Simulate Threshold Energies to Induce Photo-nuclear Reactions for Heavy Metals

Nuclear reactions take place within a probability that is proportional to cross section. The common unit of cross section is the barn b. Cross section can mathematically be defined as the number of light particles produced per unit time, per unit incident flux of photon (monoenergetic) beam or the incident particles, per target nucleus [1]. Sample activation is one of the nuclear techniques used in this study. In Photon Activation Analysis (PAA), photons are captured by the target nucleus that excites it to the higher energy level. The excited nucleus decays to a lower energy level (possibly the ground state) with the emission of photons, charged particles, or neutrons. The absorption of incident photons by the target nucleus is dependant on nuclear reactions cross section and threshold energy of incident bremsstrahlung photons. Isolated energy states of the target nuclei are excited below 10 MeV however there is a broad resonance region called the Giant Dipole Resonance (GDR) between 10 MeV and 30 MeV in which the collective vibrational motion of nucleons is observed [2]. Goal: To find the GDR and threshold energies of incoming bremsstrahlung photons to induce (gamma, n), and (gamma, 2n) reactions. Target samples such as 23g U-238, 10g Pu-239, and 10g ThC will be used during simulation. By using GEANT4 simulation, my goal is to plot total cross section verses energy of incident photons (1-1000 MeV) and based on the plot I will identify the approximate GDR range for U-238, Pu-239, and ThC that could help to optimize beam energy for PAA experiments. The cross sections obtained from Monte Carlo simulations will be compared with the data published in ”Atomic and Nuclear Data Reprints”.