SCHOOL OF ENGINEERING, TOHOKU UNIVERSITY Driving Force THE POWER TO MAKE TOMORROW INTERVIIEW REPORT
SCHOOL OF ENGINEERING, TOHOKU UNIVERSITY Driving Force THE POWER TO MAKE TOMORROW INTERVIIEW REPORT

Developing a new radiation sensor
using semiconductors.
From Rokkasho to the world.

Professor
Department of Quantum Science
and Energy Engineering
Graduate School of Engineering,
Tohoku University
Keitaro Hitomi REPORT #37

© School of Engineering, Tohoku University

A hub for nuclear research, education, and regional industry promotion.

Situated at the base of the Shimokita Peninsula in the eastern part of Aomori Prefecture, Rokkasho is regarded as a strategic area for Japan’s energy sector, hosting nuclear facilities including a reprocessing plant, a national oil reserve base, and large-scale wind power generation facilities that take advantage of its Pacific coastline’s geographic location.

In 2010, the Tohoku University Cyclotron and Radioisotope Center Rokkasho Branch opened in this area, which later became the Department of Quantum Science and Energy Engineering Rokkasho Branch of the Graduate School of Engineering. “This branch has three missions: nuclear research, education, and regional industry promotion,” says Professor Keitaro Hitomi, who has been working at the branch since its opening and has contributed to its development. The branch has two laboratories: the Nuclear Fuel Science Division which works on advanced separation technologies for radioisotopes contained in high-level radioactive liquid waste; and the Advanced Radiation Application Division which develops advanced usage technologies for the application of separated radioisotopes in wide-ranging fields from engineering to medicine. Moreover, it accepts working graduate students from nearby nuclear-related facilities, actively engaging in education and research.

For his current activities, Professor Hitomi’s base is the Aomori Prefecture Quantum Science Center (QSC), established in 2017. He says, “QSC is equipped with research facilities that can be applied in engineering, medical, and agricultural fields, including a cyclotron (circular accelerator) that can supply accelerated proton beams to experimental devices. In terms of research environment, QSC is on par with the Aobayama Campus in Sendai. Rokkasho also has the advantage of easier access to materials and equipment necessary for research, since it hosts facilities such as a nuclear fuel reprocessing plant and a nuclear fusion energy research institute. Furthermore, it’s quiet compared to the hustle and bustle of a large city, which allows us to concentrate on our research.”

QSC also offers accommodation, attracting many researchers from overseas. “Young researchers come here to engage in research exchange—some of these are the students of my colleagues when I was a visiting researcher at the University of Michigan in the US, and some are students of researchers I’ve met at conferences. This is quite indicative of Tohoku University’s “Open Doors” policy,” Professor Hitomi says.

Developing a gamma ray detector using a thallium bromide semiconductor.

In 2024, Professor Hitomi achieved one of his research goals with the publication of his paper on developing a gamma ray detector using a thallium bromide semiconductor. Gamma rays, which are high-energy photons, have extremely high penetrating power, leading to their widespread application in fields such as medicine and engineering. Especially in medicine, gamma rays are used in positron emission tomography (PET), which is effective in the early detection of cancer and in brain function imaging.

Currently, germanium semiconductors are widely used in gamma ray detection. In contrast, Professor Hitomi’s group has been developing detectors using thallium bromide semiconductors. Professor Hitomi explains the background this way: “Germanium semiconductor sensors require continuous cooling with liquid nitrogen, which requires large-scale equipment. I thought, couldn’t we make a smaller sensor that operates at room temperature even if it has slightly lower resolution? That’s when we shifted our attention to thallium bromide semiconductors which have the highest efficiency in gamma ray absorption.”

The detector Professor Hitomi and his team initially developed required switching the applied voltage direction after a certain period of operation to maintain its performance. To eliminate this requirement, improvements were made including applying a special treatment on thallium bromide crystals. They succeeded in making the detector work stably for longer periods of time, and these results were published in his paper in 2024.

Professor Hitomi says, “In terms of the development of gamma ray sensors using thallium bromide semiconductors, the sensor itself is almost complete.” According to him, the next goal is applying the sensor to next-generation semiconductor PET devices and gamma cameras (devices that receive gamma rays emitted by radioactive drugs administered into a human body as signals, computer-process these signals and turn them into images). “Application of the thallium bromide semiconductor detector makes it possible to pinpoint a lesion’s location more accurately. Moreover, this detector has high sensitivity to gamma rays, so we can expect reduced radiation exposure for the patient. However, the size of the sensor we developed is currently about 1 square centimeter. To view images of a person’s brain or body, we need a larger, more stable, high-performance device. In terms of imaging, our research has just begun,” he explains.

School of Engineering, Tohoku University Driving Force, The Power to Make Tomorrow. INTERVIIEW REPORT

An encounter during his undergraduate years led to research into compound semiconductors.

Professor Hitomi’s primary research is in thallium bromide compound semiconductors. Unlike semiconductors composed of one element such as silicon, compound semiconductors made by combining two or more elements have characteristics such as high-speed operation, high heat resistance, low power consumption, and light-emitting properties. They are widely used in high-frequency devices for smartphones, blue LEDs, and laser diodes for optical fiber communications.

Professor Hitomi’s first encounter with thallium bromide semiconductors can be traced back to his undergraduate days at the Electronic Engineering Course of the Faculty of Engineering of the Tohoku Institute of Technology. He says, “In our lab that was producing semiconductor crystals and devices, there were groups researching semiconductors made of silicon, and there were those working on various compound semiconductors. At the time, the star in our lab was a lead iodide semiconductor made of a lead and iodine compound. Even though I was supposed to study lead iodide, I was advised to try doing something different to start with, so that’s how I got involved with thallium bromide.”

Thallium bromide crystals have been produced since the beginning of the 20th century. Because infrared rays easily pass through them, they have been used in infrared lenses, infrared prisms and materials for windows. American researchers have also reported that thallium bromide can be used to measure radiation. Professor Hitomi says, “To use thallium bromide as a semiconductor material, high purity crystals must be produced by thoroughly removing impurities from the crystals. Drawing on previous research, I increased the purity by using the zone refining method wherein such crystals were repeatedly melted in a quartz tube using an electric furnace. This method yielded surprisingly high purity thallium bromide crystals with an even better performance. I published these findings in 1998 when I was studying for my master’s degree at the Tohoku Institute of Technology.”

“Before publishing my paper, I would read papers and think ‘Wow, there are amazing people out there.’ But now I’m the one writing and publishing,” Professor Hitomi laughs. He worked really hard on his experiments, achieved world-class results, and compiled them in a paper. This sequence of events then determined his subsequent path as a researcher.

Nothing is impossible if you do it with passion.

When Professor Hitomi entered the Electronic Engineering Course of the Faculty of Engineering of the Tohoku Institute of Technology, he had a vague idea of possibly becoming an electrical engineer in the future. The turning point happened in his sophomore year during a hands-on class in a lab. He says, “We were asked to make a semiconductor sensor that measured radiation, and we were allowed to use Tohoku University’s experimental equipment such as accelerators. Welcoming students from other universities such as myself, the lab members did the preparations, experiments, and research with us. This too embodies Tohoku University’s “Open Doors” policy. Meeting Tohoku University professors, graduate students, and undergraduate students, I experienced firsthand and was stimulated by the passion that they poured into their research, and I still remember thinking, ‘I didn’t realize there was such passion in research!’”

According to Professor Hitomi, the interesting thing about engineering is alternating classroom learning and hands-on experiments as you proceed through your studies. “As for me, I make everything from scratch—from developing semiconductor materials to producing sensors, so I have full responsibility for the results. This is what’s interesting about my research, and it’s the rewarding part as well. As a researcher, it’s been my experience that nothing is impossible if you do it with passion. My message to highschoolers is this: it doesn’t matter what it is, just jump in headfirst. You’ll surely find something interesting, including challenges, as well as the joy of achievement in it.”

Professor Hitomi is determined to “promote new industries in Rokkasho using technologies developed at the Rokkasho Branch.” He says, “Without preconceptions and assumptions, I will continue to wholeheartedly engage in research and contribute to realizing one of the branch’s missions—promoting regional industry.”