Synopsis: Neutron Knockout

Knocking out neutrons or protons from an isotope of oxygen provides a successful test of an equation that relates nuclear masses.
Synopsis figure
Courtesy R. J. Charity/Washington University, St Louis

Unstable nuclear states that decay by emitting protons or neutrons provide important tests for models that are based on properties of nuclei close to the line of stability. They also permit tests of the isospin symmetry—a quantity related to the dominant component of the strong nuclear force—to determine if certain nuclear properties are unchanged if a neutron is replaced by a proton and vice versa.

One of the rarest decay modes of unstable nuclei is the essentially simultaneous emission of two protons. In a paper in Physical Review C, Marieke Jager at Washington University, Missouri, and collaborators report results from an experiment in which they observed two different 2-proton decay modes. In a beam of an isotope of oxygen, 13O, either a neutron or a proton was knocked out, leaving 12O and 12N, respectively. The 2-proton decay of these to 10C and 10B gave valuable information on the mass and lifetime (equivalent to a property called the width of the state) of states in 12O and 12N. The mass and width for the 12O ground state turn out to be considerably smaller than what was known from previous measurements and resolve a discrepancy behind the theory for the width. The properties of the isobaric analog state in 12N—analogous to one in 12O, except that a neutron replaces a proton—are measured here for the first time.

These results permit a successful test of a 3-parameter equation, called the isobaric multiplet mass equation, that relates the masses of nuclei in the same state (here 12Be, 12B, 12C, 12N, and 12O) with the same total number of nucleons, simply in terms of the respective numbers of protons and neutrons. – Rick Casten and John Millener


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Nuclear Physics

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