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. Pb ions ͑g 168͒ 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 ϳ4) predicted by LS cannot be deconvoluted from the multiple Coulomb scattering distribution. [S0031-9007(96) Bloch [3], and it was shown that at relativistic velocities a "Mott" correction for spin changing collisions [4] was required. The proper combination of these effects was shown to match quantitatively with experiments on the stopping power of heavy ions with energies from 700 to 1000 MeV A [5].The energy loss DE of a totally stripped ion Z 1 passing through a thickness Dx with an electron density n e can be expressed asAt relativistic energies the term L is given bywhere g is the Lorentz factor ͑1 2 b 2 ͒ 21͞2 , b y͞c, m 0 is the electron rest mass, and I is the mean ionization potential of the target electrons. The term d arises from the so-called density effect [6,7] due to the relativistic increase in the transverse field and the attendant target screening of the projectile charge in distant collisions. For g $ 100 as in our experiments, the density effect correction can be closely approximated bywhere v p is the plasmon frequency of the total density of the electrons of the medium. Then with b Х 1 andandThe new term here is DL NS , the correction for nuclear size effect that has just recently been proposed by Lindhard and Sørensen [8,9]. Lindhard and Sørensen (LS) performed exact quantum mechanical calculations on the basis of the Dirac equation to produce values for the average energy loss and straggling which are stated to be accurate for any value of projectile charge. Note that the various DL terms are just a consequence of the calculation. Using a point Coulomb potential, they are able to reproduce the results of Bohr, Bethe, Bloch, and Mott. However, they show that at sufficiently high energies the finite nuclear size effects the stopping power.It is convenient to view the projectile nucleus as a stationary scattering point for a flux of electrons moving at the velocity of the ion in the laboratory system. According to LS, an electron will encounter the nucleus when its angular momentum pR Х gm 0 cR where R is the nuclear radius (R ϳ 1.2 3 10 213 A 1͞3 cm, where A is the atomic weight). When pR ϳh͞2 modification of the first few quantum phase shifts will be needed, i.e., nuclear size effects should be important when 2gm 0 cR͞h gA 1͞3 ͞160 Х 1. Alternatively, one may consider that the effect will become important when the deBroglie wavelength of the electron l -h͞gm 0 c beco...