RADSAFE: Chart of Nuclides Question - black square

["Donivan Porterfield, LANL CST-3" <dporterfield@lanl.gov>]

>On the GE chart of the nuclides a black triangle in the lower
>right corner indicates a fission product. There are some
>nuclides that have a black square in the lower right corner,
>e.g.  Eu-154, Pm -150 and Cs-136. It is not explained in the
>symbols section. What does this square indicate?
>
>luke.mccormick@mrd01.usace.army.mil