A Grand Tour of Nuclear Fission Physics
263
the SF half-life tends to decrease with increasing Z 2 /A, and nuclei with an odd
number of either protons or neutrons have systematically larger half-lives than the
neighboring even-even nuclei [11].
In induced fission, an incident particle fuses with a target to form a compound
system. 1 If the excitation energy of the compound system is sufficiently large, the
parent nucleus will likely emit neutrons, and may undergo multiple-chance fission
(fission without prior neutron emission is called “first-chance” fission, if one neutron
is emitted the process is referred to as “second-chance” fission, etc.). Once there
is no longer enough excitation energy left to emit additional neutrons, the parent
nucleus can continue to de-excite through gamma emission or proceed toward
scission (the breaking point of the nucleus). Various models predict a transition
time to scission (also known as the saddle-to-scission time) of 10 −20 − 10 −19 s for
low-energy fission [22–24]. This is a relatively long transition time compared to
the orbital time of 10 −22 s of a nucleon in the nucleus, as expected for a collective
process involving the coherent motion of many or all nucleons. There are various
experimental techniques that attempt to measure the time scale of the fission process.
Some of these techniques probe the time scale of the entire fission process from
compound-nucleus formation to scission. For example, one such approach takes
advantage of the rearrangement of the electronic structure that occurs when the
fragments formed, and the corresponding change in the x-rays produced [25].
Another approach, the crystal blocking technique, relies on the ordered structure
of a crystal to gauge how far the parent nucleus has recoiled before the fragments
are produced and fly apart, and to deduce the time delay since compound-nucleus
formation [26]. Other experimental methods, looking at the properties of neutrons
and gammas emitted before scission, can give a better sense of the saddle-to-scission
time scale [27, 28].
The fragments produced at scission are called “primary fragments” until they
emit prompt neutrons and become “secondary fragments.” These secondary fragments can then beta decay into stable nuclei, which are then referred to as “fission
products” [29]. The gamma rays emitted by the fragments can be prompt, or “late
prompt” [30] if they are issued from an isomeric state, and can help identify the
fragments thanks to their precisely known energies. The total number of fragments
with a given mass (A), charge (Z), and isomeric state (I ), produced in each fission
event after prompt neutron emission but before any delayed decays, is called the
“independent yield” Y (A, Z, I ) and is normalized to 2 (or 200%) when summed
over all values of A, Z, and I [29]. The total number of nuclei produced by each
fission over all time is called the “cumulative yield” Y cu (A, Z, I ), and is also
normalized to 2 (or 200%) [29]. The sum yield Y (A) is the sum of independent
yields over all Z and I for a given mass A. The chain yield Y ch (A) is the cumulative
yield of the last (i.e., stable) member of a decay chain of given mass A.
1 Direct reaction mechanisms can also lead to fission, but without initially going through
compound-nucleus formation [21].
263
the SF half-life tends to decrease with increasing Z 2 /A, and nuclei with an odd
number of either protons or neutrons have systematically larger half-lives than the
neighboring even-even nuclei [11].
In induced fission, an incident particle fuses with a target to form a compound
system. 1 If the excitation energy of the compound system is sufficiently large, the
parent nucleus will likely emit neutrons, and may undergo multiple-chance fission
(fission without prior neutron emission is called “first-chance” fission, if one neutron
is emitted the process is referred to as “second-chance” fission, etc.). Once there
is no longer enough excitation energy left to emit additional neutrons, the parent
nucleus can continue to de-excite through gamma emission or proceed toward
scission (the breaking point of the nucleus). Various models predict a transition
time to scission (also known as the saddle-to-scission time) of 10 −20 − 10 −19 s for
low-energy fission [22–24]. This is a relatively long transition time compared to
the orbital time of 10 −22 s of a nucleon in the nucleus, as expected for a collective
process involving the coherent motion of many or all nucleons. There are various
experimental techniques that attempt to measure the time scale of the fission process.
Some of these techniques probe the time scale of the entire fission process from
compound-nucleus formation to scission. For example, one such approach takes
advantage of the rearrangement of the electronic structure that occurs when the
fragments formed, and the corresponding change in the x-rays produced [25].
Another approach, the crystal blocking technique, relies on the ordered structure
of a crystal to gauge how far the parent nucleus has recoiled before the fragments
are produced and fly apart, and to deduce the time delay since compound-nucleus
formation [26]. Other experimental methods, looking at the properties of neutrons
and gammas emitted before scission, can give a better sense of the saddle-to-scission
time scale [27, 28].
The fragments produced at scission are called “primary fragments” until they
emit prompt neutrons and become “secondary fragments.” These secondary fragments can then beta decay into stable nuclei, which are then referred to as “fission
products” [29]. The gamma rays emitted by the fragments can be prompt, or “late
prompt” [30] if they are issued from an isomeric state, and can help identify the
fragments thanks to their precisely known energies. The total number of fragments
with a given mass (A), charge (Z), and isomeric state (I ), produced in each fission
event after prompt neutron emission but before any delayed decays, is called the
“independent yield” Y (A, Z, I ) and is normalized to 2 (or 200%) when summed
over all values of A, Z, and I [29]. The total number of nuclei produced by each
fission over all time is called the “cumulative yield” Y cu (A, Z, I ), and is also
normalized to 2 (or 200%) [29]. The sum yield Y (A) is the sum of independent
yields over all Z and I for a given mass A. The chain yield Y ch (A) is the cumulative
yield of the last (i.e., stable) member of a decay chain of given mass A.
1 Direct reaction mechanisms can also lead to fission, but without initially going through
compound-nucleus formation [21].