Ultra-trace analysis of plutonium by thermal ionization mass spectrometry with a continuous heating technique without chemical separation

Abstract

Thermal ionization mass spectrometry (TIMS) with a continuous heating technique is known as an effective method for measuring the isotope ratio in trace amounts of uranium. In this study, the analytical performance of thermal ionization mass spectrometry with a continuous heating technique was investigated using a standard plutonium solution (SRM 947). The influence of the heating rate of the evaporation filament on the precision and accuracy of the isotope ratios was examined using a plutonium solution sample at the fg level. Changing the heating rate of the evaporation filament on samples ranging from 0.1fg to 1000fg revealed that the influence of the heating rate on the precision and accuracy of the isotope ratios was slight around the heating rate range of 100–250mA/min. All of the isotope ratios of plutonium (SRM 947), 238Pu/239Pu, 240Pu/239Pu, 241Pu/239Pu and 242Pu/239Pu, were measured down to sample amounts of 70fg. The ratio of 240Pu/239Pu was measured down to a sample amount of 0.1fg, which corresponds to a PuO2 particle with a diameter of 0.2μm. Moreover, the signals of 239Pu could be detected with a sample amount of 0.03fg, which corresponds to the detection limit of 239Pu of 0.006fg as estimated by the 3-sigma criterion. 238Pu and 238U were clearly distinguished owing to the difference in the evaporation temperature between 238Pu and 238U. In addition, 241Pu and 241Am formed by the decay of 241Pu can be discriminated owing to the difference in the evaporation temperature. As a result, the ratios of 238Pu/239Pu and 241Pu/239Pu as well as 240Pu/239Pu and 242Pu/239Pu in plutonium samples could be measured by TIMS with a continuous heating technique and without any chemical separation processes.