| Popis: |
Cosmic-ray-produced 37 Ar ( t 1/2 = 35 days), and 39 Ar ( t 1/2 = 269 years) in the Fe-Ni phase of meteorites have been studied in connection with their implications for the radial gradient of cosmic ray particles. For the chondrite, Can˜on City, which fell on October 27, 1973, we found 20.1 ± 1.5dpm/kg FeNi of 37 Ar and 22.5 ± 1.4dpm/kg FeNi of 39 Ar. Usually, the 37 Ar/ 39 Ar ratio is used to deduce a spatial gradient. However, 37 Ar data reported in the literature are inconsistent. They fluctuate much more than could be accounted for by different orbits and the anticipated correlation with the cosmic ray neutron registrations is rather weak. Consequently, the 37 Ar/ 39 Ar-derived gradient has a low confidence level. On the other hand, 39 Ar activities group close to a mean value of 22.5 dpm/kg FeNi and appear to be almost independent of the different meteoroid orbits. A comparison of measured 39 Ar activities in meteorites with those calculated for orbits obtained from fireball data shows that a gradient of 10%/Au or less is reconcilable with the experimental findings. The average gradient ( E > 200MeV) during the last 500 years was probably not much larger than that measured presently by the Pioneer 10 and 11, and Helios spacecrafts. |
