Radioactive 26Al measured on LDEF satellite clamps - What is its source?

Since 1984 astrophysicists have speculated on the original source of the 26Al measured by the HEAO 3 satellite gamma-ray telescope. With a half-life of 717 000 years it cannot be primordial, and the Sun cannot produce it. Possible sources suggested have included gamma radiation directly from various astronomical entities, such as novae, supernovae, AGB stars or Wolf-Rayet stars, or radiating particles from some of those sources present in the local interstellar medium. It has also been suggested that some of the 26Al gamma radiation being observed might be coming from grains produced in nearby supernovae, or from cosmic-ray spallation reactions occurring on heavier atoms in space. These atoms or particles might have been swept down into the local interstellar medium by the solar or stellar winds. Using a multidimensional gamma-ray spectrometer we have measured the disintegration rates of 26Al on four clamps which flew for 5.8 years on the Long Duration Exposure Facility satellite. Evaluation of possible local internal production of 26Al by cosmic-rays during the known flight path of the satellite has determined that the observed 26Al disintegration rates of the four clamps range from 17 to 32 times the rates from local production. Thus, the 26Al observed is likely from space, confirming earlier discoveries of its presence in the local interstellar medium. Based on our measurements, a tentative maximum value of 0.000052 ± 0.000006 atoms cm-3 is derived for the density of 26Al in near Earth space.