Beta Decays of Heavy Nuclear Species for S-Process Studies

There are some 300 naturally occurring nuclides. In addition, over 3000 radioactive isotopes have become known. The s(low) and r(apid) processes of neutron capture synthesize the nuclides heavier than iron. The synthesis, namely the increase in the atomic numbers Z, is actually governed by β decays. A “flow” of successive neutron captures in the chart of the nuclides is intercepted by a nucleus whose β decay half-life is short enough. In this review, I discuss the s-process exclusively. The neutron capture rate to be compared with the β decay rate is represented by λ=nnvT<σ>, where nn is the neutron number density, vT is the neutron thermal velocity at the temperature T, and <σ> is the Maxwellian averaged (around vT) radiative neutron capture cross-section, which depends on the nucleus of interest. The classical analysis of the solar system abundances of nuclides leads to canonical combinations like nn∼108/cm3 and T∼3×108 K for the s-process. The s-process flow becomes intricate when the neutron capture and β decay timescales are comparable, causing a branch of the flow. Subsequently, an evaluation of β decay rates is required, which is difficult to do straightforwardly. In this review, I will discuss the historical developments and the current status of predicting β decay rates under s-process environments (specified basically by temperature, density, and composition). Those conditions are inaccessible in the laboratory. Embedded in high-temperature environments, even a very massive atomic species could be highly ionized, and its atomic and nuclear excited states could be thermally populated. I will exemplify the consequent difficulties of β decay rate evaluations for s-process studies.