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.     

Experimental study of some important characteristics of the thermal neutron induced fission of 237Np

Fission fragment mass and kinetic energy distributions and their correlations have been studied for the thermal neutron induced fission of ²³⁷Np. The global mass distribution is rather smooth, apart from a weak shoulder at μ_H = 140–141. When low excitation events are selected, fine structures associated with the charge of the fragments are observed. Furthermore, there is a sudden increase in E_k for μ_H > 155, which is probably due to a spherical shell N = 50 in the light fragment and the corresponding deformed (but stable) heavy fragments with masses in the rare earth region. For the average (pre-neutron emission) total fragment kinetic energy, a value of 176.4 ± 0.6 MeV has been obtained, in agreement with the systematics.Also the prompt neutron emission curve $\text{v(m}{}^1\text{)}$ has been calculated, which shows the well-known saw-tooth shape. Finally, the energy distribution and the emission probability of the ternary α-particles have been determined.     

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.     

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.     

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 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.     

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.     

Elastic and Raman scattering of 8.5–11.4 MeV photons from 159Tb, 165Ho and 237Np

Differential cross sections for elastic and inelastic Raman scattering from the deformed heavy nuclei ¹⁵⁹Tb, ¹⁶⁵Ho and ²³⁷Np were measured at five energies between 8.5 and 11.4 MeV. Angular distributions at four angles between 90° and 140° for both elastic and inelastic scattering at 9.0 and 11.4 MeV were also measured. The monoenergetic photons were obtained from thermal neutron capture in Ni and Cr. All the angular distributions and the elastic and Raman scattering at the higher energies are in good overall agreement with theoretical predictions. The theory is based on a modified simple rotator model of the giant dipole resonance in which the effect of Delbrück scattering was included. A trend of both the elastic and Raman scattering at lower energies to be stronger than expected are suggested by the data. However, the ratio between the Raman and elastic scattering seem to be in good agreement with theory throughout the whole energy range. This shows that there is no need to introduce a direct nonresonant component to the imaginary part of the elastic scattering amplitude to explain the experimental data.     

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.     

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 $\text{K =}\text{1}\text{2}$.     

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.     

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.     

Excited levels of 233Pa by alpha decay of 237Np

α-spectrometry using Si(Au) detectors has been performed with thin samples of ²³⁷Np; αγ bi-parametric coincidences have permitted an analysis and unfolding of the spectrum and give data about the level scheme. Direct and α-coincident γ-spectra have been measured using Ge(Li) and Si(Li) detectors, obtaining the characteristic electromagnetic transitions in ²³³Pa. Nine new γ-transitions are found, relative to the most recent compilation made by Ellis. In the level scheme, the existence of a level at 306.1 keV is confirmed. A level at 257.1 keV, previously suggested, is evidenced. The level at 300.5 keV is proposed as the $\text{7}\text{2}{}^{\mathrm{?}}$ component of the $\text{5}\text{2}\text{(523)}$ band, hence proposing the 306.1 keV level as the $\text{7}\text{2}{}^{\mathrm{+}}$ term of the $\text{5}\text{2}\text{(642)}$ band.     

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.     

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.     

Ground-state α-decay of N = 128 isotones 216Ra, 217Ac and 218Th

The α-decay properties of very short-lived N = 128 isotones, ²¹⁶Ra, ²¹⁷Ac and ²¹⁸Th, were investigated by the pulsed-beam method. Alpha emitters of interest were produced in the bombardment of ²⁰⁸Pb or ²⁰⁹Bi with 65–96 MeV ¹²C or ¹⁴N ions and α-decays were measured between natural beam bursts of the cyclotron. The results obtained are _Eα = 9.349±0.008 MeVand $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 182±10}\text{ns for}{}^{216}\text{Ra}\text{, 9.650±0.010}\text{MeV}$ and $\text{111±7}\text{ns for}{}^{217}\text{Ac}\text{, 9.665±0.010}\text{MeV}$ and $\text{96±7}\text{ns for}{}^{218}\text{Th}$. The experimental reduced α-widths of N = 128 isotones from ²¹²Po to ²¹⁸Th are shown to agree well with the simple shell model calculation.     

Angular momentum transfer in 12C-, 20Ne- and 40Ar-induced fission

Angular momentum transfer in a variety of ¹²C-, ²⁰Ne- and ⁴⁰Ar-induced fission reactions has been investigated using γ-ray multiplicity techniques. Fission fragments were detected in coincidence using a pair of solid-state detectors. The fragment masses were deduced from the kinetic energies and emission angles using two-body kinematics. The γ-ray multiplicities (M_γ) of the fission fragments were measured utilizing an array of eight NaI detectors. For most of the systems studied, M_γ is nearly independent of the exit-channel mass asymmetry. The strongest dependence on mass is observed in the systems ¹⁵⁴sm + 240 MeV ⁴⁰Ar, where a minimum exists at symmetry, and ¹⁹⁷Au + 164 MeV ²⁰Ne, where nuclear structure effects are suggested by the data. For all the reactions the quantity M_γ tends to decrease gradually with increasing fragment kinetic energy. The magnitude of M_γ generally appears to be larger than expected on the basis of rigid rotation, suggesting a spin enhancement effect. The data are compared with a simple model which assumes the statistical excitation of a variety of angular momentum bearing collective modes. Reasonable agreement is obtained with the experimental results. The roles of other collective effects, such as shape fluctuations and angular momentum fractionation, are also considered.     

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.     

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.     

Fission fragment energy correlation measurements for the sub-barrier (nth, f) of 241Am and 243Am

Fission-fragment mass and kinetic-energy distributions and their correlations have been studied for the sub-barrier fission of the odd-odd ²⁴²Am and ²⁴⁴Am nuclei resulting from thermal neutron capture in ²⁴¹Am and ²⁴³Am. Unwanted events were eliminated by a coherence test based on the time of flight. The ²⁴³Am mass distribution is more asymmetric and shows structures at μ_H ≈ 139 and 144, compared with that for ²⁴¹Am which is smooth and structureless. The structure at μ_H ≈ 144 is caused by deformed neutron shells at N ≈ 88 in the heavy fragment and $\text{N ≧ 60}$ in the light fragment. While the 〈E_K〉 at symmetry for ²⁴¹Am is ≈ 19 MeV higher than that for ²³⁵U, it is shifted upwards only ≈ 6.5–10 MeV in the other mass regions. However, 〈E_K〉 at symmetry for ²⁴³Am is ≈ 6 MeV lower than for ²⁴¹Am and this decrease tapers down to ≈ 1 MeV for μ_H > 135. These data show a decrease in the total fragment deformation for ²⁴¹Am at symmetry as predicted by calculations. However, the ²⁴³Am data show a sudden change back to higher deformation.