But let us get back to the bombardment of uranium with neutrons research
that Fermi had begun. After he had reported his work, other physicists
repeated it and also got a variety of beta particles and were also
unable to decide what was going on.
[Illustration: _Lise Meitner and Otto Hahn in their laboratory in the
1930s._]
One way to tackle the problem was to add to the system some stable
element that was chemically similar to the tiny traces of radioactive
isotopes that might be produced through the bombardment of uranium.
Afterwards the stable element could probably be separated out of the
mixture and the trace of radioactivity would, it was hoped, be carried
along with it. The stable element would be a “carrier”.
Among those working on the problem were Otto Hahn and his Austrian
co-worker, the physicist Lise Meitner (1878-1968). Among the potential
carriers they added to the system was the element, barium, which has an
atomic number of 56. They found that a considerable quantity of the
radioactivity did indeed accompany the barium when they separated that
element out of the system.
A natural conclusion was that the isotopes producing the radioactivity
belonged to an element that was chemically very similar to barium.
Suspicion fell at once on radium (atomic number 88), which was very like
barium indeed as far as chemical properties were concerned.
Lise Meitner, who was Jewish, found it difficult to work in Germany,
however, for it was then under the rule of the strongly anti-Semitic
Nazi regime. In March 1938 Germany occupied Austria, which became part
of the German realm. Meitner was no longer protected by her Austrian
citizenship and had to flee the country and go to Stockholm, Sweden.
Hahn remained in Germany and continued working on the problem with the
German physical chemist Fritz Strassman (1902- ).
Although the supposed radium, which possessed the radioactivity, was
very like barium in chemical properties, the two were not entirely
identical. There were ways of separating them, and Hahn and Strassman
busied themselves in trying to accomplish this in order to isolate the
radioactive isotopes, concentrate them, and study them in detail. Over
and over again, however, they failed to separate the barium and the
supposed radium.
Slowly, it began to seem to Hahn that the failure to separate the barium
and the radioactivity meant that the isotopes to which the radioactivity
belonged had to be so much like barium as to be nothing else _but_
barium. He hesitated to say so, however, because it seemed unbelievable.
If the radioactive isotopes included radium, that was conceivable.
Radium had an atomic number of 88, only four less than uranium’s 92. You
could imagine that a neutron being absorbed by a uranium nucleus might
make the latter so unstable as to cause it to emit 2 alpha particles and
become radium. Barium, however, had an atomic number of 56, only a
little over half that of uranium. How could a uranium nucleus be made to
turn into a barium nucleus unless it more or less broke in half? Nothing
like that had ever been observed before and Hahn hesitated to suggest
it.
While he was nerving himself to do so, however, Lise Meitner, in
Stockholm, receiving reports of what was being done in Hahn’s laboratory
and thinking about it, decided that unheard-of or not, there was only
one explanation. The uranium nucleus _was_ breaking in half.
Actually, when one stopped to think of it (after getting over the
initial shock) it wasn’t so unbelievable at that. The nuclear force is
so short-range, it barely reaches from end to end of a large nucleus
like that of uranium. Left to itself, it holds together most of the
time, but with the added energy of an entering neutron, we might imagine
shock waves going through it and turning the nucleus into something like
a quivering drop of liquid. Sometimes the uranium nucleus recovers,
keeps the neutron, and then goes on to beta-particle emission. And
sometimes the nucleus stretches to the point where the nuclear force
doesn’t quite hold it together. It becomes a dumbbell shape and then the
electromagnetic repulsion of the two halves (both positively charged)
breaks it apart altogether.
It doesn’t break into equal halves. Nor does it always break at exactly
the same place, so that there were a number of different fragments
possible (which was why there was so much confusion). Still, one of the
more common ways in which it might break would be into barium and
krypton. (Their respective atomic numbers, 56 and 36, would add up to
92.)
Meitner and her nephew, Otto Robert Frisch (1904- ), who was in
Copenhagen, Denmark, prepared a paper suggesting that this was what was
happening. It was published in January 1939. Frisch passed it on to the
Danish physicist Niels Bohr (1885-1962) with whom he was working. The
American biologist William Archibald Arnold (1904- ), who was also
working in Copenhagen at the time, suggested that the splitting of the
uranium nucleus into halves be called “fission”, the term used for the
division-in-two of living cells. The name stuck.
In January 1939, just about the time Meitner and Frisch’s paper was
published, Bohr had arrived in the United States to attend a conference
of physicists. He carried the news of fission with him. The other
physicists attending the conference heard the news and in a high state
of excitement at once set about studying the problem. Within a matter of
weeks, the fact of uranium fission was confirmed over and over.
One striking fact about uranium fission was the large amount of energy
it released. In general, when a very massive nucleus is converted to a
less massive one, energy is released because of the change in the mass
defect, as Aston had shown in the 1920s. When the uranium nucleus breaks
down through the ordinary radioactive processes to become a less massive
lead nucleus, energy is given off accordingly. When, however, it breaks
in two to become the much less massive nuclei of barium and krypton (or
others in that neighborhood) much more energy is given off.
It quickly turned out that uranium fission gave off something like ten
times as much nuclear energy per nucleus than did any other nuclear
reaction known at the time.
Even so, the quantity of energy released by uranium fission was only a
tiny fraction of the energy that went into the preparation of the
neutrons used to bring about the fission, if each neutron that struck a
uranium atom brought about a single fission of that 1 atom.
Under those conditions, Rutherford’s suspicion that mankind would never
be able to tap nuclear energy probably still remained true. (He had been
dead for 2 years at the time of the discovery of fission.)
However, those were not the conditions.