Insensitivity of the elastic proton-nucleus reaction to the neutron radius of 208Pb

The sensitivity-or rather insensitivity-of the elastic proton-nucleus reaction to the neutron radius of Pb-208 is investigated using a non-relativistic impulse-approximation approach. The energy region (T-lab = 500 MeV and T-lab = 800 MeV) is selected so that the impulse approximation may be safely assumed. Therefore, only free nucleon-nucleon scattering data are used as input for the optical potential. Further, the optical potential includes proton and neutron ground-state densities that are generated from accurately-calibrated models. Even so, these models yield a wide range of values (from 0.13 fm to 0.28 fm) for the poorly known neutron skin thickness in Pb-208. An excellent description of the experimental cross section is obtained with all neutron densities. We have invoked analytic insights developed within the eikonal approximation to understand the insensitivity of the differential cross section to the various neutron densities. As the diffractive oscillations of the cross sections are controlled by the matter radius of the nucleus, the large spread in the neutron skin among the various models gets diluted into a mere 1.5% difference in the matter radius. This renders ineffective the elastic reaction as a precision tool for the measurement of neutron radii. (c) 2006 Elsevier B.V. All rights reserved.