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The University of Tennessee

Department of Physics and Astronomy

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    Dr. Kate Jones
    406A Nielsen Physics Building
    Knoxville, TN  37996
    Phone: (865) 974-4022
    Email: kgrzywac@utk.edu

 

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Experimental Nuclear Astrophysics

See also :  Decay Spectroscopy : High Spin Gamma Ray Spectroscopy

All of the elements contained in the materials around us were originally synthesized in the cosmos. Most of the universe consists of the light elements hydrogen and helium that were produced in the big bang. Heavier elements, such as carbon and oxygen that are essential to life, as well as iron, gold and elements all the way up to uranium, were made, and are still being made, through nuclear reactions in stars and stellar explosions.

As nuclear astrophysicists, we perform experiments in Earth-based laboratories to try to understand the nuclear reactions that occur in stars. This information is essential to unraveling observational data such as light curves and spectra in order to better understand the conditions for nucleosynthesis.

Elements lighter than iron can be produced through nuclear fusion as this is energetically favorable. However, iron is so tightly bound that it requires energy to add nucleons to it. Two processes bypass this problem by adding neutrons to make an unstable nucleus that subsequently β-decays to a nucleus that is of a heavier element. The first of these processes is the slow neutron capture process, or s-process, that stays close to stability. The second, rapid neutron capture process, or r-process, involves very neutron-rich unstable nuclei (see above figure). Half of the elements heavier than iron are produced in this process, that occurs in a hot, dense, neutron-rich environment, possibly a Type II supernova.

In order to understand how nuclei are synthesized in the r-process, we need to know the masses, β-decay half-lives and neutron capture rates on a host of neutron-rich nuclei, all of which are difficult to produce, and many cannot be produced in current laboratories. It is not possible to perform the relevant neutron-capture reactions in the laboratory as the nuclei of interest are too short lived to be made into targets, and we cannot make targets out of neutrons. The best approach to understanding the r-process is to measure the most important nuclei that can be reached and those that have the most influence on mass models, such that the properties of out-of-reach nuclei can be calculated.

Our group uses a recent method to gain information on these nuclei and on the neutron-capture reactions they undergo. We perform neutron-transfer reactions using radioactive ion beams. Currently the HRIBF at Oak Ridge National Laboratory is the only facility in the world that can accelerate r-process nuclei to the energies required to perform transfer reactions. We have concentrated our efforts to nuclei close to the nuclear shell closures where the nuclear structure has the greatest impact on element production. Through our collaboration with the Center of Excellence for Radioactive Ion Beam Studies, and built upon the first measurements of this type using beams of 82Ge and 84Se [1] we have been central to measurements around the double-shell closure at 132Sn [2]. By measuring protons emerging from neutron-transfer reactions in large arrays of detectors such as SIDAR, ORRUBA in coincidence with auxiliary detectors such as MCPs, small silicon detectors (mini/micro/QQQs), we are able to measure the Q-value of the reaction and extract excitation energies. The angular distributions of proton ejectiles provide information on the nature of the state populated in the final nucleus.

We are working with theoretical astrophysicists to determine the sensitivity of r-process abundance patterns on individual neutron-capture rates to guide our program. As the precise path of the r-process is determined by the conditions present in the supernova (or neutron-star collision), predicting abundances resulting from this process requires knowledge of many different factors. Nuclear data is an essential input into astrophysics models and ultimately should be used to constrain the macroscopic conditions.

[1]. "Single-neutron excitations in neutron-rich 83Ge and 85Se",
J. S. Thomas, G. Arbanas, D. W. Bardayan, J. C. Blackmon, J. A. Cizewski, D. J. Dean, R. P. Fitzgerald, U. Greife, C. J. Gross, M. S. Johnson, K. L. Jones, R. L. Kozub, J. F. Liang, R. J. Livesay, Z. Ma, B. H. Moazen, C. D. Nesaraja, D. Shapira, M. S. Smith, & D. W. Visser
Phys. Rev. C 76, 044302 (2007)

[2]. "Single neutron transfer experiments close to the r-process path",
K.L. Jones, A.S. Adekola, D.W. Bardayan, J.C. Blackmon, K.Y. Chae, K. Chipps, J.A. Cizewski, D.J. Dean, L. Erikson, R.P. Fitzgerald , A.L. Gaddis, U. Greife, C. Harlin, R. Hatarik, J.A. Howard, M.S. Johnson , R.L. Kozub, J.F. Liang, R.J. Livesay, Z. Ma, B.H. Moazen, P.D. O’Malley, C.D. Nesaraja, S.D. Pain, N.P. Patterson, S.V. Paulauskas, D. Shapira, J.F. Shriner Jr, D.J. Sissom, M.S. Smith, T.P. Swan,& J.S. Thomas.
Acta Physica Polonica Volume 38 (4), 1205 (2007)