Exploring the Low-ZShore of the Island of Inversion atN=19

Abstract

The technique of invariant mass spectroscopy has been used to measure, for the first time, the ground state energy of neutron-unbound 28 F, determined to be a resonance in the 27 F + n continuum at 220(50) keV. States in 28 F were populated by the reactions of a 62 MeV/u 29 Ne beam impinging on a 288 mg/cm 2 beryllium target. The measured 28 F ground state energy is in good agreement with USDA/USDB shell model predictions, indicating that p f shell intruder configurations play only a small role in the ground state structure of 28 F and establishing a low-Z boundary of the island of inversion for N = 19 isotones.PACS numbers: 21.10.Dr, 21.10.Pc A hallmark of the nuclear shell model is its reproduction of large energy gaps at nucleon numbers 2, 8, 20, 28, 50, 82, and 126. Although well established in stable nuclei, these magic numbers begin to disappear for nuclei far from stability. For example, it has been known for over 30 years that the large shell gap at N = 20 diminishes for neutron-rich nuclei [1][2][3][4]. The change in shell structure around N = 20 is now known to be a result of the tensor force, which is strongly attractive for the spin flip pairs πd 5/2 -νd 3/2 and strongly repulsive for the pairs πd 5/2 -ν f 7/2 [5][6][7]. For nuclei in the region of N ∼ 20 and Z 13, the reduced N = 20 gap allows p f shell intruder configurations, in the form of multi-particle, multi-hole (npnh or nℏω) cross shell excitations, to compete with standard sd only configurations if the gain in correlation energy is on the same order as the size of the shell gap [8][9][10]. This has led to the establishment of the "island of inversion"-a region of nuclei near N = 20 for which the intruder configuration is dominant in the ground state.

Spectroscopy of neutron-unbound27,28F

“…A 28 keV resonant decay from 25 F was assigned to a 1/2 − excited state in 25 F at an excitation energy of 4249 keV [8]. In the present paper we report the observation of the unbound ground state of 28 F and an unbound excited state in 27 F. The results of the 28 F experiment have been reported in a recent article [9].…”

“…The overall resolution and acceptance for the decay of 28 F into 27 F + n has already been presented in Ref. [9], and the corresponding shapes are essentially identical in the case of 27 F * breakup. Due to the low statistics of the present data set, an unbinned maximum likelihood technique was used for parameter estimation [29].…”

“…Separate analyses using ℓ = 1 and ℓ = 3 resonances yield results that do not differ significantly from the ℓ = 2 case. The lack of sensitivity to ℓ values is largely due to experimental resolution, which is limited primarily by uncertainty of the reaction position within the 9 Be target. The width of the measured resonance is almost completely determined by experimental response, over- 26,27 F, including the present observation of an unbound excited state in 27 F at 2500 keV, decaying to the ground state of 26 F. The shaded grey boxes around the various levels indicate the total uncertainty in their placement relative to the 27 F ground state.…”

“…[9], the measured 28 F decay energy is best described as a sum of two independent ℓ = 2 BreitWigner resonances, with the lower resonance at 220(50) keV (Γ 0 ≡ 10 keV), the upper resonance at 810 keV (Γ 0 ≡ 100 keV), and the lower resonance composing 28% of the total area. As with 27 F, the width of each measured resonance was dominated by experimental resolution, making sensitivity to the resonance ℓ value minimal.…”

Ground-state properties and symmetry energy of neutron-rich and neutron-deficient Mg isotopes

“…Previously, the Coulomb excitation studies of 32 Mg also confirmed the large deformation and pointed out the vanishing of the N = 20 shell gap [6]. The concept of island of inversion has been explored, for instance, experimentally also to isotones with N = 19 [7] and complementary information about its extent to at least N = 22 for the Ne isotopes was obtained [8].…”