Studying neutron-rich 18 N in fusion-evaporation reactions

Light neutron-rich nuclei provide an excellent opportunity to study the changes in nuclear shell structure that occur with increasing neutron number and are an important testing ground for shell model theories. Probably one of the most striking examples of shell modification is the occurrence of intruder ground states, which signal an inversion of the normal shell ordering. Intruder ground states are observed around 32Mg (Z=10-12), ''the island of inversion'', and in 11Be. An analogous situation appears in the Z=2 He isotopes, where the intrusion of sd excitations in p-shell configurations becomes important in the heavy helium isotopes. Finally, for Z=8, recent data on 20O [1] show a reduction in the p-sd shell gap with increasing neutron number. It remains an open question whether the observed diminishing of the p-sd shell gap is restricted to O and F isotopes or extends also to neighboring nuclei. Here, we report preliminary results on 18N (Z=7), which is sufficiently far from stability to exhibit modified shell structure and yet still within the reach of stable beam facilities utilizing state-of-the art detector systems. 18N was produced in the 9Be(11B,2p)18N reaction at the 88'' Cyclotron at LBNL and studied using the LIBERACE-STARS detector array--an array of large area segmented silicon detectors (E-ΔE) and six HPGe Clover detectors. This experiment was the first to use a fusion-evaporation reaction to populate 18N. Previous information on the excited states of 18N came from 18C beta-decay [2] and charge-exchange reactions [3]. These are highly selective reactions and the fusion-evaporation reaction used here can provide a more comprehensive picture of the excitation spectrum. The beam energy of 50 MeV was chosen to optimize the cross section for the evaporation of 2 protons while simultaneously suppressing the evaporation of additional neutrons in conjunction with the 2p channel. The two proton tag cleanly selects the weak (sub milli-barn) 18N products. A natural lead catcher foil was mounted between the target and Silicon detectors (3 cm distance) to detect gamma-rays emitted from long lived (t1/2 < 1 (micro)s) states. The 18N γ-ray spectrum is shown in figure 1 and a preliminary level scheme in figure 2. New transitions were observed at 628 and 155 keV. The 628 and 114 keV transitions are shown to be in coincidence. The origin of the 298 keV line is currently being investigated. In ref. [2] a lifetime of > 600 ns was assigned to the first excited state at 114 keV. However, from our measurement we estimate a lifetime value of < 30 ns for this state; far shorter than the value of > 600 ns given from the beta decay experiment