Cosmogenic Nuclides In Meteorites Research Articles

Galactic Cosmic Rays, Cosmic-Ray Variations, and Cosmogenic Nuclides in Meteorites

Abstract We present a new generation of model calculations for cosmogenic production rates in various types of solar system bodies. The model is based on the spectra for primary and secondary particles calculated using the INCL++6 code, which is the most reliable and most sophisticated code available for spallation reactions. Thanks to the recent improvements (extending the code to lower and higher energies and considering light charged particles as ejectiles and projectiles), we can for the first time directly consider primary and secondary Galactic α particles. We calculate production rates for 22Na, 10Be, and 26Al in an L-chondrite with a radius of 45 cm and in the Apollo 15 drill core, and we determine the long-term average Galactic cosmic-ray (GCR) spectrum (represented by the solar modulation potential Φ) in the meteoroid orbits at ∼3 au of Φ = 600 MV and at 1 au, i.e., for Earth and Moon of Φ = 660 MV. From this, we calculate a long-term average GCR gradient in the inner solar system of ∼5% au−1. Finally, we discuss the possibility of studying temporal GCR variations and meteoroid orbits using production rate ratios of short- and long-lived radionuclides.

James R. Arnold: From the Manhattan Project to the moon and beyond

On January 6, 2012, the field of cosmochemistry (more specifically, meteoritics and lunar and planetary sciences) lost one of its founding scientists. Jim Arnold's scientific endeavors ranged widely, ranging from work on the Manhattan Project to the origin and use of cosmogenic nuclides in meteorites and lunar samples and to sampling cosmic dust with balloons.

The Galactic Cosmic Ray Intensity over the Past 106–109 Years as Recorded by Cosmogenic Nuclides in Meteorites and Terrestrial Samples

Concentrations of stable and radioactive nuclides produced by cosmic ray particles in meteorites allow us to track the long term average of the primary flux of galactic cosmic rays (GCR). During the past ∼10 Ma, the average GCR flux remained constant over timescales of hundreds of thousands to millions of years, and, if corrected for known variations in solar modulation, also during the past several years to hundreds of years. Because the cosmic ray concentrations in meteorites represent integral signals, it is difficult to assess the limits of uncertainty of this statement, but they are larger than the often quoted analytical and model uncertainties of some 30%. Time series of concentrations of the radionuclide 10Be in terrestrial samples strengthen the conclusions drawn from meteorite studies, indicating that the GCR intensity on a ∼0.5 million year scale has remained constant within some ±10% during the past ∼10 million years. The very long-lived radioactive nuclide 40K allows to assess the GCR flux over about the past one billion years. The flux over the past few million years has been the same as the longer-term average in the past 0.5–1 billion years within a factor of ∼1.5. However, newer data do not confirm a long-held belief that the flux in the past few million years has been higher by some 30–50% than the very long term average. Neither does our analysis confirm a hypothesis that the iron meteorite data indicate a ∼150 million year periodicity in the cosmic ray flux, possibly related to variations in the long-term terrestrial climate.

Interpretation of cosmogenic nuclides in meteorites on the basis of accelerator experiments and physical model calculations

Cosmogenic nuclides in extraterrestrial matter provide a wealth of information on the exposure and collision histories of small objects in space and on the history of the solar and galactic cosmic radiation. The interpretation of the observed abundances of cosmogenic nuclides requires detailed and accurate knowledge of their production rates. Accelerator experiments provide a quantitative basis and the ground truth for modeling cosmogenic nuclide production by measurements of the relevant cross sections and by realistic simulations of the interaction of galactic protons with meteoroids under completely controlled conditions, respectively. We review the establishment of physical model calculations of cosmogenic nuclide production in extraterrestrial matter on the basis of such accelerator experiments and exemplify this approach by presenting new experimental and theoretical results for the cosmogenic nuclide44Ti. The model calculations describe all aspects of cosmogenic nuclide production and allow the determination of long-term solar and galactic cosmic ray spectra and a consistent interpretation of cosmogenic nuclides in extraterrestrial matter.

On Cosmic-Ray Exposure Ages of Terrestrial Rocks: A Suggestion

An important recent development in the field of geomorphology has been the application ofin-situcosmic-ray-produced nuclides to obtain modelerosion ratesandsurface exposure ages.These concepts emerged some four decades ago in studies of cosmogenic nuclides in meteorites, but cannot generally be used analogously for terrestrial rocks. The differences in the two cases are outlined. For the case ofsteady-state erosional histories, the terrestrialsurface exposure agesdepend on the half-life of the radionuclide studied. A suggestion is made for presenting thesurface exposure ages, which allows a clear definition of the meaning of the estimated exposure ages. In the case of adiscrete exposure history, the meaning of “exposure age”—which should more appropriately be called “event age”—is however quite unambiguous.

Inversion of cosmogenic-nuclide data from iron meteorites

The interpretation of the observed abundances of cosmogenic nuclides in meteorites involves two considerations, namely, the cosmic-ray-exposure ages and the past behaviour of the particle flux. Traditional methods of analysis do not have the mathematical structure to simultaneously deal with both questions in a convenient and rigorous fashion. Reformulating this problem within the framework of inverse theory allows for a new and powerful perspective: it enables us to make simultaneous, self-consistent estimates of the flux prehistory and the exposure ages, and of their uncertainties. In this paper we have focussed on inversions that would, if possible, give constant-flux prehistories, and, in any case, would give the flattest model in a least-squares sense. This allows us to show by rigorous examination that the available data are inconsistent with the assumption that the flux was constant over a period of 109 years. While this is not an original conclusion, it is reached here by a much more direct analysis, and we are able to make a defensible estimate for the change in the 109 years. The analysis also yields new estimates of the exposure ages that are consistent with the time-varying flux. That is, one does not need to presuppose any particular functional form for the flux. The application of inverse theory to such problems also makes available additional benefits that have been widely recognized in geophysical analysis. For example, it forces one to explicitly acknowledge the inherent assumptions, it provides quantitative estimates of the uncertainty of the results, and it provides a means of anticipating the value of new experimental data. We have based our demonstration on published data from four individual iron meteorites, since these data apparently provide a self-consistent set. Smoothed estimates are derived of the long-term prehistory of the particle flux at similar depths within these bodies (1000 kg m−2 as inferred from the cosmogenic nuclides 21Ne and 40K). With our preferred choice for the present value of the particle flux, the results independently support a model in which the magnitude of the particle flux 109 years ago was about 40% less than the present value; the individual estimates for the four meteorites of the ratio of the present flux to the flux 109 years ago are 1.75 (Mount Ayliff), 1.66 (Yardymly), 1.64 (Williamstown), and 1.72 (Carbo). The exposure ages consistent with this history are smaller, by some 10–20%, than previous estimates. One can choose a smaller value for the present particle flux to bring the ages into agreement with previously estimates values, but then a substantially larger change in flux with time is required. Thus our analysis provides a sophisticated test of the possibility of a constant-flux model.

Production of stable and radioactive nuclides in thick stony targets ( R = 15 and 25 cm) isotropically irradiated with 600 MeV protons and simulation of the production of cosmogenic nuclides in meteorites

Two artificial meteoroids made out of gabbro with a density of 3 g cm −3 with radii of 15 and 25 cm were isotropically irradiated with 600 MeV protons in order to simulate the production in meteoroids of cosmogonie nuclides by galactic cosmic ray protons. The depth dependent production of a wide range of radionuclides from target elements O, Mg, Al, Si, Ti, Fe, Co, Ni, Cu, Ba, Lu, and Au was measured. Furthermore, the production of He and Ne isotopes from Al, Mg, Si as well as from degassed meteoritic material was determined. Together with earlier results on an artificial meteoroid with a radius of 5 cm, and with data derived from thin-target experiments, the depth dependence of production rates is investigated for radii from 0 to 75 g cm −2. 60Co from Co shows the strongest size dependence; the center production rates differ by a factor of 100 for radii of 5 and 25 cm. Other low-energy products, like 58Co from Co and 24Na produced from Al, increase only up to a factor of 3.5 over the entire range of radii. For extreme high-energy products, in contrast, the center production rates decrease by up to a factor of 10. The observed depth profiles show a wide varity of shapes. Low-energy products have pronounced maxima in the center, high-energy products exhibit strong decreases from surface to center and, in between, essentially flat profiles are seen as well as such with a transition maximum. The spectra of primary protons and of secondary protons and neutrons in the artificial meteoroids were calculated using Monte Carlo techniques. The fluxes of secondary protons and neutrons depend strongly on depth and size, the spectral shapes being different for protons and neutrons. Calculating also the nucleon spectra which result from irradiation with real GCR p-spectra, the differences between simulation experiments and cosmic irradiation conditions are quantitatively described. On the basis of all these spectra and of thin-target excitation functions, production rates were calculated and compared with the experimental ones. The theoretical depth profiles allow to distinguish the different contributions of primary and secondary particles and to unravel the various production modes of cosmogenic nuclides in meteoroids. Our investigation shows that it is possible to model the production of residual nuclides in artificial meteoroids with excellent accuracy by thin-target calculations, provided that reliable thin-target excitation functions are at hand.

On the production of cosmogenic nuclides in meteorites by primary galactic particles: Cross sections and model calculations

Cross sections for the proton‐induced production of radionuclides (40≦A≦60) from natural titanium, iron, and nickel were measured for proton energies from 80 to 200 MeV. To critically survey the theoretical predictions of unknown excitation functions, the experimental data were compared with calculations according to Blann's hybrid model of nuclear reactions and to the semiempirical formulas of Rudstam and of Silberberg and Tsao. Then, based mainly on the new excitation functions, the production rates of cosmogenic radionuclides in meteorites by primary galactic protons were calculated. Since the interaction of primary solar and galactic particles with meteorites can be described with accuracy, the model calculations allow for a quantitative estimate of the action of secondary particles in meteorites. A method of completely describing the production of cosmogenic nuclides in meteorites that takes into account all sources of production is proposed.

Measurement of cross sections for 22Na, 20-22Ne and 36-42Ar in the spallation of Mg, Al, Si, Ca and Fe. Production ratios of some cosmogenic nuclides in meteorites

Measurement of cross sections for 22Na, 20-22Ne and 36-42Ar in the spallation of Mg, Al, Si, Ca and Fe. Production ratios of some cosmogenic nuclides in meteorites

Spallation products distributed in a thick iron target bombarded by 3-Bev protons

The distribution of several spallation products, covering a wide range of atomic number, has been measured in a thick iron target (30×30×90 cm) bombarded by 3-bev protons at the Brookhaven cosmotron. Measurements were made of the radioactivity of γ-, β-, and X-ray emitters, either directly or after chemical separation, in iron and nickel foils placed at different depths. Products such as Co isotopes in Ni and Fe55, Mn54, and Cr51 in Fe, which are close to the target mass, were produced in important amounts at large angles to the primary beam. Production of nuclides of lower A showed gradually better collimation, Na22 and Be7 showing the smallest spread. These data provide useful information on the production rates of cosmogenic nuclides in meteorites and on their variation with depth and size. The results of this experiment may be compared with the available meteorite data and with other calculations.