Radioactive Sample Effects on Edxrf Spectra

Energy dispersive X-ray fluorescence (EDXRF) is a rapid, straightforward method to determine sample elemental composition. A spectrum can be collected in a few minutes or less, and elemental content can be determined easily if the detector has adequate energy resolution. Radioactive alpha emitters, however, emit X-rays during the alpha decay process that complicates spectral interpretation. This is particularly noticeable when using a portable instrument where the detector is located in close proximity to the instrument analysis window held against the specimen. A portable EDXRF instrument was used to collect spectra from specimens containing plutonium239 (a moderate alpha emitter) and americium-241 (a heavy alpha emitter). These specimens were also analyzed with a wavelength dispersive XRF (WDXRF) instrument to compare spectra collected from instruments with very different instrumental geometries. INTRODUCTION Pulse pile-up, escape peaks, and sample diffraction effects can complicate EDXRF spectra, but these artifacts are recognizable by an experienced analyst. Radioactive alpha emitters, however, emit X-rays due to internal processes that result from alpha decay, which further complicates spectral interpretation. (Gamma rays emitted by radioactive samples are indistinguishable to the detector from X-rays of the same energy, but gamma energies are normally too high to be of concern in EDXRF.) This alpha decay effect can be seen in plutonium-239 spectra which exhibit a prominent uranium L X-ray emission peak series. When a plutonium-239 atom alpha decays, there is some probability that the uranium-235 daughter nucleus will be left in an excited state that can then transfer its excess energy to an electron through internal conversion. If the electron is then ejected upon absorption of this energy, an X-ray photon or Auger electron will be emitted. Hence, the uranium L X-ray series is present due to sample radioactive decay rather than source-induced X-ray fluorescence. Therefore, quantifying the uranium based on observed peak intensities can lead to erroneous results. Portable instruments are particularly susceptible to this effect due to the short distance between the specimen and the detector. In the present study, Pu-239 metal was analyzed with a portable EDXRF instrument. The same specimen was also examined by WDXRF, where the detector was much further from the specimen, and a diffraction crystal was located between the specimen and detector. This comparison demonstrated that using WDXRF is more advantageous for analyzing alpha emitters. With WDXRF a high power X-ray source is used to increase the number of fluorescence X-ray photons and counteract signal attenuation resulting from the relatively large distance between the specimen and detector. Because using higher source power 90 Copyright ©JCPDS-International Centre for Diffraction Data 2009 ISSN 1097-0002