Argon-39 in the Sikhote-Alin Meteorite Fall

Abstract

THE radioactive isotope argon-39 (260-year half-life) has been determined in a specimen of the Sikhote-Alin meteorite fall which occurred on February 12, 1947. This fall¹ was very large, consisting of more than a hundred fragments weighing up to 300 kgm. The measurements were made on samples taken from a 150-gm. bar of the meteorite obtained from Truman P. Kohman, who in turn received the bar from E. L. Krinov. The bar was piece No. 5, which was sawn from specimen No. 2052, according to E. L. Krinov's designation. Argon-39 was not detected in the Carbo meteorite which was found in Mexico in 1923. The detection and measurement of argon-39 in the Sikhote-Alin meteorite makes it possible to determine a new type of age (argon-38/argon-39), which can be called an argon exposure age. The amount of argon-39 also gives an estimate of the cosmic-ray intensity that irradiated the Sikhote-Alin sample in space. From the lack of argon-39 in the Carbo meteorite, the date of fall of which is unknown, one can conclude that it fell more than 1,500 years ago.