Mass Yield Distribution Research Articles

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.

Degrees of deformation at scission and correlated fission properties of atomic nuclei

The degrees of the deformation of atomic nuclei at scission configurations of the mass-symmetric and mass-asymmetric fission modes are studied. The \ensuremath{\beta} shape elongation of the fissioning nucleus, associated with the asymmetric fission is found to be constant for ${A}_{f}=213\char21{}260,$ while that associated with the symmetric fission is larger but also constant for a wide range of fissioning nuclei ${(A}_{f}=205\char21{}260)$ except for the low-energy induced and spontaneous fission of heavy nuclei with ${A}_{f}=245\char21{}262$ where a change of the nucleon number by one unit is found to cause a very rapid change in \ensuremath{\beta}. A systematic correlation between the scission deformation and the fission properties such as the mass-yield distribution and the TKE are found. The constancy of the \ensuremath{\beta} gives rise to new formulas of ${\mathrm{TKE}}_{\mathrm{sym}}=0.1173\ifmmode\times\else\texttimes\fi{}{(Z}_{f}^{2}{/A}_{f}^{1/3})+7.5\mathrm{MeV}$ for symmetric fission, and of ${\mathrm{TKE}}_{\mathrm{asym}}=0.1217\ifmmode\times\else\texttimes\fi{}{(Z}_{f}^{2}{/A}_{f}^{1/3})+3.5\mathrm{MeV}$ for asymmetric fission. The physical origin of the well-known simple linear function for the TKE released in the fission process is found to be the invariance of the degrees of the elongation of scissioning nuclei. The mass dispersion of fission products was found to have a direct correlation with the \ensuremath{\beta} value; as \ensuremath{\beta} becomes larger, the width of the mass yield distribution becomes wider. Our understanding of the spontaneous fission properties of heavy nuclides in the ${}^{258}\mathrm{Fm}$ region is presented. In this region, two different scission configurations, one giving $\ensuremath{\beta}\ensuremath{\sim}1.53$ which is the characteristic value for the asymmetric deformation and the other giving $\ensuremath{\beta}\ensuremath{\sim}1.33$ which is for the symmetric deformation, are found.

Electron-capture delayed fission properties of242Es

Electron-capture delayed fission of ${}^{242}\mathrm{Es}$ produced via the ${}^{233}\mathrm{U}{(}^{14}\mathrm{N}{,5n)}^{242}\mathrm{Es}$ reaction at 87 MeV (on target) was observed to decay with a half-life of $11\ifmmode\pm\else\textpm\fi{}3\mathrm{s},$ consistent with the reported \ensuremath{\alpha}-decay half-life of ${}^{242}\mathrm{Es}$ of ${16}_{\ensuremath{-}4}^{+6}\mathrm{s}.$ The mass-yield distribution of the fission fragments is highly asymmetric. The average pre-neutron emission total kinetic energy of the fragments was measured to be $183\ifmmode\pm\else\textpm\fi{}18\mathrm{MeV}.$ Based on the ratio of the measured number of fission events to the measured number of \ensuremath{\alpha} decays from the electron-capture daughter ${}^{242}\mathrm{Cf}$ (100% \ensuremath{\alpha} branch), the probability of delayed fission was determined to be $0.006\ifmmode\pm\else\textpm\fi{}0.002.$ This value for the delayed fission probability fits the experimental trend of increasing delayed fission probability with increasing Q value for electron capture.

Mass yield distributions of target fragments from the reactions of iron with 135 MeV/nucleon 12C and 80 MeV/nucleon 16O ions

Cross sections for the production of target fragments in the reactions of iron with 135 MeV/nucleon 12C and 80 MeV/nucleon 16O ions have been measured by off-line γ-ray spectroscopy. Through these data, the mass yield distributions have been obtained. The result of the experiment for the reaction with 135 MeV/nucleon 12C ions is compared with theoretical calculations using the fusion-fragmentation model and the GEMINI code for sequential binary decay, following a calculation with the fireball model.

Photofission of 197Au at Intermediate Energies

The reaction yields of 58 radionuclides with the mass number A = 42‐131 produced in the photofission of 197 Au by bremsstrahlung of end-point energies (E0) from 300 to 1100 MeV have been measured using the catcher foil technique. The reaction yields increase steeply with an increase of E0 up to 600 MeV and increase gradually above 600 MeV, indicating the resonance-type excitation function attributed to the (3,3) resonance. The charge distribution was found to be well reproduced by a Gaussian function with the most probable charge ( Zp) expressed by a linear function of A, i.e., Zp = RA + S, and with the A-independent full width at half maximum (FWHMCD). The charge distribution parameters R, S, and FWHMCD were found to be independent of E0 above 600 MeV, reflecting the resonance nature in photonuclear reactions at intermediate energies. The weighted mean values at E0 ≥ 600 MeV were R = 0.424 ± 0.001, S = 0.7 ± 0.1, and FWHMCD = 2.2 ± 0.1 charge unit (c.u.). Based on these parameters, the symmetric mass yield distributions with the most probable mass Ap of 92 ± 1 mass unit (m.u.) and the width FWHMMD of 39 ± 1 m.u. were obtained. The characteristics of the charge and mass yield distributions were discussed by referring to the literature data including those of proton-induced reactions.

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.

Projectile-mass dependence of the linear momentum transfer in central collisions

Target residues from the interaction of copper with 40MeV/nucleon 40Ar ions have been studied using thick-target, thick-catcher recoil techniques. Cross sections and average forward ranges were measured for 42 products. The data were used to obtain the mass yield distribution, the linear momentum transfer, and the excitation energy of composite systems. Comparison to similar studies for the interaction of copper with 12C ions at the same projectile energy per nucleon shows that the linear momentum transfer in central collisions depends strongly on the projectile mass.

Mass yield distributions of target fragments from the reactions of iron with 135 MeV/nucleon

Mass yield distributions of target fragments from the reactions of iron with 135 MeV/nucleon

Mass Yield Distributions in Proton-Induced Fission of 248Cm

Mass Yield Distributions in Proton-Induced Fission of ²⁴⁸Cm

Two deformation paths in fission of light actinides

The correlation among the fission threshold energy, the scission configuration and the mass yield distribution has been studied in proton-induced fission of light actinides. It was found that there exist at least two fission paths from the threshold region to the scission. The elongated scission configuration is related with the fission process that goes over a higher threshold energy and results in a symmetric mass division mode, while the compact scission configuration with the process that experiences a lower threshold and ends up with an asymmetric mass division mode.

Characteristcs of binary scission configurations in proton-induced fission of actinides

Two kinds of scission configurations for certain mass division have been verified by the accurate measurements of fragment velocities in protoninduced fission of actinides: compact and elongated scission configurations. A correlation between the binary scission configurations and mass yield distributions reveals that elongated scission configurations are associated with the symmetric mass distribution and compact scission configurations with the asymmetric mass distribution. The elongation properties of nuclei at scission in a wide range of actinides fissions are studied. The results suggest that the compact and the elongated scission configurations in light actinides fission smoothly change to the scission properties of the symmetric and the asymmetic modes in heavy actinides fission.

A high-fluence 0.6 GEV proton irradiation experiment with thin uranium and thorium targets

High-fluence proton irradiation eperiments with thin actinide targets are part of PSI's programme related to accelerator-based transmutation studies. The aim is to provide experimental data for the validation of theoretical models in nucleon-meson transport codes, with emphasis on the mass yield distribution of spallation and fission products. The paper presents results of an irradiation experiment carried out with 238UO 2 and 232ThO 2. Based on mass spectrometric measurements, isobaric production cross-sections for fission and spallation products, as well as total fission cross-sections of the target nuclides, are reported. The experimentally deduced total fission cross-sections for both target nuclides are found to be approximately twice as large as values reported in certain earlier experiments, as well as with respect to calculational results obtained using the HETC code and the RAL high-energy fission model.

Influence of the odd neutron on the fragment characteristics in the photofission of 235U

Cumulative and independent post-neutron product yields were measured for the photofission on 235U with 6.5, 7.0, and 8.4 MeV bremsstrahlung, applying γ-spectrometric methods on catcherfoils and pneumatically transported 235U-samples. The independent yields for the heavy ( A = 128–149) fragments were deduced. Post-neutron mass and isotonic yield distributions were studied together with the elemental yield distributions and the proton odd-even effect. The mass distribution shows about an equal yield for the mass regions around A = 134 ( N = 82) and A = 142 ( N = 86–88). The proton odd-even effect for the fission of this odd- A nucleus remains at a constant value δ p = 25% in the studied compound nucleus excitation energy region, indicating that the odd neutron does not influence the energy transfer between the different degrees of freedom during the descent from saddle to scission point in a substantial way.

The mass yield distribution of fragments studied with improved antisymmetrized molecular dynamics

The mass yield distribution of fragments studied with improved antisymmetrized molecular dynamics

Precision determination of charge dispersion and distribution in the proton-induced fission of238U at 13.9MeV excitation

By means of a gas-jet transport system and the ordinary radiochemical stacked-target technique incorporated with the Geγ-ray spectrometry, 126 product nuclides with half-lives ranging from seconds through a year were identified and their yields were determined in the proton-induced fission of238U at 13.9 MeV excitation. The charge dispersion width was determined for mass chains and concluded to be constant with respect to the mass. The average of the deduced widths was 0.69±0.04 charge unit which was just equal to that of thermal-neutron fission of235U and also consistent with values reported for proton-induced fission of238U. From the width thus obtained, we determined the most probable chargesZ p for observed isobaric multiplets. Considering the pre-scission neutron systematics and an appropriate charge polarization between the complementary fragments we deduced from the obtainedZ p the post-scission neutron numberν post that turned out fairly consistent with Terrell’s neutron systematics for thermal-neutron-induced and spontaneous fission. Thus obtainedν post enabled us in turn to derive equations describingZ p as functions ofA. The obtained charge dispersion and charge distribution resulted in the mass-yield distribution resembling that of thermal-neutron fission.

Mass yield distribution in proton-induced fission of238U

An analytical expression is obtained for the mass yield distribution in fission induced by high-energy protons (100–340 MeV) on238U. The reaction is treated within the context of a two-step model. In the first step, the proton hits the target, knocks out some of its nucleons, yielding a residual nucleus with a distribution of excitation energies. In the second step, a fraction of the excitation energy of the residual nucleus is exhausted in the emission of several nucleons before it fissions. The multiplicity of the pre-scission neutrons is assumed to follow a Poisson distribution. The mass yield distribution, resulting from the fission of the remnant nucleus, is described within the hypothesis of the two independent fission modes, with parameters extracted from the analysis of low-energy fission, assuming that post-scission neutron emission weakly depends on excitation energy. Comparison with experimental data allows us to extract values for the excitation energy at which shell effects disappear.

The mass yield distribution of fragments studied with improved antisymmetrized molecular dynamics

An improved antisymmetrized molecular dynamics model has been applied to the study of the mass yield distribution for fragments of the 12C + Fe reaction at 135MeV/nucleon. It is shown that the experimental data are well reproduced by the theoretical calculation, especially for the mass yield of light fragments. The stochastic splitting of the nucleon wave packet according to its diffusion predicted by Vlasov equation has drastic effect on the description of fragmentation process.

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.

Electron-capture delayed fission properties ofNp228

Electron-capture delayed fission was positively identified for the first time in $^{228}\mathrm{Np}$ produced via the $^{233}\mathrm{U}$(50-MeV p,6n${)}^{228}$Np reaction. The half-life of the observed fission activity was measured to be 61.4\ifmmode\pm\else\textpm\fi{}1.4 s. The ECDF mode was verified by an x-ray--fission coincidence experiment. Only about 25% of the expected x-ray--fission coincidences were detected which indicates that $^{228}\mathrm{U}$ does not have a fission isomer populated by the electron-capture and that the overall lifetime of the high-energy states populated by the electron-capture is less than about ${10}^{\mathrm{\ensuremath{-}}17}$ s. The fission activity was assigned to ECDF of $^{228}\mathrm{Np}$ based on chemical separation and observation of known progeny activities. The ratio of electron-capture to alpha decay is 1.5\ifmmode\pm\else\textpm\fi{}0.4 for $^{228}\mathrm{Np}$. The production cross section for $^{228}\mathrm{Np}$ is 35\ifmmode\pm\else\textpm\fi{}10 \ensuremath{\mu}b and the delayed fission probability is (2.0\ifmmode\pm\else\textpm\fi{}0.9)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$. The kinetic energies of coincident fission fragments were measured and a pre-neutron emission, most probable total kinetic energy, of 169\ifmmode\pm\else\textpm\fi{}6 MeV was obtained. The mass-yield distribution was found to be predominantly asymmetric.

Projectile mass dependence for the interaction of copper with 14N, 16O, and 22Ne at ~540 MeV total projectile kinetic energy.

Using thick-target, thick-catcher recoil range techniques, target residues from the interaction of copper with 40 MeV/nucleon [sup 14]N, 35 MeV/nucleon [sup 16]O, and 25 MeV/nucleon [sup 22]Ne have been studied. Isobaric and mass yield distributions as well as longitudinal momentum transfer information have been determined from measured cross section, average forward range, and forward-to-backward ratios. Comparisons with previously reported reactions involving copper and lighter ions have been made. Comparisons of the data with the interaction-evaporation model Boltzmann-Vehling-Uhlenbeck-PACE are presented.