Robert D. Loss

Atomic weights of the elements 2001 (IUPAC Technical Report)

The biennial review of atomic-weight, Ar(E), determinations and other cognate data have resulted in changes for the standard atomic weights of the following elements: From To Zinc 65.39 ± 0.02 65.409 ± 0.004 Krypton 83.80 ± 0.01 83.798 ± 0.002 Molybdenum 95.94 ± 0.01 95.94 ± 0.02 Dysprosium 162.50 ± 0.03 162.500 ± 0.001 Presented are updated tables of the standard atomic weights and their uncertainties estimated by combining experimental uncertainties and terrestrial variabilities. In addition, this report again contains an updated table of relative atomicmass values and half-lives of selected radioisotopes. Changes in the evaluated isotope abundance values from those published in 1997 are relatively minor and will be published in a major review of each element in 2003. Many elements have a different isotopic composition in some nonterrestrial materials. Some recent data on parent nuclides that might affect isotope abundances or atomic-weight values are included in this report for the information of the interested scientific community.

Atomic weights of the elements 2013 (IUPAC Technical Report)

Abstract The biennial review of atomic-weight determinations and other cognate data has resulted in changes for the standard atomic weights of 19 elements. The standard atomic weights of four elements have been revised based on recent determinations of isotopic abundances in natural terrestrial materials: cadmium to 112.414(4) from 112.411(8), molybdenum to 95.95(1) from 95.96(2), selenium to 78.971(8) from 78.96(3), and thorium to 232.0377(4) from 232.038 06(2). The Commission on Isotopic Abundances and Atomic Weights (ciaaw.org) also revised the standard atomic weights of fifteen elements based on the 2012 Atomic Mass Evaluation: aluminium (aluminum) to 26.981 5385(7) from 26.981 5386(8), arsenic to 74.921 595(6) from 74.921 60(2), beryllium to 9.012 1831(5) from 9.012 182(3), caesium (cesium) to 132.905 451 96(6) from 132.905 4519(2), cobalt to 58.933 194(4) from 58.933 195(5), fluorine to 18.998 403 163(6) from 18.998 4032(5), gold to 196.966 569(5) from 196.966 569(4), holmium to 164.930 33(2) from 164.930 32(2), manganese to 54.938 044(3) from 54.938 045(5), niobium to 92.906 37(2) from 92.906 38(2), phosphorus to 30.973 761 998(5) from 30.973 762(2), praseodymium to 140.907 66(2) from 140.907 65(2), scandium to 44.955 908(5) from 44.955 912(6), thulium to 168.934 22(2) from 168.934 21(2), and yttrium to 88.905 84(2) from 88.905 85(2). The Commission also recommends the standard value for the natural terrestrial uranium isotope ratio, N(238U)/N(235U)=137.8(1).

Mass discrimination during MC-ICPMS isotopic ratio measurements: investigation by means of synthetic isotopic mixtures (IRMM-007 series) and application to the calibration of natural-like zinc materials (including IRMM-3702 and IRMM-651).

A long known way of anchoring isotope ratio values to the SI system is by means of gravimetrically prepared isotopic mixtures. Thermal ionization mass spectrometry (TIMS) is the traditionally associated measurement technique, but multi-collector double focusing inductively coupled plasma (MC-ICP)-MS now appears to be an attractive alternative. This absolute calibration strategy necessitates that mass discrimination effects remain invariant in time and across the range of isotope ratios measured. It is not the case with MC-ICPMS and the present work illustrates, in the case of Zn isotopic measurements carried out using locally produced synthetic Zn isotope mixtures (IRMM-007 series), how this calibration strategy must be adjusted. First, variation in mass discrimination effects across the measurement sequence is propagated as an uncertainty component. Second, linear proportionality during each individual measurement between normalized mass discrimination and the average mass of the isotope ratios is used to evaluate mass discrimination for the ratios involving low abundance isotopes. Third, linear proportionality between mass discrimination and the logarithm of the isotope ratio values for n(67Zn)/n(64Zn) and n(68Zn)/n(64Zn) in the mixtures is used iteratively to evaluate mass discrimination for the same ratios in the isotopically enriched materials. Fourth, ratios in natural-like materials (including IRMM-3702 and IRMM-651) are calibrated by external bracketing using the isotopic mixtures. The relative expanded uncertainty (k = 2) estimated for n(68Zn)/n(64Zn) and n(67Zn)/n(64Zn) ratio values in the synthetic isotopic mixtures and the natural-like zinc samples was in the range of 0.034 to 0.048%. The uncertainty on the weighing (0.01%, k = 1) was the largest contributor to these budgets. The agreement between these results and those obtained with a single detector TIMS and with another MC-ICPMS further validated this work. The absolute isotope ratio values found for IRMM-3702—material also proposed as “delta 0” for δ-scale isotopic measurements—are n(66Zn)/n(64Zn) = 0.56397 (30), n(67Zn)/n(64Zn) = 0.082166 (35), n(68Zn)/n(64Zn) = 0.37519 (16), and n(70Zn)/n(64Zn) = 0.012418 (23). The derived Zn atomic weight value Ar(Zn) = 65.37777 (22) differs significantly from the current IUPAC value by Chang et al. [1]. Remeasurement, with isotopic mixtures from the IRMM-007 series, of the Zn isotope ratios in the same Chang et al. [1] material have revealed large systematic differences (1.35 (27)% per atomic mass unit) that suggest unrecognized measurement biases in their results.

Fission product retention in the Oklo natural fission reactors

Eight samples from a planar cross section of one of the natural fission reactor zones at the Oklo U mine were analyzed to determine the abundances and isotopic composition of U, Mo, Ru, Pd, Ag, Cd, Sn, Te and Nd. Fission product concentrations were calculated from the isotopic compositions. The relative abundances of these fission products are different from those produced by fission i.e. portions have been lost or gained from all the samples. The proportions of Te, Ru, 99Ru, Pd and Mo in the eight samples are invariant. We attribute this regularity to retention at the site of production—primary retention. Based upon this interpretation, we suggest that fixed proportions of Ru, Tc, Pd and Mo have been removed from the reactor zone. In contrast, Nd and Sn have been depleted in some regions of the zone and enriched in others. Portions of Ag and virtually all the fissiogenic Cd have been removed from the reactor zone. By analogy with anthropogenic spent fuel, we suggest that the degree of primary retention was controlled by phase assemblages formed in the Oklo natural reactor fuel in response to microscale conditions of pressure, temperature and composition produced by the nuclear reactions. The ability of these putative minerals to retain nuclear products was a function of their stability under physiochemical conditions established by the geological environment over the last 2 Ga. An integrated study of the natural reactors would test our hypothesis and provide valuable information for evaluating the long-term effectiveness of anthropogenic spent fuel as a container of nuclear waste in the geochemical environment.

The isotopic composition of zinc, palladium, silver, cadmium, tin, and tellurium in acid-etched residues of the Allende meteorite

The isotopic and elemental abundances of Zn, Pd, Ag, Cd, Sn, and Te have been measured in three acid-resistant residues extracted from the Allende meteorite. High-efficiency, low-contamination ion-exchange procedures were developed to separate and purify the nanogram amounts of these elements present. Elemental-abundance determinations performed by Mass Spectrometric Isotope Dilution agree with previously published work for similarly derived residues. No isotope anomalies similar to those found for Xe (Xe-HL) in these samples were detected for any of these elements, which is consistent with the residues not being derived directly from the Xe-HL carriers. The lack of major Te-isotope anomalies does not support earlier reports of {sup 126}Te and {sup 130}Te excesses which were measured by neutron activation in similar samples. Small excesses were detected in the minor isotopes of Sn and Te, but these may be due to measurement problems associated with the small ion currents obtained for these samples. Two of the residue solutions contain Cd with up to several percent excesses for {sup 106}Cd and {sup 108}Cd. Interpretations of these results are limited by the unknown nature of the carrier minerals in the residues but may indicate the presence of a p-process component in Allende residues.

The isotopic composition of tin

Abstract Devillers et al. recently measured the isotopic composition of tin in order to establish its absolute isotope abundance and the atomic weight. However, because of interferences, particularly at 115, and possibly at 114, accurate abundances could not be assigned to these very low abundance isotopes. The astrophysical and cosmochemical significance of the minor tin isotopes has prompted us to remeasure the isotopic composition of tin. A silica/alumina/ boric acid activator was developed which permitted submicrogram samples of tin to be analysed free of 115 and 114 interferences. The abundance of 115 Sn was found to be 5.5% lower and that of 114 Sn to be 1.1% higher than the corresponding values reported by Devillers et al. The abundances of the other isotopes are in excellent agreement with their values and the accurate isotopic data now available provides a sound basis for a study of isotopic anomalies to be undertaken in meteorites.

Fission-product retentivity in peripheral rocks at the Oklo natural fission reactors, Gabon

Abstract Thermal ionisation mass spectrometry has been used to measure the isotopic and elemental abundances of Mo, Ru, Pd, Ag, Cd, Sn and Te in a number of sandstones and shales in the immediate vicinity of Reactor Zone 9 at the Oklo mine site. The mass spectrometric isotope dilution technique was used to measure the elemental abundances of these elements. The data show that considerable amounts of the fissiogenic Mo, Ru, Pd, Ag and Te which escaped from the reactor zone, have been retained in the peripheral rocks, although Cd and Sn were not contained to any significant extent in these samples. Some information on the retention of Sb and Tc was also obtained. Evidence of element fractionation has also been demonstrated in that 99 Tc and Sn were fractionated from Ru and Te, respectively within 1 Ma of the end of reactor criticality. The presence of excess 125 Te in some of the peripheral rock samples indicates that Sb was mobilised from Reactor Zone 9 within tens of years of the completion of reactor criticality. The implications of these results to the storage of radioactive wastes in natural geological repositories is discussed.

The Oklo natural reactors: cumulative fission yields and nuclear characteristics of Reactor Zone 9

The isotopic composition of molybdenum, ruthenium, palladium, silver, cadmium, tin, tellurium, neodymium and uranium have been measured by solid source mass spectrometry in eight uraninite samples from Reactor Zone 9 at the Oklo natural reactors. Cumulative fission yields have been derived for most of these elements, after correcting where necessary for the primordial component of the element concerned. Neutron capture reactions on a number of nuclides with significant thermal cross sections, and fission chains in which one of the precursor nuclides has a lengthy half-life, have also been examined to provide information on the relative mobilities of the elements involved. The Pd fission yields have been utilised to calculate the proportions of235U,238U and239Pu fission to be approximately 88%, 8% and 4% respectively in Reactor Zone 9. It has also been shown that nearly 50% of the fissioning235U nuclides were produced from the α-decay of239Pu. The integrated neutron flux in this zone was calculated to be approximately 3.6 × 1020 n cm−2. Estimates of the spectral index show that approximately two-thirds of the neutrons in Zone 9 were thermalised. Although it is unlikely that the Oklo reactors operated continuously, this study has shown that the duration of criticality in Zone 9 was approximately 2.2 × 105 years. During this period the average fission density was 0.92 × 1020 fissions cm−3, which represent a total energy output of 4 × 108 J g−1 of sample with an average power output of 8.1 × 10−5 W g−1 of sample. The accurate analysis of the isotopic abundances of nine elements, has enabled the nuclear characteristics of Reactor Zone 9 to be established, thus providing a base for estimating the nuclear inventory of the various fission products. This will allow a comparison to be made with the present-day elemental abundances and hence an evaluation of the mobility of these elements in the geological environment at Oklo.

Mass spectrometric isotope dilution analyses of palladium, silver, cadmium and tellurium in carbonaceous chondrites

Abstract The mass spectrometric isotope dilution technique was used to measure the elemental abundances of Pd, Ag, Cd and Te in Orgueil (C1), Ivuna (C1), Murray (C2) and Allende (C3) chondrites. The Pd abundance of 554 ppb for the C1 chondrites is almost identical to the recommended value of Anders and Ebihara (1982); that for Cd (712 ppb) is approximately 5% higher, whereas that for Ag (198 ppb) is approximately 10% lower than the recommended values. A smooth distribution for the abundances of the odd- A nuclides between65 ≦ A ≦ 209 have been observed except for small irregularities in the Pd-Ag-Cd and the Sm-Eu mass regions (ANDERS and Ebihara, 1982). The results from the present work have the effect of smoothing out the dip in the Pd-Ag-Cd region and indicate that there is no systematic fractionation of cosmochemical element groups in this mass region. A Te abundance of 2.25 ppm has been determined for the C1 chondrites Orgueil and Ivuna in agreement 2+with the value of Smith et al. (1977). This value is some 30% lower than the value of Krahenbuhl et al . (1973) but is in good agreement with the more recent measurements from Chicago. The Krahenbuhl et al . value causes 128 Te and 130 Te to lie approximately 30% above the r -process peak at A = 130 (Kappeler el al. , 1982), whereas the new value fits smoothly into the general trend.

Measurement of Ag, Te and Pd in geochemical reference materials by mass spectrometric isotope-dilution analysis.

Procedures are described which permit mass spectrometric isotope-dilution analysis to be used to determine Ag, Te and Pd in rock samples at the ng g level. The concentrations (ng g ) of Ag, Te and Pd were found to be 25.7 +/- 0.7, 1.2 +/- 0.6 and 0.08 +/- 0.05 respectively in BCR-1 and 3.5 +/- 0.2, 4.2 +/- 0.7 and 2.9 +/- 1.7 respectively in PCC-1.