N. Ishikawa

Accumulation and recovery of defects in ion-irradiated nanocrystalline gold

Abstract Effects of 60 MeV 12 C ion irradiation on nanocrystalline gold (nano-Au) are studied. The experimental results show that the irradiation-produced defects in nano-Au are thermally unstable because of the existence of a large volume fraction of grain boundaries. This suggests a possibility of the use of nanocrystalline materials as irradiation-resistant materials.

Ion tracks on LiF and CaF2 single crystals characterized by scanning force microscopy

Abstract This work presents results of scanning force microscopy (SFM) of tracks induced by energetic heavy ions on the surface of LiF and CaF 2 single crystals. The samples have been irradiated at normal incidence with several ion species with kinetic energies in the range from several hundred MeV to some GeV. The analysis of the SFM micrographs provides the mean diameters and heights of tiny hillocks created by ion impact. In the case of LiF, a previously published data set of the diameters and heights as a function of energy loss has been extended. Furthermore, the topographic height distribution of an image containing numerous tracks is analyzed by plotting a grey-value histogram of the complete image. Fitting a Gaussian function to each of the two peaks of this distribution allows us to determine the relative surface coverage with ion tracks. Displaying the relative area covered with tracks versus increasing fluence results in an exponential saturation curve containing an average track diameter. This procedure examines the consistency of the different mean diameters extracted from the series of single SFM images underlying the data set presented here.

Transport characteristics near the glass–liquid transition temperature before and after heavy-ion irradiation in a YBa2Cu3O7−δ thin film

Abstract We investigated the influence of flux pinning on the behavior of flux lines in a c-axis oriented YBa2Cu3O7−δ thin film, by analyzing the critical scaling of current–voltage characteristics at various temperatures under the magnetic field parallel to the c-axis before and after irradiation. In the irradiation process, splayed columnar defects were produced in the film by tilting 200 MeV Au-ion beams off the c-axis by ±15°. The enhancement of the glass–liquid transition temperature and the increment of the dynamic critical exponent z were observed after the irradiation. The depinning model taking into account a distribution of pinning strength can reasonably describe the origin of the changes of both the glass–liquid transition temperature and z owing to the introduction of defects. This fact indicates that the flux line dynamics is governed by the pinning interaction rather than the intrinsic property of the flux line system.

Anomalous shift of Curie temperature in iron–nickel Invar alloys by high-energy heavy ion irradiation

Fe0.68Ni0.32 Invar alloys are irradiated with 3.54 GeV Xe ions, and with 2.71 GeV U ions at room temperature. Measurements of the magnetic moment of specimens under AC magnetic field before and after irradiation show that the Curie temperature, Tc, for the irradiated region increases with increasing the ion fluence. This phenomenon appears even at low ion fluence, implying that it is attributed to ion-induced high-density electronic excitation. It is well known that Tc of such Fe–Ni Invar alloys strongly depends on the Ni concentration and the external pressure. With increasing the Ni concentration from ∼30%, Tc increases, and with decreasing the lattice parameter by the external pressure, Tc decreases. Therefore, the increase in Tc by high-energy ion irradiation can be explained as originating from the lattice expansion and/or the composition change, which are induced by the high-density electronic excitation.

Hardening of Fe–Cu alloys by swift heavy ion irradiation

Two kinds of model alloys, Fe–0.6 wt.% Cu and Fe–1.2 wt.% Cu, are irradiated with GeV heavy ions at 250 C and at room temperature. Vickers microhardness test at room temperature shows that the change in hardness due to irradiation strongly depends on the irradiation temperature and the copper content. For Fe–1.2 wt.% Cu alloy irradiated at 250 C, the hardness change is much larger than that for 2.5 MeV electron irradiation at the same dpa (displacements per atom through elastic collisions). The experimental result implies that the electronic excitation by GeV ions enhances the copper precipitation. This effect is, however, hardly observed for Fe–0.6 wt.% Cu alloy or for the irradiation at room temperature. 2002 Elsevier Science B.V. All rights reserved.

Modification of Fe–Ni Invar alloys by high-energy ion beams

Abstract Measurements of temperature dependence of AC-susceptibility were made in Fe–30.2at.%Ni Invar alloy before and after 100 MeV Xe ion irradiation up to the dose of 10 14 ions/cm 2 . Measurements were performed at various angles θ between the direction of the ion beam and the external AC-magnetic field. It was found that in partially irradiated area, locally ferromagnetic parts exist at temperatures above the Curie temperature of the body where high-energy ions did not penetrate. The easy axis of the locally ferromagnetic parts was determined to be parallel to the beam direction. Those locally ferromagnetic parts can be considered to be in thin needle-like shape. This type of modification has a possibility of applications for perpendicular high-density memory and giant magneto-resistance materials.

Use of high energy ions for the mechanistic study of irradiation embrittlement in pressure vessel steels using Fe–Cu model alloys

Abstract To study the mechanism of irradiation embrittlement in pressure vessel steels, it is necessary to sort out the irradiation parameter dependence, which is particularly complex in the neutron irradiation case. Irradiation with high energy heavy ions allows us to carry out single parameter experiments, thereby providing basic data for the modelling of irradiation embrittlement. In this study using Fe–Cu model alloys, three important parameters; ion fluence, irradiation temperature and copper concentration are selected and irradiation induced Vickers hardness change is measured. The hardness change shows a steep increase with fluence after a certain incubation dose. The irradiation temperature dependence of hardness change at fixed fluence shows a peak at around 250–300 °C. Shift of irradiation temperature dependence from neutron irradiation case may be ascribed to the difference in dose rate. Copper concentration dependence shows that the change in hardness seems to depend on the square root of copper concentration. This supports the dispersed barrier hardening model.

HIGH ENERGY HEAVY ION IRRADIATION DAMAGE IN OXIDE SUPERCONDUCTOR EUBA2CU3OY

Defect production and recovery are studied in oxide superconductor, EuBa2Cu3Oy, irradiated with high energy (80 MeV–3.80 GeV) heavy ions. X-ray diffraction measurements show that the c-axis lattice parameter increases linearly with increasing ion-fluence. The rate of the increase in lattice parameter can be scaled with the primary ionization rate, dJ/dx, and not with the electronic stopping power, Se. This result implies that a Coulomb explosion process triggers the atomic displacements in EuBa2Cu3Oy. From the electrical resistivity change at 100 K as a function of ion-fluence, the diameter and the resistivity of a cylindrically-shaped damage region can be determined. Irradiation induced resistivity change is markedly recovered during the annealing of the specimen up to 300 K. The dJ/dx dependence of the diameter and the resistivity of damaged region is discussed.

ELECTRONIC EXCITATION EFFECTS IN ION-IRRADIATED HIGH-TC SUPERCONDUCTORS

Abstract We have measured the fluence dependence of the c -axis lattice parameter in EuBa 2 Cu 3 O y (EBCO) irradiated with various ions from He to Au over the wide energy range from 0.85 MeV to 3.80 GeV. We have observed a linear increase of the c -axis lattice parameter with increasing fluence for all irradiations. The slope of c -axis lattice parameter against fluence, which corresponds to the defect production rate, is separated into two contributions; the effect via elastic displacement and the effect via electronic excitation. The former contribution exhibits a linear increase against the nuclear stopping power, S n . The latter contribution is scaled by the primary ionization rate, d J /d x , rather than by the electronic stopping power, S e , and is nearly proportional to (d J /d x ) 4 .

Lattice expansion in EuBa2Cu3Oy irradiated with energetic ions

Abstract We report an irradiation effect on the c -axis lattice parameter in EuBa 2 Cu 3 O y oxide superconductors when irradiated with ions of energy ranging from 0.85 to 200 MeV. For the irradiation with low energy (0.85-2 MeV) ions, the defect production and the resultant c -axis lattice expansion were dominated by elastic collisions. On the other hand, for irradiation with high energy (120–200 MeV) ions, the change in the c -axis lattice parameter was found to be much greater than expected from the elastic displacement of target atoms. For high energy ion irradiation we could observe an excessive increase of the c -axis lattice parameter reflecting additional production of defects which can be attributed to the electronic excitation. The large increase in the c -axis lattice parameter due to high energy ion irradiation should be taken into account for the study on the interaction between vortices and irradiation-induced defects.