A. Iwase

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.

Effect of electron excitation on radiation damage in fce metals

Abstract Defect production, radiation annealing and defect recovery are studied in several fcc metals (Al, Cu, Ni, Ag and Pt) irradiated with low-energy (∼ 1 MeV) and high-energy (∼ 100 MeV) ions. Irradiation of the metals with strong electron-lattice interaction (Al, Ni and Pt) by ∼ 100 MeV ions causes an anomalous reduction, or even a complete disappearance of stage-I recovery. This experimental result shows that the energy transferred from excited electrons to lattice atoms through the electron-lattice interaction contributes to the annihilation of single interstitials. This effect is also observed in Ni as a large cross section for radiation annealing, and a decrease of the damage efficiency. On the other hand, in Cu and Ag thin foils, we find that lattice defects are produced not only through elastic interactions, but also through a process strongly associated with electron excitation. In the latter process, the defect production cross section is proportional to Se1.7 in Cu and Se1.5 in Ag. The nearly quadratic dependence of the cross section on Se suggests that the mutual Coulomb repulsion of ions positively charged by electron excitation causes the defect production.

Electronic excitation induced sputtering of insulating and semiconducting oxides by high energy heavy ions

Abstract We have measured the electronic sputtering yields of eight samples of insulating and semiconductor oxides by high energy heavy ions for systematic investigation of the electronic excitation effect on atomic displacement, applying a carbon-film collector method. We have found that the sputtering yields, which include the oxygen contribution, increase super-linearly with the electronic stopping power S e and that they are larger by 30–2000 than the calculated yields based on the elastic collision cascade. In the plot of the sputtering yields at a given electronic stopping power versus the band gap E g , the upper limits of the yields are suggested and the maximum yields follow E g 4 dependence.

Hardening of Fe–Cu alloys at elevated temperatures by electron and neutron irradiations

Abstract Comparative experiments using 2.5 MeV electron irradiations to Fe–Cu model alloys have been undertaken to study the gamma-ray induced hardening of RPV steels. Hardness changes induced by Cu precipitates for Fe–0.6%CU model alloy was confirmed. The difference between electron and neutron irradiation hardening was very small on a dpa basis except for very low dpa.

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.

Effect of damage cascades on the irradiation-induced amorphization in graphite

Abstract In-situ observation of the irradiation-induced amorphization and measurements of the critical amorphization dose in graphite under irradiation with ions and electrons have been performed to reveal the effects of damage cascades. The critical amorphization dose ( D c ) increases exponentially with temperature and shows an apparent critical amorphization temperature ( T c ). T c increases with increase in mass of projectiles and with decrease in energy of ions. Dose rate dependence of D c and T c was also observed. No amorphization was detected above 860 K. Accumulation of vacancies and/or interlayer carbon molecules is attributable to the amorphization, while annealing of amorphous regions is dominated by kinetics of lattice defects. Defect annealing is associated with recombination of interlayer carbon molecules with vacancies. Damage cascades leave high density of interlayer molecular clusters ((C 2 ) n , n = 1, 2, 3…) and stable di-vacancies, resulting in stable amorphous regions at high temperature.

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.

Irradiation effects on MgB2 bulk samples and formation of columnar defects in high-Tc superconductor

Abstract Irradiation effects on MgB2 were studied to investigate the pinning property of this new material. It is pointed out that flux pinning at grain boundaries is important for MgB2 bulk samples. We confirmed this with electron irradiation. Degradation of inter-grain coupling by electron irradiation resulted in degradation of pinning properties. Heavy ion irradiation was also accomplished on MgB2. Columnar defects introduced by the irradiation improve pinning in higher field regions. Concerning the formation of columnar defects in high-Tc superconductors, we conducted a systematic analysis of Bi2212 single crystal with different irradiation ions. Applying the time dependent line source model to our results, only a third of the electronic losses contributed to the formation of columnar defects.

Electronic sputtering of oxides by high energy heavy ion impact

Abstract We have measured sputtering yields of electrically insulating oxides, i.e. amorphous SiO 2 , Yb-doped SrCeO 3 and SrTiO 3 in order to investigate the electronic excitation effects on ion-induced atomic displacements, applying a carbon (C)-film collector method. It is found that the sputtering yields are larger by a factor of ∼1000 than the calculated sputtering yields due to the elastic collision cascade, indicating a huge contribution of the electronic excitation effects to the sputtering yields. It also appears that the sputtering yields scale with the square of the electronic excitation density and that the sputtering yields increase steeply with increasing the band gap of the oxides.