The Determination of minor isotope abundances in naturally occurring uranium materials

Abstract

The mass spectrometric determination of minor abundant isotopes, $/sp [234] $U and $/sp [236] $U in naturally occurring uranium materials requires instruments of high abundance sensitivity and the use of highly sensitive detection systems. In this study the thermal ionisation mass spectrometer Finnigan MAT 262RPQ was used. It was equipped with 6 Faraday cups and a Secondary Electron Multiplier (SEM), which was operated in pulse counting mode for the detection of extremely low ion currents. The dynamic measurement range was increased considerably combining these two different detectors. The instrument calibration was performed carefully. The linearity of each detector, the deadtime of the ion counting detector, the detector normalisation factor, the baseline of each detector and the mass discrimination in the ion source were checked and optimised. A measurement technique based on the combination of a Gas Source Mass Spectrometry (GSMS) and a Thermal Ionisation Mass Spectrometry (TIMS) was developed for the accurate determination of isotopic composition in naturally occurring uranium materials. The absolute $n (/sp [235] [/rm U]) /n (/sp [238] [/rm U]) $ amount ratio (precision 0. 05 percent) determined by GSMS by Mr. Willy De Bolle was used as an Internal Ratio Standard for TIMS measurements. Because the expected ratio of $n (/sp [234] [/rm U]) /n (/sp [238] [/rm U]) $ exceeded the dynamic measurement range of the Faraday detectors of the TIMS instrument, an experimental design using a combination of two detectors was developed. The $n (/sp [234] [/rm U]) /n (/sp [235] [/rm U]) $ and $n (/sp [236] [/rm U]) /n (/sp [235] [/rm U]) $ ratios were determined using ion counting in combination with the decelerating device. The $n (/sp [235] [/rm U]) /n (/sp [238] [/rm U]) $ ratio was determined by the Faraday detector. This experimental design allowed the detector cross calibration to be circumvented. Precisions of less than 1 percent for the $n (/sp [234] [/rm U]) /n (/sp [235] [/rm U]) $ ratios and 5-25 percent for the $n (/sp [236] [/rm U]) /n (/sp [235] [/rm U]) $ ratios were achieved. The purpose of the study was to establish a register of isotopic signatures for natural uranium materials. The amount ratios and isotopic composition of 18 ore concentrates, collected by the International Atomic Energy Agency (IAEA) from uranium milling and mining facilities (Australia, Canada, Gabon, Namibia, Czech Republic, France), were determined. These signatures form the basic register. The isotopic signatures are feasible in identifying the sample origin and in separating naturally occurring or background contributions from local anthropogenic sources. With the comparison of fingerprints of unknown samples to the isotopic fingerprints of samples of known origin, it is possible to trace back unknown samples to their origin or at least to exclude suspected origins in the case of non-identity of fingerprints. This was successfully demonstrated with a number of samples of unknown origin, which were measured during the study. Generally, no significant variability was observed in the $n (/sp [235] [/rm U]) /n (/sp [238] [/rm U]) $ ratios except in the well known case of samples originating from Oklo (Gabon). Small variations in the $n (/sp [234] [/rm U]) /n (/sp [238] [/rm U]) $ amount ratios were understood from the radiochemical mother-daughter relationship of the two isotopes involved. The detection limit for the $n (/sp [236] [/rm U]) /n (/sp [235] [/rm U]) $ amount ratio (DL = 0. 000001) was derived from blank measurements. The limit of quantitation 0. 000003 was calculated as LQ = 3DL. When the measured ratio exceeded the quantitation limit, the presence of $/sp [236] $U is explained