There is disclosed a fluid treatment device comprising a housing for receiving a flow of fluid. The housing comprises a fluid inlet, a fluid outlet, a closed fluid treatment zone disposed between the fluid inlet and the fluid outlet. Disposed in the housing is at least one elongate radiation source assembly having a longitudinal axis disposed in the fluid treatment zone substantially parallel to a direction of the flow of fluid through the housing. The radiation source assembly comprises an elongate radiation source disposed in a protective sleeve to define a substantially annular passageway. The protective sleeve has opposed open ends configured to permit heat to exit the passageway and the housing through at least one of the opposed open ends of the sleeve.