  We explore in detail the semiclassical environment of collapsing shells of
matter, and determine the semiclassical flux measured by a variety of
observers. This study is a preliminary step in a broader investigation of
thermodynamic properties of the geometry of collapsing objects. Specifically,
in this paper we consider spherically symmetric null and timelike collapsing
shells which form an event horizon, and calculate the flux measured by
observers both inside and outside the shell, and both inside and outside the
event horizon, and find nontrivial results in most of the cases. Additionally,
we also investigate the environment of a shell which collapses but \emph{does
not} form a horizon, halting at some radius larger than the Schwarzschild
radius, and find that such an object generically gives rise to a pulse of
radiation which is sharply peaked as it travels inwards and is reflected at the
origin, and eventually emerges from the shell in a "thermalized" form. Our
results have potential consequences in addressing questions pertaining, e.g. to
black hole entropy and backreaction.
