Geology Reference
In-Depth Information
The nuclear fission of uranium had been discovered in 1939 by
Enrico Fermi, a Nobel-Prize-winning Italian physicist then
working in America to escape the rise of Fascism. Nuclear
fission is the splitting of the atomic nucleus, which is accom-
panied by the emission of two or three neutrons and the release
of large amounts of nuclear energy. While the process does
occur spontaneously in nature, it can also be induced by bom-
barding nuclei with neutrons. The neutrons released in this
process are then used to induce further fissions, setting up a
chain reaction that must be controlled if it is not to result in
a nuclear explosion.
The potential for nuclear fission to be used as a weapon dur-
ing the war was obvious, but initially there was some question
as to which of the uranium isotopes was responsible for fission.
So Fermi urged Nier to try and separate the uranium isotopes
using his mass spectrometer, and help determine which one
could account for the slow neutron fission he had observed, stat-
ing that to know which was which was 'of considerable theo-
retical and possible practical interest'. When the analysis was
duly performed (using a geological sample originally desig-
nated for age determinations as the source of the uranium) and
uranium 235 was shown to be the culprit, Fermi went o¬ to
build the first nuclear reactor. Following that, in 1941, when it
became apparent that enriched uranium 235 might be used for
making atom bombs, Nier's laboratory contained the only mass
spectrometer in the world that could analyse uranium isotopes.
Inevitably, he was required to perform analyses for various
groups working on such problems and so found no time for
geologically related research. But it was during this period that
Holmes renewed their correspondence.
Intrigued by Nier's results from the Manitoba pegmatite,
Holmes had re-calculated Nier's age data for himself and found
the pegmatite to be 2480 million years old, a value closer to the
older result determined by Nier. He wrote to tell Nier in May
1945, and cautiously expressed his views:
