Researchers from Norway, Hungary, Russia, Turkey, and the U.S. have studied the least energetic light emitted by heated iron nuclei. This kind of light is also called gamma radiation. It is about one million times more energetic than visual light and cannot be seen with the naked eye.

Surprisingly, heated iron nuclei like to emit low-energy radiation, the more so, the lower its energy. The researchers found ten times more low-energetic light than previously assumed. This result was first found out in an experiment at the University of Oslo. There, researchers from five institutions investigated the previously unaccessible region of low-energy (0.5--3 MeV) gamma radiation emitted by warm (6--8 MeV excited) iron nuclei. The MeV or Mega electron Volt is a traditional measure of energy for nuclear physicists. It requires 26 quadrillion (a one followed by 15 zeros) MeV equal one food calorie.

Often color is used to represent the preference to absorb, scatter, and re-emit light at different energies, with red light having less energy than blue light. According to this picture, the experiment showed that heated iron nuclei are 'redder' than thought, by giving out a lot of low-energy gamma radiation. Standard nuclear theory based on vast experimental data predicts that radiation should be emitted mostly at much higher energies of 10--15 MeV, which represent the region of 'blue light' in nuclear physics. This high-energy light is governed by the well known phenomena of the Giant Dipole Resonance connected to the swinging of protons against neutrons, the two constituents of nuclei. The results from Oslo suggested that besides this 'blue light' (which is also seen in the experiment with the expected intensity) a second, weaker source of 'red light' exists in heated iron nuclei with a fifth of the energy.

The researchers kept their results quiet for two years since nobody was able to understand the findings. They contacted their Hungarian colleagues and a second experiment was carried out at a Budapest reactor. It confirmed the earlier results by using a different experimental method, including a different nuclear reaction and different detection instruments.

Theorists in several countries will now try to understand why heated iron nuclei emit so much low-energy gamma radiation. Parallel to this, the experimentalists will continue their effort and investigate new nuclei like molybdenum. Their results might impact our understanding of giant star explosions like supernovae where chemical elements are produced by nuclear reactions. The outcomes will also be important for the projected Rare Isotope Accelerator, a Department-of-Energy funded, one-billion-dollar factory of very short-lived nuclei.

The present results will be published shortly in the prestigious physics journal 'Physical Review Letters'. Among the participating U.S. institutions are Ohio University, Lawrence Livermore National Laboratory, Triangle University Nuclear Laboratory, North Carolina State University, and Michigan State University.

Reference: ``Large Enhancement of Radiative Strength for Soft Transitions in the Quasicontinuum,'' A. Voinov, E. Algin, U.Agvaanluvsan, T.Belgya, R.Chankova, M.Guttormsen, G.E.Mitchell, J.Rekstad, A.Schiller, S.Siem, Phys. Rev. Lett. 93, 142504 (2004).

More information here.

Contact: voinov@ohio.edu