On the origin of heavy species

Ani Aprahamian gave us a nice, quick chronological review of everlasting quest for the answer to the question: what is the world made of?

We quickly see the evolution in this investigation from the ancient Greeks’ four elements (earth, fire, air, and water) to the present Rare Ion Beam facilities, which has allowed us to build the chart of nuclides. There are ~300 stable isotopes known, and lots of unstable ones, which number, some theorist claim could be up to 10 thousand!

OK, so there are way too many different nuclei … now, where are they made? So far we’ve figured out that the lightest elements were created shortly after the Big Bang, other low A elements were synthesized in early stars, and then up to Fe are created in main sequence stars. The creation of heavier elements requires a neutron-rich environment. We’ve come to classify the different nucleosynthesis processes according to the mass range of isotopes they create, for instance the s-process, and the r-process (the location of which is still an open question).

So, it becomes pretty obvious that nuclear physics plays a key role in getting an answer for our question. Nuclear physics experiments aim to learn about the structure of nuclei, the shell closures, the limits of stability, etc. The link between precision astronomical observations and nucleosynthesis models (such as the r-process in supernova, neutrino winds, jets, explosive burning, prompt explosion, GRB) is the nuclear physics! So we do experiments, simulations and nuclear theoretical models. An example to study nucleosynthesis in the r-process: Ani and collaborators ran a network calculation taking Fe as seed nuclei, and considered n-captures competing with beta decays. The process time scale is around 8^4 sec. Does this give the right peaks? We see the accumulation at bottlenecks (nuclei with slow beta-decays). Then, one performs experiments to study such bottlenecks, for instance 78Ni, which is doubly magic. The experiment was performed at the NSCL, and 11 events were recorded, which allowed the first determination of the half-life of this nucleus (Paul Hosmer’s Ph.D. thesis). Another NSCL experiment studied 90Se, 89As, 88,87As (Matt Quinn’s Ph.D. Thesis).

How do we know what to measure next? Ani’s group made a sensitivity study using the FRDM mass model to find out what number gives us the biggest effect on final elemental abundances. They found the most interesting cases to be around closed shells (N=50,and N=82).

Here’s some further reading:

Brian Fields 2002 – Big Bang Nucleosynthesis

Anna Frebel 2006 – Early stars

Grevesse and Noels 1995 – Solar abundance pattern