ATOMIC-COLLISIONS WITH 33-TEV LEAD IONS

Recent availability of relativistic and ultrarelativistic beams of hea vy ions has permitted the first controlled studies of atomic collision s at energies sufficient to measure effects of several new basic pheno mena. These include measurements substantiating recently predicted fin ite nuclear size effects resulting in a reduction in the total electro nic energy loss of heavy ions in matter, and measurements of Coulomb c ollisions in which electrons are excited from the Dirac negative energ y continuum. Measurements of total energy loss, free electron-positron pair production, and electron capture from pair production have been recently performed using 33-TeV Pb82+ ions from the CERN SPS accelerat or in Geneva. Results of these studies are presented, along with compa risons with relevant theory.