Jeremy Bernstein in Inference:
MEITNER SPENT HER first Christmas in exile in Kungälv, a small town near Gothenburg. She was joined there by her nephew, Otto Frisch, a physicist who had taken refuge at the Niels Bohr Institute in Copenhagen. In his biography, Frisch described their famous walk in the woods and their discussion about the nature of nuclear fission. Before recounting this conversation, a few background details may prove useful.
In 1932, James Chadwick discovered the neutron. It was immediately clear that this new particle could penetrate and transform nuclei due to its electrical neutrality. Experimental studies were subsequently undertaken by Enrico Fermi and his group in Rome. They examined a number of elements before they turned their attention to uranium and began bombarding it with neutrons. But Fermi already knew what he was going to find before any experiments had taken place: neutron absorption would transform the uranium nuclei. As we now know, uranium becomes neptunium, which in turn becomes plutonium. Fermi was certain that his team had created these transuranic elements. Before she left Germany, Meitner, along with Hahn and a young collaborator named Fritz Strassmann, were engaged in repeating these experiments. After her departure, Hahn and Meitner had managed to maintain communication. It was Hahn and Strassmann’s latest experimental results that Meitner and Frisch were discussing when they took a walk in the woods.