D. R. Haenni

Alignment processes and shape variations in 184Pt

Abstract High-spin states in the transitional nucleus 184Pt were populated via the reactions 154Sm(34S, 4n)184Pt and 172Yb(16O, 4n)184Pt. The yrast band was extended up to I = 28 ħ and six new side bands built on both neutron and proton quasiparticle configurations were observed. Shell correction-type calculations indicate variations of the nuclear shape in different bands, especially as a result of band crossings due to the process of angular momentum alignment. Comparison of the band characteristics are made between 184Pt and eight adjacent nuclei. The pattern of band crossings in these nine nuclei is considered from the viewpoint of blocking comparisons and of theoretical calculations. The competition between low-frequency vi 13 2 and πh 9 2 band crossings is discussed.

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

Abstract 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 (Mγ) of the fission fragments were measured utilizing an array of eight NaI detectors. For most of the systems studied, Mγ 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 Mγ tends to decrease gradually with increasing fragment kinetic energy. The magnitude of Mγ 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.

Continuous distribution of αt relative kinetic energies in 7Li breakup reactions

Abstract A newly developed technique has been used to probe a previously inaccessible region of the continuum in the at channels of 7 Li breakup reactions. This was used to investigate reactions of 63 MeV 7 Li with 58 Ni, 120 Sn, 144 Sm, and 208 Pb targets. Breakup pairs of α-particles and tritons emitted within a finite opening angle were analyzed according to their magnetic rigidity using a K = 150 Enge split-pole spectrograph. The αt coincidences were taken with two single-wire proportional counters mounted at different positions along the focal plane of the spectrograph. The large energy acceptance ( E max / E min ≈ 8) of the spectrograph is crucial to the present coincidence measurements involving two particles with different q / m values. Very small relative energy components were studied without any detection threshold. A pronounced bump was observed immediately above the αt particle threshold. No strong indication of the post Coulomb acceleration was observed. The excitation of the continuum states as well as the 7 − 2 state in 7 Li is discussed in terms of the distorted-wave Born approximation. The astrophysical implications of the data are also discussed regarding the Coulomb excitation of the continuum. The present technique offers the promise of very high sensitivity at small relative energies if it were applied in conjunction with ray-tracing, a thin target, and a dispersion-matched beam.

Observation of α-t continuum states with relative energies of 0–2 MeV in7Li breakup reactions

Abstract The breakup of 7 Li at 63 MeV was studied in an α-t coincidence experiment using an Enge split pole spectrograph. For the first time, the entire spectrum of the α-t continuum states of 7 Li was observed in the range of the relative energy ɛ from zero to 2 MeV. A pronounced bump was found at ɛ ≈ 0.4MeV. The astrophysical implications of the data are discussed.

K-equilibration and spin enhancement effects in 120 MeV 16O- and 214 MeV 32S-induced fission

Abstract The gamma-ray multiplicities (Mγ) of fission fragments have been investigated in a variety of 16O- and 32S-induced reactions. The dependence of Mγ on target mass and excitation energy is well explained by a model which assumes the thermal excitation of angular momentum bearing collective modes. As predicted by the statistical description of fission fragment angular distributions, the value of Mγ is found to vary with fission fragment detection angle. The observed variation is considerably weaker than predicted by simple rigid rotation models. A large part of this discrepancy can be explained by the excitation of collective modes. For heavy targets, sequential fission contributions also weaken the angular dependence of the fragment spins. However, even with these corrections, some systems still seem to exhibit a weaker angular dependence than predicted by the model calculations. Possibly this is due to a suppression of high K-states.

Exploring the k-distribution in fission

Abstract Statistical models of fission angular distributions predict that the spins of the fragments should vary with angle. This effect has been investigated in several 4 He-induced fission reactions using γ-ray multiplicity techniques. While M γ does vary with emission angle, the effect is smaller than expected. The excitation of additional angular momentum bearing modes accounts for much of this discrepancy.

Linear momentum and angular momentum transfer in the reactions of O 16 with Sm 154

From coincidence measurements between projectile-like fragments or heavy residues and their associated ..gamma.. rays, the angular momentum transfers for a variety of incomplete fusion reactions of 180 and 310 MeV /sup 16/O with /sup 154/Sm have been derived. At the higher energy, the correlation between angular momentum transfer and linear momentum transfer has been obtained over the entire range of linear momentum transfer. A comparison of the data with calculations of both the sum-rule and geometric overlap models indicates that each makes reasonable predictions of the observed trend even though the assumptions of the models are quite different, and very different initial partial waves are predicted to contribute to particular reaction channels. This results primarily from prescriptions relating fractional mass transfer to fractional angular momentum transfer. The reconstruction of the initial partial wave distributions from correlated measurements of linear momentum and angular momentum transfers is addressed. Comparisons are also made with more recent model calculations which focus on nucleon-nucleon scattering as the mechanism of momentum transfer.

Angular momentum alignment in the reaction154Sm+214 MeV32S

Angular momentum alignment in the reaction154Sm+214 MeV32S has been investigated using discrete-lineγ-ray techniques. The anisotropies of stretchedE2 transitions from the target-like fragments reach approximately three at the largest energy losses, implying rather strong alignment. The results are compared with the predictions of a simple model.

Angular momentum bearing modes and nuclear structure effects in12C-induced fission

The angular momenta of the fragments produced in several 100 and 180 MeV12C induced fission reactions have been investigated usingγ-ray multiplicity (Mλ) techniques. The averageMλ increases with both the bombarding energy and the total mass of the system. The dependence ofMλ on mass asymmetry is generally rather weak, except in regions near shell closures, where local minima are observed. The magnitudes ofMλ tend to be larger than expected on the basis of rigid rotation, possibly due to the excitation of collective modes. A comparison is made with a statistical model.

Possible evidence for the statistical excitation of collective modes in heavy ion induced fission

Abstract Angular momentum transfer in the fission of 194 Hg has been studied utilizing gamma-ray multiplicity techniques. The results indicate that the fragment spins are considerably larger than expected from rigid rotation. The data are compared with the predictions of a simple model which assumes the statistical excitation of a variety of collective modes.