Evolution of Single-Particle Energies for N=9 Nuclei at Large N/Z

Abstract

Abstract.We have studied the nucleus 14 B using the 13 B(d,p) 14 B and 15 C(d, 3 He) 14 B reactions. The two reactions provide complementary information about the negativeparity 1s 1/2 and 0d 5/2 neutron single-particle states in 14 B. The data from the (d,p) reaction give neutron-spectroscopic strengths for these levels, and the (d, 3 He) results confirm the existence of a broad 2 -excited state suggested in the literature. Together these results provide estimates of the sd-shell neutron effective single-particle energies in 14 B.The effective single-particle energies (ESPE) of shell-model orbitals are crucial ingredients for nuclear-structure calculations. In shell-model calculations they set the scale for excitation energies, as well as help determine the degree of configuration mixing for multi-particle states. The ESPE depend on the filling of neutron and proton orbitals, and it is particularly interesting to determine how the values of these energies evolve as a function of N/Z, especially at the limits of nuclear stability.In light nuclei, the values of the ESPE are sensitive to many aspects of the nuclear force responsible for modifications in heavier systems, such as the tensor force [1,2]. The nucleus 14 B, with a neutron separation energy of 0.969 MeV, is the lightest bound N=9 isotone. With a single neutron outside the closed p shell it provides an excellent opportunity to track the 1s 1/2 and 0d 5/2 ESPE to, and beyond, the limits of stability. There exists a well-known inversion between these two orbitals moving from 17 O to 15 C, and we wish to learn how the energies of these orbitals evolve in 14 B. To study this evolution, we have investigated the structure of 14 B using two nucleon-transfer reactions: neutron adding with 13 B(d,p) 14 B, and proton removal with 15 C(d, 3 He) 14 B. These two reactions provide complementary information about 14 B and together, the results provide information that can be used to determine the sd-shell ESPE in 14 B.Our measurements were conducted using the HELIcal Orbit Spectrometer (HELIOS) [3,4] at the ATLAS facility at Argonne National Laboratory. HELIOS is a device specifically designed to study reactions in inverse kinematics. HELIOS utilizes the uniform magnetic field produced by a large solenoid magnet to transport light-charged particles from the target to an array of position-sensitive silicon detectors. The magnetic-field axis is aligned with the beam direction, and the target and silicon-detector array also lie on this axis. Conceptual and operational details about HELIOS may be found in [3,4]. In our measurements, unstable 13 B and 15 C beams were produced from a primary 14 C beam at 17