The rough schematic does not imply that the Nazis built or even were close to building a nuclear bomb, but it shows they had progressed farther toward that goal than is conventionally thought, they said.

The article appears in the June issue of the British monthly Physics World, and can be seen on its website.

The 60-year-old document is part of a report that appears to have been produced just after the end of the war in Europe in May 1945 and describes the work on nuclear weapons that had been carried out during the conflict.

The report is undated and in addition lacks a title page, which means its author or authors cannot be identified. It came from a "private archive," the historians said.

In addition to the rough plan for a bomb, the Nazi report estimates, comparatively accurately, that a plutonium warhead of just over five kilospounds) was needed to achieve critical mass -- the chain reaction that leads to a nuclear blast.

The historians are Rainer Karlsch, an independent historian based in Berlin, and Mark Walker of Union College, Schenectady, New York state.

Karlsch stirred controversy earlier this year when he published a book in Germany, "Hitlers Bombe," in which he claimed that the Nazis had successfully tested a primitive nuclear device in the last days of World War II as Allied troops were closing in on both sides.

The device, tested in Thuringia, eastern Germany, killed several hundred prisoners of war and concentration camp inmates, the book said.

That assertion was scoffed at by other historians, who said that by the end of the war, the Nazis still did not understand some of the physics of chain reaction.

The knowledge gaps include the way in which fast neutrons emitted by plutonium or uranium-235 atoms trigger further fission reactions.

This ignorance in turn caused the Nazi scientists to grossly over-estimate the amount of nuclear material needed to achieve critical mass, according to the critics' view.

Karlsch and Walker rebut this, saying that new sources of historical material, such as documents squirrelled away in Russian archives, are trickling out, showing that the Nazis were farther down the road to acquiring nuclear capability than was thought.

Germany's "uranium project" was launched in 1939 to investigate nuclear reactors, isotope separation and nuclear explosives.

Unlike the US-led Manhattan Project, which harnessed thousands of people and several billion dollars to devise the bombs that were dropped on Hiroshima and Nagasaki in August 1945, the German effort amounted to no more than a few dozen scattered scientists.

Scientific files seized after the war, and bugged conversations of 10 German nuclear scientists held in a British prison camp in 1945, forged the conventional belief, prevailing today, that even though Hitler craved a nuclear bomb, he was still months or more likely years from ever getting one.

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