D.K. Scott

Nuclear structure of light nuclei using the selectivity of high energy transfer reactions with heavy ions

Abstract We present data obtained by bombarding light nuclei with beams of 11B, 12C and 14N at energies of about 10 MeV/nucleon. Reactions involving transfers of various numbers of nucleons were investigated and the spectra of the detected ions correspond to a wide range of excitation energies in the residual nuclei. The results characteristically show only a few strong peaks in each spectrum and the cross sections exhibit many of the features of direct reactions. We explain the apparent high degree of selectivity in terms of the interplay of reaction mechanisms and the structure of the strongly populated states. Firstly, it is shown that a simple semiclassical model of the reaction process indicates that states of the final nucleus with high spin are greatly favoured. On the other hand, it is demonstrated that the shell-model gives a reasonably complete explanation for the occurence of such high spin states at the appropriate excitation energies. These shell-model states are found to be dominated by configuration with the structure of a multi-nucleon cluster, in the ground state of its internal motion, orbiting the core nucleus so that the centre of mass motion of the cluster carries all the excitation quanta.

Liquid-gas phase instabilities in nuclear systems☆

Abstract The conventional approach to composite fragment production in heavy ion collisions from a single gaseous phase may require modification at temperatures below 20 MeV due to the onset of a liquid-gas phase instability. Clusters heavier than the α-particle are necessary for an unambigous experimental signature.

Bragg curve spectroscopy

Abstract A new method named “Bragg curve spectroscopy”, based on the Bragg curve of the heavy ion being stopped in a gaseous ionization chamber, has been developed for identifying the particle and measuring its energy. The design of such a chamber and the experience obtained with the chamber using various kinds of fast heavy ions are described. In conclusion, several problems to be solved in this method are pointed out.

Energy dependence of peripheral reactions induced by heavy ions

Abstract Collisions of 16 O ions on targets of 94 Zr, 197 Au, 208 Pb, and 232 Th are investigated at incident energies of 140 and 315 MeV and compared to published data on peripheral collisions at 33.6 GeV. At 140 MeV, the isotope-production cross sections resemble the ground-state Q -value systematics characteristic of a partially-equilibrated, di-nuclear system formed in deeply-inelastic scattering. The yields are independent of shell and pairing effects in the target and residual nuclie. These Q -value systematics fail at 315 MeV, where the relative element yields are similar to those at 33.6 GeV. A theory of projectile fragmentation accounts for the shapes of the inclusive energy spectra as well as for the isotope yields at both energies. The relative cross sections are target independent (i.e., factorize) for the data at 315 MeV incident energy, but limiting fragmentation (i.e. isotope yields independent of energy) applies only at higher energies. The differential cross sections peal at the grazing angle or decrease exponentially, depending on the incident energy and the mass of the ejectile; the distributions are described by classical trajectories and diffraction models for which the reaction time is estimated to be typical of direct reactions. The kinetic energies of the reaction products depend primarily on their charge and only slightly on their mass number. For the 315 MeV results, the velocities of the reaction products at the maximum of the spectrum and the grazing angle are slightly less than the beam velocity and decrease rapidly for larger scattering angles. The results are interpreted with simple fraction and fragmentation models, and are compared to observations at 33.6 GeV.

Energy dependence of nuclear matter disassembly in heavy ion collisions

Abstract Measurements of light charged particle spectra from 20 Ne + Au at 100 and 156 MeV/nucleon are compared with results for similar systems at 9, 13, 19, 43, 241, 393, and 800 MeV/nucleon. Spectra at each energy are fitted with a moving source model to extract the temperatures, cross sections and source velocities for protons and light nuclei in the intermediate rapidity region. The 4 He/p production ratio decreases drastically with incident energy, whereas the d/p and t/p ratios are almost constant.

INFLUENCE OF INTRINSIC NUCLEON MOTION ON ENERGY SPECTRA AND ANGULAR DISTRIBUTIONS FOR 16O-INDUCED REACTIONS AT 20 MEV/A

Abstract Energy spectra and angular distributions for peripheral reactions induced by 16 O on 208 Pb at 20 MeV/ A can be explained in terms of the intrinsic nucleon motion of the excited projectile, in a manner similar to the interpretation of projectile fragmentation reactions at 2.1 GeV/ A .

Nuclear collisions at intermediate energy

Abstract Two types of nuclear instability, which could play a role in intermediate energy heavy ion collisions, are discussed. One is a fast mechanical instability leading to break-up of the system when the compressibility becomes negative, and the other is governed by a chemical instability between liquid and gaseous phases occurring on a slower time scale. Observable experimental consequences are suggested. Both types of behavior rely on a hydrodynamical description of nuclear collisions in which an initial heated and compressed zone undergoes an expansion and rarefaction. Compared to phase transitions of more exotic kinds, e.g. density isomeric states or, quark-gluon plasmas, the transitions we discuss are based on conventional aspects of nuclear behavior. They may, however, be useful in establishing whether hydrodynamical approaches and the time scales required for phase transitions are relevant for nuclear collisions.

Response of pilot U scintillator to heavy ions

Abstract The light output response of Pilot U scintillator for stopped charged particles has been measured for 4He, 6Li, 12C, 16O, 20Ne and 40Ar ions incident at various energies up to 20 MeV/nucleon. From these we derived a systematic description of the variation of the scintillation parameters with the charge and energy of the projectile. The suitability of such a detector for the focal plane of a heavy ion magnetic spectrometer is discussed.

Particle-particle angular correlations in peripheral heavy-ion reactions☆

Abstract Angular correlations between fast α-particles and outgoing heavy reaction products have been measured for the reactions ( 16 O, 12,13,14 Cα) and ( 16 O, 14 Nα) on 208 Pb and 197 Au targets at 140 and 310 MeV laboratory energy. For a wide range of energy losses, coincident particles are preferentially emitted on the same side of the beam axis. The results are compared with recent qualitative predictions.

High energy proton emission in reactions induced by 315 MeV 16O ions

Abstract Inclusive proton spectra have been measured for the reaction 197Au(16O, p)X at 315 MeV. The data, which are consistent with emission from a moving source, are compared with the fireball model and with models preequilibrium emission.