Alpha imaging: first results and prospects

For more than ten years the CEA has been using γ imaging systems for radiological characterization purposes during decommissioning operations. This technology is very effective for detecting high-energy γ emitters, but less so for characterizing actinides, which are mainly α emitters. In order to provide a full range of decommissioning tools, the γ-cameras were supplemented by another imaging system capable of generating images directly from α emissions. This article describes the initial laboratory and field test results obtained with a prototype camera capable of displaying α radioactivity. The system is based on the detection of ultraviolet radiation emitted by nitrogen when irradiated by alpha particles. The α contamination is localized by superimposing it on a visible-light image. All measurements must be carried out in complete darkness. Laboratory tests showed that the system is capable of detecting point sources and extended sources at levels as low as 430 Bq·cm^-2. Profile measurements of point sources revealed a scintillation bubble with dimensions corresponding to the range length of α particles in air. The device is also capable of detecting the phenomenon through translucent materials such as glove box panels. The camera has been implemented for in situ examination of various fuel cycle facilities, and under these conditions has revealed α contamination without any breach of containment through several millimeters of plexiglas.