VanishingN=20Shell Gap: Study of Excited States inNe27,28

Abstract

This Letter reports on the (1)H((28)Ne, (28)Ne) and (1)H((28)Ne, (27)Ne) reactions studied at intermediate energy using a liquid hydrogen target. From the cross section populating the first 2(+) excited state of (28)Ne, and using the previously determined BE(2) value, the neutron quadrupole transition matrix element has been calculated to be M(n)=13.8 +/- 3.7 fm(2). In the neutron knockout reaction, two low-lying excited states were populated in (27)Ne. Only one of them can be interpreted by the sd shell model while the additional state may intrude from the fp shell. These experimental observations are consistent with the presence of fp shell configurations at low excitation energy in (27,28)Ne nuclei caused by a vanishing N=20 shell gap at Z=10.

Single-neutron knockout from intermediate energy beams ofMg30,32: Mapping the transition into the “island of inversion”

“…For sodium with just one proton less, recent systematic calculations [23], which agree with measured observables for this isotopic chain, suggest a more gradual transition. The N = 19 nuclide 30 Na is concluded to lie within the island of inversion [23], whereas measurements for 29 Na, predicted to be outside of the island, FIG. 2.…”

“…SDPF-M calculations predict a dramatic change in the single-particle structure of these nuclei. Whereas the SDPF-M 30 Mg ground state wave function is dominated by normal configurations with an occupancy of only 0.6 neutrons in fp single-particle states, the 32 Mg ground-state wave function is a nearly pure 2p-2h configuration across the N = 20 shell gap with a neutron fp occupancy of 2.15. This shift in structure is illustrated in Fig.…”

“…Sufficient data do not exist to systematically track the transition in the neon isotopic chain. However, already at N = 18, 28 Ne shows characteristics indicating that it is on the boundary of the island of inversion [5,28], and for 27 Ne, intruder states have been observed at low excitation energy [13,29,30].…”

“…For N > 17, a growing amount of empirical evidence has been deduced in support of this transition from normal to intruder dominance for magnesium and sodium isotopes. Although 30 Mg (N = 18) is well described without considering cross-shell configurations [5,25] indicating negligible intruder content in the ground state, at N = 19 the low-energy level structure [26,27] and the recently measured magnetic moment of its ground state [11] indicate a strong admixture of intruder configurations in 31 Mg. Therefore, for Z = 12 isotopes there appears to be a sharp transition at N = 19 into the island of inversion.…”

“…In the present measurement the single-particle structures of the 30,32 Mg ground states are probed by single-neutron knockout at intermediate beam energy. SDPF-M calculations predict a dramatic change in the single-particle structure of these nuclei.…”