C. Tuniz

Beryllium-10 contents of core samples from the St. Severin meteorite

Abstract Eleven samples taken from along the 35-cm core AIII of the LL-chondrite St. Severin have 10 Be contents ranging from 20 to 28 dpm/kg and averaging 24.5 ± 0.9 dpm/kg. The 10 Be contents increase with depth in the outermost 8 cm while at depths greater than 8 cm the 10 Be contents change little. Calculations based on cross sections for proton-induced reactions that make 10 Be disagree seriously with the measured values. Relatively large cross sections were constructed for neutron-induced reactions below 1 GeV. They give calculated 10 Be production rates that agree with the measured values to within 10%. Cosmogenic 10 Be in stony meteorites is better described as a medium-energy product than as a high-energy one.

Beryllium-10 in Australasian Tektites: Evidence for a Sedimentary Precursor

Each of seven Australasian tektites contains about 1 x l08 atoms of beryllium-10 (half-life, 1.53 x 106 years) per gram. Cosmic-ray bombardment of the australites cannot have produced the measured amounts of beryllium-10 either at the earths surface or in space. The beryllium-10 contents of these australites are consistent with a sedimentary precursor that adsorbed from precipitation beryllium-10 produced in the atmosphere. The sediments must have spent several thousand years at the earths surface within a few million years of the tektite-producing event.

Beryllium-10 contents of shergottites, nakhlites, and Chassigny

Accelerator mass spectrometry gives the following 10Be contents (dpm/kg) for the SNC meteorites: Shergotty, 13.0 ± 1.5 and 17.3 ± 2.7; Zagami, 18.6 ± 2.5 and 20.0 ± 3.2; ALHA 77005, 15 ± 3; EETA 79001A, 7.8 ± 1.1 and 6.3 ± 0.5; EETA 79001B, 8.5 ± 1.1; Nakhla, 19.7 ± 3.3; Lafayette, 18.1 ± 2.5; Governador Valadares, 25.6 ± 3.6; Chassigny, 20.5 ± 3.1. The 10Be contents of the NC meteorites indicate that significant accumulation of cosmogenic nuclides occurred in decimeter rather than planetary-size bodies. The agreement of the 3He, 21Ne, and 10Be exposure ages of the shergottites also supports small-body irradiation. A long terrestrial age for EETA 79001 appears unlikely.

Production of 10Be in stony meteorites: Composition dependence

Abstract We measured, by accelerator mass spectrometry, the 10Be contents of 21 samples from 17 meteorites in an effort to estimate production rates of 10Be (dpm/kg) from the major elements (wt.%) present in stones. Regression analyses and other considerations yielded the equation 10Be = (0.32 ± 0.05) O + (0.30 ± 0.08) (Mg + 1.1 Al + 0.5 Si) + 0.15 S + 0.10 Ca + 0.05 (Fe + Ni) which reproduces the measured 10Be contents with an accuracy of about 10%. The high coefficient for Mg relative to O conflicts with production rates estimated from nuclear cross sections and from simulation experiments.

Spallogenic10Be in the Jilin chondrite

Abstract The 10 Be contents measured in eight bulk samples of the Jilin meteorite range from 5.05 ± 0.55 to 16.3 ± 1.7 dpm/kg, more than three fold, and correlate with the 21 Ne contents. A least-squares fit of these data has a positive intercept on the 10 Be axis which can be explained in terms of a two-stage irradiation. The second, more recent stage lasted 0.57 ± 0.14 Myr. Model calculations give first-stage 10 Be/ 21 Ne ages ranging from 6.2 to 7.1 Myr.

Production of 7Be, 22Na, 24Na and 10Be from Al in a 4π-irradiated meteorite model

A model of a small meteorite, 10 cm in diameter, made of granodiorite was rotated in a beam of 600 MeV protons to simulate 4π-irradiation conditions in space. Analysis of Al foils placed at known locations within the model give depth profiles of high energy (7Be, 10Be) and low energy (24Na, 22Na) spallation products. These isotopes were measured by means of high-resolution gamma-ray or accelerator mass spectroscopy (AMS). While the production rate profiles of the high energy products are flat within the limits of uncertainty, the profiles of the low energy products increase from the surface to the center by a factor of approximately 1.2. This increase indicates the importance of secondary neutrons and protons for the production of low energy spallation products, even within small meteorites.

Accelerator mass spectrometry II: Background reduction with a Wien (velocity) filter; 10Be contents of Central American basalts

Abstract Our limit of detection for 10 Be by accelerator mass spectrometry has been lowered to a few million atoms by the installation of a Wien filter which screens out co-transmitted 9 Be. Analyses of six Central American basalts gave 10 Be contents ranging from 1 × 10 6 to 60 × 10 6 atom/g. The sample richest in 10 Be has been contaminated by sediments. The other samples have 10 Be contents in the range reported in the literature by Brown and co-workers.

Accelerator mass spectrometry of10Be to study the fossil record of cosmic radiation

SummaryThe ultrasensitive accelerator mass spectrometry is applied to measure as few as 106÷107 atoms of10Be, which is present in terrestrial and extraterrestrial matter, and provides a time scale for many terrestrial and solar-system events of the last few million years. Some successful results are presented.RiassuntoSi applica la spettrometria di massa ultrasensibile con acceleratori per misurare fino a 106÷107 atomi di10Be, che è presente sia nella materia terrestre che extraterrestre e fornisce una scala temporale per molti eventi terrestri e del sistema solare degli ultimi milioni di anni. Si presentano alcuni soddisfacenti risultati.