Effect of Nuclear Size on the Stopping Power of Ultrarelativistic Heavy Ions

A new formulation for the theory of electronic stopping power of ions at relativistic energies has been proposed by Lindhard and Sørensen (LS). In it, they find that, at sufficiently high energy, nuclear size effects should act to reduce the momentum transfer to electrons and hence the stopping power. To test this result, we passed beams of 33.2-TeV ${}^{208}$Pb ions $(\gamma =168\mathrm{})$ from the CERN-SPS through targets of C, Si, Cu, Sn, and Pb, and measured energy loss and beam broadening. The LS theory for stopping power is confirmed, but with a slight drift upward from theory for high-Z targets. A drastic decrease in energy straggling (factor of $\sim 4$) predicted by LS cannot be deconvoluted from the multiple Coulomb scattering distribution.