Nuclear reactions: General approach An outline of the general theory and modeling of nuclear reactions can be given in many ways. A common classification is in terms of time scales: short reaction times are associated with direct reactions and long reaction times with compound nucleus processes. At intermediate time scales, pre-equilibrium processes occur. An alternative, more or less equivalent, classification can be given with the number of intranuclear collisions, which is one or two for direct reactions, a few for pre-equilibrium reactions and many for compound reactions, respectively. As a consequence, the coupling between the incident and outgoing channels decreases with the number of collisions and the statistical nature of the nuclear reaction theories increases with the number of collisions. Figs. 3.1 and 3.2 explain the role of the different reaction mechanisms during an arbitrary nucleon-induced reaction in a schematic manner. They will all be discussed in this manual. This distinction between nuclear reaction mechanisms can be obtained in a more formal way by means of a proper division of the nuclear wave functions into open and closed configurations , as detailed for example by Feshbach’s many contributions to the field. This is the subject of several textbooks and will not be repeated here. When appropriate, we will return to the most important theoretical aspects of the nuclear models in TALYS in Chapter 4.
As specific features of the TALYS package we mention • In general, an exact implementation of many of the latest nuclear models for direct, compound,pre-equilibrium and fission reactions. • A continuous, smooth description of reaction mechanisms over a wide energy range (0.001- 200MeV) and mass number range (12 A 339). • Completely integrated optical model and coupled-channels calculations by the ECIS-06 code . • Incorporation of recent optical model parameterisations for many nuclei, both phenomenological (optionally including dispersion relations) and microscopical. • Total and partial cross sections, energy spectra, angular distributions, double-differential spectra and recoils. • Discrete and continuum photon production cross sections. • Excitation functions for residual nuclide production , including isomeric cross sections . • An exact modeling of exclusive channel cross sections, e.g. (n, 2np), spectra, and recoils. • Automatic reference to nuclear structure parameters as masses, discrete levels, resonances, level density parameters, deformation parameters, fission barrier and gamma-ray parameters , generally from the IAEA Reference Input Parameter Library . • Various width fluctuation models for binary compound reactions and, at higher energies, multiple Hauser-Feshbach emission until all reaction channels are closed. • Various phenomenological and microscopic level density models. • Various fission models to predict cross sections and fission fragment and product yields, and neutron multiplicities. • Models for pre-equilibrium reactions, and multiple pre-equilibrium reactions up to any order. • Generation of parameters for the unresolved resonance range. • Astrophysical reaction rates using Maxwellian averaging. • Medical isotope production yields as a function of accelerator energy and beam current. • Option to start with an excitation energy distribution instead of a projectile-target combination,helpful for coupling TALYS with intranuclear cascade codes or fission fragment studies. • Use of systematics if an adequate theory for a particular reaction mechanism is not yet available or implemented, or simply as a predictive alternative for more physical nuclear models. • Automatic generation of nuclear data in ENDF-6 format (not included in the free release). • Automatic optimization to experimental data and generation of covariance data (not included in the free release). • A transparent source program. • Input/output communication that is easy to use and understand. • An extensive user manual. • A large collection of sample cases.
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