Session FP: Mini-Symposium: Invariant Mass Spectrometry for the Study of Rare Isotopes

Probing the neutron dripline: challenges and prospects

The study of neutron-unbound nuclei is one of the major research fields to probe the nuclear structure. The most important characteristic of these nuclei is their immediate decay via the emission of one or more neutrons after being populated. Their extreme short-lived nature makes their direct measurement impossible, and thus requires the use of specific experimental techniques, such as the invariant-mass spectroscopy. Thoroughly used to explore unknown states and isotopes near and beyond the neutron dripline, the invariant-mass technique has also revealed prominent features of 3-body decay mechanisms in the continuum. A short overview of some recent results will be presented, paying special attention to three-body correlations in neutron decays of nuclei like $^{16}$Be or $^{26}$O.   

Search for an S-Wave Resonance in $^{\mathrm{7}}$Li Just Above the Proton Decay Threshold

Near threshold resonances play an outsized role in nucleosynthesis and applied nuclear science. The study of nuclei removed from stability has greatly extended the list of resonances very close to decay thresholds. A recent theory paper employing a continuum-extended version of the no-core shell model indicates three quarters of a century of nuclear science may have missed a resonance in $^{\mathrm{7}}$Li just above the proton decay threshold at an excitation energy of 10 MeV [1]. This is plausible as this positive parity J $=$ 1/2 resonance would not be easy to populate, resulting in being obscured by both broader and more easily populated neighboring states. We will present our plans to search for this resonance using a secondary-beam transfer experiment and the invariant-mass technique. The latter is a technique that is ideally suited to the observation of narrow resonances just above decay thresholds. [1] M. Vorabbi, P. Navra'til, S. Quaglioni and Q. Hupin, Phys. Rev. C 100, 024304 (2019).   

Neutron-Unbound States in the N=20 Island of Inversion

Studies of neutron-rich nuclei around N=20 investigate a region of the nuclear chart with anomalous structural properties known as the “Island of Inversion”. It is characterized by a level inversion not predicted in the spherical shell model and associated with enhanced binding energy, deformation, and novel structure. Nuclei in this region are short-lived and require radioactive beams in order to study them and their neutron-unbound states. An experiment was performed at the National Superconducting Cyclotron Lab (NSCL) to study unique nuclear structure characteristics of nuclei with A/Z ~3 using the MoNA-LISA neutron arrays and the Sweeper magnet to perform invariant mass spectroscopy. Current status of data analysis will be discussed.   

Structure Studies of $^{\mathrm{13}}$Be

A variety of structure phenomena such as alpha clustering, neutron halos, and the breakdown of the N=8 shell gap, are observed in the beryllium nuclei marking them attractive for nuclear structure studies. The structure of $^{\mathrm{13}}$Be offers insights into the N=8 shell gap, nature of the Borromean $^{\mathrm{14}}$Be nucleus, the influence of the continuum and the nature of neutron-drip line nuclei. We performed the $^{\mathrm{12}}$Be(d,p)$^{\mathrm{13}}$Be transfer reaction in inverse kinematics at ISAC II at TRIUMF. The $^{\mathrm{12}}$Be beam at 9.5 MeV/u interacted with the novel IRIS solid D$_2$ target, and ejectiles and recoils were detected in an annular silicon detector array and two $\Delta$E -E telescopes. A Q-value plot showing the resonances in the $^{\mathrm{13}}$Be continuum, and preliminary results from fitting the plot with a fixed-line shape and the response function obtained from GEANT4 simulations will be presented here.   

First study of single-neutron excitations of $^{\mathrm{207}}$Hg via the $^{\mathrm{206}}$Hg(d,p) reaction

We report the first exploration of the single-neutron shell structure of $^{\mathrm{207}}$Hg, which located below Z $=$ 82 and with N \textgreater 126 in an almost unexplored region of the nuclear chart. The $^{\mathrm{206}}$Hg(d,p) neutron-adding reaction in inverse kinematics at 7.4 MeV/u was performed at CERN's HIE-ISOLDE facility. The single-neutron excitations in $^{\mathrm{207}}$Hg were determined from the scattered protons, which were detected using the new ISOLDE Solenoidal Spectrometer at a magnetic field strength of 2.5 T. The Q-value resolution was 140 keV FWHM. Angular distributions suggest that the 0g$_{\mathrm{9/2}}$, 2d$_{\mathrm{5/2}}$, 3s$_{\mathrm{1/2}}$, 2d$_{\mathrm{3/2}}$ and 0g$_{\mathrm{7/2}}$ states were observed. An extrapolation of the single-particle energies using $^{\mathrm{209}}$Pb and $^{\mathrm{207}}$Hg as anchors towards the neutron threshold plays an important role in improving our understanding the r-process nucleosynthesis in this region.