  Radioactive isotopes such as 44Ti offer probably the most direct probe into
nucleosynthesis environments in the interior of exploding stars, when the
associated gamma-ray activities in the explosion remnant are detected and
translated back to the isotopic abundances at the time of the explosion. In
this paper, we assert that the procedure may not necessarily be
straightforward, at least in the case of 44Ti, an orbital-electron capture
decay isotope. Using the analytic model of McKee and Truelove (1995) for young
supernova remnants, and assuming the existence of overdense 56Fe-dominated
clumps that contain also 44Ti, we show that a high degree of ionization may be
caused by the reverse shock so that the electron-capture rate of 44Ti could be
significantly reduced from its laboratory value. When applied to Cas A, this
increases under certain conditions the current 44Ti-activity by a factor 1.5 -
2.5, which yields a better compatibility between the COMPTEL observation of the
1.16 MeV line activity associated with the 44Ti decay and the SN model
predictions of the initial 44Ti abundance. This possibility is, however,
subject to various uncertainties, and in particular to the unknown properties
and radial distribution of the the clumps in the ejecta.
