238U Reaction Research Articles

Fission cross-section measurement for the 238U(n,f)89Rb reaction induced by D-T neutrons

The fission reactions of 238U induced by D-T neutrons were investigated by neutron activation technique and low-background HPGe-γ spectrometer. The half-life was used to identify the possible daughter nuclide. The fission cross-sections of 238U(n,f)89Rb reaction induced at the neutron energy around 14 MeV, i.e., 14.1 ± 0.3, 14.5 ± 0.3 and 14.7 ± 0.3 MeV, were measured precisely. The neutron flux was monitored by accompanying α-particle of the T(d,n)4He reaction and the neutron energies were determined by the cross-section ratio of 90Zr(n,2n)89Zr to 93Nb(n,2n)92mNb reaction. The fission cross-sections of the 238U(n,f)89Rb reaction are 33.2 ± 2.1 mb, 32.2 ± 2.0 mb, and 32.6 ± 2.1 mb, corresponding to the above neutron energy points.

First results of the 230Th(n,f) cross section measurements at the CERN n_TOF facility

The study of neutron-induced reactions on actinides is of considerable importance for the design of advanced nuclear systems and alternative fuel cycles. Specifically, 230Th is produced from the α-decay of 234U as a byproduct of the 232Th/233U fuel cycle, thus the accurate knowledge of its fission cross section is strongly required. However, few experimental datasets exist in literature with large deviations among them, covering the energy range between 0.2 to 25 MeV. In addition, the study of the 230Th(n,f) cross-section is of great interest in the research on the fission process related to the structure of the fission barriers. Previous measurements have revealed a large resonance at En=715 keV and additional fine structures, but with high discrepancies among the cross-section values of these measurements. This contribution presents preliminary results of the 230Th(n,f) cross-section measurements at the CERN n_TOF facility. The high purity targets of the natural, but very rare isotope 230Th, were produced at JRC-Geel in Belgium. The measurements were performed at both experimental areas (EAR-1 and EAR-2) of the n_TOF facility, covering a very broad energy range from thermal up to at least 100 MeV. The experimental setup was based on Micromegas detectors with the 235U(n,f) and 238U(n,f) reaction cross-sections used as reference.

Measurement of the 236U(n,f) cross section at fast neutron energies with Micromegas Detectors

In the present work, the measurement of the 236U(n,f) cross section was performed, with reference to the 238U(n,f) reaction. The measurements took place at the neutron beam facility of the National Centre for Scientific Research "Demokritos" (Greece) and the quasi-monoenergetic neutron beams were produced via the 2H(d,n)3He reaction in the energy range 4–10 MeV. Five actinide targets (two 236U, two 238U and one 235U) and the corresponding Micromegas detectors for the detection of the fission fragments were used. Detailed Monte Carlo simulations were performed, on one hand for the study of the neutron flux and energy distribution at the position of each target, and on the other hand for the study of the energy deposition of the fission fragments in the active volume of the detector. The mass and homogeneity of the actinide targets were experimentally determined via alpha spectroscopy and the Rutherford Backscattering Spectrometry, respectively. The experimental procedure, the analysis, the methodology implemented to correct for the presence of parasitic neutrons and the cross section results will be presented and discussed.

What can we learn from (n,xnγ) cross sections about reaction mechanism and nuclear structure?

Inelastic (n,n') cross section is a key quantity to accurately simulate reactor cores, and its precision was shown to need significant improvements. To bypass the experimental difficulties to detect neutrons from (n,xn) reaction and to discriminate inelastically scattered neutrons from those following the fission process in case of fissile targets, an indirect but yet powerful method is used: the prompt γ-ray spectroscopy. Along this line, our collaboration has developed the GRAPhEME setup, optimized for actinides, at the GELINA facility to measure partial (n,xn γ) cross sections, from which the total (n,xn) cross section can be inferred. (n,xn γ) experiments with actinides are still particularly challenging, as their structure presents a high level density at low energy, and the competing neutron-induced fission reaction contaminates the γ-energy distribution. New precise measurements of the partial (n,xn γ) cross sections provide a stringent test to theoretical model and offer a way to improve them. This is a path to a better determination of the total inelastic scattering cross sections. In this contribution we discuss modeling aspects of the 238U and 182W (n,n' γ) reactions, also measured with GRAPhEME, using the three codes TALYS, EMPIRE and CoH. We will highlight the needed/expected improvements on reaction modeling and nuclear structure input.

Evaluation of the Prompt Fission Gamma Properties for Neutron Induced Fission of 235,238U and 239Pu

We present details of the prompt fission gamma property evaluations included in the released ENDF/B-VIII.0 files. The average prompt gamma multiplicity, the average prompt gamma spectrum, the average total prompt gamma energy released and the prompt gamma multiplicity distributions, included in the ENDF/B-VIII.0 files, are presented for 235U(n,f), 238U(n,f), and 239Pu(n,f) reactions, as a function of incident neutron energy up to 30 MeV. The evaluation is based on available experimental data and model calculations. Notable data include recent measurements by experimental groups at Los Alamos and Lawrence Livermore National Laboratories, at Geel and Budapest, at CEA labs, as well as seminal historical gamma production measurements by Drake at Los Alamos.

NEUTRON-PHYSICAL CHARACTERISTICS OF A SYSTEM OF QUASI-INFINITE DEPLETED URANIUM BLANKET – PARTICLE ACCELERATOR. MONTE CARLO SIMULATION

Simulation of a quasi-infinite uranium target irradiated by 1 GeV proton and deuteron particles with the help of FLUKA simulation package was carried out. Neutron spectra and neutron flux in a target volume were obtained. 235U(n,f), 238U(n,f), 238U(n,g) reaction rates in a target were determined. Beam particle power multiplication were calculated.

ADVANTG hybrid code application for the analysis of neutron field depression in uranium foils activated in the MARK III converter of the BR1 reactor

This paper deals with the self-shielding correction calculation for natural and 40% enriched uranium foils activated in a fast-epithermal neutron field in the MARK III converter of the BR1 research reactor. Although the irradiated uranium foils are thin (~70 μm to 90 μm), neutron self-shielding is not negligible. The calculation of the neutron field depression in the foils was challenged by unreliable results and long computing times using the MCNP6.2 Monte Carlo code. These simulations require the use of the variance reduction techniques to compute a detector response with low uncertainties within an acceptable computing time. An additional problem was the convergence of the spectrum averaged cross section of the 238U(n,γ) reaction, even with a high number of particle histories (e.g. 1 × 1010), even when the average neutron flux has low statistical uncertainty. To speed up the calculations and to test the reliability of variance reduction techniques, a comparison was made with the tally results and statistical uncertainties of the MCNP6.2 weight window technique, ADVANTG-accelerated MCNP6.2 and default MCNP6.2 calculations. To generate variance reduction parameters with the ADVANTG code for the final calculations, the consistent adjoint driven importance sampling (CADIS) method is used. The neutron self-shielding has been found considerable in the 238U(n,γ) reaction due to three strong resonances (at 7 keV, 21 keV and 37 keV) leading to a correction factor of 1.57 for the natural uranium foil. The sensitivity study has shown a negligible impact from the selected nuclear data libraries (JEFF and ENDF/B) on the 238U(n,γ) and 235U(n,f) spectrum averaged cross sections in the MARK III converter.

Possibilities of producing superheavy nuclei in multinucleon transfer reactions based on radioactive targets * * Supported by the National Natural Science Foundation of China (11605296); the Natural Science Foundation of Guangdong Province, China (2016A030310208); and Fundamental Research Funds for the Central Universities (18lgpy87)

The multinucleon transfer (MNT) process has been proposed as a promising approach to produce neutron-rich superheavy nuclei (SHN). MNT reactions based on the radioactive targets 249Cf, 254Es, and 257Fm are investigated within the framework of the improved version of a dinuclear system (DNS-sysu) model. The MNT reaction 238U + 238U was studied extensively as a promising candidate for producing SHN. However, based on the calculated cross-sections, it was found that there is little possibility to produce SHN in the reaction 238U + 238U. In turn, the production of SHN in reactions with radioactive targets is likely.

Computational study of the neutron flux produced by the 2H(d,n)3He reaction

Quasi-monoenergetic neutron beams, in the energy range of 4-10 MeV produced via the 2H(d,n)3He reaction, have been used for cross section measurements at the 5.5 MeV tandem T11/25 Accelerator Laboratory of NCSR “Demokritos”. For accurate cross section measurements, it is important that the energy profile of the neutron beam and the experimental conditions are well characterized. Therefore, an investigation of the energy dependence of the neutron fluence, especially taking into account the deuteron break up phenomenon, has been carried out in the present work, using the recently developed analytical and Monte-Carlo simulation code NeusDesc. Fission experimental data from 238U ultra-thin targets, were used to validate the simulated results. The fission fragments of the 238U(n,f) reaction have been detected via MicroMegas detectors and using the well known cross section of the 238U isotope, the neutron flux has been deduced. The simulated reaction rates were in good aggrement with the experiment ones, thus confirming the validity of the simulations.

Measurement of the 237Np(n,f) cross section with the FIC detector at the CERN n_TOF facility

The cross section of the 237Np(n,f) reaction has been experimentally determined at the n_TOF facility at CERN, in the neutron energy range 100 keV – 10 MeV, relative to the standard 235U(n,f) and 238U(n,f) reaction cross sections with use of a Fast Ionisation Chamber. The reproduction of the high accuracy data obtained from this experiment was attempted within the Hauser-Feshbach formalism and phenomenological models with use of the code EMPIRE 3.2.

Monte-Carlo simulation of ion distributions in a gas cell for multinucleon transfer reaction products at LENSHIAF spectrometer

The multinucleon transfer (MNT) reaction is one promising way to produce neutron-rich heavy nuclei and even super heavy nuclei and attracts more and more attentions theoretically and experimentally. A low energy nuclear structure spectrometer called LENSHIAF specific to the MNT reactions will be designed and constructed in the ongoing big project HIAF in China. In the LENSHIAF spectrometer, the most challenge part is how to collect and stop efficiently the high-energy MNT products into the gas cell. By using Monte-Carlo method, the geometry of the gas cell, the thickness of the titanium window/degrader, and the optimal gas pressure filled in the gas cell have been calculated and estimated. For neutron-rich nuclei around N = 126 from 136Xe + 198Pt reaction, with a titanium window/degrader with a thickness of 2.5–3.5 μm, a cylindrical helium gas cell with a length of 0.6 m and a diameter of 1.2 m can satisfy the requirements to stop the target-like fragments. For heavier and super heavy nuclei from 238U + 238U reaction, with a 5–8 μm thick titanium window/degrader, the cylindrical gas cell has to be as big as a length of at least 1.6 m and a diameter of 1.6 m.

Spectroscopy of Neutron Induced Reactions with the $\nu $-ball Spectrometer

The ʋ-ball is a high-efficiency hybrid spectrometer which consists of both germanium (Ge) detectors and associated anti-Compton BGO shields, coupled to lanthanum bromide (LaBr3) detectors. The hybrid configuration provides a combination of both excellent energy and timing resolutions. The ʋ-ball geometry allows the coupling with the LICORNE directional neutron source at the ALTO facility of the IPN, Orsay. This opens the possibility to perform precise spectroscopy of neutron induced reactions and was used for two experiments during the recent experimental campaign. These two experiments are described here: 1. Spectroscopy of the neutron-rich fission fragments produced in the 238U(n; f) and 232Th(n; f) reactions; 2. Spectroscopy above the shape isomer in 238U. The 238U(n; f) and 232Th(n; f) reactions produce hundreds of neutron-rich nuclei on which gamma-ray spectroscopy can be performed. The main goal of the experiment aiming to populate the shape isomer in 238U is the measurement of the gamma-ray and fission decay branches as well as determination of level scheme in the super-deformed minimum. The shape isomer is populated by 238U(n; n') reaction, which gives a very advantageous population cross section over other reactions. More detailed descriptions of these two ʋ-ball experiments will be presented here.

Feedback from experimental isotopic compositions of used nuclear fuels on neutron cross sections and cumulative fission yields of the JEFF-3.1.1 library by using integral data assimilation

Comparisons of calculated and experimental isotopic compositions of used nuclear fuels can provide valuable information on the quality of nuclear data involved in neutronic calculations. The experimental database used in the present study − containing more than a thousand isotopic ratio measurements for UOX and MOX fuels with burnup ranging from 10 GWd/t up to 85 GWd/t − allowed to investigate 45 isotopic ratios covering a large number of actinides (U, Np, Pu, Am and Cm) and fission products (Nd, Cs, Sm, Eu, Gd, Ru, Ce, Tc, Mo, Ag and Rh). The Integral Data Assimilation procedure implemented in the CONRAD code was used to provide nuclear data trends with realistic uncertainties for Pressurized Water Reactors (PWRs) applications. Results confirm the quality of the 235U, 239Pu and 241Pu neutron capture cross sections available in the JEFF-3.1.1 library; slight increases of +1.2 ± 2.4%, +0.5 ± 2.2% and +1.2 ± 4.2% are respectively suggested, these all being within the limits of the quoted uncertainties. Additional trends on the capture cross sections were also obtained for other actinides (236U, 238Pu, 240Pu, 242Pu, 241Am, 243Am, 245Cm) and fission products (103Rh, 153Eu, 154Eu) as well as for the 238U(n,2n) and 237Np(n,2n) reactions. Meaningful trends for the cumulative fission yields of 144Ce, 133Cs, 137Cs and 106Ru for the 235U(nth,f) and 239Pu(nth,f) reactions are also reported.

The Effect of Super-Radiance on the C-A Transition of the Excimer Molecule XeCl* at 352 nm Excited Only by <sup>235</sup>U Fission Fragments

The work is devoted to the study of spectral, temporal and generation characteristics of plasma generated in dense gas mixtures by the products of neutron nuclear reaction 235U(n, f). This plasma differs in its properties from the discharge plasma, as has the track structure and low temperature of electrons, and the presence in gas mixtures of electronegative gas can become without electrons. On the basis of such nuclear-excited plasma excimer gas lasers with the nuclear pumping, carrying out direct transformation of nuclear energy to the laser radiation UV range of lengths of waves, can be created. We found out high efficiency of formation of excimer molecules of XeCl* in dense (~ 760 Торр) Ar - Xe - CCl 4 of gas mixture with low (~ 1015 mol/cм3) concentration of CCl4 at the nuclear pumping. Here we report the first clear observation of super-radiance at 352 nm on C-A transition of excimer molecule XeCl* excited solely by nuclear pumping Ar-Xe-CCl4 gas mixture at a pressure of 760 Torr with a low concentration of CCl4 (0.15Torr). The mixture was excited by products of neutron nuclear reaction 235U(n,f) pumped at a rate of 2kW/cm3. A laser cell with a length of 100cm was placed inside an optical resonator tuned at 352 nm. Super-radiance had a very sharp increase and low neutron threshold (about 2÷4.1014 neutron/cm2. s) when we used the tuning resonator at 352 nm wavelength with 10.8% reflection of output and 62.1% reflection of back spherical mirrors, and super-radiance disappeared if the laser output mirror was replaced by a quartz window.

Measurement of 232Th and 238U neutron capture cross-sections in the energy range 5–17 MeV

The neutron capture cross sections of 232Th and 238U at the average neutron energies of 5.08 ± 0.17, 8.96 ± 0.77, 12.47 ± 0.83, and 16.63 ± 0.95 MeV have been measured by using the activation technique and off-line γ-ray spectroscopy. The 232Th and 238U were irradiated with neutrons produced from the 7Li(p, n) reaction using the proton energies of 7, 11, 15 and 18.8 MeV from the 14UD BARC-TIFR Pelletron facility in Mumbai, India. Detailed covariance analysis was also performed to evaluate the uncertainties in the measured cross-sections. The excitation function of the 232Th(n, γ) and 238U(n, γ) reactions were calculated using the theoretical model code TALYS-1.9. The experimental and theoretical results from the present work were compared with the ENDF/B-VII-1 and JENDL-4.0 nuclear data libraries and were found to be in good agreement.

Influence of the shell effects on evaporation residue cross section of superheavy nuclei

In order to study the influence of the shell effects on the formation and fission of superheavy elements, we applied multidimensional Langevin equations. The evaporation residue cross sections have been calculated for 3n, 4n, and 5n evaporation channels using three ( $$K=0$$ )- and four ( $$K \ne 0$$ )-dimensional Langevin equations. Calculations were done for 48Ca + 238U and 48Ca + 244Pu hot fusion reactions with 3n, 4n evaporation channels and 70Zn + 208Pb, and 54Cr + 209Bi cold fusion reactions with 1n and 2n evaporation channels. The calculations were performed for 4n and 5n evaporation channels of the 26Mg + 238U reaction, as well. Our results show that with increasing dimension of Langevin equations the residue cross section increases, whereas the fission cross section decreases. The obtained results with four-dimensional Langevin and considering shell effects are in better agreement with experimental data in comparison with three- and four-dimensional Langevin equations without shell effects.

Investigation on Neutron-Induced Fission Fragment Angular Distribution of 232Th and 238U

Abstract Fission fragment angular distribution (FFAD) data would help obtain new insights into both the fission process and the mechanism of the projectile’s interaction with the nucleus. Recently, a structure has been reported in neutron-induced FFAD of even-even actinide nuclei near threshold. Statistical modelling is used in this article to analyse FFAD from neutron-induced fission of 238U and 232Th. The statistical variance K0 2 is obtained by fitting the measured fragment anisotropies with a theoretical model. Accurate analysis is performed to deduce the variance K0 2 of the K-distribution of the levels in the transition nucleus at neutron energies from threshold to 50 MeV. We show the method by which quantitative values of K0 2 can be obtained. The results not only present high-resolution data in these even-even nuclei but also show that for the 238U(n,f) reaction, the strength for the K-transition states comes mainly from the higher angular momenta, in agreement with the Nilsson model orbitals. We also study the periodic structure of anisotropy related to the set of (n,f) reactions in comparison with the related cross section performed with the TALYS code. The comparison of the variance with the cross section clearly illustrates the strong correlation between the value of the variance and the opening of a fission chance. We show that whenever the probability of reaction in a new channel and cross section increases, K0 2 decreases versus the incident neutron energy, so the minimum of K0 2 can show the maximum probability of the (n,f)xn reaction.

Systematics of Nd cumulative fission yields for neutron-induced fission of 235U, 238U, 238Pu, 239Pu, 240Pu and 241Pu

Systematics of cumulative fission yields of the neodymium isotopes for the thermal and fast neutron fission of 235U, 238U, 238Pu, 239Pu, 240Pu and 241Pu were obtained by combining integral results from the PROFIL experiments with theoretical calculations from the GEF code. The systematic behavior with the neodymium mass number (A = 143, 145, 146, 148, 150) deduced from the experimental trends is consistent with the smooth variation predicted by the GEF calculations, excepted for the 238U(n,f) reaction. For this system, isotopic and isotonic effects in fission-fragment shell structures are not adequately taken into account in the theoretical calculation. The obtained results also confirm the weak energy dependence of the Nd cumulative fission yields in the energy range of interest for thermal and sodium fast reactors. They suggest an energy dependency comparable to the experimental uncertainty which lies below 3%, for the 235U, 239Pu and 241Pu fissile isotopes.

Formation of Superheavy Elements: Study Based on Dynamical Approach

Using multi-dimensional Langevin equations for the probability distribution of the distance between the surfaces of two approaching nuclei, we have studied the formation of superheavy elements via calculation of evaporation and fission cross sections of these elements. Evaporation residue cross sections have been calculated for the 1n, 2n, 3n, 4n, and 5n evaporation channels using one and four dimensional Langevin equations for the 48Ca+226Ra, 232Th, 238U, 237Np, 239,240,242,244Pu, 243Am, 245,248Cm, 249Bk, and 249Cf reactions. Our results show that with increasing dimension of Langevin equations the evaporation residue cross section is increased. Also, obtained results based on fourdimensional Langevin are in better agreement with experimental data in comparison with one-dimensional model.

Decay of Plutonium isotopes via spontaneous and heavy-ion induced fission paths

Based on the collective clusterization approach, we have extended our earlier study on α-decay, exotic cluster-decay, and heavy particle radioactivity, to the phenomenon of spontaneous fission (SF) in the ground-state (g.s.) decays of even mass 234−246Pu parents. The calculations for the SF half-lives of these Pu-isotopes have been made within the framework of preformed cluster model (PCM), both for spherical as well as β2-deformed choices of shapes, and a comparison is made with the relevant available experimental data, which prefer spherical shapes. The importance of the orientation degree of freedom (hot compact or cold elongated configurations) is also explored. Next, in order to look for the exclusive role of heavy-ion induced fission, the dynamics of 6He + 238U reaction forming 244Pu⁎ is studied over the center of mass energy range of Ec.m.=15.0–28.8MeV, using the dynamical cluster-decay model (DCM), an extension of the PCM with temperature T- and angular momentum ℓ-effects included. The β2-deformed fragments of 244Pu⁎ in the mass range A2=106–113 (plus their complementary heavy fragments), corresponding to asymmetric fission peaks, are found contributing towards the fission cross-section. Finally, the potential energy surfaces and barrier modification effects are presented for the relative comparison of spontaneous and the heavy-ion induced fission processes. Both are found to behave similar with respect to the probable emission of fragments and hence point out to the shell closure property of the decay fragments.