Hahn,
Otto
Hahn, Otto (1879-1968), German physical chemist and Nobel
laureate, best known for his contributions in the field of radioactivity. Hahn
was born in Frankfort am Main and educated at the universities of Marburg and
Munich. In 1911 he became a member of the Kaiser Wilhelm Institute for Physical
Chemistry in Berlin. He served as director of the institute from 1928 to 1945,
when it was taken into Allied custody after World War II. In 1918 he
discovered, with Austrian physicist Lise Meitner, the element protactinium. Hahn, with his coworkers, Meitner and German
chemist Fritz Strassmann, continued the research
started by Italian physicist Enrico Fermi: bombarding
uranium with neutrons. Until 1939 scientists believed that elements with atomic
numbers higher than 92 (known as transuranium
elements) were formed when uranium was bombarded with neutrons. In 1938,
however, Hahn and Strassmann, while searching for transuranium elements in a sample of uranium that had been
irradiated with neutrons, found traces of the element barium. This discovery,
announced in 1939, was irrefutable evidence, confirmed by calculations of the
energies involved in the reaction, that the uranium had undergone fission,
splitting into smaller fragments consisting of lighter elements. Hahn was
awarded the 1944 Nobel Prize in chemistry for his work in nuclear fission. It
was proposed in 1970 that the newly synthesized element number 105 be named hahnium in his honor, but another
naming system was adopted for transuranium elements
with atomic numbers 104 and higher.
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Hahn,
Otto
b. March 8, 1879, Frankfurt am Main, Ger.
d. July 28, 1968, Göttingen,
W.Ger.
German chemist who, with the radiochemist
Fritz Strassmann, is credited with the discovery of nuclear fission. He was awarded the Nobel Prize for Chemistry in 1944 and shared
the Enrico Fermi Award in 1966 with Strassmann and Lise Meitner.
Early life
Hahn was the son of a glazier. Although his parents wanted him to
become an architect, he eventually decided to study chemistry at the University
of Marburg. There Hahn worked hard at chemistry, though he was inclined
to absent himself from physics and mathematics lectures in favour of art and
philosophy, and he obtained his doctorate in 1901. After a year of military
service, he returned to the university as chemistry lecture assistant, hoping
to find a post in industry later on.
In 1904 he went
to London, primarily to learn English, and worked at University College with Sir William Ramsay, who was
interested in radioactivity. While working on a crude radium preparation that
Ramsay had given to him to purify, Hahn showed that a new radioactive
substance, which he called radiothorium, was present. Fired by this early
success and encouraged by Ramsay, who thought highly of him, he decided to
continue with research on radioactivity rather than go into
industry. With Ramsay's support he obtained a post at the University of Berlin.
Before taking it up, he decided to spend several months in Montreal with Ernest
Rutherford (later Lord Rutherford of Nelson) to gain further experience with
radioactivity. Shortly after returning to Germany in 1906, Hahn was
joined by Lise Meitner,
an Austrian-born physicist, and five years later they moved to the new Kaiser
Wilhelm Institute for Chemistry at Berlin-Dahlen.
There Hahn became head of a small but independent department of
radiochemistry.
Feeling that his
future was more secure, Hahn married Edith Junghans,
the daughter of the chairman of Stettin City Council,
in 1913; but World War I broke out the next year, and Hahn was posted to
a regiment. In 1915 he became a chemical-warfare specialist, serving on all the
European fronts.
After the war, Hahn
and Meitner were among the first to isolate
protactinium-231, an isotope of the recently discovered radioactive element protactinium. Because nearly all
the natural radioactive elements had then been discovered, he devoted the next
12 years to studies on the application of radioactive methods to chemical
problems.
In 1934 Hahn
became keenly interested in the work of the Italian physicist Enrico Fermi, who found that when the
heaviest natural element, uranium, is bombarded by neutrons,
several radioactive products are formed. Fermi supposed these products to be
artificial elements similar to uranium. Hahn and Meitner, assisted by the young Strassmann,
obtained results that at first seemed in accord with Fermi's interpretation but
that became increasingly difficult to understand. Meitner
fled from Germany in July 1938 to escape the persecution of Jews by the Nazis,
but Hahn and Strassmann continued the work. By the
end of 1938, they obtained conclusive evidence, contrary to previous
expectation, that one of the products from uranium was a radioactive form of
the much lighter element barium, indicating that the uranium atom had split
into two lighter atoms. Hahn sent an account of the work to Meitner,
who, in cooperation with her nephew Otto Frisch, formulated a
plausible explanation of the process, to which they gave the name nuclear fission.
The tremendous
implications of this discovery were realized by scientists before the outbreak
of World War II, and a group was formed in Germany to study possible military
developments. Much to Hahn's relief, he was allowed to continue with his own
researches. After the war, he and other German nuclear scientists were taken to
England, where he learned that he had been awarded the Nobel Prize for 1944 and
was profoundly affected by the announcement of the explosion of the atomic bomb
at Hiroshima in 1945. Although now aged 66, he was still a vigorous man; a
lifelong mountaineer, he maintained physical fitness during the enforced stay
in England by a daily run.
On his return to
Germany he was elected president of the former Kaiser Wilhelm Society (renamed
the Max Planck Society for the Advancement of Science) and became a respected
public figure, a spokesman for science, and a friend of Theodor
Heuss, the first president of the Federal Republic of
Germany. He campaigned against further development and testing of nuclear
weapons. Honours came to him from all sides; in 1966 he, Meitner,
and Strassmann shared the prestigious Enrico Fermi Award. This period of his life was saddened,
however, by the loss of his only son, Hanno, and his
daughter-in-law, who were killed in an automobile accident in 1960. His wife
never recovered from the shock. Hahn died in 1968, after a fall; his wife
survived him by only two weeks.
Advances in nuclear and subatomic
physics
The 1920s witnessed further
advances in nuclear physics with Rutherford's discovery of induced
radioactivity. Bombardment of light nuclei by alpha particles produced new
radioactive nuclei. In 1928 the Russian-born American physicist George Gamow explained the lifetimes in alpha radioactivity using
the Schrödinger equation.
Discovery
of neutrons
The constitution of the
nucleus was poorly understood at the time because the only known particles were
the electron and the proton. It had been established that nuclei are typically
about twice as heavy as can be accounted for by protons alone and thus have to
contain more than just such particles. A consistent theory was impossible until
the English physicist James Chadwick
discovered the neutron in 1932. He found that alpha particles reacted with
beryllium nuclei, ejecting neutral particles with nearly the same mass as
protons. Almost all nuclear phenomena can be understood in terms of a nucleus
composed of neutrons and protons. Surprisingly, the neutrons and protons in the
nucleus behave to a large extent as though they were in independent wave
functions, just like the electrons in an atom. Each neutron or proton is
described by a wave pattern with peaks and nodes and angular momentum quantum
numbers. The theory of the nucleus based on these independent wave functions is
called the shell
model. It was introduced in 1948 by Maria
Goeppert Mayer of the United States and J. Hans
D. Jensen
of West Germany, and it developed in succeeding decades into a comprehensive
theory of the nucleus.
The interactions of neutrons
with nuclei had been studied during the mid-1930s by the Italian-born American
physicist Enrico Fermi and others. Nuclei readily capture
neutrons, which, unlike protons or alpha particles, are not repelled from the
nucleus by a positive charge. When a neutron is captured, the new nucleus has
one higher unit of atomic mass. If a nearby isotope of that atomic mass is more
stable, the new nucleus will be radioactive, convert the neutron to a proton,
and assume the more stable form.
Nuclear
fission was discovered by the German chemists Otto
Hahn and Fritz
Strassmann in 1938. In fission, a uranium nucleus
captures a neutron and gains enough energy to trigger the inherent instability
of the nucleus, which splits into two lighter nuclei of roughly equal size. The
fission process releases more neutrons, which can be used to produce further
fissions (see the article nuclear
fission). The first nuclear reactor, a device designed to permit controlled
fission chain reactions, was constructed at the University
of Chicago under Fermi's direction, and the first self-sustaining chain reaction
was achieved in this reactor in 1942. In 1945 American scientists produced the
first atomic bomb, which used uncontrolled fission reactions in either uranium
or the artificial element plutonium.
The basic
principle of the fusion weapon (also called the thermonuclear or hydrogen bomb) is to produce
ignition conditions in a thermonuclear fuel such as deuterium, an isotope of
hydrogen with double the weight of normal hydrogen, or lithium deuteride. The Sun may be considered a
thermonuclear device; its main fuel is deuterium, which it consumes in its core
at temperatures of 18,000,000
to
36,000,000 F
(10,000,000 to
20,000,000 C). To
achieve comparable temperatures in a weapon, a fission triggering device is
used.
The development of fission weapons
Following the
discovery of artificial radioactivity in the 1930s, the Italian physicist Enrico Fermi performed a series of
experiments in which he exposed many elements to low-velocity neutrons. When he
exposed thorium and uranium, chemically different
radioactive products resulted, indicating that new elements had been formed,
rather than merely isotopes of the original elements. Fermi concluded that he
had produced elements beyond uranium (element 92), then the last element in the
periodic table; he called them transuranic elements and named two of them ausenium (element 93) and hesperium
(element 94). During the autumn of 1938, however, when Fermi was receiving the
Nobel Prize for his work, Otto Hahn and Fritz Strassmann
of Germany discovered that one of the "new" elements was actually
barium (element 56).
The Danish
scientist Niels Bohr
visited the United States in January 1939,
carrying with him an explanation, devised by the Austrian refugee scientist Lise Meitner and her nephew Otto
Frisch, of the process behind Hahn's surprising data. Low-velocity neutrons caused the uranium
nucleus to fission, or break apart, into two smaller pieces; the combined
atomic numbers of the two pieces--for example, barium and krypton--equalled
that of the uranium nucleus. Much energy was released in the process. This news
set off experiments at many laboratories. Bohr worked with John Wheeler at Princeton; they
postulated that the uranium isotope uranium-235 was the one undergoing
fission; the other isotope, uranium-238, merely absorbed the
neutrons. It was discovered that neutrons were produced during the fission
process; on the average, each fissioning atom
produced more than two neutrons. If the proper amount of material were
assembled, these free neutrons might create a chain reaction. Under special
conditions, a very fast chain reaction might produce a very large release of
energy; in short, a weapon of fantastic power might be feasible.