'Epsilon' was a Allied undertaking at the end of World War II in which 10 German scientists, all believed to have worked on Germany’s nuclear programmes, were held at Farm Hall, a house in Godmanchester, England, which had been outfitted with secret microphones and recording equipment (3 July 1945/3 January 1946).

The German scientists captured and detained during 'Epsilon' were Erich Bagge, Kurt Diebner, Walther Gerlach, Otto Hahn, Paul Harteck, Werner Heisenberg, Horst Korsching, Max von Laue, Carl Friedrich von Weizsäcker and Karl Wirtz who, after their capture in Germany, were flown to England.

All of the scientists were shocked when they learned of the atomic bombing of Hiroshima on 6 August 1945, and some initially doubted that the report was genuine. They were told at first of an official announcement that an 'atomic bomb' had been dropped on Hiroshima, with no mention of uranium or nuclear fission. Harteck said that he would have understood the words 'uranium' and 'nuclear bomb', but he had worked with atomic hydrogen and atomic oxygen and thought that US scientists might have succeeded in stabilising a high concentration of separate atoms in a bomb that would have had a tenfold increase in explosive power over that of a conventional bomb.

The scientists then considered how the US bomb was made and why Germany did not produce one. The transcripts seem to indicate that the scientists, and in particular Heisenberg, had either overestimated the amount of enriched uranium that an atomic bomb would require or consciously overstated it, and that the German project was at best in a very early, theoretical stage of thinking about how atomic bombs would work.

Some of the scientists indicated that they were happy that they had not been able to build a nuclear bomb for Adolf Hitler while others, such as Diebner and Gerlach, more sympathetic to the Nazi ideology were dismayed at having failed. One of those who were grateful that Germany had not built a bomb, Hahn criticised those who had worked on the German project, saying that 'if the Americans have a uranium bomb then you are all second-raters'.

With the exception of Hahn and Harteck, who were chemists, the scientists were physicists, and all except von Laue had been involved in the German nuclear project. While he was incarcerated at Hall Farm, Hahn was awarded the 1944 Nobel Prize for Chemistry for the discovery of nuclear fission.

A group of eight people, led by Major T. H. Rittner, was responsible for eavesdropping, recording, copying and translating the words of the German scientists. Only relevant technical or political information, representing some 10% of all words heard, was recorded, transcribed and translated. The recordings were made with six to eight machines on shellac-coated metal discs. After their contents had been transcribed, the discs were deleted and reused.

The transcripts were sent as reports to London and the US consulate, and were then forwarded to Major General Leslie R. Groves, head of the 'Manhattan Project' in 24 reports totalling more than 250 pages.