Hidefumi Takeshita

Conductivities of a sintered pellet and a single crystal of Li2O

Abstract The conductivities of a single crystal and a sintered pellet of Li 2 O were measured in argon atmosphere by the two-terminal technique with an ac Wheatstone bridge in the temperature range 300–1150°C. The conductivity of Li 2 O depended strongly on the concentration of the OH − impurity in the sample and the transition temperature T tr between the extrinsic and intrinsic regions increased with as increasing amount of OH − impurity in the sample. The activation energies of conductivity in the extrinsic and intrinsic regions were 0.4–0.5 eV and 1.2–1.3 eV, respectively.

In situ tritium recovery experiment from lithium oxide under high neutron fluence

In situ measurement of tritium release during irradiation provides a more realistic understanding of the tritium release mechanism than that obtained by the post-irradiation annealing method. The tritium release curves reached saturated values after irradiation of a few days at the reactor operation of 10 MW. The saturated values were in good agreement with the predicted values of the tritium generation rate obtained from neutronics calculation. Temperature dependence of tritium release under the temperature of 550°C at the pellet center was interpreted by the decomposition of LiOH(T) at the pellet surface. The ratio of tritium gas and tritiated water (HT/HTO) was strongly affected by the moisture generated from the piping system and the pellet. The retained tritium in Li2O was 0.5–0.9 wppm after the final stage of the irradiation of 760°C and the burnup of 0.24% of total Li.

Reaction of several iron and nickel based alloys with sintered Li2O Pellets

Abstract The reaction of type 316 stainless steel, Incoloy 800, Hastelloy X-R, Inconel 600 and pure Ni with sintered Li 2 O pellets has been studied between 800 and 1100°C under dynamic vacuum. The reaction products were analyzed by means of metallographic, microprobe and X-ray diffraction methods. The reactions proceeded measurably between 800 and 950°C and appreciably at 1000°C, being greatest with Incoloy 800 and least with Hastelloy X-R. Among the primary alloy constituents, chromium was exclusively attacked by lithium and oxygen diffusing from the Li 2 O into the alloys to form LiCrO 2 . This phase grew into a reaction zone (subscale) of uniform thickness beneath the surface of each alloy. Preferential growth of LiCrO 2 along the grain boundaries was observed only in the case of Inconel 600 below 950°C. On the other hand, iron diffused toward the Li 2 O pellets to form volatile Li 5 FeO 4 . However, any reaction product associated with Ni was not detected and Ni metal was little attacked by the Li 2 O pellet over the whole range of reaction temperature.

Analysis of hydrogen in Nb/Cu multilayers using ion beams

Abstract The dependence of the H concentration on the layer thickness in H-charged Nb/Cu multilayer samples has been studied using 15 N nuclear reaction analysis and high-energy elastic recoil detection. This system shows a smooth increase in H concentration in the Nb layers with increasing layer thickness. The H concentration shows a tendency towards saturation as the thickness increases. The relative concentrations are independent of the total amount of incorporated H in the measured range up to about 40 at.%. These results are also independent of the stacking order of the layers.

Charge states of fast heavy ions in glancing collisions with aligned atoms in single crystals

Abstract Spectroscopy of ion-induced secondary electrons emitted from single crystals allows observation of high-energy shadowing which is the initial stage of ion channeling. The high-energy shadowing effect is sensitive to short-range (∼ 0.1 A) screening of the ions nuclear charge by inner-shell electrons and, accordingly, reflects the charge states of the ions moving in the crystal. This article reports recent progress in the study of charge states of MeV/u ions in glancing collisions with crystal atoms.

Compatibility between several heat resistant alloys and sintered Li2O in static helium gas environment

The reaction of sintered Li2O discs with several commercial heat resistant alloys has been investigated under the conditions of 3.3 × 104Pa (13 atm) static He gas atmosphere in the temperature range of 500 and 750° C. Reaction products were identified by X-ray diffraction analysis to be two phases of Li5FeO4 and LiCrO2. The former was dominant below 650° C and the latter was dominant above 650° C. The activation energies of the reaction were determined by the Arrhenius plots for weight changes and sub-scale thickness measurements. The reactivity of the four Fe-Ni-Cr alloys decreased according to the order of Incoloy 800, 316 SS, Hastelloy X-R and Inconel 600. Grain boundary penetration was observed above 500° C for Incoloy 800, 550° C for 316 SS and 600° C for Inconel 600. There was no grain boundary penetration in Hastelloy X-R.

RBS/channeling analysis of damage and annealing processes of Nb-implanted TiO2

Abstract Single crystalline TiO2 (0 0 1) was implanted with Nb+ ions RT (room temperature) and at 130 K to the doses of 1.1 × 1016 and 0.9 × 1016Nb/cm2, respectively. The damage and its recovery processes during isochronal annealing were analyzed by the RBS/channeling method. As a result, the TiO2 crystal was found to be amorphized by 130 K implantation while a crystalline part remained after RT-implantation. The partially damaged layer produced by RT-implantation almost recovered with one annealing stage at 500°C. The layer amorphized by 130 K implantation showed two annealing stages, i.e., 300–600°C and above 800°C. Between these two stages, 800°C annealing made the substitution-ality of Nb decrease to almost 0%, which suggests a phase transition of the crystal. After annealing at 1200°C, the mixed oxide of NbxTi1−xO2 was formed in either sample, which was confirmed by RBS angular scan measurement and XPS analysis.

Stopping-powers and straggling of 15N ions for nuclear reaction analysis at 6.385 MeV

Abstract Stopping-powers and straggling has been measured for 15 N incident ions at 6.4 MeV using the well known 1 H( 15 N,αγ) 12 C resonant nuclear reaction at 6.385 MeV. Samples have been prepared using electron beam evaporation of various elements on H terminated silicon substrates under UHV conditions with high layer quality. The H monolayer remains stable at the silicon substrate during evaporation and ion irradiation. The layer thicknesses were ranging from 500 to 2000 A. At this thickness range the energy straggling is between 15 and 40 keV (σ) and is the main contribution to the measured width of the H profile. Surface roughness, interface quality and Doppler broadening are negligible. The measured H profiles were fitted with a Gaussian normal distribution function. In the case of target materials, as e.g. Nb, which incorporate H easily, the layers were H charged and an integrated Gauss function was used for fitting. The stopping-powers and straggling were compared with ZBL data for electronic stopping, Bohr straggling and TRIM simulations for nuclear stopping and straggling and with former experimental data obtained with same method. The data for stopping-power and straggling showed remarkable differences to former measurements.

Mass spectrometric study of the evaporation of LiCrO2 as a corrosion product in the compatibility experiment of Li2O pellets with Fe-Ni-Cr alloys

Abstract The measurement of the vapor species and pressures over Li 5 FeO 4 as a corrosion product in the compatibility experiment of Li 2 O pellets with Fe-Ni-Cr alloys has been done in the temperature range 1200–1500 K by means of the Knudsen effusion mass spectrometric technique. The evaporation process of liquid Li 5 FeO 4 was established and expressed by the reaction Li 5 FeO 4 (1) = LiFeO 2 (s) + 4 Li(g) + O 2 (g). The enthalpy of formation of Li 5 FeO 4 at 298 K was derived to be 1950 kJ/mol from the third-law treatment. Although a congruent process was suggested for the sublimation, it could not be confirmed in detail.

L X-ray spectra of Fe and Cu by 0.75 MeV/u H, He, Si and Ar ion impacts

Abstract Spectra of L X-rays emitted from Fe and Cu targets have been measured by 0.75 MeV/u H, He, Si and Ar ion impacts with a crystal spectrometer. The spectra produced by H and He ion impacts are very similar. The X-ray transition energies in the Fe target for Lι, Lη, Lα1,2 and Lβ1 lines are determined to be 616, 628, 704.9 and 717.7 eV, respectively, which are in good agreement with those given in the reference. The spectra produced by heavier Si and Ar ions have more complicated structures due to multiple vacancy production. These spectra are compared with the calculated ones based on the multiconfiguration Dirac-Fock method. The origin of the broadening of the Lα1,2 line to the lower energy is attributed to one 2p plus one 3d electron vacancy production.