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.     

Fission of 232Th, 238U and 235U induced by negative muons

The absolute yields of prompt and delayed fission induced by negative muons in ²³²Th, ²³⁸U and ²³⁵U have been measured. The delayed fission yields are much lower than could be predicted from Γ_n/Γ_f systematics for 15–20 MeV nuclear excitation. The systematics of prompt fission yields are compared with recently obtained photofission data. It is suggested that prompt fission can be used for investigating the channel structure of the fission barrier.     

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.     

Fragment angular distributions in electron-induced fission of 232Th

Fission fragment angular distributions have been observed in electron-induced fission of ²³²Th for electron energies 8.7 MeV ≦ E_e ≦ 30 MeV. For low energies (though above the fission threshold) the angular distributions contain both a dipole and a quadrupole component. The (90°/0°) anisotropy decreases rapidly for higher electron energies but reveals smaller peaks after the onset of second-, third- and fourth-chance fission suggesting that the effective fission barriers for ²³¹Th and ²²⁹Th in second- and fourth-chance fission, respectively, are both characterized by $\text{K =}\text{1}\text{2}$.     

Search for E2 strength in electrofission of 238U and 232Th

We have measured the cross-section ratio σ^?/σ⁺ for the electron and positron induced fission of uranium and thorium in the energy range 10–40 MeV with detectors at 90° and in a 2π geometry. The observed periodic structure is discussed with respect to E2 admixtures and an anisotropy of the angular distribution, after the onset of each higher chance fission threshold, as well as with respect to the shape of the photofission cross section.     

Fission fragment angular distributions for neutron fission of 232Th and their interpretation with a triple-humped fission barrier

The fission fragment angular distributions have been measured for the neutron fission of ²³²Th at a number of energies near the neutron threshold. An exhaustive search has been made for a set of transition states and barrier parameters that would simultaneously fit the angular distributions and reproduce quantitatively the structure seen in the neutron fission cross section. No satisfactory fit to both types of data could be obtained with a double-humped fission barrier. However, use of a triple-humped fission barrier does provide a reasonable fit to all the experimental data.     

The thermal neutron induced fission cross section of 232Th

The thermal neutron induced fission cross section of ²³²Th was measured using the highly pure thermal neutron beam from the 87 m curved neutron guide at the High Flux Reactor of the ILL (Grenoble). An upper limit of 4 μb was obtained for the fission cross section, which is an order of magnitude smaller than previous results. This result is discussed in terms of the double-humped fission barrier theory.     

Photofission cross sections of 232Th and 236U from threshold to 8 MeV

Measurements of the photofission cross sections of ²³²Th and ²³⁶U for an energy range of 5.0 to 8.0 MeV are reported. The Compton scattering facility at the Ames Laboratory Research Reactor was used for these measurements. Resonance structure at γ-ray energies of 6.3 and 7.6 MeV is observed in ²³²Th and at 5.3, 6.0, 6.5, and 7.5 MeV in ²³⁶U. The present results are compared with other photofission experiments and particle-induced fission studies. Suggestions for the interpretation of the resonances are given on the basis of Bohr's fission-channel theory and the existence of a double-humped fission barrier.     

Symmetric and asymmetric fission in electron induced fission of 232Th

We have measured fragment kinetic energies in electron induced fission of ²³²Th for electron energies in the range 7 MeV ≦ E_e ≦ 66 MeV. The relative contribution of the distribution peak associated with high fragment kinetic energies decreases continuously with electron energy. This is interpreted as a relative increase of the symmetric fission yield as compared to the asymmetric fission yield; this fact in turn indicates a non-negligible increase in the average excitation of the fissioning nucleus, with the energy of the bombarding electrons, even above the giant dipole resonance.     

Study of some characteristics of the thermal-neutron-induced sub-barrier fission of 231Pa, 232Th and 237Np

Some characteristics of the thermal-neutron-induced fission of ²³¹Pa, ²³²Th and ²³⁷Np were studied using a highly pure thermal-neutron beam of the Grenoble High Flux Reactor. For the thermal-neutron-induced-fission cross section we obtained 20 ± 4 mb for ²³⁷Np, 19 ± 4 mb for ²³¹Pa and less than 2.5 μb for ²³²Th, in agreement with calculations based on a double humped fission barrier. We also determined a mean total fission fragment kinetic energy of 172.8 + 2.5 MeV for ²³⁷Np, which fits into the semi-empirical systematic of $\text{E}\text{?}{}_{\mathrm{<mtext>K</mtext>}}$ versus $\text{Z}{}^2\text{/A}{}^{\mathrm{<mtext>case1</mtext><mtext>3</mtext>}}$. Finally, we determined LRA/B ratios of (1.94±0.17) × 10^?3 for ²³⁷Np and (1.67±0.11) × 10^?3for ²³¹Pa, in disagreement with the theoretical predictions of Feather, but consistent with the semi-empirical systematic of Halpern.     

Absolute (p, xn) cross-section measurements on 232Th

Excitation functions have been measured for both the (p, 6n) and (p, 7n) reactions on ²³²Th at incident energies up to 100 MeV using the external beam of the McGill synchrocyclotron. The natural activity of the target material is used to determine the target thickness, making it unnecessary to use monitor foils, or to carry out chemical separations following bombardment. The shape of both excitation functions is consistent with the prediction of the Jackson model. If fission competition is taken into account the observed cross sections are in reasonable quantitative agreement with the theoretical predictions. The (p, 6n) excitation function is found to be in disagreement with a number of earlier measurements.     

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.     

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.     

Study of the giant dipole resonance of 232Th and 238U using elastic and raman photon scattering

Differential cross sections for elastic and Raman scattering of photons from ²³²Th and ²³⁸U targets were measured. Eight photon energies in the range 7.9–11.4 MeV were used and were obtained from thermal neutron capture in Fe, Cr, Cu and Ni. The angular distribution of the elastic and Raman scattered radiation from ²³²Th was measured. The results are compared with calculations of the simple rotator model and the dynamical collective model of the giant dipole resonance after incorporating the effect of Delbruck scattering.     

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.     

Multiparameter study of 1H, 3H and 4He in fast neutron induced fission of 235U

The emission probabilities per fission of α-particles, tritons and protons have been measured in fast neutron induced fission of ²³⁵U. The measurements were carried out at neutron energies of 120, 180, 230 and 550 keV. AΔE-E semiconductor detector telescope was used to identify different light charged particles and the fission fragments were detected with an ionization chamber. The three-parameter data corresponding to the pulse heights from the ΔE-E detectors and the ion-chamber were recorded event by event on magnetic tape and were analyzed off-line by computer. No significant variation in the most probable energy ($\text{E}$) and the standard deviation (σ_E) of the energy spectra of different light charged particles with incident neutron energy was observed, although $\text{E}{}_{\mathrm{\alpha }}$ was seen to have a slightly higher value beyond E_n = 230 keV. The yield of α-particles in fission induced by neutrons of E_n ～ 200 keV was found to be higher by about 20 % than that in thermal neutron induced fission. The yields of tritons and protons were found to increase significantly with neutron energy.     

High-spin states in 222Th

Levels to spin 16⁺ in the ground band and 17^? in the octupole band in ²²²Th have been identified using the reaction ²⁰⁸Pb(¹⁸O, 4n)²²²Th at 93 MeV. To suppress intense γ-ray background from fission a residue detector was built and operated in coincidence with the Ge(Li) and NaI(Tl) detectors. The apparent moment of inertia in the ground-state rotational band rises very rapidly with increasing spin, however, no indications of backbending were observed. The octupole band at low spin has an aligned angular momentum $\text{J ≈ 3}\text{h}\text{?}$ relative to the ground-state band. Strong cross-band E1 transitions competed with collective E2 transitions within each band. Analysis showed that the ratios $\text{B(}\text{E}\text{1)}\text{B(}\text{E}\text{2)}$ were within experimental uncertainty independent of both the spin and parity of the parent state.The average γ-ray multiplicity per cascade was measured for ²²²Th. The results were in reasonable agreement with the computer code ORNL-ALICE.     

Linear momentum transfer in the reaction 232Th(14N,light particles) AT 208 MeV

The longitudinal momentum transferred to a target nucleus (ΔP) has been studied for light-particle emission in the ¹⁴N-induced reaction on ²³²Th at 208 MeV. The transferred momentum was deduced by measuring folding angles between two fission fragments resulting from the sequential decay of the target-like nucleus. It was found that the fraction $\text{?}{}_2\text{=}\text{(〈ΔP〉 + P}{}_{\mathrm{<mtext>out</mtext>}}{}^{\mathrm{\parallel }}\text{)}\text{P}{}_{\mathrm{<mtext>beam</mtext>}}$ averaged over light-particle energy was about 0.68, almost independent of light-particle species and detection angle, where P_beam is the initial momentum of the beam particle and P_out^∥, the momentum component parallel to the beam carried away by the observed light particle. A possible mechanism for describing the phenomena is discussed.     

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.     

Fission fragment energy correlation measurements for 229Th(nth,f)

Fission-fragment mass and kinetic energy distributions and their correlations have been measured for ²²⁹Th. The results are compared with those for the neighbouring nuclei. The ²²⁹Th yield data show a peak for the symmetric fission. The mass yield peak/valley ratio is 531 ± 49. The 〈E_K〉(μ_H ≈ 115?22) is flat and the dip ΔE_K at symmetry is 18.0 ± 1.1 MeV. In the yields for high kinetic energy selected events, the mass 144 dominates. A complete systematic for the μ_H≈ 144 structure as a function of the fissioning nuclei is presented and discussed.