Visualization of plutonium contamination in HEPA filters and various samples with a newly developed alpha imaging system

Morishita, Yuki; DiPrete, D.*; Deason, T.*; Pham, P.*; Abbott, A.*; Nagaishi, Ryuji

no journal, , 

Visualization of alpha contamination is useful for identifying contamination distribution and supporting decontamination efforts in nuclear decommissioning sites and nuclear fuel facilities. Most Pu isotopes are alpha-emitting radionuclides. Direct measurement of alpha particles with a survey meter is challenging because their short range makes them easily shielded by surrounding structures. To achieve precise visualization of alpha contamination, we developed an alpha imaging system incorporating a new CMOS camera. Two types of scintillators were used: a ZnS(Ag) scintillator and Revvity Ultima Gold AB liquid scintillation cocktail (LSC). The newly developed alpha imaging system was used to measure plutonium samples at the Savannah River National Laboratory, and its effectiveness was confirmed through these measurements. A qCMOS camera (C15550-22UP, Hamamatsu Photonics K.K., Hamamatsu, Japan) was employed as the CMOS detector. A lens with a focal length of 8 mm was mounted on the camera. This lens was used to provide a larger field of view. The distance from the scintillator to the lens surface was fixed at 16 cm. The field of view was approximately 16 cm x 16 cm. The sample, scintillator, and lens were enclosed in a dark box to block external light. The lid of the dark box was tightly closed during scintillation light measurements. The sample was oriented vertically during the measurements. The samples were prepared in the radiation controlled area at SRNL. First, a standard Pu-239 source with an activity of 1 kBq was measured to evaluate the system performance. The measurement duration was varied between 5 and 300 seconds. Then, Pu-238 solution that was fixed on a plate was measured. We also measured a sample in which multiple Pu-238 particles were fixed to a sheet. Additionally, HEPA filters containing Pu-238 solution were measured with LSC. A clear image of the standard Pu-239 source was obtained even with the 5-second measurement. The intensity ratio between the source and the background was 1.3 with the 5-second measurement. The Pu-238 solution images appeared along the flow of the solution due to the tilt of the plate. The Pu particle image made it possible to distinguish two neighboring particles separated by a few hundred micrometers. The distribution of radioactivity could be estimated from the intensity values of the images. The image of the HEPA filter visualized the distribution of the Pu solution that had penetrated into the filter. Overlaying the optical and alpha images enabled the identification of contamination locations. Since ordinary survey meters cannot measure contamination that has penetrated filters, this method is particularly effective for understanding the distribution of airborne contamination.