Absolute Isotopic Composition Research Articles

Feasibility study of a compact neutron resonance transmission analysis instrument

Neutron Resonance Transmission Analysis (NRTA) uses resonant absorption of neutrons to infer the absolute isotopic composition of a target object, enabling applications in a broad range of fields such as archeology, enrichment analysis of nuclear fuel, and arms control treaty verification. In the past, NRTA involved large user facilities and complex detector systems. However, recent advances in the intensity of compact neutron sources have made compact neutron imaging designs increasingly feasible. This work describes the Monte Carlo (MC) based design of a compact epithermal NRTA radiographic instrument, which uses a moderated, compact deuterium-tritium neutron source and an epithermal neutron detector. Such an instrument would have a wide range of applications and would be especially impactful for scenarios such as nuclear inspection and arms control verification exercises, where system complexity and mobility may be of critical importance. The MC simulations presented in this work demonstrate accurate time-of-flight reconstructions for transmitted neutron energies, capable of differentiating isotopic compositions of nuclear material with high levels of accuracy. A new generation of miniaturized and increasingly more intense neutron sources will allow this technique to achieve measurements with greater precision and speed, with significant impact on a variety of engineering and societal problems.

Accurate Determination of the Absolute Isotopic Composition and Atomic Weight of Molybdenum by Multiple Collector Inductively Coupled Plasma Mass Spectrometry with a Fully Calibrated Strategy.

A fully calibrated strategy has been investigated for the first time for the accurate determination of absolute isotopic composition and atomic weight of molybdenum using multiple-collector inductively coupled plasma mass spectrometry. The correction for instrumental mass bias was performed using synthetic isotope mixtures, which were gravimetrically prepared with all of the seven high-purity and isotopically enriched molybdenum isotope materials together. Six natural molybdenum materials, including molybdenum standard solution NIST SRM 3134, were accurately measured and yielded the absolute isotopic composition (in atom %, k = 1) of 92Mo-14.690(18), 94Mo-9.173(6), 95Mo-15.865(5), 96Mo-16.666(3), 97Mo-9.588(4), 98Mo-24.307(16), and 100Mo-9.711(13). These isotopic data enable an atomic weight Ar(Mo) of 95.9466(34) (k = 2) to be calculated, which is slightly lower than the current standard atomic weight 95.95(1) and with a much improved uncertainty. The associated uncertainties were evaluated according to the Guide to Expression of Uncertainty in Measurement of ISO/BIPM and Monte Carlo simulation to ensure that all sources of uncertainty were fully accounted for. A particular characteristic of the proposed new approach is that mass bias correction factor K for each isotope ratio of molybdenum can be achieved via fully experimental determination without using the traditional semiempirical correction mathematical models. In addition, the relationship between mass of isotope and bias per mass unit β was investigated based on the thorough measurement data.

The absolute isotopic composition and atomic weight of ytterbium using multi-collector inductively coupled plasma mass spectrometry and development of an SI-traceable ytterbium isotopic certified reference material

The absolute isotopic composition of ytterbium in six varieties of (terrestrial source) materials were determined using MC-ICP-MS.

The absolute isotopic composition and atomic weight of molybdenum in SRM 3134 using an isotopic double-spike

Analytical techniques have been developed to perform a fully calibrated measurement of the isotopic composition of molybdenum reference materials using multiple collector inductively coupled plasma mass spectrometry. A correction for instrumental mass bias was performed using a molybdenum double-spike prepared from gravimetric mixtures of 92Mo and 98Mo isotope spikes. The absolute isotopic composition of the NIST SRM 3134 molybdenum reference material with 2 s.d. uncertainty was determined to be 92Mo = 14.65(4), 94Mo = 9.187(12), 95Mo = 15.873(10), 96Mo = 16.673(3), 97Mo = 9.582(6), 98Mo = 24.29(3), and 100Mo = 9.74(2). The atomic weight of molybdenum in SRM 3134 was calculated as Ar = 95.949(3). Delta values (reported as δ98/95Mo relative to SRM 3134) were determined for SCP Science – PlasmaCal molybdenum as −0.42(10)‰ and for Johnson-Matthey pure molybdenum rod as −0.45(12)‰. The total natural variation of molybdenum of −1.5‰ to +3‰ results in a calculated atomic weight interval of Ar = [95.944, 95.956].

Improvements in 230Th dating, 230Th and 234U half-life values, and U–Th isotopic measurements by multi-collector inductively coupled plasma mass spectrometry

We have developed techniques for measuring 234U and 230Th on Faraday cups with precisions of 1–3 epsilon units (1 ε-unit=1 part in 104) using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Using a Thermo-Scientific Neptune with desolvation nebulization, we obtained ionization/transmission efficiencies of 1–2% for both U and Th. We set up protocols to correct for tailing, prepared U and Th gravimetric standards, tested a Th mass fractionation correction procedure based on U isotopes, and identified natural calcite samples likely to be in U–Th isotopic secular equilibrium. The measured atomic ratios, 234U/238U=54.970 (±0.019)×10−6 and 230Th/238U=16.916 (±0.018)×10−6, for these calcite samples were identical within errors (quoted 2σ uncertainties calculated combining all sources of error). Half-life values calculated from these ratios are consistent with previous values, but have much smaller errors: 245,620±260a for 234U and 75,584±110a for 230Th (quoted 2σ uncertainties calculated using all sources of error). In calculating a 230Th age, some of the systematic errors included in estimating the full error in the half-lives effectively cancel. Removing these uncertainties (uncertainty in the 238U half-life value, uncertainty in our gravimetric uranium and thorium standards, and uncertainty in the absolute isotopic composition of the uranium standard), yields effective uncertainties for the purposes of 230Th dating of ±70a for the 234U half-life value and ±30a for the 230Th half-life value. Under ideal circumstances, with our methods, the 2σ uncertainty in age, including uncertainty in half-life values is ±10a at 10ka, ±100a at 130ka, ±300a at 200ka, ±1ka at 300ka, ±2ka at 400ka, ±6ka at 500ka, and ±12ka at 600ka. The isotopic composition of a sample with an age <800ka can clearly be resolved from the isotopic composition of a sample in secular equilibrium, assuming closed system behavior. Using these techniques, we analyzed a Sanbao Cave (Hubei, China) stalagmite that formed between 510 and 640ka ago. As the half-life values were determined independent of the Sanbao Cave ages, the observed co-variation between stalagmite δ18O and Northern Hemisphere summer insolation is consistent with accurate ages and half-life values.

Precise and direct determination of the half-life of 41Ca

Calcium-41 plays an important role in the long-term evaluation of the safety of final repositories for nuclear waste and is used to study the fine-scale chronology of the formation of the Solar System. Both applications are hindered by insufficient precision and poor consistency of previous determinations of the half-life. This work reports a half-life for 41Ca of (9.94±0.15)×104years, which was determined with a combination of methods, chosen to provide the best possible precision. The activity was measured by liquid scintillation counting (LSC) exploiting the triple-to-double coincidence ratio method (TDCR); the absolute isotopic composition was determined by thermal ionization mass spectrometry (TIMS) and isotope dilution. Enhanced precision and accuracy of the 41Ca half-life will allow the improvement of safety analyses for final deposit sites of nuclear waste and of dating first solids, and better constrain the stellar environment of the formation of the Solar System.

Absolute isotopic composition and atomic weight of selenium using multi-collector inductively coupled plasma mass spectrometry

The isotopic composition of selenium was measured with high precision using a collision cell multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). Gravimetric synthetic mixtures prepared from three highly enriched isotope of 76Se, 78Se and 82Se with well defined purity were used to calibrate a MC-ICP-MS. Measurements of seven various natural selenium materials including Se NIST SRM 3149 yielded the absolute isotopic composition (in at.%) of 74Se 0.8623(38), 76Se 9.228(10), 77Se 7.5975(50), 78Se 23.693(12), 80Se 49.800(17) and 82Se 8.8188(85). The new atomic weight of selenium was calculated as 78.9711(9).

Improving stable carbon and oxygen isotope geochemical measurements in dolomite: reference material and acid fractionation factor

The analysis of stable carbon and oxygen isotope composition is one of the most commonly used techniques in stratigraphic and diagenetic research of carbonate rocks. The wide-spread use and easy access of this long-established method has the side effect that little attention is paid to fundamental calibrations. Dolomite is often measured against a calcite standard (NBS19), and the acid fractionation factor used to calibrate is based on the one for calcite. To date, no reference material exists for dolomite. In this study, which is part of dolomite research in the Qatar Carbonates and Carbon Storage Research Centre project, we focus on two main goals. First, we characterize a current standard of dolomite used for major and minor elemental geochemistry, and assess its suitability as a new dolomite standard for δ18O and δ 13C. Second, we attempt to better constrain the acid fractionation factor for dolomite and assess the influence of different dolomite types on this fractionation factor. As only two thirds of the total oxygen in the carbonate is released in the form of CO2 during acid reaction, a fractionation between the reacting carbonate and the resulting gas will occur. A recent study improved on the acid fractionation factors for calcite and aragonite. Often, the acid fractionation factor for dolomite is used to calculate δ18O and δ 13C from the values obtained by calibration with the calcite standard. Only two studies (from the 1980&apos;s) have attempted to constrain the acid fractionation factor for dolomite, of which only one did experiments not only at 25ºC, but also at 50 and 100ºC. The dataset of the latter experiment is, however, very limited and contains only two dolomite samples. We aim at improving the constraints on the acid fractionation factor of dolomite by reacting a wide range of different types of dolomite at a wide range of acid temperature, and compare this to the absolute isotopic composition of the samples measured on a fluorination line.

Development of enriched 106CdWO 4 crystal scintillators to search for double β decay processes in 106Cd

A cadmium tungstate crystal scintillator enriched in 106Cd was developed for an experiment to search for double beta processes in 106Cd. With this aim samples of cadmium with natural isotopic composition and enriched in 106Cd were purified by vacuum distillation. Cadmium tungstate compounds were synthesized from solutions. The absolute isotopic composition of the enriched cadmium was accurately determined as 66.4% by thermal ionisation mass-spectrometry. A 106CdWO 4 crystal boule was grown by the low-thermal-gradient Czochralski technique. The total irrecoverable loss of the enriched cadmium in all the stages of the crystal scintillator production does not exceed 2.3%. The produced 106CdWO 4 crystal scintillator with mass of 216 g exhibits good optical and scintillation properties.

Certification of natural isotopic abundance inorganic mercury reference material NIMS-1 for absolute isotopic composition and atomic weight

A candidate reference material of natural isotopic composition for inorganic mercury has been characterized by the Institute for National Measurement Standards of the National Research Council Canada. The material, designated NIMS-1 (natural inorganic mercury standard) is certified for isotope ratios, isotopic abundances and atomic weight of mercury. The certification was achieved using multi-collector ICP-MS based on a state-of-the-art regression model for calibration. The certified isotopic composition is x196 = 0.001 55(4), x198 = 0.100 38(10), x199 = 0.169 38(9), x200 = 0.231 38(6), x201 = 0.131 70(12), x202 = 0.297 43(9) and x204 = 0.068 18(6) with the corresponding atomic weight of mercury Ar(Hg) = 200.5924(8). Values are presented in a concise notation whereby the expanded uncertainty with a coverage factor of two is given in parenthesis next to the least significant digits to which it applies. A full disclosure of all raw data pertinent to certification is presented in order to afford a fully transparent process. Care was taken to ensure high metrological quality in the evaluation of the accuracy and the uncertainty of the certified results. In this regard, it is superior to the best measurement from a single terrestrial source as currently recognized by IUPAC. Considering the known natural variations of Hg isotopic composition, we propose 200.592(3) as a revised assessment of the standard atomic weight of mercury.

Molybdenum isotope mass fractionation in iron meteorites

An examination of the isotope fractionation of molybdenum in a range of iron meteorites has been carried out by thermal ionization mass spectrometry incorporating the double spike technique. Ten iron meteorites were dissolved in hydrochloric acid and double spiked with a 92Mo– 98Mo mixture of enriched isotopes. An ion exchange-based chemical separation process was then used to remove any vestiges of zirconium and ruthenium from the samples, so as to eliminate any possibility of isobaric interferences, and to provide a final molybdenum extract suitable for mass spectrometric analysis. The molybdenum from each extracted meteorite was analysed under similar mass spectrometric conditions to a Laboratory Standard. The double spike technique enabled the magnitude of the isotope fractionation in the meteorites to be determined with high accuracy relative to the Laboratory Standard, whose absolute isotopic composition is known. The ten iron meteorites gave isotope fractionations in the range −0.5‰ to +1.2‰/mass unit with respect to the terrestrial standard, the isotopic composition of which is identical, within experimental uncertainty, to other terrestrial samples.

The role of mass spectrometry in atomic weight determinations

The 1914 Nobel Prize for Chemistry was awarded to Theodore Richards, whose work provided an insight into the history of the birth and evolution of matter as embedded in the atomic weights. However, the secret to unlocking the hieroglyphics contained in the atomic weights is revealed by a study of the relative abundances of the isotopes. A consistent set of internationally accepted atomic weights has been a goal of the scientific community for over a century. Atomic weights were originally determined by chemical stoichiometry--the so-called "Harvard Method," but this methodology has now been superseded by the "physical method," in which the isotopic composition and atomic masses of the isotopes comprising an element are used to calculate the atomic weight with far greater accuracy than before. The role of mass spectrometry in atomic weight determinations was initiated by the discovery of isotopes by Thomson, and established by the pioneering work of Aston, Dempster, and Nier using sophisticated mass spectrographs. The advent of the sector field mass spectrometer in 1947, revolutionized the application of mass spectrometry for both solids and gases to other fields of science including atomic weights. Subsequently, technological advances in mass spectrometry have enabled atomic masses to be determined with an accuracy better than one part in 10(7), whilst the absolute isotopic composition of many elements has been determined to produce accurate values of their atomic weights. Conversely, those same technological developments have revealed significant variations in the isotope abundances of many elements caused by a variety of physiochemical mechanisms in natural materials. Although these variations were initially seen as an impediment to the accuracy with which atomic weights could be determined, it was quickly realized that nature had provided a new tool to investigate physiochemical and biogeochemical mechanisms in nature, which could be exploited by precise and accurate isotopic measurements. Atomic weights can no longer be regarded as constants of nature, except for the monoisotopic elements whose atomic weights are determined solely by the relative atomic mass of that nuclide. Stable isotope geochemists developed mass spectrometric protocols by the adoption of internationally accepted reference materials for the light elements, to which measurements from various laboratories could be compared. Subsequently, a number of heavy elements such as iron, molybdenum and cadmium have been shown to exhibit isotope fractionation. The magnitude of such isotope fractionation in nature is less than for the light elements, but technological developments, such as multiple collector-inductively coupled plasma-mass spectrometry, have enabled such fractionation effects to be determined. Measurements of the atomic weights of certain elements affect the determination of important fundamental constants such as the Avogadro Constant, the Faraday Constant and the Universal Gas Constant. Heroic efforts have been made to refine the accuracy of the atomic weight of silicon, with the objective of replacing the SI standard of mass--the kilogram--with the Avogadro Constant. Improvements in these fundamental constants in turn affect the set of self-consistent values of other basic constants through a least-squares adjustment methodology. Absolute isotope abundances also enable the Solar System abundances of the s-, r-, and p-process of nucleosynthesis to be accurately determined, thus placing constraints on theories of heavy element nucleosynthesis. Future developments in the science of atomic weight determinations are also examined.

Absolute isotopic composition and atomic weight of neodymium using thermal ionization mass spectrometry

Synthetic mixtures prepared gravimetrically from highly enriched isotopes of neodymium in the form of oxides of well-defined purity were used to calibrate a thermal ionization mass spectrometer. A new error analysis was applied to calculate the final uncertainty of the atomic weight value. Measurements on natural neodymium samples yielded an absolute isotopic composition of 27.153(19) atomic percent (at.%) 142Nd, 12.173(18) at.% 143Nd, 23.798(12) at.% 144Nd, 8.293(7) at.% 145Nd, 17.189(17) at.% 146Nd, 5.756(8) at.% 148Nd, and 5.638(9) at.% 150Nd, and the atomic weight of neodymium as 144.2415(13), with uncertainties given on the basis of 95% confidence limits. No isotopic fractionation was found in terrestrial neodymium materials.

Absolute measurements of neodymium isotopic abundances and atomic weight by MC-ICPMS

Gravimetric synthetic mixtures prepared from highly enriched isotopes of neodymium in the form of oxides of well-defined purity were used to calibrate a multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). Measurements on natural neodymium samples yielded an absolute isotopic composition 27.147(20) at.% 142Nd, 12.182(21) at.% 143Nd, 23.803(14) at.% 144Nd, 8.297(6) at.% 145Nd, 17.190(13) at.% 146Nd, 5.755(8) at.% 148Nd, 5.626(10) at.% 150Nd, and the atomic weight of neodymium as 144.2409(17) both with an uncertainty given on the basis of 95% confidence limit. No isotopic fractionation was found in terrestrial neodymium materials. Thermal ionization mass spectrometry (TIMS) analyses were carried on the same samples measured by MC-ICPMS and the essentially identical results were obtained, both for the abundances and the atomic weights.

Mass spectrometric method for the absolute calibration of the intramolecular nitrogen isotope distribution in nitrous oxide.

A mass spectrometric method to determine the absolute intramolecular (position-dependent) nitrogen isotope ratios of nitrous oxide (N2O) has been developed. It is based on the addition of different amounts of doubly labeled 15N2O to an N2O sample of the isotope ratio mass spectrometer reference gas, and subsequent measurement of the relative ion current ratios of species with mass 30, 31, 44, 45, and 46. All relevant quantities are measured by isotope ratio mass spectrometers, which means that the machines' inherent high precision of the order of 10(-5) can be fully exploited. External determination of dilution factors with generally lower precision is avoided. The method itself can be implemented within a day, but a calibration of the oxygen and average nitrogen isotope ratios relative to a primary isotopic reference material of known absolute isotopic composition has to be performed separately. The underlying theoretical framework is explored in depth. The effect of interferences due to 14N15N16O and 15N14N16O in the 15N2O sample and due to 15N2+ formation are fully accounted for in the calculation of the final position-dependent nitrogen isotope ratios. Considering all known statistical uncertainties of measured quantities and absolute isotope ratios of primary isotopic reference materials, we achieve an overall uncertainty of 0.9 per thousand (1 sigma). Using tropospheric N2O as common reference point for intercomparison purposes, we find a substantially higher relative enrichment of 15N at the central nitrogen atom over 15N at the terminal nitrogen atom than measured previously for tropospheric N2O based on a chemical conversion method: 46.3 +/- 1.4 per thousand as opposed to 18.7 +/- 2.2 per thousand. However, our method depends critically on the absolute isotope ratios of the primary isotopic reference materials air-N2 and VSMOW. If they are systematically wrong, our estimates will also necessarily be incorrect.

Absolute isotopic composition and atomic weight of commercial zinc using inductively coupled plasma mass spectrometry.

Inductively coupled plasma mass spectrometry is introduced as a method for determining the absolute isotopic composition of zinc. The high ionization efficiency and time-independent characteristics of the mass spectrometry permit the absolute isotopic composition of high ionization potential elements. The mass discrimination of the instrument is calibrated by synthetic isotope mixtures prepared from highly enriched isotopes of zinc. The resulting isotope ratios yield atomic percents of 64Zn, 49.188 +/- 0.030; 66Zn, 27.792 +/- 0.041; 67Zn, 4.041 +/- 0.009; 68Zn, 18.378 +/- 0.050; and 70Zn, 0.600 +/- 0.003. This isotopic composition is different from those of conventional mass spectrometric measurements. Their differences depend on the mass differences about 0.8-1.2%/amu with enhancement of heavier isotopes. The atomic weight calculated from our isotopic composition is 65.3756 +/- 0.0040. The obtained atomic weight is fully consistent with that of a precise coulometric measurement in contrast to the previous mass spectrometric measurements. An isotopic variation of commercial zinc reagents has been investigated. A mass-dependent fractionation of 0.12%/amu is observed in a high-purity metal zinc, NIST-SRM 682, among five reagents. This mass dependence is probably inherited through their purification process.

Absolute isotopic composition and atomic weight of samarium

Gravimetric synthetic mixtures prepared from highly enriched isotopes of samarium in the form of oxides of well defined purity were used to calibrate a thermal ionization mass spectrometer. Measurements on natural samarium samples yielded an absolute isotopic composition 3.08(1) at.% 144 Sm , 15.02(5) at.% 147 Sm , 11.25(3) at.% 148 Sm , 13.83(4) at.% 149 Sm , 7.35(2) at.% 150 Sm , 26.74(3) at.% 152 Sm , and 22.73(5) at.% 154 Sm , and the atomic weight of samarium as 150.363(8) both with an uncertainty given on the basis of 95% confidence limit. No isotopic fractionation was found in terrestrial samarium materials.

Absolute isotopic composition and atomic weight of zinc

Gravimetric synthetic mixtures prepared from highly enriched isotopes of zinc in the form of oxides of well defined purity were used to calibrate a thermal ionization mass spectrometer. Measurements on natural zinc samples yielded an absolute isotopic composition of 48.27 (21) at. % 64Zn, 27.98 (5) at. % 66Zn, 4.10 (1) at. % 67Zn, 19.02 (8) at. % 68Zn, and 0.63 (1) at. % 70Zn, and the atomic weight of zinc as 65.409 (6) both with an uncertainty given on the basis of 95% confidence limit.

Absolute isotopic composition and atomic weight of dysprosium

Gravimetric synthetic mixtures of highly enriched isotopes of 162Dy and 164Dy in the form of oxides of well defined purity were used to calibrate a thermal ionization mass spectrometer. Measurements on five natural samples of dysprosium yielded an absolute isotopic composition of 0.056 (2) at. % 156Dy, 0.095(2) at. % 158Dy, 2.329(12) at. % 160Dy, 18.889 (28) at. % 161Dy, 25.475(24) at. % 162Dy, 24.896(28) at. % 163Dy and 28.260(36) at. % 164Dy, and the atomic weight of dysprosium as 162.4995(17) with an uncertainty given on the basis of 95% confidence limit. No isotopic fractionation was found in terrestrial normal dysprosium materials.

Absolute isotopic composition and atomic weight of germanium

Gravimetric synthetic mixtures prepared from highly enriched isotopes of germanium in the form of oxides of well defined purity were used to calibrate a thermal ionization mass spectrometer. Measurements on natural germanium samples yielded an absolute isotopic composition of 20.38(10) at. % 70Ge, 27.34(9) at. % 72Ge, 7.75(5) at. % 73Ge, 36.71(9) at. % 74Ge, and 7.82(4) at. % 76Ge, and the atomic weight of germanium as 72.639(7) both with an uncertainty given on the basis of 95% confidence limit. No isotopic fractionation was found in terrestrial normal germanium materials.