Determination of the proton and neutron densities at the nuclear periphery with antiprotonic X-rays and p-bar-nucleus reactions

The nucleon density at the nuclear periphery has been investigated with two different methods. The first one is the measurement of the energy shift and the width of antiprotonic X-ray lines. Besides various other nuclides the X-rays from the isotopes 172Yb and 176Yb have been measured very extensively. From these data the absorption widths of six antiprotonic levels and the shifts of the lowest visible transitions could be determined. The comparison of the experimental intensities of the transitions with results from calculations of the antiprotonic cascade showed a good agreement. The second method is the determination of the yield of the annihilation products from the reaction of the antiprotons with nuclei. From this the neutron-to-proton density ratio at a distance of about 3 fm outside the half-density radius can be determined. This method was extended to short-lived residual nuclei with a half-life down to 5 s. The experiment confirmed the previously found negative correlation between the size of the neutron skin and the binding energy of the last neutron. The results have been compared with calculations. For many nuclei a good agreement was found between experiment and calculation, however for some nuclei measured and calculated values are at variance