An on-line measurement system was designed and constructed for low level tritium gas. The system consists of a proportional counter, amplifiers, a discriminator and a scaler interfaced to a micro-computer. Characteristics of the proportional counter were examined for pure He and Ar counting gases using 5.9 keV 55Fe X-rays; it worked with pure rare gases in the proportional mode up to the gas multiplication factor of about 100. The developed system was used to measure the release rate of tritium out of Al-Mg-Li alloys irradiated by thermal or 14 MeV neutrons. Applicability of the system with a proportional counter has been confirmed for measurements of low level tritium gas carried by pure He or Ar gas. The system has the maximum sensitivity of 0.1 pCi/cm3 for a 50 cm3 sensitive volume.
