Migration of Nuclear Shell Gaps Studied in thed(Ne24,pγ)Ne25Reaction

Abstract

The transfer of neutrons onto 24 Ne has been measured using a reaccelerated radioactive beam of 24 Ne to study the ðd; pÞ reaction in inverse kinematics. The unusual raising of the first 3=2 þ level in 25 Ne and its significance in terms of the migration of the neutron magic number from N ¼ 20 to N ¼ 16 is put on a firm footing by confirmation of this state's identity. The raised 3=2 þ level is observed simultaneously with the intruder negative parity 7=2 À and 3=2 À levels, providing evidence for the reduction in the N ¼ 20 gap. The coincident gamma-ray decays allowed the assignment of spins as well as the transferred orbital angular momentum. The excitation energy of the 3=2 þ state shows that the established USD shell model breaks down well within the sd model space and requires a revised treatment of the proton-neutron monopole interaction. DOI: 10.1103/PhysRevLett.104.192501 PACS numbers: 21.10.Hw, 21.10.Jx, 23.20.Lv, 25.60.Je The monopole part of the nucleon-nucleon interaction is now recognized as having a major effect on nuclear shell structure far from stability [1,2]. The interaction between valence protons and neutrons is sufficient to alter the energies of single-particle levels so that different magic numbers (or shell gaps) appear, and this can substantially affect the collective [3] and magnetic [4] properties and basic quantities such as the lifetime [5]. Nucleon transfer reactions induced by light ions are an established experimental tool for studying single-particle structure [6]. Here we employ the ðd; pÞ reaction in inverse kinematics to explore the disappearance of the N ¼ 20 magic number (and its replacement by N ¼ 16) in the neutron-rich neon isotones. As will be shown, the measurement of the differential cross sections of the light ejectiles plus the coincident gamma decays of the residual nucleus brings a new power to this type of study.Recent work using other techniques has provided evidence for the emergence of N ¼ 16 as a magic number in this region, but has not identified the single-particle structure in an unambiguous manner through measurements of the spectroscopic factors and spins. In a study of the decay of 25 F [7] the increased energy of the 0d 3=2 neutron orbital was inferred. This made use of a preliminary analysis of the present work [8] and concluded that the energy shift was consistent with the monopole effect [7]. In a study of 27 Ne using the ðd; pÞ reaction but without detecting the protons [9], a reduced gap between the 0d 3=2 and higher negative parity orbitals was deduced. This agreed with nucleon removal studies [10]. Finally, in recent studies of 23 O by transfer [11] and 25 O by proton removal [12] the 0d 3=2 state was found to have an increased excitation energy, but the required modifications to the shell-model interaction were not mutually consistent [11,12]. While an extensive review including the emergence of the N ¼ 16 magic number has recently been published [2], further quantitative data are needed in order to understand this monopole effect properly....

One-neutron removal fromNe29: Defining the lower limits of the island of inversion

“…In addition to 31 Ne, there is now a body of experimental data on the very neutron-rich Ne isotopes [12,[17][18][19][20][21][22][23]. The structures of these isotopes evolve considerably as one moves toward the neutron drip line and are complex.…”

Structure and decay of the pygmy dipole resonance in Ne26

“…3(b). In the low-lying positive-parity states, similar to 26 [41,42,[47][48][49][50]. The observed and calculated spectroscopic factors for those negative-parity states are large, and hence, their excitation energies are good measures for the p 3/2 and f 7/2 single-particle energies.…”

Investigating Single-Particle Structure in 26Na Using the New SHARC Array

“…Previous experiments have used a spectrometer at zero degress [1,8], but in this instance, a different solution was needed. An in-beam zero-degree detector, the TRIFOIL, was mounted at a distance of 40 cm downstream of the target.…”

“…Experimental studies of 24 Ne [1] and 27 Ne [2] using the TIARA array have shown the reduction of the N=20 shell gap in favour of N=16. Deep inelastic studies have also been motivated, including one using a radioactive 24 Ne beam [3,4], where the 26 Na nucleus was populated but no spectroscopic results were published.…”