HEAVY-RESIDUE PRODUCTION IN AR-TH COLLISIONS AT 44, 77, AND 95 MEV NUCLEON/

The yields and recoil properties of the targetlike fragments produced in the interaction of 44, 77, and 95 MeV/nucleon Ar with Th-232 have b een measured. From these data, the fragment mass distributions, averag e energies, and the linear momentum transfer leading to their formatio n have been deduced. The residue yields are large at all energies [790 mb, 790 mb, and 710 mb (+/-30%) at 44, 77, and 95 MeV/nucleon, respec tively] while the average residue energies are low [mean fractional li near momentum transfer (FLMT) is 0.14, 0.12, and 0.13 for the 44, 77, and 95 MeV/nucleon experiments]. Because of the low mean FLMT Values a ssociated with the residues, we conclude that, on average, they do not result from fusionlike events. The residue energies and their depende nce on fragment mass number agree with predictions of BUU calculations . The substantial survival of the residues of excited, fissionable nuc lei is shown to be a result of retardation of fission decay and the br oad primary fragment distributions. The intranuclear cascade model is shown to give an incorrect description of the primary fragment propert ies in these reactions.