Nuclear Beta Decay

My graduate research focused on radioactive properties of the very heaviest atomic nuclei, whose half-lives are an important puzzle piece in the search for the astrophysical origin of the elements heavier than iron. (Our sun, and those stars like it, do not generate any elements heavier than about Z = 26 because the process becomes energy-consuming rather than energy-producing.) In particular, I worked exclusively on β-decay, a process by which a neutron (bound within the atomic nucleus in my case) decays into a proton and an electron. This process changes the mass-energy of the nucleus along with the electrical forces, since a neutral particle in the nucleus is exchanged for a charged one, and heavier nuclei (especially nuclei with an imbalance in the numbers of protons and neutrons) are rather unstable against this decay mode.

To calculate these nuclear half-lives, our group developed a program, a proton-neutron extension of the finite amplitude method (FAM), that significantly reduced the computational cost of certain half-life calculations (Phys. Rev. C, local, preprint). A postdoc in our group applied the program to calculate the properties of thousands of nuclei (Phys. Rev. C, preprint), and I extended the method to compute the half-lives of particularly important nuclei having odd numbers of protons and/or neutrons (Phys. Rev. C, local, preprint). A nucleus having an odd number of particles of one type or another breaks some very useful symmetries that nuclear calculations often rely on, and adding these symmetries back into our FAM calculations was a nontrivial piece of work.

This research program was one aspect of a larger collaboration supporting astrophysical investigations into the r process, the mechanism (thought to be) responsible for producing many of the heaviest elements that make up the periodic table. Excitingly, this research is closely linked to cutting-edge experiments including the 2017 detection of a neutron star merger by LIGO. These neutron star collisions are hypothesized to produce copious amounts of r-process elements, and subsequent electromagnetic observations are attempting to determine if this particular merger exhibits a telltale signature of such element formation.


Finite-amplitude method for charge-changing transitions in axially deformed nuclei
M. T. Mustonen, T. Shafer, Z. Zenginerler, and J. Engel
Phys. Rev. C  90 024308 (2014)

β decay of deformed r-process nuclei near A = 80 and A = 160, including odd-A and odd-odd nuclei, with the Skyrme finite-amplitude method
T. Shafer, J. Engel, C. Fröhlich, G. C. McLaughlin, M. Mumpower, and R. Surman
Phys. Rev. C  94  055802  (2016)

Doctoral Dissertation

Calculation of beta-decay rates in heavy deformed nuclei and implications for the astrophysical r process
T. Shafer
College of Arts and Sciences, Department of Physics and Astronomy (May 2016)

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