R. Vandenbosch

The energy dependence of the fissionability of neptunium isotopes and the level density of highly deformed nuclei

Abstract The relative probability, Γ n Γ f , of neutron evaporation and fission was determined for neptunium isotopes produced in bombardments of 238 U with protons of energy up to 22 MeV. The measurement involved the comparison of the numbers of neutrons emitted at 0° to fission fragments with those emitted at 90°. The observations are incompatible with the statistical theory expressions for Γ n Γ f , using the Fermi-gas expression for nuclear level densities with deformation-independent values for the level density parameter, a . They can be brought into accord with the theory if the value of a at the saddle point distortion is 10 to 15 % larger than that near the normal ground state. A difference of about this magnitude is suggested by microscopic calculations of the relevant level densities when they are allowed to include single-particle and pairing effects.

Excitation energy dependence of neutron yields and fragment kinetic energy release in the proton induced fission of 233U and 238U

Abstract The yields and energies of neutrons emitted from fission fragments of known mass were measured as a function of proton bombarding energy for 233 U and 238 U. The familiar sawtooth dependence of v , the number of neutrons emitted per fragment, on the fragment mass was found to be considerably washed out with increasing bombarding energy, with the depth of the minimum at A ≈ 132 decreasing more rapidly than that at A ≈ 80. As the bombarding energy is increased the yield of neutrons from the heavy fragment increases more rapidly than does that of the light fragment. The results are interpreted in terms of qualitative considerations on energy-apportionment between fragments at the time of scission. Structure in the variation of the total fragment kinetic energy with mass ratio was observed to decrease at higher bombarding energies. The nuclear level density parameter a of the final fragments has been obtained as a function of fragment mass from the neutron kinetic energy and yield data. The a -values are observed to increase for fragments near closed shells and to decrease for mid-shell fragments as the excitation energy is increased.

Gamma decay of the 238U shape isomer

Abstract A search for a competing γ-decay branch of the spontaneously fissioning 238mU isomer is reported. The 200 nsec 238U shape isomer was produced by the (d, pn) reaction with a pulsed beam of 18 MeV deuterons incident on natural uranium targets. A γ-ray line at 2.514 MeV has been observed which is attributed to the decay of the shape isomer to the 0.045 MeV 2+ rotational state of 238U. It has a half-life in agreement with that observed for the fission branch and is produced with a yield consistent with predictions of a more penetrable inner barrier. A second weaker line has been observed at 1.879 MeV corresponding in energy to the transition from the shape isomer to the first excited 1− state at 0.680 MeV in 238U. The decay of this line is also consistent with the measured half-life of 238mU.

Magnitude and alignment of transferred angular momentum in both quasi and deeply inelastic scattering

Abstract In-plane and out-of-plane angular correlations of fission fragments in coincidence with projectile-like fragments have been measured for the 86 Kr + 238 U reaction at 730 MeV. The dependence of the magnitude and alignment of the angular momentum transferred to the fissioning heavy reaction product has been determined. Both quantities decrease strongly with decreasing energy loss in the quasi elastic region, in agreement with the predictions of a transport model.

Alkali carbide fragmentation. A new path to doubly-charged negative ions

Abstract C 9 2− ions have been observed as products in the collision of 48 keV CsC 9 − ions with hydrogen and helium.

Energy, angular and charge distributions for deeply inelastic scattering of Xe by Ta and Pb☆

Abstract Energy spectra, angular distributions, and Z -distributions for the scattering of 1120 MeV 136 Xe ions by 181 Ta and 208 Pb have been measured. The elastic scattering angular distributions are fitted by optical model potentials whose strength in the tail region of the potential is in good agreement with theoretical estimates. The angular distributions for non-elastic processes are sidewise peaked for both the quasi-elastic and deeply inelastic components. The sum of the integrated cross sections for these components accounts for all of the expected total reaction cross section, with no evidence for a significant fusion-fission component. For most angles the energy spectra exhibit a broad peak several hundred MeV below the elastic peak. At very forward and very backward angles the most probable total kinetic energy is sub-barrier with respect to the entrance channel. The Z -distributions of the light reaction product exhibit a most probable Z that is remarkably close to that of the projectile for all angles and energy losses. A linear dependence of the width of the Z -distribution on energy loss is found that is common to all angles.

Elastic and inelastic scattering of 18O by 16O and 18O

Abstract Angular distributions have been measured for the elastic and inelastic scattering of 18 O by 16 O and 18 O at laboratory bombarding energies of 42 and 52 MeV. The inelastic scattering data are analyzed in terms of collective excitations using a coupled channel approach. Deformation parameters are obtained for the strongly excited states. The relationship between the strength of inelastic scattering and the amount of structure in the elastic scattering distributions is discussed.

On the origin of oscillations in the fusion cross section of 12C + 12C☆

Abstract It is suggested that a necessary condition for fusion from a particular partial wave be the availability of overlapping compound states of the appropriate angular momentum. A calculation employing this criterion accounts for the magnitude and structure in the 12 C + 12 C fusion excitation function.

Fusion cross section for the system6Li+28Si atE ∼ 36 MeV

The fusion cross section for the system6Li+28Si has been measured atE∼36 MeV. Combining this with the data available at lower energies, the nucleus-nucleus real potentials have been determined for a range of interaction distances.

Heavy-ion emission from light nuclei

Abstract Reactions involving the emission of heavy ions leading to low-lying states of the residual nucleus have been studied using 42 MeV α-particles incident on light target nuclei. The heavy ions resulting from these reactions were identified by mass number using the time-of-flight technique. With this method, products with a mass number as high as A = 14 could generally be separated from neighbouring isobars. Reactions leading to different particle masses from a particular target were studied simultaneously. Angular distributions and total cross sections were obtained for the reactions 12 C( 4 He, 6 Li) 10 B, 12 C( 4 He, 7 Be) 9 Be, 16 O( 4 He, 6 Li) 14 N, 16 O( 4 He, 7 Li) 13 N, 16 O( 4 He, 7 Be) 13 C and 16 O( 4 He, 9 Be) 11 C. A comparison of the experimental angular distributions and cross sections with statistical model calculations was carried out. We show that as the mass of the emitted heavy ion increases for a particular reaction, the fractional contribution of compound-nuclear processes increases. Considerable compound nuclear contributions to all the reactions studied are indicated. Comparison of the calculated and experimental results for the 16 O( 4 He, 7 Be) and 16 O( 4 He, 7 Li) reactions with those for the reaction 16 O( 4 He, 8 Be) 12 C indicate that there is no enhancement of the latter reaction due to α-clustering. Evidence is given for the conservation of isotopic spin in complex reactions proceeding through a short-lived compound nucleus. The possible astrophysical significance of low-energy α-particle-induced reactions in the nucleosynthesis of Li, Be and B is discussed.