1. The negative part of the p(r) comes from pairs of one scatterer from the core (negative ΔSLD) and one scatterer from the shell (positive ΔSLD).
  2. At small distances, the scattereres are next to scatterers of the same type, i.e., having the same ΔSLD. Therefore p(r) is positve at small r.
  3. The largest distances are between scatters in the shell, and it is positive since the scatterers in these pair have the same sign of ΔSLD.