Colloquium Seminar: Structure of the exotic magnesia
Speaker: Heather Crawford, LBNL
Location: Auditorium
Time: 14:00
The elementary modes of excitation in atomic nuclei are a central theme of study in nuclear structure physics. One of the most interesting findings of these studies is the competition of single- particle and collective degrees of freedom, in particular the emergence of deformation and associated rotations. With the development of exotic beams, we have started to chart the character and evolution of these excitation mechanisms with isospin as one moves toward the neutron dripline. Due largely to the monopole part of the spin-isospin components of the nuclear force, effective single-particle levels for a given isotope change dramatically as protons or neutrons are added to the system. The competition between monopole shifts of single particle energies and pairing plus quadrupole correlations leads to competition between spherical and deformed configurations. In the region around 32Mg the change in the effective single particle spacing reduces the N=20 shell gap and the deformed intruder configuration, with neutron pairs promoted from the sd to the fp shell, is energetically favoured. Near N=28 and 40Mg, there is a similar development of deformation along the isotonic chain below 48Ca, with the removal of protons driving rapid shape oscillations between the N=28 nuclei. I will discuss new experimental results along the Mg chain from N=20 to N=28. The region near 32Mg has been a subject of intense work in recent years, both experimental and theoretical. In spite of the fact that there is clear evidence for the existence of deformed ground states, band structures, which are considered the fingerprints of rotational motion, have not been observed and a basic question on what exactly are the excitation spectra and collective modes remains largely unanswered. I will present results from a measurement with GRETINA [1] at the S800, studying the yrast structures in 32,33Mg populated via a fragmentation reaction [2], and discuss the results, consistent with a rotational description, but requiring a strong reduction in pairing already at the 6+ state in 32Mg. I will also present results for proton knockout into 40Mg [3], for which the inclusive reaction cross-section provides a mechanism to constrain the nature of the deformation in this most exotic Z=12 nucleus. [1] S. Paschalis, et al., NIM A, 709, 44 (2013). [2] H. L. Crawford, et al., Phys. Rev. C, 93, 031303(R) (2016). [3] H. L. Crawford, et al., Phys. Rev. C, 89, 041303(R) (2014)