  Recent developments of the Cascade-Exciton Model (CEM) of nuclear reactions
are described. The improved cascade-exciton model as implemented in the code
CEM97 differs from the CEM95 version by incorporating new approximations for
the elementary cross sections used in the cascade, using more precise values
for nuclear masses and pairing energies, using corrected systematics for the
level-density parameters, and several other refinements. We have improved
algorithms used in many subroutines, decreasing the computing time by up to a
factor of 6 for heavy targets. We describe a number of further improvements and
changes to CEM97, motivated by new data on isotope production measured at GSI.
This leads us to CEM2k, a new version of the CEM code. CEM2k has a longer
cascade stage, less preequilibrium emission, and evaporation from more highly
excited compound nuclei compared to earlier versions. CEM2k also has other
improvements and allows us to better model neutron, radionuclide, and gas
production in ATW spallation targets. The increased accuracy and predictive
power of the code CEM2k are shown by several examples. Further necessary work
is outlined.
