Studying neutron-rich 18 N in fusion-evaporation reactions [electronic resource].

Published:: Washington, D.C. : United States. Dept. of Energy, 2006.
Oak Ridge, Tenn. : Distributed by the Office of Scientific and Technical Information, U.S. Dept. of Energy.
Physical Description:: PDF-file: 4 pages; size: 0 Kbytes
Additional Creators:: Lawrence Berkeley National Laboratory, United States. Department of Energy, and United States. Department of Energy. Office of Scientific and Technical Information

Access Online

www.osti.gov

Availability

Finding items...

Restrictions on Access:

Free-to-read Unrestricted online access

Summary:

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 ²°O [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 (t{sub 1/2} < 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.

Report Numbers:

E 1.99:ucrl-tr-223252
ucrl-tr-223252

Subject(s):

Other Subject(s):

Note:

Published through SciTech Connect.
07/28/2006.
"ucrl-tr-223252"
Lyles, B; Clark, R; Fallon, P; Lee, I; Burke, J; Ward, D; Bernstein, L; Bleuel, D; Gibelin, J; Macchiavelli, A; Phair, L; Rodriguez-Vieitez, E; Wiedeking, M; Cromaz, M; Deleplanque, M; Moretto, L.

Funding Information:

W-7405-ENG-48

View MARC record | catkey: 14669136