There is disclosed a coupling for a radiation source assembly that comprises an elongate radiation source and an elongate radiation transparent protective sleeve for receiving the elongate radiation source. The coupling disengages in two stages when it is desired to remove the elongate radiation source for servicing (or any other purpose). The coupling is disengaged from a first position in which a seal is made between the sleeve bolt element and the lamp plug element. When this action takes place, the lamp plug element is still secure with respect to the sleeve bolt element but since there is no seal between the two, any fluid which has flooded the elongate radiation source (e.g., due to breakage or other damage to the protective sleeve) will emerge from the coupling warning the operator not to fully disengage the lamp plug element from the sleeve bolt element. If no such fluid is seen by operator, the operator may continue to disengage the lamp plug element from the sleeve bolt element to withdraw the elongate radiation source from the elongate radiation transparent protective sleeve.