THE night the B-29 bombers came, Ryohei Nakane had been enriching uranium for Japan's "super bomb".
By the next morning - April 13, 1945 - all that remained of his samples and his laboratory at the Riken Institute was charred, splintered wood and broken glass.
For nearly six decades, historians have been unable to solve one of the mysteries of Japan's World War II A-bomb project: How close were Japanese scientists to building the bomb before the US air raid stopped them?
All official records were believed to have been burned in the closing days of the war, forcing historians to piece together an answer from less reliable clues.
However, long-lost wartime documents are setting the record straight.
The 23 pages of Imperial Army papers returned to Japan in April offer convincing evidence that Japanese scientists were years away from completing their 20-kiloton A-bomb which would have had more force than the US's 15 kiloton bomb dropped on Hiroshima but less than the 22-kiloton one that hit Nagasaki.
Historians say not only had Japan's scientists underestimated how much of the rare isotope uranium-235 they would need for the bomb, they misunderstood the mechanics of an atomic explosion.
"The documents are one-of-a-kind. We can finally prove that even if Japan had built a bomb, it would not have been powerful at all," said Masakatsu Yamazaki, a professor of science history at the Tokyo Institute of Technology, who analysed the papers.
"And it might have taken them another decade to complete one."
Dr Nakane has been telling a similar story for years.
"We were carrying out our research so leisurely. None of us thought we would finish before the war ended," Dr Nakane, now 83, said.
More than a half century after the war, Japan's A-bomb project is only a historical footnote. Few Japanese have even heard of their wartime government's nuclear program.
Scientists and military officers who were there have written memoirs and talked publicly about their work.
But, over the years, speculation and conspiracy theories have clouded the facts and raised doubts about the participants' accounts.
Japan's own efforts to build a bomb are difficult for many here to accept because of the bombings of Hiroshima and Nagasaki, and the widespread feeling that Japan would never have even considered such a brutal attack.
The treasure trove of wartime papers could change that.
Sneaked out of the country just after the war by former Tokyo University professor Kazuo Kuroda, who left for the US, the papers were sent to the Riken Institute, north of Tokyo, by Professor Kuroda's widow months after his death in Las Vegas, Nevada, in 2001.
The documents - the only surviving record of Japan's A-bomb research - read like a blueprint for the bomb.
Scholars estimate that Japan's army spent $1 million - a pittance compared to the roughly $4 billion the US shelled out for the Manhattan Project.
A parallel Japanese navy project, which had no chance of success, cost $300,000.
So stretched were the country's resources that, at one point, military leaders considered scrapping a battleship to supply steel to the army's A-bomb team.
by Editor on March 11, 2003 at 9:23 PM