Looking at nuclei in the black sphere picture

We review our method for systematically analyzing proton elastic scattering data for stable nuclei, by assuming that the target nucleus is a ``black'' sphere. This method gives a length scale, $a$, the values of which are determined so as to reproduce the angle of the first diffraction maximum in the scattering data. We find that the absorption cross section, $\pi a^2$, agrees with the empirical total reaction cross section for C, Sn, and Pb to within error bars for proton incident energies, $T_p$, higher than $\sim800$ MeV. Our preliminary results suggest that this agreement is retained for $T_p$ down to a few hundreds MeV. For $T_p>\sim800$ MeV, we also find that $\sqrt{3/5}a$ almost completely agrees with the empirically deduced values of the r.m.s. matter radius for nuclei having mass larger than about 50, while it systematically deviates from the deduced values for $A<\sim50$. This tendency suggests a significant change of the nuclear matter distribution from a rectangular one for $A<\sim50$. Possible application of our method to neutron-rich unstable nuclei is finally discussed.