Effects of bulk composition on production rates of cosmogenic nuclides in meteorites

The bulk chemical composition of meteorites has been suggested as a main factor influencing the production of cosmogenic nuclides. Numerical simulations with Los Alamos Monte Carlo production and transport codes were done for Ne-21/Ne-22 ratios and Ar-38 production rates in meteorites with a wide range of compositions. The calculations show that an enhanced flux of low-energy secondary particles in metal-rich phases is the essential key for the explanation of experimentally observed differences in nuclide production processes in various meteorite classes

Radioactivities induced in some LDEF samples

Radioactivities induced in several Long Duration Exposure Facilities (LDEF) samples were measured by low-level counting at Los Alamos and elsewhere. These radionuclides have activities similar to those observed in meteorites and lunar samples. Some trends were observed in these measurements in terms of profiles in trunnion layers and as a function of radionuclide half-life. Several existing computer codes were used to model the production by the protons trapped in the Earth's radiation belts and by the galactic cosmic rays of some of these radionuclides, Mn-54 and Co-57 in steel, Sc-46 in titanium, and Na-22 in alloys of titanium and aluminum. Production rates were also calculated for radionuclides possibly implanted in LDEF, Be-7, Be-10, and C-14. Enhanced concentrations of induced isotopes in the surfaces of trunnion sections relative to their concentrations in the center are caused by the lower-energy protons in the trapped radiation. Secondary neutrons made by high-energy trapped protons and by galactic cosmic rays produce much of the observed radioactivities, especially deep in an object. Comparisons of the observed to calculated activities of several radionuclides with different half-lives indicate that the flux of trapped protons at LDEF decreased significantly at the end of the mission

A thermodynamically motivated optimization algorithm: Circular wheel balance optimization

summary:The author investigates a Monte Carlo algorithm for finding suboptimal solutions for a wide clase of complicated optimization problems characterized by a large combinatorial complexity. This algorithm was applied to one specific problem: circular wheel balance optimization. The slow increase of the effort along with the increasing size of the problems and the generality of the method promise that the thermodynamically motivated optimization will become a very universal and effective optimization method

Cosmogenic nuclides in stony meteorites revisited

We present new model calculations for depth and size dependent cosmogenic production rates in ordinary and carbonaceous chondrites by galactic cosmic rays. This model, essentially that of Leya et al. (2000a), folds together particle spectra and cross sections for the relevant nuclear reactions, but has been significantly improved due to major improvements in the neutron cross section database and better Monte Carlo modeling of the primary and the secondary particle spectra. The data presented here replace (and extend) the results of our earlier model predictions. Here we give for ordinary and carbonaceous chondrites elemental production rates for the cosmogenic radionuclides 10Be, 14C, 26Al, 36Cl, 41Ca, 53Mn, 60Fe, and 129I as well as for the noble gas isotopes 3He, 4He, 20Ne, 21Ne, 22Ne, 36Ar, and 38Ar. Using the new data and expressing size and depth scales to the unit [g/cm2], we are able to demonstrate that the matrix effect for both chondrite types is negligible for all target product combinations, except for those which are dominated by thermal or very low energy neutron reactions. Based on the new model predictions, we present a variety of elemental and isotopic production rate ratios allowing for a reliable determination of preatmospheric sizes, shielding depths, cosmic-ray exposure ages, and diffusive losses.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202

Thermal neutron capture effects in radioactive and stable nuclide systems

Neutron capture effects in meteorites and lunar surface samples have been successfully used in the past to study exposure histories and shielding conditions. In recent years, however, it turned out that neutron capture effects produce a nuisance for some of the short-lived radionuclide systems. The most prominent example is the 182Hf-182W system in iron meteorites, for which neutron capture effects lower the 182W/184W ratio, thereby producing too old apparent ages. Here, we present a thorough study of neutron capture effects in iron meteorites, ordinary chondrites, and carbonaceous chondrites, whereas the focus is on iron meteorites. We study in detail the effects responsible for neutron production, neutron transport, and neutron slowing down and find that neutron capture in all studied meteorite types is not, as usually expected, exclusively via thermal neutrons. In contrast, most of the neutron capture in iron meteorites is in the epithermal energy range and there is a significant contribution from epithermal neutron capture even in stony meteorites. Using sophisticated particle spectra and evaluated cross section data files for neutron capture reactions we calculate the neutron capture effects for Sm, Gd, Cd, Pd, Pt, and Os isotopes, which all can serve as neutron-dose proxies, either in stony or in iron meteorites. In addition, we model neutron capture effects in W and Ag isotopes. For W isotopes, the GCR-induced shifts perfectly correlate with Os and Pt isotope shifts, which therefore can be used as neutron-dose proxies and permit a reliable correction. We also found that GCR-induced effects for the 107Pd-107Ag system can be significant and need to be corrected, a result that is in contrast to earlier studies

Noble gases in Grant and Carbo and the influence of S- and P-rich mineral inclusions on the 41K-40K dating system

Cosmogenic He, Ne, and Ar were measured in the iron meteorites Grant (IIIAB) and Carbo (IID) to re-determine their preatmospheric geometries and exposure histories. We also investigated the influence of sulphur- and/or phosphorus-rich inclusions on the production rates of cosmogenic Ne. Depth profiles measured in Grant indicate a preatmospheric center location 117 mm left from the reference line and 9 mm below bar B, which is clearly different (~10 cm) from earlier results (~165 mm left from the reference line on bar F). For Carbo the preatmospheric center location was found to be 120 mm right of the reference line and 15 mm above bar J, which is in agreement with literature data. The new measurements indicate a spherical preatmospheric shape for both meteorites and, based on literature 36Cl data, the radii were estimated to be about 32 cm and 70 cm for Grant and Carbo, respectively. We demonstrate that minor elements like S and P have a significant influence on the production rates of cosmogenic Ne. In our samples, containing on average 0.5% S and/or P, about 20% of 21Ne was produced from these minor elements. Using measured 21Ne concentrations and endmember 22Ne/21Ne ratios for Fe + Ni and S + P, respectively, we show that it is possible to correct for 21Ne produced from S and/or P. The thus corrected data are then used to calculate new 41K-40K exposure ages--using published K data--which results in 564 +/- 78 Ma for Grant and 725 +/- 100 Ma for Carbo. The correction always lowers the 21Ne concentrations and consequently decreases the 41K-40K exposure ages. The discrepancies between 36Cl-36Ar and 41K-40K ages are accordingly reduced. The existence of a significant long-term variation of the GCR, which is based on a former 30-50% difference between 41K-40K and 36Cl-36Ar ages, may warrant re-investigation.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202

Presence of the solar de Vries cycle (~205 years) during the last ice age

Certain characteristic periodicities inthe $\Delta ^{14}$C record from tree rings, such as the well-known 11-yr Schwabe cycle, are known to be of solar origin. The origin of longerperiod cycles, such as the 205-yr de Vries cycle, in the $\Delta ^{14}$C record was less certain, and it was possible to attribute it either to solar or climatic variability. Here, we demonstrate that the de Vries cycle is present in $^{10}$Be data from the GRIP ice core during the last ice age (25 to 50 kyr BP). Analysis of the amplitude of variation of this cycle shows it to be modulated by the geomagnetic field, indicating that the de Vries cycle is indeed of solar, rather than climatic, origin