Takeshi Nihira

Atomic Displacements by Electron Irradiation in Pyrolytic Graphite

The anisotropic threshold energy for atomic displacements and the displacement cascade process in electron-irradiated pyrolytic graphite are studied in the energy range from 0.12 to 1.0 MeV at three different temperatures by observing the a -axis electrical resistivity increases. The threshold energy T d is represented as \(T_{\text{d}}(\varPsi){=}A\cos^{2}\varPsi+B\sin^{2}\varPsi+C(1-\cos 4\varPsi){\equiv}(A,B,C)\) in eV, where \(\varPsi\) is the angle between the c -axis and the displacement direction. In the 6 and 80 K irradiations T d is given as a set of (23, 30, 0) and (31, 30, -2)±2 eV, and the cascade obeys the Harrison-Seitz Replacement model for \(\varTheta{=}0\)°, but obeys the Kinchin-Pease model for \(\varTheta{=}90\)°, where \(\varTheta\) is the angle between the c -axis and the electron direction. In the 285 K irradiations T d is either (28, 42, 0)±2 eV, or a set of (28, 42, 0) and (32, 42, -1)±2 eV, and the cascade obeys the Harrison-Seitz (no replacement) model for \(\varTheta{=}0\)°, 30°...

Thermal Resistivity Changes in Electron-Irradiated Pyrolytic Graphite

Changes in the a-axis and the c-axis thermal resistivity of pyrolytic graphite caused by electron irradiation at 82 K and by subsequent isochronal pulse annealing have been measured. The additive thermal resistivity at 78.5 K increases as 9.6×102N0.85f and (7.9±0.6)×103N0.75f cm deg/W in the a-axis and the c-axis directions, respectively, in the region of 2 ppm<Nf<70 ppm, where Nf is the concentration of nearly isolated Frenkel defects produced by irradiation. Annealing of the additive thermal resistivity occurs around 100 K and above about 220 K. The characteristics of annealing stages are different in some important respects from those obtained by Goggin and Reynolds. It is inferred that most interstitials do not recombine with vacancies below about 220 K.

Low Temperature Electron-Irradiation Damage and Recovery in Pyrolytic Graphite

The electrical resistivity changes by electron irradiation at 5 K and the isochronal annealing of resistivities in the temperature range 5-85 K (Stage I) in pyrolytic graphite are studied in order to get information on point defects. Both the a -axis and the c -axis resistivity changes by irradiation are explained by the ordinary transport theory with defect scattering. Small recoveries in Stage I are shown to be composed of four substages: I A (5-15 K); I B (15-45 K); I C (45-65 K); and I D (65-85 K). Effects of the irradiating electron energy, the radiation-doping and the graphitization temperature of samples on these substages are also studied. Substages I A and I B may be caused by the correlated rearrangement of close Frenkel pairs, where interstitial atoms form loose coupling with their own vacancies. The activation energy for I A is roughly estimated to be 0.027±0.004 eV. In I C the long-range free migration of interstitial atoms seems to occur.

Stored Energy Release in Electron-Irradiated Graphite

The stored energy release in electron-irradiated pyrolytic graphite was measured during annealing from around 80 K to 320 K by using a method of differential thermal analysis. For converting the recorded temperature difference into the energy release rate, the heat transfer coefficients and heat capacities of samples were measured as a function of temperature. A useful method of data analysis in differential calorimetry is presented; it gives the reproducible base line of stored energy release curves even if the heating rate is not strictly controlled. The energy release curves show peaks near 105, 150, 200, 250 and 295 K. The amount of the energy released around each peak is obtained for a pair of Frenkel defects produced by irradiation. The formation energy of a Frenkel defect and annealing of defects are discussed.

Defect Production by Electron Excitation in Cu and Ag

It has been found in ion-irradiated Cu and Ag thin foils that lattice defects are produced not only through elastic collisions, but also through a process strongly associated with electronic stopping power S e . In the latter process, the defect production cross section is proportional to S e 1.7 in Cu and S e 1.5 in Ag. The nearly quadratic dependence of the defect production cross section on S e suggests that the mutual Coulomb repulsion of ions positively charged by electron excitation causes the defect production in Cu and Ag.

Defect production and recovery in nickel irradiated with energetic ions

Abstract Nickel foils were irradiated with 0.5-1.8 MeV 1 H, 3 He, 4 He, 14 N and 40 Ar ions at liquid helium temperatures. During the irradiations, resistivity changes were measured to the total resistivity change of about 500 nΩ cm, where the saturation behavior of defect production could be observed. After the irradiations, annealing experiments were performed up to 300 K. From these measurements, the defect production cross sections, the spontaneous recombination volumes and the defect recovery spectra were obtained. The damage efficiency, the spontaneous recombination volume, and the total fraction and the structure of stage I recovery depended on the PKA energy.

Radiation annealing in nickel and copper by 100 MeV iodine ions

Radiation annealing by 100 MeV 127 I-ions is studied in pre-doped Ni and Cu samples through the defect production rate measurements below 10 K. The experimental results are analyzed by using a new model which describes the production and radiation annealing of two or more types of defects. The subthreshold recombination cross section and the concentration of doped-in defects are obtained for each type of defects. In Ni, the defect annihilation corresponding to the stage-I thermal annealing occurs during the I-ion irradiation with the subthreshold recombination cross section of 6.5×10 -12 to 1.4×10 -12 cm 2 . In Cu, the stage-I defects are annihilated with the cross section of 4.6×10 -13 cm 2 . The large radiation annealing in Ni shows the contribution of the electron excitation by irradiating ions to the subthreshold recombination of defects through the electron-phonon interaction.

Irradiation and Annealing Effects on the c-Axis Electrical Resistivity of Graphite

Changes in the c -axis electrical resistivity of pyrolytic graphite due to electron irradiations at 80 K and due to subsequent isochronal pulse annealings up to 400 K wave measured. The average carrier mobility in the c -axis direction was calculated from the c -axis resistivity by the use of the carrier density obtained through the a -axis galvanomagnetic property measurements. The c -axis resistivity decreased by irradiation at 80 K. The reciprocal of the average carrier mobility in the c -axis direction increased by 7±2 mV ·sec·cm- 2 per ppm of Frenkel defects. As to thermal annealings, the c -axis electrical resistivity showed peculiar features compared with the a -axis one. The reciprocal of the average carrier mobility in the c -axis direction, however, showed nearly the same changes in relative values as that in the a -axis direction with annealing temperatures.

Defect production and defect saturation behavior in nickel irradiated with heavy ions in the energy range 84–120 MeV

Abstract Nickel foils are irradiated at liquid helium temperatures with 84–120 MeV 12 C, 19 F, 28 Si, 35 C1, 81 Br and 127 I ions. During the irradiation, electrical resistivity changes are measured as a function of ion fluence. The defect production cross section and the recombination volume are determined from this measurement for each irradiation. The effect of electron excitation by ions on the defect production process is discussed.

Non-ohmic resistive state in ion-irradiated YBa2Cu3O7−x

Abstract We have studied the effect of 120 MeV 16 O ion irradiation on the non-ohmic electrical resistive state at 77.3 K in YBa 2 Cu 3 O 7− x . For small measuring currents, the voltage V varies as a power of the current I and the ion-fluence Ω, i.e. V ∼ I n and V∼Ω m , where the exponents n and m are functions of the ion-fluence and the current, respectively. For larger currents, the voltage obeys another power-law: V ∼ I n , where n ′ is lower than n at a given ion-fluence. Analysis of the experimental data usinga model of current-induced unbinding of thermally excited vortex pairs shows that ion-irradiations cause the enhancement of dissociation of the bound vortex-antivortex pairs.