Abstract: An apparatus for treating flowable material falling through an irradiation zone in the form of a thin, substantially nonturbulent, cohesive layer including in combination an outlet reservoir adapted to catch the falling material after it is irradiated; a source of radiation adapted to direct radiation against the layer as it falls through the irradiation zone; and means for forming flowable material flowing through a pipe or similar material transfer means into a thin, substantially nonturbulent, cohesive layer and directing the layer so formed through the irradiation zone, said means having an inlet orifice adapted to receive material from the transfer means and convey it to an upwardly extending transition zone wherein the flow is gradually widened prior to its entry into a gradually upwardly sloped approach zone which terminates at a forward edge over which the material passes in the appropriate form for treatment in the irradiation zone.