Mass Measurements Demonstrate a StrongN=28Shell Gap in Argon

Abstract

We present results from recent time-of-flight nuclear mass measurements at the National Superconducting Cyclotron Laboratory at Michigan State University. We report the first mass measurements of 48 Ar and 49 Ar and find atomic mass excesses of -22.28(31) MeV and -17.8(1.1) MeV, respectively. These masses provide strong evidence for the closed shell nature of neutron number N = 28 in argon, which is therefore the lowest even-Z element exhibiting the N = 28 closed shell. The resulting trend in binding-energy differences, which probes the strength of the N = 28 shell, compares favorably with shell-model calculations in the sd-pf shell using SDPF-U and SDPF-MU Hamiltonians.The "magic" numbers of protons and neutrons, which enhance nuclear binding for isotopes near the valley of β-stability, can evolve for more neutron-rich or neutrondeficient nuclei [1][2][3]. The neutron magic number N = 28 has been the subject of extensive recent experimental and theoretical investigations [4][5][6][7][8]. Since neutron-rich N = 28 nuclei are within experimental reach and are computationally tractable for shell-model calculations, they are ideal candidates for illuminating the fundamental forces at work in exotic nuclei. It is known that the N = 28 shell gap, which stabilizes doubly magic Mg 28 suggests it has a prolate deformed ground state [18], which would be consistent with the absence of a neutron shell gap.The existence of the N = 28 shell gap for argon is a matter of some controversy. Several previous experimental studies have assessed the shell structure of neutronrich argon [19][20][21][22][23][24][25][26][27][28][29] [19,20] and one at Coulomb-barrier beam energy [29], deduce a low B(E2), corresponding to a reduced quadrupole collectivity. In this case quadrupole collectivity reflects a propensity for neutrons to be excited across the N = 28 shell gap, and thus a low B(E2) may be expected for a semi-magic nucleus. State-of-the-art shell-model calculations that properly account for the breakdown of the N = 28 magic number in silicon and sulfur isotopes predict a markedly higher B(E2) for 46 Ar [28]. A low-statistics lifetime measurement of the 2 + 1 state of 46 Ar deduced a high B(E2) value in agreement with theory [27], but at odds with the three consistent, independent Coulomb excitation measurements [19,20,29].However, B(E2) measurements are not necessarily unambiguous probes of neutron shell structure, since they are sensitive to proton degrees of freedom and proton-neutron interactions. In contrast, mass measurements, and the neutron separation energies derived from them, directly probe the neutron shell gap in a modelindependent way.We report here results from the first [31] mass measurements of 48 Ar and 49 Ar, which provide robust evidence for the persistence of the N = 28 shell gap for argon. These results were obtained with the time-offlight (TOF) technique at the National Superconducting Cyclotron Laboratory (NSCL) [32][33][34]. Neutron-rich isotopes of silicon to zinc were produced by fragmentation of a 140 ...

Single-particle structure of the N = 20, 28 isotones within the dispersive optical model

“…Then, the global parameters KD were applied to predict the evolution of the single-particle structure of neutron-rich N =20, 28 isotones. Argon with Z = 18 is the lowest even-Z nucleus exhibiting the N = 28 closed shell according to the experimental data [38]. At further Z decrease, excitations through the N = 28 gap intensify so that 42 Si is characterized by the low experimental excitation energy E(2 + 1 ) = 742(8) keV [1].…”

Coulomb excitation ofCa44andAr46

“…Being located two protons away from the doubly magic 48 Ca, 46 Ar should in principle be considered as a semimagic nucleus, as confirmed by the following experimental results. A significant drop in the one-neutron separation energy S n is observed in the Ar chain after having passed N = 28 [16], which is suggestive of a shell closure. Moreover, studies of its neutron Fermi surfaces via various transfer and knockout reactions [17][18][19] on 46 Ar led to the following conclusions: (i) the neutron f 7/2 orbit is about 75% full in the ground state of 46 Ar, the remaining 25% being in the p 3/2 orbit, (ii) the [20], obtained from intermediate energy Coulomb excitation measurements.…”

Toward precision mass measurements of neutron-rich nuclei relevant to r-process nucleosynthesis

“…For instance, the masses of Borromean drip-line nuclei 19 B, 22 C, 29 F, and 34 Na have been measured only by TOF-Bρ-MS to date [62]. New results from GANIL [62,63] and NSCL [64,65] are summarized in Fig. 6.…”