The Rapid Neutron-Capture Process


The origin of the chemical elements that make up our world is one of the oldest most fundamental scientific questions. The universe after the Big Bang consisted only of hydrogen and helium with traces of lithium. All the other elements, including the carbon in our bodies, the iron, silicon, and oxygen that makes up most of our earth, have been created later by nuclear reactions in stars. When stars explode they eject their freshly made nuclei into space. This stardust can then form new stars that continue the process of the synthesis of elements and planets like the earth.

However, the origin of many elements beyond iron, including gold and uranium, is still a mystery.

These elements are attributed to a process called the r process: rapid neutron capture process. In this process iron nuclei are bombarded with neutrons, which they rapidly capture until a very exotic nucleus is formed. Such an exotic nucleus decays quickly (within 10-100 milliseconds) before another neutron can be captured. In this decay a neutron is converted into a proton and a new element is created. Then the process continues with more neutron captures and decays creating heavier and heavier elements. Eventually large amounts of iron are converted into gold, uranium, and many other elements.

The mystery: While we have a rough idea about what the r-process might be like, we don't understand how this sequence of reactions really works, and we don't know where in the universe this process occurs.

The Role of NSCL

While the site of the r-process is not known, there are prime suspects such as supernova explosions or colliding neutron stars. However, none of these models can produce r-process elements in the correct proportions as we find them, for example, in the solar system or in certain very old stars.

Does this mean we have to discard those models and find something different? We cannot tell, because we don't know whether our description of the nuclear reactions during the process is correct.

The overwhelming majority of the nuclei involved in the r-process have never been observed in a laboratory. These nuclei might even have very unusual properties, for example skins of neutrons, that still need to be discovered.

This is where NSCL will make a huge impact. At NSCL, we can produce many of the nuclei in the r-process for the first time, and we can measure their properties with sophisticated detector systems. NSCL scientists also run astrophysical models to identify the most important measurements needed and to determine the impact of new results. This is done in close collaboration with MSU astronomers, who are among the leading observers of r-process elements in stars.