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

Abstract

The breakdown of the N = 20 magic number in the so-called island of inversion around 32 Mg is well established. Recently developed large-scale shell-model calculations suggest a transitional region between normal-and intruder-dominated nuclear ground states, thus modifying the boundary of the island of inversion. In particular, a dramatic change in single-particle structure is predicted between the ground states of 30 Mg and 32 Mg, with the latter consisting nearly purely of 2p-2h N = 20 cross-shell configurations. Single-neutron knockout experiments on 30,32 Mg projectiles have been performed. We report on a first direct observation of intruder configurations in the ground states of these very neutron-rich nuclei. Spectroscopic factors to low-lying negative-parity states in the knockout residues are deduced and compare well with shell-model predictions.

Two-proton knockout fromMg32: Intruder amplitudes inNe30Fallon1, Rodriguez‐Vieitez2, Macchiavelli3 et al. 2010Phys. Rev. CSelf Cite441231View full textAdd to dashboardCite

“…The result was that theoretical calculations overestimated the data by a factor of 2, as shown in Fig. 3 for the test case examples: 26 Si, 30 S, 34 Ar, 28 Mg, and 54 Ti. For these nuclei σ ex = R s (2N ) × σ th and the two-nucleon suppression factor R s (2N ) took the value 0.5.…”

“…Two-nucleon removal cross-section ratios, σ ex /σ th , derived from inclusive cross section measurements and calculations (note 54 Ti-2p is for 0 + → 0 + ). The data for the 26 Si-2n, 30 S-2n, 34 Ar2n, 28 Mg-2p, and 54 Ti-2p two-nucleon removal reactions correspond to the test-case nuclei discussed in the text and are taken from [15,19], and references therein. The σ ex /σ th value for the 32 Mg-2p 0…”

“…We make the simplifying assumption, | = |φ π |φ ν , and write the neutron wave function as a combination of npnh excitations: |φ ν = α|0p0h + β|2p2h + γ |4p4h . One-neutron knockout data [26] indicate 32 Mg has a neutron intruder occupancy n(fp) ∼ 2, and all evidence to date points to both 32 Mg and 30 Ne ground state wave functions being primarily npnh (n 2) with little 0p0h. The mismatch in σ ex and σ th arising from the neutron overlap function is then mainly due to differences in the 2p2h and 4p4h neutron components.…”

Single-particle structure atN=29: The structure ofAr47and first spectroscopy ofSGade1, Tostevin2, Bader3 et al. 2016Phys. Rev. CSelf Cite15050View full textAdd to dashboardCite

“…3 (upper left) of Ref. [40] shows how a 336 keV τ =1.4 ns γ-ray transition would appear in a spectrum taken with SeGA in the relevant geometry and velocity regime.…”

Islands of insight in the nuclear chart

“…4(a)]. The transfer mechanism is described solely by a one-step correlated pair transfer, since sequential two-step transfer is strongly suppressed due to the large negative Q value (À 3:9 MeV) of the tð 30 Mg; dÞ 31 Mg reaction. For both states the protons clearly show an angular distribution of a ÁL ¼ 0 transfer.…”