1.5 Artificially-Induced Radioactivity and the Path to Fission
17
In October, 1934, Fermi discovered accidentally that if the bombarding neutrons
were caused to be slowed (“moderated”) before hitting the target element by having
them first pass through water or paraffin, the strength of the induced radioactivities
could in some cases be drastically increased. Fermi attributed this to the neutrons
having more time in the vicinity of target nuclei, and hence a greater probability
of reacting with them. As a result, the Rome group began re-investigating all those
elements which they had previously subjected to fast (energetic) neutron bombardment. Uranium was one of many elements which proved to yield greater activity upon
slow neutron bombardment. Ironically, slow neutron bombardment of uranium-238
does create plutonium, which is an excellent material for fueling nuclear weapons;
Fermi initially thought this was happening with fast neutron bombardment, which
tends to lead to fission of this isotope.
The possibility that new elements were being created was treated with some skepticism within the nuclear research community. Among the leaders of that community
were Otto Hahn and Lise Meitner at the Kaiser Wilhelm Institute for Chemistry in
Berlin, who between them had accumulated years of experience with the chemistry
and physics of radioactive elements. In 1935, they and chemist Fritz Strassmann
began research to sort out to what elements uranium transmuted under slow neutron
bombardment. By 1938, the situation had become extremely muddled: No less than
ten distinct half-lives had been identified. To complicate things further, Irène Curie
and Paul Savitch, working in Paris, identified an approximately 3.5 hr beta half-life
resulting from slow neutron bombardment of uranium, an activity which Hahn and
his group had not found. Curie and Savitch suggested that the 3.5 hr decay might
be attributed to thorium, element 90. If this were true, it would mean that neutrons
slowed to the point of possessing less than one eV of kinetic energy (see Sect. 3.2)
were somehow capable of knocking alpha-particles out of uranium nuclei.
Further research by Curie and Savitch showed that the 3.5 h beta-emitter had
chemical properties similar to those of element 89, actinium. This observation would
eventually be realized as another missed chance in the discovery of fission. To
isolate the beta-emitter from the bombarded uranium target, Curie and Savitch used
a lanthanum-based chemical analysis. Lanthanum is element 57, which is in the
same column of the periodic table as actinium. Chemists were long familiar with the
fact that elements in the same column of the table behave similarly as far as their
chemical properties are concerned. That the beta emitter “carried” with lanthanum
in a chemical separation indicated that it must have chemistry similar to lanthanum,
and since the element nearest uranium in the periodic table with such chemistry
is actinium, it was assumed that the beta-decayers must be nuclei of that element.
The possibility that uranium might in fact be transmuting to lanthanum would have
seemed ludicrous, as U and La differ by a factor of nearly two in mass. Curie and
Savitch were probably detecting
141 La, which is now known to have a half-life of
3.9 h.
Hahn, Meitner, and Strassmann resolved to try to reproduce the French work.
Tragically, in July, 1938, Meitner was forced to flee to Holland. Born into a Jewish
17
In October, 1934, Fermi discovered accidentally that if the bombarding neutrons
were caused to be slowed (“moderated”) before hitting the target element by having
them first pass through water or paraffin, the strength of the induced radioactivities
could in some cases be drastically increased. Fermi attributed this to the neutrons
having more time in the vicinity of target nuclei, and hence a greater probability
of reacting with them. As a result, the Rome group began re-investigating all those
elements which they had previously subjected to fast (energetic) neutron bombardment. Uranium was one of many elements which proved to yield greater activity upon
slow neutron bombardment. Ironically, slow neutron bombardment of uranium-238
does create plutonium, which is an excellent material for fueling nuclear weapons;
Fermi initially thought this was happening with fast neutron bombardment, which
tends to lead to fission of this isotope.
The possibility that new elements were being created was treated with some skepticism within the nuclear research community. Among the leaders of that community
were Otto Hahn and Lise Meitner at the Kaiser Wilhelm Institute for Chemistry in
Berlin, who between them had accumulated years of experience with the chemistry
and physics of radioactive elements. In 1935, they and chemist Fritz Strassmann
began research to sort out to what elements uranium transmuted under slow neutron
bombardment. By 1938, the situation had become extremely muddled: No less than
ten distinct half-lives had been identified. To complicate things further, Irène Curie
and Paul Savitch, working in Paris, identified an approximately 3.5 hr beta half-life
resulting from slow neutron bombardment of uranium, an activity which Hahn and
his group had not found. Curie and Savitch suggested that the 3.5 hr decay might
be attributed to thorium, element 90. If this were true, it would mean that neutrons
slowed to the point of possessing less than one eV of kinetic energy (see Sect. 3.2)
were somehow capable of knocking alpha-particles out of uranium nuclei.
Further research by Curie and Savitch showed that the 3.5 h beta-emitter had
chemical properties similar to those of element 89, actinium. This observation would
eventually be realized as another missed chance in the discovery of fission. To
isolate the beta-emitter from the bombarded uranium target, Curie and Savitch used
a lanthanum-based chemical analysis. Lanthanum is element 57, which is in the
same column of the periodic table as actinium. Chemists were long familiar with the
fact that elements in the same column of the table behave similarly as far as their
chemical properties are concerned. That the beta emitter “carried” with lanthanum
in a chemical separation indicated that it must have chemistry similar to lanthanum,
and since the element nearest uranium in the periodic table with such chemistry
is actinium, it was assumed that the beta-decayers must be nuclei of that element.
The possibility that uranium might in fact be transmuting to lanthanum would have
seemed ludicrous, as U and La differ by a factor of nearly two in mass. Curie and
Savitch were probably detecting
141 La, which is now known to have a half-life of
3.9 h.
Hahn, Meitner, and Strassmann resolved to try to reproduce the French work.
Tragically, in July, 1938, Meitner was forced to flee to Holland. Born into a Jewish