The mass and kinetic energy distributions in the fission of 237Np, 232Th, 238U, 235U and 209Bi induced by 110 MeV electrons

Fission of ²³²Th, ²³⁷Np, ²⁰⁹Bi, ²³⁵U and ²³⁸U induced by 110 MeV electrons has been studied by means of surface barrier detectors. The resulting mass and kinetic energy distributions are presented. Comparison with the liquid drop model predictions shows reasonable agreement in the case of ²⁰⁹Bi. The data are analysed in terms of a two component model of fission and the mean total kinetic energies of the components are shown to depend linearly on $\text{Z}{}_1\text{Z}{}_2\text{(A}{}_1{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{+ A}{}_2{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{)}$. Interesting differences are found when the present results are compared with the recent photofission experiments of Areskoug et al. and features in both sets of data correlate with changes of fragment deformation implied by the calculations of Wilkins et al.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Study of α-particles produced in thermal neutron induced reactions on 235U

The ²³⁵U(n_th, α) reaction and the α-particles emitted in the thermal neutron induced fission of ²³⁵U were measured using a very pure thermal neutron beam of the Grenoble high flux reactor. An upper limit of 0.66 mb was determined for the ²³⁵U(n_th, α) reaction cross section, which is lower than all the previous results. The energy distribution of the α-particles produced by the ²³⁵U(n_th, f) reaction was measured down to 7 MeV with a non-shielded surface-barrier detector telescope assembly. The measured distribution has a quasi-Gaussian shape; it reveals, however, a pronounced deviation from such a shape at lower α-energies. Several possible explanations for this deviation are discussed.     

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.     

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.     

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.     

Fission fragment yield distributions as a function of their excitation energies for 235U(nth, f)

The recent ²³⁵U(n_th, f) data taken at the Grenoble high flux reactor have been reanalysed to determine the fragment mass distributions as a function of their excitation energies $\text{E}{}^1$. The mass distributions for low $\text{E}{}^1$ values show the known fine structures. However, the distributions for $\text{E}{}^1\text{> 30}\text{MeV}$ do not show the fine structure reported by Newson. Beyond $\text{E}{}^1\text{≈ 35}\text{MeV}$, the most probable masses of distributions decrease monotonously as $\text{E}{}^1$ goes up.     

Search for A γ-branch from shape isomers in 236U and 238Np

It is pointed out that for excitations involving magnetic transitions, pion scattering from nuclei should in principle allow a separation of the orbital and spin contributions. This is not possible in electron scattering.     

Photoinduced fission of the doubly even uranium isotopes 234U, 236U and 238U

A measurement of the angular distributions and yields of fission fragments in the photofission of ²³⁴U has been performed between 5.2 and 6.4 MeV. As γ-source, the bremsstrahlung from a microtron was used. For the detection of the fission fragments, solid-state track detectors were used. The present data for ²³⁴U have been analysed together with earlier obtained data for ²³⁶U and ²³⁸U. The values of the fission barrier parameters obtained are compared to results in theoretical macroscopic and microscopic fission potential energy calculations.     

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.     

Multiparameter study of the long-range α-particles emitted in the thermal neutron induced fission of 235U

The correlations between the various parameters in the thermal neutron induced fission of ²³⁵U accompanied by long-range α-particles (LRA) have been measured employing a back-to-back gridded ionization chamber. Semiconductor detectors were placed symmetrically along the electric field direction of the chamber behind the thin windows of the collector plates for the measurement of the LRA energies. A distinguishing feature of the present method is that the angle between the fission fragments and the electric field direction (hence the direction of α-particle emission) is also determined electronically by the analysis of the coincident collector and grid pulses of the ionization chamber. About 5 × 10⁵ LRA-accompanied fission events and about 2 × 10⁶ binary fission events were recorded and analyzed to obtain a number of correlations of interest between the fragment mass, total kinetic energy, LRA energy and fragment-LRA angle. These experimental results are presented and discussed with regard to the emission mechanism of the LRA.     

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.     

The muon capture rate in 232Th and 238U studied in the fission mode

We find that for 0.4 GeV protons nucleon-nucleon data have enough accuracy for detailed analyses of proton-helium elastic scattering. Continued sensitivity to nucleon-nucleon inputs and an unexpected discrepancy below the first minimum suggest a need for caution in extracting nuclear information.