Determination of primordial and cosmogenic radioactivity in achondritic meteorites by low-level, gamma-ray spectrometry

A high-sensitivity, low-background gamma-ray spectrometer containing two 23 cm by 13 cm thallium-activated, sodium iodide detectors was used to measure long-lived primordial and cosmogenic radioactivity in a suite of achondritic meteorites. Potassium, thorium, uranium, and ²⁶Al abundances were established for sixteen brecciated eucrites, two unbrecciated eucrites, a nakhlite, a chassignite, and a unique meteorite from Antarctica by nondestructive counting techniques. In several cases, multiple samples of the same meteorite fall were examined. Concentrations ranged from 79.8 ppm to 1150 ppm for potassium, 55.6 ppb to 663 ppb for thorium, 18.1 ppb to 190 ppb for uranium, and 45.0 dpm/kg to 99.0 dpm/kg for ²⁶Al. In addition, a ¹³⁷Cs concentration of 264 dpm/kg was observed in the Allan Hills 77005,9 specimen.

Standards were prepared by dispersing small quantities of solids containing potassium, thorium, uranium, ²⁶Al, and ¹³⁷Cs into finely-divided iron powder. These mixtures were placed in polyethylene bags, which were then counted in the same geometries as those of the corresponding meteorite specimens. Counting times for the standards varied from approximately one to three hours, while those for the achondrites ranged from about five to fifteen days.

The densities of the meteorite specimens were determined using water phantoms constructed from aluminum foil coated with epoxy resin. In addition, documented specimen weights were verified.

vi Data reduction was accomplished with the aid of programs MOON and ALPHA-M. Experimental data were transferred from the memory of a multichannel analyzer unit to paper tapes and then were preprocessed through program MOON. Data cards returned as part of the output package from program MOON were sorted and subsequently used in ALPHA-M, a least-squares fitting program designed for the analysis of gamma-ray spectra.

Correlations between potassium, a volatile element, and thorium and uranium, both refractory elements, were made in relation to current models of meteorite genesis based on fractional crystallization and partial melting processes. Cumulate species, the first achondrites to form in underground magma chambers, exhibited decreased abundances of these elements compared to noncumulate species.

In addition to experimentally determined values, theoretical abundances of ²⁶Al were calculated for each meteorite specimen based on target element concentrations. Anomalously low experimental to-calculated ²⁶Al / ²⁶Al ratios were observed for many of the achondrites. Based on ²⁶Al measurements, an upper limit of 4.94 x 10 yr was suggested for the terrestrial age of the Allan Hills 77005 achondrite from Antarctica.