Laser spectroscopy of Radioactive Isotopes

Nuclear properties such as the size and magnetic field strength slightly shift or perturb the orbits of the atomic electrons.These shifts are small and at the level of a part per million of the electron binding energies. They can be measured with great precision by a variety of techniques using tunable laser light, which can be controlled with part per billion precision. When the laser light is tuned to exactly match an electronic transition frequency, it resonantly excites an isotope of the selected element. The excited atom quickly decays with the emission of fluorescence light which may be detected by photomultipliers sensitive to single photons. In this way the frequency of the laser resonance can be measured experimentally. The analysis of the resonant frequencies across an isotope series allows us to deduce the change in nuclear size as a function of neutron number. It is possible to detect the change of the proton distribution caused by the removal of a single neutron. The same data can show in a clear way whether the nucleus is changing its shape as neutrons are added or subtracted. The magnetic field strength around the nucleus, also measured in these experiments, is produced by the motion of both protons and neutrons. The theory of the magnetic field is well understood and clear conclusions can be drawn about the nucleon orbits occupied by the protons and neutrons. Thus the laser method provides fundamental information on basic properties of the nucleus - size, shape, magnetic properties, nuclear 'spin' angular momentum - which helps to refine our theoretical approaches to explain observed properties, and predict properties of nuclei we can not yet synthesize in the laboratory. The predictive power of these theories is very important. Many of the nuclei unavailable for study on Earth are involved in supernova processes. We cannot begin to understand the processes without first having an understanding of the nuclei involved. The laser techniques we