Another very important characteristic of spallation reactions is the nature of the heavy residues. This reflects the light particle emission rates and deter­mines the radiotoxicity of the wastes of the spallation reaction. In particular, for heavy target nuclei like lead, the amount of 194Hg created is a key quantity since 194Hg has a long half-life of 520 years and decays to 194Au which has a short lifetime of 38 hours and a large decay energy of

2.5 MeV. Thus the decay of 194Hg is both long lived and energetic. Up until recently determination of residue mass and charge distributions used standard radiochemical techniques, mostly gamma ray counting of the reac­tion products. The most extensive study has been that by Michel et al. [111]. This technique is based on the observation of gamma decays following beta decays, and thus misses stable residues as well as very long-lived ones whose activity is too weak to be visible. Recently, the very beautiful inverse kinematic technique promoted in GSI, for fission studies and neutron-rich nuclei synthesis, has been used to tackle the residue question. This technique, essentially, allows determination of all yields of heavy residues and fission fragments. These yields are measured immediately after the reaction and thus cannot be compared directly with those obtained from radiochemical measurements. However, for the so-called shielded isotopes which are directly produced by the reaction mechanism, a direct comparison between the two techniques can be done, and is found to be satisfactory [112]. Figure 6.5 shows a comparison between the experimental data and a few cal­culations for the reaction p + Au (0.8 GeV/A). All calculations reported use the Dresner evaporation code. The figure shows a significant improvement when using modern INC calculations for the region of spallation products. However, fission appears to be under-evaluated. Figure 6.6 shows that an

even better agreement is obtained if the Dresner evaporation code is replaced by a modern evaporation code elaborated at GSI [113].