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absorbing an additional neutron. Other nuclei, notably^238 U (which makes up 99.3 per-

cent of natural uranium, with^235 U as the remainder) need more excitation energy for

fission than the binding energy released when another neutron is absorbed. Such nu-

clei undergo fission only by reaction with fast neutrons whose kinetic energies exceed

about 1 MeV.

Fission can occur after a nucleus is excited by means other than neutron capture,

for instance by gamma-ray or proton bombardment. Some nuclides are so unstable as

to be capable of spontaneous fission, but they are more likely to undergo alpha decay

before this takes place.

A striking aspect of nuclear fission is the magnitude of the energy given off. As we

saw earlier, this is in the neighborhood of 200 MeV, a remarkable figure for a single

atomic event; chemical reactions liberate only a few electronvolts per event. Most of

the energy released in fission goes into the kinetic energy of the fission fragments. In

the case of the fission of^235 U, about 83 percent of the energy appears as kinetic energy

of the fragments, about 2.5 percent as kinetic energy of the neutrons, and about 3.5 per-

cent in the form of instantly emitted gamma rays. The remaining 11 percent is given

off in the subsequent beta and gamma decays of the fission fragments.

Shortly after nuclear fission was discovered it was realized that, because fission

leads to other neutrons being given off, a self-substaining sequence of fissions should

be possible (Fig.12.21). The condition for such a chain reactionto occur in an as-

sembly of fissionable material is simple: at least one neutron produced during each

fission must, on the average, cause another fission. If too few neutrons cause fis-

sions, the reaction will slow down and stop; if precisely one neutron per fission

causes another fission, energy will be released at a constant rate. (which is the case

in a nuclear reactor); and if the frequency of fissions increases, the energy release

will be so rapid that an explosion will occur (which is the case in an atomic bomb).

Nuclear Transformations 453

60 70 80 90 100 110 120 130 140 150 160 170

0.001

0.01

0.1

1.0

10

Mass number

Yield, %

Figure 12.20The distribution of mass numbers in the fragments from the fission of^23592 U.

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