Heisenbergʹs critical mass calculations for an explosive nuclear reaction

Based on the declassified records of Heisenbergʹs Farm-Hall “lecture”, we are attempting in the present work to follow Heisenbergʹs thoughts and results and show in a more transparent way how he arrived at his surprisingly quite accurate estimation of the critical mass of a reflected U235 sphere undergoing explosive neutrons fission. We conclude with a discussion regarding the contraversy concerning the so-called German Uranium bomb and the role played by Heisenberg and C.F. von Weizsäcker in this connection during and after the war.