Young Faculty： Associate Professor Shinichi Yamashita
Young Faculty / 014
Associate Professor Shinichi Yamashita, Uesaka-Yamashita lab., Nuclear Professional School
2008 PhD, Engineering, Department of Quantum Engineering and Systems Science, the University of Tokyo (UTokyo).
2008 Postdoctoral fellow, Advanced Science Research Center, Japan Atomic Energy Agency (JAEA).
2011 Postdoctoral fellow, Quantum Beam Science Directorate, JAEA.
2012 Specially-appointed Assistant Professor, Nuclear Professional School (NPS), UTokyo.
2013 Assistant Professor, NPS, UTokyo.
2017 Associate Professor, NPS, UTokyo.
<About the Research>
Some of you might have impression that ionizing radiations are scaring. This would be probably due to radiation diseases, invisibility of ionizing radiations, or the fact that not all of their features are clarified yet. However, even if exposure to ionizing radiations results in radiation-induced death, energy given by them is very tiny, which corresponds to temperature rise less than 0.01 ℃ assuming that all the energy is used to increase temperature. Ionizing radiations can give tiny energy to any matter including human bodies, however the energy deposition is instantaneous and localized. Temporal and special density of energy deposition is quite high, leading to highly efficient induction of chemical reactions or biological effects. Of course, the “locality” of radiation-induced phenomena is being lost and diluted with time goes on, so it is quite important to know radiation-induced initial dynamics at early stages in order to clarify unrevealed features of ionizing radiations.
Large fraction of issues in nuclear engineering are closely related to ionizing radiations. We have worked through “chemical forms of halide ions (Cl−/Br−/I−) in aqueous solutions under radiation environment”, “gas evolution from water radiolysis under boiling conditions”, “water radiolysis at water/metal oxide interfaces”, and so on. Ionizing radiations are now an essential tool in medicine and industry. For example, ionizing radiations are used in cancer treatment, diagnosis, material processing, micro-/nano-fabrication, and so on. We have worked through “initial stages of radiation enhancement/protection by small amount of additives”, “oxidative and reductive radiation damage to DNA”, “water radiolysis with therapeutic high-energy heavy-ion beams”, “fundamental investigation for development of polymer gel dosimeter”, and so on. Anyhow, it is essential to comprehend details of radiation-induced phenomena in order to minimize bad points of ionizing radiations and to maximize their good points. We are working to clarify radiation-induced initial dynamics. Not only early physical and physicochemical events occurring from picoseconds (10−12 s) to microseconds (10−6 s) but also chemical and biochemical events occurring after microseconds are in our scope.
Specifically, pulse radiolysis setup is used to observe fast phenomena induced by ionizing radiations, and final products in irradiated samples are analyzed after their exposure to ionizing radiations. Pulse radiolysis method requires short pulse of radiation typically from picoseconds to nanoseconds (10−9 s) otherwise fast phenomena cannot be observed. This method enables us to observe directly short-lived reactive species such as radicals (species having unpaired electron(s)) and excited states produced by ionizing radiations. Currently time-resolved UV-Vis absorption spectroscopy is used in such observation.
Recently we have started a new project to install time-resolved resonance Raman spectroscopy apparatus in our pulse radiolysis set up. As mentioned above, time-resolved UV-vis absorption spectroscopy is, of course, a useful tool for our goal however it is expected that time-resolved resonance Raman spectroscopy can provide more detailed information.
In general, peaks in UV-vis absorption spectra are relatively broad and often some of them overlap with others. On the other hand, peaks in Raman spectra are much sharper and can give detailed information of vibrational states related to chemical bonds, three-dimensional structure, and so on. In addition, Raman spectroscopy can be used for non-transparent samples such as dispersion of nanoparticles, suspension, solid-liquid interfaces, etc. while absorption spectroscopy is limited to transparent samples. Thus, this project is a challenge to pioneer into unexplored filed of “radiation effects at interfaces”.
There is another viewpoint of ionizing radiations. They are good and ideal sources of radicals (species having unpaired electron(s)). They can deposit energy to any molecules non-selectively. Radicals are found not only in radiation-induced phenomena but also in photochemistry, atmospheric chemistry, electrochemistry, burning, explosion, pyrolysis, catalysis, surface reactions, etc. Ionizing radiations can be regarded as a tool to investigate radicals. We want to contribute not only to radiation science or nuclear engineering but also to other fields not directly related to ionizing radiations.
Uesaka-Yamashita lab : http://www.tokai.t.u-tokyo.ac.jp/kiki/