Self Absorption of Alpha and Beta Particles in a Fiberglass Filter

Abstract

Environmental air sampling uses fiberglass filters to collect particulate matter from the air and then a gas flow detector to measure the alpha and beta activity on the filter. When counted, the filter is located close to the detector so the alpha and beta particles emerging from the filter travel toward the detector at angles ranging from zero to nearly 90 degrees to the normal to the filter surface. The particles at small angles can readily pass through the filter, but particles at large angles pass through a significant amount of filter material and can be totally absorbed. As a result, counting losses can be great. For 4 MeV alpha particles, the filter used in this experiment absorbs 43% of the alpha particles; for 7.5 MeV alphas, the absorption is 13%. The measured beta activities also can have significant counting losses. Beta particles with maximum energies of 0.2 and 2.0 MeV have absorptions of 44 and 2%, respectively.

Assessment of the Losses Due to Self Absorption by Mass Loading on Radioactive Particulate Air Stack Sample Filters

“…Higby [1984] provides self-absorption data for filters loaded only with radioactive aerosol, considered a light loading. The measured losses of 0 to 24% (depending on particle size and face velocity) compare well with the calculations by Luetzelschwab et al [2000] using the thickness of the front filter layer alone.…”

“…Once mass loading reaches a certain value, loss increases exponentially with increasing mass loading until loss is 100%. Another method for determining percent loss given a certain mass loading could be a linear relationship, which appears to be closer to that shown in the absorption curves presented by Luetzelschwab et al [2000]. mg cm -2 for the filters used in this study.…”

“…A study by Higby [1984] calculated a minimum burial depth for an alpha particle to be lost due to absorption (100% loss) of about 3.7 mg cm -2 based on calculations for the range of 239 Pu alpha particles in glass fiber filters. Luetzelschwab et al [2000] recommended assuming a 40% loss at a loading of 3.3 mg cm -2 and a 28% loss for a loading of 2.3 mg cm -2…”

“…Under normal operating conditions at the stacks monitored by Effluent Management, the mass loading of sample filters averages 0.09 + 0.12 (2σ) mg cm -2 (excluding negative values and outliers) and ranges from 0 mg cm -2 to 0.24 mg cm -2 . This study presents two different methods of relating percent loss due to selfabsorption to filter mass loading: exponential and linear relationships based on data from Luetzelschwab et al [2000] and Higby [1984]. In general, samples have losses of less than 19% using the conservative exponential model and less than 7% using the linear model; therefore, a correction factor of 0.85 remains conservative.…”

“…As the mass loading decreases, less of the radiation from alpha particles is blocked, and a greater percentage of alpha particles will be counted. Luetzelschwab et al [2000] suggest that when the mass loading is 3.3 mg cm -2 , 40% of alpha particles will not be counted, and if mass loading is 2.3 mg cm -2 , 28% of alpha particles will not be counted. These values are from calculations of absorption curves as a function of absorber thickness for alpha particles using an energy of 5.0 MeV.…”