Multi-modal Random Neck-rupture Model Research Articles

Fission fragment spectroscopy of 235U(nth,f)

Simultaneous measurements of the relative fission fragment charge and mass yield distributions have been performed for the even-even fission fragments produced from the reaction, 235U(nth,f). The measurements have been carried out using the conventional fission fragment spectroscopic technique. The extracted results are interpreted on the basis of the Multi-Modal Random Neck Rupture Model (MM-RNRM). The results from the analysis bring out the necessary experimental evidence for the influence of the deformed proton shell closures at Z = 52 and ∼ 56 along with the neutron shell closures at N = 82 (spherical) and 88 (deformed) in controlling the respective S1 and S2 fission modes occurring in the heavier group of asymmetric fission fragments. The new findings from the present investigation provide the crucial inputs for understanding the features of different fission modes that persist in the low-energy fission dynamics of the lighter actinides. The pair wise neutron multiplicity distribution profiles for the five correlated fission fragment pairs have been presented, and the corresponding extracted neutron multiplicity values are also reported.

Modified Boltzmann Factor on Rupture Probability

Fission Products are data that is very much needed for developing nuclear technology. Considering that the experimental results of nuclear data are minimal, theoretical modeling and calculations are needed. One of the theoretical models is the "multimodal random neck-rupture model. (M-RNRM)" However, although it has completed the fission product data, it needs closer to the reference value. For this reason, the idea of modifying the Boltzmann factor on rupture probability was developed. This modification is in the form of adding a polynomial factor to the Boltzmann factor. This work has succeeded in showing better fission product calculation results closer to the reference value.

Fragment-mass, kinetic energy, and angular distributions forU234(n,f)at incident neutron energies fromEn=0.2MeV to 5.0 MeV

This work investigates the neutron-induced fission of $^{234}\mathrm{U}$ and the fission-fragment properties for neutron energies between ${E}_{\mathrm{n}}=0.2$ and 5.0 MeV with a special highlight on the prominent vibrational resonance at ${E}_{\mathrm{n}}=0.77$ MeV. Angular, energy, and mass distributions were determined based on the double-energy technique by means of a twin Frisch-grid ionization chamber. The experimental data are parametrized in terms of fission modes based on the multimodal random neck-rupture model. The main results are a verified strong angular anisotropy and fluctuations in the energy release as a function of incident-neutron energy.

Fluctuations of Mass Yield Distribution for the Epithermal Fission of 235U and Its Impact on HCLWR Calculations

The need for accurate nuclear data represents a permanent challenge to improve the calculation tools used in reactor physics. Uranium-235 is one of the most important isotopes, and its related nuclear data need to be known with a high degree of accuracy. In this context, many studies have been undertaken to improve the fission yields of 235U for the main fission products in spent fuels.For a few years, an increasing interest has been observed for high-conversion light water reactors, for better use of the fuel and for nonproliferation considerations. These concepts are based on a low moderation ratio (˜0.9) and the use of highly enriched mixed oxide fuels (>8%). Because of a neutron flux much harder than in pressurized water reactors, calculations require a good knowledge of nuclear data in the epithermal range to accurately predict the fuel depletion with burnup. In particular, the energy dependence of the fission yields must be considered, because of the existence of fluctuations of the fragment fission yields in the resonances of 235U(n,f). Unfortunately, the fluctuations are incorrectly taken into account in calculation codes because of the rough energy description in the usual nuclear data libraries.In this paper, integral experiments are presented for the identification and the quantification of this effect in an almost 1/E neutron spectrum. The experiments consist of the irradiation of 235U samples in the MINERVE reactor (CEA Cadarache), first in a mostly thermal neutron spectrum and second in a mostly epithermal one. Measurements of some abundant fission products are realized with gamma-ray spectrometry and show fluctuations of the 235U cumulative fission yields between the two experiments. Results are interpreted with the multimodal random neck rupture model as a variation of 5% of the weight ratio w1/w2 between the standard I and standard II fission modes, and compared with the differential experiments.

Indication of anisotropic TKE and mass emission in 234U(n,f)

Abstract The neutron-induced fission of 234 U has been studied for neutron energies ranging from 200 keV to 5 MeV. Special focus was put around the prominent vibrational resonance in the sub-barrier region around 800 keV incident neutron energy. The aim was to investigate the fission fragment (FF) characteristics and search for fluctuations in energy and mass distributions. The strong angular anisotropy in the case of 234 U(n,f) was verified and correlations with changes in energy and mass distributions were found. The TKE around the resonance increases contrary to earlier literature data. Furthermore, the TKE and mass distribution were found to be dependent on emission angle. At the resonance, the TKE was smallest near the 0° emission of the FF. This effect was consistent and coherent with a change in the mass distribution around the resonance. The mass distribution was observed to be less asymmetric near 0° emission. From a fitting analysis based on the Multi-Modal Random Neck-Rupture (MMRNR) model, we found the yield of the standard-1 mode increasing around the resonance. Because the TKE is increasing at larger angles and the mass distribution becomes more symmetric also at larger angles, we conclude that this behavior is due to an increase of the standard-1 mode at these larger angles. Based on the formalism of MMRNR, such difference in angular distribution may be an indication of a different outer barrier height for the standard-1 and standard-2 modes.

Fission properties of actinide nuclei from proton-induced fission at 26.5 and 62.9 MeV incident proton energies

Fission properties of proton-induced fission on {sup 232}Th, {sup 237}Np, {sup 238}U, {sup 239}Pu, and {sup 241}Am targets, measured at the Louvain-la-Neuve cyclotron facility at proton energies of 26.5 and 62.9 MeV, are compared with the predictions of the state-of-the-art nuclear reaction code talys. The code couples the multimodal random neck-rupture model with the pre-equilibrium exciton and statistical models to predict fission fragment mass yields, pre- and post-scission neutron multiplicities, and total fission cross sections in a consistent approach. The sensitivity of the calculations to the input parameters of the code and possible improvements are discussed in detail.

Calculation of fission product yields for some nuclei with TALYS code

We have carried out a calculation of the transmission coe cients per fission mode and determined the fission product yields for some nuclei using the Talys-0.72 version of the code(1). For fission, this code uses a statistical approach based on a revised version of the multi-modal random neck-rupture model (MM-RNRM). The fragment mass distribution is the sum of the three dominant fission modes: the superlong (SL), the standard I (ST I) and the standard II (ST II). The relative contribution of each mode is deduced from the calculation of the transmission coe cients according to the Hill-Wheeler expression taking into account the relevant transition states. The shell and pairing corrections are introduced in the level density parameter according to the Ignatyuk(2) formula. The calculated transmission coe cients values per fission mode are compared with the existing values. First, we reproduced the fragment mass distributions obtained using theTalys-0.64 version of the code for the fission of 238 U induced by 1.6 and 5.5 MeV neutrons with the 'constant temperature model' of Gilbert and Cameron(3), then we extended the calculations to other actinides with other level density models(4)(5). Finally, these calculated values of fission yields are compared with the experimental data.

Measurement of Fragment Mass Yields in Neutron-Induced Fission of 232Th and 238U at 33, 45, and 60 MeV

Incineration of spent nuclear fuel in Accelerator Driven Systems requires nuclear data on neutron-induced fission of actinides at energies above 20 MeV - the upper limit of conventional evaluated nuclear data files. Over the past years, a significant effort has been devoted to measurements of neutron-induced fission cross-sections at intermediate energies but there is a lack of experimental data on fission yields. Here we describe recent measurements of pre-neutron emission fragment mass distributions from intermediate energy neutron-induced fission of (232)Th and (238)U. The measurements have been carried out at the neutron beam of the Louvain-la-Neuve cyclotron facility CYCLONE. Quasi-monoenergetic neutrons with peak energies at 32.8, 45.3 and 59.9 MeV were produced by protons impinging on a thin lithium target. A multi-section Frisch-gridded ionization chamber was used as a fission fragment detector. The measurement results are compared with available experimental data as well as with predictions of the multi-modal random neck-rupture model implemented into the TALYS code.

Monte Carlo calculation of the de-excitation of fission fragments ofCf252(sf) within multimodal random neck rupture model

The multimodal random neck rupture model has been employed to study the energy distribution in nuclear fission. Monte Carlo calculations have been carried out on the de-excitation of fission fragments of {sup 252}Cf(sf) using this model. In this study special attention has been paid to the law of conservation of energy. Results for the neutron multiplicities and kinetic energies, average gamma energies are in good agreement with the experimental data. Mass and charge distributions of secondary fission fragments are also well reproduced in the calculations.

An Interpretation of Energy Dependence of Delayed Neutron Yields in the Resonance Region for 235U and 239Pu

Possible fluctuation in the delayed neutron yields (DNYs) in the resonance region was predicted on the basis of experimental data of mass distribution of fission fragments at resonances. Analyzed according to the multimodal random neck rupture model of fission, the small variations in the experimental mass distribution were attributed to fluctuations in the branching ratios to different modes of fission. Using the results of analysis of measured data for 235U and 239Pu, the DNYs were calculated for each resonance by the summation method, considering 271 precursors and evaluated data of delayed neutron emission probability. It was found that the DNYs should have local dips for 235U and spikes for 239Pu at resonances.

Analysis of Incident Neutron Energy Dependence of Fission Product Yields for 235U by the Selective Channel Scission Model

The change of fission product (FP) yields from the n+235U reaction for different incident neutron energies was analyzed by the selective channel scission (SCS) model. Gaining a complete understanding of why FP yields from the n+235U reaction have two peaks has been difficult. However, by considering proper channel-dependent fission barriers including the tunnel fission effect, FP yields for thermal and 1 MeV neutron-induced fissions of 235U were analyzed. The incident neutron energy dependence of FP yields for 235U was understood consistently. Then, we compared the SCS model with the multimodal random-neck rupture model that is conventionally used to explain the characteristics of fission successfully and phenomenologically.

Nucleon-Induced Fission at Intermediate Energies

Temperature-dependent fission barriers and fission-fragment mass distributions are calculated with an extended ver sion of the multi-modal random neck-rupture model (MM-RNRM). It is shown how the distinction between the different fission modes vanishes in going towards higher excitation energies, due to the melting of shell effects. The fission-fragment mass yield calculations are coupled to the nuclear reaction code ALICE-91, which computes the fission cross section in competition with the other reaction channels. With the combination of the temperature-dependent MM- RNRM and ALICE-91 nucleon-induced fission is investigated at energies between 10-200 MeV. Results are included for several (sub)actinide nuclei.

Fission cross-section evaluations in the frame of the multi-modal fission model for 237Np(n, f)

The statistical model for fission cross-section evaluation is extended by including the concept of multi-modality of the fission process. The three most dominant fission modes, the two asymmetric standard I (S1) and standard II (S2) modes and the symmetric superlong (SL) mode are taken into account. Based on calculations of the nuclear surface in the frame of the multi-modal random neck-rupture model, separate outer fission barriers are considered for each mode, while the inner barriers and isomeric wells are assumed to be the same. Deconvoluted fission cross sections for S1, S2 and SL modes for 237Np(n, f), based on experimental branching ratios, were calculated for the first time in the incident energy range 0.01 to 5.5 MeV providing good agreement with the experimental fission cross-section data. Including the concept of multi-modal fission into nuclear reaction models could lead to a better understanding and provides a more consistent description of the fission process.

Mass distributions in nucleon-induced fission at intermediate energies

Temperature-dependent fission barriers and fission-fragment mass distributions are calculated in the framework of the multimodal random neck-rupture model (MM-RNRM). It is shown how the distinction between the different fission modes disappears at higher excitation energies, due to the melting of shell effects. The fission-fragment mass yield calculations are coupled to the nuclear reaction code ALICE-91, which takes into account the competition between the other reaction channels and fission. With the combination of the temperature-dependent MM-RNRM and ALICE-91 nucleon-induced fission is investigated at energies between 10 and 200 MeV for nuclei varying from Au to Am.

Study of the 237 Np(n,f)-reaction at MeV neutron energies

The fission fragment properties of the reaction 237 Np(n, f ) have been investigated at IRMM in the neutron energy range E n=0.3 to 5.5 MeV. Above E n=0.5 MeV, the TKE averaged over all fission fragment masses shows a continuously decreasing trend. The observed variations in the TKE as a function of the compound nuclear excitation energy have been attributed to a combination of the changes in the mass yield and TKE (A) distributions. A description of the two-dimensional mass–TKE distributions has been obtained in terms of fission modes in the frame of the multimodal random neck-rupture model. The inclusion of the superlong symmetric mode is necessary to explain the observed dip in the experimental TKE close to symmetry.

Investigation of the fission fragment properties of the reaction 238U(n,f) at incident neutron energies up to 5.8 MeV

The fission fragment properties of the reaction 238U(n,f) have been studied, at different incident neutron energies ranging from E n=1.2 to 5.8 MeV. The pre-neutron emission mass, kinetic energy and fission fragment angular distributions have been investigated with a double Frisch-gridded ionization chamber. The influence of the subthreshold vibrational resonances and of the proton pairing effect on the fission fragment properties is clearly visible. The total kinetic energy averaged over all fission fragment masses ( TKE ) shows an increasing trend up to E n=3.5 MeV with a sudden drop at roughly E n=3.8 MeV which has been attributed to the onset of pair breaking at the barrier. Above E n=3.8 MeV, the TKE is again continuously increasing. The changes in the mass yield and TKE ( A) distributions have been studied as a function of the compound nuclear excitation energy and their contribution to the observed variations in the TKE have been determined. The two-dimensional mass-TKE distributions have been described in terms of fission modes and compared with theoretical calculations performed recently in the frame of the multi-modal random neck-rupture model. Although theoretically six asymmetric fission modes are predicted which all surpass individual outer barriers, an interpretation in terms of only two asymmetric modes has physical meaning. This points to an influence of shell structure effects to the observed distributions. In any case, the super-long symmetric mode has to be included, in order to explain the dip in TKE ( A) distribution close to symmetry.

Fission-mode calculations for 239U, a revision of the multi-modal random neck-rupture model

In the course of an experimental study of the fragment characteristics after neutron-induced fission of 238U with incident neutron energies between 1.2 and 5.8 MeV fission-mode calculations in the frame of the multi-modal random neck-rupture model have been performed. During these calculations some technical parts of the model have been revised. The identification of fission modes is now based on unequivocal and reproducible criteria. The Rayleigh criterion has consequently been applied to determine each possible scission configuration. The most remarkable new results are that all physically relevant fission modes branch off in the second potential minimum exhibiting different outer barriers, and that the total kinetic energy distribution of the fission fragments is a direct consequence of the Rayleigh criterion. The fragment characteristics as the mean mass, the mean total kinetic energy and the corresponding width obtained from the fission-mode calculations compare reasonably well with the experimental findings. For the first time the weighted fission cross sections through each particular fission mode have been analyzed simultaneously using a Hill-Wheeler type expression for the transmission through a double-humped fission barrier. The results support the picture of individual outer barriers with slightly different penetrabilities and a slightly lower inner barrier.

Fission modes in the compound nucleus 238Np

Potential energy calculations in conjunction with the multi-modal random neck-rupture model of Brosa, Großmann and Müller (BGM-model) indicate the existence of four fission modes in the compound nucleus 238Np. For the first time the splitting of the standard mode into three submodes could be demonstrated theoretically. The existence of a third standard mode in fission has already been proposed for heavier nuclei, where a nonnegligible yield of more asymmetric fission fragments is experimentally observed. The calculated fission fragment properties for 238Np are discussed and compared to experimental results.

252Cf: neutron multiplicities in correlation with fission-fragment mass and energy

The number of neutrons emitted in the spontaneous fission of 252Cf has been measured, using a large Gd-doped liquid scintillator tank. A total of 1.7 × 107252Cf fission events were assayed, and the correlations between the number of neutrons emitted by the fission fragments and the total kinetic energy (TKE) released in a particular fission event were investigated. From these correlations, spontaneous-fission parameters were derived and compared to exit-channel predictions given by the multi-modal random neck-rupture model of Brosa and co-workers.