Investigations of Exotic Nuclei with Large Arrays

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Almost every element on earth was formed at the core of a star. Nuclear reactions continually convert light elements into heavier ones, but the exact mechanism and astrophysical site where this takes place remain unclear. This situation arises because there are many ambiguities concerning the exact nature of the nuclear strong force, which is responsible for binding the protons and neutrons together at the centre of an atom. A primary goal of contemporary nuclear physics is to reveal a comprehensive unified description of this force so that we can understand the origin and evolution of all nuclear matter. The vast majority of elements existing on earth are stable nuclei, forming the so-called valley of nuclear stability. These stable nuclei are well described in the nuclear shell model developed by Nobel laureates M. Goppert-Mayers and J. H. Jensen in the late 1940s. This model has successfully explained many experimental observations, including the existence of the so-called magic numbers, a special set of proton and neutron numbers (2, 8, 20, 28, 82 and 126) that brings extra stability to nuclear matter, similar to the electron configurations of the noble gases in chemistry. For decades, the shell model has been the backbone to our understanding of nuclear structure. In recent years, experimental investigations of nuclei with larger numbers of neutrons compared to protons have revealed new magic numbers in these exotic nuclei. The challenge is to understand why exactly the magic numbers shift with different combinations of protons and neutrons, and to successfully predict whether other more exotic nuclei that might only exist for fractions of a second can be formed far from stability. The evolution of shell structure could lead to a new frontier of nuclear stability comprehension. In order to develop new models that will describe the structure of nuclear matter far from stability, the evolution of magic numbers, and the creation of elements in stars, scientists need detailed knowledge of nuclear masses, lifetimes, and decay probabilities. Experimental nuclear science, such as that described in this proposal, is responsible for the critical gathering of information for nuclei situated far from stability. In particular, the multitude of tin nuclei makes an excellent case study because they all have a magic number of 50 protons, and many tin isotopes exist far from stability with neutron numbers beyond the magic number of 82. The proposed experiments will take place mainly at TRIUMF in Vancouver, BC. Unstable nuclei are created on-site with the world’s largest cyclotron. These nuclei are then investigated with large state-of-the-art detectors that are able to observe the radiation emitted when the nuclei decay. The information extracted from these experiments provides clues into the nuclear matter and produces essential information for nuclear astrophysics about the creation of elements in the universe.