ACTINIDE AND ULTRA-HEAVY ABUNDANCES IN THE LOCAL GALACTIC COSMIC RAYS: AN ANALYSIS OF THE RESULTS FROM THE LDEF ULTRA-HEAVY COSMIC-RAY EXPERIMENT

The LDEF Ultra-Heavy Cosmic-Ray Experiment (UHCRE) detected Galactic cosmic rays (GCRs) of charge Z >= 70 in Earth orbit with an exposure factor of 170 m(2) sr yr, much larger than any other experiment. The major results include the first statistically significant uniform sample of GCR actinides with 35 events passing quality cuts, evidence for the existence of transuranic nuclei in the GCR with one Cm-96 candidate event, and a low Pb-82/Pt-78 ratio consistent with other experiments. The probability of the existence of a transuranic component is estimated as 96%, while the most likely U-92/Th-90 ratio is found to be 0.4 within a wide 70% confidence interval ranging from 0 to 0.96. Overall, the results are consistent with a volatility-based acceleration bias and source material which is mainly ordinary interstellar medium material with some recent contamination by freshly synthesized material. Uncertainty in the key U-92/Th-90 ratio is dominated by statistical errors resulting from the small sample size and any improved determination will thus require an experiment with a substantially larger exposure factor than the UHCRE.