Nuclear physicists discovered twenty years ago that some of the lightest nuclei can have a matter radius as large as those found in the heaviest naturally occurring elements [1]. This was explained by weakly bound nucleons that form a dilute cloud around a central nuclear core [2]. Such a structure is called a neutron halo. A schematic picture for the structure of the two-neutron halo nucleus ¹¹Li is shown in Figure 1. The wave-function of the halo neutrons extends far away from the ⁹Li core, much further than it would be classically allowed. Indeed the neutrons spend more than half of the time outside the range of the strong nuclear force. These findings initiated a new field of nuclear physics research. The most prominent representatives of halo nuclei are ¹¹Li, ^6,8He, ¹¹Be, and ⁸B. The halo structure is particularly sensitive to the effective forces acting between nucleons, and the charge radius of these nuclei provides important information about the influence of the halo on the nuclear core. However, the charge radius of the most famous halo nucleus ¹¹Li cannot be measured by the usual method of electron scattering because the abundance of this isotope is too low and its lifetime is too short (only 8 milliseconds). In a combined effort of theoretical and experimental atomic physics, an international collaboration with groups from Germany, Canada, and the US has overcome both theoretical and experimental difficulties. They developed a sensitive and accurate method to determine the charge radius via a measurement of the isotope shift - the change in an electronic transition frequency between isotopes.

   Normally, this method cannot be applied to radioactive isotopes of elements lighter than sodium because the tiny effect of the charge radius on the transition frequency cannot be separated from other mass-dependent corrections to the electronic energy levels. On the theoretical side, the intractable problem had to be solved to calculate the three-electron energy levels to the required accuracy of 1 part in 10⁹, including the combined effects of electron correlation, relativity and quantum electrodynamics [3]. On the experimental side, a method was developed at GSI Darmstadt (Germany) that is both very accurate and extremely sensitive and is able to handle the small production rates and the short lifetime of ¹¹Li.[4] In June 2004 the experiment was installed at TRIUMF in the ISAC experimental hall. After first tests with ^8,9Li in September, a ¹¹Li beam of approximately 35,000 ions/s was delivered by the ISAC surface ion source during the beamtime in October 2004. With an overall efficiency of 10^-4, resonance signals like that shown in Figure 2 were recorded within 15 minutes. Analysis of these curves will result in the first model-independent value for the charge radius of ¹¹Li. It will provide a critical test of nuclear models that have been proposed for the effective forces holding this fragile halo structure together. It is of great interest because the understanding of effective nuclear forces is important to determine, for example, the structure of neutron stars or the pathways of stellar nucleo-synthesis to form the elements of which we are made.•

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