Correlation of tritium outgassing from metal surfaces with surface smearing

Abstract

Smearing is the most frequently used technique to determine surface activity. The technique is destructive, does not account for all the tritium adsorbed on surfaces and cannot track changes in surface concentration in real time. A relationship correlating the outgassing of tritiated species with surface activity as measured by smearing is sought. In vacuum technology significant efforts[ 1-31 have been made to measure and derive relationships between outgassing rates and time, material and temperature. One major difference between outgassing in vacuum systems and outgassing from tritiated surfaces is the presence of one atmosphere of air. The interaction between air and the active surface may alter the outgassing spectrum commonly observed for unbaked vaccum systems. The tritium outgassing rate is measured by pumping room air continuously over the active surface in a chamber and through a tritium monitor. The real time airborne tritium concentration is correlated with the surface activity. Surface activities in the range of S to 200,000pCi/m2 have been examined. Outgassing rates in the range of 3 to 8,000 pCi/m2.hr have been measured. Desorption of physisorbed HTO is most likely the dominant outgassing mechanism. For times shorter than 200 hours following a tritium exposure the surface outgassing rate decreases with a negative half power dependence on time.