  The variation of the coherent branching ratio $R_{\mu e}$ (ratio of the
$\mu^-\to e^-$ reaction rate divided by the total muon-capture rate) through
the periodic table is studied by using exact muon wave functions. It was found
that, by using very heavy nuclei (e.g. \nuc{197}Au, the SINDRUM II target) as
$\mu^-\to e^-$ conversion stopping-targets, the above ratio is favored by a
factor of about four to five than by using light ones (e.g. \nuc{48}Ti, chosen
as PRIME target).

Nuclear Theory;High Energy Physics - Experiment;High Energy Physics - Phenomenology