  We have measured the energy spectra of pions and protons emitted in the weak
decay of 12_Lambda_C, 28_Lambda_Si, and Lambda_Fe hypernuclei produced via the
(pi+, K+) reaction. The decay widths of the pi- mesonic decay (Lambda -> p +
pi-) and the nonmesonic decay (Lambda + N -> N + N) were extracted. The present
results demonstrate an increase of the mesonic decay width due to a distortion
of the pion wave function in nuclear medium for the first time. The ratios of
the neutron- to proton-induced nonmesonic decay widths, Gamma_n(Lambda + n -> n
+ n)/Gamma_p(Lambda + p -> n + p), were evaluated by a direct comparison of the
measured proton energy spectra with the calculated ones. No theoretical
calculation which has been proposed so far can simultaneously account for both
the nonmesonic decay widths and the Gamma_n/Gamma_p ratios in the present data.
