264
W. Younes
The mass and charge distributions of fission products have been measured for
many actinides and for both SF and induced fission at various energies. Some
characteristic features can be gleaned from these studies. For thermal fission, the
distributions as a function of A tend to be bimodal, with one peak near the doubly
magic 132 Sn due to quantum shell effects, and another corresponding to the lighter
complementary nuclei. The two peaks are fairly broad and are separated by a dip
near symmetric fission for thermal and low-energy fission [31, 32]. As the energy
of the incident neutron is increased, the central dip at symmetric fission tends to
fill in, and the mass peaks broaden somewhat [33]. Another feature of the fission
products is that the ratio Z/A of a product tends to have a value close to the Z/A
ratio of the parent. This is known as the unchanged charge distribution (UCD)
rule [34]. The distribution of products as a function of Z sometimes displays an
even-odd staggering pattern for low-energy fission that has been used to estimate
the energy dissipated by the parent nucleus into non-collective modes of excitation
before scission (see, e.g., chapter 8 in [35]).
The total energy released in a fission event is defined as the difference in restmass energy between the parent nucleus and the final products. This energy can also
be written as the sum of the total kinetic energy (TKE) and the total excitation
energy (TXE) of the primary fragments, minus any energy contributed by the
incident particle in the case of induced fission (i.e., its kinetic energy plus any
excitation energy gained in the formation of the compound nucleus). The TKE of
the fragments can be estimated as the Coulomb repulsion energy between centers
of charge of the fragments at their separation distance at scission, however, this
estimate ignores any pre-scission energy acquired during the transition from saddle
to scission. Precisely how the initial energy of the parent nucleus is partitioned
between TKE and TXE of the fragments, and how the TXE is divided among the
fragments remain open questions [36, 37].
In addition to the information that can be gathered from the fragments and
products (e.g., mass distributions and TKE), the neutrons and gammas that they
emit can also provide useful data that shed light on the fission process. In principle,
the excitation energy and initial angular momentum imparted to the fragments
could be reconstructed by measuring the neutrons and gammas they emit. The
average number of prompt neutrons (multiplicity, ¯
ν) emitted as a function of primary
fragment mass (A), typically follows a characteristic “sawtooth” shape and can
be used to estimate the average fragment excitation energy, removed by neutron
emission [38]. The energy spectrum of the neutrons emitted by the fragments is
expected to have a Maxwellian shape in the center of mass frame of the fragment
which, when transformed to the laboratory frame, takes on a Watt functional form
[39]. In addition to the prompt neutrons emitted by the fragments, and multichance neutrons emitted by the parent nucleus, there are two additional sources
of neutrons produced by fission: scission neutrons and delayed neutrons. Scission
neutrons are thought to be emitted by the parent nucleus as it breaks apart, and the
frequency with which they occur remains a source of debate [40, 41]. Their angular
distribution is expected to be isotropic in the laboratory frame because they are
emitted by the slowly recoiling parent nucleus, in contrast to the neutrons emitted
W. Younes
The mass and charge distributions of fission products have been measured for
many actinides and for both SF and induced fission at various energies. Some
characteristic features can be gleaned from these studies. For thermal fission, the
distributions as a function of A tend to be bimodal, with one peak near the doubly
magic 132 Sn due to quantum shell effects, and another corresponding to the lighter
complementary nuclei. The two peaks are fairly broad and are separated by a dip
near symmetric fission for thermal and low-energy fission [31, 32]. As the energy
of the incident neutron is increased, the central dip at symmetric fission tends to
fill in, and the mass peaks broaden somewhat [33]. Another feature of the fission
products is that the ratio Z/A of a product tends to have a value close to the Z/A
ratio of the parent. This is known as the unchanged charge distribution (UCD)
rule [34]. The distribution of products as a function of Z sometimes displays an
even-odd staggering pattern for low-energy fission that has been used to estimate
the energy dissipated by the parent nucleus into non-collective modes of excitation
before scission (see, e.g., chapter 8 in [35]).
The total energy released in a fission event is defined as the difference in restmass energy between the parent nucleus and the final products. This energy can also
be written as the sum of the total kinetic energy (TKE) and the total excitation
energy (TXE) of the primary fragments, minus any energy contributed by the
incident particle in the case of induced fission (i.e., its kinetic energy plus any
excitation energy gained in the formation of the compound nucleus). The TKE of
the fragments can be estimated as the Coulomb repulsion energy between centers
of charge of the fragments at their separation distance at scission, however, this
estimate ignores any pre-scission energy acquired during the transition from saddle
to scission. Precisely how the initial energy of the parent nucleus is partitioned
between TKE and TXE of the fragments, and how the TXE is divided among the
fragments remain open questions [36, 37].
In addition to the information that can be gathered from the fragments and
products (e.g., mass distributions and TKE), the neutrons and gammas that they
emit can also provide useful data that shed light on the fission process. In principle,
the excitation energy and initial angular momentum imparted to the fragments
could be reconstructed by measuring the neutrons and gammas they emit. The
average number of prompt neutrons (multiplicity, ¯
ν) emitted as a function of primary
fragment mass (A), typically follows a characteristic “sawtooth” shape and can
be used to estimate the average fragment excitation energy, removed by neutron
emission [38]. The energy spectrum of the neutrons emitted by the fragments is
expected to have a Maxwellian shape in the center of mass frame of the fragment
which, when transformed to the laboratory frame, takes on a Watt functional form
[39]. In addition to the prompt neutrons emitted by the fragments, and multichance neutrons emitted by the parent nucleus, there are two additional sources
of neutrons produced by fission: scission neutrons and delayed neutrons. Scission
neutrons are thought to be emitted by the parent nucleus as it breaks apart, and the
frequency with which they occur remains a source of debate [40, 41]. Their angular
distribution is expected to be isotropic in the laboratory frame because they are
emitted by the slowly recoiling parent nucleus, in contrast to the neutrons emitted