Amy M. Gaffney

Rb-Sr, Sm-Nd and Lu-Hf isotope systematics of the lunar Mg-suite: the age of the lunar crust and its relation to the time of Moon formation

New Rb-Sr, 146,147Sm-142,143Nd and Lu-Hf isotopic analyses of Mg-suite lunar crustal rocks 67667, 76335, 77215 and 78238, including an internal isochron for norite 77215, were undertaken to better define the time and duration of lunar crust formation and the history of the source materials of the Mg-suite. Isochron ages determined in this study for 77215 are: Rb-Sr=4450±270 Ma, 147Sm-143Nd=4283±23 Ma and Lu-Hf=4421±68 Ma. The data define an initial 146Sm/144Sm ratio of 0.00193±0.00092 corresponding to ages between 4348 and 4413 Ma depending on the half-life and initial abundance used for 146Sm. The initial Nd and Hf isotopic compositions of all samples indicate a source region with slight enrichment in the incompatible elements in accord with previous suggestions that the Mg-suite crustal rocks contain a component of KREEP. The Sm/Nd—142Nd/144Nd correlation shown by both ferroan anorthosite and Mg-suite rocks is coincident with the trend defined by mare and KREEP basalts, the slope of which corresponds to ages between 4.35 and 4.45 Ga. These data, along with similar ages for various early Earth differentiation events, are in accord with the model of lunar formation via giant impact into Earth at ca 4.4 Ga.

235U–231Pa age dating of uranium materials for nuclear forensic investigations

Age dating of nuclear material can provide insight into source and suspected use in nuclear forensic investigations. We report here a method for the determination of the date of most recent chemical purification for uranium materials using the 235U-231Pa chronometer. Protactinium is separated from uranium and neptunium matrices using anion exchange resin, followed by sorption of Pa to an SiO2 medium. The concentration of 231Pa is measured by isotope dilution mass spectrometry using 233Pa spikes prepared from an aliquot of 237Np and calibrated in-house using the rock standard Table Mountain Latite and the uranium isotopic standard U100. Combined uncertainties of age dates using this method are 1.5 to 3.5 %, an improvement over alpha spectrometry measurement methods. Model ages of five uranium standard reference materials are presented; all standards have concordant 235U-231Pa and 234U-230Th model ages.

A unique glimpse into asteroidal melting processes in the early solar system from the Graves Nunatak 06128/06129 achondrites

Abstract The recently recovered Antarctic achondrites Graves Nunatak 06128 and 06129 are unique meteorites that represent high-temperature asteroidal processes in the early solar system never before identified in any other meteorite. They represent products of early planetesimal melting (4564.25 ± 0.21 Ma) and subsequent metamorphism of an unsampled geochemical reservoir from an asteroid that has characteristics similar to the brachinite parent body. This melting event is unlike those predicted by previous experimental or geochemical studies, and indicates either disequilibrium melting of chondritic material or melting of chondritic material under volatile-rich conditions.

Round-robin 230Th–234U age dating of bulk uranium for nuclear forensics

In an inter-laboratory measurement comparison study, four laboratories determined 230Th–234U model ages of uranium certified reference material NBL U050 using isotope dilution mass spectrometry. The model dates determined by the participating laboratories range from 9 March 1956 to 19 October 1957, and are indistinguishable given the associated measurement uncertainties. These model ages are concordant with to slightly older than the known production age of NBL U050.

Application of the 226Ra–230Th–234U and 227Ac–231Pa–235U radiochronometers to uranium certified reference materials

Uranium certified reference materials (CRM) issued by New Brunswick Laboratory were subjected to dating using four independent uranium-series radiochronometers. In all cases, there was acceptable agreement between the model ages calculated using the 231Pa–235U, 230Th–234U, 227Ac–235U or 226Ra–234U radiochronometers and either the certified 230Th–234U model date (CRM 125-A and CRM U630), or the known purification date (CRM U050 and CRM U100). The agreement between the four independent radiochronometers establishes these uranium certified reference materials as ideal informal standards for validating dating techniques utilized in nuclear forensic investigations in the absence of standards with certified model ages for multiple radiochronometers.

Nuclear forensic analysis of uranium oxide powders interdicted in Victoria, Australia

Abstract Nuclear forensic analysis was conducted on two uranium samples confiscated during a police investigation in Victoria, Australia. The first sample, designated NSR-F-270409-1, was a depleted uranium powder of moderate purity (∼ 1000 μg/g total elemental impurities). The chemical form of the uranium was a compound similar to K2(UO2)3O4 · 4H2O. While aliquoting NSR-F-270409-1 for analysis, the body and head of a Tineid moth was discovered in the sample. The second sample, designated NSR-F-270409-2, was also a depleted uranium powder. It was of reasonably high purity (∼ 380 μg/g total elemental impurities). The chemical form of the uranium was primarily UO3 · 2H2O, with minor phases of U3O8 and UO2. While aliquoting NSR-F-270409-2 for analysis, a metal staple of unknown origin was discovered in the sample. The presence of 236U and 232U in both samples indicates that the uranium feed stocks for these samples experienced a neutron flux at some point in their history. The reactor burn-up calculated from the isotopic composition of the uranium is consistent with that of spent fuel from natural uranium (NU) fueled Pu production. These nuclear forensic conclusions allow us to categorically exclude Australia as the origin of the material and greatly reduce the number of candidate sources.

The application of radiochronometry during the 4th collaborative materials exercise of the nuclear forensics international technical working group (ITWG)

In a recent international exercise, 10 international nuclear forensics laboratories successfully performed radiochronometry on three low enriched uranium oxide samples, providing 12 analytical results using three different parent-daughter pairs serving as independent chronometers. The vast majority of the results were consistent with one another and consistent with the known processing history of the materials. In general, for these particular samples, mass spectrometry gave more accurate and more precise analytical results than decay counting measurements. In addition, the concordance of the 235U–231Pa and 234U–230Th chronometers confirmed the validity of the age dating assumptions, increasing confidence in the resulting conclusions.

Improved protactinium spike calibration method applied to 231Pa–235U age-dating of certified reference materials for nuclear forensics

The application of multiple radiochronometers in a forensic investigation of bulk uranium provides increased confidence for interpreting age-dating results. We have developed a streamlined method for the purification of Pa and Th from bulk uranium and have applied this method in analyzing uranium certified reference materials (CRMs) IRMM-1000 and CRM U100, using the 231Pa–235U and 230Th–234U radiochronometers. Our improved 233Pa spike calibration technique has replaced the time-intensive, conventional calibration using geologic rock standards. Paired Pa–U and Th–U age-dating analyses of CRM U100 produce concordant results. In contrast, analyses of IRMM-1000 demonstrate reproducibly-discordant Th–U and Pa–U age-dates.

A case study in plutonium radiochronometry using multiple isotope systems

Radiochronometry analyses of two Pu metals were performed using the 237Np–241Am–241Pu, 234U–238Pu, 235U–239Pu, and 236U–240Pu decay series. For one sample, all radiochronometers yield concordant model dates in 1959–1960, indicating that the aqueous processing method used to purify Pu effectively removed U, Am and Np decay products from the bulk Pu. The second sample yields discordant model dates that are also older than the known production date in 1982. The excess U, Am and Np present in the sample indicate that the sample was purified during at least two different episodes, using a combination of aqueous methods and molten salt extraction.

New determination of the 229Th half-life

A new determination of the 229Th half-life was made based on measurements of the 229Th massic activity of a high-purity solution for which the 229Th molality had previously been measured. The 229Th massic activity was measured by direct comparison with SRM 4328C using 4παβ liquid scintillation counting, NaI counting, and standard addition liquid scintillation counting. The massic activity was confirmed by isotope dilution alpha spectrometry measurements. The calculated 229Th half-life is (7825 ± 87) years (k = 2), which is shorter than the three most recent half-life determinations but is consistent with these values within uncertainties.