Surko trap technology, a pulsed positron beam has been used to obtain preliminary measurements of low energy, differential elastic scattering cross sections from helium. The operation of the beamline is described and preliminary absolute cross section values for scattering energies of 5, 10 and 15 eV are presented and compared with contemporary theoretical calculations.     

Spectroscopic investigations of new metallic complexes with leucine as ligand

The [Cu(L)₂] · H₂O (1), [Co(L)₂] · 2H₂O (2) and [Zn(L)₂] · H₂O (3) complexes with leucine (L) as ligand, were synthesized in water solution and analyzed by physical-chemical and spectroscopic means (UV-VIS, FT-IR, ESR). The comparative analysis of the IR spectra for the ligand and the complexes indicate the coordination of the metallic centre to the carboxylic oxygen atom and the nitrogen atom of the amino group due to the shift of the ν_s(CO) and ν_s(NH) stretching vibrations. Spectral UV-VIS and ESR data confirmed the covalent metal-ligand bonds, the pseudotetrahedral symmetry around the copper and zinc ions and the octahedral environment for the cobalt ion.     

X-ray spectroscopy of highly-charged heavy ions at FAIR

In the current contribution, we give an overview of the envisioned X-ray spectroscopy program within the atomic physics research collaboration SPARC (Stored Particle Atomic Research Collaboration) at FAIR (Facility for Antiproton and Ion Research). These activities comprise, among others, the investigation of relativistic collision dynamics, electron correlation in the presence of strong fields, the test of Quantum Electrodynamics in extremely strong electromagnetic fields, and ideas to test the predictions of fundamental theories besides Quantum Electrodynamics. The state of the art X-ray spectroscopy will be of key importance for realization of these challenging goals. The world-wide unique experimental conditions and opportunities offered by the future FAIR facility will be combined with advanced X-ray detection devices, i.e. large-area, segmented solid-state detectors, high-resolution crystal spectrometers, calorimetric detectors etc.     

Recovery and restructuring induced by fission energy ions in high burnup nuclear fuel

In light water commercial reactors, extensive change of grain structure was found at high burnup ceramic fuels. The mechanism is driven by bombardment of fission energy fragments and studies were conducted by combining accelerator based experiments and computer-science. Specimen of CeO₂ was used as simulation material of fuel ceramics. With swift heavy ion (Xe) irradiation on CeO₂, with 210 MeV, change of valence charge and lattice deviation of cations were observed by XPS and XRD. Combined irradiations of Xe implantation and swift heavy ion irradiation successfully produced sub-micrometer sized sub-grains, similar as that observed in commercial fuels. Studying components of mechanism scenarios, with first principle calculations using the VASP code, we found stable hyper-stoichiometric defect structures of UO_2+x. Molecular dynamics studies revealed stability of Xe planar defects and also found rapid transport mode of oxygen-vacancy clusters.     

Evaluation of non-Rutherford proton elastic scattering cross-section for magnesium

The experimental data available for magnesium (p,p) elastic scattering cross-section at angles and energies suitable for Ion Beam Analysis have been evaluated using the theoretical model approach together with additional measurements and benchmark experiments. The results obtained provide the evaluated differential cross-sections for magnesium (p,p) elastic scattering in the energy region up to 2.7 MeV.     

Plasma-surface interactions of hydrogenated carbon

We present a review of our study of interactions of plasma particles (atoms, molecules) with hydrogenated amorphous carbon surfaces typical of plasma-facing divertor tiles and deposited layers in magnetic-fusion reactors. Our computer simulations of these processes are based on classical molecular dynamics simulations, using the best currently available multibody bond-order hydrocarbon potentials. Our research in this field has been focused on the chemical sputtering of carbon surfaces at low impact energies, the most complex of the plasma-surface interactions (PSI). Close collaboration with beam-surface and plasma-surface experiments provides not only theoretical support for the experiments, but also builds suitable benchmarks for our methods and codes, enabling production of theoretical plasma-surface data with increased reliability.     

Polarization effects in non-relativistic ep scattering

The cross section which addresses the spin-flip transitions of a proton (antiproton) interacting with a polarized non-relativistic electron or positron is calculated analytically. In the case of attraction, this cross section is greatly enhanced for sufficiently small relative velocities as compared to the result obtained in the Born approximation. However, it is still very small, so that the beam polarization time turns out to be enormously large for the parameters of e^± beams available now. This practically rules out a use of such beams to polarize stored antiprotons or protons.