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Studying Neutron Rich Nuclei
Nuclei with extra numbers of neutrons (neutron-rich) compared to their stable naturally abundant brethren are difficult to produce, particularly in high-spin states. Traditionally, high angular momentum states are produced almost exclusively by heavy-ion induced fusion reactions using stable beam and target combinations. These reactions produce only neutron-deficient nuclei and so radioactive beams are required if fusion reactions are to produce neutron-rich nuclei at high spin. Recently, two alternative production methods for neutron-rich nuclei were used at GAMMASPHERE involving studies of fission fragments from spontaneous fission and deep-inelastic reactions. These reactions produce many neutron-rich nuclei, however they are created in low numbers, ie., with low cross sections. Only with GAMMASPHERE does one have enough sensitivity to observe high spin states in these nuclei.
As shown in the upper figure the spontaneous fission of 252Cf produces neutron-rich nuclei in the mass regions of A = 100 and A = 140. Nuclei with up to 10 neutrons greater than stable have been observed with a yield of 10-5 per fission event. In many nuclei, new levels up to spin I = 16 were observed for the first time. A large range of nuclear structure phenomena, from high-deformation to spherical double-magic nuclei (132Sn), were studied. Evidence was also found in both even-even and odd-A nuclei for a new region of stable octupole deformation. In addition, direct measurement of isotopic yields and neutron multiplicities have been made for many fragment pairs from Sr-Nd to Pd-Te. These data revealed both a fission mode with zero neutron multiplicity, implying a form of cluster radioactivity, and a mode with high neutron multiplicity, implying a large elongation at scission.
Three deep-inelastic reactions, 48Ca + 176Yb, 154Sm + 176Yb and<P>154Sm + 208Pb, have been studied. Gamma-ray transitions were observed from nuclei several neutrons richer than the target. These reactions<P>produced 60 new transitions in 12 neutron-rich nuclei. Yrast states with spins up to 20 were populated for the first time in nuclei such as 175-178Yb and 153-156Sm. The lower left figure shows the triple coincidence spectrum of 178Yb. The production cross section of neutron-rich nuclei, the gamma-ray yield as a function of spin, and the correlation of the yield of projectile- and target-like nuclei have been determined. The yield as a function of spin for the 48Ca + 176Yb reaction in the lower right figure shows that deep-inelastic reactions lead to higher spins compared with Coulomb excitation.