Accelerator based characterization of fusion reactor materials

It is widely recognized that light atoms such as hydrogen isotopes and helium play the critical role in the fusion reactor materials as well as those of the fission reactor. They are stored in the reactor materials to cause deterioration such as hydrogen enbrittlement and swelling, and when bombarded with plasma particles, the atoms stored are reemitted into the plasma to deteriorate the performance of the core plasma.

One of the most effective method to analyze non-destructively the distribution of these atoms in the near-surface region of material is the accelerator analysis using MeV ions relying on various nuclear interaction with material atoms; Rutherford Backscattering Spectroscopy (RBS), Elastic Recoil Detection Analysis (ERDA), Nuclear Reaction Analysis (NRA) and Particle Induced X-ray Emission (PIXE) analysis. The resolution of the method is no less than tens of nm, and its application is limited to samples with microscopically flat surface.

In our laboratory, efforts are made to improve and diversify the method to extend its application to more realistic samples, especially to those found in fusion experimental devises.

Nuclear fusion reaction in condensed matter

In 1989 an astonishing interpretation of the experimental results was reported on newspapers: heat produced in the Pd cathode during an electrolysis of heavy water exceeds the input power, and it is due to the nuclear reaction. This is called the cold fusion or the Fleishmann-Pons effect. In spite of extensive experiments made since then, this phenomenon has not yet been confirmed by other researchers.

Being stimulated by this phenomenon, research on nuclear fusion reactions in solid materials was begun from a refined point of view. Prof. A. Takahashi et al., Osaka University, have reported greatly enhanced rates of 3-body deuterium nuclear reaction: the ratio of the D+D+D reaction rate to the D(d,p)t reaction rate reached 10^-4 for the DD(d,³He)t channel. Prof. J. Kasagi et al., Tohoku University, have reported the ratio reaching 10^-7 for the DD(d,⁴He)pn channel. In the latter laboratory they have also made precise measurements on low-energy D(d,p)t reaction rate in various metals/metal oxides: the reaction rate at the deuteron energy of 2.5 keV in a PdO sample was about 50 times greater than that calculated with use of the conventional cross section. Recently Prof. H. Ikegami, Uppsala University, has claimed "buffer energy nuclear fusion"; enormous enhancement of 10¹⁴ of Li(d,n)2⁴He reaction driven by thermodynamic force.

In our laboratory the fusion reactions in condensed matter induced by bombardment of keV - MeV hydrogen isotope ions are investigated in conjunction with the isotope distribution analysis in the targets. A target system which enables in situ control of deuterium absorption and simultaneous characterization of the sample by accelerator-based analyses has been prepared to investigate greatly enhanced nuclear reactions in low energy region in metal deuterides. An enhanced rate of D(d,p)t reaction has been observed during 7.5-25 keV D²⁺ irradiation of deuterated Au/Pd and CaO/Pd samples. Ratio of the measured yield to calculated one depends on energy, charge-to-mass ratio of the incident ion and irradiation history.

Interaction of low energy hydrogen isotopes with wall materials in fusion devices

Between a core plasma with a temperature of several hundred million K and the containing wall with a temperature of several hundred K in a nuclear fusion plasma confinement device, there exists a buffer zone called the Scrape-Off-Layer. Energy and ash particles produced in the core plasma are transferred to the wall through the SOL, and impurity and fuel ions/atoms reemitted from the wall material stream back into the core plasma through the SOL, which induces the crucial effects on the confinement of the core plasma.

The SOL plasma is thus very important both for the plasma confinement and the wall material performance. As a method to diagnose the SOL plasma, we have made improvement of a collector probe method (see the last three references below), where carbon collector plates/sheets imersed in the SOL are analyzed with use of accelerator analyses after several shots of JFT-2M Tokomak operation, to investigate spatial distribution and temporal behavior of the SOL particle density and temperature.

As a wall material low-Z(atomic number) elements are preferable from an energetical point of view. This is because Bremsstrahrung loss of the plasma energy scales as Z². We are now making experimental study on properties of Li interacting with low energy hydrogen isotopes. Based on thermal desorption mass spectra (TDS), etc., not only nuclear but also chemical characteristics are are discussed.

Behavior of radicals induced by charged particle bombardment of polymers and application to production of functional materials

When a polymer film is irradiated with an ion beam, a lot of hydrogen molecules are liberated leaving chemically active tracks in the substrate. The evolution of the hydrogen release from polymers has been examined in terms of track formation (ref.1). As a result, a number of carbon radicals, C=C bonds and cross linking are formed in the track.

The radicals are chemically active and could be utilized to produce functional materials with use of graft polymerization technique. One of the examples of the groups introduced into the grafted polymer chains is chelate-forming groups such as amidoxime groups suitable for collecting doubly charged ions of rare metal elements, such as uranium and vanadium dissolved abundantly in seawater. They could be also usefull for recovery of pollutive elements from seawater.

Low LET (linear energy transfer) radiations such as gamma rays and electron beams are usually used to produce carbon radicals in the polymer, from which the graft polymerization is initiated. We are studying ion produced radicals to open up a possibility of unique application of the graft polymerization technique.

Production of pulsed high-power beams of quantum particles

Ion beams with particle energy of several keV - MeV have potential applications in a variety of fields. They could be used not only as an energy driver for inertial confinement nuclear fusion but also as an impulse source and an instantaneous heat source in the field of material science.

Photon beams with photon energy of several tens of eV or greater also, XUV/soft-X-ray laser, have a variety of potential applications. They could be used not only as an energy driver for material processing, e.g., lithography, but also as a probing light for a microscope visualizing live cells.

Using the ns-GW (nanosecond-gigawatt) power source, ERIDATRON-2, we are making research on production of pulsed ion beams (PIB) and photon beans/laser.

Intense (kA) PIB of protons and medium mass ions, e.g., Na⁺, with energy of several hundred keV have been successfully produced (ref.1, 2 and 5 below). Mechanism of ablation induced by the PIB in the near-surface region of the irradiated material has been investigated (ref.3, 4 and 6) in connection with its application to deposition of thin-films having unique properties from the ablation plume.

We have recently been studying on pulsed capillary discharge to realize a compact high-power XUV laser (ref.7).

Accelerator-based analysis of interaction of poluting materials and marine microorganisms

We have started research on application of accelerator analysis technique to investigate dynamic behavior of pollutive elements in marine environment. The techniques used for the theme (1), RBS, ERDA, NRA and PIXE, are also employed in this research.

Organotin compounds such as tributyltin oxide, tributyltin chloride (TBTCl), and triphenyltin chloride are potent antifouling agents. Many self-polishing antifouling paints containing these compounds were developed and used worldwide in 1970s to prevent fouling on ships' bottoms. In the same era, the toxicity of such organotin compounds was reported in France (Champ & Lowenstein, 1987). Severe shell thickening in Pacific oysters and abnormally progressed male characteristics, e.g., formation of a penis and a sperm duct in females of common dogwhelks (imposex) were observed in Arcachon Bay, France. Horiguchi et al. (1995) reported that 30 of the 38 species of Japanese gastropods were affected by imposex, and it could be induced in adult females of Thais clavigera exposed to seawater containing just 1 ppt (10^-12 g/ml) of organotin compounds. As a result, the possibility that tributyltin compounds can act as endocrine disrupters has been recognized (Oberdorster et al., 1998). Organotin compounds also have mutagenicity (Hamasaki et al., 1993). Therefore, the International Maritime Organization has prohibited the usage of organotin compounds as anti-fouling agents after 2008.

Organotin compounds released from ships' bottoms are stable, and exist in marine environments for prolonged periods (Tao et al., 1999), and this may cause an increase in frequency for the occurrence of TBT resistant marine microorganisms (Suzuki et al., 1992). A TBTCl resistant marine Vibrio, which can grow in the presence of 125 microM of TBTCl, has been isolated from coastal seawater (Fukagawa et al., 1992).

In order to clarify the degradation processes in TBTCl resistant marine microorganisms, development of the analytical instrument is necessary to detect cell associated TBTCl and the small amounts of the biodegradation products. Accelerator analyses can provide direct information on the composition of atomic elements, including isotopes. Moreover, there are few pre-treatment processes required when these methods are used for the measurement of biological samples.

We have successfully applied the accelerator analyses to the quantification of Sn, Cl and Na elements associated with a TBTCl resistant marine microorganism that was newly isolated from sediment in a ship's ballast water tank.

Application of environmental radioactivity to functional activation of energy systems

Growing interest is taken in effect of natural radioactivity not only on biological but also inorganic matter in recent years. We are looking for unique application of concentrated energy specific to radiation available in the environment.