Nuclear Transformations 455
(Fig. 12.22). The greater the difference between the masses, the greater the number of
collisions needed to slow a neutron down, and the longer the period in which it is in
danger of being captured by a^238 U nucleus. The majority of today’s commercial reactors
use light water both as moderator and as coolant. Each molecule of water contains two
hydrogen atoms whose proton nuclei have masses almost identical with that of the
neutron, so light water is an efficient moderator.
Unfortunately protons tend to capture neutrons to form deuterons in the reaction(^1) H(n, ) (^2) H. Light-water reactors therefore cannot use natural uranium for fuel but need
enriched uraniumwhose^235 U content has been increased to about 3 percent. Enriched
uranium can be produced in several ways. Originally all enriched uranium was produced
by gaseous diffusion, with uranium hexafluoride (UF 6 ) gas being passed through about
2000 successive permeable barriers. Molecules of^235 UF 6 are slightly more likely to dif-
fuse through each barrier than^238 UF 6 because of their smaller mass. A more recent
method uses high-speed gas centrifuges for the separation. Still other processes are
possible.
Enrico Fermi (1901–1954) was
born in Rome and obtained his
doctorate at Pisa. After periods at
Göttingen and Leiden working
with leading figures in the new
quantum mechanics, Fermi re-
turned to Italy. At the University
of Rome in 1926 he investigated
the statistical mechanics of parti-
cles that obey Pauli’s exclusion
principle, such as electrons; the
result is called Fermi-Dirac statistics because Dirac independ-
ently arrived at the same conclusions shortly afterward. In
1933 Fermi introduced the concept of the weak interaction
and used it together with Pauli’s newly postulated neutrino (as
Fermi called it) to develop a theory of beta decay able to ac-
count for the shape of the electron energy spectrum and the
decay half-life.
Later in the 1930s Fermi and a group of collaborators car-
ried out a series of experiments in which radionuclides were pro-
duced artificially by bombarding various elements with neutrons;
they found slow neutrons especially effective. Some of their re-
sults seemed to suggest the formation of transuranic elements.
In fact, as Meitner and Hahn were to find later, what they were
observing was nuclear fission. In 1938 Fermi received the No-
bel Prize for this work, but instead of returning to Mussolini’s
Fascist Italy, he went to the United States. As part of the atomic-
bomb program, Fermi directed the design and construction of
the first nuclear reactor at the University of Chicago, which be-
gan operating in December 1942, four years after the discovery
of fission. After the war Fermi shifted to a different field, high-
energy particle physics, where he made important contributions.
He died of cancer in 1954, one of the very few physicists of the
modern era to combine virtuosity in both theory and experi-
ment. The element of atomic number 100, discovered the year
after his death, is called fermium in his honor.
0 2 4 5
0.25
0.50
0.75
1.00
Mass ratio m 2 /m 1
Kinetic energy ratio
KE
′^2
KE
1
13
Figure 12.22Energy transfer in an elastic head-on collision between a moving object of mass m 1 and
a stationary object of mass m 2 (see Exercise 59).
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