Neutron Induced Fission Of 235U Research Articles

Energy-Dependent Fission-Product Yields in the Region of Second-Chance Fission

The energy dependence of high-yield fission products has been measured using quasi-monoenergetic neutron beams at energies between 5.5 and 11.0 MeV. The absolute number of fissions during the irradiation period was determined via dual-fission ionization chambers, while the fission products were measured via direct g-ray spectroscopy. This paper presents absolute fission product yields from neutron-induced fission of 235U, 238U, and 239Pu isotopes for five incident energies in the second chance fission region.

Directional-polarization correlations using EXILL

Eight EXOGAM-type, Ge-clover detectors of the EXILL array, arranged on an octagon plan and acting as Compton polarimeters, were used to measure directional-polarization correlations of γ rays populated in various processes. Measurements of γ radiation following neutron-induced fission of 235U and 241Pu targets; β --decay following fission; (n,γ) reactions on various stable targets and measurements using radioactive sources of 60Co, 133Ba and 152Eu were performed at the PF1B cold-neutron facility of the Institut Laue-Langevin in Grenoble. Digital electronics and a trigger-less acquisition system allowed the collection of signals from Ge detectors down to about 20 keV, providing measurements of linear polarization down to 120 keV. The precise sensitivity calibration was determined for the set of eight EXILL clover polarimeters in a range from 100 keV to 10000 keV. A new formula for calculating the directional-polarization correlations for the upper transition in a γγ cascade is derived from first principles. Directional-polarization correlations were measured for a few dozens of transitions in a number of nuclei, including values for twenty transitions in the 152Sm nucleus measured for the first time, and compared against the calculated values, to illustrate the quality of the technique. The combined analysis of angular and directional-polarization correlations is presented, which helps the determination of spin-parity assignments to excited levels.

Lifetime measurements in 131Sb at LOHENGRIN

The neutron-rich 131Sb nucleus was studied at Institut Laue-Langevin, with the LOHENGRIN spectrometer, via neutron-induced fission of 235U and lifetime measurements using LaBr3(Ce) detectors. The lifetimes of the and states were measured with the generalized centroid difference and the decay slope methods, yielding T½=3(2) ps and T½=1347(171) ps, respectively. The measured value for the is at the limit of the fast-timing technique. The level scheme of 131Sb and the decay properties of the state were described by realistic shell-model calculations, which reproduce well both the experimental spin and energy sequences of 131Sb and the measured B(E2; ) value, pointing to an almost pure 2+(130Sn) ⊗ πg7/2, core-proton coupled configuration for the state.

Core-proton coupled nature of the 11/2+ state in 131 Sb probed by lifetime measurements

The structure of the state in 131Sb was investigated at the LOHENGRIN spectrometer of Institut Laue-Langevin via neutron-induced fission of 235U and lifetime measurements, yielding , at the edge of the sensitivity of the experimental method. This first result for the state half-life in neutron-rich Sb isotopes provides a quadrupole reduced transition probability to the ground state of W.u., very close to the in 130Sn. Realistic shell-model calculations reproduce well both the experimental level scheme of 131Sb and the value, indicting a dominant core-proton coupled configuration for the state.

The SINQ gas-jet facility as a source for radionuclides from neutron-induced fission of 235U

The Swiss Spallation Neutron Source SINQ of the Paul Scherrer Institute provides neutrons via proton-induced spallation reactions in a lead target. Produced neutrons are thermalized and impinge on 235U-targets, enclosed in a three-part chamber assembly, which is located in the inner wall of the SINQ shielding. The thermal-neutron-induced fission products can be readily transported from this chamber assembly to a radiochemical laboratory using the gas-jet technique either with a pure carrier gas or with an aerosol-particle-loaded carrier gas. In the past, mainly radioisotopes of the elements Se, Br, Rb, and Kr were retrieved and used for gas-phase chemistry experiments. Here, we present first experiments after the commissioning of the SINQ gas-jet facility as a source of recoverable, non-volatile and volatile, carrier-free fission products for general radiochemical studies and other applications.

Activity Measurement of Iodine Fission Products in 2LiF-BeF2 Salt and Its Application in Redox Potential Surveillance for a Thorium Molten Salt Reactor.

The activities of 131I, 132I, 133I, and 135I produced by neutron-induced fission of 235U in 2LiF-BeF2 (FLiBe) eutectic salt and their dependence on the redox potential were studied. The dependence observed experimentally suggested that the activity ratio for 131I to 132I could be used as an indicator of the redox potential for FLiBe salt. Relying on the selective adsorption of iodine ions on the activated silver probe by ion exchange, a novel method for activity distribution measurement of the iodine isotopes in FLiBe salt was founded. The method is simple, fast, and easy to operate and would be suitable particularly to in situ monitor the redox potential of a thorium molten salt reactor, where the redox potential should keep at a high level to avoid possible safety risk induced by 233Pa deposition in the reactor.

Coprecipitation with samarium hydroxide using multitracer produced through neutron-induced fission of 235U toward chemical study of heavy elements

For new chemical studies on heavy elements, we previously investigated the coprecipitation behaviors with samarium hydroxide for various elements. Herein, we report the coprecipitation experiment using multitracer produced by neutron-induced fission of 235U. The coprecipitation behaviors of 10 elements were investigated: new data were obtained for Sr, Ru, I, Pm, and Np. The present results support the previously obtained conclusion that the hydroxide precipitation properties of various elements can be qualitatively investigated through their coprecipitation behaviors.

Low- Z boundary of the N=88 –90 shape phase transition: Ce148 near the critical point

Excited states of the neutron-rich nucleus Ce148 have been populated by neutron-induced fission of a U235 target. Its electromagnetic decays were studied by means of γ-ray coincidence spectroscopy with fast-timing capabilities. Lifetimes of the 21+ and 41+ states of Ce148 were obtained and their E2 decay rates deduced. The B4/2=B(E2;41+→21+)/B(E2;21+→01+) ratio indicates that Ce148 is a transitional nucleus while the N=88/90 shape phase transition evolves into a gradual change of nuclear deformation for proton numbers Z<60.2 MoreReceived 29 October 2019DOI:https://doi.org/10.1103/PhysRevC.101.014303©2020 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCollective levelsFissionLifetimes & widthsNuclear structure & decaysNucleon distributionProperties90 ≤ A ≤ 149Nuclear Physics

Prompt Fission Neutron Spectra for Neutron-Induced Fission of 239Pu and 235U

We report the current results of a large effort to accurately measure the Prompt Fission Neutron Spectra (PFNS) for neutron-induced fission of 235U and 239Pu for incident neutrons with energies from 1 to 20 MeV. The Chi-Nu experiment at the Los Alamos Neutron Science Center used an unmoderated, white spectrum of neutrons to induce fission in actinide samples that were placed inside a parallel plate avalanche counter to provide a fast fission signal. A double time-of-flight technique was used to determine the incoming and outgoing neutron energies. Two neutron detector arrays, one with 54 liquid scintillators and another with 22 lithium glass detectors, were used to detect the outgoing neutrons and measure the PFNS distributions over a wide range in outgoing neutron energy, from below 100 keV to 10 MeV. Extensive Monte Carlo modeling was used to understand the experiment response and extract the PFNS. Systematic errors and uncertainties in the method have been examined and quantified. A summary of these results for incoming energies from 1 to 5 MeV is presented here.

Prompt Fission Gamma-Ray Measurements at UML Research Reactor

Neutron-induced fission of 235U was studied at the thermal column of the UMass Lowell 1 MW Research Reactor. A collimated, 2.25-inch diameter beam of thermal neutrons with the flux of ~5x105 n/cm2/sec induced fission reaction on a plate of low-enriched uranium with the areal density ~25 mg/cm2 of 235U. We have used the prompt fission-neutron tagging method to identify the fission reaction in the off-line analysis. The method employs the pulse-shape discrimination of neutrons and gamma-ray events in stilbene scintillator and enables identification of coincidence events of prompt fission gamma-rays and prompt fission neutrons in coincidence time intervals less than 20-30 ns. The prompt gamma-ray radiation was detected using two co-linear NaI(Tl) detectors. The measured spectra of prompt-fission gamma rays between 150 keV and 6 MeV are presented. The results from these initial measurements demonstrate the feasibility of the experimental method. Future measurements with extended arrays of detectors are planned.

Lifetimes and shape-coexisting states of Zr99

Lifetimes of intermediate-spin states in two rotational bands of 99Zr have been measured. These states were populated following the neutron-induced fission of 235U at the PF1B beamline of the Institut Laue-Langevin, Grenoble, during the EXILL-FATIMA campaign. The nucleus 99Zr59 exhibits shape coexistence and lies precisely on the border of an abrupt change in ground-state deformation when going from N=58 to N=60, making its study interesting for understanding the mechanisms involved in the rapid onset of deformation here. The B(E2) values extracted for decays in the ν3/2[541] band allow quadrupole deformations of β2=0.34(1) and 0.26(3) to be determined for the 821.6- and 1236.6-keV members, whereas β2=0.32(3) was found for the 850.5-keV member of the ν3/2[411]band. Some of the excited states known in 99Zr have been reasonably described with interacting boson-fermion model (IBFM) calculations. Type-II shell evolution is proposed to play a major role in modifying single-particle energies in 99Zr.

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.

Evaluations of Energy Spectra of Neutrons Emitted Promptly in Neutron-induced Fission of 235U and 239Pu

The energy spectra of neutrons emitted promptly in the neutron-induced fission reactions of 235U and 239Pu were re-evaluated for ENDF/B-VIII.0. These evaluations are based on a careful modeling of all relevant physics processes, an extensive analysis of experimental data and a detailed quantification of pertinent uncertainties. Energy spectra of neutrons emitted in up to fourth chance fission are considered and both compound and pre-equilibrium processes are included. Also, important nuclear model parameters, such as the average total kinetic energy of the fission fragments and the multiple chance fission probabilities, and their uncertainties are estimated based on experimental knowledge, model information and evaluated data. In addition to experimental information already available for ENDF/B-VII.1, these new evaluations make use of recently published experimental data either of high precision or spanning a broad incident energy range, information on legacy measurements explaining discrepancies and recently measured data of the average total kinetic energy as a function of incident neutron energy. The resulting evaluated data and covariances agree well with the experimental database used for the evaluation. However, the evaluated spectra are softer than the 235U and 239Pu ENDF/B-VII.1, JENDL-4.0 and JEFF-3.2 evaluations for incident neutron energies Einc≤1.5 MeV and Einc≤5 MeV, respectively. For Einc>5 MeV, the evaluated spectra show structures due to the improved modeling which are not present in ENDF/B-VII.1 and JEFF-3.2 but can be observed in JENDL-4.0 evaluations. Part of these new evaluations were adopted for ENDF/B-VIII.0, while the ENDF/B-VII.1 239Pu PFNS was retained for Einc≤5 MeV awaiting more conclusive experimental evidence.

Fission fragment yields and total kinetic energy release in neutron-induced fission of235,238U,and239Pu

Different aspects of the nuclear fission process have been studied at Los Alamos Neutron Science Center (LANSCE) using various instruments and experimental techniques. Properties of the fragments emitted in fission have been investigated using Frisch-grid ionization chambers, a Time Projection Chamber (TPC), and the SPIDER instrument which employs the 2v-2E method. These instruments and experimental techniques have been used to determine fission product mass yields, the energy dependent total kinetic energy (TKE) release, and anisotropy in neutron-induced fission of U-235, U-238 and Pu-239.

Measurements of the Prompt Fission Neutron Spectrum at LANSCE: The Chi-Nu Experiment

The goal of the Chi-Nu experiment at the Los Alamos Neutron Science Center is to measure the prompt fission neutron spectra from major actinides using a double time-of-flight method with a pulsed, white incoming neutron source. Fission events are detected with a parallel-plate avalanche counter and outgoing neutrons are detected with either a 6Li-glass or liquid scintillator detector array for low- or high-energy neutrons, respectively. A detector response matrix for the interaction of neutrons with the experimental environment for neutrons measured with the Chi-Nu 6Li-glass detector array has been calculated to obtain a full understanding of the measured Chi-Nu data and also to allow for nearly instantaneous production of simulated Chi-Nu data spectra. Prompt fission neutron spectra corresponding to 19 incoming neutron energy ranges from 0.7-20 MeV have been extracted using the ratio-of-ratios method with Chi-Nu 6Li-glass data on the neutron-induced fission of 235U.

Excited States and Collectivity in 88Se

The γ decays of excited states in the very neutron-rich nucleus 88Se have been observed following the cold neutron-induced fission of 235U at the PF1B facility of the Institut Laue-Langevin (ILL), Grenoble. The EXILL array was used to measure γ - γ - γ coincidences, which were then analysed to build a level scheme of 88Se. A low (2+1) energy hints at the onset of quadrupole deformation and the identification of possible members of a (2+2) band provide evidence for γ vibrations. Shell-model calculations using a 78Ni core reproduce the experimental decay scheme well, implying that interactions between particles in the π f5/2 p and vsd orbits are responsible for much of the collectivity present. The algebraic collective model has also been used to interpret the experimental data, showing that that the experimental spectrum is consistent with 88Se being a transitional nucleus, possessing weak static β deformation and γ instability.

Shape coexistence in the odd-odd nucleus Y98 : The role of the g9/2 neutron extruder

Excited states in Y98, populated in neutron-induced fission of U235 and in spontaneous fission of Cm248 and Cf252, have been studied by means of γ spectroscopy using the Lohengrin fission-fragment separator at ILL Grenoble and the EXILL, Eurogam2, and Gammasphere Ge arrays. Two new isomers have been found in Y98: a deformed one with T1/2=180(7) ns and a rotational band on top of it, and a spherical one with T1/2=0.45(15)μs, analogous to the 8+ isomer in Y96, corresponding to the (νg7/2,πg9/2)8+ spherical configuration. Using the JYFLTRAP Penning trap, an accurate excitation energy of 465.7(7) keV has been determined for the 2.36-s isomer in Y98. This result and the studies of excited levels in Zr98, populated in β− decay of the isomer, indicate a new spin-parity, Iπ=(7)+ for the isomer. The high spin and the decay properties of this isomer suggest the presence of the 9/2+[404] neutron extruder orbital in its structure. This is consistent with the large deformation of the isomer, reported recently. The present work does not provide arguments to support the special role of the νg7/2−πg9/2 interaction (the spin-orbit-partner, or SOP, mechanism).13 MoreReceived 9 May 2017Revised 8 September 2017DOI:https://doi.org/10.1103/PhysRevC.96.044333©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasEnergy levels & level densitiesIsomer decaysLifetimes & widthsNuclear structure & decaysProperties90 ≤ A ≤ 149Nuclear Physics

Fission Reaction Event Yield Algorithm FREYA 2.0.2

FREYA (Fission Reaction Event Yield Algorithm) is a fission event generator which models complete fission events. As such, it automatically includes fluctuations as well as correlations between observables, resulting from conservation of energy and momentum. The purpose of this paper is to present the main differences between FREYA versions 1.0 and 2.0.2 : additional fissionable isotopes, angular momentum conservation, Giant Dipole Resonance form factor for the statistical emission of photons, improved treatment of fission photon emission using RIPL database, and dependence on the incident neutron direction. FREYA 2.0.2 has been integrated into the LLNL Fission Library 2.0.2, which has itself been integrated into MCNP6.2, TRIPOLI-4.10, and can be called from Geant4.10. New version program summaryProgram title:FREYA 2.0.2Program Files doi: ” http://dx.doi.org/10.17632/2mssy7r3gt.1”Licensing provisions: BSD 3-clauseProgramming language: Fortran 90, C++Journal Reference of previous version: J. M Verbeke, J. Randrup, R. Vogt, “Fission Reaction Yield Algorithm FREYA for Event-by-Event Simulation of Fission,” Comp. Phys. Comm. 191, pp. 178–202; doi:10.1016/j.cpc.2015.02.002 (2015).Does the new version supersede the previous version? Yes.Reasons for the new version: New physics and more fissionable isotopes.Summary of revisions: Additional fissionable isotopes, angular momentum conservation, Giant Dipole Resonance form factor for the statistical emission of photons, improved treatment of fission photon emission using RIPL database, and dependence on the incident neutron direction.Nature of problem: Modeling of fission events.Solution method: Simulation of complete fission events, production of secondary fission fragments, fission neutrons and photons.Restrictions: Restricted to spontaneous fission of 238U, 238Pu, 240Pu, 242Pu, 244Cm, 252Cf; neutron-induced fission of 233U, 235U, 238U, 239Pu, 241Pu, for incident neutron energies less than 20 MeV.

Angular distributions and anisotropy of the fragments from neutron-induced fission of 233U and 209Bi in the intermediate energy range of 1–200 MeV

New results of the neutron-induced fission experiments carried out at the neutron time-of-flight spectrometer GNEIS of the PNPI are given. Angular distributions of fission fragments from the neutron-induced fission of 233U and 209Bi nuclei have been measured in the energy range 1–200 MeV using position sensitive multiwire proportional counters as fission fragment detector. The recent improvements of the measurement and data processing procedures are described. The data on anisotropy of fission fragments deduced from the measured angular distributions are presented in comparison with the experimental data of other authors.

Near-yrast Excitations in Nucleus $^{83\mskip -\thinmuskip }$As: Tracing the Deformation in the $^{78}$Ni Region

Medium-spin, yrast excitations in nucleus 83As have been studied in neutron-induced fission of 235U. The experiment took place at the Institut Laue-Langevin in Grenoble where gamma rays were registered using the EXILL detector array. Recently, we proposed new tentative (9=2+) spin assignment for 2777 keV level in 83As, which is now discussed in the context of deformation showing up in 78Ni region.