The so-called gamma-echo effect has been
observed experimentally and analyzed using the semiclassical
optical theory. Here the effect is reinterpreted using
a new 1D quantum mechanical model. This leads to a different
interpretation of the effect as a π phase-shift induced
transparency. In the basic time-differential Mössbauer
spectroscopic technique the forward-scattered recoil-free radiation
is observed, in delayed coincidence, after passing through a
nuclear-resonant absorber. The effect in question is produced
most efficiently when the source of recoil-free radiation is moved
abruptly causing a π phase shift of the source radiation during
its radiative lifetime. Using the 1D model the effect is seen to
arise from the constructive interference between the source radiation
at a later time, and the radiation coming from the absorber excited
at an earlier time. The exact form of the source modulation and the
nuclear-resonant thickness of the resonant absorber determines the
shape of the time-differential resonant gamma ray transmission
spectrum. Numerical results are given using the familiar 57Fe
recoil-free resonant transition. The π phase-shift-induced
transparency allows the resonant gamma radiation, incident on the
resonant absorber, to be transmitted through the absorber without
appreciable attenuation.