Evidence from Nuclear Masses on Proposed Closed Shells at 20 Nucleons

Combining microwave measurements of sulfur and chlorine masses with mass spectroscopic and nuclear reaction information, the effect of nuclear shell structure on masses in the region of 20 neutrons or protons is investigated. It is found that except for ${\mathrm{Ca}}^{40}$, nuclei with 20 neutrons or protons do not show any special stability.