Anomalous Mass Number Dependence of Nuclear Transparency

We examine target mass number (A) dependence of the nuclear transparency calculated for (e, e'p) reactions. Generally, the transparency is proportional to A-113 for large A's and has weaker A-dependence as A decreases. The conventional nuclear physics calculation yields the transparency whose A - 113 dependence is reached in rather light nuclei (around A =20). If either the proton mean-free path in nuclei becomes substantially longer, or the internal structure of the proton is explicitly incorporated in the calculation, however, the A - 113 dependence is reached only in extremely heavy nuclei. The A-dependence should thus serve as a signature of the so-called color-transparency phenomenon.