These were stable, monoisotopic elements, such as thulium, inserted into the test device. These reactions produced radioactive isotopes, which were observed by drilling into the test region, removing samples, and analyzing them in the laboratory. In order to understand the observed isotope distributions, accurate cross sections for all the reactions are needed, including reactions on radioactive isotopes. S-process path near 151 Sm. Red indicates unstable nuclei, and blue indicates stable nuclei observed in nature.
Heavy outlines indicate nuclei that are produced only by the s-process.
Capture reactions are also of great interest to the study of the synthesis of elements that takes place in stars. The basic mechanism of the s-process appears to be understood. This is illustrated in Figure 2, which shows the path of sequential capture in the region of the element samarium. The branching of the capture path that occurs at radioactive isotopes is of particular interest. An important example is the radioactive nucleus 151 Sm.