Ranking and validation of the spallation models for description of intermediate mass fragment emission from p + Ag collisions at 480 MeV incident proton beam energy

Abstract

Abstract. Double-differential cross-sections d 2 σ/dΩ dE for isotopically identified intermediate mass fragments (6 Li up to 27 Mg) from nuclear reactions induced by 480 MeV protons impinging on a silver target were analyzed in the frame of a two-step model. The first step of the reaction was described by the intranuclear cascade model INCL4.6 and the second one by four different models (ABLA07,GEM2, GEMINI++, and SMM). The experimental spectra reveal the presence of low-energy, isotropic as well as high-energy, forward-peaked contributions. The INCL4.6 model offers a possibility to describe the latter contribution for light intermediate mass fragments by coalescence of the emitted nucleons. The qualitative agreement of the model predictions with the data was observed but the high-energy tails of the spectra were significantly overestimated. The shape of the isotropic part of the spectra was reproduced by all four models. The GEM2 model strongly underestimated the value of the cross-sections for heavier IMF whereas the SMM and ABLA07 models generally overestimated the data. The best quantitative description of the data was offered by GEMINI++, however, a discrepancy between the data and the model cross-sections still remained for almost all reaction products, especially at forward angles. It indicates that non-equilibrium processes are present which cannot be reproduced by the applied models. The goodness of the data description was judged quantitatively using two statistical deviation factors, the H-factor and the M -factor, as a tool for ranking and validation of the theoretical models.

Non-equilibrium processes in p + Ag collisions at GeV energies

“…Moreover, it may be conjectured that they are present also at other proton beam energies. For example, the above discussed effects are very similar to those found in the recent analogous analysis of the intermediate mass fragments spectra and angular distributions from p+Ag collisions at 480 MeV proton beam energy [18] as well as isotopically identified production cross sections of heavy products from 136 Xe+H reactions at 500 MeV/nucleon energy of 136 Xe [19]. Thus the explanation of the nature of the non-equilibrium processes in proton induced reactions at GeV energies remains an important and intriguing problem to be solved.…”

“…The same physical picture was extended with success also to light IMFs (A < 9) in the new version (INCL4.6) of this model [17]. However, it was recently shown for 0.48 GeV proton induced spallation reactions on a silver target [18] that, while the model ensures the qualitative agreement with experimental IMF data, the high-energy tails of the spectra are strongly overestimated. It was also observed [19] that the production cross sections of isotopically identified residual nuclei with Z = 41-45 from reactions induced by 136 Xe projectiles at 0.5 GeV/nucleon on a hydrogen target are systematically underestimated by the model.…”

Odd--Even Staggering in the Yields of Intermediate Mass Fragments from $p$+Ag collisions at $Ep=480$ MeV

“…It was shown in Ref. [23] that the use of INCL plus GEMINI models enables one to reproduce to large extend the experimental spectra and their energy dependence for heaviest products of the p+Ag reaction analysed in the present study. Furthermore, it was found that these two models always underestimate the differential cross sections for light intermediate mass fragments, i.e.…”

“…The origin of the difference between the data and model predictions might be attributed to the lack of knowledge of non-equilibrium processes which seem to be quite important for the studied reactions [23]. However, it was checked that the δ function does not practically change when the coalescence of nucleons from the intranuclear cascade into complex ejectiles was extended from particles with A < 5 to those with A < 9 for all models of the second step of the reaction.…”

“…A possibility of the emission of complex light charged particles in this stage of the process has been taken into account to assure achieving a realistic mass, charge and excitation energy distribution of the residual compound nuclei. However, the coalescence of the nucleons escaping from the intranuclear cascade with creation of intermediate mass fragments was not allowed because of two reasons: (i) the INCL++ enables one to perform efficiently such calculations only for the lightest IMF and (ii) it was found in the earlier study of these reactions [23] that high-energy spectra of IMF are significantly overestimated by this model. It is important to emphasize that the above decision does not modify significantly the mass, charge and energy distribution of the excited nuclei -the remnants of the cascade since the cross sections for production of IMF are orders of magnitude smaller than those for nucleons and complex LCP.…”