Nuclear masses and the origin of the elements

The origin of the elements beyond iron and nickel, which are not produced by fusion reactions in stars, is still not fully understood. The solar abundance distribution shows peaks at the mass numbers 130, 138, 195 and 208. It is the nuclear masses and the resulting neutron binding energies that point to the origin of heavy elements by neutron capture. We note that the abundance maxima occur pair wise. From this, one can conclude that heavy elements are formed by two different neutron capture processes: a slow process (s-process) and a rapid process (r-process). The s- and r- processes cross the closed neutron shells at different elements, so that one obtains two distinct abundance maxima at different mass number for each neutron shell closure. During the s-process neutron captures are in most cases much slower than beta decays. Radioactive isotopes formed after a neutron capture decay quickly into stable nuclei before the next neutron is captured. During the r-processes neutrons densities are so large that neutron captures are much faster than beta decays so that extremely beta unstable nuclei can be formed. After their creation these nuclei are converted into stable nuclei through a long chain of beta decays. Understanding the r-process is one of the greatest challenges of modern nuclear astrophysics. Extremely neutron-rich and short-lived isotopes close to the r-process path have already been produced and the masses have been measured with unprecedented accuracy in a Penning trap or with the storage ring method. (A.C.)