2008

Low-Energy Nuclear Physics

 
 

Nuclear Structure and nuclear astrophysics are subfields of low-energy nuclear physics. In contrast to intermediate- and high-energy nuclear physics, which probe the Quark-Gluon interactions and the QCD (quantum chromodynamic) vacuum, respectively, in low-energy nuclear physics, the most relevant degrees of freedom are the protons and neutrons. The structure of nuclei is then interpreted in terms of the renormalized bare nucleon-nucleon interactions (derived from nucleon-nucleon scattering phase shifts and effective field theory) applied to the nuclear many-body problem. Recently, such a microscopic ab initio approach has been very successful in describing the ground states and lowest excitations of the lightest nuclei on or near the stability line. The goal of experimental nuclear physics is now to provide data on nuclei far from stability, at high excitations (temperatures), and larger masses where such a level of understanding has not yet been achieved. In each of these three directions new phenomena emerge which present different challenges for theory.

  1. (i)Some nuclei far from stability exhibit a systematic re-ordering of nucleon shells of the nuclear shell model which has yet to be fully explained based on the underlying forces. On the proton-rich side, new phenomena such as proton and di-proton radioactivity have been observed. On the neutron-rich side, some weakly-bound nuclei have been observed to display an extended neutron-halo structure. Investigation of nuclear structure far from stability is currently on the forefront of low-energy nuclear physics.

  2. (ii)High excitations of nuclei are mostly chaotic, due to a thorough mixing of quantum levels through small, residual interactions within the shell model. Statistical concepts such as entropy and temperature are therefore useful to describe this regime. However, due to the finite size of the nucleus, fluctuations around the averages often play an important role and cannot be neglected. The study of average level density and its fluctuations (among others) can provide insight in this modern, interdisciplinary field of mesoscopic physics.

  3. (iii)In medium and heavy nuclei collective phenomena emerge. Such excitations are characterized by a coherent motion of a large number of nucleons. Examples are: rotation, vibrational motion, giant resonances and other resonant excitations. Such collective motions are very hard to describe in a fundamental way. On the other hand, mean-field models based on phenomenological, effective interactions and simple, but easily visualized   geometric models, where the nuclear shape is parameterized and surface excitations are the relevant degrees of freedom, have been quite successful in the past. Certain giant and other resonances can be investigated as function of temperature by measuring the corresponding radiative strength functions, which describe the nuclear response to simple electromagnetic excitations. Of particular interest is research on M1 (magnetic dipole) and E1 (electric dipole) resonances and other enhancements of the strength functions which are of importance.

Introduction to Experimental Low-Energy Nuclear Physics

Radiative strength of 57Fe in units of 10-9 MeV-3 vs. γ energy in units of MeV. For more information see story on ‘red iron’.

Rendering of the unwrapped MoNA scintillator bars. Figure taken from the MoNA collaboration website at Concordia College.

Current projects:


Students (Ph.D., M.Sc., and undergrads) are encouraged to inquire (Edwards, office 205) about research possibilities in any of the areas mentioned in the main text. Specific projects are

• Nuclear structure investigation of N=16 dripline nuclei with MoNA, SeGA, and the S800 at the NSCL/MSU

• Measurements of level densities with the particle-evaporation method at the Edwards Accelerator Lab, and with primary γ-rays at the Oslo Cyclotron Lab

• Measurements of γ-ray strength functions at the Oslo Cyclotron Lab, and development of new methods for alternative experimental techniques

• Measurements of specific nuclear reaction cross sections at the Edwards Accelerator Lab and detector/device development for experiments at modern radioactive beam facilities

Nuclear Reactions: Means and Goals of Nuclear Physics

Figure taken from A. Richter, keynote presentation at the INPC, Göteborg, 2004. For detailed explanations, see text below.

Close-up of the CACTUS statistical γ detector array. Figure taken from the Oslo Cyclotron Laboratory Website.

4.5 MeV Tandem Accelerator at John E. Edwards Accelerator Lab, Ohio University. This and similar accelerators are used to induce nuclear reactions.

Image: NASA

Virtually any measurement in nuclear physics is facilitated by a nuclear reaction. Therefore, nuclear reactions are an indispensable tool of nuclear physics. In other instances, the actual determination of a nuclear reaction cross section by direct or indirect means might be the goal of a particular experiment. The three main applications of nuclear physics which are power generation, defense, and production of radioactive isotopes for medical and technical purposes, all rely on the knowledge of specific nuclear reaction cross sections, and our ability to induce such nuclear reactions in a predictable and controllable manner. A different, but conceptually similar branch or application of nuclear physics is nuclear astrophysics which is concerned with the nuclear reactions which power energy and isotope productions in stellar objects. There, the goal is to measure or otherwise infer nuclear reaction cross sections which are of importance for stellar or chemical evolution within a galaxy.


The use of nuclear reactions to study nuclear structure requires the untangling of such structure information from the nuclear cross section. In order to obtain the most precise structure data, reaction theorists have to incorporate realistic many-component wave-functions into sophisticated coupled-channel, multi-step reaction calculations. In simpler, more general cases, the challenge lies in developing new types of reactions as useful and reliable tools for nuclear-structure investigations.


In cases where the measurement of an actual reaction cross section is the goal, the direct study of the reaction is the most straightforward approach. However, sometimes the reaction cross-section is too low or the participants are short-lived radioactive isotopes which precludes a direct measurement. In such cases, indirect methods, which often rely on a mix of theoretical considerations and the experimental knowledge of specific nuclear-structure data, such as detailed spectroscopic information, resonance properties, nuclear level densities and radiative strength functions have been applied. Often, experimental nuclear-structure data which replaces or supplants theoretical considerations help to dramatically decrease systematic uncertainties connected to such reaction cross-section estimates.