The relative abundance of actinides in the cosmic radiation

The DIAS-ESTEC Ultra Heavy Cosmic Ray Experiment (UHCRE) on the Long Duration Exposure Facility (LDEF) collected approximately 3000 cosmic-ray nuclei with Z>65 in the energy region E>1.5 GeV/nucleon during a six year exposure in Earth orbit. The entire accessible collecting area of the solid-state nuclear-track detector (SSNTD) array has been scanned for actinides, yielding a sample of thirty from an exposure of ≈150 m2 sr yr. The observed charge spectrum is presented. The current best value for the cosmic-ray actinide relative abundance, (Z>88)/(74≤Z≤87), is given. This value is considered in the context of the predictions of source/propagation models.