Isotopic analysis of plutonium by mass spectrometry

Abstract

This paper describes the results of a study on the necessary amounts of plutonium in order to make an accurate determination of isotopic composition of plutonium by mass spectrometry.
We made an L-type glove box and a sample-loading apparatus (for trial). The glove box is used to exchange the filament assembly and to set up or remove a vacuum lock of the mass spectrometer. The sample-loading apparatus is used to load a sample solution onto the filament in the glove box. A Nuclide Analysis Associates 12-90-SU mass spectrometer was connected with the glove box.
The glove box has two pair gloves and one port. These gloves and the port are used to exchange the filament assembly, to set up or remove the vacuum lock system, and to carry in or out the materials.
The sample-loading apparatus is used to deposit the sample solution on one of the side filaments of the assembly, and about 10 μl solution can be treated. A capillary type loading-pipet is drawn from glass tubing which is 5 mm in diameter. The pipet is fixed by a holder in such a way that it can be freely moved up and down, right and left, and around the center. A filament assembly mounting can also be freely moved toward and backward. The most suitable contact angle between the surface of the firament and the outlet of the pipet is 45°. For drying and converting the sample to oxide, the mounting is connected to an electrical lead from D. C. power supply, which can be adjusted from 0 to 5 amp. currents, at an output voltage 3 volts.
A portion of plutonium nitrate solution is loaded onto a filament with the sample-loading apparatus. A current applied to the filament was set at about 1 amp. About 10 μl of the solution was automatically deposited on the filament from a small pipet under this condition, and was evaporated to dryness. After the evaporation, the filament current was increased to 2.0 to 2.2 amp. in order to decompose plutonium nitrate to oxide, and then the filament assembly was set up into the ion source by using the vacuum lock system. All the above procedures were carried out in the glove box.
The plutonium sample was analyzed by surface ionization method under the following conditions.
analytical ion Pu⁺
measuring peak numbers 21 pairs filament currents 3A (sample filament)
4A (ionization filament)
quantity of plutonium 0.2μg resolving power 500 M/e
accelerating voltage 10 kV
The suitable amount of plutonium was 0.2μg under the above conditions. Where, PuO⁺ and Pu₂O⁺ peak are lower than that of Pu⁺; the Pu⁺ peak was 1000 times as high as peaks of other Pu ion, and it was very stable. A mixed solution which consists of a 10 : 1 mixture of uranium and plutonium could be analyzed without any trouble.
The precision of the results under the above conditions was as follows;
atomic % of plutonium isotope precision (relative)
8090% ±0.1±0.2%
515% ±0.2±0.4%
15% ±0.4±0.6%
0.11% ±0.6±1.0%
smaller than 0.1% ±1± 5%
The limit of detection for the isotopic abundance is 0.01% and samples in which the abandance is greater than this can be measured by the present method.