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Register a new userEvaluation of the total photoabsorption cross sections for actinides from photofission data and model calculations
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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.
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An improved procedure is suggested for finding the total photoabsorption cross section on the neutron from data on the deuteron at energies < 1.5 GeV. It includes unfolding of smearing effects caused by Fermi motion of nucleons in the deuteron and also takes into account non-additive contributions to the deuteron cross section due to final-state interactions of particles in single and double pion photoproduction. This procedure is applied to analysis of existing data.
Measurements of neutron total cross-sections are both extensive and extremely accurate. Although they place a strong constraint on theoretically constructed models, there are relatively few comparisons of predictions with experiment. The total cross-sections for neutron scattering from $^{16}$O and $^{40}$Ca are calculated as a function of energy from $50-700$~MeV laboratory energy with a microscopic first order optical potential derived within the framework of the Watson expansion. Although these results are already in qualitative agreement with the data, the inclusion of medium corrections to the propagator is essential to correctly predict the energy dependence given by the experiment.
The 5-dimensional spin-0 form of the Kemmer-Duffin-Petiau (KDP) equation is used to calculate scattering observables [elastic differential cross sections ($dsigma/dOmega$), total cross sections ($sigma_{Tot}$), and reaction cross sections ($sigma_{Reac}$})] and to deduce $sigma_{Tot}$ and $sigma_{Reac}$ from transmission data for $K^+ + $ $^{6}$Li, $^{12}$C, $^{28}$Si, and $^{40}$Ca at several momenta in the range $488 - 714 MeV/c$. Realistic uncertainties are generated for the theoretical predictions. These errors, mainly due to uncertainties associated with the elementary $K^+ +$ nucleon amplitudes, are large, so that the disagreement that has been noted between experimental and theoretical $sigma_{Tot}$ and $sigma_{Reac}$ is not surprising. The results suggest that the $K^+ +$ nucleon amplitudes need to be much better determined before unconventional medium effects are invoked to explain the data.
It is well known that two fission fragments (FFs) are emitted essentially back to back in the laboratory frame. That can be used widely in many applications as a unique signature of fissionable materials. However, such fission fragments are difficult to detect. The energy and angular distributions of neutrons, on the other hand, are easy to measure, and that distribution will carry information about the fission fragments energy and angular spectra, as well as the neutron spectra in the fission fragment rest frame. We propose to investigate the two neutron correlation yield resulting from two FFs as a function of different targets, the angle between the two neutrons and the neutron energies. The preliminary calculation of the two neutron correlation shows a huge asymmetry effect: many more neutrons are emitted anti-parallel to each other than parallel to each other. That asymmetry becomes even more if the energy cut on each neutron is done. This study will potentially permit a new technique for actinide detection for homeland security and safeguards applications as well as improve our knowledge of correlated neutron emission.
We perform a parameter-free calculation for the high-energy proton-nucleus scattering based on the Glauber theory. A complete evaluation of the so-called Glauber amplitude is made by using the factorization of the single-particle wave functions. The multiple-scattering or multistep processes are fully taken into account within the Glauber theory. We demonstrate that proton- $^{12}$C, $^{20}$Ne, and $^{28}$Si elastic and inelastic scattering ($J^pi=0^+ to 2^+$ and $0^+ to 4^+$) processes are very well described in a wide range of the incident energies from $sim$50 MeV to $sim$ 1 GeV. We evaluate the validity of a simple one-step approximation andfind that the approximation works fairly well for the inelastic $0^+ to 2^+$ processes but not for $0^+ to 4^+$ where the multistep processes become more important. As an application, we quantify the difference between the total reaction and interaction cross sections of proton-$^{12}$C, $^{20}$Ne, and $^{28}$Si collisions.
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