Another line of research at Liverpool was measuring the fission cross section of uranium-235, on which Frisch and Peierls' calculations depended. They had assumed that almost every collision between a neutron of any energy and a uranium-235 nucleus would produce a fission. The value they used for the fission cross section was that published by French researchers in 1939, but data published by the Americans in the 15 March and 15 April 1940 issues of the American journal "Physical Review" indicated that it was much smaller.