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W. Younes
observables. The Bohr and Wheeler paper directly addressed the questions: how
much energy is released in fission? How much energy has to be supplied, and is
spontaneous fission possible? What is the cross section for induced fission? Which
isotope in natural uranium is primarily responsible for the observed fission cross
section? What is the origin of delayed neutrons? What is the mass distribution of
the fragments? These and other questions will be further explored in the remainder
of this lecture.
2 General Features of Fission
Various characteristic features of fission have emerged from over 80 years of study.
These features are important in formulating a coherent understanding of the fission
process. That being said, it is important to keep in mind that, despite overall
systematic trends, observed fission properties can differ significantly between
neighboring nuclei.
Nuclear fission is an extreme example of large-amplitude collective motion
[10] that results in the division of a parent nucleus into two or more fragment
nuclei. Binary fission (fission into two fragments) is far more common than ternary
fission, which typically occurs in fewer than about one in a thousand events in
thermal fission [11]. The third fragment in ternary fission is most often an α
particle that tends to be emitted in a direction roughly perpendicular to the axis
connecting the other two fragments [11]. The collective character of the fission
process distinguishes it from other processes that break up the nucleus, such as
spallation [12]. In a spallation reaction, a high-energy incident nucleon (e.g., an
800-MeV proton [13]) interacts with individual protons and neutrons in the nucleus
causing the emission of secondary particles via an intranuclear cascade mechanism
[12]. Once the nucleus loses enough energy through this process, particle (mostly
neutron) emission continues via statistical evaporation. The different mechanisms
underlying fission and spallation lead to differences in their observable properties:
spallation tends to produce fragments that are close to the target in mass, whereas
(low-energy) fission more often results in a pair of fragments significantly lighter
than the target nucleus. In another example, the quasi-fission process where the
reacting system re-separates before fission can begin differs from fission and in fact
can compete with it in the formation of superheavy nuclei [14, 15].
The fission process can occur spontaneously, or it can be induced by an incident
particle. Historically, induced fission was discovered first and spontaneous fission
(SF) was observed in uranium nuclei sometime later [16, 17]. The SF process in
actinides typically competes with α decay and is often dwarfed by the α branch but
not always, as in the case of 250 Cm where SF dominates α decay. When viewed
as a barrier penetration mechanism, the SF half-life can be calculated using the
semi-classical Wentzel-Kramers-Brillouin (WKB) approximation [18] in terms of
the action integral for the system [11, 19, 20]. The SF half-lives, when plotted as
a function of the Z 2 /A ratio of the parent nucleus, display two important features:
W. Younes
observables. The Bohr and Wheeler paper directly addressed the questions: how
much energy is released in fission? How much energy has to be supplied, and is
spontaneous fission possible? What is the cross section for induced fission? Which
isotope in natural uranium is primarily responsible for the observed fission cross
section? What is the origin of delayed neutrons? What is the mass distribution of
the fragments? These and other questions will be further explored in the remainder
of this lecture.
2 General Features of Fission
Various characteristic features of fission have emerged from over 80 years of study.
These features are important in formulating a coherent understanding of the fission
process. That being said, it is important to keep in mind that, despite overall
systematic trends, observed fission properties can differ significantly between
neighboring nuclei.
Nuclear fission is an extreme example of large-amplitude collective motion
[10] that results in the division of a parent nucleus into two or more fragment
nuclei. Binary fission (fission into two fragments) is far more common than ternary
fission, which typically occurs in fewer than about one in a thousand events in
thermal fission [11]. The third fragment in ternary fission is most often an α
particle that tends to be emitted in a direction roughly perpendicular to the axis
connecting the other two fragments [11]. The collective character of the fission
process distinguishes it from other processes that break up the nucleus, such as
spallation [12]. In a spallation reaction, a high-energy incident nucleon (e.g., an
800-MeV proton [13]) interacts with individual protons and neutrons in the nucleus
causing the emission of secondary particles via an intranuclear cascade mechanism
[12]. Once the nucleus loses enough energy through this process, particle (mostly
neutron) emission continues via statistical evaporation. The different mechanisms
underlying fission and spallation lead to differences in their observable properties:
spallation tends to produce fragments that are close to the target in mass, whereas
(low-energy) fission more often results in a pair of fragments significantly lighter
than the target nucleus. In another example, the quasi-fission process where the
reacting system re-separates before fission can begin differs from fission and in fact
can compete with it in the formation of superheavy nuclei [14, 15].
The fission process can occur spontaneously, or it can be induced by an incident
particle. Historically, induced fission was discovered first and spontaneous fission
(SF) was observed in uranium nuclei sometime later [16, 17]. The SF process in
actinides typically competes with α decay and is often dwarfed by the α branch but
not always, as in the case of 250 Cm where SF dominates α decay. When viewed
as a barrier penetration mechanism, the SF half-life can be calculated using the
semi-classical Wentzel-Kramers-Brillouin (WKB) approximation [18] in terms of
the action integral for the system [11, 19, 20]. The SF half-lives, when plotted as
a function of the Z 2 /A ratio of the parent nucleus, display two important features: