d extended the scope of the experiments, the picture became
more confused rather than clarified. Careful studies soon showed that
the activities did _not_ decay logarithmically--which means that they
were caused by mixtures, not individual pure substances--and the
original four activities reported by Fermi grew to at least nine.

As a matter of fact, the way out of the difficulty had been indicated
soon after Fermi's original announcement. Dr. Ida Noddack pointed out
that no one had searched among the products of Fermi's experiment for
elements _lighter_ than lead, but no one paid any attention to her
suggestion at the time. The matter was finally cleared up by Dr. Otto
Hahn and F. Strassmann. They were able to show that instead of uranium
having small pieces like helium nuclei, fast electrons, and super-hard
x-rays, knocked off as expected, the atom had split into two roughly
equal pieces, together with some excess neutrons. This process is called
nuclear fission. The two large pieces were unstable and decayed further
with the loss of electrons, hence the [beta] activity. This process is
so complicated that there are not, as originally reported, only four
half-lives, but at least 200 different varieties of at least 35
different elements. The discovery of fission attended by the release of
enormous amounts of energy led to feverish activity on the part of
physicists and chemists everywhere in the world. In June 1940, McMillan
and Abelson presented definite proof that element 93 had been found in
uranium penetrated by neutrons during deuteron bombardment in the
cyclotron at the University of California Radiation Laboratory.

The California scientists called the newly discovered element neptunium,
because it lies beyond the element uranium just as the planet Neptune
lies beyond Uranus. The particular isotope formed in those first
experiments was {93}Np<239>; this is read neptunium having a nuclear charge
of 93 and an atomic mass number of 239. It has a half-life of 2.3 days,
during which it gives up another electron ([beta] particle) and becomes
element 94, or plutonium (so called after Pluto, the next planet beyond
Neptune). This particular form of plutonium ({94}Pu<239>) has such a long
half-life (24,000 years) that it could not be detected. The first
isotope of element 94 to be discovered was Pu<238>, made by direct deuteron
bombardment in the Berkeley 60-inch cyclotron by Radiation Laboratory
scientists Seaborg, McMil