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Register a new userEquilibrium and pre-equilibrium processes in the 55Mn(6Li,xp) and 57Fe(a,xp) reactions
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Spectra of outgoing neutrons and protons from the 6Li+55Mn reaction and protons from the a+57Fe reaction have been measured with beams of 15 MeV 6Li ions and 30 MeV alpha-particles. These reactions proceed through the same 61Ni nucleus at the same excitation energy, thus allowing the difference in reaction mechanism to be studied. It is shown that spectra from the first reaction measured at backward angles are due to emission from a traditional compound nucleus reaction, in which the intermediate nucleus has reached statistical equilibrium; the spectra from the second reaction contain a significant fraction of pre-equilibrium emission at all angles. Level density pa- rameters of the residual nucleus 60Co have been obtained from the first reaction. Both emission spectra and angular distributions have been measured for the second reaction. It was found that the pre-equilibrium component exhibits a forward-peaked angular distribution, as expected, but with a steeper slope than predicted and with an unusual slight rise at angles above 120deg. The backward- angle rise is explained qualitatively by the dominance of the multi-step compound mechanism at backward angles.
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Excitation functions were measured for the $^{55}$Mn(n,2n)$^{54}$Mn, $^{55}$Mn(n,$alpha$)$^{52}$V, $^{63}$Cu(n,$alpha$)$^{60}$Co, $^{65}$Cu(n,2n)$^{64}$Cu, and $^{65}$Cu(n,p)$^{65}$Ni reactions from 13.47 to 14.83 MeV. The experimental cross sections are compared with the results of calculations including all activation channels for the stable isotopes of Mn and Cu, for neutron incident energies up to 50 MeV. Within the energy range up to 20 MeV the model calculations are most sensitive to the parameters related to nuclei in the early stages of the reaction, while the model assumptions are better established by analysis of the data in the energy range 20-40 MeV. While the present analysis has taken advantage of both a new set of accurate measured cross sections around 14 MeV and the larger data basis fortunately available between 20 and 40 MeV for the Mn and Cu isotopes, the need of additional measurements below as well as above 40 MeV is pointed out. Keywords: 55Mn, 63,65Cu, E$leq$40 MeV, Neutron activation cross section measurements, Nuclear reactions, Model calculations, Manganese, Copper
New experimental data for the inclusive reactions (p,xp) and (p,xd) on isotopes of the nuclei $^{90,92}$Zr and $^{92}$Mo, have been measured at E$_{p}$=30.3 MeV, which has not been investigated in detail so far. We show the extension of the pre-equilibrium reactions to this energy region and interpret the results of these experiments. Moreover, we display the mechanism of the reaction and the level of energy-dependence. The adequacy of the theoretical models in explaining the measured experimental data is also discussed. In our theoretical analysis, the contributions of multi-step direct and compound processes in the formation of cross-sections are determined and we assert that the traditional frameworks are valid for the description of the experimental data.
The MEDLEY setup based at The Svedberg Laboratory (TSL), Uppsala, Sweden has previously been used to measure double-differential cross-sections for elastic nd scattering, as well as light ion production reactions for various nuclei in the interaction with neutrons around 95 MeV. When moved to the new beam line, the first experimental campaign was on light-ion production from Ca at 94 MeV in February 2005. These data sets have been analyzed for proton production in forward and backward angles. The Delta E - Delta E - E technique have been used to identify protons, and a cutoff as low as 2.5 MeV is achieved. Suppression of events induced by neutrons in the low-energy tail of the neutron field is achieved by time-of-flight techniques. The data are normalized relative to elastic np scattering measured in the 20-degree telescope. Results from an accepted neutron spectrum are presented and some methods to correct for events from low energy neutrons are presented and evaluated. The data are compared with calculations using the nuclear code TALYS. It was found that TALYS systematically overestimates the compound part, and underestimates the pre-equilibrium part of the cross-section.
A number of accelerator-based isotope production facilities utilize 100- to 200-MeV proton beams due to the high production rates enabled by high-intensity beam capabilities and the greater diversity of isotope production brought on by the long range of high-energy protons. However, nuclear reaction modeling at these energies can be challenging because of the interplay between different reaction modes and a lack of existing guiding cross section data. A Tri-lab collaboration has been formed among the Lawrence Berkeley, Los Alamos, and Brookhaven National Laboratories to address these complexities by characterizing charged-particle nuclear reactions relevant to the production of established and novel radioisotopes. In the inaugural collaboration experiments, stacked-targets of niobium foils were irradiated at the Brookhaven Linac Isotope Producer (E$_p$=200 MeV) and the Los Alamos Isotope Production Facility (E$_p$=100 MeV) to measure $^{93}$Nb(p,x) cross sections between 50 and 200 MeV. The measured cross-section results were compared with literature data as well as the default calculations of the nuclear model codes TALYS, CoH, EMPIRE, and ALICE. We developed a standardized procedure that determines the reaction model parameters that best reproduce the most prominent reaction channels in a physically justifiable manner. The primary focus of the procedure was to determine the best parametrization for the pre-equilibrium two-component exciton model. This modeling study revealed a trend toward a relative decrease for internal transition rates at intermediate proton energies (E$_p$=20-60 MeV) in the current exciton model as compared to the default values. The results of this work are instrumental for the planning, execution, and analysis essential to isotope production.
A theoretical approach was developed to describe secondary particle emission in heavy ion collisions, with special regards to pre-equilibrium {alpha}-particle production. Griffins model of non-equilibrium processes is used to account for the first stage of the compound system formation, while a Monte Carlo statistical approach was used to describe the further decay from a hot source at thermal equilibrium. The probabilities of neutron, proton and {alpha}-particle emission have been evaluated for both the equilibrium and pre-equilibrium stages of the process. Fission and {gamma}-ray emission competition were also considered after equilibration. Effects due the possible cluster structure of the projectile which has been excited during the collisions have been experimentally evidenced studying the double differential cross sections of p and {alpha}-particles emitted in the E=250MeV 16O +116Sn reaction. Calculations within the present model with different clusterization probabilities have been compared to the experimental data.
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