METROLOGY OF PLUTONIUM FOR ENVIRONMENTAL MEASUREMENTS

Since its discovery in 1940, plutonium has become widely distributed i n 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 be ings and the environment. Plutonium is, however, difficult to measure because all of the commonly encountered isotopes have very low abundan ce gamma-ray emissions, making conventional absolute 4 pi alpha-gamma- coincidence counting techniques impractical for the nuclides Pu-236, P u-238, Pu-239, Pu-240, Pu-242 and Pu-244. Instead, the technique of us ing a tracer nuclide (in this case Am-241) has been employed at the Na tional Physical Laboratory (NPL) to standardize Pu-239 and Pu-242 and is applicable to the other isotopes mentioned above. Pu-241 has some d ifferent problems in that it is a beta-emitter with a low (similar to 20 keV) end point energy and, again, no associated gamma-ray. It decay s to (241)m which can be measured quite easily, but to achieve calibra tion by ingrowth with an acceptable uncertainty could take many years. Pu-241 is calibrated using liquid scintillation counting by compariso n with aqueous H-3, with some corrections made for differing end point energies and beta 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 advanta ges in the routine, high-throughput analysis of plutonium. However, wo rk commissioned by NPL suggests that there are significant problems in the calibration of ICP-MS equipment, with large variations in the res ponse obtained from different plutonium nuclides. In the U.K. measurem ent community, the most pressing need is for suitable matrix reference materials and standardized chemical yield tracers, incorporating Pu-2 36, Pu-242 or Pu-244. Each of these have some problems in that they ar e difficult to produce or procure and that isotopic purity can be a pr oblem. The questions to be asked here are; ''Is there a continuing glo bal demand for these yield tracers?'' if so ''Who is prepared to fund production and purification?'' and ''Are National Standards Laboratori es prepared to take on such work?''. These questions and some possible answers will be discussed in the paper.