A process and system for the rapid concentration, separation and measurement of Kr-85 and Xe-133. By measuring the amount of Kr-85 radioisotope which is present, false positive signals for the vacuum sipping process can be eliminated. The fission gases are trapped by passing the recirculating gas stream through an adsorbing media (6). During this process, a known mixture of nonradioactive krypton and xenon is added to the gas stream which enters a thermal conductivity (TC) detector (11) to calibrate the latter. The trapped gases are desorbed and injected into the inlet of a separating column (9). The column discharges into the TC detector. The TC detector discharge is diverted for counting to an evacuated beta detector chamber (14) during the time that krypton is sensed by the TC detector. Any minute amount of Kr-85 is captured in the chamber, free of Xe-133. The detector discharge is diverted for counting to another evacuated beta detector chamber (15) during the time that xenon is sensed by the TC detector. Any minute amount of Xe-133 is captured in the chamber, free of Kr-85.