De-excitation of the projectile and target nuclear pre-fragments by nuclear ablation

A nuclear ablation model, based largely on the description provided by Wilson et al [1], has been developed to provide a better approximation for the final nuclear fragment from an abrasion interaction. The algorithm implemented in G4WilsonAblationModel uses the same approach for selecting the final-state nucleus as NUCFRG2 and determining the particles evaporated from the pre-fragment in order to achieve that state. However, use is also made of classes in Geant4's evaporation physics to determine the energies of the nuclear fragments produced.

The number of nucleons ablated from the nuclear pre-fragment (whether as nucleons or light nuclear fragments) is determined based on the average binding energy, assumed by Wilson et al to be 10 MeV, i.e.:

$$A_{abl} = \left\{ {\begin{array}{*{20}c} {Int\left( {\frac{... ...A_{PF} } \hfill & {:otherwise} \hfill \\ \end{array}} \right.$$ (26.23)

Obviously, the nucleon number of the final fragment, , is then determined by the number of remaining nucleons. The proton number of the final nuclear fragment () is sampled stochastically using the Rudstam equation:

$$\sigma (A_F ,Z_F ) \propto \exp \left( { - R\left\vert {Z_F ... ...\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$2$}}} } \right)$$ (26.24)

Here = , =, and = . Once and have been calculated, the species of the ablated (evaporated) particles are determined again using Wilson's algorithm. The number of $\alpha$-particles is determined first, on the basis that these have the greatest binding energy:

$$N_\alpha = \left\{ {\begin{array}{*{20}c} {Int\left( {\frac... ...left( {\frac{{A_{abl} }}{4}} \right)} \\ \end{array}} \right.$$ (26.25)

Calculation of the other ablated nuclear/nucleon species is determined in a similar fashion in order of decreasing binding energy per nucleon of the ablated fragment, and subject to conservation of charge and nucleon number.

Once the ablated particle species are determined, use is made of the Geant4 evaporation classes to sample the order in which the particles are ejected (from G4AlphaEvaporationProbability, G4He3EvaporationProbability, G4TritonEvaporationProbability, G4DeuteronEvaporationProbability, G4ProtonEvaporationProbability and G4NeutronEvaporationProbability) and the energies and momenta of the evaporated particle and the residual nucleus at each two-body decay (using G4AlphaEvaporationChannel, G4He3EvaporationChannel, G4TritonEvaporationChannel, G4DeuteronEvaporationChannel, G4ProtonEvaporationChannel and G4NeutronEvaporationChannel). If at any stage the probability for evaporation of any of the particles selected by the ablation process is zero, the evaporation is forced, but no significant momentum is imparted to the particle/nucleus. Note, however, that any particles ejected from the projectile will be Lorentz boosted depending upon the initial energy per nucleon of the projectile.