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Register a new userEffects of Neutron Emission on Fragment Mass and Kinetic Energy Distribution from Thermal Neutron-Induced Fission of $^{235}U$
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The mass and kinetic energy distribution of nuclear fragments from thermal neutron-induced fission of 235U have been studied using a Monte-Carlo simulation. Besides reproducing the pronounced broadening in the standard deviation of the kinetic energy at the final fragment mass number around m = 109, our simulation also produces a second broadening around m = 125. These results are in good agreement with the experimental data obtained by Belhafaf et al. and other results on yield of mass. We conclude that the obtained results are a consequence of the characteristics of the neutron emission, the sharp variation in the primary fragment kinetic energy and mass yield curves. We show that because neutron emission is hazardous to make any conclusion on primary quantities distribution of fragments from experimental results on final quantities distributions.
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Focused on the generation and evolution of vast complementary pairs of the primary fission fragments at scission moment, Dinuclear and Statistical Model (DSM) is proposed. (1) It is assumed that the fissile nucleus elongates along a symmetric coaxis until it breaks into two primary fission fragments. (2) Every complementary pair of the primary fission fragments is approximatively described as two ellipsoids with large deformation at scission moment. (3) The kinetic energy in every complementary pair of the primary fragments is mainly provided by Coulomb repulsion, which is explicitly expressed through strict six-dimensional integrals. (4) Only three phenomenological coefficients are obtained to globally describe the quadrupole deformation parameters of arbitrary primary fragments both for $^{235}$U($n_{th}, f$) and $^{239}$Pu($n_{th}, f$) reactions, on the basis of the common characteristics of the measured data, such as mass and charge distributions, kinetic energy distributions. In the framework of DSM, the explicit average total kinetic energy distribution $overline{TKE}(A)$ and the average kinetic energy distribution $overline{KE}(A)$ are consistently represented. The theoretical results in this paper agree well with the experimental data. Furthermore, this model is expected as the reliable approach to generally evaluate the corresponding observebles for thermal neutron-induced fission of actinides.
The mass and kinetic energy distribution of nuclear fragments from thermal neutron induced fission of 235U have been studied using a Monte-Carlo simulation. Besides reproducing the pronounced broadening on the standard deviation of the final fragment kinetic energy distribution $sigma_{e}(m)$ around the mass number m = 109, our simulation also produces a second broadening around m = 125, that is in agreement with the experimental data obtained by Belhafaf et al. These results are consequence of the characteristics of the neutron emission, the variation in the primary fragment mean kinetic energy and the yield as a function of the mass.
We have measured the total kinetic energy (TKE) release for the $^{235}$U(n,f) reaction for $E_{n}$=2-100 MeV using the 2E method with an array of Si PIN diode detectors. The neutron energies were determined by time of flight measurements using the white spectrum neutron beam at the LANSCE facility. (To calibrate the apparatus, the TKE release for $^{235}$U(n$_{th}$,f) was also measured using a thermal neutron beam from the OSU TRIGA reactor). The TKE decreases non-linearly from 169.0 MeV to 161.4 MeV for $E_{n}$=2-90 MeV. The standard deviation of the TKE distribution is constant from $E_{n}$=20-90 MeV. Comparison of the data with the multi-modal fission model of Brosa indicates the TKE decrease is a consequence of the growth of symmetric fission and the corresponding decrease of asymmetric fission with increasing neutron energy. The average TKE associated with the Brosa superlong, standard I and standard II modes for a given mass is independent of neutron energy.
We have measured the total kinetic energy (TKE) release for the $^{235}$U(n,f) reaction for $E_{n}$=2-100 MeV using the 2E method with an array of Si PIN diode detectors. The neutron energies were determined by time of flight measurements using the white spectrum neutron beam at the LANSCE facility. To benchmark the TKE measurement, the TKE release for $^{235}$U(n$_{th}$,f) was also measured using a thermal neutron beam from the Oregon State University TRIGA reactor, giving pre-neutron emission $E^*_{TKE}=170.7pm0.4$ MeV in good agreement with known values. Our measurements are thus absolute measurements. The TKE in $^{235}$U(n,f) decreases non-linearly from 169 MeV to 161 MeV for $E_{n}$=2-100 MeV. The multi-modal fission analysis of mass distributions and TKE indicates the origin of the TKE decrease with increasing neutron energy is a consequence of the fade out of asymmetric fission, which is associated with a higher TKE compared to symmetric fission. The average TKE associated with the superlong, standard I and standard II modes for a given mass is independent of neutron energy. The widths of the TKE distributions are constant from $E_{n}$=20-100 MeV and hence show no dependence with excitation energy.
Several sources of angular anisotropy for fission fragments and prompt neutrons have been studied in neutron-induced fission reactions. These include kinematic recoils of the target from the incident neutron beam and the fragments from the emission of the prompt neutrons, preferential directions of the emission of the fission fragments with respect to the beam axis due to the population of particular transition states at the fission barrier, and forward-peaked angular distributions of pre-equilibrium neutrons which are emitted before the formation of a compound nucleus. In addition, there are several potential sources of angular anisotropies that are more difficult to disentangle: the angular distributions of prompt neutrons from fully accelerated fragments or from scission neutrons, and the emission of neutrons from fission fragments that are not fully accelerated. In this work, we study the effects of the first group of anisotropy sources, particularly exploring the correlations between the fission fragment anisotropy and the resulting neutron anisotropy. While kinematic effects were already accounted for in our Hauser-Feshbach Monte Carlo code, $mathtt{CGMF}$, anisotropic angular distributions for the fission fragments and pre-equilibrium neutrons resulting from neutron-induced fission on $^{233,234,235,238}$U, $^{239,241}$Pu, and $^{237}$Np have been introduced for the first time. The effects of these sources of anisotropy are examined over a range of incident neutron energies, from thermal to 20 MeV, and compared to experimental data from the Chi-Nu liquid scintillator array. The anisotropy of the fission fragments is reflected in the anisotropy of the prompt neutrons, especially as the outgoing energy of the prompt neutrons increases, allowing for an extraction of the fission fragment anisotropy to be made from a measurement of the neutrons.
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