Nucleon density of172Yband176Ybat the nuclear periphery determined with antiprotonic x rays

The antiprotonic x-ray cascade in ${}^{172}\mathrm{Yb}$ and ${}^{176}\mathrm{Yb}$ was studied and the widths and shifts of the levels which are affected by the strong interaction were deduced. A large number of transitions up to $\Delta n=5\mathrm{}$ could be observed. This opened the possibility to determine for the first time also the widths of noncircular orbits. In ${}^{172}\mathrm{Yb}$ and ${}^{176}\mathrm{Yb}$ four level shifts and seven level widths were measured in each case. The widths are slightly increasing from ${}^{172}\mathrm{Yb}$ to ${}^{176}\mathrm{Yb}$ whereas the shifts stay roughly constant. The experimental intensities of the transitions are compared with results from calculations of the antiproton cascade. Using a Fermi distribution for the nucleon densities the neutron diffuseness parameter was deduced to be for ${}^{172}\mathrm{Yb}\hspace{0.5em}(0.58\mathrm{}\pm 0.04)\hspace{0.5em}\mathrm{fm}$ larger than the proton diffuseness parameter. For ${}^{176}\mathrm{Yb}$ this value is $(0.71\mathrm{}\pm 0.04)\hspace{0.5em}\mathrm{fm}.$