There is described In another of its aspects, the present invention provides a radiation source assembly comprising: (i) an elongate radiation source; (ii) a positioning element connected to a proximal portion of the elongate radiation source; and (iii) a connecting portion secured to a proximal portion of the positioning element and configured to engage a support element to maintain a distal portion of the elongate radiation source in a cantilevered position. The present radiation source assembly is configured such that the distal portion of the radiation source is cantilevered with the respect to the distal portion of the protective sleeve in which it is disposed. This feature obviates the need to use spacers, stops, springs and the like in a distal portion of the protective sleeve to maintain correct position of the radiation source within the protective sleeve. Further, the present radiation source assembly is advantageous in that allows for withdrawal of the radiation source from the radiation source assembly without the need to disengage all of the components. Thus, it is possible to replace a single radiation source by removing it from the protective sleeve during operation of the fluid treatment system. This operation can be accomplished quickly without the need to shut down the fluid treatment system or otherwise compensate for the fact that one of the radiation source is being serviced.