Very heavy fission fragments produced in the spallation reaction 238U+p at 1 A GeV

Fission fragments from 1 A GeV U-238 ions impinging a hydrogen target are investigated by using the fragment separator FIRS for magnetic selection of reaction products including ray-tracing, and Delta E-ToF techniques. On the basis of the kinematics of the fragments, binary fission can be assigned as production process up to very heavy fragments. Fission fragments are separated and identified as far as Re-184 for the first time. Cross sections are measured and found in the range of 500 mu b to 5 mu b. Isobaric cross sections decrease smoothly with increasing masses. Beyond tungsten, fission fragments are hidden in the more abundant spallation evaporation residues. 283 isotopes of elements gadolinium to rhenium in the mass range 147 to 184 are observed and cross sections measured. All the fragments are formed by fission of excited parent nuclei following emission of a cascade of secondary neutrons. (c) 2005 Elsevier B.V. All rights reserved.