The ratio of pre-equilibrium neutrons to protons from collisions of neutron-rich nuclei is studied as a function of their kinetic energies. This ratio is found to be sensitive to the density dependence of the nuclear symmetry energy, but is independent of the compressibility of symmetric nuclear matter and the inmedium nucleon-nucleon cross sections. The experimental measurement of this ratio thus provides a novel means for determining the nuclear equation of state of asymmetric nuclear matter. *

We adopt a personal approach here reviewing several calculations over the years in which we have experienced confrontations between cluster models and the shell model. In previous cluster conferences we have noted that cluster models go hand in hand with Skyrme Hartee-Fock calculations in describing states which cannot easily, if at all, be handled by the shell model. These are the highly deformed (many particle-many hole) intruder states, linear chain states e.t.c. In the present work we will consider several topics; the quadrupole moment of 6 Li, the non-existence of low lying intruders in 8 Be, and then jumping to the f 7/2 shell, we discuss the two-faceted nature of the nuclei-sometimes displaying shell model properties, other times cluster properties. I. THE QUADRUPOLE MOMENT OF THE J = 1 + STATE IN 6 LI Whereas the quadrupole moment of the deuteron is positive(Q = +2.74mb), that of the J=1 + state of 6 Li is negative, Q=-0.818(17)mb. The magnetic moment of the deuteron is µ = 0.85741 nm while that of 6 Li is 0.822 nm. There appears to be a big discrepancy between cluster model calculations and the shell model calculations. In nearly all cluster model calculations Q comes out positive. However in many shell model calculations Q comes out negative, sometimes too negative. This is an important problem that deserves further attention. See for example arguments in the literature between the cluster group[1] and the shell model group[2]. See also the recent compendium of A=6 by D.R. Tilley et. al.[3].

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The masses of 31 neutron-rich nuclei in the range A = 29-47 have been measured. The precision of 19 masses has been significantly improved and 12 masses were measured for the first time. The neutron-rich Cl, S, and P isotopes are seen to exhibit a change in shell structure around N = 28. Comparison with shell model and relativistic mean field calculations demonstrate that the observed effects arise from deformed prolate ground state configurations associated with shape coexistence. Evidence for shape coexistence is provided by the observation of an isomer in 43S.