A method for determining leakage of 133Xe gas from septum-sealed glass vials

We have developed a method for determining the leakage of 133Xe gas from septum-sealed glass vials that are supplied for medical examinations. Twenty vials each originally containing 370 MBq of 133Xe and 20 vials each originally containing 740 MBq 133Xe were measured daily for 26 d. Retention of 133Xe within the vial was modeled as a first order process with a constant rate coefficient, lambdaT. The value of lambdaT was estimated for each vial using a regression analysis. The leakage rate, lambdaL, was then determined assuming that lambdaT = lambdaL + lambda(r) where lambda(r) represents the physical decay of 133Xe. Monte Carlo simulations were performed using uncertainties in the estimates of each vial to obtain the mean and tails of the distribution for the average leakage rate, lambdaL. The average leakage rate for the complete sample of vials was 0.00007 d(-1) with an upper, one-sided, 95% confidence limit of 0.0011 d(-1). Uncertainties in the published values of lambda(r) for 133Xe made a significant contribution to the uncertainties of the leakage rate for this sample of vials. The methods described can be applied to other situations where leakage of radioactive materials may be of concern.