  We present measurements of the ab-plane magnetic penetration depth,
\lambda(T), in five optimally doped Pr_{1.855}Ce_{0.145}CuO_{4-y} films for 1.6
K \leq T \leq T_c \sim 24 K. Low resistivities, high superfluid densities
n_s(T)\propto \lambda^{-2}(T), high T_c's, and small transition widths are
reproducible and indicative of excellent film quality. For all five films,
\lambda^{-2}(T)/\lambda^{-2}(0) at low T is well fitted by an exponential
temperature dependence with a gap, \Delta_{min}, of 0.85 k_B T_c. This behavior
is consistent with a nodeless gap and is incompatible with d-wave
superconductivity.

Superconductivity