A connection assembly between two portions of a line including an inner conduit for transporting a cryogenic fluid, the assembly including two flanges which are respectively arranged at the ends of the line portions and which are configured to be held in contact with one another by virtue of a fixing arrangement. The contact zone between the two flanges includes at least two grooves for positioning seals and at least one conduit opening out, for the one part, between the two seals and, for the other part, into an expansion chamber for the cryogenic fluid. The expansion chamber is arranged in one, or in the vicinity of one, of the first connection flange and the second connection flange. The expansion chamber includes a detector for detecting the presence of the cryogenic fluid in the expansion chamber or a recess configured for the installation of such a detector.