Hello to all, I am new to this forum. I am webmaster for the Nucleon Cluster Model of the atomic nucleus of the late nuclear physicist Ronald Brightsen (MIT, 1950). You may view the site at this link: http://www.brightsenmodel.phoenixrising-web.net In his papers Mr. Brightsen asked the following question: Why are no beta-stable isotopes found at neutron numbers N = 19,21,35,39,45,61,71,89,112,123,147. Do any current "models" of nuclear physics provide an answer ? Thanks for any help you can provide.