Hiroki Tobimatsu

Helium trapping in aluminium near the critical dose on blister formation

Blistering and flaking caused by energetic He ions emitted from the plasma in fusion reactors possibly contribute to first-wall erosion. In order to study their characteristics, the numbers of He atoms trapped in He-ion-irradiated Al samples have been measured by a He atom measurement system and every sample has been observed by a scanning electron microscope. The samples have been prepared from a polycrystalline plate and irradiated with 20 keV He ions at room temperature. The saw-tooth like variation of the trapped He atoms with the dose has three edges corresponding to the blistering, flaking and double flaking, respectively. The critical doses for the three events are found to be 4 × 1021, 7 × 1021, 12 × 1021 He atoms m−2, respectively. The average number of He atoms included in an event is 5.4 × 1010 He atoms in the case of the blistering and 2.1 × 1011 He atoms in the case of flaking.

Helium trapping and surface deformation of helium-ion-implanted aluminium

Abstract Helium ions accelerated to 20 keV were implanted into Al samples at room temperature. The formation of blisters and flakes at the surface was observed as a function of He + irradiation fluence by scanning electron microscopy and the number of He trapped in the Al was also determined. To examine the influence of microstructure studies were carried out for 400°C, 2 h (well-annealed) samples, 300°C, 0.5 h (moderately-annealed) samples, and compared to previous results for as-received (cold-rolled) Al. The number of trapped He varies in a saw-toothed form with He ion fluence for the samples with flakes and the surface deformation is predominantly by flaking for the well-annealed samples. The moderately-annealed samples undergo blistering and flaking. Results are discussed from the viewpoint of a correlation between the predominant surface deformation mode and the near-surface microstructural condition.

Absolute Calibration by Helium-Ion-Implanted Samples for Helium Accumulation Method Applied to Neutron Dosimetry

A He accumulation method applied to neutron dosimetry is a useful means to measure a fast-neutron fluence in a high-flux neutron field. The number of He atoms produced by (n, xα) reactions should be absolutely measured in this method. One of the methods of the calibration of a He atom measurement system is a standard He gas method which is based on the measurement of the pressure, volume and temperature of the He gas. The uncertainty in measuring the number of He atoms is largely influenced by the standard gas purity. In this paper, a He-ion-implanted sample method has been developed as a new method for the calibration. The number of implanted He ions can be determined by measuring the current of ions analyzed by a magnetic field where uncertainty of the measurement can be made reasonably small as it depends only on the ion current measurement. The two methods have been compared by using Al samples implanted with He ions. Both the methods can be applied as a mutually complementary calibration each. The He...

He 集積法による中性子照射量測定のための He 絶対量測定装置

In the helium accumulation method for neutron dosimetry, it is applied that the number of helium atoms produced through (n,xα) reaction is proportional to neutron fluence in fast neutron irradiation fields. Besides, activities measuring in the activation method saturate to the value depending on their half-lives. A helium atom measurement, system for detecting of helium atoms accumulated in neutron-irradiated samples has been constructed and tested. The samples are vaporized in a furnace to release helium contained in them. The number of helium atoms is measured with a quadrupole mass spectrometer calibrated by a standard helium gas supply which is consist of a known-volume vessel, a pressure gauge and a thermometer. It has been proposed in this work that helium-ion implanted aluminum disks are applied to calibrate the helium atom measurement system. This method has succeeded to obtain results agreed with the standard helium gas supply system.