Session J12: Interesting Near-Threshold States in Nuclear Physics

The elusive Near-threshold resonance in 11B.

The understanding of the dynamics of open quantum systems (OQS) has become one of the most intriguing topics in nuclear physics. Near-threshold resonances in light nuclei are a prime example of them. Coupling between bound- and continuum-states plays a significant role in the structure and reactions of these systems. In this talk, I'll discuss the experimental status of the elusive near-threshold resonance in 11B which provides a less exotic explanation to the controversial observation of an unexpectedly large beta delayed proton emission in 11Be.   

Structure of 13Be studied through the 12Be(d,p)13Be reaction

The low-lying structure of ¹³Be has remained an enigma for decades. Despite numerous experimental and theoretical studies, inconsistencies remain. Being both unbound and one neutron away from ¹⁴Be, the heaviest bound beryllium nucleus, ¹³Be is difficult to study through simple reactions with weak radioactive-ion beams or more complex reactions with stable-ion beams. Data from a study of ¹³Be using the ¹²Be(d,p)¹³Be reaction in inverse kinematics using a 9.5 MeV per nucleon ¹²Be beam from the ISAC-II facility will be presented. The solid deuteron target of IRIS was used to achieve an increased areal thickness and reduced background compared to conventional deuterated polyethylene targets. The Q-value spectrum below −4.4 MeV was analyzed using a Bayesian method with GEANT4 simulations. Two possible scenarios to explain the strength below 1 MeV above the neutron separation energy are proposed.  Comparisons of the Q-value spectrum with GEANT4 simulations obtained using the energies and widths of states reported in four previous works will be presented.   

Nigrogen-9 - a ground-state five-proton emitter at the edge of the chart of nuclides

For light nuclei just beyond the proton drip line, one finds a region of both ground-state single- and two-proton emitters. Even further out from this drip line, a small number of three- and four-proton emitters have been observed.  At present there are only two known 4p emitters; ⁸C and ¹⁸Mg.   On the other side of the chart of nuclides, one 4n emitter (²⁸O) has been observed beyond the neutron drip line.  In the regions even further removed from both these drip lines one may find nuclides that decay by emitting even more nucleons. Such exotic states are expected to be located near the boundary of the chart of nuclides where the decay widths of such exotic states are so large so that they can be not be well differentiated from the rest of continuum. In this talk we will show strong evidence for the existence of ⁹N, with five unbound protons outside of an alpha-particle core.  This resonance was produced in the fragmentation of a secondary ¹³O beam provided by the National Superconducting Cyclotron Laboratory. The decay products were detected in a highly-pixelated array and the invariant masses of events with 5 protons and an alpha particle were determined. Great care must be taken in understanding the background in the invariant-mass distribution from detected 5p+α events where some of the protons are produced promptly in the fragmentation reaction and not from resonance decay. Evidence for one, or more likely two, ⁹N states have been found in the invariant-mass spectrum. These exotic nuclides are great testing grounds for nuclear-structure models that include the coupling of the continuum.  Nitrogen-9 was also studied theoretically in the Gamow shell model and  ½⁺ and ½^- states are predicted at the energies of the two likely  invariant-mass peaks in the experimental distribution.