Fission Product Yield Data for the Transmutation of Minor Actinide ...

However, this CRP entered an entirely new field of
research, as usable models and systematics do not
exist over such an energy range and the outcome of
the work was deemed to be unpredictable.
Therefore, the goals of the CRP were subsequently
limited to the development of appropriate nuclear
models and systematics for the prediction of fission
yields as tools for the evaluation of energy
dependent fission yields up to 150 MeV.
1.2. MODELLING OF FISSION FRAGMENT
MASS DISTRIBUTIONS
This brief introduction should help in better
understanding the problems to be addressed, along
with the results of the benchmark exercise
(Section 6). More detailed descriptions can be
found in Sections 3.1, 4.1 and 4.6.
1.2.1. Fission process
Fission is a slow process on a nuclear
timescale, involving deformation of the whole
nucleus, and is always a compound process. A
captured low energy neutron leads directly to an
equilibrated compound system (first chance
fission). For sufficiently high incident neutron
energies (E n ) of a few MeV, the emission of a preequilibrium
neutron becomes possible (second
chance fission) at an about 100-fold faster timescale
than fission. Still higher E n can lead to the emission
of two (third chance fission) or more (multi-chance
fission) pre-equilibrium neutrons before fission.
This behaviour is also applicable to other projectiles
except that there is always a threshold for fission. At
such high energies, a ‘composite nucleus’ (target
nucleus plus projectile) is formed that emits fast
particles and gradually loses excitation energy and
memory of the incident particle by many nucleon–
nucleon interactions before reaching an equilibrated
compound stage of the reaction, where
fission may occur (multi-chance or emissive fission).
Thus the fissioning nucleus has a mass lower than
the composite nucleus by the number of preequilibrium
particles emitted.
Fission occurs when the saddle point
deformation of the nucleus is reached. On the
descent from saddle to scission, neutrons may be
emitted and reduce the excitation energy. The
highly excited primary fission fragments are deexcited
by the emission of prompt neutrons,
resulting in secondary fission fragments, followed
2
by prompt γ rays to form the primary fission
products. The latter are generally neutron rich and
reach stability by the emission of delayed neutrons
and/or by radioactive β decay. We distinguish
between the ‘pre-neutron emission mass distribution’
of primary fission fragments and the ‘postneutron
emission mass distribution’ of secondary
fission fragments.
1.2.2. Fission fragment mass distributions
Mass distributions from low energy neutron
induced actinide fission are predominantly
asymmetric, and such an effect is reflected by the
light and heavy mass peaks corresponding to
complementary fission fragments. These mass distributions
have successfully been represented by five
Gaussians, accounting for the observed fine
structure in the asymmetric peak regions, whereas
seven Gaussians gave a better fit for spontaneous
fission and less than five Gaussians were adequate
for the pre-actinides and higher actinides where
increased symmetric fission is observed [1.2].
Brosa et al. [1.3] have developed a model that
relates the above representations of fission
fragment mass distributions to different fission
modes (corresponding to separate fission channels)
through which an excited nucleus in the actinide
region can undergo fission: a symmetric ‘super long’
mode and two asymmetric modes, ‘standard 1’
(ST-1) and ‘standard 2’ (ST-2). The super long mode
corresponds to the symmetric peak; both the ST-1
and ST-2 modes correspond to two Gaussians, each
mode being composed of a light and a heavy mass
peak in asymmetric fission. ST-1 and ST-2 are
responsible for the observed fine structure in the
asymmetric peaks.
The positions of the asymmetric mass peaks
are determined by shell effects: the ST-1 contribution
to the heavy mass peak at about A = 134 is
attributed to the formation of spherical heavy
fragments close to Z = 50 and N = 82; that of ST-2 at
about A = 142 is identified with the deformed shell
closure at N ≅ 88. As a consequence, these positions
have been observed to be stable with respect to the
change in mass of the fissioning nuclide, as is also
confirmed by the Brosa model [1.3] that predicts a
change of the mean heavy fragment mass for ST-2
from 142 in 238 U to 140 in 226 U fission. Thus, only the
position of the light mass peak shifts with a change
of the mass of the fissioning nuclide.
The symmetric fission contribution has been
observed to increase for lower mass actinides