Photofission and total photoabsorption cross sections in the energy range of shadowing effects

Abstract

The energy dependence of photofission cross section for heavy nuclei has recently been well described in terms of a Monte Carlo calculation at energies from the pion photoproduction threshold up to 1 GeV [see, for instance, A. Deppman et al., Phys. Rev. Lett. 87 (2001) 182701]. Recent experimental data from CLAS (CEBAF Large Angle Spectrometer) collaboration have extended the measured photofission cross section up to 3.5 GeV for actinide and preactinide nuclei. In this work we address the calculation of photoabsorption and photofission cross sections for actinide and preactinide nuclei above 1 GeV, a region where the shadowing effect plays an important role in the nuclear photoabsorption process. DOI: 10.1103/PhysRevC.73.064607 PACS number(s): 25.20.−x, 25.85.Jg, 24.10.Lx, 24.85.+p Photon-nucleus reactions are excellent tools for investigating nuclear and nucleonic structures. The photonuclear absorption process, for instance, has been used to study the formation and propagation of baryonic resonances inside nuclear matter [1-6], the photon hadronization process that gives rise to the shadowing effect in the photoabsorption cross section [1,7], or the formation and propagation of hyperons in the nucleus [6]. The simplicity of photonuclear reactions as compared to reactions induced by other probes, from both the theoretical and experimental points of view, is attractive to those willing to study nuclear and subnuclear structures.However, the various nuclear processes taking place during the reaction, mainly at intermediate and high energies, cause some problems in the comprehension of the nuclear or subnuclear mechanisms. One example is the fissility of heavy nuclei. It was supposed that fissility was an increasing function of the photon energy, and that for actinide nuclei, which present the highest fissility values among the stable nuclei, one could expect their fissility to be 1 for energies above a few hundred MeV [8]. In fact, the measurement of fission cross section was proposed as a reliable method for measuring the total photoabsorption cross section [9,10]. A fine, although incomplete, overview on the theoretical approaches for calculating photofission cross sections is presented in Ref. [11].Experimental results obtained at Frascati [9,10], Mainz [12,13], Bonn [1,14], Saskatoon [15], and Thomas Jefferson Laboratory [16,17] have shown, however, that this was not the case. The fissility for thorium and several uranium isotopes was found to be lower than that for neptunium, showing that nuclear fissility does not saturate for those nuclei, remaining at a value below 100% even at high incident photon energies. This result was fully explained by a Monte Carlo study of the intranuclear cascade and evaporation/fission competition processes that follows the photon absorption, as implemented by the MCMC and MCEF codes [18,19]. The important feature for explaining the nonsaturation of the heavy-nuclei fissility was the inclusion of protons and α-particles evaporation in the evaporation-fission competition proc...

Transport-theoretical description of nuclear reactions

“…Deppman et al [315] have successfully applied the so-called MCMC/MCEF cascade model to evaluate photon-fission cross sections using the photoabsorption cross section as given input. Complementary to this first work, Deppman achieved with the CRISP code [316,317], where the photon absorption is modeled via a microscopic resonance model, also satisfactory results for photon absorption.…”

Photon and proton induced fission on heavy nuclei at intermediate energies

“…This code has been developed for more than ten years [8][9][10][11][12][13][14] and it has been applied in the study of fission induced by photons and protons, and for the study of hyper-nuclear decay [15]. It has also been used in the development of new nuclear reactor technologies [16][17][18].…”

Evidence of subnucleonic degrees of freedom inJ/ψphotoproduction in ultraperipheral collisions at energies available at the CERN Large Hadron Collider

“…Our tool for investigating J/ψ production in UPCs is the CRISP model (acronym for Collaboration Rio -Ilhéus -São Paulo), which is implemented through a cascade of hadronic collisions using Monte Carlo techniques [14,15]. The CRISP model describes the nuclear reaction as a two step process, namely the intranuclear cascade and the evaporation/fission competition.…”