Key benchmark nucleus produced and studied

Noble gasses with their filled shells of electrons are the benchmarks for atomictheory. Their existence, not discovered until 1898, became a cornerstone in the development of the atomic shell model. The equivalent "benchmarks" in nuclear models are so-called doubly magic nuclei, which have closed shells of protons and neutrons. The few doubly magic nuclei in nature, helium-4,oxygen-16, calcium-40,48 and lead-208 are the starting points for modeling more complex nuclei. Nuclear scientists at the Michigan State University National Superconducting Cyclotron Laboratory (NSCL) have now synthesized a new doubly magic nucleus with special characteristics – tin-100.


The production and study of tin-100 and cadmium-96 were enabled by a NSF-funded radio frequency fragment separator used to filter the beam of reaction products and separate out proton-rich nuclei previously out of reach at NSCL.


What makes this nucleus special is that tin-100 contains 50 protons and 50 neutrons and is almost certainly the heaviest doubly magic nucleus containing equal numbers of protons and neutrons. The properties of tin-100 are of great interest to nuclear scientists, who can use it constrain parameters in the attempt to develop predictive nuclear models.


The experimental result entailed breaking apart a tin-116 projectile traveling roughly 40% of the speed of light and removing 16 neutrons in the process. The experiment was made possible by the NSF-funded MRI project to build a new type of radio frequency isotope separation device. This was the first time tin-100 had been produced in a U.S. facility. The isotope is exceedingly difficult to produce; more than a million billion tin-116 ions were needed to make the 14 tin-100 nuclei measured in the experiment. The half-life of tin-100 nuclei was determined to be slightly less than one second and represents half of the basic information needed on this nucleus; the other is the binding energy.


A significant side result of this experiment, published in Physical Review Letters, was the first-ever measurement of the half-life of cadmium-96, relevant to understanding the role of the isotope in the rapid proton capture (rp) process. This process, along with slow neutron capture and rapid neutron capture, accounts for many of the universe's heavy elements. The half-life of cadmium-96 was found to be shorter than expected, meaning that the pathway to production of heavier elements in the rp-process is not impeded by this nuclide as had been surmised by researchers. 


The rp process occurs in supernovae, X-ray bursts and perhaps other astrophysical environments where seed nuclei join with free protons to form nuclei of increasing atomic number. Build-up stalls at specific stages when the binding of another proton is energetically unfavorable. Nuclei accumulate at these so-called waiting points, generating a spike in the observed isotope abundance.


Such a spike exists at ruthenium-96, the product of beta decay from cadmium-96, which suggests a waiting point at cadmium-96. However, because the half-life measured at NSCL is only a tenth of the value required to account for the observed abundance of ruthenium-96, there must be another explanation – perhaps an unexplored astrophysical process.