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Knowledge Co-Creation - Profiles of researchers

From Space to the Human Body--
Challenging the Frontiers of High-Energy Radiation Physics

Jun Kataoka
Associate Professor, Faculty of Science and Engineering, Waseda University

Fermi Space Telescope - Shedding Light on High Energy Astrophysics

X-rays and gamma-rays are the same electromagnetic waves as visual light, but possess much higher energy that cannot be seen by the human eye. High-energy astronomy is a field of astrophysics that searches where in the universe such high-energy photons are produced, how it is formed, and its origins. And within that field is “gamma-ray astronomy”, my major, in which we “examine” the universe with gamma-rays, which contain 100 million times the high-energy of visible light.

Figure 1. Fermi Gamma Ray Space Telescope The Fermi telescope project was led by America's Stanford Linear Accelerator Center (SLAC), with a five nation research team containing researchers from the US, Japan, Italy, France and Sweden. From equipment development to data analysis and management, the Japanese team made a huge contribution. Participating as full members (formal members) contributing to the development of the LAT extraction unit prior to launch, was a team made up of researchers from Hiroshima University, Tokyo Institute of Technology, Japan Aerospace Exploration Agency (JAXA), and the sole representative from a private university, Waseda's own, Associate Professor Kataoka.

In the past few years, high-energy astronomy has been gaining attention as a hot field of science. The reason behind this was the June, 2008 launch of the Fermi gamma-ray space telescope (Fermi satellite for short) (Fig. 1). The Fermi satellite has excellent sensitivity that is about 100 times better than a previous satellite mission, the Compton Gamma-ray Observatory (CGRO) launched in 1990s. In only a year since its launch, Fermi satellite has discovered more than 1,500 astrophysical objects that emit gamma-rays. This has contributed to us further understanding celestial emissions in our Galaxy, such as those from supernova remnants (the remains of a star's final explosion), ultra-compact stars including pulsars (neutron stars), and diffused emissions from interaction between interstellar gases and high-energy cosmic-rays, and furthermore, a giant black hole and gamma-ray burst far beyond our Galaxy.

Figure 2. New-Type Gamma Ray Galaxies Discovered by the Fermi Telescope. Due to all-sky monitoring ability with a wide field of view 10 times that of conventional telescopes, various gamma-ray objects are being discovered. Through research of the group led by Associate Professor Kataoka, a joint international research group including NASA, two new-type “gamma-ray galaxies” have been found. Irrespective of the fact that the galaxies were bright enough to be picked up by CGRO satellite built in the 1990s, because this is the first time to be discovered, there is the possibility that the phenomenon of gamma-rays appearing and disappearing has been occurring over this 10 year time span. (Press release, May 30, 2009)

By continuous observation of Fermi satellite over years, we expect to discover thousands of gamma-ray sources. The scientists in the world are so excited because most of the gamma-ray sources detected with Fermi satellite is brand-new objects, and only about 200 gamma-ray sources were known by CGRO. The ability of Fermi satellite is not limited to just observing astrophysical objects, but also provide tight constraints on dark matter annihilation, and providing a limit on the variation of the speed of light etc, greatly influencing fundamental physics. The Fermi space telescope was ranked second in the 2009 “Breakthrough of the Year” category in the American journal, “Science”, for its involvement in the progress of astrophysics.

Trying to catch gamma-rays in outer space is more difficult than trying to pick up a single grain from a thousand grains of sand. Moreover, amount of gamma-rays that are emitted from astrophysical objects are much smaller than noise, which inevitably requires delicate technology. The Fermi satellite is equipped with two types of gamma-ray detectors, the Large Area Telescope (LAT) and Gamma Bursting Monitor (GBM). Of these, the Large Area Telescope (LAT) was jointly developed by Japanese team led by Hiroshima University, with all the silicone sensors manufactured by Hamamatsu Photonics K.K. In other words, it isn't an overstatement to say that the materialization of the Fermi telescope's advanced observational capacity is due to the excellent Japanese technology, and Japanese members should be proud of their contributions.

Discovery of Gamma-rays from the Giant Lobes in Radio Galaxy

Extremely high-energy gamma-rays can be produced by particles of even higher energy. It is known that many high-energy particles called cosmic-rays exist, especially in outer space. Cosmic-rays were discovered in the 1910s, but we are yet to come to a conclusion on where these particles are born in space, or how they are formed. It can be said that determining the origin of cosmic-rays is one of our biggest issues for the future. Several ideas, such as acceleration of particles in pulsars, which rotate in “ultra” fast cycles from a few milliseconds to a few seconds, and the strong shockwaves producing supernova explosions, have been proposed. But unfortunately, these objects are so small in size. For example, neutron stars are only 10 kilometers in diameter. Even if these kind of “small” stars were able to accelerate relatively low energy cosmic-rays, it would be impossible for them to produce energetic particles called “highest-energy cosmic-rays”. Larger, more effective accelerators are definitely lurking somewhere in the universe.

Figure 3. Discovery of a Giant Particle Cloud (lobe) Emitting Gamma Rays. In joint research with Hiroshima University's Professor Yasushi Fukazawa, Associate Professor Kataoka, using data from the Fermi telescope, and discovered giant gamma ray objects (particle cloud) in the Centaurus A galaxy region, which went beyond gamma ray objects that were already known. (Press release, April, 2010)

After the discovery of the new-type of gamma-ray galaxies in 2009 (refer to Fig. 2 above), joint research with Hiroshima University in 2010 has lead the detection of diffuse gamma-rays from a giant particle cloud (called “lobes”) in the vicinity of a galaxy (Centaurus A), approximately 12 million light years away. This cloud is so enormous that it would take 2 million years to travel from one end to the other, even at the speed of light, and is 50 times the size of the galaxy itself (Fig.3). Centaurus A is known to be a giant black hole (active galactic nucleus; AGN) with a mass 100 million times that of our sun, and produces particle beams (called jets). It is imaginable that the giant lobe is made from high-energy particles carried by jets, but nobody could predict that the particles would emit gamma-rays because they “cool” (lose energy) as they expanded. With the discovery that gamma-rays are emitted from the whole cloud, we now understand that the particles, even inside the lobe, continue to be accelerated. It may be inside such giant clouds where the highest energy cosmic-rays are efficiently produced.

In addition, the Fermi satellite has made various discoveries such as old pulsars being “recycled” and coming to life again, giving off strong gamma ray emissions, and galaxies flourishing with star formations, called starburst galaxies, producing gamma rays. More than 60 articles have been published, including two in “Nature” magazine, and eight in “Science”. The Fermi team is a large organization with over 500 people, including graduate school students and post-doctoral scholars, but in the middle of it all is the Japanese team, including Waseda University, which I think deserves a special mention for taking the lead in many major articles.

Enjoying Cooperation with Researchers in the World

My involvement in astrophysics and high-energy astronomy originally began with my interest in observing the stars as a child. As a primary school student, I would spend long hours on the veranda in the middle of winter, peering through my telescope, and ended up being hospitalized for a month with nephritis.

While doing graduation research at university, I spent half a year in an elementary particle experiment laboratory, and I was so impressed by preciseness and smartness of these physical experiments. I wanted to study astronomy, but I also thought the preciseness of physics was important, so I decided to learn high-energy astrophysics. It may be only me, but to me, particle physics was “too inflexible”, and on the other hand, I felt that astronomy was “seductive” education, like a girl chasing after her dreams (laughs). “High-energy astrophysics” is astronomy with a scent of particle physics, but when I was a student, the field of gamma-ray astrophysics had yet to be fully established in Japan, so I started out by entering X-ray astronomy and gradually shifted my interest toward the higher energy gamma-ray astrophysics.

Generally speaking, celestial objects which emit gamma-rays also produce visible light, infrared and X-rays, all types (wavelengths) of light simultaneously. In other words, to look at gamma rays alone is a case of being unable to see the trees for the forest, and by becoming involved in self-centered, one-man research, can bring about misgivings. On the contrary, by linking up and cooperating with researchers of other wavelengths, you can construct, above all, a strong research group. The interesting part of this field is that you can build a positive, cooperative team, not only from within Japan, but with overseas researchers as well, discussing and writing articles together. This is a real feeling but, I get the feeling that low-energy physics researchers have a calm, laid back temperament, while the more a researcher becomes involved in high-energy physics, the rougher he tends to become. Is human energy inversely proportional to wavelengths?

Also, I believe, the low walls separating the theorists and observers, and thriving cooperation, is a good point of high-energy astrophysics. Frankly speaking, I am an observer rather than theorist, but if I can't find a theory or model to agree with observation data, I try rewriting them, or develop new calculation codes. I have been doing this kind of work since I was writing my doctorate thesis, so I am able to conduct joint research from the same viewpoint as theoretical researchers. We invite theoretical professors to our research society, or are invited ourselves, and continue progress on our research by helping each other.

An observer, on one hand may be seen as conducting basic research, but on the other hand, we must develop experimental and observation equipment, so we are actually very busy. But is it only the Japanese researchers who are this busy? At North American and European research institutions, the theorists and observers are entirely made up of scientists, and don't appear to do their own soldering or develop circuits. Specialist engineers are employed to develop equipment, and scientists usually take the stance of only complaining about the completed blueprints and specifications. I enjoy building my own detectors and designing circuits, so I am never bothered by spending a day in the laboratory, but even so, I do sometimes envy overseas researchers who can become immersed in basic research alone. I feel that the reason behind Japanese publishing fewer articles than westerners comes down to, even before mentioning the English language barrier, the difference in the environment for research advancement. But my personal opinion is that, “commanders not at the scene of development should stand down”. That being the case, I may be a “pure Japanese” researcher by nature.

Application of Leading Edge Technology in Nuclear Medicine

Figure 4. Waseda University Developed High-Sensitivity APD Array for Next Generation PET scanner. In Conventional PET (positron-emission tomography), the resolution is about 5mm, but with Associate Professor Kataoka's technology, it has become possible to raise the accuracy closer to theoretical limits with a resolution of 0.9mm. Costs have also been drastically cut through mass production of APD (photodiodes), and reducing the unit size to less than 10cm.

It is even harder for me, as in my case, I have also been involved in research development to be applied in cancer detection equipment for medical use, using the development technology of gamma-ray detection cultivated in astrophysics. Since 2006, with backing from the Japan Science and Technology Agency (JST) and the New Energy and Industrial technology Development Organization (NEDO), we have successfully developed new generation photo-sensors like avalanche photodiodes (APD) (Fig. 4) that could be used in various experiments, with Hamamatsu Photonics and JAXA. By installing this technology in next generation PET scanner, we can produce an epoch-making contribution to early cancer detection, and aim at making Japanese technology the global standard. Current PET scanner has a poor resolution and is only able to detect 5-10mm large cancers, but the APD-PET equipment we have developed, has achieved the utmost limit of the so-called sub-millimeter.

At present, it could be said that the distribution of my research stands at 30% for basic research using the Fermi gamma-ray space telescope, 30% for research and development of radiation detectors for nuclear medicine, and 40% for development of observation equipment to be installed in Japan's X-ray observation satellite, ASTRO-H, planned for launch in 2014. Simply put, lab work for the observation equipment development takes time, so it receives a higher distribution of my effort. But in my opinion, it would be nice to aim for 300% effort, putting 100% into each of them. To proceed in various fields at the required pace, it is especially important to quickly switch your brain over. No matter how much time you spend on lab work, there is always measurement “waiting time”. Depending on how you spend this time, whether you take a break for a cup of tea, or carry on with a little data analysis as a different program, the outcomes will be completely different.

Although it is an applied physics laboratory, physics and engineering run side-by-side simultaneously, and the students in the lab, like myself, involve themselves in various areas. There are students who want to research medicine and go to the National Institute of Radiological Research (NIRS) in Chiba to study, and also those who visit JAXA. The educator's motto is to go on-site as much as possible, and attend faculty students' experimentations. I thought those kind of teachers were great when I was a student, so I would like to become like that myself.

Jun Kataoka
Associate Professor, Faculty of Science and Engineering, Waseda University

Born in 1972. Graduated from Faculty of Science Physics Department, University of Tokyo in 1995 and completed his Master's at the Science and Technology Department, Graduate School of University of Tokyo in 1997, before receiving his doctorate from the same department in 2000. Doctor of Science. After spending time as an assistant and assistant professor at the Tokyo Institute of Technology Graduate School of Engineering, he entered his present position in 2009. Majors in gamma ray astrophysics and radiation applied physics. Participated as a full member of the America-Japan-Europe joint research program for the Fermi Gamma-ray Space Telescope, publishing major articles which gained worldwide attention. Awarded cosmic physics grant in 2001, and the Astronomical Society of Japan's research grant in 2004.

Kataoka Laboratory

WASEDAResearch Promotion Division, Waseda University  http://www.waseda.jp/rps/