Particle collection

Soil and sediment samples containing particles were collected in the Yenisei floodplain during 2002–2012. The procedure of particle collection includes in-field gamma measurements to identify anomalously high radioactivity (hot spots) followed by sampling of bulk material (soil/sediments). All soil samples were dried at room temperature, and then a combination of numerous sample splittings and sequential radioactivity measurements (gamma) was used to detect and isolate particles. The procedure of splitting and counting is repeated until a sufficient small fraction containing the radioactive particle is reached. In the present work, portable devices with scintillation crystals (SRP-97 survey radiometers, Russia) were utilised for the field measurements. In the laboratory, samples of minimal mass and maximal activity (due to the presence of the particle) were isolated by sample splitting using an SRP-97 radiometer and a Radiagem 2000 portable dose rate meter (Canberra, USA). Then, the active soil (sediment) samples containing particles were placed under a binocular microscope or a light microscope, to visually isolate radioactive particles as their colour or shape differed from the surrounding soil particles. Samples were screened for heterogeneities using digital phosphor imaging (image plate and image plate scanner, Typhoon 8600, Molecular Dynamics) to detect small-size radioactive particles.