Nuclear Structure and Dynamics

Testing modern nuclear structure theories

Testing modern nuclear structure theories: As published in Physics Letters B (2018), this recent TIGRESS result measured the gamma-ray excitation transition rate in magnesium-22 (22Mg) and its mirror sodium-22 (22Ne). Mirror nuclei are pairs of nuclei where there are as many protons in one as neutrons in the other and vice versa. Based on “isospin symmetry”, the relative transition rates in mirror pairs ought to be well understood. However, in selected nuclei around atomic mass 21 to 24, there appears to be a much higher transition rate in the proton-rich pair compared to the neutron-rich mirror partner. This discrepancy cannot be explained within the phenomenological shell model, the standard model for nuclear structure, but for gamma-ray transition rates, it invokes an empirical “effective charge” parameter. The 22Mg experiment was two-fold: 1) measure the transition rate in 22Mg with high enough precision to determine if the anomalous proton-rich transition rate was an anomaly in selected nuclei or a trend across the whole mass region; and, 2) compare results to modern calculations that do not use effective charges but that calculate transition rates from first principles. The results clearly showed that, indeed, 22Mg’s transition rate is anomalously high compared to 22Ne, and furthermore, the first principles also were also unable to reproduce it.