Characterizing Neutron-Proton Equilibration in Nuclear Reactions with Subzeptosecond Resolution.

We study neutron-proton equilibration in dynamically deformed atomic nuclei created in nuclear collisions. The two ends of the elongated nucleus are initially dissimilar in composition and equilibrate on a subzeptosecond time scale following first-order kinetics. We use angular momentum to relate the breakup orientation to the time scale of the breakup. The extracted rate constant is 3 zs^{-1}, which corresponds to a mean equilibration time of 0.3 zs. This technique enables new insight into the nuclear equation of state that governs many nuclear and astrophysical phenomena leading to the origin of the chemical elements.

Study of two-neutron radioactivity in the decay of 26O.

A new technique was developed to measure the lifetimes of neutron unbound nuclei in the picosecond range. The decay of 26O→24O+n+n was examined as it had been predicted to have an appreciable lifetime due to the unique structure of the neutron-rich oxygen isotopes. The half-life of 26O was extracted as 4.5(-1.5)(+1.1)(stat)±3(syst) ps. This corresponds to 26O having a finite lifetime at an 82% confidence level and, thus, suggests the possibility of two-neutron radioactivity.

Unresolved question of the 10He ground state resonance.

The ground state of (10)He was populated using a 2p2n-removal reaction from a 59 MeV/u (14)Be beam. The decay energy of the three-body system, (8)He+n+n, was measured and a resonance was observed at E=1.60(25) MeV with a 1.8(4) MeV width. This result is in agreement with previous invariant mass spectroscopy measurements, using the (11)Li(-p) reaction, but is inconsistent with recent transfer reaction results. The proposed explanation that the difference, about 500 keV, is due to the effect of the extended halo nature of (11)Li in the one-proton knockout reaction is no longer valid as the present work demonstrates that the discrepancy between the transfer reaction results persists despite using a very different reaction mechanism, (14)Be(-2p2n).

Evidence for the ground-state resonance of 26O.

Evidence for the ground state of the neutron-unbound nucleus (26)O was observed for the first time in the single proton-knockout reaction from a 82 MeV/u (27)F beam. Neutrons were measured in coincidence with (24)O fragments. (26)O was determined to be unbound by 150(-150)(+50) keV from the observation of low-energy neutrons. This result agrees with recent shell-model calculations based on microscopic two- and three-nucleon forces.

First observation of ground state dineutron decay: 16Be.

We report on the first observation of dineutron emission in the decay of 16Be. A single-proton knockout reaction from a 53 MeV/u 17B beam was used to populate the ground state of 16Be. 16Be is bound with respect to the emission of one neutron and unbound to two-neutron emission. The dineutron character of the decay is evidenced by a small emission angle between the two neutrons. The two-neutron separation energy of 16Be was measured to be 1.35(10) MeV, in good agreement with shell model calculations, using standard interactions for this mass region.

Near-barrier fusion of Sn + Ni and Te + Ni systems: examining the correlation between nucleon transfer and fusion enhancement.

The fusion excitation functions for radioactive (132)Sn + (58)Ni and stable (130)Te + (58,64)Ni were measured at energies near the Coulomb barrier. The coupling of transfer channels in heavy-ion fusion was examined through a comparison of Sn + Ni and Te + Ni systems, which have large variations in the number of positive Q-value nucleon transfer channels. In contrast with previous experimental comparisons, where increased sub-barrier fusion cross sections were observed in systems with positive Q-value neutron transfer channels, the reduced excitation functions were equivalent for the different Sn + Ni and Te + Ni systems. The present results suggest a dramatically different influence of positive Q-value transfer channels on the fusion process for the Sn + Ni and Te + Ni systems.