Seminar Details

Discovery of Two-Neutron Halo Nucleus 22C

Krystin Stiefel, MSU NSCL/Chemistry
Thursday, October 31, 11:00 AM - Research Discussion
NSCL Lecture Hall

Understanding nuclear structure is a major concern in nuclear science. Typically, nuclei are relatively compact due to the nuclear force binding together the nucleons. However, there exist nuclei that do not follow this common structure, instead exhibiting a normal core with one or more nucleons that are loosely bound. These nuclei are found along the proton and neutron driplines, where there is an overabundance of one of the nucleon types. Because of their exotic configuration, halo nuclei challenge the reliability of typical nuclear structure theoretical models. The traditional example for halo nuclei is 11Li [1], a two-neutron halo. Similarities between 11Li and 22C, such as their separation energies [2], suggested that 22C would also have a halo structure, consisting of a bound Borromean system made of a 20C core plus two valence neutrons. The first evidence of the 22C halo structure was provided by the work of Tanaka et. al, [3] who bombarded a 22C beam, produced at RIKEN, on a liquid hydrogen cell. The proton reaction cross section for 22C was found to be 1338274 millibarns. This is significantly larger than the reaction cross sections observed for 19C and 20C. Furthermore, the root-mean-square (rms) matter radius was determined to be 5.40.9 fm, well above the theoretical value extrapolated from other carbon isotope rms matter radii. Using direct time-of-flight based mass measurement techniques, Gaudefroy et. al [4] were able to determine the mass of 22C for the first time. The mass excess for 22C was measured to be 53.640.38 MeV. By comparing the measured neutron separation energy with the calculated matter radius, 22C was described as a two-neutron halo with an s-wave configuration combined with a fully occupied d5/2 orbit. The experimental studies leading to the discovery of 22C as well as the subsequent probing of the halo structure will be discussed. References: 1. I. Tanihata et al., Phys. Rev. Lett. 55, 2676 (1985) 2. G. Audi, A.H. Wapstra, and C. Thibault, Nucl. Phys. A729, 337 (2003) 3. K. Tanaka et al., Phys. Rev. Lett. 104, 062701 (2010) 4. L. Gaudefroy et al., Phys. Rev. Lett. 109, 202503 (2012)