Measurements of Radioactivity in the Meteorite of April 25, 1969

Abstract

TWO fragments of the meteorite of April 25, 1969 (ref. 1), have been examined by scintillation spectrometry for gamma-ray emission. The first (weight 231 g, loaned by Mr A. D. Andrews of Armagh Observatory) was placed between two NaI(Tl) scintillation counters, 15 cm in diameter and 9 cm thick, at a separation of 4.6 cm. Counting was started 115 h after the fall and a rate of 102 c.p.m. was observed above a background of 1,293 c.p.m.; the gamma-ray spectrum showed that the response was confined to the region below 3.15 MeV. Despite the low activity, seven peaks were apparent at 0.511, 0.66, 0.84, 1.02, 1.28, 1.46 and 1.81 MeV, and these were attributed to annihilation radiation, caesium-137, manganese-54, annihilation radiation coincidences, sodium-22, potassium-40 and aluminium-26, respectively. The presence of caesium-137 was unexpected; it was considered to be associated with adherent fragments of the asbestos roof which the meteorite had pierced, and when these were removed the peak at 0.66 MeV was no longer observed. Because of the low counting rate and the relatively long interval between the fall and the start of the observations, it was not possible to detect short-lived radionuclides (for example, ²⁴Na) or those of low abundance (for example, ⁵²Mn, ⁴⁶Sc, ⁶⁰Co); also, because of summation effects in the gamma-ray spectrum it was only possible to derive approximate values for the contents of ⁵⁴Mn and potassium and for the ¹³⁷Cs contamination. These were 75–110 d.p.m. kg⁻¹, 0.06–0.10 per cent and 4.5–7.0 pCi respectively, depending on exact values for the geometrical efficiency and self-absorption. More detailed and accurate data must await the results of calibration measurements which are planned.