Atmospheric Fragmentation of the Gold Basin Meteoroid as Constrained from Cosmogenic Nuclides

Since the discovery of the Gold Basin L4 chondrite shower almost ten years ago in the northwestern corner of Arizona, many thousands of L-chondrite specimens have been recovered from an area of approx.22 km long and approx.10 km wide. Concentrations of cosmogenic 14C and 10Be in a number of these samples indicated a terrestrial age of approx.15,000 years and a large pre-atmospheric size [1]. Additional measurements of cosmogenic Be-10, Al-26, Cl-36, and Ca-41 in the metal and stone fractions of fifteen Gold Basin samples constrained the pre-atmospheric radius to 3-5 m [2]. This implies that Gold Basin is by far the largest stone meteorite in the present meteorite collection, providing us with an opportunity to study the fragmentation process of a large chondritic object during atmospheric entry. Knowledge about the fragmentation process provides information about the mechanical strength of large meteoroids, which is important for the evaluation of future hazards of small asteroid impacts on Earth and possible defensive scenarios to avoid those impacts

Accelerator mass spectrometry of Strontium-90 for homeland security, environmental monitoring, and human health

Strontium-90 is one of the most hazardous materials managed by agencies charged with protecting the public from radiation. Traditional radiometric methods have been limited by low sample throughput and slow turnaround times. Mass spectrometry offers the advantage of shorter analysis times and the ability to measure samples immediately after processing, however conventional mass spectrometric techniques are susceptible to molecular isobaric interferences that limit their overall sensitivity. In contrast, accelerator mass spectrometry is insensitive to molecular interferences and we have therefore begun developing a method for determination of {sup 90}Sr by accelerator mass spectrometry. Despite a pervasive interference from {sup 90}Zr, our initial development has yielded an instrumental background of {approx} 10{sup 8} atoms (75 mBq) per sample. Further refinement of our system (e.g., redesign of our detector, use of alternative target materials) is expected to push the background below 10{sup 6} atoms, close to the theoretical limit for AMS. Once we have refined our system and developed suitable sample preparation protocols, we will utilize our capability in applications to homeland security, environmental monitoring, and human health

Labeling the human skeleton with 41Ca to assess changes in bone calcium metabolism

Bone research is limited by the methods available for detecting changes in bone metabolism. While dual X-ray absorptiometry is rather insensitive, biochemical markers are subject to significant intra-individual variation. In the study presented here, we evaluated the isotopic labeling of bone using 41Ca, a long-lived radiotracer, as an alternative approach. After successful labeling of the skeleton, changes in the systematics of urinary 41Ca excretion are expected to directly reflect changes in bone Ca metabolism. A minute amount of 41Ca (100nCi) was administered orally to 22 postmenopausal women. Kinetics of tracer excretion were assessed by monitoring changes in urinary 41Ca/40Ca isotope ratios up to 700days post-dosing using accelerator mass spectrometry and resonance ionization mass spectrometry. Isotopic labeling of the skeleton was evaluated by two different approaches: (i) urinary 41Ca data were fitted to an established function consisting of an exponential term and a power law term for each individual; (ii) 41Ca data were analyzed by population pharmacokinetic (NONMEM) analysis to identify a compartmental model that describes urinary 41Ca tracer kinetics. A linear three-compartment model with a central compartment and two sequential peripheral compartments was found to best fit the 41Ca data. Fits based on the use of the combined exponential/power law function describing urinary tracer excretion showed substantially higher deviations between predicted and measured values than fits based on the compartmental modeling approach. By establishing the urinary 41Ca excretion pattern using data points up to day 500 and extrapolating these curves up to day 700, it was found that the calculated 41Ca/40Ca isotope ratios in urine were significantly lower than the observed 41Ca/40Ca isotope ratios for both techniques. Compartmental analysis can overcome this limitation. By identifying relative changes in transfer rates between compartments in response to an intervention, inaccuracies in the underlying model cancel out. Changes in tracer distribution between compartments were modeled based on identified kinetic parameters. While changes in bone formation and resorption can, in principle, be assessed by monitoring urinary 41Ca excretion over the first few weeks post-dosing, assessment of an intervention effect is more reliable ∼150days post-dosing when excreted tracer originates mainly from bon

California GAMA Special Study: Nitrate Fate and Transport in the Salinas Valley

Abstract not provide

Cosmogenic radionuclides in L5 and LL5 chondrites from Queen Alexandra Range, Antarctica: Identification of a large L/LL5 chondrite shower with a preatmospheric mass of approximately 50,000 kg

The collection of approximately 3300 meteorites from the Queen Alexandra Range (QUE) area, Antarctica, is dominated by more than 2000 chondrites classified as either L5 or LL5. Based on concentrations of the cosmogenic radionuclides 10Be, 26Al, 36Cl, and 41Ca in the metal and stone fraction of 16 QUE L5 or LL5 chondrites, we conclude that 13 meteorites belong to a single meteorite shower, QUE 90201, with a large preatmospheric size and a terrestrial age of 125 kyr. Members of this shower have properties typical of L (e.g., pyroxene composition) and LL chondrites (e.g., metal abundance and composition), as well as properties intermediate between the L and LL groups (e.g., olivine composition), and is thus best described as an L/LL5 chondrite. Based on comparison with model calculations, the measured radionuclide concentrations in the metal and stone fractions of QUE 90201 indicate irradiation in an object with a preatmospheric radius of approximately 150 cm, representing one of the largest chondrites known so far. Based on the abundance of small L5 and LL5 chondrites at QUE and their distinct mass distribution, we conclude that the QUE 90201 shower includes up to 2000 fragments with a total recovered mass of 60-70 kg, \u3c 1% of the preatmospheric mass of approximately 50,000 kg. The mass distribution of the QUE 90201 shower suggests that the meteoroid experienced catastrophic atmospheric fragmentation(s), either because it was a fragile object or it had a high entry velocity

Accelerator mass spectrometry of Strontium-90 for homeland security, environmental monitoring, and human health Nuclear Instruments and Methods in Physics Research B: Beam Interactions with Material and Atoms Accelerator mass spectrometry of Strontium-90

Abstract Strontium-90 is one of the most hazardous materials managed by agencies charged with protecting the public from radiation. Traditional radiometric methods have been limited by low sample throughput and slow turnaround times. Mass spectrometry offers the advantage of shorter analysis times and the ability to measure samples immediately after processing, however conventional mass spectrometric techniques are susceptible to molecular isobaric interferences that limit their overall sensitivity. In contrast, accelerator mass spectrometry is insensitive to molecular interferences and we have therefore begun developing a method for determination of 90 Sr by accelerator mass * Corresponding author: 7000 East Avenue, L-397; Livermore, CA 94551; +1-925-423-9012; [email protected] 2 spectrometry. Despite a pervasive interference from 90 Zr, our initial development has yielded an instrumental background of ~10 8 atoms (75 mBq) per sample. Further refinement of our system (e.g., redesign of our detector, use of alternative target materials) is expected to push the background below 10 6 atoms, close to the theoretical limit for AMS. Once we have refined our system and developed suitable sample preparation protocols, we will utilize our capability in applications to homeland security, environmental monitoring, and human health

Identifying large chondrites using cosmogenic radionuclides

We measured the concentrations of the cosmogenic radionuclides Be-10, Al-26, Cl-36 and Ca-41 in the metal and stone fractions of three large chondrite showers to determine their pre-atmospheric size. Large chondrites are characterized by substantial contributions of neutron-capture Ca-41 in the stone fraction (up to similar to 2 dpm/gCa), low radionuclide concentrations in the metal fraction and high Be-10(stone)/Be-10(metal) ratios. Based on the measured concentrations in comparison with calculated cosmogenic nuclide depth profiles, using a semi-empirical and a purely physical model, we conclude that these objects had pre-atmospheric radii ranging from similar to 80 cm to \u3e3 m. We conclude that the semi-empirical model is more reliable for spallogenic production rates in large objects, while the purely physical model is more reliable for neutron-capture products. (C) 2009 Elsevier B.V. All rights reserved