Metrology of plutonium for environmental measurements

Abstract

Since its discovery in 1940, plutonium has become widely distributed in the world's environment by a number of routes. Due to the perceived hazard of the element, considerable resources have been devoted to the measurement of plutonium in nuclear waste, nuclear effluent, human beings and the environment. Plutonium is, however, difficult to measure because all of the commonly encountered isotopes have very low abundance γ-ray emissions making conventional absolute 4πα-γ-coincidence counting techniques impractical for the nuclides 236Pu, 238Pu, 239Pu, 240Pu, 242Pu and 244Pu. Instead, the technique of using a tracer nuclide (in this case 241Am) has been employed at the National Physical Laboratory (NPL) to standardize 239Pu and 242Pu and is applicable to the other isotopes mentioned above. 241Pu has some different problems in that it is a ß-emitter with a low (∼20 keV) end point energy and, again, no associated γ-ray. It decays to 241Am which can be measured quite easily, but to achieve calibration by ingrowth with an acceptable uncertainty could take many years. 241Pu is calibrated using liquid scintillation counting by comparison with aqueous 3H, with some corrections made for differing end point energies and β spectrum shapes. In recent years, the technique of Inductively Coupled Plasma Mass Spectrometry (ICP-MS) has been applied to the field of plutonium measurement and it appears to offer advantages in the routine, high-throughput analysis of plutonium. However, work commissioned by NPL suggests that there are significant problems in the calibration of ICP-MS equipment, with large variations in the response obtained from different plutonium nuclides. In the U.K. measurement community, the most pressing need is for suitable matrix reference materials and standardized chemical yield tracers, incorporating 236Pu, 242Pu or 244Pu. Each of these have some problems in that they are difficult to produce or procure and that isotopic purity can be a problem. The questions to be asked here are; “Is there a continuing global demand for these yield tracers?” if so “Who is prepared to fund production and purification?” and “Are National Standards Laboratories prepared to take on such work?”. These questions and some possible answers will be discussed in the paper.