Red Nuclei

Experiments conducted in Oslo and Budapest have determined that the gamma rays streaming out of excited iron nuclei come in all different energies---relatively low energy (3 MeV) as well as the expected higher energy (10 MeV). In other words, the nuclei proved to be (if one can impute colors to the gamma spectrum equivalent to the visible spectrum) “redder” than thought. Why is this a surprise?

First of all, knowledge of energy levels in the nuclear realm is not nearly as detailed as it is for atoms. Quantum electrodynamics (QED), the theory which rules the atomic world, can specify energy levels with uncertainties in parts per trillion. By contrast, quantum chromodynamics (QCD), the theory that attempts to grapple with the strong nuclear force, is rather vague, a shortcoming owing chiefly to the strength of the nuclear force. The best predictions of energy levels, in some nuclei, are only good to about 10%.

Not only that, but when a nucleus such as iron is “heated” (via particle interactions) through a “temperature” corresponding to 1 MeV, thousands of higher energy levels can be populated. When researchers observe the subsequent cooling of such nuclei what they see is not the spectrum of discrete lines one gets with atoms but instead a quasi-continuum of gamma lines.

According to Andreas Schiller of Michigan State University (schiller@nscl.msu.edu, 517-324-8142), the unexpected red gamma rays might correspond to the excitation energy of some new robust, collective, low-frequency oscillation in the iron nucleus. The collaboration includes scientists from the Joint Institute of Nuclear Research (Russia), the University of Oslo (Norway), Chemical Research Centre (Hungary), Osmangazi University (Turkey), and several US institutions---Ohio University, Lawrence Livermore National Lab, North Carolina State, and MSU. (Voinov et al., Physical Review Letters,1 October 2004.)