Size measurement of plutonium particles from internal sputtering into air

During the past century, the results of spontaneous translocation of radioactivity in air, biological media and groundwater have been reported. Here, we report the first measurements of the size characteristics in air of the particles participating in this translocation phenomenon. For the plutonium material powering radioisotope thermal generators, we find two narrow, well-separated fractions, one corresponding to particles below a nanometer and one at or below 10 nm. These results are interpreted as a gas-phase nucleation phenomenon arising from internal sputtering. They suggest fruitful directions for further research with immediate implications for accounting for the effects of radiological terrorism, for identifying new signatures for nuclear materials of possible use in antiterrorism and other covert nuclear materials operations, for radioactive and mixed materials storage handling, for reactor safety and source term modeling and for other materials processes.