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Logs of Nobel Prize-winning physicist Yukawa show clues on wartime nuke research

A page of Hideki Yukawa's diary from the year 1945 is pictured here on Dec. 21, 2017. (Mainichi)

Journals kept around the end of World War II in 1945 by Japan's first Nobel Prize winner, physicist Hideki Yukawa (1907-1981), were released by the Kyoto University Yukawa Institute for Theoretical Physics on Dec. 21.

The diaries include entries about atomic bomb research, so-called "F Research," which Yukawa almost never discussed during his lifetime, and about the dropping of the atomic bomb on Hiroshima and the political and social climate of the time, prompting researchers to deem the journals an important primary historical source.

During the Pacific theater of World War II, the then Japanese military commissioned physicists to carry out research into the development of nuclear weapons. The Imperial Japanese Navy commissioned Bunsaku Arakatsu (1890-1973), a nuclear physicist at Kyoto Imperial University (present-day Kyoto University) to work on "F Research," referred to as such by taking the first letter of the word "fission." It is generally said, however, that the research was far from reaching the production of nuclear weapons.

Yukawa won the Nobel Prize in physics in 1949 for his theory of mesons. While it has been widely known that the Imperial Japanese Navy had theoretically supported "F Research" being conducted at Kyoto Imperial University, there had hardly been any of Yukawa's own writing about it.

Hideki Yukawa is pictured at his Columbia University lab in this file photo taken in February 1951. (Mainichi)

Following Yukawa's death in 1981, his family donated 15 notebooks including his research logs written between 1938 and 1948. Following analyses of the contents, the Yukawa Institute released the contents of three B5-sized notebooks that had been written in 1945. "F Research" is said to have begun around 1944, but the notebooks show that in early 1945, Yukawa participated in numerous meetings related to the project.

The first time the term "F Research" appears in the notebooks is in an entry dated Feb. 3, 1945, in which Yukawa writes that he consulted with Arakatsu. On June 23 of that year, Yukawa met with Arakatsu and 10 other researchers. In an entry dated July 21, Yukawa writes, "I went to Biwako Hotel," in Otsu, Shiga Prefecture. "F Research" is not mentioned in the entry, but July 21 is the date on which the Kyoto Imperial University and the Imperial Japanese Navy held a joint meeting at the hotel -- corroborating Yukawa's presence at the meeting.

In an entry for Aug. 7, the day after the dropping of the atomic bomb on Hiroshima, Yukawa writes that he had been asked for commentary on the atomic bomb by a newspaper, but that he declined. On Aug. 9, however, he cited a newspaper, writing, "The power of the heat waves from the bomb is said to have reached several kilometers. The bomb was dropped by parachute, and exploded several hundred meters above the ground."

In his journals, Yukawa expressed great interest in what was happening in the war. When there were air raids on Osaka in June 1945, he wrote that the burning of the city could be seen from Kyoto. Yukawa also showed concern for the devastation of Iwo Jima and in the Battle of Okinawa.

"Especially because the journals correspond to a period in which Yukawa refrained from speaking publicly, the writings provide great clues into what kind of research he was doing at the time," Masakatsu Yamazaki, an honorary professor on the history of science at Tokyo Institute of Technology says. "This should be made into an opportunity to promote the conservation and release of historical sources about wartime military research."

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