Effects of proton irradiation on electronic structure of NdFeB permanent magnets

Effects of proton irradiation on electronic structure and atomic local structure of N35EH-type NdFeB permanent magnet were investigated by soft X-ray absorption spectrometry and Mössbauer spectrometry. The local coordination environment of Fe atoms changes after proton irradiation, and the average hyperfine field H_in of the magnets decreases from 288.4 to 286.9 kOe. The effects of irradiation on Fe atoms local environment at different lattice sites are different. The near edge structure of Fe L₃ edge is changed, indicating the density of unoccupied state of Fe 3d electrons increases after proton irradiation.     

Fabrication of microfluidic devices using MeV ion beam Programmable Proximity Aperture Lithography (PPAL)

MeV ion beam lithography is a direct writing technique capable of producing microfluidic patterns and lab-on-chip devices with straight walls in thick resist films. In this technique a small beam spot of MeV ions is scanned over the resist surface to generate a latent image of the pattern. The microstructures in resist polymer can be then revealed using a chemical developer that removes exposed resist, while leaving unexposed resist unaffected. In our system the size of the rectangular beam spot is programmably defined by two L-shaped tantalum blades with well-polished edges. This allows rapid exposure of entire rectangular pattern elements up to 500 × 500 μm in one step. By combining different dimensions of the defining aperture with the sample movements relative to the beam spot, entire fluidic patterns with large reservoirs and narrow flow channels can be written over large areas in short time. Fluidic patterns were written in PMMA using 56 MeV ¹⁴N³⁺ and a 3 MeV ⁴He²⁺ beams from K130 cyclotron and a 1.7 MV Pelletron accelerators, respectively, at the University of Jyväskylä Accelerator Laboratory. The patterns were characterized using SEM, and the factors affecting patterns quality are discussed.     

Spectroscopic and thermal studies of 8 MeV electron beam irradiated HPMC films

Radiation-induced changes in hydroxypropyl methylcellulose (HPMC) films under electron irradiation were investigated and correlated with dose. Polymer samples were irradiated in air at room temperature by an electron beam accelerator in the range of 0-100 kGy. Various properties of the irradiated films were studied using a Ultraviolet-Visible spectrophotometer and Fourier transform infrared spectroscopy and thermogravimetric analysis. Electron irradiation was found to induce changes in the physical, chemical and thermal properties, depending on the irradiation dose.     

Irradiation effects in an HfO2/MgO/HfO2 tri-layer structure induced by 10 MeV Au ions

In this report, we present radiation damage effects in a thin film, tri-layer structure, HfO₂/MgO/HfO₂. Irradiations were performed with 10 MeV Au ions in a recently developed medium energy ion irradiation facility at Los Alamos National Laboratory, which is described in this paper. Energy deposition by 10 MeV Au ions corresponds to a mixed regime, wherein electronic and nuclear stopping contribute to radiation damage. In this study, we investigated modifications of both surface and bulk properties in order to assess the structural stability of our oxide tri-layers under the severe irradiation conditions employed here. The most dramatic structural changes were observed to occur on the surfaces of the tri-layer samples. Surface features consisted of large craters and spires. The dimensions of these craters and spires exceed those of the individual ion tracks by almost three orders of magnitude. As for the bulk tri-layer structure, our conclusions are that this structure is stable in terms of: (i) resistance to amorphization; (ii) resistance to compositional mixing and (iii) resistance to pronounced nucleation and growth of extended defects. The main effect observed in the tri-layer structure was the transformation of the first HfO₂ layer from a monoclinic to either a tetragonal or cubic form of HfO₂.     

10 MeV Au ion irradiation effects in an MgO-HfO2 ceramic-ceramic (CERCER) composite

Room temperature ion irradiation damage studies were performed on a ceramic composite intended to emulate a dispersion nuclear fuel. The composite is composed of 90-mole% MgO and 10-mole% HfO₂. The as-synthesized composite was found to consist of Mg₂Hf₅O₁₂ (and some residual HfO₂) particles embedded in an MgO matrix. X-ray diffraction revealed that nearly all of the initial HfO₂ reacted with some MgO to form Mg₂Hf₅O₁₂. Ion irradiations were performed using 10 MeV Au³⁺ ions at room temperature over a fluence range of 5 × 10¹⁶-5 × 10²⁰ Au/m². Irradiated samples were characterized using both grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM), the latter using both selected-area electron diffraction (SAED) and micro-diffraction (μD) on samples prepared in cross-sectional geometry. Both GIXRD and TEM electron diffraction measurements on a specimen irradiated to a fluence of 5 × 10²⁰ Au/cm², revealed that the initial rhombohedral Mg₂Hf₅O₁₂ phase was transformed into a cubic-Mg₂Hf₅O₁₂ phase. Finally, it is important to note that at the highest ion fluence used in this investigation (5 × 10²⁰ Au/m²), both the MgO matrix and the Mg₂Hf₅O₁₂ second phase remained crystalline.     

Photoconduction investigations in 75 MeV oxygen ion irradiated kapton-H polyimide

Photoconduction behaviour of 75 MeV oxygen ion irradiated (Fluences: 1.8 × 10¹¹, 1.8 × 10¹² and 1.8 × 10¹³ ions/cm²) kapton-H polyimide film in the visible region has been investigated at different temperatures ranging 400-2500 °C and at various electric fields ranging 40-600 kV/cm. A photoinduced exciton formation is the major source for providing charge carriers through thermolization and field-assisted dissociation processes. An attempt has been made to fit the field dependence of the steady state photocurrent to one of the several possible conduction mechanisms. In the high and low fluence (1.8 × 10¹³ and 1.8 × 10¹¹ ions/cm²) irradiated samples there exists a possibility of Poole-Frankel type of photoconduction mechanism, whereas at intermediate fluence (1.8 × 10¹² ions/cm²) a Schottky type photoconduction mechanism may be operative. The log I_ps versus 1/T plots consist of two straight lines with a knee point around 800-1000 °C. The activation energy estimated from the slope of these lines is field dependent varying from 0.40 to 0.73 eV and 0.18 to 0.23 eV above and below the knee point, respectively. This indicates the presence of more than one type of trapping levels in irradiated kapton-H polyimide.     

Oxygen defects created in CeO2 irradiated with 200 MeV Au ions

CeO₂ films were irradiated with 200 MeV Au ions in order to investigate the damages created by electronic energy deposition. In the Raman spectra of the ion-irradiated films, a broad band appears at the higher frequency side of the F_2g peak of CeO₂. The band intensity increases as ion fluence increases. Furthermore, the F_2g peak becomes asymmetric with a low-frequency tail. In order to understand the origin of these spectral changes, an unirradiated CeO₂ film was annealed in vacuum at 1000 °C. By comparing the results for the irradiation and for the annealing, it is concluded that the broad band obtained for irradiated samples contains the peak observed for the annealed sample. The F_2g peak becomes asymmetric with a low-frequency tail by the irradiation as well as the annealing. Therefore, the above-mentioned changes in the Raman spectra caused by 200 MeV Au irradiation is closely related to the creation of oxygen vacancies.     

Effect of 50 MeV Li ion irradiation on electrical, optical and mechanical properties of 4,4′-dimethylbenzophenone

4,4′-Dimethylbenzophenone (DMBP) single crystals were irradiated at room temperature and at liquid nitrogen temperature with 50 MeV Li³⁺ ions at fluences 1 × 10¹² and 1 × 10¹³ ions/cm². The dielectric constant and dielectric loss as a function of frequency of the applied ac field in the range from 20 Hz to 1 MHz and at temperatures ranging from 313 to 353 K were analyzed. The dielectric constant decreases with increase in frequency for all the temperatures. The dielectric constant and dielectric loss increase with fluence. Optical absorption was measured at different conditions. UV-Vis studies reveal the decrease in bandgap. The unirradiated as well as irradiated crystals were characterized by photoluminescence. Ion-induced changes were also studied with respect to their mechanical response using the Vicker’s microhardness technique and parameters including fracture toughness, brittleness index and yield strength are calculated.     

Effects of 0.28-2.80 MeV proton irradiation on GaInP/GaAs/Ge triple-junction solar cells for space use

GaInP/GaAs/Ge triple-junction solar cells were irradiated with 0.28, 0.62 and 2.80 MeV protons with fluences ranging from 1 × 10¹⁰ cm⁻² to 1 × 10¹³ cm⁻². Their performance degradation is analyzed using current-voltage characteristics and spectral response measurements. The degradation rates of the short circuit current, open circuit voltage, and maximum power output increase with fluence, but decrease with increasing proton energy. It was also observed that the spectral response of the GaAs middle cell degrades more significantly than that of the GaInP top cell.     

Experimental study of the residual activity induced by 950 MeV/u uranium ions in stainless steel and copper

As a preparatory work for constructing the FAIR facility at GSI, samples of stainless steel and copper were irradiated by 950 MeV/u ²³⁸U ions and depth-profiles of residual activity were measured by gamma-ray spectroscopy. The isotopes with dominating contribution to the residual activity were identified and their contributions were quantified. In contrast to the previous study performed at lower energies, the activities could no longer be determined from the full-assembly target measurements. Depth-profiling of residual activity of all identified isotopes had to be completed by measurements of individual target foils. The activity contributions were then obtained by integration of the depth-profiles.     

The effect of copper and manganese on magnetic minor hysteresis loops in neutron irradiated Fe model alloys

Changes of magnetic minor hysteresis loops in pure Fe, Fe-1 wt% Mn, Fe-0.9 wt% Cu, and Fe-0.9 wt% Cu-1 wt% Mn model alloys after neutron irradiation have been studied. Minor-loop coefficients which are obtained from scaling relations between minor-loop parameters and in proportion to internal stress, were found to decrease in all model alloys after the irradiation to a fluence of 3.32 × 10¹⁹ n cm⁻². The decrease of the coefficients is larger for alloys including Cu and is enhanced by 1 wt% Mn addition. Such decrease implying the reduction of internal stress during irradiation is in contrast with changes of yield strength after the irradiation that increase with Cu and Mn contents. A qualitative explanation was given on the basis of the preferential formation of Cu precipitates along pre-existing dislocations which reduces internal stress of the dislocations.     

Competition of terrace and step-edge sputtering under oblique-incidence ion impact on a stepped Pt(1 1 1) surface

Using molecular-dynamics simulation, we study the sputtering of a Pt(1 1 1) surface under oblique and glancing incidence 5 keV Ar ions. For incidence angles larger than a critical angle ?_c, the projectile is reflected off the surface and the sputter yield is zero. We discuss the azimuth dependence of the critical angle ?_c with the help of the surface corrugation felt by the impinging ion. If a step exists on the surface, sputtering occurs also for glancing incidence ?>?_c. We demonstrate that for realistic step densities, the total sputtering of a stepped surface may be sizable even at glancing incidence.     

Surface roughness of ion-bombarded Si(1 0 0) surfaces: Roughening and smoothing with the same roughness exponent

We have carried out scanning tunneling microscopy experiments under ultrahigh vacuum condition to study the roughness of pristine as well as ion-bombarded Si(1 0 0) surfaces and of ultrathin Ge films deposited on them. One half of a Si(1 0 0) sample (with native oxide layer) was irradiated at room temperature using 45 keV Si⁻ ions at a fluence of 4 × 10¹⁵ ions/cm² while the other half was masked. STM measurements were then carried out on the unirradiated as well as the irradiated half of the sample. Root-mean-square (rms) roughness of both the halves of the sample has been measured as a function of STM scan size. Below a length scale of ∼30 nm we observe surface smoothing and surface roughening is observed for length scales above this value. However, the surface is self-affine up to length scales of ∼200 nm and the observed roughness exponent of 0.46 ± 0.04 is comparable to earlier cases of ion sputtering studies where only roughening [J. Krim, I. Heyvart, D.V. Haesendonck, Y. Bruynseraede, Phys. Rev. Lett. 70 (1993) 57] or only smoothing [D.K. Goswami, B.N. Dev, Phys. Rev. B 68 (2003) 033401] was observed. Preliminary results involving morphology for Ge deposition on clean ion-irradiated and pristine Si(1 0 0) surfaces are presented.     

Strain-driven (0 0 2) preferred orientation of ZnO nanoparticles in ion-implanted silica

We present an experimental and theoretical study on the structural properties of ZnO nanoparticles embedded in silica. The ZnO-SiO₂ nanocomposite was prepared by ion implanting a Zn⁺ beam in a silica slide and by annealing in oxidizing atmosphere at 800 °C. From an experimental point of view, the structural properties of the ZnO-SiO₂ nanocomposite were studied by using glancing incidence X-ray diffraction. According to the results, zinc crystalline nanoclusters with an average diameter of 13 nm are in the as-implanted sample. The annealing in oxidizing atmosphere promotes the total oxidation of the Zn nanoclusters and increases their size until to an average of 22 nm. Moreover, the formed ZnO nanocrystals have a preferential (0 0 2) crystallographic orientation. From a theoretical point of view, the preferential orientation of the ZnO nanoparticles can be explained satisfactory by the minimization of the strain energy of the nanoparticles placed in proximity of the surface of the matrix.     

Plutonium measurements on the 1 MV AMS system at the Centro Nacional de Aceleradores (CNA)

Plutonium isotopes have been recently added to the list of radionuclides that can be measured with the new generation of compact AMS facilities. In this paper we present first experimental results concerning the development of the plutonium AMS technique at 680 kV on the 1 MV AMS system at the Centro Nacional de Aceleradores (CNA) in Sevilla, Spain. This is the first compact AMS machine designed and manufactured by High Voltage Engineering Europa. As we demonstrate, the obtained backgrounds for ^239,240Pu, of about 10⁶ atoms, and the ²³⁹Pu/²³⁸U mass suppression factor, in the range of 10⁻⁹, compare to the ones achieved on other AMS facilities. With the measurement of reference materials provided by the International Atomic Energy Agency (IAEA-375, IAEA-Soil-6, IAEA-381) and samples already studied on the 600 kV compact ETH/PSI AMS system at Zürich, we show that the CNA system can be perfectly used for the routine measurement of plutonium isotopes at environmental levels.     

Enhanced susceptibility of CaF2(1 1 1) to adsorption due to ion irradiation

We have investigated morphological changes of freshly cleaved CaF₂(1 1 1) single crystal surfaces before and after ion irradiation. We show that with or without irradiation the surface undergoes serious changes within minutes after the cleavage if the samples are exposed to ambient conditions. This is most likely due to the adsorption of water and could be avoided only if working under clean ultra-high-vacuum conditions. Ion-induced modifications on this surface seem to act as centers for an increased rate of adsorption so that any quantitative numbers obtained by atomic force microscopy in such experiments have to be treated with caution.     

Cross section measurements for As isotope at neutron energies from 13.5 to 14.8 MeV

Cross sections which is relevant in research of neutron transmutation doping of some important semiconducting materials were measured at neutron energies from 13.5 to 14.8 MeV for the reactions ⁷⁵As(n, 2n)⁷⁴As, ⁷⁵As(n, p)^75m+gGe, ⁷⁵As(n, α)^72m+gGa by activation relative to the ²⁷Al(n, α)²⁴Na reaction. Measurements were carried out by γ-detection using a coaxial HPGe detector. Natural realgar (As₂S₂) powder of 99.9% purity was used as samples. Fast neutrons were produced by the T(d, n)⁴He reaction. The results obtained are compared with existing data.     

Effect of ion beam irradiation on magnetic and structural properties of Pt/Cr/Co multilayers

This paper discusses the effect of ion beam irradiation on the magnetic and structural properties of Pt/Cr/Co multilayers. We observe Co-Cr-Pt ternary alloy phase formation in 1 MeV N⁺ ion irradiated [Pt (2.5 nm)/Cr (0.8 nm)/Co (3.0 nm)]_×6/Si multilayers for a fluence of 1 × 10¹⁶ ions cm⁻² and beyond. The observed phase formation is accompanied by an enhancement in the average grain size, surface roughness and coercivity. Monte Carlo simulation has been performed to study ion-induced defect evolution and atomic displacements to correlate the above observed effects.     

Neutron interrogation of high-enriched uranium by a 4 MeV linac

For revealing unauthorized transport (illicit trafficking) of nuclear materials, a non-destructive method reported earlier, utilizing a 4 MeV linear accelerator for photoneutron interrogation, was further developed. The linac served as a pulsed neutron source for assay of highly enriched uranium. Produced in beryllium or heavy water by bremsstrahlung, neutrons subsequently induced fission in the samples. Delayed neutrons were detected by a newly designed neutron collar built up of 14 ³He counters embedded in a polyethylene moderator. A PC controlled multiscaler served as a time analyzer, triggering the detector startup by the beam pulse. Significant progress was achieved in enhancing the detector response, hence the sensitivity for revealing illicit material. A lower sensitivity limit of the order of 10 mg ²³⁵U was determined in a 20 s measurement time with a reasonable amount of beryllium (170 g) or of heavy water (100 g) and a mean electron current of 10 μA. Sensitivity can be further enhanced by increasing the measurement time.