Muon induced fission of 237Np, 239Pu and 242Pu

The absolute yields for prompt and delayed fission induced by negative muons in ²³⁷Np, ²³⁹Pu and ²⁴²Pu have been measured and compared with photofission data.     

A subnanosecond and a nanosecond fission isomer in 238Pu

A subnanosecond fission isomer has been found in ²³⁸Pu by means of a new fission-inflight technique. An isomer with a longer half-life reported previously in the same nucleus has been detected simultaneously. The half-lives are 0.5 ± 0.2 ns and 5 ± 2 ns respectively. Excitation functions show that the two isomers are separated by 1.3 ± 0.3 MeV, the short-lived isomer having the lowest excitation energy. It is assigned as the ground state while the 5 ns state is interpreted as a quasiparticle state in the second potential well. The difference of 1.3 MeV in excitation energy defines within the uncertainty the neutron energy gap at the isomeric deformation.     

Self-consistent calculation of the fission barrier of 240Pu

Completely self-consistent calculations using the Skyrme force have been carried out for the fission energy curve of ²⁴⁰Pu. We use a deformed oscillator basis including 13 major shells and convergence has been checked by extending the size of the basis to 15 shells. We obtain a double humped barrier with energies E_A = 9 MeV for the first barrier, E_B = 13 MeV for the second barrier and E_II = 4 MeV for the isomeric state. Corrections to our calculation, such as inclusion of non-axial and symmetric shapes and zero-point rotational motion, are likely to improve quantitative agreement with experimental data.     

The quadrupole moment of the 40 ps fission isomer in 236Pu

A quadrupole moment of 37_?8⁺¹⁴ b has been deduced for the 40 ps shape isomer in ²³⁶Pu. The value has been derived from a measured delayed fission fragment angular distribution with an anisotropy of 1.48 ± 0.15 for the isomeric decays at 30°. The deformation of the second minimum expressed by the axes ratio of a spheroid is found to be 2.0 ± 0.3.     

Prompt γ-rays emitted in the thermal-neutron induced fission of 233U and 239Pu

The average number and average energy of γ-rays emitted within ≈ 5 nsec after fission have been determined as functions of fragment mass and as functions of total kinetic energy. They were obtained from a four-parameter experiment that recorded, event-by-event, correlated of γ-rays and of fission-fragment pairs and the time, relative to fission, at which a γ-ray was detected. For ²³³U(n_th, f) the average total number and energy emitted per fission were found to be 6.31 ± 0.3 and 6.69 ± 0.3 MeV, respectively, giving an average photon energy of 1.06 ± 0.07 MeV. The results for ²³⁹Pu(n_th, f) given in the same order, are 6.88 ± 0.35,6.73 ± 0.35 MeV, and 0.98 ± 0.07 MeV.     

Spectroscopic properties of 237,239Pu fission isomers from self-consistent calculations

We have studied ^237,239Pu isomeric states: energy levels, M1 and E2 spectroscopic moments and reduced transition probabilities have been calculated within the rotor+quasi-particle model from self-consistently determined single-particle states. The electromagnetic properties of these states have been especially investigated. Without any ad hoc parameter adjustment, a very good reproduction of most of the known spectroscopic data is yieldedwhich assesses the predictive power of the whole approach.     

Fragment mass and kinetic energy distribution in isomeric fission of 240Pu

Fragment mass and kinetic energy distributions have been measured for isomeric fission of ²⁴⁰Pu. The mass distribution is asymmetric with the average heavy fragment mass nearly equal to that found for ground state spontaneous fission of ²⁴⁰Pu, but slightly lower than for n_th + ²³⁹Pu-fission. The average total fragment kinetic energy appears to be higher in isomeric fission (179.5_?0.7^+1.5 MeV) than in spontaneous fission from the ground state (176.8 ± 1.8 MeV).     

Charge distribution and average masses in the spontaneous and isomeric fission of 234U, 236U and 240Pu

Percentage yields for various elements and their associated average masses in the spontaneous and isomeric fission of ²³⁴U, ²³⁶U and ²⁴⁰Pu are calculated for the same set of parameters which account for the percentage yield of different masses, half-lives, and the kinetic energy spectra of their respective fragments.     

Fission of muonic 232Th and 239Pu

The neutron emission is studied following the formation of muonic atoms of ²³²Th and ²³⁹Pu. Energy and time distributions are measured. Various processes which contribute to the measured spectra are considered. A collective resonance model of the muon capture is used to calculate the nuclear excitation function. The probability of the radiationless nuclear excitations and the influence of the presence of the bound atomic muon on the fission barrier are discussed. The existing data for the $\text{Γ}{}_{\mathrm{n}}\text{Γ}{}_{\mathrm{f}}$, are analysed. As a result of the analysis the rates of the prompt and delayed fission events (due to the radiationless mu-atomic transitions and the nuclear muon capture, respectively) are deduced from the experimental data to be 0.006/muon and 0.045/muon for ²³²Th and 0.10/muon and 0.49/muon for ²³⁹Pu, respectively. The increase of the fission barrier for muonic atoms is confirmed. The experimental neutron rates can be consistently explained only if it is assumed that in both nucleides the K_α radiationless transitions do not induce fission. The increase of the fission barrier for ²³⁹Pu is hence deduced to be not less than 1.2 MeV. The fate of the atomic muon after the nuclear fission is briefly discussed. Its influence on the interpretation of the present results is found to be small.     

Electron induced fission of the 238U, 237Np, 239Pu and 243Am nuclei in the energy region 100–1000 MeV

Electrofission cross sections for ²³⁸U, ²³⁷Np, ²³⁹Pu and ²⁴³Am have been measured over the energy range 100–1000 MeV. Relative flssilities are evaluated. Analysis in terms of the virtual photon spectra technique involving a nuclear size effect is made. Contributions of various electronuclear excitation mechanisms in the large-energy region are discussed.     

Correlation between fragment mass-distribution fine structure,charge distribution and nuclear structure for thermal-neutron-induced fission of 233U and 235U

Masses corresponding to observed fine-structure peaks in the fragment mass distributions for thermal-neutron-induced fission of ²³³U and ²³⁵U are shown to correspond to average measured masses for even-even nuclear charge splits. Evidence is presented that the yield enhancement for even-Z fragments is not restricted just to fission events with higher-than-average total kinetic energy. The anomalously high yield of fragments with mass 134 in ²³⁵U(n, f) as opposed to ²³³(n, f) is tentatively correlated with rapidly changing nuclear structure properties as a function of the mass of the complementary light (Z = 40) fragments.     

The nuclear structure of 225Ra

The level structure of ²²⁵Ra has been studied using the (d, t) reaction with both unpolarised and polarised deuterons and the β^? decay of ²²⁵Fr. Cross sections and excitation energies have been measured for 41 levels below 1800 keV in the (d, t) reaction, and levels up to 724 keV have been established from the β^? decay measurements. The level structure is interpreted in terms of the Nilsson model.     

Ratio of the ternary to binary fission induced by thermal and resonance neutrons in 239Pu

Measurements were performed of the ratio of ternary to binary fission (T/B) in ²³⁹Pu by neutrons in the energy region from 0.020 eV to 50 eV. The ternary and binary fission time-of-flight spectra were recorded at a 8.1 m flightpath at the Geel Linac with large gold-silicon surface-barrier detectors on both sides of a 1 mg/cm² double-faced ²³⁹Pu target. From these time-of-flight spectra the ratios of the areas of the strongest isolated resonances in ternary and in binary fission were calculated; moreover in the region 0.02–1 eV ratios were calculated for about 40 zones throughout the spectrum. The T/B ratio does not vary significantly in the energy region below 1 eV; this is not the case in the region 1–50 eV, where the T/B value for the 15.5 eV resonance is significantly larger than for the other resonances. These T/B values seem to be correlated with the resonance spin J and with some other fission characteristics.     

Energy density functional analysis of the fission properties of 240Pu: The effect of pairing correlations

We have calculated the potential energy surfaces for ²⁴⁰Pu up to the scission point using the density functional theory with different pairing strengths to investigate the effect of pairing correlations on its fission properties. An enhancement in the pairing correlations lowers the barrier heights, isomeric state, and ridge between the symmetric and asymmetric fission valleys significantly. Moreover, it weakens the microscopic shell structure around the Fermi surface, shrinks the scission frontiers, especially for the symmetric and very asymmetric fission regions, and lifts the total kinetic energies (TKEs) for the symmetric fission region. It is also emphasized that the microscopic calculation qualitatively reproduces the trend of the distribution of the measured TKEs, especially for the positions of the peaks at begin{document}$A_{rm{frag}}simeq132$end{document} and begin{document}$A_{rm{frag}}simeq108$end{document}.     

Intermediate structure in the 239Pu(d,pf) reaction

Sub-barrier resonances in the ²³⁹Pu(d, pf) reaction have been investigated with a resolution of 3 keV. Many class II resonance groups with an average spacing of 11 keV have been observed in the region of the 5.0 MeV transmission resonance. The coincident fission fragment angular correlations indicate a spin 2⁺ for all lines resolved. The data are analyzed on the basis of a simple model of intermediate structure which also explains the observed broadening of the class II groups.     

Continuum shell model calculations for 15C

Microscopic calculations of the discrete and continuous spectrum of ¹⁵C are carried out within the framework of the continuum shell model, using a recently developed generalization of the extended R-matrix formalism. Comparison is made with the predictions of a conventional shell model calculation and also with the presently rather meager experimental data. The calculated total elastic and total reaction cross sections for the ¹⁴C+n reaction exhibit some interesting resonance phenomena which it would be desirable to investigate experimentally. The calculated stripping form factors for the ¹⁴C(d, p) reaction are similar to Woods-Saxon eigenfunctions for those states in ¹⁵C which have strong single-particle character, but differ markedly from this shape when the single-particle component is small. The effect on the calculated results of the addition or removal of closed channels is shown to be small. The present calculational method is found to be convenient, flexible and economical.     

The spontaneous fission of 224Fm and 246Fm

The average total kinetic energies for the fission products of the neutron-deficient isotopes ²⁴⁴Fm and ²⁴⁶Fm have been measured as 196 ± 4 and 199 ± 4 MeV, respectively. The values agree well with those expected on the basis of systematics.     

Microscopic study of neutron-induced fission process of 239Pu via zero- and finite-temperature density functional theory

To study the neutron-induced fission of \begin{document}$ ^{239} $\end{document}Pu, potential energy surface (PES) calculations were performed using zero and finite-temperature density functional theory (FT-DFT) with the Skyrme force. The energy of the incident neutron was simulated by the temperature of the FT-DFT. The variations of the least-energy fission path, fission barrier, total kinetic energy, scission line, and mass distribution of fission fragments with the incident neutron energy were analyzed. It was learned that an increase in the temperature lowers the barrier height, the isomeric-state energy, and the ridge between symmetric and asymmetric fission valleys. Additionally, the gaps of the single particle levels become smaller with an increase in the temperature. As the temperature increases, the pre-fission region shrinks, and the scission occurs at smaller deformation around the symmetric fission channel. At low temperatures, the pairing correlations in the collective space are similar to those in zero-temperature DFT, and when the temperature is \begin{document}$ T> $\end{document} 0.3 MeV, the pairing gaps decrease rapidly. Two different methods were used to calculate the fission yields of the neutron-induced fission \begin{document}$ ^{239} $\end{document}Pu (n, f) with different incident neutron energies, in the framework of time-dependent generator coordinate method (TDGCM). One way to calculate the fission yield of \begin{document}$ ^{239} $\end{document}Pu (n, f) is to solve the collective equation of the TDGCM by using the PES from the FT-DFT with the corresponding temperature. The other involves using the PES from the zero-temperature DFT and adjusting the initial collective energy of the wave packet in the TDGCM according to the incident neutron energy. For the cases of the lower incident neutron energies, these two methods gave similar results and reproduced the experimental peak and width of fission fragment distribution. However, for the highest incident neutron energy considered in this study, the results from the TDGCM using the PES from zero-temperature DFT deviated explicitly from the experimental data, whereas those obtained by using the PES from FT-DFT remained close to the experimental data. This indicated that, with the increase in the incident neutron energy, the shell structure of the compound nuclei changed explicitly; thus, it may not be effective to use the PES from zero-temperature to perform the fission dynamic calculation.     

Gamma rays of primary fission products from 235U(n,f) and 239Pu(n,f)

Gamma rays of primary fission products in thermal-neutron-induced fission of ²³⁵U and ²³⁹Pu were investigated. Isotopic assignments of several lines were made by comparing fission yield ratios and relative γ-ray intensities of the two fissioning nuclei. Differences between ²³⁵U and ²⁵²Cf fission product γ-ray lines are discussed.     

The 6He emission accompanying the spontaneous fission of 252Cf

The results of a multiparameter investigation of the ⁶He emission accompanying the spontaneous fission of ²⁵²Cf are presented. The energy spectrum and the yield of the ⁶He particles are found to be in accord with previous measurements, but their angular distribution is observed to be narrower at 13° ± 3° (FWHM) than the value of ? 32° deduced in a previous investigation.Comparisons of the experimental with published calculated energy spectra and angular distributions for ³H, ⁴He and ⁶He particles are shown to provide evidence for a compact scission configuration in ternary fission.