First Spectroscopy of the Near Drip-line Nucleus ${}^{40}\mathrm{Mg}$

One of the most exotic light neutron-rich nuclei currently accessible for experimental study is ${}^{40}\mathrm{Mg}$, which lies at the intersection of the nucleon magic number $N=28$ and the neutron drip line. Low-lying excited states of ${}^{40}\mathrm{Mg}$ have been studied for the first time following a one-proton removal reaction from ${}^{41}\mathrm{Al}$, performed at the Radioactive Isotope Beam Factory of RIKEN Nishina Center with the DALI2 $\gamma$-ray array and the ZeroDegree spectrometer. Two $\gamma$-ray transitions were observed, suggesting an excitation spectrum that shows unexpected properties as compared to both the systematics along the $Z=12$, $N\ge 20\mathrm{Mg}$ isotopes and available state-of-the-art theoretical model predictions. A possible explanation for the observed structure involves weak-binding effects in the low-lying excitation spectrum.

Figure 1

Particle identification for (a) incoming beam components as identified in the beam line detectors of BigRIPS and (b) reaction residues following interaction of incoming ${}^{41}\mathrm{Al}$ with the secondary target, as identified by the beam line detectors of ZeroDegree. ${}^{41}\mathrm{Al}$ secondary beam is highlighted in (a); ${}^{40}\mathrm{Mg}$ reaction residues are highlighted in (b).

Figure 2

Top panel: Spectrum of prompt $\gamma$-ray transitions associated with ${}^{36}\mathrm{Mg}$, populated in $-1p3n$ removal from ${}^{40}\mathrm{Al}$. Middle panel: Prompt $\gamma$-ray spectrum associated with ${}^{38}\mathrm{Mg}$, populated in $-1p1n$ removal from ${}^{40}\mathrm{Al}$. Bottom panel: Prompt $\gamma$-ray spectrum associated with ${}^{40}\mathrm{Mg}$, populated in $-1p$ removal from ${}^{41}\mathrm{Al}$. All spectra include nearest-neighbor addback and are restricted to $\gamma$-ray multiplicity $\le 3$. Spectra were fit using the DALI2 response modeled in geant4 (red dashed curves) and a double-exponential decay (dotted blue line) to model the smooth background; the solid black line represents the total fit.

Figure 3

Known experimental $E(2_1^{+})$ (filled symbols) and $E(4_1^{+})$ (open symbols) energies for ${}^{36}\mathrm{Mg}$ and ${}^{38}\mathrm{Mg}$ [10, 21]. Also included is $E(2_1^{+})$ for ${}^{40}\mathrm{Mg}$ inferred here (filled star). Open star indicates energy of second excited state in ${}^{40}\mathrm{Mg}$ under assumption that two observed $\gamma$-ray transitions form a cascade (see text for details). Data are compared to shell-model calculations using SDPF-MU [24] and SDPF-U-MIX [11] effective interactions.