[en] The interaction properties of projectile fragments from the fragmentation of 0.9 A GeV and 1.8 A GeV ⁴⁰Ar with Cu have been studied using radioactivation techniques. Projectile fragments formed in the interaction of the primary Ar beam with a thick Cu disk (the target disk) interacted with a second detector Cu disk. The relative reaction product yields in the two disks were measured as a function of the separation between the disks. A large increase in the yield of deep spallation products (²⁴Na, ²⁸Mg) in the detector disk was observed as the projectile energy increased from 0.9 A GeV to 1.8 A GeV. At 1.8 A GeV (but not at 0.9 A GeV) the yields of the deep spallation products decreased as the separation between the disks increased indicating either a dramatic broadening of the relevant projectile fragment angular distribution between 0.9 A GeV and 1.8 A GeV or a decay in flight of some of the projectile fragments at 1.8 A GeV (but not at 0.9 A GeV). Calculations will be discussed that cast doubt on the first possibility. 15 references