Photonuclear reactions at intermediate energies investigated via the Monte Carlo multicollisional intranuclear cascade model

Abstract

The Monte Carlo multicollisional (MCMC) intranuclear cascade model is used to study photonuclear reactions at intermediate energies ͑20ഛ E ␥ ഛ 140 MeV͒. This version of the code differs from previous versions in the following aspects: (i) the quasideuteron model of photoabsorption is consistently included by taking into account relative momentum correlations of the neutron-proton pair in a relativistic kinematics; (ii) a realistic treatment of the Pauli-blocking mechanism at the initial photoabsorption and at each binary nucleon-nucleon scattering during the cascade process is incorporated throughout the calculations; (iii) a criterion based on energy considerations is required by the end of the cascade. Differently from other transport models used so far, which are based on a randomly generated nuclear ground state with a stochastic treatment of the Pauli blocking, the present model incorporates a shell constrained momentum space of the nucleons which is preserved as the cascade evolves along time. The transition between the pre-equilibrium and evaporation phases is energetically determined, allowing the description of the cascade process without any free parameter, such as some ambiguous stopping time parameters adopted in similar time-structured cascade models. The occupation number distribution after the cascade corresponds to a typical Fermi distribution at a finite nuclear temperature, and the long-standing spurious depletion of the Fermi sphere, usually present in other cascade models, no longer appears. The Pauli-blocking factors are calculated and compared with previous approaches based on Fermi gas level density calculations. The evaporation-fission process of the compound nucleus is described in the framework of a Monte Carlo algorithm. Experimental data of the total photoabsorption cross section and the neutron multiplicities for Sn, Ce, Ta, and Pb in the 20-140-MeV range are described fairly well by the present calculations.

The nuclear matter effects in pi0 photoproduction at high energies

“…Some improvements were incorporated to accommodate the Final State Interactions of the produced pions also for the nuclear coherent mechanisms, as well as the relativistic nuclear recoil for both the nuclear coherent and the electromagnetic (Primakoff) processes. The MCMC model was first developed to study high energy photonuclear reactions [4][5][6][7], being more recently successfully extended to intermediate energies [8,9]. A special and very sophisticated version of the MCMC code were also proposed in a recent paper [10] in order to study the incoherent π 0 photoproduction both at the ∆(1232) resonance energy region, as well as in the 4.0 − 6.0 GeV range and forward angles [10].…”

Nuclear incoherent photoproduction ofπ0andηfrom 4 to 12 GeV

“…The Pauli suppression reduces the cross sections in the forward direction (meson photoproduction) and also dictates the dynamics of secondary meson-nucleon scatterings (meson-nucleus FSI) during the cascade process. As we have shown in a work dedicated to the quasideuteron channel [49], the evaluation of the Pauli blocking is crucial for low-energy photonuclear reactions. In this work, a nonstochastic Pauli blocking was implemented propitiating a time-dependent analysis of multiple particlehole excitations during the cascade process.…”

Nuclear Photoproduction of Pseudoscalar Mesons at Forward Angles up to 6.0 GeV

“…The absorption effects in the Glauber model can be calculated by the well known formula: p(b, z))}db = 121., (3) where one immediately finds an absorption of about 40% (NaiaubeM ~ 0.6). This result shows very nicely that two completely different models -the Glauber and the cascade -are consistent with each other in describing attenuation effects of hadrons in the nuclei.…”