Using Coherent Pion Photoproduction to Measure Neutron Distributions in Nuclei

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The properties of neutron distributions in atomic nuclei are currently only known for a small number of nuclei. The neutron distribution in a nucleus is often quantified as a "neutron skin", defined as the difference between the neutron and proton root mean square radii.High energy photons produced by the MAMI accelerator facility in Mainz, Germany, will allow measurements of the decay of coherently photoproduced neutral pions on several nuclei targets, so neutron distributions for these nuclei can be established. In recent years, the method of pion photoproduction has successfully elucidated the neutron distribution of several nuclei, including the size and shape of the neutron skin of the heavy nucleus, 208Pb.One of the targets to be used is the doubly magic nucleus, 48Ca, of significant interest due to the proton-neutron asymmetry in the system, allowing insight into several poorly defined nuclear parameters. Models of the 48Ca neutron distribution have given widely varying results, and thus a neutron skin measurement would help to establish which models are closest to reality.In addition, parameters established from the neutron skin measurements, including the dependency of the neutron skin size on the nuclear symmetry energy, will have a strong impact on models of the equation of state of nucleonic matter. This gives information on several physical phenomena, including the structure and cooling mechanisms of neutron stars, which have recently been confirmed to be a major source of the r-process elements. The neutron skins measured in this experiment will thus have a wide impact, with the run scheduled for Summer 2018.