Multi-modal analysis for low energy neutron-induced fission of²³⁵U

Abstract

Low energy neutron induced fission of 235U is studied in the framework of the multi-modal fission model. The fission fragment properties, such as the yields, the average total kinetic energy distribution and the average neutron separation energy, are investigated for incident neutron energies from thermal to 6.0 MeV. The multi-modal fission approach is also used to evaluate the prompt fission neutron multiplicity and spectra for the neutron-induced fission of 235U with an improved version of the Los Alamos model for incident neutrons below the (n, nf) threshold. The three most dominant fission modes are taken into account. The model parameters are determined on the basis of experimental data. The calculated results are in good agreement with the experimental data.

Mass yield distributions and fission modes in photofission ofU238below 20 MeV

“…Although this behavior gives general description of the energy dependence of the contribution of asymmetric modes STI and STII, it does not provide quantitative estimates of probabilities of the modes. Furthermore, it is shown in the works that analyzed the photofission modes of 238 U [19] and neutroninduced fission of 235 U [15,23] that the width of the STI mode is approximately two times less than the width of STII, and, hence, it not enough to compare just the yields of specific mass chains, i.e., the amplitudes of the modes, but one has to take into account the areas of the corresponding distributions. Only one work, namely Ref.…”

“…(11), the contributions, or weights, of each mode were obtained in each case as W = K √ 2πσ 2 , where K and σ correspond to the parameters of the mode. In addition, the mode weights were also obtained from mass yield distributions of neutron-induced fission of 238 U [24,25] and 235 U [23,24].…”

“…However, one has to bear in mind that the systematics of Ref. [16] is composed in such a way that the STI contribution is zero at the energy E = 22 MeV, and [17,18,[20][21][22]; stars: contributions of the STII, STI, SL modes of neutron-induced fission of 235 U from [15,23,24]; squares: 238 U [24,25]. Mode yields are normalized per 100 fission reactions:…”

“…Using the obtained results and available data from other works [17,18,[20][21][22] the relative contributions of fission modes in the excitation energy range 8.5-17 MeV are obtained. The behavior of photofission modes of 238 U is compared to the behavior of the modes of the neutron-induced fission of 235 U from [15,23,24] and 238 U from [24,25] and the results of the systematics [16] for neutron-induced fission of the 238 U compound nucleus.…”

Prompt fission neutron spectra of n+ 235 U above the (n,nf) fission threshold

“…The PFNS for the 235 U(n,f) reaction have been investigated by several theoretical approaches [3][4][5][6][7], but most of them focus on the incident-neutron energy below the (n,nf) fission threshold, namely, below 6 MeV, which can be described by a relatively simple model calculation. For the calculation above the threshold, there are only the original Los Alamos model [3] calculation and Hauser-Feshbach statistical model calculation [6] at E n =7.0 MeV and 14.7 MeV, respectively.…”