The Natural Abundances of the Uranium Isotopes

Abstract

Using a surface-ionization rnass spectrometer, the natural U238/U235 abundance ratio in uranium obtained from Great Bear Lake Pitchblende has been measured t o be 137.80&0.14. From a survey of results reported in the literature 011 Uz3+ best value of 17,325&550 was estimated for the natural U238/U234 abundance ratio. The corresponding isotopic abundances are 0.7204&0.0007 atom yo and 0.00573&0.00018 atom yo for U235 and UZ3l, respectively. INTRODUCTIONThe first mass spectrographic analysis of ~~r a n i u m was made by Aston (1) in 1931. He observed only U?38, and estimated the abundance of other isotopes t o total not more than two or three per cent. In 1932, von Grosse (9), from studies of the actinium (4n + 3) series of natural radioactivity, predicted that U?35 existed in natural uranium to the extent of 0.4 atom yo. Dempster (5) in 1935 discovered U235 mass spectrographically and determined its abundance to be less than 1 atom yo. He also predicted the existence of US4 from studies of the uranium (4n + 2) series of natural radioactivity, and calculated that its abundance would be 0.008 atom yo, "and therefore too faint to be observed by the mass spectrograph".Nier (14) in 1939 made the first mass spectrometric analysis of natural uranium and obtained a value of 138.9f 1.4 for the U?38/U235 abundance ratio. Because it was a t one time thought that the ratio might be a function of the age of the mineral in which the uranium was found, he measured the ratio for three samples of widely different geologic age. He found that for Swedish Kolnl (4 X lo8 yr.), Wilberforce Uraninite (1.0 X 10"~r.), and Dakeite (1 X lo3 yr.), the ratio was constant to 0.07%, and therefore did not depend on the age of the mineral or its source. Nier also discovered the very rare isotope U"', and determined its abundance to be 0.0059 atom yo. Further determinations of the abunclance have been made bj7 several authors (3,6,7, 8, 12) sing both mass spectrometric and specific activity methods.Subseq~ient mass spectron~etric determinations of the U238/U235 abundance ratio have been reported by Inghram ( l l ) , Fox and Rustad (7), and Chamberlain, Williams, and Yuster (3), all in 1946. Inghram's value of 137.8 was lower than the earlier determination by Nier by almost one per cent. Fox and Rustad actuall~, obtained two values, 137.0f0.7 by electron bombardment ionization, and 1 3 8 . 0 f 0.3 by thermal ionization. Using the latter ion source they found, as did Kier, that the ratio was constant t o 0.03% for two African ores, one Canadian ore, and one Colorado carnotite. Chamberlain et al. obtained a value of 139 for the U238/U235 ratio, in agreement with the first determination blNier. ln/Ianuscript

Investigating uranium isotopic distributions in environmental samples using AMS and MC-ICPMS

“…The isotopic ratios varied slightly from the accepted natural levels, but were not significantly enriched or depleted. The variation of U isotopes reported in natural deposits [28,[34][35][36][37] is narrower than the range measured in our samples. Since the samples were acquired from a variety of sources, anthropogenic activities may have skewed the natural isotopic abundances from those found in ores and minerals.…”

A comparison of methods in geochronology

“…When dating samples more than several tens of millions years old, it is not necessary to take into account the existence of the intermediate disintegration Earih-Sci. Rev., 4 (1968) 5-38 (KovARiK and ADAMS, 1938;FLEMING et al, 1952;LOUNSBURY, 1956;PICCIOTTO and WILGAIN 1956).…”

A Review on Analysis of Natural Uranium Isotopes and Their Application

“…태양계 초기의 238 U/ 235 U 비는 약 3 이었 으며, 현재는 137.88로 널리 받아들여 지고 있다 (Steiger and Jager, 1977). 그러나 2000년 대 중반 이후 지구 내 물질들이 고정된 238 U/ 235 U 비가 아닌 다른 값을 가질 수 해설 ("원자력과 지질환경과학" 특별호) 있다는 연구 결과들이 본격적으로 보고되기 시작하였다 (Stirling et al, 2007;Weyer et al, 2008) Chen and Wasserburg, 1981;Lounsbury, 1956). 그러나 2000년대 중반 이후 질량분석기의 발달과 더불어 MC-ICP-MS을 이용한 우라늄 동위원소 분석이 본격적으로 이뤄지기 시작했다 (Brennacka et al, 2010;Rademacher et al, 2006;Richter et al, 2008;Stirling et al, 2005;Weyer et al, 2008).…”