Elemental Compositions and Ages of Lunar Samples by Nondestructive Gamma-Ray Spectrometry

Abstract

A gamma-ray spectrometry system with low background was used to determine the radioactivity of crystalline rocks, breccias, and fine material. Nuclides identified were ⁴⁰K, ²³²Th, ²³⁸U, ⁷Be, ²²Na ²⁶A1, ⁴⁴Ti, ⁴⁶Sc, ⁴⁸V, ⁵²Mn, ⁵⁴Mn, and ⁵⁶Co. Concentrations of K, Th, and U ranged between 480 and 2550, 1.01 and 3.30, and 0.26 and 0.83 parts per million, respectively. Concentrations of thorium and uranium were those of terrestrial basalts, while the potassium concentrations were near values for chondrites. Products of low-energy nuclear reactions showed pronounced concentration gradients at rock surfaces. Concentrations of K and of ²²Na determined here were combined with concentrations of rare gases to estimate gas-retention ages and cosmic-ray exposure ages with ranges of 2200 to 3200 and 34 to 340 million years, respectively, for three rocks.