Laser spectroscopy of Radioactive Isotopes

核的性质，如大小和磁场强度，会轻微地改变或扰乱原子电子的轨道。这些变化很小，仅为电子结合能的百万分之一。它们可以通过使用可调谐激光的各种技术以极高的精度进行测量，其精度可以控制在十亿分之一。当激光被调谐到与电子跃迁频率完全匹配时，它会共振地激发选定元素的同位素。被激发的原子随着荧光的发射而迅速衰变，荧光可以被对单光子敏感的光电倍增管探测到。用这种方法可以实验测量激光共振的频率。通过对同位素系列共振频率的分析，我们可以推断出原子核大小的变化是中子数的函数。有可能探测到由于去掉一个中子而引起的质子分布的变化。同样的数据可以清楚地显示，随着中子的增加或减少，原子核是否在改变其形状。这些实验也测量了原子核周围的磁场强度，它是由质子和中子的运动产生的。磁场理论已被很好地理解，关于质子和中子所占据的核子轨道可以得出明确的结论。因此，激光方法提供了关于原子核的基本性质的基本信息——大小、形状、磁性、原子核“自旋”角动量——这有助于完善我们的理论方法来解释观察到的性质，并预测我们还不能在实验室合成的原子核的性质。这些理论的预测能力非常重要。许多无法在地球上进行研究的原子核都与超新星过程有关。如果不首先了解所涉及的原子核，我们就不能开始理解这些过程。我们的激光技术

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