26Al in iron meteorites and the constancy of comic ray intensity in the past

Cosmic-ray-produced 26Al in iron meteorites has been measured by low-level γγ-coincidence counting. The 26Al activities, in dpm/kg, are: Aroos 3.0 +- 1.0, Braunau 2.6 +- 0.5, Kayakent 4.6 +- 1.5, N'Goureyma 4.4 +- 1.1, Okahandja 3.6 +- 0.9, Treysa 4.0 +- 0.5. Exposure ages based on 26Al/21Ne are in agreement, within experimental error (+-20%) , with those based on 36Cl/36Ar and 39Ar/38Ar but the ages based on 40K/41K are higher by about 50%. The difference in exposure ages is probably caused by a real change of the cosmic ray intensity in the inner solar system. (Auth.)