Towards More Predictive Nuclear Reaction Modelling
7
angular distributions. Historically, the OM has been first determined postulating
functional forms whose parameters have to be adjusted until a good agreement with
data is obtained. This type of approach is still currently used [5] in particular because
of its ability to allow very accurate description of experimental data (less than 1%
accuracy on total cross sections). However, it depends very much on experimental
data availability. An alternative to the pure phenomenological approach is the
microscopic approach. Such approaches enable to determine the OM without any a
priori knowledge of any related experimental data. Therefore, it enables predictions
even for nuclei far from the valley of stability. The disadvantage of such microscopic
approaches is of course a lower accuracy. One of the most employed one is the socalled JLM approach [6] which is based on nuclear matter data obtained from mean
field or beyond mean field nuclear structure descriptions. Such structure methods
generally provide a nuclear structure description which is hoped to be precise
enough to guaranty that predictions far from the valley of stability should not be
too far from the reality.
The choice between phenomenology and microscopy is guided by the goal one
has in mind. Within the framework of nuclear data evaluation where accuracy is one
of the key issues, availability of experimental data will make it preferable to use
the first option because of its fitting power. For more fundamental research or when
there is a lack of data, the microscopic option is preferred.
3.3 The Pre-equilibrium Model
Once the OM has treated the various direct processes, the remaining cross section,
corresponding to all processes which have not been explicitly accounted for, is
“feeding” the second model of Fig. 2, the PE model. This reaction cross section
reflects the probability that the projectile be captured in the continuum of the
target to form a “composite” system. At this stage, the system still remembers the
way it was formed and is going to de-excite either by re-emitting a particle or
by distributing step by step the incident projectile energy between one or several
nucleons of the target. In the latter case, several particles and holes are going
to be created sequentially, holding on towards more complex configurations, to
reach, after sufficient time, a situation corresponding to the CN approximation,
where the projectile energy has been shared among all the constituents of the
composite system. At each step of this process, the probability to emit a particle
has to be accounted for. Again, one has the choice between more or less refined
models. The most employed one is the so-called exciton model, introduced in
the seventies, which has been successively improved to account for more and
more physical features either because the appearance of new experimental data
evidenced a lack of predictive power or simply because initially missing, though
important, features were introduced [7]. Quantum mechanical approaches have also
been developed but they are clearly more complex and less flexible and do not
provide results of better quality as those obtained with the exciton model. However,
7
angular distributions. Historically, the OM has been first determined postulating
functional forms whose parameters have to be adjusted until a good agreement with
data is obtained. This type of approach is still currently used [5] in particular because
of its ability to allow very accurate description of experimental data (less than 1%
accuracy on total cross sections). However, it depends very much on experimental
data availability. An alternative to the pure phenomenological approach is the
microscopic approach. Such approaches enable to determine the OM without any a
priori knowledge of any related experimental data. Therefore, it enables predictions
even for nuclei far from the valley of stability. The disadvantage of such microscopic
approaches is of course a lower accuracy. One of the most employed one is the socalled JLM approach [6] which is based on nuclear matter data obtained from mean
field or beyond mean field nuclear structure descriptions. Such structure methods
generally provide a nuclear structure description which is hoped to be precise
enough to guaranty that predictions far from the valley of stability should not be
too far from the reality.
The choice between phenomenology and microscopy is guided by the goal one
has in mind. Within the framework of nuclear data evaluation where accuracy is one
of the key issues, availability of experimental data will make it preferable to use
the first option because of its fitting power. For more fundamental research or when
there is a lack of data, the microscopic option is preferred.
3.3 The Pre-equilibrium Model
Once the OM has treated the various direct processes, the remaining cross section,
corresponding to all processes which have not been explicitly accounted for, is
“feeding” the second model of Fig. 2, the PE model. This reaction cross section
reflects the probability that the projectile be captured in the continuum of the
target to form a “composite” system. At this stage, the system still remembers the
way it was formed and is going to de-excite either by re-emitting a particle or
by distributing step by step the incident projectile energy between one or several
nucleons of the target. In the latter case, several particles and holes are going
to be created sequentially, holding on towards more complex configurations, to
reach, after sufficient time, a situation corresponding to the CN approximation,
where the projectile energy has been shared among all the constituents of the
composite system. At each step of this process, the probability to emit a particle
has to be accounted for. Again, one has the choice between more or less refined
models. The most employed one is the so-called exciton model, introduced in
the seventies, which has been successively improved to account for more and
more physical features either because the appearance of new experimental data
evidenced a lack of predictive power or simply because initially missing, though
important, features were introduced [7]. Quantum mechanical approaches have also
been developed but they are clearly more complex and less flexible and do not
provide results of better quality as those obtained with the exciton model. However,