Evaluation of the total photoabsorption cross sections for actinides from photofission data and model calculations

We have calculated the fission probabilities for 237-Np, 233,235,238-U, 232-Th, and nat-Pb following the absorption of photons with energies from 68 MeV to 3.77 GeV using the RELDIS Monte-Carlo code. This code implements the cascade-evaporation-fission model of intermediate-energy photonuclear reactions. It includes multiparticle production in photoreactions on intranuclear nucleons, pre-equilibrium emission, and the statistical decay of excited residual nuclei via competition of evaporation, fission, and multifragmentation processes. The calculations show that in the GeV energy region the fission process is not solely responsible for the entire total photoabsorption cross section, even for the actinides: ~55-70% for 232-Th, \~70-80% for 238-U, and ~80-95% for 233-U, 235-U, and 237-Np. This is because certain residual nuclei that are created by deep photospallation at GeV photon energies have relatively low fission probabilities. Using the recent experimental data on photofission cross sections for 237-Np and 233,235,238-U from the Saskatchewan and Jefferson Laboratories and our calculated fission probabilities, we infer the total photoabsorption cross sections for these four nuclei. The resulting cross sections per nucleon agree in shape and in magnitude with each other. However, disagreement in magnitude with total-photoabsorption cross-section data from previous measurements for nuclei from C to Pb calls into question the concept of a ``Universal Curve'' for the photoabsorption cross section per nucleon for all nuclei.Comment: 39 pages including 11 figure

Three-dimensional representation of the photofission process on heavy nuclei, induced by real photons with energy between 0.1 and 0.6 GeV

La fisión de núcleos pesados inducida por fotones reales (fotofisión), en el rango de energías entre 0,1 GeV y el umbral de fotoproducción de un pión (0,6 GeV), se representa usando un motor de gráficos en 3D creado por el autor. Para modelar los núcleos pesados se usan los modelos nucleares de gota líquida y gas de Fermi. El proceso de fotofisión se lleva a cabo en dos etapas: la etapa de cascada intranuclear y la etapa de cascada evaporativa. En la primera etapa, el haz de fotones incidente da inicio a una cascada de reacciones nucleares en las que algunas de las partículas involucradas pueden escapar del núcleo. Como resultado, el núcleo queda en un estado excitado (núcleo compuesto), dando paso a la segunda etapa de fotofisión. En esta etapa, el núcleo compuesto es desexcitado por medio de los mecanismos de evaporación de partículas y/o de fisión. Así, contando la tasa de fisión, se calcula la probabilidad de fotofisión. Las etapas antes mencionadas son representadas usando ilustraciones tridimensionales a través de una aplicación creada en C/C++ y vinculada con OpenGL.The fission of heavy nuclei induced by real photons (photofission) in the energy range between 0.1 GeV and the one-pion-photoproduction threshold (0.6 GeV) is represented using a 3D graphics engine created by the author. Nuclear models: liquid drop and Fermi gas are used to represent the heavy nuclei. The photofission process is divided into two stages: the intranuclear cascade and the evaporative cascade. In the first stage, the incident photon beam initiates an intranuclear cascade in which some of the particles involved can escape from the nucleus. The nucleus is then left in an excited state (compound nucleus), triggering the evaporative cascade in which the compound nucleus is de-excited via particle evaporation and/or fission mechanisms. The photofission probability is computed by counting the fission rate. The aforementioned stages were depicted using three-dimensional illustrations through an application written in C/C++ and linked to OpenGL