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Documenting Hiroshima of 1945: In October, study conducted of rooftiles melted by A-bomb’s thermal rays

by Maho Yamamoto, Staff Writer

In October 1945, Yoshio Suga, a professor at Tokyo Imperial University (present-day University of Tokyo) who died in 1985 at the age of 83, arrived in Hiroshima for the work of investigating the radiant heat and blast pressure resulting from the detonation of the atomic bomb. Mr. Suga was an expert in applied physics and member of a mechanical and metal engineering subcommittee, one of nine making up a special task force established by the Japan Ministry of Education’s Scientific Research Council for investigation of damages resulting from the atomic bombing.

Mr. Suga’s study notebook, which started with an entry dated October 5, is now held at the Hiroshima Peace Memorial Museum. His focus was on rooftiles that had been melted near the hypocenter, resulting in blisters forming on their surfaces. In a notebook entry dated October 14, 1945, he wrote about how he had “collected one rooftile near Shima Hospital” as he walked amidst the ruins. In the same day’s entry regarding Shirakami Shrine, located around 500 meters from the hypocenter, he wrote, “Few of the rooftiles in this vicinity have blistered surfaces.” For his work of studying the blast from the atomic bombing, he made sketches of misaligned stone pillars of Motoyasu Bridge and a stone guardian lion-dog at Hiroshima Gokoku Shrine.

Starting on October 8, Mr. Suga also spent some time in Nagasaki City. It was not an easy task to investigate the two ruined A-bombed cities while they were under occupation by Allied forces. According to Mr. Suga’s personal notes collected in Genshi Bakudan (in English, ‘Atomic Bomb’), published in 1973, “Rough-hewn Allied soldiers snatched my watch and my camera. My eyes lit up when I heard the rumor that there was a place selling baked sweet potatoes, with words insufficient for expressing the hunger, fatigue, and poor lodging I experienced at that time.”

Mr. Suga and Masaichi Majima, a professor at Tokyo Imperial University who served as head of the subcommittee, put together a study report with other subcommittee members. As detailed in the report, rooftiles with melted surfaces caused by radiant heat from the atomic bombing were observed within around a 700-meter radius from the hypocenter in Hiroshima.

When he looked at how much heat would be required to reach the same condition by placing A-bombed rooftiles he had collected in the area into a small, electric furnace, he found the temperature to be between around 1,200 and 1,250 degrees Celsius. However, different from the A-bomb explosion, it took the furnace at least 10 minutes to attain that level of heat. The report concluded it would be “appropriate to interpret those temperatures as a minimum value.”

The report revealing the ferocity of the A-bomb’s thermal rays, which took countless lives of citizens, was compiled in the Collection of Reports on the A-bomb Damage, published in 1953, the year after the end of Japan’s occupation.

(Originally published on October 22, 2024)

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