Mass-yield Curve Research Articles

Total kinetic energy and mass yields from the fast neutron-induced fission of $$^{239}\hbox {Pu}$$

The total kinetic energy (TKE) release in fission is an important observable, constituting over 80% of the energy released in fission (E$_{f}$ $\approx$ 200 MeV). While the TKE release in the $^{239}$Pu(n,f) reaction was previously measured up to 50 MeV incident neutron energy (E$_{n}$), there were features in TKE release at the highest values of E$_{n}$ that were puzzling. There was a marked flattening of TKE release from E$_{n}$ = 30 to 50 MeV, in disagreement with the clearly decreasing TKE observed from E$_{n}$ = 0.5 to 30 MeV. To verify and clarify this trend, TKE measurements at higher values of E$_n$ were made. We present absolute measurements of TKE release in $^{239}$Pu(n,f) from E$_{n}$ = 2.4 to 100 MeV. We used silicon PIN detectors to measure the fragment energies and deduce mass-yield curves using the 2E-method. We also discuss fission asymmetry and the relationships between approximate fission fragment mass and distortion.

Oversize of the average prompt neutron multiplicity measured by the 1V1E method in the symmetric region of thermal neutron-induced fission of 239Pu

Using a Monte Carlo method, the measurement by the 1V1E technique of the average prompt neutron multiplicity as a function of the mass of fragments from the thermal neutron-induced fission of 239Pu is simulated. The input data, associated with the masses of complementary primary fragments (A1, A2), consist of the yield (Y), the average total kinetic energy (TKE-) and its standard deviation (σTKE), the average prompt neutron multiplicity (ν-s, a sawtooth approach of an experimental curve ν-), and the slope of neutron multiplicity against total kinetic energy, -dν/d<TKE>, plotted as a function of fragment mass. The output data, associated with the provisional masses of complementary fragments (m1∗, m2∗) calculated with the 1V1E method, consist of the simulated quantities Y, TKE-, σTKE, N-, comparable to the measured ones. In comparison with ν-s, N- and ν-are oversized in the region of light fragment masses near the symmetric fission. To interpret the results, the neighboring masses ratio RA =YA+1/Y(A) and the term FA=A-m∗, where m∗ is the average of provisional mass as a function of A, are defined. It is shown that N--ν-s and ν--ν-s are correlated with -F(R-1). It is concluded that the oversize of ν- relative to ν-s is due to the interplay of the prompt neutron emission and the slope of the mass yield curve.

Quantification of Trace-Level Fissile Samples via Short-Lived Delayed Gamma Spectroscopy

We describe a generalized means of isotopic assay and quantification of multicomponent fissile mixtures based on the observation of short-lived fission-product gamma signatures. Given that the characteristic mass yield curves differ for individual fissile species, this can be used to construct an overdetermined system of equations corresponding to the observed photopeak yields, the solution of which yields information on the individual isotopic masses of parent fissile isotopes within the system. This method is then demonstrated for single-isotope samples of 233U, 235U, and 239Pu; binary fissile mixtures of U and Pu; and trace fissile samples contained within intact typical environmental sampling media, each irradiated and measured using the High Flux Isotope Reactor at the Oak Ridge National Laboratory. Our findings indicate that this method can reliably be used to quantify nanogram-level fissile masses in binary mixtures with uncertainties on the order of 2%–7%. Finally, observed discrepancies with individual fission-product indicators and likely sources of bias are discussed.

Systematic study of proton-induced spallation reactions with the Constrained Molecular Dynamics (CoMD) model

Proton – induced spallation reactions on 238U, 208Pb, 181Ta and 197Au targets at high energies were studied and investigated using the microscopic Contrained Molecular Dynamics (CoMD) model. Total fission cross sections, the ratio fission cross section to residue cross section, mean kinetic energy of fission fragments, mass yield curves and the number of nucleons emitted, before and after scission, as well as the total nucleon multiplicity were calculated using the CoMD model and compared with experimental data from the literature. Some of our calculations showed satisfactory agreement with available experimental data.The calculations of cross sections and the ratio fission cross section to residue cross section as a function of the proton energy gave us the opportunity to estimate observables for unmeasured nuclides.

Systematic study of proton-induced spallation reactions with microscopic and phenomenological models

Proton induced spallation reactions on 238U, 208Pb, 181Ta and 197Au targets at high energies were investigated using the microscopic Contrained Molecular Dynamics (CoMD) model and the phenomenological models INC and SMM. We have calculated the total fission cross sections, the ratio fission cross section to residue cross section, the mass yield curves and the excitation energy after the intranuclear cascade using the CoMD model and the INC/SMM phenomenological models. We made a comparison between the models and the experimental data from the liter- ature. Our calculations showed satisfactory agreement with available experimental data and suggest further improvements in the models. Our study with the CoMD code represents the first complete dynamical description of the spallation process with a microscopic code based on an effective nucleon-nucleon interaction.

Neutron-fragment and Neutron-neutron Correlations in Low-energy Fission

A computational method has been developed to simulate neutron emission from thermal-neutron induced fission of 235U and from spontaneous fission of 252Cf. Measured pre-emission mass-yield curves, average total kinetic energies and their variances, both as functions of mass split, are used to obtain a representation of the distribution of fragment velocities. Measured average neutron multiplicities as a function of mass split and their dependence on total kinetic energy are used. Simulations can be made to reproduce measured factorial moments of neutron-multiplicity distributions with only minor empirical adjustments to some experimental inputs. The neutron-emission spectra in the rest-frame of the fragments are highly constrained by ENDF/B-VII.1 prompt-fission neutron-spectra evaluations. The n-f correlation measurements of Vorobyev et al. (2010) are consistent with predictions where all neutrons are assumed to be evaporated isotropically from the rest frame of fully accelerated fragments. Measured n-f and n-n correlations of others are a little weaker than the predictions presented here. These weaker correlations could be used to infer a weak scission-neutron source. However, the effect of neutron scattering on the experimental results must be studied in detail before moving away from a null hypothesis that all neutrons are evaporated from the fragments.

Impact of organically bonded potassium on torrefaction: Part 2. Modeling

In torrefaction, the mass yield depends on the biomass type, size of the biomass, torrefaction temperature, and residence time. Mass yield curves vs. residence time are usually modeled based on biomass type at different torrefaction temperature. This work is the second part of a study on the effect of alkali metals on torrefaction in which the K content affects the degradation of biomass during torrefaction.In this part of the study, the mass loss of spruce wood with different content of K was modeled using a two-step reaction model based on four kinetic rate constants. The results show that it is possible to model the mass loss of spruce wood doped with different levels of K using the same activation energies but different pre-exponential factors for the rate constants. Three of the pre-exponential factors increased linearly with increasing K content, while one of the pre-exponential factors decreased with increasing K content. Therefore, a new torrefaction model was formulated using the hemicellulose and cellulose content and K content. The new torrefaction model was validated against the mass loss during the torrefaction of aspen, miscanthus, straw and bark. There was good agreement between the model and the experimental data for the other biomasses, except bark. For bark, the mass loss of acetone extractable material was also needed to be taken into account. The new model can describe the kinetics of mass loss during torrefaction of different types of biomass. This is important for considering fuel flexibility in torrefaction plants.

Symmetric and asymmetric fission modes in proton-induced fission at 660 MeV of 238U

Fission product cross sections of intermediate-energy fission of 238U were used in order to construct the charge and mass yield distributions. Enriched target of 238U was irradiated by proton beam with energy 660 MeV for several hours at the LNP Phasotron, Joint Institute for Nuclear Research (JINR), Dubna, Russia. The charge distribution of the fission fragments was analyzed for calculation of isobaric cross sections. The mass yield curves were expanded into symmetric and asymmetric components according multimodal fission approach. The fissility values of actinides were calculated at given proton energy. The obtained results have been compared to the same data for targets 237Np and 241Am.

Origin of the narrow, single peak in the fission-fragment mass distribution forFm258

We discuss the origin of the narrowness of the single peak at mass-symmetric division in the fragment mass-yield curve for spontaneous fission of $^{258}\mathrm{Fm}$. For this purpose, we employ the macroscopic-microscopic model and calculate a potential-energy curve at the mass-symmetric compact scission configuration, as a function of the fragment mass number, which is obtained from the single-particle wave-function densities. In the calculations, we minimize total energies by varying the deformations of the two fragments, with constraints on the mass quadrupole moment, and by keeping the neck radius zero. The energies thus become functions of mass asymmetry. Using the obtained potential, we solve the one-dimensional Schr\odinger equation with a microscopic coordinate-dependent inertial mass to calculate the fragment mass-yield curve. The calculated mass yield, expressed in terms of the microscopic mass density, is consistent with the extremely narrow experimental mass distribution.

The Percolation Mechanism for Fragmentation of Nuclei Using Monte Carlo Technique

A statistical bond-percolation model for the fragmentation has been applied to the proton-induced reactions assuming a lattice structure to the prefragment nucleus and using the Monte Carlo technique to determine the bond to be broken. The model succeeded to reproduce the essential features of the mass yield curves for the p-Cu reaction at 3.9 GeV and to describe qualitatively the charge and the multiplicity distributions of the projectile fragments for the 28Si interactions with the quasi-free emulsion nucleon at 3.7 AGeV.

Characteristics of two fission modes

Characteristics of nuclear deformation properties at scission of two fission modes, symmetric and asymmetric, are reviewed. Existence of two kinds of scission configurations associated with the symmetric and asymmetric fission modes is pointed out: elongated and compact configurations. Each symmetric and asymmetric scission property is discussed in terms of shape elongation evaluated from fragment total kinetic energy (TKE). Fragment deformation at scission is also discussed based on neutron multiplicity data. From the systematic study of the scission properties in a wide range of actinide fission, the bimodal fission observed in spontaneous fission (SF) of the heavy actinides is interpreted as the result of the presence of the two fission paths of the ordinary asymmetric mode and a strongly shell-influenced symmetric one. The correlation between the fragment shell structures and the asymmetric mass-yield curves is described.

Target-dependence of light fragment production in photonuclear reactions at intermediate energies

Yields of7Be,10Be,22Na,24Na and28Mg produced fromnatB,natC,natO,23Na,27Al,natSi,natS,natCl,natK,natCa,natV,55Mn,natFe,59Co,natNi,natCu,89Y,natAg,natIn,141Pr,159Tb,165Ho and197Au in irradiations with bremsstrahlung having maximum end-point energies up to 1200 MeV were measured by γ-ray spectrometry (7Be,22Na,24Na and28Mg) and accelerator mass spectrometry (10Be). The yields of heavier fragments,39Cl,43, 44m, 44g, 46, 47, 48Sc,59Fe, and56, 57, 58, 60Co, were also measured by γ-ray spectrometry in order to survey the relative contribution of fragmentation to spallation and/or fission in the mass yield curves. A distinct difference of the target mass-dependence of these yields has made it possible to disentangle the contributions of photospallation and photofragmentation processes. It was found that the neutron-to-proton ratios of targets strongly affect the formation of the nuclei by fragmentation as revealed in the yields of the isotopic pairs as well as of the single isotopes. The photon results were compared with those of proton-induced reactions, and no clear effect due to the difference of the initial interactions of photons and protons with nuclei was found in the fragment yields.

Mass and charge distributions in the very asymmetric thermal neutron induced fission of the odd-Z nucleus 242mAm

Yields of light fission products ( A = 68, 70–84, 87, 88, 94, 96, 98, 102 and 106–108), their kinetic energies and nuclear charge distributions ( A = 71–84, 87 and 88) in the thermal neutron induced fission of the odd-Z nucleus 242mAm(Z = 95) were measured using the mass-separator Lohengrin at the Institute Laue-Langevin in Grenoble (France). The mass yield curve shows a fine structure at A = 70, probably due to shell and/or odd-even effects affecting also the nuclear charge distribution. The analysis of isotopic chain yields gives evidence for a very low excitation energy of the lightest fission fragments observed. A preferential formation of fragments with even Z is found for this odd-Z compound nucleus. Calculated values for the local odd-even effect are comparable with those for the neighbouring even-Z fissile nuclides and increase from 13% to 30% with increasing asymmetry of the mass split ( A L = 84 to 68 and Z L = 35 to 28). The neutron odd-even effect shows a similar but less pronounced behaviour.

Measurement of Fission Product Yields from Neutron-Induced Fission of Americium-241

To determine the fission product yields from thermal and epithermal neutron-induced fissions of 241Am by the Cd-ratio technique, highly-purified 241Am targets and neutron flux monitors of Co/Al and Au/Al alloys were irradiated in an aluminum capsule by using Japan Material Testing Reactor. The neutron fluxes and their epithermal neutron fractions were determined by measuring γ-rays of 60Co and 198Au. The radioactivities of 95Zr, 99Mo, 103Ru, 106Ru, 127Sb, 129mTe, 131I, 132Te, 133Xe, 140Ba, 141Ce, 147Nd and 156Eu as fission products were measured by γ-ray spectrometry after irradiation. The fission-product yields from 241 Am were calculated from the radioactivities. In the calculation, the contribution from the fissions of 242mAm, 242gAm and 242Cm produced by the neutron capture reaction of 241Am was corrected for, assuming that the mass yield curves in thermal and epithermal neutron-induced fissions are identical among 241Am, 242mAm, 242gAm and 242Cm. The fluctuations of the yields by the neutron capture reactions of the fission products during irradiation were also considered for the determination of the precise fission yields.

Proton-induced fission at 190 MeV ofnatW,197Au,natPb,208Pb,and232Th

Proton-induced fission at 190 MeV of ${}^{\mathrm{nat}}\mathrm{W},$ ${}^{197}\mathrm{Au},$ ${}^{\mathrm{nat}}\mathrm{Pb},$ ${}^{208}\mathrm{Pb},$ and ${}^{232}\mathrm{Th}$ is studied by means of an innovative method based on activation analysis. The fission-product mass distribution is reconstructed from the fission-product yields, which are obtained from off-line observed $\ensuremath{\gamma}$-ray spectra. For ${}^{\mathrm{nat}}\mathrm{W}{,}^{197}\mathrm{Au}{,}^{208}\mathrm{Pb},$ and ${}^{\mathrm{nat}}\mathrm{Pb}$ the fission process results in a symmetric mass distribution. In the case of ${}^{232}\mathrm{Th}$ the mass yield curve is decomposed into a mixed symmetric-asymmetric contribution originating from fissioning nuclides in the neighborhood of the target mass and a purely symmetric contribution from very neutron-deficient nuclides. The fission cross sections are extracted by integrating over the reconstructed mass distribution. The width of the mass distribution is determined as a function of target mass. The results for the fission cross sections are compared with other experimental data as well as with theoretical model calculations using the LAHET and the ALICE-91 codes.

The role of off-line mass spectrometry in nuclear fission.

The role of mass spectrometry in nuclear fission has been invaluable since 1940, when A. O. C. Nier separated microgram quantities of (235) U from (238) U, using a gas source mass spectrometer. This experiment enabled the fissionable nature of (235) U to be established. During the Manhattan Project, the mass spectrometer was used to measure the isotope abundances of uranium after processing in various separation systems, in monitoring the composition of the gaseous products in the Oak Ridge Diffusion Plant, and as a helium leak detector. Following the construction of the first reactor at the University of Chicago, it was necessary to unravel the nuclear systematics of the various fission products produced in the fission process. Off-line mass spectrometry was able to identify stable and long-lived isotopes produced in fission, but more importantly, was used in numerous studies of the distribution of mass of the cumulative fission yields. Improvements in sensitivity enabled off-line mass spectrometric studies to identify fine structure in the mass-yield curve and, hence, demonstrate the importance of shell structure in nuclear fission. Solid-source mass spectrometry was also able to measure the cumulative fission yields in the valley of symmetry in the mass-yield curve, and enabled spontaneous fission yields to be quantified. Apart from the accurate measurement of abundances, the stable isotope mass spectrometric technique has been invaluable in establishing absolute cumulative fission yields for many isotopes making up the mass-yield distribution curve for a variety of fissile nuclides. Extensive mass spectrometric studies of noble gases in primitive meteorites revealed the presence of fission products from the now extinct nuclide (244) Pu, and have eliminated the possibility of fission products from a super-heavy nuclide contributing to isotopic anomalies in meteoritic material. Numerous mass spectrometric studies of the isotopic and elemental abundances of samples from the Oklo Natural Reactor have enabled the nuclear parameters of the various reactor zones to be calculated, and the mobility/retentivity of a number of elements to be established in the reactor zones and the surrounding rocks. These isotopic studies have given valuable information on the geochemical behavior of natural geological repositories for radioactive waste containment. © 1997 John Wiley & Sons, Inc.

Systematic variation of fission barrier heights for symmetric and asymmetric mass divisions.

The excitation functions and the mass yield curves for proton-induced fissions of $^{233}\mathrm{U}$, $^{235}\mathrm{U}$, $^{236}\mathrm{U}$, $^{237}\mathrm{Np}$, $^{239}\mathrm{Pu}$, $^{242}\mathrm{Pu}$, $^{244}\mathrm{Pu}$, $^{241}\mathrm{Am}$, and $^{243}\mathrm{Am}$ targets were studied. The excitation functions for near-symmetrically divided fragments were found quite different in shapes and threshold energies from those of asymmetrically divided ones in all the fissioning systems studied. In order to obtain the fission barrier heights for symmetric and asymmetric mass division, the conventional Bohr-Wheeler type calculations for the competition between fission and neutron emission were performed. In the calculation, two independent fission channels, symmetric and asymmetric, were assumed, and fission barrier heights and level density parameters that could best reproduce the shapes of the excitation functions (incident energy dependence) of typical symmetric and asymmetric products were deduced. Barrier heights for symmetric and asymmetric mass divisions both decrease as the mass of fissioning nuclide ${\mathit{A}}_{\mathit{f}}$ increases, and they also exhibit slight dependence on the atomic number. Difference between the two barrier heights is 2.0--2.5 MeV in the region of ${\mathit{A}}_{\mathit{f}}$=236--245. From similar analysis of excitation function data reported in literature, it is found that the difference changes from a large positive value for N\ensuremath{\sim}150, neutron number of the fissioning nuclide, to a large negative value for N\ensuremath{\sim}126, and the two barrier heights become comparable at N=135--138.

Mass yield curves in low-energy proton-induced fission of 233U, 235U, 236U, 237Np, 239Pu, 242Pu, 244Pu, 241Am, and 243Am.

Mass yield curves in low-energy proton-induced fission of $^{233}\mathrm{U}$, $^{235}\mathrm{U}$, $^{236}\mathrm{U}$, $^{237}\mathrm{Np}$, $^{239}\mathrm{Pu}$, $^{242}\mathrm{Pu}$, $^{244}\mathrm{Pu}$, $^{241}\mathrm{Am}$, and $^{243}\mathrm{Am}$ were investigated in the energy range of 8--32 MeV. The gross features of mass yield curves were compared either at the same excitation energy or for the same charge of the compound nucleus with different neutron number. The results are briefly discussed in terms of (1) the weighted mean mass number of the light and heavy asymmetric peak, (2) the FWHM of the heavier asymmetric peak, and (3) the peak-to-valley ratio.

Uranium and thorium target fragmentation by 3.65 AGeV 12C ions

The production cross section of residual nuclei from the interaction of 238U and 232Th with 3.65 AGeV 12C ions has been studied by off-line gamma-ray spectroscopy. Mass-yield distributions were deduced from these data. The mass-yield curves show a large fission peak for 80<or approximately=A

Unified multifragmentation-evaporation nuclear reaction models.

A simple and exactly solvable unified model for fragmentation was recently proposed. It involved a single parameter which served to tune relative contributions of evaporation and multifragmentation to the mass-yield distribution for nuclear reactions. We have measured mass-yield curves for intermediate-mass products from medium-mass nuclei by radioactivation techniques. They are compared to the model's predictions. No choice of the single parameter is able to reproduce the observed trend. It is argued that by disregarding initial intranuclear cascading, the model is incomplete. Its advantage might lie in combining it with intranuclear cascade/evaporation calculations.