bei48482_FM

452 Chapter Twelve

Figure 12.19Nuclear fission according to the liquid-drop model.

unable to bring back together the now widely separated groups of protons, and the

nucleus splits into two parts. This picture of fission is illustrated in Fig. 12.19.

The new nuclei that result from fission are called fission fragments.Usually fission

fragments are of unequal size (Fig. 12.20). Because heavy nuclei have a greater

neutron/proton ratio than lighter ones, the fragments contain an excess of neutrons.

To reduce this excess, two or three neutrons are emitted by the fragments as soon as

they are formed, and subsequent beta decays bring their neutron/proton ratios to stable

values. A typical fission reaction is

235

92 U

1

0 n→

236

92 U*→

140

54 Xe

94

38 Sr

1

0 n

1

0 n

which was illustrated in Fig. 12.17.

A heavy nucleus undergoes fission when it has enough excitation energy (5 MeV or

so) to oscillate violently. A few nuclei, notably^235 U, are able to split in two merely by

Lise Meitner (1878–1968),

the daughter of a Viennese

lawyer, became interested in

science when she read about

the Curies and radium. She

earned her Ph.D. in physics

in 1905 at the University of

Vienna, only the second

woman to obtain a doctor-

ate there. She then went to

Berlin where she began re-

search on radioactivity with

the chemist Otto Hahn.

Their supervisor refused to

have a woman in his labo-

ratory, so they started their

work in a carpentry shop.

Ten years later she was a professor, a department head, and,

with Hahn, the discoverer of a new element, protactinium.

In the 1930s the Italian physicist Enrico Fermi found that bom-

barding heavy elements with neutrons led to the production of

other elements. What happened in the case of uranium was es-

pecially puzzling, and Meitner and Hahn tried to find out by re-

peating the experiment. At the time the German persecution of

Jews had begun, but Meitner, who was Jewish, was protected by

her Austrian citizenship. In 1938 Germany annexed Austria, and

Meitner fled to Sweden but kept in touch with Hahn and their

younger colleague Fritz Strassmann. Hahn and Strassmann finally

concluded that neutrons interact with uranium to produce

radium, but Meitner’s calculations showed that this was impossi-

ble and she urged them to persist in their work. They did, and

found to their surprise that the lighter element barium had in fact

been created. Meitner surmised that the neutrons had caused the

uranium nuclei to split apart and, with her nephew Otto Frisch,

developed the theoretical picture of what they called fission.

In January 1939 Hahn and Strassmann published the dis-

covery of fission in a German journal; because Meitner was

Jewish, they thought it safer for themselves to ignore her con-

tribution. Meitner and Frisch later published their own paper

on fission in an English journal, but it was too late: Hahn dis-

gracefully claimed full credit, and not once in the years that fol-

lowed acknowledged her role. Hahn alone received the Nobel

Prize in physics for discovering fission. Unfortunately Meitner

did not live to see a measure of justice: the element of atomic

number 109 is called meitnerium in her honor, while the ten-

tative name of hahnium for element 105 was changed in 1997

to dubnium, after the Russian nuclear research center in Dubna.

Niels Bohr carried the news of the discovery of fission to the

United States later in 1939, just before the start of World War II,

where its military possibilities were immediately recognized.

Expecting that German physicists would come to the same con-

clusion and would start work on an atomic bomb, such a program

began in earnest in the United States. By the time it was suc-

cessful, in 1945, Germany had been defeated, and two atomic

bombs exploded over Hiroshima and Nagasaki then ended the

war with Japan. It was later learned that the German atomic-

bomb effort had amounted to very little. Not long afterward the

Soviet Union, Great Britain, and France also developed nuclear

weapons, and later China, Israel, South Africa, India, and Pakistan

did so as well.

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