18
1 Energy Release in Nuclear Reactions, Neutrons, Fission, and Characteristics …
family in Austria, her Austrian citizenship had protected her against German antiSemitic laws, but this protection ended with the German annexation of Austria in
March, 1938. Hahn and Strassmann carried on with the work and corresponded with
her by letter, but her career was essentially destroyed.
By December, 1938, Hahn and Strassmann had refined their chemical techniques
and had become convinced that they were detecting barium (element 56) as a result
of slow-neutron bombardment of uranium. Barium is adjacent to lanthanum in the
periodic table, and is another common product of uranium fission. On December 19,
Hahn wrote to Meitner (who was by then living in Sweden) of the barium result, and
two days later followed up with a second letter indicating that they were also detecting
lanthanum. By chance, Meitner’s nephew, physicist Otto Frisch, was then working
at Niels Bohr’s Institute for Theoretical Physics in Copenhagen. Frisch traveled to
Sweden to spend Christmas with his aunt, and they conceived of the fission process
around Christmastime, working out an estimate of the energy that could be expected
to be released. By this time, Hahn and Strassmann had already submitted their barium
paper to the journal Naturwissenschaften (Hahn and Strassmann 1939). Otto Hahn
was awarded (solely) the 1944 Nobel Prize in Chemistry for the discovery of fission;
Meitner and Strassmann did not share in the recognition.
Soon after returning to Copenhagen on New Year’s Day 1939, Frisch informed
Niels Bohr of the discovery. Bohr was about to depart for a semester at Princeton
University, and it is he who carried the word of the discovery to the New World on the
same day, January 16, that Meitner and Frisch submitted a paper to Nature with their
interpretation of the fission process. This was published on February 11 (Meitner
and Frisch 1939), by which time the process had been duplicated in a number of
laboratories in Europe and America.
Otto Frisch is credited with appropriating the term “fission” from the concept of
cell division in biology to describe this newly-discovered phenomenon. He is also
credited with being the first person to set up an experiment to deliberately demonstrate
it and measure the energy of the fragments, work he did in Copenhagen on Friday,
January 13, 1939. After replicating the Hahn & Strassmann uranium results, he also
tested thorium. This element proved to act like uranium in that it would fission under
bombardment by fast neutrons, but at the same time to act unlike uranium in that it did
not do so at all when bombarded with slow neutrons. This asymmetry would catalyze
a crucial revelation on the part of Niels Bohr a few weeks later as to which isotope of
uranium is responsible for slow-neutron fission. Uranium comprises two isotopes, the
“even/even” (in the sense Z/N) one
238 U, and the much rarer “even/odd” one
235 U,
whereas thorium has only one naturally-occurring isotope,
232 Th, an “even/even”
nuclide. Bohr realized that, as a matter of pure logic,
235 U must be responsible for
slow-neutron fission, as it is the one “parity” of isotope that thorium does not possess.
The difference in behavior between
238 U and
235 U under neutron bombardment
and how it relates to parity is examined further in Sect. 1.9. In the meantime, we
examine in more detail the energetics of the fission process itself.
1 Energy Release in Nuclear Reactions, Neutrons, Fission, and Characteristics …
family in Austria, her Austrian citizenship had protected her against German antiSemitic laws, but this protection ended with the German annexation of Austria in
March, 1938. Hahn and Strassmann carried on with the work and corresponded with
her by letter, but her career was essentially destroyed.
By December, 1938, Hahn and Strassmann had refined their chemical techniques
and had become convinced that they were detecting barium (element 56) as a result
of slow-neutron bombardment of uranium. Barium is adjacent to lanthanum in the
periodic table, and is another common product of uranium fission. On December 19,
Hahn wrote to Meitner (who was by then living in Sweden) of the barium result, and
two days later followed up with a second letter indicating that they were also detecting
lanthanum. By chance, Meitner’s nephew, physicist Otto Frisch, was then working
at Niels Bohr’s Institute for Theoretical Physics in Copenhagen. Frisch traveled to
Sweden to spend Christmas with his aunt, and they conceived of the fission process
around Christmastime, working out an estimate of the energy that could be expected
to be released. By this time, Hahn and Strassmann had already submitted their barium
paper to the journal Naturwissenschaften (Hahn and Strassmann 1939). Otto Hahn
was awarded (solely) the 1944 Nobel Prize in Chemistry for the discovery of fission;
Meitner and Strassmann did not share in the recognition.
Soon after returning to Copenhagen on New Year’s Day 1939, Frisch informed
Niels Bohr of the discovery. Bohr was about to depart for a semester at Princeton
University, and it is he who carried the word of the discovery to the New World on the
same day, January 16, that Meitner and Frisch submitted a paper to Nature with their
interpretation of the fission process. This was published on February 11 (Meitner
and Frisch 1939), by which time the process had been duplicated in a number of
laboratories in Europe and America.
Otto Frisch is credited with appropriating the term “fission” from the concept of
cell division in biology to describe this newly-discovered phenomenon. He is also
credited with being the first person to set up an experiment to deliberately demonstrate
it and measure the energy of the fragments, work he did in Copenhagen on Friday,
January 13, 1939. After replicating the Hahn & Strassmann uranium results, he also
tested thorium. This element proved to act like uranium in that it would fission under
bombardment by fast neutrons, but at the same time to act unlike uranium in that it did
not do so at all when bombarded with slow neutrons. This asymmetry would catalyze
a crucial revelation on the part of Niels Bohr a few weeks later as to which isotope of
uranium is responsible for slow-neutron fission. Uranium comprises two isotopes, the
“even/even” (in the sense Z/N) one
238 U, and the much rarer “even/odd” one
235 U,
whereas thorium has only one naturally-occurring isotope,
232 Th, an “even/even”
nuclide. Bohr realized that, as a matter of pure logic,
235 U must be responsible for
slow-neutron fission, as it is the one “parity” of isotope that thorium does not possess.
The difference in behavior between
238 U and
235 U under neutron bombardment
and how it relates to parity is examined further in Sect. 1.9. In the meantime, we
examine in more detail the energetics of the fission process itself.