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Description
Preliminary assessment of the dose due to potential atmospheric release from MINERVA Proton Target Facility (PTF) being designed at SCK CEN has identified isotopes of Iodine (notably 125I, 126I and 131I) as major contributors [1]. Iodine is a volatile element and its retention in the structures is limited. Therefore, its production in the PTF actinide ISOL (Isotope Separation On-Line) targets must be accurately modelled. Thorium (ThC2-2C) and Uranium (UC2-2C) carbide targets are being considered to produce a variety of radionuclides under irradiation by 100 MeV proton beams. The production rate of any residual nucleus is governed by the particle fluxes and corresponding production cross sections (excitation functions) for each projectile particle on a given nuclide target (Th and U in this case) for the whole energy range up to the energy of the primary particle. In MINERVA, only protons are responsible for Iodine production, due to low yield of secondary neutrons and thus the negligible neutron flux in the target compared to the proton beam. The incoming proton flux is well simulated by radiation transport codes. The critical point is the estimation of the excitation functions.
Due to short half-lives and low energy of emitted gamma, production cross sections of the isotopes of interest in relevant energy range (predominantly below 100 MeV) are difficult to measure. More specifically, there exists only one experiment where the yield of 126I from 232Th was measured at 190 MeV [2]. No data on proton-induced Iodine production is available at all for Uranium. Therefore, nuclear reaction model simulations are the only way to obtain excitation functions for Iodine isotopes, and the question on their reliability arises.
When a proton with energy around 100 MeV hits the nucleus, either it triggers the emission of energetic nucleons leaving the compound nucleus in highly excited state, or the proton can directly be absorbed by the compound nucleus. The de-excitation occurs mainly by neutron emission, however, heavy nuclei can fission and this is how Iodine appears from proton-induced reactions on actinides. Nuclear reaction codes model the whole process in either 3-step process (intranuclear cascade, pre-equilibrium and statistical evaporation-fission) or in 2 steps (pre-equilibrium followed by statistical evaporation-fission). There exist various models describing each step, so one can employ more than 10 models or model combinations to simulate interactions of 100 MeV protons with actinides. With this in mind, the calculations with different event generators or their combinations included in MCNP [3], PHITS [4], FLUKA [5] radiation transport codes, as well as INCL++ [6] and CASCADEX [7] standalone nuclear reaction codes, were performed. A Posteriori statistical analysis allowed to construct best estimate excitation functions for Iodine isotopes and their uncertainty bands. However, a closer look on the resulting cross sections allowed to identify outliers that all made use of the RAL fission model [8]. More profound analysis of Iodine and Xenon (many Xenon isotopes are precursors for Iodine) isotopic distributions has driven us to the conclusion that the RAL model implemented in MCNP is unable to accurately simulate yields of certain Iodine and Xenon isotopes. Therefore the excitation functions were updated removing these outliers. Such analysis will be extended to the whole range of fission products with ultimate goal of creating proton induced evaluated files for 232Th, 235U and 238U.