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

Self Cite

— Thick spherical targets made of gabbro (R = 25 cm) and of steel (R = 10 cm) were irradiated isotropically with 1.6 GeV protons at the Saturne synchrotron at Laboratoire National Saturne (LNS)/CEN Saclay in order to simulate the interaction in space of galactic cosmic‐ray (GCR) protons with stony and iron meteoroids. Proton fluences of 1.32 × 1014 cm−2 and 2.45 × 1014 cm−2 were received by the gabbro and iron sphere, respectively, which corresponds to cosmic‐ray exposure ages of about 1.6 and 3.0 Ma. Both artificial meteoroids contained large numbers of high‐purity target foils of up to 28 elements at different depths. In these individual target foils, elementary production rates of radionuclides and rare gas isotopes were measured by x‐ and γ‐spectrometry, by low‐level counting, accelerator mass spectrometry (AMS), and by conventional rare gas mass spectrometry. Also samples of the gabbro itself were analyzed. Up to now, for each of the experiments, ∼500 target‐product combinations were investigated of which the results for radionuclides are presented here. The experimental production rates show a wide range of depth profiles reflecting the differences between low‐, medium‐, and high‐energy products. The influence of the stony and iron matrices on the production of secondary particles and on particle transport, in general, and consequently on the production rates is clearly exhibited by the phenomenology of the production rates as well as by a detailed theoretical analysis. Theoretical production rates were calculated in an a priori way by folding depth‐dependent spectra of primary and secondary protons and secondary neutrons calculated by Monte Carlo techniques with the excitation functions of the underlying nuclear reactions. Discrepancies of up to a factor of 2 between the experimental and a priori calculated depth profiles are attributed to the poor quality of the mostly theoretical neutron excitation functions. Improved neutron excitation functions were obtained by least‐squares deconvolution techniques from experimental thick‐target production rates of up to five thick‐target experiments in which isotropic irradiations were performed. A posteriori calculations using the adjusted neutron cross sections describe the measured depth profiles of all these simulation experiments within 9%. The thus validated model calculations provide a basis for reliable physical model calculations of the production rates of cosmogenic nuclides in stony and iron meteorites as well as in lunar samples and terrestrial materials.

Section: Introductionmentioning

confidence: 99%

Section: Model Calculations Nuclide Production In Mixed Particle Fieldsmentioning

confidence: 99%

Simulation of the interaction of galactic cosmic‐ray protons with meteoroids: On the production of radionuclides in thick gabbro and iron targets irradiated isotropically with 1.6 GeV protons

Leya

Lange

Lüpke

et al. 2000

Self Cite

“…The additional uncertainties for Ne and Ar are much lower, i.e., 3% and 2%, respectively. However, when comparing noble gas data of different laboratories, additional uncertainties of 5% for concentrations and 1% for isotopic ratios have to be taken into account (Michel et al 1989). Tables 1 and 2 give the measured cosmogenic noble gas concentrations and isotopic ratios for He, Ne, and Ar for Grant and Carbo together with the sample locations.…”

Section: Uncertaintiesmentioning

confidence: 99%

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

Ammon

Masarik

Leya

2008

Abstract-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 36 Cl 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 21 Ne was produced from these minor elements. Using measured 21 Ne concentrations and endmember 22 Ne/ 21 Ne ratios for Fe + Ni and S + P, respectively, we show that it is possible to correct for 21 Ne produced from S and/or P. The thus corrected data are then used to calculate new 41 K-40 K 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 21 Ne concentrations and consequently decreases the 41 K-40 K exposure ages. The discrepancies between 36 Cl-36 Ar and 41 K-40 K 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 41 K-40 K and 36 Cl-36 Ar ages, may warrant re-investigation.

“…A surface discrepancy was already observed in St SCverin (Lavielle and Marti, 1988) and suggests a possible way to separate the parameters of shielding depth of the samples and of meteoroid sizes. This possibility will be further investigated in model calculations that are in progress, based on simulation experiments done at LNS (accelerator SATURNE, Saclay, France; Michel et al, Michel et al, 1989). Finally, Fig.…”

Section: Depth Profilesmentioning

confidence: 99%

“…Exposure histories of these objects can be derived by the determination of the production rates of the nuclides, properly corrected for shielding depth effects and abundances of the target elements. Such corrections can be obtained by using semi-empirical methods (Nishiizumi et al,, 1980;Baros and Regnier, 1984;Reedy, 1985;Eugster, 1988;Graf et al, 1990b;Graf and Marti, 1995), or by model calculations (Michel et al, 1986;Michel et al, 1989;Reedy et al, 1993;Masarik and Reedy, 1994). Measurements of cosmogenic nuclides in samples from several known positions in the same meteorite are still necessary to test and improve all methods of production rate determination.…”

Section: Introductionmentioning

confidence: 99%

Noble gas measurements in the L/LL5 chondrite Knyahinya

Lavielle

Toe

Gilabert

1997

Abstract-The LLL5 chondrite Knyahinya had an approximately spherical shape, and! as it experienced a single stage exposure history, it represents a very interesting object to study depth profiles of cosmic-rayproduced nuclide concentrations. Such data are required to improve and to validate model calculations of production rates. We report Ne, Ar, Kr and Xe isotopic abundances in five bulk samples. The adopted procedure of noble gas extraction included two pyrolysis steps at 450 "C and 650 "C, respectively, followed by a combustion step in pure 0 2 at 650 "C before melting the sample. This procedure allows for the separation of a significant fraction of the trapped Kr and Xe, leading to an enrichment of the cosmic-ray produced component, which is released in the melting step. The isotopic composition of the trapped Xe component measured in the combustion step is found to be identical with the OC-Xe composition (Lavielle and Marti, 1992) and supports the suggestion that ordinary chondrites formed in a homogeneous trapped noble gas reservoir. Cosmic-ray produced Kr and Xe components and depth profiles were measured, including for the first time a 81Kr profile. The calculated exposure age of 39.5 2 1.0 Ma, based on the 81Kr-G method, is found to be in excellent agreement with previous determinations. The concentrations of trapped and fissiogenic noble gas components are clearly lower than those generally observed in type 5 ordinary chondrites and may suggest diffusion losses before a meter-sized object was exposed to the cosmic radiation.