Figure 2 (IMAGE) Osaka University Caption Evidence for a proton magic number of six (indicated by blue arrows) obtained through systematic evaluation of experimental data. (a) Mass-independent proton radius, (b) electronic transition probability, (c) proton shell gap, and (d) proton shell gap as a function of neutron and proton numbers. Credit Hooi Jin Ong Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.