Characterisation of hot particles remaining in soils from Palomares (Spain) using a nuclear microprobe

More than 40 years ago, an aircraft accident took place in Palomares (Spain) which involved the destruction of two nuclear weapons. A portion of the remaining transuranic contamination in the affected soils is present in the form of small high activity concentration particles (10-100 μm), also called 'hot particles', which contain plutonium and uranium. Several hot particles have been isolated and identified from the superficial soils recently collected in the zone affected by the accident. The isolation was carried out by screening the soil using gamma-ray spectrometry, through discrimination of the high activity concentrations of 241Am in the samples which indicates the presence of plutonium. The hot particles, composed of several elements with very high atomic number, could be easily identified by scanning electron microscopy (SEM) in backscattering electron image (BSE) mode. Moreover, their morphology and size were also studied using SEM in secondary electron (SE) mode. In this work, the hot particles have been investigated with the nuclear microprobe of the National Accelerator Centre (CNA) in Seville. Compositional analysis, mapping and depth distribution of different elements have been performed by a simultaneous combination of Particle Induced X-ray Emission (PIXE) and Rutherford Backscattering Spectrometry (RBS). Uranium and plutonium have been identified in the PIXE spectra as the main components of the particles, whereas the concentration of Americium is two orders of magnitude smaller. In addition, an estimation of the particles density has been obtained by comparison of the RBS results with the particles thickness directly determined by SEM